Claire Polo

Jean-Marc Colletta

The Multimodal Mediation of Knowledge: instructors' explanations in a scientific café

In this paper, we present an in depth study of the interplay between various semiotic modes in the specific instructional setting of a scientific café. We analyzed the multimodal performance of five female instructors delivering a monologue explanation during instruction on the following dimensions: speech, gesture (hand gestures, head orientation and gaze) and use of written didactical material. Results first point out the crucial role played by referential hand gesture together with gaze-body behavior both in representing new concepts (conceptual mediation) and in building bridges between information displayed in several modes (semiotic mediation). They also show crossindividual differences in instructors' multimodal performance, that we propose to interpret as three diverse modes of mediating knowledge, guiding being the only one providing both conceptual and semiotic mediation.

State of the art: multimodality at the heart of cognitive processes

. For the speech-gesture system, it has been shown that any mode can contribute to both the

Introduction

Recent literature emphasizes the role of multimodality in teaching, but how do instructors actually use and combine diverse semiotic modes in authentic face-to-face interactions with learners? This exploratory study offers insights into the different ways novice instructors multimodally mediate knowledge when using slides in a one to many lecturing scenario. The multimodal dimension of teaching was first studied as using complementary external representations of concepts like pictures, diagrams, algebraic formula, and such [START_REF] Duval | Semiosis et Pensée Humaine[END_REF]. This perspective was enlarged to encompass oral communicative resources within what Arzarello calls the 'semiotic bundle' [START_REF] Arzarello | Semiosis as a Multimodal Process[END_REF][START_REF] Flevares | How many do you see? The use of nonspoken representations in first-grade mathematics lessons[END_REF][START_REF] Lund | La reformulation multimodale et polysémiotique comme aide à la compréhension de la physique[END_REF][START_REF] Bécu-Robinault | Modèles implicates dans les reformulations multimodales des enseignants[END_REF]. Some studies focused on face-to-face resources, notably speech and gesture [START_REF] Alibali | How Teachers Link Ideas in Mathematics Instruction Using Speech and Gesture: A Corpus Analysis[END_REF][START_REF] Goldin-Meadow | Gesture's role in the learning process[END_REF][START_REF] Singer | Children learn when their teacher's gestures and speech differ[END_REF][START_REF] Valenzeno | Teachers' gestures facilitate students' learning: A lesson in symmetry[END_REF]. Works on discursive skills also show that the use of multimodal resources gets more complex as a child grows (e. g. [START_REF] Colletta | Comparative analysis of children's narratives at different ages, a multimodal approach[END_REF][START_REF] Colletta | The development of multimodal explanations in French children[END_REF][START_REF] Graziano | Le développement des gestes pragmatiques et leur relation avec le développement de la compétence textuelle chez l'enfant âgé de 4 à 10 ans[END_REF], and may vary depending on the interactional setting (Mazur-Palandre, [START_REF] Mazur-Palandre | Context sensitive 'how' explanation in children's multimodal behavior[END_REF]. A common instructional practice consists in asking some tutor students to explain something to their peers, or to younger students. Nevertheless, how they manage to do so has still not been precisely studied. Our paper explores this issue on the basis of videotaped explanations elaborated by senior students (here after named 'instructors') leading a scientific café about drinking water management, for junior students (here after named 'students'), in France and the USA. How do they make use of diverse semiotic resources to mediate the knowledge that they have to explain? As interactional linguists, we investigated this issue with an inductive approach, starting from data sessions and then defining our analytical categories according to two methodological principles: their data adequacy (ability to actually describe the observed phenomena) and their interpretative power (ability to make sense of the observed phenomena in the perspective of the participants) (e. g. [START_REF] Mondada | Chercheurs en interaction[END_REF].

primary and the secondary building of reference: gestures can contribute the same meaning as the words, or they can "refine, qualify, or make more restricted the meaning of the words... and sometimes even provide aspects of reference that are not present at all in the verbal component" (Kendon, 2004: 161). Our interest in the building of reference corresponding to the concepts explained in the situation studied led us to do what some authors call 'a semantic feature analysis': "A semantic feature analysis is the systematic evaluation of how gestures and words represent meaning" (Gerwing & Allison, 2009: 5). As a result, we focused on students' gestures playing a referential function, i.e. representational and pointing gestures. Representational gestures directly "exploit imagery" (Goldin-Meadow, 2004: 314), and provide embodied images of the referent in a diversity of ways. Some representational hand gestures depict a concrete physical referent by drawing in space its imaginary shape and dimension, or by locating parts of it or tracing its trajectory in an imaginary space, while others use a part of the body, or the whole body to mime actions from an animated referent [START_REF] Colletta | Multimodal Data Transcription and Annotation with ELAN. Coding Manual, ANR "Multimodality[END_REF][START_REF] Cosnier | Sémiotique des gestes communicatifs[END_REF][START_REF] Kendon | Gesture: Visible action as utterance[END_REF][START_REF] Mcneill | Hand and Mind: What Gestures Reveal about Thought[END_REF][START_REF] Streeck | Gesturecraft. The manu-facture of meaning[END_REF]. Other representational gestures help express ideas by providing visual metaphors for abstract concepts (e.g. the gripping / giving / trenching /chasing gestures to express respectively founding / sharing / dividing / chasing) [START_REF] Calbris | Elements of Meaning in Gesture[END_REF], Cienki & Muller, 2008[START_REF] Mcneill | Hand and Mind: What Gestures Reveal about Thought[END_REF]. Representational gestures can be organized sequentially, in a way that gives a specific meaning to the repetition of similar gestures, or to the use of a given section of the gestural space. Pointing gestures are defined as "a communicative bodily movement which projects a vector whose direction is determined, in the context, by the conceived spatial location, relative to the person performing the gesture, of a place or thing relevant to the current utterance" [START_REF] Enfield | Primary and secondary pragmatic functions of pointing gestures[END_REF]: 1724). Among pointing gestures, a distinction is needed between concrete pointing, directed to a person or to an object physically present and abstract pointing, directed to an empty section of the gestural space. Concrete pointing appears early in language development [START_REF] Colletta | The development of multimodal explanations in French children[END_REF] and has mostly a referential function [START_REF] Enfield | Primary and secondary pragmatic functions of pointing gestures[END_REF]. When concrete pointing is directed to a person, it can serves a different, interactive function [START_REF] Bavelas | Interactive gestures[END_REF]. In the context of the present paper, all the concrete pointing gestures studied are directed to the screen where written data is projected, thus clearly referring to referential information provided on the slide. Abstract pointing belongs to the category of abstract deictics [START_REF] Mcneill | Hand and Mind: What Gestures Reveal about Thought[END_REF], and, as all the abstract gestures, it appears later in language development (e. g. [START_REF] Colletta | The development of multimodal explanations in French children[END_REF]. Abstract pointing can be used in combination with representational gestures in gestural sequences. Pointing gestures are then directed to sections of the gestural space with specific attributed meaning and serve a referential function. In instructional settings, multimodality and gesture have been studied with the idea that varying the modes of representation of concepts improves the teaching-and-learning process [START_REF] Alibali | How Teachers Link Ideas in Mathematics Instruction Using Speech and Gesture: A Corpus Analysis[END_REF][START_REF] Goldin-Meadow | Gesture's role in the learning process[END_REF][START_REF] Singer | Children learn when their teacher's gestures and speech differ[END_REF][START_REF] Arzarello | Semiosis as a Multimodal Process[END_REF][START_REF] Flevares | How many do you see? The use of nonspoken representations in first-grade mathematics lessons[END_REF][START_REF] Lund | La reformulation multimodale et polysémiotique comme aide à la compréhension de la physique[END_REF][START_REF] Bécu-Robinault | Modèles implicates dans les reformulations multimodales des enseignants[END_REF][START_REF] Valenzeno | Teachers' gestures facilitate students' learning: A lesson in symmetry[END_REF]. Among the consensual results, it is well established that as teachers get experienced, or trained, they tend to make a greater use of multimodal combinations, at least in terms of speech-gesture system (e. g. [START_REF] Caswell | Body language for competent teachers[END_REF]. Empirical studies also brought growing evidence that gestures improve the efficiency of teaching discourse (i. g. [START_REF] Alibali | Students learn more when their teacher has learned to gesture effectively[END_REF][START_REF] Cook | Consolidation and transfer of learning after observing hand gesture[END_REF][START_REF] Perry | Constructing shared understanding: The role of nonverbal input in learning contexts[END_REF][START_REF] Singer | Children learn when their teacher's gestures and speech differ[END_REF][START_REF] Tellier | The effect of gestures on second language memorisation by young children[END_REF][START_REF] Valenzeno | Teachers' gestures facilitate students' learning: A lesson in symmetry[END_REF].

Therefore, educational settings seem to be particularly appropriate places for the development of rich multimodal communication. The explanatory sequences studied in this paper are made by instructors with no experience in teaching and limited content knowledge about the topic. Through an empirical study, we try to identify whether some specific multimodal combinations are associated with deeper explanations in terms of the amount and nature (conceptual deepening or semiotic bridging) of the displayed information. We then propose a model of knowledge mediation as a multimodal performance.

Context & Method

Instructional context and data set

We investigated an extracurricular activity held at school in the USA and France, led by instructors aged 17-18, for students aged 12-15. Based on an international project of environmental education, this scientific café aimed at helping the students explore a socio-scientific issue (here drinking water management) and debate about it as future citizens. The senior students were volunteers, and they were trained for 7 hours in order to become 'instructors'. The cafés lasted between 90 and 120 minutes and alternated class discussion, group discussion, group voting and individual voting, on the basis on a multiple-choice questionnaire. The whole activity was divided into three topical sections: 1) water resources; 2) human uses of water; 3) water management, distribution and pricing. Some questions, called 'knowledge questions' (KQ), provided students with basic information, while others introduced debates about socio-scientific issues.

To choose an answer about a KQ, the students used an electronic voting system. The percentages of votes for each option was then displayed on the screen, and the right answer turned green. In the next step, the instructor explained why this answer was considered true, on the basis of a slide called 'information desk' (consisting of written data and information source). This is the specific task that we studied, on the basis of an audiovisual corpus of the cafés, which were fully recorded. We chose to focus on two KQ. The first one (here after referred to as Q1) deals with how much water is needed to make products of common use, or the 'virtual water' embedded in such products. The second one (here after referred to as Q2) consists of assertions about the price of drinking water in different places of Manila in comparison with its price in developed countries. The slides corresponding to Q1 and Q2, for the American and the French version of the activity, are reproduced in appendices 1-4. All the activity was videotaped with several cameras, one of them recording a global view of the classroom combined with the sound from the instructor's microphone. This is the track that we used for the analyses presented here. Out of the 6 fully recorded cafés (3 in each country), one was left aside for this analysis because the instructor moved out of the camera frame while explaining the selected questions. Therefore, our final data set included the explanations made by 5 people, 3 American and 2 French female instructors (renamed for this research Cathy, Marlene and Iris, and Océane and Sylvie). In the instructional sequence, the phase of explanation of a KQ is considered as interactive: the instructor must explain the right answer, answer students' questions and doubts, and make sure that they have well understood the involved concepts. Nevertheless, the time dedicated to this dialogue varied a lot. In order to work on comparable data, we decided to limit our analysis to the monologue part of this phase, when the instructor explains the right answer to the audience. Table 1 presents the data collection. The 10 phases of 'information desk' lasted over than 18 minutes, while the monologue explanations totalized half of this.

Table 1. Overview of the video data.

Methodological path

The whole explanatory task can be considered as supplementing the 'information desk' slide in order to help the students understand the target knowledge. We defined such activity as mediation. Three research questions guided our work: 1) How do instructors make use of the available multimodal resources to mediate the target knowledge? 2) What are the effects of the multimodal combinations that they use regarding the information provided to the students? 3) Can we determine a general model of the different multimodal practices used to mediate knowledge? We studied instructors' multimodal mediation practice regarding three types of semiotic resources: speech, gestures, and the data written on the slide. The boundaries of what constitutes a mode in terms of channels providing different information pieces of the global message can be discussed. Even in speech, the information contained in words can be completed or disambiguated thanks to elements of prosody. The 'gesture' label may also refer to a large range of phenomena, from the study of gaze to kinesics and proxemics, including the restrictive use of the term 'gestures' corresponding to hand movements in an individual gestural space. The third type of semiotic objects that we study here, data to be visualized on the screen, also presents a diversity of representational forms, including both text and tables. For Q1, the written data source consisted both in text and a table (cf. appendix 2), while it was only text for Q2, (cf. appendix 4). The overall explanation performance of the instructors was studied through three analytical steps, focusing on 1) verbal explanation of the slide content, 2) gestural behavior, and 3) identification of relevant cross-individual differences among the empirically observed multimodal combinations. First, we focused on the speech dimension, and fully transcribed the instructors' speech (for transcription conventions cf. appendix 5). We then identified the parts of the transcripts that corresponded to a literal re-use of the words of the 'information desk' slide. Examples of such transcripts are provided for the case studies detailed in section 4. Most of the instructors' speech was not pure repetitions but actually embedded innovative content. These innovative elements were classified in three categories: rewording, elucidation and addition. By rewording, the instructor does not really provide new information, but she rephrases the information of the slide with her own words. Other parts of the transcripts correspond to elucidation: the instructor then relates pieces of information that are presented separately in the slide, or produces inferences based on the content of the slides. When the instructors verbally provided totally new information that was not in the slide, it was classified as addition. Additions mostly consisted in giving examples, providing reasons or anticipating implications and consequences of the ideas presented on the slide. Table 2 summarizes this classification of these verbal elements and provides an authentic example for each category. In our data, we found a total of 32 instances of rewording, 23 of elucidation, and 36 of addition.

Table 2. Typology of mediating elements in instructors' speech.

Our second analytical step focused on gestures. What we coded as gestures included the 3 typical phases of preparation, stroke and retraction [START_REF] Kendon | Gesticulation and speech: Two aspects of the process of utterance[END_REF], sometimes with a holding phase. When a gesture is hold and then serves as a basis for another gesture, then the new one is counted as another gesture, even if the first one has not finished yet. Such gestural analysis was conducted using the ELAN software. First, the attention focus of each leader all along his/her explanation was coded, on the basis of head and gaze orientation. This led us to observe an attention focus pattern distributed between the screen and the audience (this result is detailed in 4.1). For each time slot corresponding to a mediating verbal element (91 in total), we identified co-occurring representational and pointing gestures. For representational gestures and abstract pointing, a referent was systematically associated to each gesture. A table visualization of each excerpt was made in order to put gestural information in perspective with the attention focus of the leader and her co-occurring speech. Examples of this visualization are given in section 4. This analytical step made it possible to identify information gesturally added by the instructors in their explanations. Our last analytical phase consisted in stepping away from this accumulation of qualitative results, trying to identify tendencies among the explanatory practices analyzed. We empirically characterized instructional explanatory practice as combinations of two co-verbal gestural micro-activities: representational gesturing associated to focusing attention to the audience and concrete pointing associated to focusing attention to the screen. As a combination of multimodal resources, we could then define the instructors' instructional activity as enriching the building of reference by both adding new information (conceptual mediation) and establishing bridges between different semiotic representations of the involved concepts (semiotic mediation).

Results

Our first general result was to observe a clear pattern of attention focus alternation between the screen and the audience (4.1). Through a first case study we explore what typically occurs when the leaders are facing the students (4.2), making use of representational gestures to complement their speech. We then turn to another case study (4.3), corresponding to a transitional phase in the focus alternation pattern, in which both speech, gestures and head orientation serve as bridges to mediate concepts between diverse semiotic modes. Finally, we propose a multimodal model characterizing the explanatory practice of instructors as providing the students with more or less conceptual and/or semiotic mediation of knowledge (4.4).

4.1 Paying attention to both the students and the data to explain: attention focus alternation A first striking result is the observation that all the instructors alternate phases of attention focus directed to the screen and phases of attention focus directed to the students, the attention focus being defined according to the head and gaze orientation. Figures 1,2, 3, 4 and 5, based on screen captures of ELAN time lines show this pattern respectively for each instructor. Moments spent on the 'information desk' slide which do not correspond to monologue explanation were not coded and appear in black. The moments of attention focus directed to the students are highlighted in grey, the rest of time corresponding to an attention focused on the screen. Still, although the alternating pattern was found for the five instructors, the time dedicated to each single episode directed to one or the other varies a lot. Both occurring during Q2, the longest student-focused episode's duration is 18s. and shows in Océane's instruction, and the shortest student-focused episode's duration is 0.5 s. and shows in Sylvie's instruction. While only two other episodes last more than 10 s., there are 20 episodes lasting 1 s. or less. No typical duration can be established for a single episode of attention focus. Nevertheless, it is worth mentioning the general time proportion of the explanation directed to the screen vs focused on the students. This information is provided in table 3. Table 3 also presents the time proportion of each explanation corresponding to representational and pointing gestures. The instructors' behavior is especially rich in referential gestures, since, summing up pointing and representational gestures, the instructors (except Iris, who gestures very little) spent from 42% to 77% of their time providing referential information gesturally. Table 3. Time proportions of student-focused attention, representational gesturing and concrete pointing in explanations and associated gesture rates.

In table 3, clear cross-individual variation appears. Iris spent relatively little time facing the students (less than 10% for Q1 and less than 14% for Q2). She is also the one producing the least gestures, holding the record of the smallest proportion of pointing time (0%, for Q1), pointing gesture rate (0.2 for Q2), and the smallest proportion of time with representational gestures (12,9%, corresponding to the smallest rate of representational gestures too, 1 per 10 s., for Q2). Her poor gesture production goes along with a predominant attention focus toward the screen. Marlene spent one quarter to one third of her explanation time facing the students (23% for Q1 27,6% for Q2). Two instructors used this student focus about half the time, a little bit more for Cathy (54,5% in average), and a little bit less for Sylvie (46,7% in average). Océane showed greater variation, spending about 40% of her explanation time facing the students during Q1, and more than 64% during Q2. It is interesting to note that the proportion of time when she is producing representational gestures in Q2 is also about twice as large as it is for Q1 (respectively 43,8% and 22,4%), even though her representational gestures rate is only a little bigger (4.6 vs 4.2 gestures per 10 s.). As she also does a lot of pointing gestures at the screen during Q1 (spending more than 43% of the time pointing, with more than 3 gestures per 10 s), we can hypothesize that her short representational gestures are meant to help the students do semiotic transfers (from verbo-gestural concepts to written ideas), while the longer representational gestures produced during Q2 might rather foster conceptual deepening. We also found a tendency to associate attention focusing on the screen and concrete pointing on the one hand, and focusing on the audience and representational gesturing, on the other hand. This is very clear for the explanations made by Océane and Cathy. Figure 6 presents the corresponding ELAN screen captures. The first line indicates the attention focus, marked in grey when directed to the students. In the second line were coded the representational gestures, and, in the third, the concrete pointing gestures. Most of the instructors' representational gestures occurred when they were facing the students (grey zone), while most of their concrete pointing is produced facing the screen. Out of a total of 44 pointing gestures, Cathy only made 14 of them facing the students. Océane only made 6 of her 38 pointing gestures facing the students. The correlation between attention to the students and representational gestures looks even stronger than the one between attention to the screen and concrete pointing gestures. Cathy made 36 representational gestures in total, and only produced 3 of them facing the screen. Océane made a total of 68 representational gestures, and only 5 of them were produced facing the screen, 3 of which were enumeration gestures. What does this multimodal combination of speech and representational gesturing facing the audience bring to this explanatory task? The next subsection (4.2) presents a case study that is typical of the use of such resources by the instructors for their explanations.

Didactical use of representational gestures while facing the students

Here we focus on Cathy's explanation about Q1. Below is transcribed her corresponding speech (as for all the verbal transcripts reproduced in this paper, the parts of her speech corresponding to exact words or lexical expressions appearing on the slide are underlined): so: (0.9) the: w-w-of course the water footprint how much is used pound for pound varies between countries\ so: em: (0.5) as you can see in the us: it's very: very small compared to: other countries:\ and em the world average\ because most of our: er: coffee beans and everything are not grown in the us\ so then the production em: as far as we would see it here is lower/ but worldwide is what you have to keep in mind em: (0.5) is that all those coffee beans being produced and grown in other countries takes an immense amount of water\ em are they any other option that you guys: strongly thought of that er maybe you don't know how necessarily the water is used and wanna ask about it/ Even if Cathy reuses several terms from the slide, she is not reading and she also brings new vocabulary and information verbally. Seven parts of her speech were classified as innovative verbal elements, 2 of them consisting in rewording, 2 serving an elucidation function, and 3 identified as additions. These elements are presented in table 4. Table 4. Characterization of innovative elements in Cathy's explanatory speech about Q1.

For the present case study, we turn to a micro-level analysis and focus on one segment of her speech: the first addition (framed in table 4). Even if Cathy then elaborates innovative information verbally, she also produces, meanwhile, 6 gestures, using them to bring meaning that is not embedded in her speech. Her first five gestures in this excerpt are representational. They are produced facing the student, and while saying what was transcribed from "because" to "here". Her last gesture consists in pointing to the screen with her head, facing it, and occurs together with the final part of the transcript "is lower". Table 5 provides a description of each of her representational gesture, together with the accompanying speech. The parts of speech uttered during the production of the gesture appear in bold, and so does the referent identified for each gesture or part of a gesture.

Table 5. Case study 1: Cathy's speech and representational gestures. McNeill (1992: 15) calls a 'content' metaphor to represent the coffee beans as quite a big thing or quantity. This first gesture is a polysign (1), in Calbris' sense: "one gesture may (…) represent several notions simultaneously because more than one of its components has an analogical link and, thus, it contains more than one gestural sign. In this case it is a polysign gesture" (Calbris, 2011: 28). Here, the shape of the hands specifies this thing with the palms directed to Cathy's body, indicating ownership, as her simultaneous use of the word 'our'. Then, keeping the parallelism between her hands, she produces an abstract pointing gesture to her right (2), providing an interpretation of her vague speech component 'and everything' corresponding to another thing of the same size as, or in the same quantity as, or as important as the 'coffee beans'. Here, her gesture mainly disambiguates the verbal referential construction. Her third gesture (3) consists in pointing down with her two hands, a general structure that she holds all along the utterance 'are not grown in the us', with emphasizing beats on 'not', 'grown' and 'us'. Doing so, she refers to the ground, specifying an aspect of her wording 'grown' as 'coming from the ground', but she also refers metaphorically to the land, this land, elaborating on the verbal referent 'the us' with an information that does not appear in her speech, meaning that here, we are in the US. The gesture that she makes as she is pronouncing the word 'production' ( 4) is emblematic of a polysign. The first dimension of this gesture, the shape of her right hand, is similar to a claw orientated to the ground, involving the idea of taking something. The second dimension, the direction of her hand, is globally moving up depicting the image of something that is growing from the ground. Third dimension: more specifically, her hand follows a cyclic trajectory, involving the other hand, in a commonplace metaphor for "a process". Through this three-dimensional gesture, Cathy specifies what she means by 'production', as cultivation or harvest. Her last representational gesture (5) consists in pointing down again while she utters 'here', referring to this land again. In the context of her previous speech and gesture, this one can also be interpreted as referring to the US. Actually, a very interesting aspect of Cathy's representational gestures in this excerpt is that they function sequentially: the second one takes the meaning of 'another' because of a move on her side; the last pointing down takes the meaning of 'US' because a similar gesture was done earlier with a beat on 'the us'. Last but not least, this last representational gesture is produced on the right of Cathy's gestural space, as she is about to point at the screen, which is on her right. She does this concrete pointing saying 'is lower'. Somehow, this move to the right is a way to include the latest gesture in her global gestural sequence. She is literally 'bringing' the referent (US production) to the screen as a way to make sure that the audience understands what 'lower' refers to, and looks at the correct place in the slide (comparing it to the world average use of water for coffee production, cf. appendix 2).

Cathy first uses what

Mediating concepts by establishing bridges between a variety of semiotic modes

This last aspect of our first case study is similar to what some instructors do more systematically in their explanations while transiting from one attention focus to another. The global pattern of focus alternation itself works as a way to establish bridges between the semiotic modes used on the slides (text, table) and the semiotic modes used in the oral explanations (speech and gesture). Nevertheless, in some occasions, the instructors more explicitly mediate this transfer from one mode to another. In this subsection, we provide an emblematic case of such semiotic mediation, based on an analysis of an excerpt of Marlene's explanation about Q1: coffee: (0.6) requires: hum seventeen thousands/ (1.1) liters per kilogram\ (0.7) hum: (1.2) this is worldwide though\ this is no:t just the usa\ as you can see ours is: (0.5) significantly less\ hum: (0.8) it varies: depending on: (0.6) how you produce it/ and also where you're producing it\ so that might be why ours is lower\ (0.6) also: are you are you guys surprised that your second runner up was actually microchips\ (…) and also cattle\ you have to (0.8) cattle is your third one up\ you have to (1.5) the cattle has to drink\ it has to eat\ the plants\ you have to grow the plants with the water\ (0.6) so\ em: and this comes from: the: scientific journal of water resource management\ water footprint of nations\ (1.0) so that was just: some knowledge for you guys to use\ (2.6) 'kay\ Verbally, Marlene reuses some vocabulary written on the slide, but she is not just reading it, except when she mentions the title of the scientific source used. Her explanation is actually quite rich in terms of innovative verbal elements: 5 parts of this transcript were identified as rewording, 4 as elucidation, 4 as addition. Such coding is presented in table 6. Table 6. Characterization of innovative elements in Marlene's explanatory speech about Q1.

We are now going to analyze Marlene's gesturing during the second excerpt chronologically identified as playing an elucidation function, framed in table 6. Verbally, she is establishing a relation between informational elements that are presented separately on the slide, comparing water consumption for coffee production in the US and worldwide. In table 7 is reproduced her speech with a description of her accompanying gestures and attention focus. The parts of speech uttered during the production of a representational gesture appear in bold, and so does the referent identified for each gesture or component of a gesture. The parts of her speech corresponding to a concrete pointing gesture are underlined.

Table 7. Case study 2: Marlene's speech, attention focus and gestures.

With the pronoun 'that', Marlene is referring to what she has just said about the fact that the amount of water needed for coffee production varies depending on 'how you produce it/ and also where you're producing it\', a rewording that she produced facing the students. In this short excerpt, Marlene is now establishing a causal link between this previous statement and some facts presented in the table projected on the screen. With a first two-hand move to the right (1), she signifies an implication of what she has previously explained. After this first stop, the two hands complete their trajectory up, transforming into a concrete pointing at the slide (2), while Marlene is still facing the students. She then turns her head and attention to the screen, and transforms her hands pointing into an index finger pointing (3), as referring to a precise part of the slide, actually verbally indicated ('ours', stating for the water footprint of coffee produced in the USA, the last square of the second column of the table appearing on the slide). During the entire gestural sequence, Marlene is guiding the students to help them establish a link between the information given in her speech and the information presented on the slide. After clearly specifying the causal nature of this link, she gradually draws their attention to the relevant information in the table. When facing the slide, she keeps on working on this transition between semiotic modes. She uses another representational gesture addressed to the students, as if she was 'translating' to them the information content of both the slide and her speech. She then moves her right hand down, in a flat line shape (4), referring to a lower level, as she is getting to the key point of her verbal sentence: 'lower'.

Toward a multimodal description of knowledge mediation

Most of the instructors demonstrated their ability to develop rich explanatory discourse by taking advantage of a diversity of multimodal resources. Alternating their head and attention between the screen and the students, they mediated the concepts by providing innovative information in their speech and gesture. Such conceptual mediation was associated to a semiotic mediation of knowledge: by pointing at the screen, they established bridges between the oral and the written semiotic modes, and guided the students in their understanding of the slide. In this last subsection, we step away from micro-analysis and propose a general model to characterize the instructional behavior of instructors, as a combination of multimodal resources. We describe the most frequent explanatory practice as 'guiding'. Guiding can be understood as a continuum of intermediate behaviors from the two other stereotypical practices of 'showing' and 'transmitting'. Figure 4 represents these three explanatory practices and their characteristics. Showing consists of acting as if 'everything was on the slide', and as if there were no need for mediating the target knowledge through innovative information, neither verbally nor gesturally. It is associated with a verbal behavior limited to reading the slide, an attention focus only directed to the screen, and the absence of representational gestures. As this is a stereotype, no instructor in our data exactly behaved this way. Nevertheless, we have an example of an American leading student, Iris, whose instruction tended to be a showing one. For instance, see below the transcription of her explanation of Q1: so: the water footprint of a product varies between countries to countries different due to different product practices and local climate\ so as you can see: coffee: was the leader followed by microchips beef em: rice wheat and apples\ and the source was: statistics from the water research management article\ Most of her verbal production consisted in reading, and no innovative information was provided. Moreover, during all this explanation, her only representational gesturing activity was an enumeration, which did not bring much new information compared to her speech. Transmitting stands at the other end of the spectrum of explanatory multimodal performance. It corresponds to behaving as if one could explain everything from scratch, only through oral discourse (including speech and gesture), without referring to the didactical written resources. Then, almost no word appearing on the slide is reused and much innovative information is also brought verbally and gesturally. The attention is only directed to the students, with no concrete pointing at the screen. In terms of instructional effects, the knowledge is extensively conceptually mediated, but no link is established with the written semiotic modes with which the students need to become familiar: there is no semiotic mediation. Of course, no instructor totally fell into this category either. But Océane, one of the French instructors, sometimes tended to be transmitting, in parts of her explanations. For instance, during more than 15 seconds and a half, while explaining Q1, she faced the students, without pointing at the screen. She then provided a lot of new vocabulary in her speech, only reusing 5 words from the text, even if she was rewording the content of the slide. Meanwhile, she also produced 8 representational gestures, specifying a lot the few key referents that she was considering. Finally, most of instructors' authentic explanatory practices consisted in guiding. Guiding is characterized by alternating attention focus between the screen and the students, using both concrete pointing at the slide and representational gestures, together with a speech including lexical anchors from the written data. As an intermediate behavior between the two stereotypes of showing and transmitting, guiding may be embedded in a variety of communicative styles that all provide, to some extent two elements: 1) the necessary additional information to provide a conceptual mediation and 2) the bridging work to mediate their cognitive circulation between semiotic modes, and especially their understanding of the written didactical data, or semiotic mediation.

Conclusion

Novice instructors proved capable of mediating knowledge using a diversity of semiotic resources. Studying the relation between their speech, gesture, and use of the data written on the slide, we could specify different styles of multimodal explanatory performance. Except in one case, the instructors produced a speech that informed the information written on the slide, either through rewording it, elucidating it, or adding elements. Their attention focus defined by the gaze and head orientation, revealed a clear pattern of alternation between the students and the screen, for all the instructors. Through representational gestures and abstract pointing, the instructors highlighted parts of their speech, but also conveyed complementary information disambiguating their verbal expression, or specifying a referent. As in case study 1, they tended to perform such gestures facing the students, and developing complex gestural sequences. Concrete pointing always referred to the slide, either globally or to a precise part of it, and, on the contrary, tended to be produced facing the screen. Nevertheless, as in case study 2, some pointing gestures were produced facing the students, and some representational gestures facing the screen, especially in transition phases just before or after a shift of attention focus. Such behavior establishes bridges between different representations of the concepts and may help the students understand the information written on the slide. We finally propose a model of this instructional activity that includes three explanatory multimodal styles. On one end of the continuum, showing consists in minimal gestural and verbal activity. At the other end stands the transmitting style, characterized by a high level of conceptual mediation without semiotic mediation. Most of authentic studied instructional practices correspond to the third, intermediate style, guiding. Guiding is a richer instructional discourse in the sense that it provides both conceptual and semiotic mediation of knowledge, and we can hypothesize that it would turn out to be the most efficient multimodal explanatory performance. Nevertheless, further work is needed to specify this hypothesis, and notably the diverse forms of guiding. Future research should also analyze students' reception of the three styles of explanatory performance and their corresponding learning outcomes to test this hypothesis. At a theoretical level, our results strengthen the vision of didactical interactions as complex communicative situations involving real-time management of multiple goals. While studying the building of reference in instructors' explanation, we revealed the importance of facing the students, an attention focus related to the production of rich gestural referential constructions. A typical 'interactive' feature (looking at the audience) thus appeared as strongly related to the 'cognitive' dimension of the building of reference. This is consistent with previous work, for instance Azaoui's analysis of language teaching as involving the skill of addressing several interlocutors and conversations simultaneously or 'enunciative ubiquity' [START_REF] Azaoui | Analyse multimodale de l'agir professoral et degré de granularité de traitement[END_REF]. Our findings can inform the training of teachers and communicators likely to deal with similar settings to make them aware of their multimodal explanatory style and its implications for conceptual and semiotic mediation, so that they can adjust their behavior to their goals. Several studies held in instructional contexts already suggest that people can, in some conditions, learn to adapt their gestural activity (e. g. [START_REF] Alibali | Students learn more when their teacher has learned to gesture effectively[END_REF][START_REF] Goldin-Meadow | Gesture's role in the learning process[END_REF][START_REF] Tellier | Le corps et la voix de l'enseignant: théorie et pratique[END_REF].

Figure 1 .

 1 Figure 1. Attention focus during monologue explanation: Cathy.

Figure 2 .

 2 Figure 2. Attention focus during monologue explanation: Marlene.

Figure 3 .

 3 Figure 3. Attention focus during monologue explanation: Iris.

Figure 4 .

 4 Figure 4. Attention focus during monologue explanation: Sylvie.

Figure 5 .

 5 Figure 5. Attention focus during monologue explanation: Océane.

Figure 6 .

 6 Figure 6. Correlation between the attention focus and the type of gesture -Cathy and Océane (ELAN screen captures).

Figure 4 .

 4 Figure 4. Model of the multimodal explanatory performance of instructors: showing, guiding and transmitting.


Laure Wiest
email: laure.wiest@isa-lyon.fr

Barbara Giroud

Maëva Fieu

Azziz Assoumani

Emmanuelle François Lestremau

Emmanuelle Vulliet

Ultrasound-assisted sample preparation for simultaneous extraction of anionic, cationic and non-ionic surfactants in sediment

Keywords: surfactant, linear alkylbenzene sulfonates, quaternary ammoniums, ultrasound, LC-MS/MS, sediment

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Due to their specific physical and chemical properties, surfactants are used in many different fields.

The main applications are in the formulation of domestic or industrial detergents (laundry, dishwashing, cleaning products), and cosmetic products (shampoos, shower gels, toothpastes, creams, make-up...). They are also used as auxiliary products in the manufacturing processes of industry and agriculture: in the treatment of textiles and leather, the synthesis and formulation of plastics and paints, the processes of the petroleum industry, the formulation of phytosanitary products, and fertilizers. This very high consumption and these very diverse applications suggest a strong impact of these compounds on the environment, particularly on aquatic related bodies [START_REF] Jardak | Surfactants in aquatic and terrestrial environment: occurrence, behavior, and treatment processes[END_REF].

Their presence in the environment raises 2 major concerns. The first one is related to their ability to solubilize relatively insoluble xenobiotics, making them more bioavailable [START_REF] Dollinger | Effect of surfactant application practices on the vertical transport potential of hydrophobic pesticides in agrosystems[END_REF]. Hari et al. [START_REF] Hari | Effects of pH and cationic and nonionic surfactants on the adsorption of pharmaceuticals to a natural aquifer material[END_REF] and Pan et al. [START_REF] Pan | Effect of cationic and anionic surfactants on the sorption and desorption of perfluorooctane sulfonate (PFOS) on natural sediments[END_REF] studied the effect of surfactants on the adsorption of pharmaceuticals and perfluorooctanesulfonic acid (PFOS). They revealed that the impact of surfactants was compounddependent and varied with surfactant concentration. Moreover, cationic surfactants had a much stronger impact than anionic and non-ionic surfactants. The second one concerns their ecotoxicity.

Lewis [START_REF] Lewis | chronic toxicities of surfactants and detergent builders to algae -A review and risk assessment[END_REF] wrote a review on the toxicity of these 3 families of surfactants, on algae. The toxicity varied by 3 orders of magnitude depending on the species of algae studied, and on the same species, it varied by 4 orders of magnitude depending on the surfactant. It is therefore difficult to generalize and extrapolate the results, especially since the analytical verification of the exposure concentration was not often performed. However, in general the cationic molecules were the most toxic, followed by the non-ionic, and finally the anionic. Moreover, the presence of cationic surfactants such as quaternary ammonium could increase the risk of bacterial resistance, as bacteria exposed to quaternary ammoniums have been shown to acquire both quaternary ammonium resistance and antibiotic resistance [START_REF] Ostman | Screening of biocides, metals and antibiotics in Swedish sewage sludge and wastewater[END_REF]. Finally, alkylphenols, the final degradation products of the non-ionic surfactants alkylphenol ethoxylates, are persistent, bioaccumulative, and disrupt endocrine activity [START_REF] Acir | Endocrine-disrupting metabolites of alkylphenol ethoxylates -A critical review of analytical methods, environmental occurrences, toxicity, and regulation[END_REF].

Data on the occurrence of surfactants in the environment are, however, very limited [START_REF] Jardak | Surfactants in aquatic and terrestrial environment: occurrence, behavior, and treatment processes[END_REF]. The latest studies reveal high concentrations of surfactants in wastewater. In particular, anionic surfactants are found in concentrations sometimes exceeding mg/L [START_REF] Freeling | Occurrence and potential environmental risk of surfactants and their transformation products discharged by wastewater treatment plants[END_REF]. Non-ionic surfactants have also been quantified in wastewaters but at concentrations lower than anionic ones; between 0.1 and 10 µg/L [START_REF] Barber | Endocrine disrupting alkylphenolic chemicals and other contaminants in wastewater treatment plant effluents, urban streams, and fish in the Great Lakes and Upper Mississippi River Regions[END_REF][START_REF] Ciofi | Fully automated on-line solid phase extraction coupled to liquid chromatographytandem mass spectrometry for the simultaneous analysis of alkylphenol polyethoxylates and their carboxylic and phenolic metabolites in wastewater samples[END_REF]. Finally, cationic surfactants were detected at lower concentrations [START_REF] Ostman | Screening of biocides, metals and antibiotics in Swedish sewage sludge and wastewater[END_REF][START_REF] Li | Validation of sampling techniques and SPE-UPLC/MS/MS for home and personal care chemicals in the Songhua Catchment, Northeast China[END_REF]. However, these have the particularity of being in very high concentrations in solid matrices, particularly in sewage sludge [START_REF] Ostman | Screening of biocides, metals and antibiotics in Swedish sewage sludge and wastewater[END_REF][START_REF] Ruan | Identification and Composition of Emerging Quaternary Ammonium Compounds in Municipal Sewage Sludge in China[END_REF]. Their positive charge generates ionic interactions with the particles, which result in strong adsorption [START_REF] Margot | A review of the fate of micropollutants in wastewater treatment plants[END_REF][START_REF] Li | Analysis of Quaternary Ammonium Compounds in Estuarine Sediments by LC-ToF-MS: Very High Positive Mass Defects of Alkylamine Ions as Powerful Diagnostic Tools for Identification and Structural Elucidation[END_REF]. Anionic surfactants are also found in solid matrices, at slightly lower concentrations [START_REF] Huber | A first screening and risk assessment of pharmaceuticals and additives in personal care products in waste water, sludge, recipient water and sediment from Faroe Islands, Iceland and Greenland[END_REF][START_REF] Abril | Determination of perfluorinated compounds, bisphenol A, anionic surfactants and personal care products in digested sludge, compost and soil by liquid-chromatography-tandem mass spectrometry[END_REF][START_REF] Gonzalez | Degradation and environmental risk of surfactants after the application of compost sludge to the soil[END_REF]. Finally, concentrations of non-ionic surfactants between 50 µg/kg and 5 mg/kg have been reported [START_REF] Andreu | Quantitative determination of octylphenol, nonylphenol, alkylphenol ethoxylates and alcohol ethoxylates by pressurized liquid extraction and liquid chromatography-mass spectrometry in soils treated with sewage sludges[END_REF][START_REF] Koh | Fate and occurrence of alkylphenolic compounds in sewage sludges determined by liquid chromatography tandem mass spectrometry[END_REF]. These high concentrations suggest a strong impact on the receiving environment, such as river sediments [START_REF] Jardak | Surfactants in aquatic and terrestrial environment: occurrence, behavior, and treatment processes[END_REF].

Due to these possible strong interactions between the analytes and the solid environmental matrices, the extraction of these compounds requires a dedicated efficient methodology to release them from the solid material. Different types of extraction techniques were reported: mechanical agitation (Vortex [START_REF] Huber | A first screening and risk assessment of pharmaceuticals and additives in personal care products in waste water, sludge, recipient water and sediment from Faroe Islands, Iceland and Greenland[END_REF][START_REF] Koh | Fate and occurrence of alkylphenolic compounds in sewage sludges determined by liquid chromatography tandem mass spectrometry[END_REF][START_REF] Berge | Development of a multiple-class analytical method based on the use of synthetic matrices for the simultaneous determination of commonly used commercial surfactants in wastewater by liquid chromatography-tandem mass spectrometry[END_REF], oscillation with Zirconium beads [START_REF] Ostman | Screening of biocides, metals and antibiotics in Swedish sewage sludge and wastewater[END_REF]), temperature (Soxhlet [START_REF] Martinez-Carballo | Determination of selected quaternary ammonium compounds by liquid chromatography with mass spectrometry. Part II. Application to sediment and sludge samples in Austria[END_REF]), temperature and pressure (Pressurized Liquid Extraction (PLE) [START_REF] Barber | Endocrine disrupting alkylphenolic chemicals and other contaminants in wastewater treatment plant effluents, urban streams, and fish in the Great Lakes and Upper Mississippi River Regions[END_REF][START_REF] Traverso-Soto | Distribution of anionic and nonionic surfactants in a sewage-impacted Mediterranean coastal lagoon: Inputs and seasonal variations[END_REF][23][START_REF] Gonzalez | Simultaneous extraction and fate of linear alkylbenzene sulfonates, coconut diethanol amides, nonylphenol ethoxylates and their degradation products in wastewater treatment plants, receiving coastal waters and sediments in the Catalonian area (NE Spain)[END_REF]) and ultrasound [START_REF] Ruan | Identification and Composition of Emerging Quaternary Ammonium Compounds in Municipal Sewage Sludge in China[END_REF][START_REF] Abril | Determination of perfluorinated compounds, bisphenol A, anionic surfactants and personal care products in digested sludge, compost and soil by liquid-chromatography-tandem mass spectrometry[END_REF][START_REF] Lara-Martin | Multi-residue method for the analysis of synthetic surfactants and their degradation metabolites in aquatic systems by liquid chromatography-time-of-flight-mass spectrometry[END_REF][START_REF] Gonzalez | Method for the simultaneous determination of the most problematic families of organic pollutants in compost and compost-amended soil[END_REF][START_REF] Sanderson | Occurrence and hazard screening of alkyl sulfates and alkyl ethoxysulfates in river sediments[END_REF]. However, in most studies, PLE or Ultrasound-assisted Extraction (UAE) were used. Different parameters explain this choice: short extraction time, low volume of solvent, moderate cost and ease of use. Indeed, the extraction times are very variable, from 10 and 30 minutes respectively for PLE and ultrasound, to 18 hours with the Soxhlet method. Moreover, UAE can be performed using an ultrasonic bath, which is quite inexpensive. However, to our knowledge, only two surfactant analysis methods allowing the extraction of anionic, non-ionic and cationic substances from sediments and sewage sludges have been reported in the literature. Both methods required a purification step, which can be very time consuming. Gonzalez et al. [START_REF] Gonzalez | Simultaneous extraction and fate of linear alkylbenzene sulfonates, coconut diethanol amides, nonylphenol ethoxylates and their degradation products in wastewater treatment plants, receiving coastal waters and sediments in the Catalonian area (NE Spain)[END_REF] used PLE followed by solid phase extraction on C18 cartridge. Bergé et al. [START_REF] Berge | Development of a multiple-class analytical method based on the use of synthetic matrices for the simultaneous determination of commonly used commercial surfactants in wastewater by liquid chromatography-tandem mass spectrometry[END_REF] adapted a QuEChERS method [START_REF] Gonzalez-Curbelo | Evolution and applications of the QuEChERS method[END_REF], with the addition of a second extraction at acidic pH, followed by purification by SPE on C8 cartridge. Within this context, the objective of this study was to develop a sensitive and reliable analytical methodology to fill the knowledge gap regarding the occurrence of surfactants in the environment, particularly in sediments. In order to limit the number of analyses, this method must cover a wide range of surfactants, which were chosen based on a previous work [START_REF] Wiest | A multi-family offline SPE LC-MS/MS analytical method for anionic, cationic and non-ionic surfactants quantification in surface water[END_REF]. This study focuses on the optimization, validation and application of the developed analytical method. After having compared two types of extraction, salt, and vortex assisted solid-liquid extraction and UAE, different extraction solvents were tested. Then, the results of the validation of the final UAE protocol and analysis by LC-MS/MS are presented. The optimized analytical method was finally applied to 30 French river sediment samples, revealing concentrations higher than 1 mg/kg for some cationic and anionic surfactants.

Materials and methods

Standards and chemicals

Targeted surfactants were chosen from a previous study [START_REF] Wiest | A multi-family offline SPE LC-MS/MS analytical method for anionic, cationic and non-ionic surfactants quantification in surface water[END_REF] and comprise 12 families of surfactants: anionic (linear alkylbenzene sulfonate (LAS), alkylethersulfate and alkylsulfate), non-ionic (octylphenol ethoxylate (OPnEO), cocamides and 2,4,7,9-Tetramethyl-5-decyne-4,7-diol (TMDD)), cationic (benzalkonium chloride (BAC), dialkyldimethyl ammonium chloride (DDAC), alkylpyridinium, triethanolamine esterquat (TEAQ), amidoamine) and amphoteric (cetylbetaine). The suppliers of standards are reported in Table S1. Stock standards solutions were prepared as published elsewhere [START_REF] Wiest | A multi-family offline SPE LC-MS/MS analytical method for anionic, cationic and non-ionic surfactants quantification in surface water[END_REF].

LC-MS grade methanol (MeOH) and acetonitrile (ACN) were supplied by Honeywell (Seelze, Germany), while water was purchased from Fisher Chemical (Geel, Belgium). Formic acid (FA), acetic acid (AA) were supplied by Biosolve-chemicals (Dieuze, France), ammonium hydroxide by Fluka (Saint-Quentin Fallavier, France) and ethylenediaminetetraacetic acid (EDTA) by VWR Chemicals (Fontenay-sous-bois, France). Citrate buffers (Part Number: 5982-6650, Agilent Technologies, Avondale, Arizona), consisted of 4 g anhydrous MgSO4, 1 g sodium citrate, 0.5 g citric disodium salt sesquihydrate, and 1 g NaCl.

Salt-assisted solid liquid extraction

The salt-assisted solid-liquid extraction (SLE) protocol was based on the work of Bergé et al. [START_REF] Berge | Development of a multiple-class analytical method based on the use of synthetic matrices for the simultaneous determination of commonly used commercial surfactants in wastewater by liquid chromatography-tandem mass spectrometry[END_REF]. It consists of a double salt-assisted SLE, with acidic conditions: the first with a mild acid and the second with strong acidic conditions. Briefly, 500 mg of sample was introduced into a 50-mL polypropylene centrifuge tube, to which a ceramic homogenizing bar, 10 mL of ACN, and 6 mL of LC-MS water were added. After a 20-s vortex run, 3 mL of heptane was added. Then, after a 40-s vortex run, the citrate buffer was introduced into the tube, which was immediately shaken manually for 20 s and then 40 s by vortex. After centrifugation at 10,000 rpm for 5 min, 4 mL of supernatant was collected in a 12-mL calcined glass tube. 0.5 mL of sulfuric acid was then added to the 1 st extraction tube containing the solid sample, and after shaking 30 seconds and centrifugation, 4 mL of supernatant was collected and added to the 1 st supernatant. The extracts were evaporated to dryness under nitrogen at 40°C and then reconstituted in 1 mL of the injection solvent, which was composed of acetonitrile, water and a 25% ammonium hydroxide (NH4OH) solution (see section 2.4).

Ultrasound-assisted extraction

The UAE protocol was adapted from the method developed by Gago-Ferrero et al. [START_REF] Gago-Ferrero | Simultaneous determination of 148 pharmaceuticals and illicit drugs in sewage sludge based on ultrasound-assisted extraction and liquid chromatography-tandem mass spectrometry[END_REF] for pharmaceuticals in sludges. It consists in a double UAE extraction. 500 mg of sample were introduced in a 12 mL glass tube, previously calcined at 450°C for 15h. 3 mL of extraction solvent were added and the tube was then vortexed for 15 seconds, ultrasonicated for 15 minutes at 50°C, and centrifuged for 10 minutes at 5000 rpm. The supernatant was collected in a glass tube. A second 3 mL aliquot of solvent was added to the sample tube and vortexed, ultrasonicated, and centrifuged as above. The two supernatants were combined in a glass tube, evaporated to dryness under nitrogen flow at 40°C then taken up with 1 mL of the injection solvent. The tube was vortexed for 15 seconds, then ultrasonicated for 10 minutes, and centrifuged for 5 minutes at 5000 rpm. Finally, the supernatant was collected in a vial for injection. Five extraction solvents were tested: 100% MeOH, MeOH with 1% FA, 90/10 MeOH/water with 0.1 M EDTA, 100% ACN, 90/10 ACN with 1% AA/water at 0.1 M EDTA.

LC-MS/MS analysis

Analysis was carried out with an Agilent 1290 infinity LC system (Agilent Technologies, Avondale, USA) coupled with a 3200 QTrap (ABSciex, Foster City, USA) triple quadrupole mass spectrometer, with an electrospray ionization (ESI) source. The injection solvent was composed of acetonitrile, water and a 25% ammonium hydroxide (NH4OH) solution (30%, 65% and 5%, respectively). The reverse phase LC column was a Poroshell 120 (Agilent) C18 1.9 μm (50 mm x 2.1 mm) protected with a Krudkatcher from Phenomenex (Torrence, California). The injection volume was 20 µL for positive ESI mode and only 10 µL for negative ESI analyses due to high concentrations of LAS. The organic phase (B) was methanol. Aqueous phase (A) consisted in 0.1% formic acid water for positive ESI and 1 mM ammonium acetate for negative ESI. In positive mode, the gradient was a linear ramp from 50% to 100% B over 5 minutes with a 2 minute plateau and a 3 minute equilibration phase. In negative mode, it was a linear ramp from 40% to 100% B over 4 minutes with a 2 minute plateau and a 3 minute equilibration phase. Other LC parameters were the same as already published [START_REF] Wiest | A multi-family offline SPE LC-MS/MS analytical method for anionic, cationic and non-ionic surfactants quantification in surface water[END_REF]. Chromatograms are presented in Figure S1.

Two MRM transitions, parent ion > product ion, were recorded for each substance, the most sensitive for quantification (MRM1), and the second most sensitive for confirmation (MRM2). Optimized MS/MS parameters are summarized in Table S2. Temperatures of ESI sources were set to 450 and 500°C, for positive and negative modes respectively.

Validation of the whole procedure

Matrix-matched calibration was used. The calibration was performed using Fontainebleau sand [START_REF] Saadi | Characterization of Fontainebleau sandstone: quartz overgrowth and its impact on pore-throat framework[END_REF] (VWR chemicals) (calcined at 450°C for 15h) spiked at different concentrations, which underwent the entire analytical protocol. Prior to extraction, an overnight contact time was applied as follows: the tube containing the spiked sample was vortexed for 30 seconds, then the added solvent was evaporated in a hood overnight.

Validation of the analytical method was performed using the same reference matrix. The method limits of quantitation (LOQ) were determined differently, according to the contamination of method blanks: if signal to noise (S/N) was inferior to 10, LOQ was the concentration of analyte that produces a peak signal 10 times higher than the chromatogram background. If S/N was superior to 10, the LOQ was the concentration that produces a peak area twice as high as the blank peak area. LOQ were validated following the French standard NF T90-210 [START_REF]AFNOR 2009 NF T90-210 -Water quality -Protocol for the initial method performance assessment in a laboratory -May 2009 -AFNOR[END_REF]. Repeatability and trueness experiments were performed using calcined sand at LOQ and 10 times LOQ. Linearity was evaluated between LOQ and 10 times the LOQ. Inter-operator precision was evaluated at the LOQ level and inter-days precision, at 10 times the LOQ level.

Application and statistical analysis

30 sediments samples, collected in the French territory between September and October 2018, were analysed using the validated method. Their organic carbon concentration was between 2 and 256 mg/g with a mean of 39 mg/g and their dry matter content between 9 and 81% with a mean of 51%.

Sediments were first freeze-dried, grinded and sieved. Each sample was spiked with 100 µL of a solution of surrogate standards (BDDA-d5 (100 µg/L), HexPyr-d5 (1000 µg/L), SDS-d25) (1000 µg/L)) and an overnight contact time was applied before extraction. Identification of surfactants was confirmed following four criteria: MRM 1 with S/N > 10, MRM2 with S/N > 3 and the concordance of retention times and MRM ratios between the sample and calibration standards (Table S2).

Matrix-matched calibration was used for quantification. Each calibration was composed of 6 points corresponding to a method blank and the five concentration levels, 0.5*LOQ, 1*LOQ, 2.5*LOQ, 5*LOQ and 10*LOQ. The 0.5*LOQ level allowed to verify that the signal corresponding to this concentration was higher than the method blank. If this was not the case, clean-ups of the LC-MS/MS analytical system were performed with different solvent mixtures until a background compatible with the LOQ was obtained. Each analytical sequence comprised the calibration standards, samples, quality controls, injection blanks and method blanks. The creation of a correlation matrix [START_REF] Wei | R package "corrplot[END_REF] of surfactant concentrations and the calculation of the corresponding p-values [START_REF] Harrell | with contributions from Charles Dupont and many others[END_REF] were performed using R studio.

Results and discussion

Impact of sample preparation on blank contamination

Given the problems of contamination experienced during the development of the analysis of surfactants in surface water [START_REF] Wiest | A multi-family offline SPE LC-MS/MS analytical method for anionic, cationic and non-ionic surfactants quantification in surface water[END_REF], initially, method blanks from the different protocols described in sections 2.2 and 2.3 were carried out. For these tests, the reference matrix was Fontainebleau sand calcined at 450°C for 15h. Extraction blank contamination was observed for two cationic surfactants, BAC-C12 and C14, one anionic one, EHS, and two non-ionic ones, TMDD and CMEA, particularly when using the salt-assisted solid-liquid extraction. Among these compounds, a peak with a signal to noise superior to 10 was observed for BAC-C12 and C14 showing that the source of contamination is in the analytical instrument itself and/or in the mobile phase. Regardless of the extraction solvent used, ACN or MeOH, and the additives, areas from UAE were in average 10 times lower than areas from salt-assisted SLE. The highest signals in the extraction blanks were observed for BAC-C12 and EHS, and were on average 3 and 30 times lower using UAE, respectively. One source of contamination was the centrifuge tubes, which were made of polypropylene, in the salt assisted SLE, whereas calcined glass tubes were used for UAE. Migration of different additives from this kind of polymer has already been reported [START_REF] Assoumani | Nation-Wide Monitoring Campaign of 53 Contaminants of Emergent Concern in Surface Waters and Sediment (EMNAT 2018): Occurrence and PNEC Exceedance Evaluation[END_REF]. Further experiments determined that these contaminations came also from the citrate buffers, which were ready-to-use and packaged in foil pouches. Consequently, UAE was selected for the following experiments.

Ultrasound-assisted extraction optimization

One of the most critical parameters for extracting compounds from environmental solid matrices is the extraction solvent. This one must allow a good solubility of the analytes and break the interactions between the contaminants and the matrix. Considering that surfactants have both hydrophilic and hydrophobic functions, relatively polar organic solvents, MeOH and ACN, were selected. Based on another work on sludge [START_REF] Berge | Development of a multiple-class analytical method based on the use of synthetic matrices for the simultaneous determination of commonly used commercial surfactants in wastewater by liquid chromatography-tandem mass spectrometry[END_REF] and literature, these solvents were tested pure or with 10% water to which an acid and/or EDTA was added. All extraction tests were performed in triplicates with Fontainebleau sand. During preliminary experiments, differences of recoveries were observed when samples were spiked with all surfactants together and with cationic and anionic surfactants separately. Consequently, to be as close as possible to real samples, the reference matrix was spiked with cationic, anionic and non-ionic surfactants.

Figure 1 shows the comparison of the extraction recoveries obtained with the different tested conditions. Since the trends are the same for different homologues of the same surfactant family, only the recoveries of one homologue are shown in the figure. For EHS and SDS, recoveries are quite similar whatever the extraction solvent used. The addition of FA and/or EDTA improved the yields of LAS and alkyl ether sulfates. Specifically, the 90/10 acetonitrile with 1% acetic acid (AA)/water and 0.1 M EDTA mixture performed best for alkyl ether sulfates. Improved extraction efficiency with the addition of EDTA has recently been observed, particularly for pharmaceuticals [START_REF] Gago-Ferrero | Simultaneous determination of 148 pharmaceuticals and illicit drugs in sewage sludge based on ultrasound-assisted extraction and liquid chromatography-tandem mass spectrometry[END_REF] and polar pesticides [START_REF] Chamkasem | Direct determination of glyphosate, glufosinate, and AMPA in soybean and corn by liquid chromatography/tandem mass spectrometry[END_REF], but, to our knowledge, has never been tested before for surfactant extraction.

Regarding non-ionic surfactants, the trends for these three families of compounds are different. For CMEA, the UAE technique with a 100% organic solvent, MeOH or ACN, gave the best recovery. For TMDD, it was the UAE with 100% acetonitrile. Finally, for OPEO, the extraction with 100% methanol or methanol with EDTA yielded the best result.

For most cationic surfactants, extraction with MeOH and formic acid was the least efficient conditions.

The two conditions giving the best performance for most the compounds were 100% ACN and the 90/10 ACN with 1% acetic acid (AA)/water mixture with 0.1 M EDTA. However, the most efficient conditions for extracting BAC-C12 and LaurPyr were with 100% organic solvent, MeOH or ACN. The difference between BAC-C12 and its C14, C16 and C18 homologues may come from its lower hydrophobicity, making it more soluble in the tested organic solvents. Finally, for cetylbetaine, extraction with 100% MeOH yielded the best result.

In conclusion, except for anionic surfactants, no extraction condition was optimal for all target substances. These experiments show the need to find a compromise when developing multi-residue analytical methods. However, the mixture 90/10 ACN with 1% acetic acid (AA)/water with 0.1 M EDTA was retained for the rest of the study as it gave the highest recoveries regarding anionic surfactants and was the only one that allowed recoveries superior to 70%, except the TEAQ.

Validation

The optimised analytical method was evaluated in terms of sensitivity, linearity, accuracy, repeatability and intermediate precision, using Fontainebleau sand calcined at 450°C for 15 hours as the reference matrix.

The LOQs ranged from 6.4 µg/kg for LAS C10 to 158 µg/kg for 1-laureth sulfate (Table 1). Compared to the literature, these LOQs are of the same order of magnitude or higher. However, the methods developed in the other studies were specific to some families of surfactants. Concerning the method developed by Sanderson for laureth sulfates [START_REF] Sanderson | Occurrence and hazard screening of alkyl sulfates and alkyl ethoxysulfates in river sediments[END_REF], the sample mass was 35 g, compared to 500 mg in our method, and the limits of quantification were only estimated and not validated.

The highest LOQs are those of SDS and laureth sulfates. These rather high LOQs are explained by the presence of these compounds in the method blanks. Indeed, 13 substances out of the 27 targeted have a signal-to-noise ratio greater than 10 in the method blanks: LAS C10 to C13, laureth sulfates, SDS, BAC-C12 and C14, DDAC-C10 and TEAQ. The concentrations of these contaminations are highly variable, ranging from 1.6 µg/kg for LAS C10 to 31 µg/kg for SDS. In order to be above this background during the analysis, the LOQ were determined so that the peak area was twice as high as the blank peak area. Finally, extraction recoveries were superior to 80%, except for the cationic surfactants TEAQ and SADP-DMA. Cationic surfactants are well-known to be difficult to extract from environmental solid matrices, due to polar, hydrophobic and electrostatic interactions with the surfaces of negatively charged particles.

Given these results in terms of recoveries and levels of contamination of the method blanks, matrixmatched calibration was selected. Linearity range, trueness, repeatability and intermediate precision indicators are summarized in Table 2. The correlation coefficients were all higher than 0.991, except for BAC-C16 and C18. These two surfactants are both hydrophobic and cationic, the calibration curves tended toward a quadratic curve pattern. Trueness was evaluated at the LOQ level and at ten times LOQ. In both cases, trueness was higher greater than 75%, showing that matrix-matched calibration effectively corrected for the lower recoveries of TEAQ. Repeatability and intermediate precision were also evaluated at these two concentrations. At the LOQ, the inter-operator precision was evaluated, whereas at 10 times the LOQ, it was the inter-day precision on 3 days. The coefficients of variation (CV) were respectively comprised between 6 and 22% and between 10 and 32% for repeatability and inter-operator precision. Laureth sulfate and TEAQ were the compounds with the most variability, which may have two explanations: high and variable contamination levels and low mass spectrometry response factors. Indeed, the response factors for TEAQ were about ten times lower than for BAC.

Overall, these validation data were satisfactory.

Finally, in order to evaluate the matrix effects and to validate the relevance of the calibration in the Fontainebleau sand matrix, three sediment samples were extracted and analyzed, the concentrations of surfactants were calculated by matrix-matched calibration and standard addition. Regarding standard addition, spiking was performed using the same protocol as matrix-matched calibration (see section 2.5). The standard addition calibration comprised 4 points which corresponded to a non-spiked sediment, and the sediments spiked at the expected concentration (determined by matrix-matched calibration) divided by 5, by 2 and by 1. Results of quantification are presented in Table S3. The differences in the concentrations determined through the two calibration methods being less than 25%, these experiments allowed to validate the calibration in the Fontainebleau sand matrix for the surfactants quantified in these sediments.

Application to real samples

The 27 surfactants were searched for in 30 sediment samples collected in French rivers, between September and October 2018. Matrix-matched calibration with Fontainebleau sand was used for quantification. Quantification frequencies and mean concentrations are summarized in Table 3. This study was part of a nation-wide monitoring campaign of 53 contaminants of concern in surface water and sediment [START_REF] Pouech | Human exposure assessment to a large set of polymer additives through the analysis of urine by solid phase extraction followed by ultra high performance liquid chromatography coupled to tandem mass spectrometry[END_REF] Of the 27 surfactants searched for, 9 were not detected: one cationic LaurPyr, one anionic, 2-laureth sulfate, and all targeted nonionic surfactants, OP7-11EO, CMEA, and TMDD. These results can be explained either by a low use of these compounds or by a low affinity of these substances for solid particles. In the study by Loyo-Rosales et al. [23], OP1-5EO were looked for in river water and sediments and were very little quantified. In Western Europe, the use of OPEO has decreased following the identification of their main degradation products as endocrine disruptors, and their inclusion in the OSPAR convention and in the list of "substances of very high concern" (SVHC). On the contrary, TMDD has already been detected quite frequently in river waters [START_REF] Wiest | A multi-family offline SPE LC-MS/MS analytical method for anionic, cationic and non-ionic surfactants quantification in surface water[END_REF][START_REF] Guedez | Occurrence and temporal variations of TMDD in the river Rhine, Germany[END_REF][START_REF] Dsikowitzky | Identification of characteristic organic contaminants in wastewaters from modern paper production sites and subsequent tracing in a river[END_REF]. The fact that it was not quantified in sediment suggests that this compound has little affinity for the particulate phase.

Considering the anionic surfactants, alkyl and alkyl ether sulfates were found in very few samples, with quantification frequencies between 0 for 2-laureth sulfate and 6% for EHS. SDS and 1-laureth sulfate were quantified in the same sample. Among the alkyl ether sulfate homologues, the major presence in sediments of SDS and 1-laureth sulfate has already been observed [START_REF] Sanderson | Occurrence and hazard screening of alkyl sulfates and alkyl ethoxysulfates in river sediments[END_REF]. LAS were the most frequently quantified anionic surfactants. Quantification frequencies were between 23% for LAS C14 and 70% for LAS C11 to C13. They were also the surfactants with the highest concentrations, with average concentrations above mg/kg for LAS C11 to C13. These high levels of contamination are consistent with the literature [START_REF] Traverso-Soto | Distribution of anionic and nonionic surfactants in a sewage-impacted Mediterranean coastal lagoon: Inputs and seasonal variations[END_REF][START_REF] Gonzalez | Simultaneous extraction and fate of linear alkylbenzene sulfonates, coconut diethanol amides, nonylphenol ethoxylates and their degradation products in wastewater treatment plants, receiving coastal waters and sediments in the Catalonian area (NE Spain)[END_REF][START_REF] Lara-Martin | Multi-residue method for the analysis of synthetic surfactants and their degradation metabolites in aquatic systems by liquid chromatography-time-of-flight-mass spectrometry[END_REF][START_REF] Li | Distribution and diagenetic fate of synthetic surfactants and their metabolites in sewage-impacted estuarine sediments[END_REF]. The correlation matrix of surfactant concentrations (Figure 2) showed a strong correlation between the four different homologues of LAS (p-value < 0.0001 [START_REF] Harrell | with contributions from Charles Dupont and many others[END_REF]). This is consistent with the fact that they all are used in the same formulations and have similar behaviours in the aquatic environment.

Quantification frequencies of cationic surfactants were between 3% for SADP-DMA and 90% DDAC-C10. Cetylbetaine, a zwitterionic surfactant, which had never been quantified in river water, was found in 10% of the sediment samples. Average concentrations ranged from 23.5 µg/kg for TEAQ-C18 to 828 µg/kg for TEAQ-C18 saturated. These results are consistent with the literature that reports a high affinity of cationic surfactants for the particulate phase [START_REF] Margot | A review of the fate of micropollutants in wastewater treatment plants[END_REF]. To our knowledge, no sediment occurrence data are available for TEAQ, SADP-DMA, and cetylbetaine, although TEAQ are widely used compounds, mainly as softeners [START_REF]Esterquats environmental risk assessment[END_REF]. Cetylbetaine concentrations were strongly correlated with concentrations of BAC (p-value < 0.0001) (Figure 2). BAC were quantified in more than 30% of the sediment samples at average concentrations of 84 µg/kg for BAC-C18 and 355 µg/kg for BAC-C12, which is consistent with the literature [START_REF] Martinez-Carballo | Determination of selected quaternary ammonium compounds by liquid chromatography with mass spectrometry. Part II. Application to sediment and sludge samples in Austria[END_REF][START_REF] Li | Distribution and diagenetic fate of synthetic surfactants and their metabolites in sewage-impacted estuarine sediments[END_REF]. Concentrations of DDAC-C10, which was quantified surfactant in 90% of the sediment samples, were also correlated with BAC, particularly with BAC-C14. Finally, concentrations of TEAQ homologues were strongly correlated with each other, which is consistent with the fact that they are present in the same products. More surprisingly, their concentrations also strongly correlated with LAS (p-value < 0.0001). As LAS, TEAQ may be markers of heavily polluted sites.

In conclusion, this application shows that the developed analytical method is efficient enough to obtain a broad view of the surfactant contamination of sediments. However, a compromise was made during the development and it would be necessary to use more class-specific methods to lower the limits of quantification.

Conclusion

Since surfactants are present in both dissolved and particulate fractions of rivers, it is essential to develop methods for extracting them from sediments. Two types of extraction, salt-and vortexassisted, and ultrasound-assisted extraction were compared, first in terms of method blank contamination, then in terms of extraction efficiency. Different organic solvents and the addition of acid and EDTA were also tested. Except for anionic surfactants, no extraction condition was optimal for all target substances which shows the limitation of multi-residue analytical methods. However, the ultrasound-assisted extraction with the mixture 90/10 ACN/water with 1% acetic acid and 0.1 M EDTA yielded globally the highest recoveries and the lowest method blank contamination. The validation of the finalized protocol showed satisfactory performances in terms of sensitivity, linearity, precision and trueness. The limits of quantification obtained, between 6.4 µg/kg for LAS C10 and 158 µg/kg for 1-laureth sulfate, are compatible with trace analysis in sediments. Finally, more than half of the targeted surfactants were quantified in river sediment samples. The surfactants with the highest quantification frequencies were BAC, DDAC, and LAS, with mean concentrations between 75 and 4182 µg/kg. These results show that rivers are highly contaminated with different families of surfactants, which would therefore deserve increased monitoring in the environment.

Figure captions

Figure 1 :Figure 2 :

 12 Figure 1: Absolute extraction recoveries with UAE, at 10 times the LOQ, using different solvents and additives: (A) anionic, (B) non-ionic, (C) cationic surfactants

Figure 2

 2 Figure 2

Table 1 :

 1 Instrument (Instr.) and method limits of quantification (LOQ) compared to the literature, estimated method blank concentration and absolute extraction recoveries

		Instr.	Method	Literature		Method blank	Absolute extraction
	Substance	LOQ	LOQ	LOQ	Ref.	concentration	recovery (%)
		(µg/L)	(µg/kg)	(µg/kg)		(µg/kg)	at 5*LQ
	LAS C10	2	6.4	9.3	[25]	1.6	83.7
	LAS C11	20	49	27	[25]	16	98.5
	LAS C12	15	44	15	[25]	20	97.6
	LAS C13	10	31	2.7	[25]	14	97.4
	LAS C14	0.5	32	ND		S/N <10	97.5
	C12EO1S	10	158	1 (estim.) [27]	16	117.6
	C12EO2S	8	124	1 (estim.) [27]	12	109.9
	SDS	30	123	50	[16]	31	97.8
	EHS	0.5	11	ND		S/N <10	103.0
	CMEA	1	13	ND		S/N <10	107.6
	TMDD	0.2	42	ND		S/N <10	113.6
	BAC-C12	5	27	1	[21]	2.7	120.8
	BAC-C14	4	18	1	[21]	3.0	112.7
	BAC-C16	0.2	28	0.7	[21]	S/N <10	111.0
	BAC-C18	0.2	34	2	[21]	S/N <10	113.1
	DDAC-C10	1	40	0.6	[21]	15	109.1
	LaurPyr	0.3	105	ND		S/N <10	118.0
	TEAQ-C16	6	59	ND		22	55.7
	TEAQ-C18	3	32	ND		12	22.1
	TEAQ-C18 sat.	1	12	ND		4.0	94.4
	SADP-DMA	0.5	39	ND		S/N <10	79.0
	Cetylbetaine	0.5	21	ND		S/N <10	98.1
	EHS: Ethylhexylsulfate; CMEA: Cocamide MonoEthanolAmine; LaurPyr: lauryl pyridinium;
	SADP-DMA: StearAmiDoPropyl DiMethylAmine; TEAQ-C18 sat.: saturated TEAQ-C18
	ND: No Data

Table 3 :

 3 Quantification frequency and mean concentration of the targeted surfactants in 30 sediments sampled in France in 2018

	Substance	Quantification frequency (%)	Mean concentration (µg/kg)
	DDAC-C10	90	526
	LAS C13	70	4182
	LAS C11	70	1515
	LAS C12	70	3066
	BAC-C14	63	278
	BAC-C12	63	355
	LAS C10	57	74.6
	BAC-C18	37	84.2
	BAC-C16	30	93.3
	LAS C14	23	658
	TEAQ-C16	20	98.8
	TEAQ-C18 sat.	17	148
	Cetylbetaine	17	113
	TEAQ-C18	13	15.3
	EHS	6	20.4
	1-laureth sulfate (C12EO1S)	3	412
	SDS	3	236
	SADP-DMA	3	82.6
	TMDD	0
	2-laureth sulfate (C12EO2S)	0
	OP7-11EO	0
	CMEA	0
	LaurPyr	0

Acknowledgments

The authors thank the French Biodiversity Agency for the financial support for the whole EMNAT 2018 campaign. This work was performed within the framework of the EUR H2O'Lyon (ANR-17-EURE-0018) of Université de Lyon (UdL), within the program "Investissements d'Avenir" operated by the French National Research Agency (ANR).


Nicolas Blanchard

Vincent Bizet

Acid fluorides in transition metal catalysis: A good balance between stability and reactivity

Several recent reports outlined the singular reactivity of acid fluorides as excellent electrophiles in transition-metal catalysis. These species undergo oxidative addition of the metal into the C-F bond; then, retention or release of the CO moiety can occur and be controlled by tuning the catalytic system and the reaction parameters. Acid fluorides, which can be derived from carboxylic acids, show good stability and high reactivity in a wide range of possible functionalizations with nucleophiles. Their use provides an interesting alternative to that of the parent carboxylic acid derivatives (acid chlorides, esters, amides, acids, or aldehydes).

Over the last two years, the development of selective transitionmetal-catalyzed CF functionalization reactions has led to renewed interest in acid fluorides. The acid fluoride motif is a well-known functional group in organic chemistry; it is easily obtained from the corresponding carboxylic acid and mainly used for peptide couplings. [1] Most carboxylic acid derivatives, such as acids, acyl halides, anhydrides, esters, and amides, have been successfully used as electrophilic partners in cross-coupling reactions. [START_REF] Sugihara | Handbook of Organopalladium Chemistry for Organic Synthesis[END_REF][START_REF] Gooßen | [END_REF][4][5][6][7] Compared to other acid halides or anhydrides, acid fluorides show higher stability towards water and neutral oxygen nucleophiles, and they are less prone to racemization. In the presence of palladium(0), nickel(0), or iridium(I) complexes, rapid oxidative addition takes place, leading to a highly electrophilic acyl PdII, NiII, or IrIII complex. The latter can then react with a wide variety of nucleophiles in a transmetalation step, following a decarbonylative or a non-decarbonylative reaction pathway depending on the ligand structure and the reaction parameters. In specific cases, the fluoride anion generated during the transmetalation elementary step is not innocent and can act as a mild base. Herein, we highlight recently developed crosscoupling reactions with acid fluorides; the specific reactivity of acid fluorides and comparative studies with other derivatives are also disclosed when available. A groundbreaking study in this exciting field of research was reported by Rovis and Zhang. It concerns the synthesis of ketones 3 from acid fluorides 1 by a nickel(0)-catalyzed crosscoupling reaction with diorganozinc reagents 2 (Scheme 1). [8] Scheme 1. Nickel(0)-catalyzed cross-coupling reactions of acyl fluorides 1 with diorganozinc reagents 2.

The optimal catalytic system was based on (2diphenylphosphinoethyl)pyridine (pyphos) as the ligand and 4fluorostyrene as a substoichiometric additive that dramatically accelerates the reaction. The reaction proceeds, in most of the cases, in 10 to 30 min at ambient temperature, and gives the desired products in excellent yields. Remarkably, both groups of the diorganozinc species 2 are transferred, and the reaction works equally well with enantioenriched aliphatic acid fluorides without epimerization (Scheme 1, last four examples). The functional group tolerance is also worthy of note, with acetals, ethers, esters, succinimides, and alkenes being compatible. A comparative study showed that this procedure works equally well with acid chlorides, anhydrides, acyl cyanides, thioesters, pyridinyl, and aryl esters, but then usually requires slightly longer reaction times. More recently, Hiyama-type cross-coupling reactions of benzoic acid fluorides 4 with aryl trifluorosilanes 5 were proposed by Sakai and co-workers for the generation of diaryl ketones 6 with a Pd 0 /Pt-Bu3 catalytic system in the presence of cesium fluoride [Scheme 2, Eq. ( 1)]. [9] However, the yields did not exceed 63%, and the scope was relatively limited. Acid fluorides and acid chlorides proved to be much more reactive than the corresponding acid bromides and anhydrides by a factor of two to three. The same research group subsequently developed a palladium(0)-catalyzed Suzuki-Miyaura cross-coupling of acid fluorides 1 with boronic acids 7 [Scheme 2, Eq. ( 2)]. Tris(4methoxyphenyl)phosphine (L1) and tricyclohexylphosphine (L2) were found to be the best ligands for this process. Compared to the Hiyama cross-coupling [Eq. ( 1)], the Suzuki-Miyaura reaction is more general, and the formation of non-symmetric aryl, heteroaryl, alkyl, and alkenyl ketones 3 proceeded in moderate to high yields. [10] The first decarbonylative coupling of acid fluorides enabled a trifluoromethylation reaction with triethyl(trifluoromethyl)silane, as reported by Schoenebeck and co-workers (Scheme 3). [11] Indeed, for the trifluoromethylation of oxidative-addition complexes, a previous report highlighted the need for the Xantphos ligand for the reductive elimination to occur, but most importantly, the formation of the Pd II F intermediate was crucial for the transmetalation step with the CF3 anion. [12] Taking into account these considerations, the decarbonylative trifluoromethylation reaction of acid fluorides 4 proceeded in moderate to good yields in the presence of catalytic quantities of [(cinnamyl)PdCl]2, Xantphos, and K3PO4 in toluene at high temperature, yielding trifluoromethylarenes 8. It should be noted that under the same reaction conditions, Ar-Cl and Ar-Br substrates are totally unreactive, clearly indicating that the use of acid fluorides, and the presence of the fluoride thus generated, is key to the success of this transformation. Computational studies favored a mechanism in which the oxidative-addition step has a free energy of activation of 10 kcal.mol -1 , leading to complex I. The TS connecting complexes II and III is stabilized by favorable Xantphos-Pd interactions; in addition, it requires less distortion energy than the corresponding TS from ArCO [Pd II ]F complex I. This mechanistic proposal is also supported by experimental results as the ketone resulting from reductive elimination of complex II was observed below 170 °C.

This elegant strategy works also with other nucleophiles, as exemplified by the base-free nickel-catalyzed decarbonylative Suzuki-Miyaura coupling of acid fluorides 4 with boronic acids 7 with a Ni(cod)2/PPh2Me catalytic system, or with the air-stable Ni(o-tolyl)(PPh2Me)2Cl complex and a catalytic amount of CsF (Scheme 4), which was reported by Sanford and co-workers. [13] Ligand screening showed that PPh2Me was essential to reach perfect selectivity for the decarbonylative arylation product 9. The reaction proceeded smoothly with a wide variety of acid fluorides 4 and boronic acids 7 in moderate to excellent yields. It should be noted that the fluoride generated during the process acts as a mild base and activates the organoboronate intermediates; furthermore, with both nickel systems, the decarbonylation proceeds at room temperature while high temperatures are needed with palladium. A similar strategy has been used for the decarbonylative methylation or ethylation of acid fluoride using Lewis acidic trimethyl-or triethylborane and a closely related catalytic system (Ni(cod)2/dppe), which suffers from a narrower scope. [14] Soon after, the same group reported a nickel-catalyzed decarbonylative borylation of acid fluorides 4 (Scheme 5). An external source of fluoride (KF) is required to activate the B2pin2 (10), while NaCl as an additive is essential to reach satisfactory yields of products 11. A comparative experiment showed that benzoyl chloride is totally unreactive under these reaction conditions, indicating that the acid fluoride plays a crucial role and that there is no conversion of the acid fluoride into the acid chloride with NaCl during the reaction. The reaction proceeds in moderate to excellent yields with good functional group tolerance; for example, an ester group in para position remained untouched. [15] Scheme 5. Nickel-catalyzed decarbonylative borylation of acid fluorides 4

The decarbonylative coupling of acid fluorides 4 can also be performed by CH activation of arenes and heteroarenes catalyzed by a combination of [IrCl(cod)]2 and BrettPhos in mesitylene at 200°C, as shown by Tobisu and co-workers (Scheme 6). While the reaction is sensitive to steric hindrance, both electron-rich and electron-deficient (hetero)arenes 12 underwent the arylation under identical conditions, providing the arylation products 13 in modest to good yields. The same holds true with regard to the acid fluoride substates 4 even if electron-deficient ones react faster. Comparative experiments showed that acid chlorides, bromides, and anhydrides are completely unreactive in this transformation, showing once again the specific reactivity of acid fluorides. Mechanistic investigations suggested transition state A for arenes, where the fluoride abstracts a hydrogen atom in a CMD-like mechanism, and intermediate B for heteroarenes, resulting from arylation of the C=N bond. [16] The control of reactivity and selectivity in organometallic catalysis is essential, and by careful tuning of the catalytic system and the reaction conditions, acid fluorides can evolve selectively in a decarbonylative or a non-decarbonylative pathway. This crucial control of the CO release has been addressed in the palladium-catalyzed reduction of acid fluorides 1 with triethylsilane developed by Sakai and coworkers, where the ligand fully controls the decarbonylation process (Scheme 7). [17] In the presence of the monodentate PCy3 ligand (L2), acid fluorides 1 were converted into the corresponding aldehydes 14 without decarbonylation. However, replacing L2 with the bidentate 1,2-bis(dicyclohexylphosphine)ethane system (L3) inverted the selectivity and afforded the decarbonylative reduction product 15 under otherwise identical reaction conditions. The difference in selectivity was explained by the difference in the Pd/L ratios (1:3 for L2 vs. 1:1.5 for L3), leading in the former case to a coordinatively saturated oxidative-addition complex (OAC), which favors the formation of the aldehyde, while the bidentate L3 ligand gives an OAC with a vacant coordination site, which might favor the decarbonylation step. This brief overview of recently developed C-F functionalizations of acid fluorides with and without decarbonylation shows the high potential of this electrophilic coupling partner in catalytic reactions mediated by transition metals; we are confident that this concept will find many applications in catalysis for the synthesis of valuable compounds.

Scheme 2 .

 2 Scheme 2. Palladium-catalyzed Hiyama and Suzuki-Miyaura cross coupling reactions of acid fluorides 4 (eq. 1) and 1 (eq. 2) with 5 and 7.

Scheme 3 .

 3 Scheme 3. Palladium-catalyzed decarbonylative trifluoromethylation of acid fluoride 4

Scheme 4 .

 4 Scheme 4. Nickel-catalyzed decarbonylative Suzuki-Miyaura cross coupling of acid fluorides 4.

Scheme 6 .

 6 Scheme 6. Iridium-catalyzed decarbonylative C-H arylation of acid fluorides 4.

[a]Dr. Nicolas Blanchard, Dr. Vincent Bizet Université de Haute-Alsace, Université de Strasbourg, CNRS, LIMA, UMR 7042, 68000 Mulhouse (France) E-mail: vbizet@unistra.fr

Acknowledgements

We would like to warmly acknowledge our colleague and friend Dr. Philippe Bisseret for his long-lasting and insightful contributions to the group and his contagious and constant good mood.