I Serre
email: serre@lms.polytechnique.fr

N Celati

R M Pradeilles-Duval

Tribological and corrosion wear of graphite ring against Ti 6 Al 4 V disk in artificial sea water

Keywords: Planar contact, Tribocorrosion, Graphite, Ti 6 Al 4

Severe degradations result from the friction of two antagonists in sea water environment. It is proposed to evaluate materials resistance to wear with a tribocorrosion experimental set-up which is mechanically and electrochemically instrumented. The method is illustrated with graphite and Ti 6 Al 4 V.

The deposition of graphite on Ti 6 Al 4 V samples is observed and modifies the contact characteristics. Processes of graphite wear due to mechanical effect are characterised. Observations clearly indicate that Ti 6 Al 4 V degradations depend on the electrochemical potential imposed and more precisely on the electrochemical conditions in the contact zone.

Introduction

In marine environment, planar contacts which should have sealing capacity are found in ring-on-disk systems, pumps, packers of boat, etc. Therefore, the evaluation of the life durability of the structures requires to identify the degradation mechanisms and to formulate wear laws for antagonists in contact in a corrosive environment [START_REF] Lederer | Modélisation tribomécanique du frottement en milieu agressif[END_REF][START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF]. Tribocorrosion experiments should allow for the identification and the estimation of the material parameters required for the wear laws.

In the class of problems considered, two rings are in contact and rotate, one with respect to the other. In the contact region, friction induces material wear, tampering with waterproofing and the sealing capacity of the system. Classical pin-on-disk tribocorrosion experiments are not relevant to this class of problems because of the difference in the contact geometry, in the time evolution, in the spatial distribution of the pressure, in the wear debris displacement, which are all paramount wear parameters. For these reasons, a ring-on-disk tribocorrosion experimental set-up was developed with an electrochemical and a mechanical instrumentation [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF][START_REF] Serre | Modélisation d'un contact graphite contre Ti 6 Al 4 V renforcé TiC en milieu marin artificiel: essais de tribocorrosion plan/plan de type anneau/disque[END_REF]. These experiments enable us to study degradation processes of materials submitted to friction in sea water.

Classically, the tribocorrosion experiments are carried out with one antagonist made of alumina [START_REF] Lederer | Modélisation tribomécanique du frottement en milieu agressif[END_REF][START_REF] Mischler | A round robin on combined electrochemical and friction tests on alumina/stainless steel contacts in sulphuric acid[END_REF][START_REF] Jemmely | Tribocorrosion behaviour of the Fe-17Cr stainless steel in acid and alkaline solutions[END_REF][START_REF] Lizandier | Characterization of porosity of ceramic coatings assigned to applications in sea water with the evolution of their electrochemical behaviour under friction[END_REF] despite the well-known influence of the physical, chemical and mechanical reactions on wear and friction between the specific materials [START_REF] Georges | Frottement, usure et lubrification[END_REF][START_REF] Kato | Wear in relation to friction: a review[END_REF]. Alumina is chosen because it is electrochemically inert simplifying the interpretation of the electrochemical measurements. Because of its strong mechanical properties in comparison with those of the second antagonist material, no material loss and no significant degradation are observed for alumina. The tribocorrosion experiments considered here were specifically designed for the two materials used in the real structure namely, graphite and a titanium alloy, Ti 6 Al 4 V.

The purpose of this paper is to study and to understand the wear behaviour in a corrosive environment, i.e. the tribocorrosion for a ring on disk contact. In this paper, we have focused on the evolution of the friction coefficient and the wear of the two antagonists.

Materials and experimental conditions

Description of materials

The two antagonists considered are a sintered graphite and a titanium alloy, Ti 6 Al 4 V. The elastic properties of the two materials as obtained from a resonance frequency method [START_REF]Standard test method for dynamic Young's modulus and Poisson ratio for advanced ceramics by impulse excitation of vibration[END_REF] are given elsewhere. The ring is cut from a cube of Hot Isostatically Pressed (HIP) graphite. The material is elastic brittle and isotropic [START_REF] Soc | Carbons and graphites for mechanical applications[END_REF]. Elasticity limits of 38 and 160 MPa are measured in traction and in compression, respectively. Young's modulus of the graphite is 13 GPa and Poisson's ratio is 0.13 [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF].

The disk is made of Ti 6 Al 4 V, having Poisson's ratio of 0.3 and Young's modulus of 120 GPa. This material has not sustained any treatment so that its microstructure is standard ␣/␤ duplex mixture. Its elasticity limit is around 900 MPa.

Experimental device description

Experiments are carried out with ring on disk tribocorrosimeter developed by "le centre technique d'Arcueil" [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF][START_REF] Serre | Modélisation d'un contact graphite contre Ti 6 Al 4 V renforcé TiC en milieu marin artificiel: essais de tribocorrosion plan/plan de type anneau/disque[END_REF]. This tribocorrosimeter (Fig. 1) consists in a fixed disk and a rotating ring. The contact between the two antagonists is maintained by applying a normal pressure. An electrochemical cell permits corrosion experiments in artificial sea water with prescribed electrochemical conditions.

The annulus has an average radius of 8 mm. The width of the contact zone is 3 mm. The fixed disk is assembled with a sphere which has the function of a kneecap. As a result, during the annulus rotation, it is possible to obtain a planar contact by correction of the parallelism defects between the disk and the annulus. These defects are inherent to the set-up of the antagonists or are generated by the wear phenomena. A series of short experiments has permitted to establish that the friction zones and wear are independent of the angular position on the contact surface and that the contact is really planar after few seconds.

During the experiments, the friction coefficient and the cumulated wear (the sum of ring wear and disk wear) of the two antagonists are measured. After the experiments, the total wear of the disk is evaluated by profilometry. The height of the annulus is measured before and after each experiment to obtain the total wear of the graphite annulus.

All the experiments were performed at room temperature in a circulating artificial sea water, i.e. A3 solution (NaCl solution to 30 g/l buffered at pH 8 according to standard NFA 91 411). A three-electrode set-up (Fig. 1) enables to impose different potentials on the exposed surface of both samples and to measure the current [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF]. All electrochemical potential values are given in what follows with reference to the Ag/AgCl saturated KCl electrode.

The bath is stirred continuously during the experiment, with the help of a pump, to ensure the stability of the artificial sea water characteristics. As a result, there is homogeneity and stability of the pH but also of the displacements of the wear debris out of the contact area.

The average pressure applied between the two antagonists is constant during the experiments and is equal to 5.3 MPa. This load value is representative of the applied pressure in the industrial case and ensures that the two materials are in their elastic range of deformation.

The graphite annulus rotates at 40 rpm, which corresponds to a mean velocity of 0.033 m/s. In addition, most of experiments last for 6 h unless otherwise stated.

Experimental results and discussion

Friction coefficient

At the end of the experiments, the friction coefficient ranges between 0.12 and 0.22 whatever the electrochemical conditions. It increases during transient phase of approximately 20 min. The initial values of the friction coefficient given in Table 1 are obtained after this transient stage. The evolution in time of the friction coefficient beyond the first transient is towards a stable value which does not depend strongly on the electrochemical potential value (-1.010, -0.060 and 0.140 V). For the other studied prescribed potential (-0.810, -0.210 V and free potentials (around -0.100 V)), the friction coefficient does not reach a stable value during the experiment and keeps on increasing.

Wear

The two materials experience with damage and material loss during the tribocorrosion experiments. The following subsections focus on the specific wear of the two antagonists.

Graphite wear

We observe (Table 2) that the material loss of the graphite ring tends to decrease as the prescribed electrochemical potential is increased except for the experiments at an applied potential of -0.810 V. We note that, the smaller the friction coefficient, the larger the material loss of graphite.

Electrochemical reactions of the artificial sea water occur on the graphite surface. But, dissolution or oxidation of graphite cannot happen under atmospheric pressure and at room temperature [START_REF] Van Muyder | [END_REF]. Thus, the origin of graphite material loss is mechanical. Physical and chemical surface of the Ti 6 Al 4 V disk is different depending on the prescribed electrochemical potential. So this evolution of the properties of the antagonist can explain the small evolution of the graphite wear with the prescribed electrochemical potential.

After experiment, we observe small microscopic graphite particles (<10 m) and macroscopic graphite lengthened chips (about 1 mm) (Figs. 2 and3). After 5 or 20 min experiments, we observe by scanning electron microscopy (SEM) only microscopic graphite debris on the edges of the Ti 6 Al 4 V friction zone. After 6 h experiments, microscopic debris are observed in the artificial sea water solution as well as in the Ti 6 Al 4 V friction zone. Macroscopic chips are observed after 6 h experiment on the friction zone of the Ti 6 Al 4 V disk (Figs. 2 and3). On this macroscopic debris, cracks in the slip-parallel and slip-perpendicular directions are present (Fig. 4). From these after-experiment analysis, we deduce that the graphite matter removal occurs in two ways: small microscopic particles and macroscopic lengthened chips. These types of matter removal seem to occur simultaneously during experiments. But only microscopic debris appear at the very beginning of the experiments.

In the graphite friction zone, one observes microscopic cracks perpendicular to the friction movement (Fig. 5). In addition, the presence of cracks which propagate in the direction of movement suggests the possibility that the removal of graphite is carried out by macroscopic chipping according to the direction of friction. Moreover, because of the presence of cracks on the macroscopic chips and because we do not collect any macroscopic debris by filtering the artificial sea water after experiment, the macroscopic debris are destroyed in the contact. After this process, we think that they are ejected out of the contact zone. So, microscopic debris are produced during long experiments, either directly or from cracking of the macroscopic debris.

Furthermore, the macroscopic debris, observed in the contact zone, present marked scratches of friction (Fig. 6). This observation shows paradoxically that debris are in the contact area for such a long time that they participate to friction without being cracked. The adherence between the debris of graphite and the alloy of Ti 6 Al 4 V induces local friction of graphite against graphite, which explains the small friction coefficient.

It must be underline that the wear of the graphite is not homogeneous in the width of the contact zone. Indeed, along the internal edge of the graphite ring, irregular damages are present while they are not observed along external edge (Fig. 7). This might be due at least in part to the wear debris evacuation which is not of the same kind inside and outside the friction track due to the circulation of artificial sea water (from outside to inside the ring). Other explanations can be the dimension of the zone of the debris evacuation, i.e. the length of the evacuation front or the difference in linear velocity between the internal edge and the external edge of the contact zone (0.027-0.039 m/s) causing different wear rates. Lastly, non-homogeneity in pressure across contact zone might influence the spatial evolution of the wear rate too [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF][START_REF] Serre | Modelling an abrasive wear experiment by the boundary element method[END_REF].

Furthermore, a profilometry of the graphite friction zone is carried out perpendicularly to the friction direction. The roughness changes during the experiments: the Ra of the graphite increases from 0.2 m before experiment to 0.4-0.6 m after experiment while the Ra of the disk reaches approximately 0.3-0.6 m, after experiment. This increase is probably due to the heterogeneity of the wear of the graphite surface and to the roughness of the Ti 6 Al 4 V disk.

Ti 6 Al 4 V wear

The material loss of the Ti 6 Al 4 V disk is measured by profilometry. Four radial measurements are carried out every 90 • around the disk. We assume that the eroded volume corresponds to the product of the average surface under the initially measured level of the sample multiplied by the mean length of the friction track. Analyses of Ti 6 Al 4 V disks indicate that the material loss of Ti 6 Al 4 V is strongly dependent on electrochemical conditions. With electrochemical potential of 0.140 V, experimental results give a material loss of 0.22 ± 0.06 mm 3 . It must be note that this material loss is not homogeneous across the track and is located in a preferential way nearby the edges of the friction zone (Fig. 8) as yet highlighted by Walter and Plitz [START_REF] Walter | The ring on disk method: clinical significance of a wear-screening test of biomaterials for hip joint alloplasty[END_REF]. At a potential lower than -0.060 V, the Ti 6 Al 4 V wear corresponds to grooves whose depth is between 3 and 5 m. They are located along the edges of the friction zone (Fig. 9). For lower potentials (free corrosion potential around -0.100 V with our electrochemical measurement configuration, potential of -0.210, -0.810, -1.010 V), the grooves mainly have lower depth (<2 m). We note, the presence of bulb edges, along the friction track, after the experiments at 0.140 V.

Furthermore, observations by SEM of the contact track (Fig. 2) clearly indicate a low thickness graphite deposit. This deposit is adherent. Then, it corresponds to graphite transferred on the disk surface rather than graphite debris. However, the study of the trail after some experiments (Fig. 10) exhibits very thin graphite clusters. They have oval form with the major axis along the direction of the slip. Since several cleanings with ultrasounds can not remove them, we deduce that they are strongly adherent with Ti 6 Al 4 V, at least more adherent than a part of the transferred graphite which goes away with cleanings. The Ti 6 Al 4 V roughness increases from a Ra included between 0.02 and 0.04 m before experiment, to a Ra between 0.3 and 0.6 m after experiment. Probably, this increase is associated for a part to the presence of the graphite deposit. In optical microscopy we observe that the Ti 6 Al 4 V disk presents friction scratches in the slip direction which are not uniform across the contact area. Some zones do not present observed scratches by SEM. This confirms that the friction is not uniform because of the non-uniform contact pressure and also because of the graphite deposit and macroscopic graphite debris presence. Nevertheless, observations by optical and SEM of the disks show that the friction scratches, the grooves and the material loss are homogeneous according to the rotation angle.

For the experiments at free potential or at an imposed potential of 0.140 and -0.060 V, the corrosion products are observed in the friction zone and particularly on the edges of the friction track (Figs. 11 and12). These corrosion products are not due to an ageing of the samples after experiment because none is observed except on the friction track. Chipping and friction scratches are also observed (Fig. 12). In general, the damaging phenomena occur on corrosion products, located on the external edge of the wear track. A chemical analysis of these corrosion products was performed by SEM analysis. Phosphorus, oxygen, sodium and chlorine were detected. Titanium, aluminium and vanadium are also detected in some cases. According to the thickness of the corrosion products and with regards with the accuracy of the chemical analyses by SEM method, the presence of these three latter elements may be due either to the presence of Ti 6 Al 4 V which constitutes the disk or to their real presence in the corrosion products. To conclude of their presence or not in the corrosion products, new chemical analyses with different techniques should be experimented even if the roughness and the non-flatness of the wear contact of the Ti 6 Al 4 V disk prove to be awkward. Chemical analyses suggest also that some corrosion products contain carbon. It comes from graphite, in particular from the transferred graphite or the graphite debris on the Ti 6 Al 4 V disk or from the friction between the corrosion products against the graphite ring.

On the friction track, an increase of the concentration of oxygen is always observed by SEM. We find much more oxygen near the edges of this zone. Moreover, in this area, we observe an increase of the concentration of phosphorus and to a less extent, chlorine and sodium. These latter three elements are not related to the presence or not of products of corrosion. This latter observation and the presence of the corrosion products actually show the existence of electrochemical reactions for titanium alloy, namely nearby the edges of the friction zone in term of reactions between the Ti 6 Al 4 V surface and the solution A3. Different electrochemical reactions (anodic or cathodic) reactions take place, according to the electrochemical conditions.

The wear of the Ti 6 Al 4 V disk especially depends on the electrochemical potential. At a potential of 0.140 V, the Ti 6 Al 4 V disk presents a material loss which is measured by profilometry (Fig. 8). This material loss cannot be obtained through a mechanical phenomenon only, because of the higher mechanical properties of Ti 6 Al 4 V compared to those of graphite and because of the small applied contact pressure (5 MPa on average) in comparison with the limit of elasticity of the Ti 6 Al 4 V (around 900 MPa). Other experiment results with Ti 6 Al 4 V against smooth material were performed with Ti 6 Al 4 V rubbing against ultra high molecular weight polyethylene (UHMWPE) [START_REF] Magnée | Influence de traitements de surface sur le comportement tribomécanique d'un alliage de titane pour prothèses osseuses[END_REF][START_REF] Buchanan | Ion implantation of surgical Ti 6 Al 4 V for improved resistance to wear-accelerated corrosion[END_REF][START_REF] Martinella | Wear behaviour of the couple polyethylene-Ti 6 Al 4 V: effects of the metallic surface preparation and nitrogen implantation[END_REF]. The wear i.e. the material loss of Ti 6 Al 4 V was observed while polyethylene has low mechanical characteristics (Young's modulus approximately 27-67 MPa) and the loading applied is below 20 MPa. Different experiments configurations in serum or in salt solution were carried out: pin on disk [START_REF] Magnée | Influence de traitements de surface sur le comportement tribomécanique d'un alliage de titane pour prothèses osseuses[END_REF], cylinder on cylinder [START_REF] Buchanan | Ion implantation of surgical Ti 6 Al 4 V for improved resistance to wear-accelerated corrosion[END_REF], or ring on disk [START_REF] Martinella | Wear behaviour of the couple polyethylene-Ti 6 Al 4 V: effects of the metallic surface preparation and nitrogen implantation[END_REF]. Those papers underlined that titanium alloy wear was due to the low resistance of oxide films. Those experimental works highlighted that oxide removal, of strong hardness, acted like abrasive and contributed to the wear of the antagonists.

In our case, first, the electrochemical potential could induce a modification of Ti 6 Al 4 V surface properties by existence of different oxides films. Because mechanical characteristics of oxides film evolve with the electrochemical potential, consequently, a modification of the potential can induce a variation of the wear of the Ti 6 Al 4 V surface even if the two antagonists have very different mechanical characteristics. Second, in the ring-on-disk experiment, the contact is a priori macroscopically continuous between the two antagonists. So the depassivation/repassivation processes which are observed during a pin-on-disk experiment, could a priori not occur. Yet, by using a specific electrochemical montage, based on current/tension converters [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF][START_REF] Deslouis | Multi-channel potentiostatic control in electrochemistry: localised mass-transfer rates at the rotating disk electrode[END_REF], we observe that, under friction, the contact between the two antagonists is not electrochemically perfect [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF]. This means local processes of depassivation/repassivation of the Ti 6 Al 4 V by contact with the graphite. Thanks specifics experiments using current/tension converters and thanks electrochemical study of the two materials [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF], we establish that during tribocorrosion experiments at 0.140 V, the measured current mainly corresponds to contribution of the anodic reactions (formation of titanium oxides) to the Ti 6 Al 4 V surface. Moreover, we check that for the experiments at 0.140 V, measurements of the Ti 6 Al 4 V material loss (0.22 ± 0.06 mm 3 ) are consistent with the material loss calculated with the measured current and Faraday's law (0.20 ± 0.05 mm 3 ) [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF][START_REF] Landolt | Corrosion et chimie des surfaces et des métaux[END_REF]. Consequently, in our case, the Ti 6 Al 4 V material loss is mainly due, at anodic potential to locally depassivation/repassivation phenomena in spite of a permanent macroscopic contact.

The corrosion products, like the graphite clusters, probably influence the friction and the wear by their properties particularly mechanical properties. This influence is all the more important since the pressure average of contact is small [START_REF] Georges | Frottement, usure et lubrification[END_REF]. Indeed, surface properties influence the response of materials to friction and thus their wear [START_REF] De Wit | Lubricating reaction products on TiN coatings during sliding wear in phosphoric acid[END_REF][15][START_REF] Magnée | Influence de traitements de surface sur le comportement tribomécanique d'un alliage de titane pour prothèses osseuses[END_REF]. Furthermore, the graphite transferred and the graphite clusters are adherent to the Ti 6 Al 4 V disk. So, they induce local friction of graphite against graphite ring which could explain the small friction coefficient.

Conclusion

A new tribocorrosion experimental set-up has been used to study materials wear for a ring in friction with a disk in sea water environment [START_REF] Serre | Contribution à l'étude des phénomènes d'usure par frottement en milieu marin[END_REF][START_REF] Serre | Modélisation d'un contact graphite contre Ti 6 Al 4 V renforcé TiC en milieu marin artificiel: essais de tribocorrosion plan/plan de type anneau/disque[END_REF]. Observations at the end of the experiment permit to determine the process of material loss and degradation for the graphite ring and Ti 6 Al 4 V disk. We observe both graphite removal and graphite transferred on Ti 6 Al 4 V. Graphite removal consists in the creation of large chips which are either deposed on the Ti 6 Al 4 V surface or broken inside the contact area before being ejected in solution. The material loss of the Ti 6 Al 4 V disk depends on the electrochemical conditions: no significant material loss exists during experiments at cathodic potential. For the experiments at anodic potential, the electrochemical material loss is calculated using the measured current and the Faraday's law. This shows that the wear of Ti 6 Al 4 V is mainly due to the existence of depassivation/repassivation phenomena for a continuous macroscopic contact, i.e. in a ring on disk contact.

Fig. 1 .

 1 Fig. 1. Ring-on-disk tribometer principle.

6 Fig. 2 .

 62 Fig. 2. Wear track of Ti 6 Al 4 V disk: presence of graphite debris.

Fig. 3 .

 3 Fig. 3. Macroscopic graphite debris on a Ti 6 Al 4 V disk.

Fig. 4 .

 4 Fig. 4. Cracks perpendicular to the sliding direction on graphite debris.

Fig. 5 .

 5 Fig. 5. Friction track of the graphite ring: cracking of the graphite.

Fig. 6 .

 6 Fig. 6. Friction scratches on graphite debris.

Fig. 7 .

 7 Fig. 7. Friction zone of the graphite ring: different wear on internal and external edges of ring.

Fig. 8 .

 8 Fig. 8. Profilometry of the wear track for the Ti 6 Al 4 V sample: experiment at electrochemical potential of 0.140 V.

Fig. 9 .

 9 Fig. 9. Profilometry of the wear track for the Ti 6 Al 4 V sample: experiment at electrochemical potential of -0.060 V.

Fig. 10 .

 10 Fig. 10. Graphite clusters on the wear track of the Ti 6 Al 4 V sample.

Fig. 11 .

 11 Fig. 11. Products of corrosion on Ti 6 Al 4 V friction zone.

Fig. 12 .

 12 Fig. 12. Products of corrosion on the external edge of the Ti 6 Al 4 V friction zone.

Table 1

 1 Friction coefficient function to the electrochemical conditions of experiment

	Electrochemical conditions	Initial value of	End value of
		the experiment	the experiment
	Imposed potential of -1.010 V	0.15 ± 0.02	0.15 ± 0.01
	Imposed potential of -0.810 V	0.18 ± 0.02	0.16 ± 0.09
	Imposed potential of -0.210 V	0.13 ± 0.04	0.12 ± 0.01
	Open circuit	0.17 ± 0.02	0.16 ± 0.04
	Imposed potential of -0.060 V	0.13 ± 0.03	0.16 ± 0.04
	Imposed potential of 0.140 V	0.14 ± 0.04	0.22 ± 0.04

Table 2

 2 Material loss of the graphite ring after 6 h experiments function electrochemical conditions of experiment

Acknowledgements

The authors wish to thank Mr Louchart of Centre Technique d'Arcueil (France) for his technical assistance. We are indebted to Mr. Keddam and Mr. Takenouti from the laboratory "Physique des Liquides et Electrochimie" for their help about the electrochemical set-up with the current/tension converters.


Rana Chehab

Bruno Coulomb

Jean-Luc Boudenne

Fabien Robert-Peillard
email: fabien.robert-peillard@univ-amu.fr

Development of an automated system for the analysis of inorganic chloramines in swimming pools via multi-syringe chromatography and photometric detection with ABTS

Keywords: Multi-syringe chromatography, inorganic chloramines, free chlorine, post-column reaction, swimming pools

Inorganic chloramines are disinfection by-products resulting from the unwanted reaction between chlorine used as disinfectant in swimming pools and nitrogenous compounds brought by bathers. This parameter (total chloramines or combined chlorine) is currently measured on site by a colorimetric method that does not allow to measure only inorganic chloramines. In this paper, a multi-syringe chromatography system combined with a post column derivatization is applied for the first time for the specific detection of the three individual inorganic chloramines (monochloramine, dichloramine and trichloramine). These latter ones are separated using a low-pressure monolithic C18 column, and separately detected after a post-column reaction with the chromogenic reagent ABTS (2,2'-azino-bis-(3-ethyl-benzothiazoline)-6-sulfonic aciddiammonium salt). Development of two ABTS reagents provides discrimination of chlorine and monochloramine that are not separated on the column. Optimization of the experimental conditions enables determination of inorganic chloramines with very good detection limits (around 10 µg eq.Cl2 L -1 ) without interferences from other chlorinated compounds such as organic chloramines or free available chlorine. The validation of the whole procedure has been successfully applied to real swimming pools samples.

Introduction

Among disinfection by-products commonly found in swimming pools, inorganic chloramines are the most often encountered at the highest concentrations, with monochloramine (MCA, NH2Cl) mainly found in water (up to 1180 µg L -1 ), followed by trichloramine (TCA, NCl3) (up to 800 µg L -1 ) and dichloramine (DCA, NHCl2) (up to 650 µg L -1 ) [START_REF] Manasfi | Occurrence, origin and toxicity of disinfection byproducts in chlorinated swimming pools: An overview[END_REF]. Inorganic chloramines are a group of three chemical substances in aqueous solution formed by reactions between chlorine at oxidation state (+I), ammonia and amino acids coming from urea and sweat brought by bathers (such as uric acid, urea, creatinine, L-arginine, L-histidine, glycine) [START_REF] Lian | Volatile disinfection byproducts resulting from chlorination of uric acid: implications for swimming pools[END_REF].

These chloramines are present in various types of waters where chlorine is used as disinfectant or for biofouling control (drinking waters, swimming pools waters, wastewaters, industrial chlorinated effluents) [START_REF] Manasfi | Occurrence, origin and toxicity of disinfection byproducts in chlorinated swimming pools: An overview[END_REF][START_REF] Heeb | Formation and reactivity of inorganic and organic chloramines andbromamines during oxidative water treatment[END_REF][START_REF] Manasfi | Occurrence and speciation of chlorination byproducts in marine waters and sediments in a semi-enclosed bay exposed to industrial chlorinated effluents[END_REF][START_REF] Mole | Correlating effluent concentration and bench-scale experiments to assess the transformation of endocrine active compounds in wastewater by UV or chlorination disinfection[END_REF][START_REF] Chuang | Comparing industrial and domestic discharges as sources of N-nitrosamines and their chloramine or ozone-reactive precursors[END_REF]. In particular for swimming pools, respiratory issues, such as asthma, wheeze, cough and lower respiratory tract infections, have been correlated with swimming pool attendance, which is likely due to chlorinated volatile DBPs, such as chloramines and especially TCA [START_REF] Bernard | Chlorinated pool attendance, atopy, and the risk of asthma during childhood[END_REF][START_REF] Florentin | Health effects of disinfection by-products in chlorinated swimming pools[END_REF][START_REF] Westerlund | Occupational exposure to trichloramine and trihalomethanes : adverse health effects among personnel in habilitation and rehabilitation swimming pools[END_REF]. TCA is also assumed to cause skin and eye irritations [START_REF] Schmalz | Application of an optimized system for the well-defined exposure of human lung cells to trichloramine and indoor pool air[END_REF]. MCA and DCA have not been yet assumed to be toxic but their roles as precursors of the potent carcinogen N-nitrosodimethylamine (NDMA) have been emphasized by several authors [START_REF] Mitch | Formation of N-nitrosodimethylamine from dimethylamine during chlorination[END_REF][START_REF] Selbes | The roles of tertiary amine structure, background organic matter and chloramine species on NDMA formation[END_REF][START_REF] Huang | Re-Examining the Role of Dichloramine in High-Yield N-Nitrosodimethylamine Formation from N,N-Dimethyl-α-arylamines[END_REF]. Consequently, reliable, rapid and sensitive methods are required to monitor pool water quality and to assess their mitigation strategies.

Many methods have been proposed for the determination and quantification of chloramines and have been reviewed by Kinani et al. [START_REF] Kinani | Analysis of inorganic chloramines in water[END_REF]. The most commonly used are based on the redox reaction using N, N-diethyl-p-phenylenediamine (DPD) as the reducing agent with colorimetric or amperometric detection. The colorimetric method uses the red coloration produced by the reaction between DPD, free chlorine and inorganic chloramines (after addition of iodide) with a spectrophotometric measurement at 510 nm, whereas the titrimetric method employs ammonium sulfate with ferrous ion as the final reagent of the colored solution produced by DPD reaction. These methods are simple and sensitive (quantification limits (LOQ) are around 30 µg eq. Cl2 L -1 ), but they suffer from a lack of selectivity. Indeed, organic chloramines may interfere with inorganic chloramines as they react in the same way in the presence of the iodide ions, leading to an overestimation of the real amount of inorganic chloramines in samples with large amounts of organic nitrogen [START_REF] Shang | Differentiation and quantification of free chlorine and inorganic chloramines in aqueous solution by MIMS[END_REF]. Another colorimetric method using a derivatization reaction has been developed to differentiate inorganic monochloramine from organic chloramines using phenol type reagent as color reagent [START_REF] Tao | Salicylate-spectrophotometric determination of inorganic monochloramine[END_REF]. The derivatized product can be measured spectrophotometrically at a wavelength of 655 nm. However, this method suffers from a low sensitivity (LOQ > 270 µg eq. Cl2 L -1 ) and from the ability to detect only monochloramine.

To overcome these issues and to allow individual determination of the three chloramines, chromatographic separation using high performance liquid chromatography (HPLC) has been proposed. Separations of chloramines are generally performed using reversed-phase HPLC on a C18-phase column. Direct spectrophotometric detection is possible at specific wavelength for each chloramine (MCA at 240 nm, DCA at 210 or 280 nm and TCA at 220 nm) [START_REF] Brunetto | High performance reversed phase liquid chromatography of chloramines[END_REF][START_REF] Pla-Tolós | Selective and sentivive method based on capillary liquid chromatography with in-tube solid phase microextraction for determination of monochloramine in water[END_REF]. However, quantification limits of this detection method are generally very high (LOQ >200 µg eq. Cl2 L -1 ), and selectivity is also very low as many other co-eluting compounds can interfere in the wavelength range below 250-300 nm. Direct amperometric detection with a working glassy carbon electrode is also proposed with a better sensitivity (quantification limits 2-13 times lower than those obtained by spectrophotometric detection), but interference from oxygen seriously hampers application of this method [START_REF] Ge | Determination of trace amounts of chloramines by liquid chromatographic separation and amperometric detection[END_REF]. Chemical derivatization generally provides low sensitivity, except when LC-MS/MS is used with a pre-column derivatization of MCA. This method is very sensitive and specific for MCA, but drawbacks are the high cost and complexity of the detection method, and the inability to measure DCA and TCA [START_REF] Kinani | Selective and trace determination of monochloramine in river water by chemical derivatization and liquid chromatography/tandem mass spectrometry analysis[END_REF]. The most efficient derivatization method for inorganic chloramines to date is the post-column derivatization with potassium iodide and photometric or electrochemical detection of triiodide ions formed [START_REF] Gagnon | Disinfectant efficacy in distribution systems: A pilot-scale assessment[END_REF]. Although the sensitivity of this method is quite high (LOQ around 30 µg eq. Cl2 L -1 ), the post-column reaction is non-specific to inorganic chloramines (potassium iodide can react with many other compounds).

The aim of this study was to develop a low-cost automated analyzer for the individual quantification of MCA, DCA and TCA. So far, the only on-line automated analytical tool for determination of these three inorganic chloramines is membrane-introduction mass spectrometry (MIMS), which uses a semi permeable membrane coupled to a mass spectrometer [START_REF] Shang | Differentiation and quantification of free chlorine and inorganic chloramines in aqueous solution by MIMS[END_REF]. This analytical tool is very specific for inorganic chloramines, but lacks of sensitivity (LOQ =60-300 µg eq. Cl2 L -1 ), remains costly and requires qualified personnel to handle with MS interpretations. Our aim was thus to propose an alternative analytical method based on the simplicity and versatility of flow injection techniques coupled to liquid chromatography separation for the determination of individual inorganic chloramines.

A multi-syringe chromatography (MSC) system using monolithic column is thus developed to obtain a low-pressure automated portable system with relatively low-cost and simple components, able to be used as an on-line system. In order to achieve better sensitivity and specificity than direct spectrophotometric detection, a new post-column reaction for all inorganic chloramines is proposed using 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), an indicator already used by Solterman et al. [START_REF] Soltermann | Comparison of a novel extractionbased colorimetric (ABTS) method with membrane introduction mass spectrometry (MIMS): Trichloramine dynamics in pool water[END_REF] for the detection of trichloramine. Optimizations of this post-column reaction and of the chromatographic separation of chloramines are presented, and the developed method applied on real swimming pool samples.

Material and methods

Reagents and solutions

All chemicals were of analytical reagent grade and used without further purification. Ultrapure water purified with a Milli-Q system (Millipore, USA, resistivity >18 MΩ cm) was used for the preparations of the whole solutions. Sodium hypochlorite solutions were prepared by dilution of a commercially available NaOCl solution (4.00-4.99% active chlorine, Sigma-Aldrich, France) and standardized by iodometric titration. Ammonium stock standard solution (2 g L -1 ) was prepared from ammonium chloride (Alfa Aesar, Germany). Solutions of ABTS (2,2'-azinobis-(3-ethyl-benzothiazoline)-6-sulfonic acid-diammonium salt, Sigma-Aldrich) were prepared by dissolving appropriate amount in water. Acetonitrile (MeCN) (HPLC-Gradient phase, Sigma-Aldrich) was used as the organic mobile phase. Formic acid (Fisher chemical, France), sulfuric acid (Fisher chemical), hydrochloric acid (Sigma-Aldrich) and sodium hydroxide (Sigma-Aldrich) were used for pH adjustments.

Inorganic chloramines standard solutions

Inorganic chloramines are unstable and sensitive to light, so they had to be prepared daily and stored in amber glass or polytetrafluoroethylene bottles at +4 °C and kept out of direct light. Inorganic chloramines solutions prepared were standardized by using DPD/FAS titrimetric method.

MCA standard solution was prepared by slowly adding 5 mL of sodium hypochlorite solution (6 mmol Cl2 L -1 ) to 5 mL of ammonium chloride solution (12 mmol L -1 ) whose pH was prior adjusted at 9 with 0.1 M NaOH (ammonia to chlorine molar ratio was 2:1). Final pH was between 10.5 and 11. The reaction vial was then covered with aluminum foil and stored at +4 °C for 60 min before use.

DCA standard solution was prepared by acidification of the MCA solution with 0.1 M formic acid to reach a pH value between 3.5 and 4. This solution was then stirred at room temperature for 2 h while pH was monitored and readjusted as necessary, and then left for 4 h at +4 °C before use.

TCA standard solution was prepared by addition of 5 mL of a 6 mmol Cl2 L -1 sodium hypochlorite solution (whose pH was previously adjusted below 3 with 0.5 M H2SO4) to 5 mL of a 1.9 mmol L -1 ammonium chloride solution (ammonia to chlorine molar ratio was 1:3.15).

Final pH was between 2.3 and 2.5. The reaction vial was then covered with aluminum foil and stored at +4 °C for 45 min before use.

A solution of MCA and DCA was prepared by additions of 5 mL of a 6 mmol Cl2 L -1 sodium hypochlorite solution (whose pH waspreviously adjusted to 6 with 0.1 M formic acid) to 5 mL of a 6 mmol L -1 ammonium chloride solution (ammonia to chlorine molar ratio was 1:1). Final pH was between 5.2 and 5.8. The vial was then covered with aluminum foil and stored at +4 °C for 45 min before use.

For the study on inorganic bromamines, monobromamine (NH2Br), dibromamine (NHBr2) and tribromamine (NBr3) were prepared as described by Galal-Gorchev and Morris [START_REF] Galal-Gorchev | Formation and stability of bromamide, bromimide, and nitrogen tribromide in aqueous solution[END_REF] and by Lei et al. [START_REF] Lei | Bromamine Decomposition Kinetics in Aqueous Solutions[END_REF] , while bromochloramine (NHBrCl) was prepared following Allard et al.'s method [START_REF] Allard | Method development for quantification of bromochloramine using Membrane Introduction Mass Spectrometry[END_REF].

HPLC separation of inorganic chloramines with direct UV detection

Inorganic chloramines separation has been first optimized by using a HPLC system consisting of an L-2130 pump, an L-2400 UV detector, and an L-2200 autosampler (Hitachi Elite LaChrom system, Tokyo, Japan). HPLC separations were performed on a RP-18 Onyx™ monolithic column (25 mm x 4.6 mm, Merck, Germany). Detection was performed at 245 nm for MCA, 295 nm for DCA (210 nm for mixtures of MCA + DCA) and 220 nm for TCA. The column temperature was fixed at 25 °C, the flow rate was set at 0.5 mL min -1 , and the injected sample volume was 20 µL.

Reaction of inorganic chloramines with ABTS

The reaction between free chlorine or inorganic chloramines with ABTS was studied using a Jasco V-670 spectrophotometer operating with the Spectra Manager software. The solutions were analyzed in a 10 mm light path quartz cell. Solutions of ABTS were stored at 4 °C and were stable for at least 2 months. Solution A was prepared by dissolving 13.6 mg of ABTS in 100 mL 0.4 M HCl ([ABTS] = 0.25 mM). Solution B was prepared by dissolving 13.6 mg of ABTS in 100 mL of 5 mM phosphate buffer at pH 3.

For the kinetic measurements in quartz cells, 1 mL of sample and 2 mL of the appropriate ABTS solution were added and mixed in the cell, and absorbance was measured every 5 s at 405 nm.

MSC instrumentation and software

The device used is described in Fig. 1. It included a multisyringe burette (BU-4-S, Crison Instruments, Spain) equipped with four syringes (Hamilton, Switzerland). Three syringes were used: S1 (5 mL; mobile phase: MeCN/H2O: 20/80), S2 (2.5 mL; ABTS solution A) and S3 (2.5 mL; ABTS solution B). Standard solenoid valves on the head of S1 were replaced by a twoway connector made of Delrin® (Sciware System SL, Spain) in order to resist organic solvents. This connector linked S1 with a new solenoid valve (V1, Takasago, Japan) which was situated outside the syringe module and allowed a maximum operating pressure of 6 bars. V2 and V3

were standard solenoid valves. The manifold was built up with a 0. Optics) connected to a computer via an USB interface. The analytical wavelength was set at 405 nm. Two FC-UV600-2 optical fibers (Ocean Optics) were used to guide the light from the source through the flow cell and further to the spectrophotometric detector. System control, data acquisition and processing were performed using the software package Autoanalysis 5.0 (Sciware System SL).

MSC procedure

The optimized protocol used for the MSC system is presented in Table 1 and summarized as follows:

A first cycle (separation/derivatization) was run with ABTS (solution B) to measure only free chlorine. After injection of the sample in the loop (Load position, step 1), chromatographic separation was performed by switching the valve to "Inject" position (step 4) and by dispensing the mobile phase (step 6), while absorbance measurement was carried out at 405 nm. After reloading of the syringes (step 8), a second cycle was run with ABTS (solution A) to measure inorganic chloramines (step 9-13).

Swimming pools samples

Samples were collected in five indoor swimming pools located in the Aix-Marseille Provence Metropolis. These five pools were supplied with public water works and were continuously disinfected with bleach at a set point concentration of 1 mg L -1 Cl2 (expressed as free chlorine).

Water samples were taken at a depth of 30 cm, stored at 4 °C and analyzed at the laboratory as quick as possible (delay < 1 hour). The sampling took place at peak times with the objective of having a maximum bather load (free swims or classes).

3. Results and discussion.

Inorganic chloramines standard solutions

DPD/FAS titrimetric standardization of chloramines standard solutions was used to provide conversion percentages of chlorine and ammonia into chloramines. In the conditions described in section 2.2, 98 % conversion of free chlorine to monochloramine was achieved within 60 minutes, and 95 % of monochloramine was also converted to dichloramine after acidification of the monochloramine solution and completion of the reaction for 6 h.

Regarding the trichloramine solutions, different preparation methods were tested as described in the literature [START_REF] Shang | Differentiation and quantification of free chlorine and inorganic chloramines in aqueous solution by MIMS[END_REF][START_REF] Soltermann | Comparison of a novel extractionbased colorimetric (ABTS) method with membrane introduction mass spectrometry (MIMS): Trichloramine dynamics in pool water[END_REF][START_REF] Pinkernell | Methods for the photometric determination of reactive bromine and chlorine species with ABTS[END_REF]. Chlorine to ammonia molar ratio is usually 3:1 or 3.15:1 with acidic pH between 2.3 and 2.8 but proposed ageing times vary considerably, from 1 h to 1 week. The DPD/FAS determination of the solutions prepared in these various conditions in our laboratory showed that there was always free chlorine and dichloramine in the trichloramine standard solutions, even at very high chlorine to ammonium molar ratio (10:1) as was described by some authors [START_REF] Soltermann | Comparison of a novel extractionbased colorimetric (ABTS) method with membrane introduction mass spectrometry (MIMS): Trichloramine dynamics in pool water[END_REF].

Preparation of trichloramine solutions (according to the protocol described in section 2) and kinetic measurements of chloramines using DPD/FAS titration over 28 h ageing showed that both dichloramine and trichloramine were present immediately after the beginning of the experiment. After 45 min, concentration of trichloramine became stable for 6 h, followed by a slow decrease in trichloramine concentration as well as an increase in dichloramine concentration. Conversion yields of trichloramine were usually around 10 % (based on initial ammonia concentration) after 45 min. This preparation method and ageing time has been chosen

for this study in order to reduce the daily preparation time of trichloramine standards.

Inorganic chloramines separation with direct detection

Optimization of the separation of inorganic chloramines was first studied using HPLC with direct UV detection (on standard solutions with high concentrations of chloramines) as preliminary experiments. Various mobile phases were tested in order to study the separation of inorganic chloramines by HPLC using a monolithic column, which has never been documented before in the literature (only with traditional particle-packed columns). Results in Table 2 show retention times of the three chloramines and resolution between MCA and DCA whose peaks were the lower-separated ones. The best compromise between TCA retention time (which corresponds to the total analysis time for three chloramines) and MCA-DCA resolution was a mobile phase made of 20 % acetonitrile, leading to short overall retention times and good separation of the whole chloramines. Representative chromatograms for the separation of the inorganic chloramines with this mobile phase are depicted in Figure S1 in the supplementary materials section. Addition of buffers (acidic or neutral) did not improve separation and was not selected for the rest of the study as it could displace equilibrium between chloramines during the column separation. Methanol was also tested as the organic solvent of the mobile phase, but results were not as good as with acetonitrile and it generated higher pressure in the system, which is not recommended for low-pressure MSC systems. Irrespective of the conditions tested, chlorine and MCA have not been able to be separated.

The same experimental conditions (monolithic column, mobile phase made of 20% MeCN, eluent flow rate of 0.5 mL min -1 ) have then been applied to the MSC system coupled to a UV detector (without post-column reaction).Retention times and resolution of chromatographic peaks were close to those obtained with the HPLC system, giving proper pressures around 3 bars. MCA and chlorine also co-eluted with this MSC system. ABTS was used as a post-column reagent being known to be able to react with halogenated compounds to form the colored ABTS°+ radical, detectable and quantifiable at 405 nm (Ɛ= 31600 M -1 cm -1 ). ABTS has indeed already been used as a colorimetric indicator for trichloramine [START_REF] Soltermann | Comparison of a novel extractionbased colorimetric (ABTS) method with membrane introduction mass spectrometry (MIMS): Trichloramine dynamics in pool water[END_REF] and for detection of monochloramine after addition of nitrite [START_REF] Pinkernell | Methods for the photometric determination of reactive bromine and chlorine species with ABTS[END_REF] but has never been studied for the simultaneous/sequential detection of all inorganic chloramines and free chlorine.

Kinetic measurements by batch experiments

The objective of the optimization of the reaction between ABTS and chloramines was first to find experimental conditions that ensured rapid and total reaction of the three inorganic chloramines, in order to be used later as post-column reaction on our MSC system. Chlorine and inorganic chloramines react with ABTS as follows:

HOCl One must therefore bear in mind that stoichiometry of TCA and DCA reactions are three times and twice that of MCA or hypochlorous acid, respectively. Various experimental conditions were tested (pH, addition of iodide catalyst…) in batch experiments (quartz cell) and all inorganic chloramines (and hypochlorous acid) were found to react quickly and quantitatively with ABTS at pH below 2, without catalyst. Figure 2 displays the kinetics of the ABTS°+ formation for all studied compounds with ABTS dissolved in 0.4 M HCl. All reactions were complete after 30 s, which was fast enough to be used for post-column reaction. Measured absorbance after reaction of DCA (10 -5 mol L -1 ) with ABTS was twice that of MCA and chlorine (10 -5 mol L -1 ), as expected. Regarding TCA, only mixtures of TCA, DCA and HOCl have been able to be formed, and the absorbance depicted for the reaction with TCA (concentration of TCA at 10 -5 mol L -1 in the mixture) was calculated by subtracting the absorbance of the expected ABTS°+ produced by reaction with DCA and HOCl from the total absorbance (originating from the reaction with the three species).

The reaction between ABTS and inorganic chloramine and free chlorine is highly dependent on the reaction pH value, and up to now, discrimination between free chlorine and MCA was not achievable, these two compounds being not separated after the chromatographic step. In our previous experiments, ABTS reacted with all chlorinated compounds at acidic pH and with any of them at neutral or basic pH. However, a fine-tuning of the pH around 3 allowed us to separate MCA and HOCl (Fig. 3). MCA did not react with ABTS at pH 3, whereas HOCl still exhibited quantitative and fast reaction. Differentiation between MCA and HOCl was thus possible by running two experiments: one with ABTS in HCl (ABTS solution A) for free chlorine and inorganic chloramines, and one with ABTS at pH 3 (ABTS solution B) for determination of free chlorine.

3.3.2. Optimization on the MSC system.

The MSC system has then been optimized in terms of the post-column reaction by varying several operational parameters: the ratio between flow rates of mobile phase and ABTS solution, the mobile phase flow rate and the tubing volume /length between the mixer and the detector. As the mobile phase and the ABTS solution were delivered by the same piston pump (by use of a multi-syringe pump in order to simplify the experimental set-up), the only way to change the flow-rates ratio was to change the volume of the syringe of ABTS while keeping that of mobile phase constant (5 mL). Therefore, flow rate ratios of 1, 2 or 5 have been able to be obtained by using 5 mL, 2.5 mL or 1 mL syringes for ABTS. For these experiments, flow rate of mobile phase was set at 0.3 mL min -1 , pressure was 2.5 bars, and the volume of the tubing between the mixer and the detector was 100 µL. Table 3 shows that peak areas and resolution between MCA and DCA peaks were better when ABTS flow rate was half that of the mobile phase (which corresponded to the volume ratios used in quartz cells experiments).

This ratio was selected for the rest of the study. The mobile phase flow rate and the tubing volume between the mixer and the detector had a direct influence on the post-column reaction time. Volumes from 100 to 300 µL and two flow rates (0.3 mL min -1 and 0.5 mL min -1 ) were tested, and best results (peak areas and resolution)

were obtained with a flow rate of 0.3 mL min -1 and a tubing volume of 100 µL (Table 4).

Considering the mobile phase and ABTS solution flow rates (global flow rate of 0.45 mL min - 1 ), this volume corresponded to a reaction time of 13.3 sec, which seemed sufficient to obtain the best compromise between peak areas and resolution. interferences from other chlorinated compounds, especially from organic chloramines which are formed during chlorination of organic amines and amino acids [START_REF] Shang | Differentiation and quantification of free chlorine and inorganic chloramines in aqueous solution by MIMS[END_REF]. These interferences result in "false positives" for inorganic chloramines determination (appearing as MCA or DCA) with DPD standard methods, which strongly hinders application of this method to real samples with complex matrix content. To assess interferences from organic chloramines on our MSC system, glycine was selected as a model organic amino compound. This latter one was indeed already demonstrated to be chlorine-reactive forming organic chloramines (N-chloro or Ndichloroglycine) [START_REF] Mehrsheikh | Investigation of the mechanism of chlorination of glyphosate and glycine in water[END_REF].

The ability of the MSC system to differentiate inorganic and organic chloramines species was demonstrated as follows : 1 mL of a glycine solution (100 mg L -1 as N) was added to 220 mL of 0.01 M phosphate buffer (pH 7) in a flask, then aliquots of free chlorine stock solution were added so that chlorine/nitrogen mass ratio were from 2 to 16 (chlorine/nitrogen molar ratio from 0.4 to 3.2). The concentration of residual chlorine was then determined by the DPD/FAS titration method (Fig. 4a.) and by the MSC system after 30 min (Fig. 4b). As expected, the lack of selectivity of DPD/FAS titration was highlighted, as MCA and DCA were quantified in high quantities while it has already been proven by MIMS measurements (during an experiment performed under the same conditions) that none of them were able to be actually present at low Cl/N mass ratios (<14) [START_REF] Shang | Differentiation and quantification of free chlorine and inorganic chloramines in aqueous solution by MIMS[END_REF]. On the opposite, our MSC system only detected free chlorine at high Cl/N mass ratio, which proved that organic chloramines did not interfere with our system.

Indeed, organic chloramines generated by the chlorination of glycine did not react with the ABTS solution A (or B), as was demonstrated by a direct test in quartz cell. Therefore, coelution of an organic chloramine with an inorganic chloramine would not cause interference on the determination of MCA, DCA or TCA. The same results were obtained during chlorination of asparagine (data not shown).

Interferences from other chlorinated disinfection by-products or chlorine-related compounds were also assessed. N-chlorourea, chloroform, bromoform (trihalomethanes), mono-, di-and tri-chloroacetic acids (haloacetic acids) and cyanuric acid (chlorine stabilizer) showed no analytical response with our system, due to the non-reactivity of these compounds with ABTS.

Reactivities of inorganic bromamines were also studied. The results showed that NH2Br coelutes with NH2Cl, while NHBr2 and NHBrCl had the same retention time as NHCl2.

Nevertheless, NBr3 has been able to be separated from NCl3 as their retention times were different (13.9 min for trichloramine and 19.6 min for tribromamine). Further studies are in progress in our laboratory to differentiate inorganic bromamines and chloramines by optimization of the ABTS reagent.

Analytical features

The analytical procedure of the MSC system was evaluated under the optimized experimental conditions, with ABTS solution A for HOCl and inorganic chloramines determination, and with ABTS solution B for HOCl only (Table 5). Excellent linear regression coefficients were obtained for all chloramines up to at least 7100 µg eq.Cl2 L -1 (100 µmol eq.Cl2 L -1 ). The limits of detection (LD) and of quantification (LQ) were estimated using the classical 3s and 10s

approaches respectively, i.e. calculation of LD and LQ through analysis of the standard deviation of blank measurements (n=8). Limits of detection for the inorganic chloramines were all around 10 µg eq.Cl2 L -1 , which was comparable to the DPD titrimetric method and 2 to 10 times better than usual MIMS performances [START_REF] Kinani | Analysis of inorganic chloramines in water[END_REF]. Limits of detection on our MSC system without post-column reaction (direct UV detection) were 100 to 500 times lower, proving efficiency of the ABTS detection. Slope of the linear regression equations of HOCl or inorganic chloramines were all similar after reaction with ABTS (when concentrations were expressed as equivalent Cl2 L -1 , and considering stoichiometry of equations presented in section 3.1), proving that chloramines concentrations were not affected by the chromatographic separation using our experimental conditions (monolithic column for fast separation, mobile phase with water),

contrary to what has been previously described when using traditional HPLC method [START_REF] Brunetto | High performance reversed phase liquid chromatography of chloramines[END_REF].

Regarding sample throughput, the overall procedure took about 30 min, including the two cycles with ABTS solution A and B, allowing the analysis of 2 samples/h. 3.6. Application to real samples Our multisyringe analytical system was applied to real samples collected in swimming pools of Aix-Marseille Provence Metropolis. We first intended to compare our results with MIMS measurements, which was not available in our laboratory. Samples were sent to another French laboratory equipped with a MIMS for comparison purposes, but quantification for MCA and DCA was unfortunately not possible on real samples, due to interferences from other compounds and between inorganic chloramines. Standard addition method was also not possible for validation, as addition of inorganic chloramines to swimming pool samples resulted in modification of chlorine-inorganic chloramine equilibrium and very large differences between added and measured concentrations in these samples.

Therefore, we chose to assess the accuracy of the multisyringe system by comparison of the results obtained with our MSC system and those obtained by the DPD/FAS titrimetric assay (Table 6). While measurements for free chlorine were in good agreement (no difference at the 0.05 level using Student's t test), significant differences appeared for inorganic chloramines.

MCA was detected by DPD/FAS assay in two samples in low concentrations (2 and 3) but not detected by the MSC procedure. DCA measurements resulted in the largest differences between both methods, with DPD/FAS results always much higher than MSC results. This large discrepancy has been able to be explained by the inability of the DPD/FAS assay to differentiate between inorganic and organic chloramines [START_REF] Lee | Comparison of colorimetric and membrane introduction mass spectrometry techniques for chloramine analysis[END_REF], which was further demonstrated in this study on swimming pool samples. Samples were collected during highly crowded hours (except sample 1 with no inorganic chloramines detectable by the MSC method), and significant amounts of organic chloramines or other DPD/FAS interferents could have been expected due to the reaction of chlorine with organic nitrogenous compounds brought by swimmers. TCA was not quantified by either methods.

Our method could be also applied to other types of waters such as drinking waters, especially when MCA is used as a secondary disinfectant such as in the USA (where significant concentrations of MCA and DCA are expected).

Conclusion

In this study, a MSC system based on a chromatographic separation with a monolithic column and post-column derivatization with ABTS was developed for the analysis of inorganic chloramines in water. Use of a mobile phase containing acetonitrile and water enabled successful separation of the chloramines with good resolution and limited pressure in the flow system, leading only to the coelution of chlorine and monochloramine. Optimization of the ABTS reaction resulted in the preparation of two ABTS reagent: one for the quantification of free chlorine only, and one for the quantification of chlorine and the three inorganic chloramines. Mono-, di-and trichloramine were quantified by the MSC system with detection limits around 10 µg eq.Cl2 L -1 , a large linear working range and a very good repeatability.

Rather as DPD-based methods, our method was not affected by the presence of organic chloramines. This method applied and validated on real samples represents a promising and cheaper alternative to MIMS, both in laboratory (use of the same method on a widely available LC-UV equipment is also possible) and on-site analysis of inorganic chloramines.

Figure captions

  8 mm internal diameter poly(tetrafluoroethylene) (PTFE) tubing. S1 was connected to one port of a 6-port injection valve module (Crison Instruments) equipped with a 20 µL loop. The three other ports of the injection valve were connected to the sample vial, the monolithic column and waste. The injection loop was filled with the sample thanks to a solenoid micro-pump (MP1) (Bio-Chem, USA). The indicated stroke volume for the micro-pumps was 20 µL. MP1 and V1 were computer-controlled by means of a MCFIA/MPFS system (Sciware System SL) having eight digital 12V output channels. The separation of chloramines was performed on an Onyx™ monolithic C18 column (25 mm x 4.6 mm, Merck, Germany) at room temperature, using a mixture of acetonitrile and water as mobile phase (MeCN/H2O: 20/80). A PEEK (polyether ether ketone) T-mixer equipped with a PAT frit (PEEK Alloyed with Teflon, Interchim, France) was used to mix the eluent from the column with the ABTS solution during the post-column derivatization. A 100 µL PTFE tubing (0.5 mm internal diameter) was used to connect the T-mixer to the flow cell to ensure enough time to complete reaction between chloramines and ABTS. The detection system was composed of an ultra violet-visible light source (DH-2000 Deuterium Tungsten Halogen Light sources; wavelength range 215-2000 nm; Ocean Optics, USA), a FIA-ZSMA PEEK flow cell (50 mm pathlength, 130 µL internal volume, Ocean Optics) and of a USB 2000 miniature fiber-optic spectrometer detector (detector range: 200 -1100 nm, Ocean

Figure 1 -

 1 Figure 1-Descriptive scheme of the multi-syringe chromatography manifold for the analysis of

Figure 2 -

 2 Figure 2-Kinetic of the reactions between ABTS solution A (prepared in HCl 0.4M) and free

Figure 3 -

 3 Figure 3-Kinetic of the reactions between ABTS solution B (prepared in pH 3 buffer) and free

Figure 4 -

 4 Figure 4-Residual chlorine concentration as function of mass ratio Cl/N after 30 min of

Figure 1 .

 1 Figure 1.

Figure 2

 2 Figure 2

Figure 3 0

 3 Figure 3

Table 1

 1 Analytical procedure for Multi-syringe chromatography analysis of free chlorine with ABTS solution B and inorganic chloramines with ABTS solution A.

	Step	V1	V2	V3	Injection	Mode	Volume	Flow rate	MP1
					Valve		(ml)	(mL min -1 ) a	(µL)
	1	OFF	OFF	OFF	LOAD	-	-	-	500
	2	Get dark spectrum continuous
	3	Get blank spectrum continuous

a. Flow rate of S1; Flow rates of S2 and S3 are half that of S1 because of half volumes of S2 and S3 compared to S1.

Table 2

 2 Retention times of inorganic chloramines for different mobile phase using HPLC with a C18 monolithic column (flow rate was 0.5 mL.min -1 ).

	Mobile phase	Retention time (min)		MCA-DCA
		MCA	DCA	TCA	resolution
	H2O	1.11	1.89	>30	2.61
	Acetonitrile 10% -H2O 90%	1.10	1.86	8.84	2.52

Table 3

 3 Influence of the ratio between mobile phase and ABTS solution flow rates on MCA and DCA peaks.

	Monochloramine	Dichloramine	Resolution

Table 4

 4 Influence of the mobile phase flow rate and of the volume of the tubing between the mixer and the detector. Flow rate of ABTS was half that of the mobile phase.

		Volume of	Monochloramine	Dichloramine
	Flow rate
		the tubing	Retention	Area of	Retention	Area of	Resolution
	(mL min -1 )
		(µL)	time (min)	peak	time (min)	peak
	0.3	100	2.88	11.5	4.03	9.6	2.01
	0.5	100	1.8	4.05	2.6	1.57	2.15
	0.3	200	3.11	12.4	4.35	10.9	1.57
	0.5	200	2.01	7.6	2.66	3.7	1.78

Table 5

 5 Analytical features of the MSC system developed

	Analytical	MCA
			DCA a	TCA a	HOCl b
	parameter	(or HOCl) a
	Limit of detection
		9.3	9.9	10.0	29.2
	(µg eq.Cl2 L -1 )
	Limit of
	quantification (µg	31.2	33	33.5	97.4
	eq.Cl2 L -1 )
	Linear working
		31-7100	33-7100	34-7100	97-7100
	range(µg eq.Cl2 L -1 )
	Linear regression
		A= 0.00116xC+0.23 A=0.0113xC+0.25 A=0.0124xC+0.06 A=0.00926xC+0.061
	equation c

Table 6

 6 Comparative results between our multisyringe system (MSC) and titrimetric assay

	(DPD/FAS) on various swimming pool samples (mg eq.Cl2 L -1 , mean ± standard deviation,
	n=2 replicates).
		HOCl		MCA		DCA		TCA
		DPD/FAS	MSC	DPD/FAS MSC	DPD/FAS	MSC	DPD/FAS MSC
	1	1.52±0.03 1.60±0.09	<LQ	<LQ	0.58±0.00	<LQ	<LQ	<LQ
	2	1.50±0.01 1.52±0.04	0.20±0.00 <LQ	1.02±0.02 0.37±0.05	<LQ	<LQ
	3	1.7±0.04 1.60±0.11	0.1±0.01	<LQ	0.7±0.03 0.68±0.04	<LQ	<LQ
	4	1.2±0.00 1.15±0.06	<LQ	<LQ	0.98±0.02 0.06±0.01	<LQ	<LQ

Acknowledgements

The project leading to this publication has received funding from the A*Midex Foundation of Aix-Marseille University, funded by socio-economic partners.