Camille Delvigne

Abel Guihou

Bernard Angeletti

Isabelle Basile-Doelsch

Jean-Dominique Meunier

Reliable Determination of Ge in Solid Environmental Samples Using a Chemical Preparation Procedure Developed for Si Isotopes and ICP-MS Analysis

Keywords: germanium, ICP-MS, reference materials, soils, plants, sample digestion, cation exchange purification

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Reliable determination of Ge using the silicon isotopes chemical preparation and 1 ICP-MS analysis for solid environmental samples. 36 Ar 36 Ar and 38 Ar 36 Ar on 72 Ge, 73 Ge and 74 Ge, respectively) should 80 also be monitored. Germanium is then often qualified as a "tricky" element to analyze and is 81 not routinely determined in environmental samples.

82

The majority of studies published so far on Ge have been carried out using the isotope-83 dilution hydride-generation ICP-MS technique (ID-HG-ICP-MS) developed by Mortlock and 84 Froelich (1996) (Fillipelli et al., 2000;Kurtz et al., 2002;[START_REF] Evans | Quartz control of high germanium/silicon ratios in 414 geothermal waters[END_REF]Derry et al., 85 2005;Wheat and McManus, 2005;[START_REF] Ellwood | Germanium incorporation 408 into sponge spicules: Development of a proxy for reconstructing inorganic germanium 409 the Chatham Rise[END_REF]Scribner et al., 2006;Blecker et al., 86 2007; [START_REF] Makishima | Determination of Ge, As, Se and Te in Silicate 472 Samples Using Isotope Dilution-Internal Standardisation Octopole Reaction Cell ICP-473 QMS by Normal Sample Nebulisation[END_REF][START_REF] Lugolobi | Germanium-silicon fractionation in a tropical, 470 granitic weathering environment[END_REF]Sutton et al., 2010). This is 87 probably the most reliable and precise technique for Ge determination, although it requires a 88 hydride generation system, which is not widespread in laboratories. Alternative techniques 89 are: (1) graphite furnace atomic absorption spectrometry (GF-AAS) (McMahon et al., 2006);

90

(2) ICP-MS analysis with or without high resolution [START_REF] Hamade | Using Ge/Si 434 ratios to decouple iron and silica fluxes in Precambrian banded iron formations[END_REF]Delvigne et al., 91 2009; Cornelis et al., 2010;Tribovillard et al., 2011); (3) laser ablation ICP-MS analysis 92 [START_REF] Hamade | Using Ge/Si 434 ratios to decouple iron and silica fluxes in Precambrian banded iron formations[END_REF]Shen et al., 2011;[START_REF] Belissont | LA-ICP-MS analyses of 380 minor and trace elements and bulk Ge isotopes in zoned Ge-rich sphalerites from the 381 Noailhac -Saint-Salvy deposit (France): Insights into incorporation mechanisms and ore 382 deposition processes[END_REF][START_REF] Dong | Germanium/silicon of the Ediacaran-Cambrian Laobao 405 cherts: Implications for the bedded chert formation and paleoenvironment interpretations[END_REF]); and (4) X-93 ray fluorescence spectrometer analysis [START_REF] Frei | Source heterogeneity for the major components of ∼3.7 Ga 423 Banded Iron Formations (Isua Greenstone Belt, Western Greenland): Tracing the nature 424 of interacting water masses in BIF formation[END_REF]. While valid for most geological 94 samples, these techniques are working close to their limits of detection for environmental 95 samples, such as plants that commonly range from 0.07 to 20 ng g -1 [START_REF] Derry | Biological control of 402 terrestrial silica cycling and export fluxes to watersheds[END_REF] 96 [START_REF] Blecker | 377 The ratio of germanium to silicon in plant phytoliths: quantification of biological 378 discrimination under controlled experimental conditions[END_REF][START_REF] Delvigne | Distinct silicon and 393 germanium pathways in the soil-plant system: Evidence from banana and horsetail[END_REF]Cornelis et al., 2010;[START_REF] Lugolobi | Germanium-silicon fractionation in a tropical, 470 granitic weathering environment[END_REF]. One 97 option is to preconcentrate samples in order to reach concentrations above the detection 98 limits. This was investigated by Soylak and Yigit (2015), who suggested a separation-99 preconcentration procedure using a polysulfone membrane filter combined with 100 spectrophotometric measurements. This method has the disadvantage of requiring specific 101 materials. For natural water samples, ID-HG-ICP-MS remains the only reliable technique 102 thanks to its high precision and unequaled low detection limits. Samples were digested following the procedure developed for silicon isotopes using a solid 132 sodium hydroxide (NaOH) flux (Georg et al., 2006). Between 10-20 mg (200 mg up to 600 133 mg for ERM-CD281, the plant standard) of powdered samples were weighed directly into 134 silver crucibles after drying for at least 24h at 105°C. Prior to the fusion, calcination steps 135 were carried out to ensure the combustion of organic matter into CO 2 . Calcinations were 136 performed at 450°C with a 1 h stage at 250°C to ensure slow temperature increase. The 450°C 137 calcination step lasted for at least 1 hour for soils, and 16 h for plants. In the case of plants, 138 the total calcination procedure lasted at least 24 h. [START_REF] Qi | Germanium isotopic systematics 492 in Ge-rich coal from the Lincang Ge deposit, Yunnan, Southwestern China[END_REF] showed that calcination at HNO 3 could be used (Luais, 2007;2012). Acidification to pH~2 was important for Fe-rich 145 samples to ensure a complete dissolution of Fe-oxides as well as ensuring a complete 146 recovery during the following cationic purification [START_REF] Fitoussi | Si isotope 427 systematics of meteorites and terrestrial peridotites: implications for Mg/Si fractionation 428 in the solar nebula and for Si in the Earth's core[END_REF]. At this pH, the 147 dominant Ge species is the Ge(OH) 4 0 form [START_REF] Pokrovski | Thermodynamic properties of aqueous Ge (IV) 488 hydroxide complexes from 25 to 350°C: Implications for the behavior of germanium and 489 the Ge/Si ratio in hydrothermal fluids[END_REF]. 2). Before loading the sample on the resin, 161 ultrapure water was passed into the resin to remove acids (Georg et al., 2006). The matrix

162

(sodium from fusion and other cations from the sample itself) was retained on the resin, while 163 anionic and neutral species (e.g., Ge(OH) 4 0 and Si(OH) 4 0 ) passed through. To minimize the 164 dilution due to elution, the sample volume was maximized (here ~ 10 ml of fused solution)

165 while the volume of eluent was kept to a minimum of one resin bed volume (~ 2 ml) (Table 166 2). Since the Si and Ge species do not interact with the resin, the eluent is simply ultrapure 167 water. Ge was collected in a ~ 12 ml cation-free solution. standard should be revised, our recovery yield was 109±2% (Fig. 1). As the procedural blanks 208 displayed very low levels of Ge (3±2 ng l -1 ; n=11) and the given large error range associated 209 with the certified value of this standard (Table 1), we assume our recovery for this standard to

223

The separation efficiency is demonstrated by the elution curves of two samples (a single 224 element Ge standard solution and a fused BHVO-2) that were loaded onto the pre-cleaned 225 cation exchange resins (Fig. 2). Ge is not retained by the resin and migrates straight through 226 the column and nearly 90% of the loaded Ge is already recovered while the sample load is 227 still infiltrating the resin bed. The last 10% is recovered with 2 ml of ultrapure water (Fig. 2). 4). The lowest detection limit was obtained 265 for 74 Ge (6 ng l -1 ), the most abundant Ge isotope. For comparison, the detection limit for 72 Ge 266 is 5 times higher, approximately 30 ng l -1 most likely due to the ArO 2 interference.

267

Considering that 20 mg of powdered sample was dissolved in 100 ml, this corresponds to a 268 detection limit of approximately 0.03 µg g -1 in the solid sample (Table 4). For the plant 269 standard (200-600 mg dissolved in 100 ml), the detection limit is approximately 0.001 µg g -1

270

in the solid sample. Additionally, the linearity limit was demonstrated over the entire The relative repeatability of ICP-MS analysis based on a Ge single element standard solution

277

(400 ng l -1 ) and analyzed six times on a single day is 3% (n=6, confidence level 95%).

278

Similarly, four aliquots of one fused solution (BHVO-2) passed through four different 279 columns and analyzed twice on the same day gives a relative repeatability of 2%.

280

Reference materials of different composition (BHVO-2; AGV-2; BCR-2; GBW 07401; GBW 281 07404; GBW 07407) were analyzed to evaluate the accuracy of results (Supplementary Table uncorrected interferences would overestimate Ge content. In the literature, Ge contents 293 estimated using specific techniques devoted to Ge analysis (ID-HG-ICP-MS or double-spike 294 MC-ICP-MS) appear to be systematically lower than Ge contents estimated with ICP-MS 295 techniques. In the case of BHVO-2, ID-HG-ICP-MS and double spike MC-ICP-MS 296 techniques provided a compiled value of 1.56±0.07 µg g -1 (Table 5; Scribner et al., 2006; 297 [START_REF] Escoube | An 418 Intercomparison Study of the Germanium Isotope Composition of Geological Reference 419 Materials[END_REF], while ICP-MS techniques (acid attacks and interferences corrections 298 not specified) averaged approximately 1.62±0.04 µg g -1 (Table 5; [START_REF] Jochum | Reference Values Following ISO Guidelines for Frequently 450 Requested Rock Reference Materials[END_REF].

299

Similarly, BHVO-1 data show a comparable shift between the two techniques (1.46±0.12 µg 300 g -1 , n=2 for ID-HG-ICP-MS and double spike MC-ICP-MS; 1.64±0.06 µg g -1 , n=3 for ICP-301 MS; [START_REF] Jochum | Reference Values Following ISO Guidelines for Frequently 450 Requested Rock Reference Materials[END_REF]. An intermediate Ge content (1.58 µg g -1 ) is obtained by Luais 302 (2012) using dedicated acid attack removing 80% of matrix analyzed by ICP-MS suggesting 303 that discrepancy between techniques is likely induced by matrix and interferences. Despite 304 being more limited in number, data acquired using isotope dilution techniques are the most 305 robust [START_REF] Escoube | An 418 Intercomparison Study of the Germanium Isotope Composition of Geological Reference 419 Materials[END_REF]Scribner et al., 2006). Considering the compiled values from 306 these two studies, our BHVO-2 results agree within 6% (Fig. 5, Table 5).

315

Regarding the other rock standards (AGV-2 and BCR-2), there is a better agreement between 316 our data and the compiled data from [START_REF] Jochum | Reference Values Following ISO Guidelines for Frequently 450 Requested Rock Reference Materials[END_REF] (Table 5). Considering these three 317 rock standards, our data agree within ~5% with published values.

318

Regarding soil standards (GBW 07401, 07404, 07407), our results are systematically 20 to 319 30% higher than certified values provided in Wang et al. (2013). However, a better agreement Cornelis et al., 2010). However, the analysis of silicon isotopes on the same 341 chemical preparation may not be straightforward because of DOC and anions responsible for 342 isotopic bias [START_REF] Hughes | 440 Controlling the mass bias introduced by anionic and organic matrices in silicon isotopic 441 measurements by MC-ICP-MS[END_REF]. Water samples with lower Ge contents, such as freshwater 343 and seawater, might be pre-concentrated using Mg-coprecipation [START_REF] Escoube | Iron and germanium isotope geochemistry in river and hydrothermal 416 systems[END_REF]. As many 344 other elements are scavenged by this method, the Mg-coprecipitation should be followed by 345 the cationic purification step. As both Ge and Si recoveries are complete during Mg-346 coprecipitation [START_REF] Escoube | Iron and germanium isotope geochemistry in river and hydrothermal 416 systems[END_REF][START_REF] Cardinal | Relevance 384 of silicon isotopes to Si-nutrient utilization and Si-source assessment in Antarctic waters[END_REF], the analysis of silicon isotopes and Ge 347 concentrations could again be performed on the same preparation. However, as anionic 348 species will not be removed completely, care should be taken to control the bias induced by 349 anionic species on δ 30 Si analysis [START_REF] Hughes | 440 Controlling the mass bias introduced by anionic and organic matrices in silicon isotopic 441 measurements by MC-ICP-MS[END_REF].

350

In addition to silicon isotopes analysis, this method paves the way towards a new approach to 351 perform Ge isotopes is soils and plants, which has never been done before. Combining silicon 352 isotopes, germanium isotopes and Ge/Si ratios would be a powerful multi-proxy approach to 353 tackle continental Si biogeochemical cycle. shows the best precision and lowest detection limit (Table 4) but is much more time

  Aix-Marseille Univ, CNRS, IRD, Coll. de France, INRA, CEREGE, Europôle Mediterranéen de 7 l'Arbois BP 80 13545 Aix-en-Provence, cedex 4, ICP-MS, reference materials, soils, plants, sample digestion, cation exchange 14 purification 15 Mots clés: germanium, ICP-MS, matériaux de référence, sols, plantes, digestion d'échantillon, 16 purification cationique par résine échangeuse dand O isotopes ( 58 Ni=68%; 16 O=99%). In addition, Ar-based molecular 79 interferences ( 40 Ar 16 O 2 ,

  The objective of this study is to propose a simple and reliable alternative method determining 104 Ge contents in solid environmental samples, with a notable focus on soils and plants using 105 ICP-MS, a common instrument found in geochemistry laboratories. As silicon isotopes and 106 Ge/Si ratios are complementary tracers of the Si biogeochemical cycle, we investigate the 107 applicability of the chemical preparation procedure developed for silicon isotopes (Georg et 108 al., 2006) to the analysis of Ge in solid environmental samples. the recommended values for Ge concentrations are scarce. Three 112 international geological USGS reference materials have been chosen in this study for 113 intercomparison with previous studies: BHVO-2 (basalt), AGV-2 (andesite), and BCR-2 114 (basalt). Despite certified values not being available for these standards, an increasing number 115 of published data provide constraints on Ge concentrations (

  118using data published between 1995 and 2015. In addition, to better fit the matrix of targeted 119 samples of soils and plants, we selected three soil standards from the Institute of Geophysical 120 and Geochemical Exploration (IGGE, Langfang, China) (GBW 07401, GBW 07404 and 121 GBW 07407, also known as GSS-1, GSS-4 and GSS-7) and a plant standard from European 122 Reference Material (ERM-CD281). Certified Ge concentrations are available for the soils 123 standards, but with low precision (

139

  600°C does not induce Ge loss. Ashes were mixed with approximately 200 mg of NaOH and 140 placed in a furnace at 720°C for 10 minutes. Fusion cakes were then allowed to dissolve 141 overnight in ultrapure water with a brief ultrasonic bath to favor dissolution. Solutions were 142 then quantitatively transferred into pre-cleaned HDPE bottles and acidified to pH ~2 with 143 HNO 3 . Due to the volatile behavior of Ge in the presence of halogens (HCl, HClO 4 ), only

  effects and various interferences, which are complex to correct for, 150 purification of samples appears to be an ideal approach. Perhaps more importantly, the 151 elimination of matrix elements prior to analysis allows a substantial reduction in the dilution 152 factor, which is of great interest for samples with low Ge contents. In addition, performance 153 of ICP-MS can be fully exploited, as the sampling interface and the ion optics are not 154 deteriorated due to salt depositions. A cation-free solution was obtained by passing the 155 solution through a cation exchange column. This allowed for testing of the resin used for 156 germanium isotopes chemical preparation, AG50W-X8 (100-200 mesh in H + form; Bio-Rad) 157 (Luais, 2007, 2012) using the column chemistry routine of silicon isotopes (Georg et al., 158 2006). Sample solutions were loaded on a Bio-Rad type column filled with 1.8 ml of 159 AG50W-X8 cation-exchange resin, pre-cleaned with several rinsing with HCl, HNO 3 as 160 detailed in the study of Georg et al., (2006) (Table

  200concentrations (330 to 1100 ng l -1 ; Fig.1). To take into account matrix effects, recovery 201 yields for BHVO-2 and GBW 07401 were determined by Ge standard addition to the fused 202 solution before purification (Ge addition from 10 ng l -1 to 100 ng l -1 ; Fig.1). The recovery 203 yields for BHVO-2 (99±2%; n=4) indicated a complete recovery for Ge. In the case of GBW 204 07401 (Ge addition from 10 ng l -1 to 1000 ng l -1 ; Fig.1), recoveries of approximately 118±5% 205 (n=6) were found when compared to the certified value(Wang et al., 2013

210 216 Fig. 1

 2161 Fig. 1 Comparison of the Ge concentrations obtained by ICP-MS after cationic purification

Fig. 2 Fig. 3 .Fig. 4

 234 Fig. 2 Elution of germanium during chemical purification of BHVO-2 on AG50W-X8 resin.

  The linearity limit was above 1000 ng l -1 , the highest standard in the calibration curve, as the coefficient correlation of the calibration curve is above 0.995. 273 274

  282

  1). However, the absence of properly certified Ge values complicated the exercise. As shown 283 in Fig. 5, our BHVO-2 data agree within 10% with the published value from the compilation 284 of Jochum et al. (2016) (

Fig. 5

 5 Fig. 5 Ge content in BHVO-2 from our study (open diamonds), high quality data (gray

  320is noted when compared to data fromLiu et al. (2014), suggesting that certified values should The use of HClO 4 in the sample decomposition procedure calls into question the 322 accuracy of these data and may explain our systematically higher Ge contents. 323 324

  purification technique based on the Si isotopes preparation protocol (Georg et 356 al., 2006) allows precise determination of Ge concentrations in environmental samples down 357 to a detection limit of approximately 6 ng l -1 . Analyses performed on 74 Ge, the most abundant 358 isotope, show the best precision of approximately 3%. Accuracy, difficult to assess given the 359 scarcity of reliable data available for comparison, was assessed with an error of approximately 360 5%. This method provides data with equivalent or better metrological characteristics than 361 other published procedures and analytical techniques except for ID-HG-ICP-MS, which 362

  363consuming. In addition to the reliability of generated data, this technique has two major 364 G. and McManus J. (2005) The potential role of ridge-flank hydrothermal systems 521 on oceanic germanium and silicon balances. Geochimica et Cosmochimica Acta, 69, 522 2021-2029.

Table 1

 1

). A considerable 116 compilation effort was undertaken by Jochum et al. (2016), which provides reference values 117 (or informative values) determined following ISO guidelines and IAG Certification Protocol

Table 1 )

 1 , while no Ge data is provided for ERM-CD281.

	124
	125	Recently, additional data have been made available for GBW 07401-07408 standards (Liu et
	126	al., 2014) (Table 1).

Table 1 .

 1 Reference materials analyzed in this study with their Ge concentrations and

	128
	129	associated uncertainties. Values in bold are certified values while others are informative
	130	values. * refers to an uncertainty of 3σ instead of 2σ; n.s.= not specified.
	131	2.2. Sample digestion

144 Sample type Provider Published value rsd (2σ) n References

	BHVO-2	basalt	USGS	1.62±0.04 µg g -1	2%	10	Jochum et al., 2016
				1.53 µg g -1	0.2%	n.s.	Escoube et al., 2012
				1.59±0.04 µg g -1	3%	3	Scribner et al., 2006
	AGV-2	andesite	USGS	1.20±0.08 µg g -1	7%	6	Jochum et al., 2016
	BCR-2	basalt	USGS	1.46±0.26 µg g -1	18%	6	Jochum et al., 2016
	GBW 07401	soil	IGGE	1.50±0.04 µg g -1	3%	6	Liu et al., 2014
				1.34±0.2 µg g -1	15% * at least 8 Wang et al., 2013
	GBW 07404	soil	IGGE	2.10±0.02 µg g -1	1%	6	Liu et al., 2014
				1.9±0.3 µg g -1	16% * at least 8 Wang et al., 2013
	GBW 07407	soil	IGGE	1.6±0.3 µg g -1	19% * at least 8 Wang et al., 2013
	ERM-CD281	plant	ERM	no data	-	-	-
		F o
		r
			R e v i e w
				O n
					l y

Table 2 .

 2 removal of matrix element, Na is potentially the more difficult element to eliminate as it is the 170 most abundant and resin shows a weak relative selectivity for Na. To warrant a good concentrations. Indeed, a 10 ml fused solution (typically 0.5 meq) saturates the resin 173 approximately 15% of its exchange capacity. Complete removal of ambient cationic species 174 and especially problematic cations (i.e., Fe, Ni, Zn) and Na was checked with ICP-MS. Column chemistry routine of the preparation of a solid sample fused with NaOH flux robustness of the method. However, Ge contents were determined using the 74 Ge isotope, the 189 most abundant isotopes (36.28%), given higher sensitivity and lower detection limits 190 compared to other isotopes. In the sequence, 400 ng l -1 Ge standard solutions were measured 191 after every 15 samples to check for instrumental drift and evaluate reproducibility.

	175
		BioRad AG50W-X8, 1.8 ml resin bed
		Separation stage Pre-cleaning F Pre-cleaning o Pre-cleaning r Pre-cleaning	Solution matrix 3N HCl 6N HCl 7N HNO 3 10N HCl	Volume (ml) 3 ml 3 ml 3 ml 3 ml
	176	Pre-cleaning Pre-cleaning R Rinse Sample load e ultrapure water 6N HCl 3N HCl Acidified fused sample w e i v Elution ultrapure water	3 ml 3 ml 6 ml 10 ml 2 ml
			O n
				l y
	182
	183	different concentrations (50, 100, 250, 500, 1000 ng l -1 ) were used to calibrate Ge
	184	concentrations. These calibration solutions were made from single element Ge standard 1000
	185	mg l -1 solution (Chem Lab, Plasma HIQU, 2-5% HNO 3 ). To correct for instrumental drift,
		rhodium (Chem Lab, Plasma HIQU, 2-5% HNO 3 ) was introduced to sample solutions through
	188

Recovered solutions were weighed 168 to minimize uncertainties inherent to the imprecise volumes recovered. Regarding the 177 adapted from Georg et al., 2006.

178 2.4.Sample introduction and mass spectrometry 179 Ge concentrations were measured using an ICP-MS Nexion 300X (Perkin Elmer) using a PFA 180 nebulizer, a quartz cyclonic spray chamber and a SC-FAST DX-4 automated introduction 181 system. Operating conditions are detailed in Table 3. A series of five standard solutions with 186 a cross flow line injection resulting in a Rh concentration of 1.58 µg l -1 in the analyzed 187 solution. Ge isotopes 72, 73 and 74 were analyzed and provided consistent values, proving the

Table 3 .

 3 Instrumental operating conditions for ICP-MS Nexion 300X

194 3. Results and discussion 195 3.1. Assessment of Ge recovery yields during chemical procedure 196 As the dominant Ge species after fusion is the Ge(OH) 4 0 , no affinity for cation exchange resin 197 is expected and thus Ge should be quantitatively recovered through the chromatographic 198 procedure. A complete Ge recovery (102±2%; n=6) during sample purification through 199 cationic resin was obtained with single element Ge standard solution at different Ge

  ). Considering the Nebuliser Ar gas flow rate 0.95 -1.05 l min -1 (optimized to minimize 140 Ce 16 O + / 140 Ce + <0.025 and 70 Ce ++ / 140 Ce + <0.03) Ge concentrations ofLiu et al., (2014), who suggested that the Ge concentration of this

	207
	Plasma power	1600 W
	Torch	Quartz glass torch
	Plasma Ar gas flow rate	15 l min -1
	Auxiliary Ar gas flow rate 1.2 l min -1
	Nebuliser Spray chamber Sample uptake rate F o PFA 0.4 ml min -1 cyclonic (quartz) 0.337 ml min -1 Interfaces cones Nickel r
	Resolution Typical 74 Ge sensitivity Typical 103 Rh sensitivity	700 ~ 12000 cps µg -1 l ~ 680000 cps µg -1 l R e v i e w
		O n
		l y
	206

Table 4 .

 4 Comparison of metrological characteristics of different techniques used for Ge

275

determination. n.s. = not specified. DL = detection limit.

276

  Table5). However, a careful sorting of compiled data reveals that 285 most of the dissolution protocols are not adapted to Ge, except for that of Scribner et al.

		Method	Dissolution Preliminary steps	DL in solution	DL in solid samples	Reproducibility	Difference with published values previously	Samples	References
		ID-HG-ICP-MS	acid	ID	0.5 ng l -1 a	-	<3%	n.s.	natural waters	Evans and Derry, 2002
		HR-ICP-MS	fusion	-	10 ng l -1	n.s.	4%	n.s.	plants and soils	Delvigne et al., 2009
		ICP-MS	fusion	-	2 ng l	-1	0.05 µg g -1	15%	n.s.	plants and soils	Cornelis et al., 2010
		LA-ICP-MS	-	-	-		< 1 µg g -1	8% b	n.s.	rocks	Belissont et al., 2011; Shen et al., 2011
		X-ray fluorescence	-	-	-		1.5 µg g -1 c	1%	15%	rocks	Frei and Polat, 2007
		Spectrophotometry	acid	Polysulfone membrane filter	2 ng l -1	n.s.	5%	n.s.	waters and soils Soylak and Yigit, 2015
		ICP-MS	fusion	Cationic purification	6 ng l	-1	0.03 µg g -1	3%	6%	plants, soils, rocks This study
		a blank level
		b valid for Ge/Si ratio as detailed in Shen et al., 2011
		c minimum Ge concentration analyzed (no limit of detection provided)
				F o
				r
					R e v i e w
									O n
										l y
	286
	287	(2006). Indeed, all but one of the studies employed acid attacks using HCl, HClO 4 or HF (at
	288	temperature above 60°C) or a combination of these, which induce loss of Ge (Luais et al.,
		2012). Tests performed by Luais (2012) demonstrated that Ge loss is at 85% when HClO 4 is
	291

289

employed in the acid dissolution mixture. In addition, it is unclear if ICP-MS analysis were 290 corrected for spectroscopic interferences and selected Ge isotopes were not specified. Thus,

Table 5 .

 5 Comparison of Ge contents in different reference materials between this study and 325 data from the literature. n.s. = not specified. Uncertainties are provided with 95% confidence As emphasized in[START_REF] Cornelis | Tracing the 390 origin of dissolved silicon transferred from various soil-plant systems towards rivers: a 391 review[END_REF], the combination of Ge/Si ratios and silicon isotopes 335 turned out to be a great asset for tracing the sources and fate of Si in the critical zone, where

	326
	level.

327

To provide the first Ge data on a plant standard, the standard ERM-CD281 was analyzed. Its 328 Ge content is estimated to be approximately 70±3 ng g -1 (n=9; Supplementary Table

1

). 329

3.3. Perspectives 330

This method provides a convenient procedure for determining accurate and precise Ge 331 concentrations in rocks, soils and plants. Additionally, the combination of Ge concentrations 332 with δ 30 Si analyses is the great advantage of this method, as both Si cycle tracers can be 333 analyzed from a single procedure allowing a better understanding of the continental Si cycle. 334 338 detection limits. The purification step of this procedure would then be suitable for 339 hydrothermal fluids (12±13 µg l -1 up to 50 µg l -1 ;

[START_REF] Evans | Quartz control of high germanium/silicon ratios in 414 geothermal waters[END_REF]

and soil solutions

Geostandards and Geoanalytical Research

found in most laboratories unless specific needs, and (2) Ge concentrations and Si isotopic

Ellwood M.J. and Maher W.A. (2003) Germanium cycling in the waters across a frontal zone:

Acknowledgments

371

This study has been funded by the ANR project BIOSiSOL (ANR-14-CE01-002). The French

analysis can be performed on the same solution. This last point represents a precious gain of 367 time in terms of sample preparation as one procedure serves to perform both analyses. A 368 systematic coupling of Ge/Si ratios and δ 30 Si compositions is a great asset in the interpretation


M A Haroun

M A * Salaheldeen

On Improving Motor Imagery BCI Using A Novel Physical Feedback Technique

In this study, and through an understanding of neuronal system communication, A light was shed upon a novel model serves as an assistive technology for locked-in people suffering from Motor neuronal disease. MND patients, who have an intact minds, are able -till now-to make a mean of onscreen communication. Work was done upon the potential of brain wave activity patterns to be detected as electrical signals, classified and translated into commands following BCI constructing paradigm. However, The interface constructed was for the first time a device which can reconstruct this command physically. The intervention is in a software is used to classify imagined right and left limb movement into binary results, and then make use of the magnetic field produced by the amplified pulses to reconstruct the activity into ferrofluid droplets movement-these moved due rotation of a desk according to the data received. Objectives of the project is to address the challenges of the inaccurate performance and cost of user-training which are yet the main issues preventing BCI from being upgraded into more immersive and effective technology. The design requirements addressed were the communication speed of the droplet movement and the accuracy of hitting fixed targets . Tests were performed based on software programs accelerated changing motor imagery movement of left and right limbs. The tests achieved an average speed of 0.469 cm/s and average accuracy of 81.6% for the best volume from many were tried. A conclusion to be drawn was that the promise of this other point of view on BCI systems to be more Brain-Real World Systems.

INTRODUCTION

WHO (World Health Organization), about 14% of world's population have disabilities with 2-4% experience significant difficulties in functioning. Correspondingly, Egypt contain about 2 million people with disabilities representing about 3.5% of the total population; a statistic made in 1997 by central Authority for public Mobilization and Statistics. The statistic revel that about 15% of people with disabilities in Egypt have Mobility Impairment.

Although there is currently no cure, it is not true to say that, 'nothing can be done for the person with motor neuronal disease'. A great deal can be done to maintain quality of life through assistive communication technologies. Brain computer interfaces or BCIs are now the tool for MND patients. Depending on understanding of how the brain works, A BCI reads the waves produced from the brain at different locations in the human head, Classifies the signals in program, and generates commands that can control the computer.

While the importance of subject's motor imagery skills in BCIs is well recognized, most studies have focused on the computer side and improving classification algorithms and very few have attended the human side and training paradigms that can facilitate the skill acquisition process for subjects. Powerful machine-learning algorithms have been created. On the other hand, less research has focused on investigating how BCI users may optimally adapt. This Project Focuses on the latter aspect with totally new intervention into the system. More precisely, The goal of this project is to explore the influence of novel intervention of physical feedback design on enhancement of user's performance and interaction with a BCI system. ARCHITECTURE OF BCI SYSTEM the BCI system generally performs a six step process (Figure 1): brain activity measurement, preprocessing, feature extraction, classification, command translation, and feedback .

EEG Data Acquisition: Brain activity measurement is the step in which electrodes are used to obtain the user's EEG at specific regions on the scalp, to form input for the BCI system. This step involves determining the number and location of the channels, amplification, analogue filtering, and A/D conversion. Channel locations are selected according to the paradigm used and mental task performed.

Most people exhibit prominent oscilation in the 8-12 Hz band of the EEG recorded ove sensomotor areas . When they are not actively enganged in motor action, sensory processing or imagination of actions. This oscilation is usually called mu rhythm and is thought to be produced by thalamocortical circuits. Analysis of mu rhythm showed that it also usually associated with 18-25 Hz beta rythms. While some of the beta rythms are harmonics of the mu rhythm, some are separated by topography or timing from mu rhythm.

In fact,BCIs use the different types of paradigms to operate like Visually Evoked Potentials, Event Related Potentials, Readiness Potentials and Motor Imagery. Here we consider MI detection. What makes these neural signal patterns important to Human computer interfacing studies is that the mu and beta frequency drop occurs even when the planned movement is not executed-that is, the signals are still observable in paralyzed patients like those with advanced ALS .Thereby, MI BCI can provide direct communication without any limb movement or external stimulus. MI BCI uses "induced" brain activity from the cortex, rather than "evoked" brain activity. The mu wave is found over the motor cortex, in a band approximately from ear to ear. A person suppresses mu wave patterns when he or she performs a motor action or, with practice, when he or she visualizes performing a motor action. This suppression is called desynchronization of the wave because EEG wave forms are caused by large numbers of neurons firing in synchrony. The electroencephalographic (EEG) mu rhythm is an 8-13 Hz rhythm generated by the sensorimotor cortex. Motor imagery implies a thought activity of imagining of physical movement. In other words, when imagining a hand movement or actually moving the hand, ERD (Event-Related Desynchronization) occurs around the μrhythm area within the sensory motor cortex (see figure 2 )

The mu wave is hypothesized to represent an "idling" rhythm of motor cortex that is interrupted when movement occurs. The free running EEG shows characteristic changes in mu activity, which are unique for the movement of different limbs. These findings have and will -for our work-be useful in the construction of BCI systems. That is shown through (fig. 3) which shows basic phenomenon of mu/beta rhythm modulations in EEG while imagination of right hand movement.

Brain activity is picked by one of different types of electrodes. In their most simple method, they are placed on the scalp using gel as a conducting material. The placement of electrodes commonly follows the 10-20 system (2) Preprocessing After the signals being detected by the headset, normal BCI systems include g.USBamp, It is a bio signal acquisition system with 24-bit resolution that provides signal-conditioning functionality to amplify, filter, and convert electrode outputs into digital values. Its user-selectable, multichannel modules allow the simultaneous recording of electro -encephalogram (EEG) data.

(3) Feature Extraction and Classification The brain patterns used in BCIs are characterized by certain features or properties. For instance, amplitudes and frequencies are essential features of sensorimotor rhythms. The firing rate of individual neurons is an important feature of invasive BCIs using intracortical recordings. The feature extraction algorithms of a BCI calculate (extract) these features. extraction is the process of distinguishing the pertinent signal characteristics from extraneous content and representing them in a compact and/or meaningful form, amenable to interpretation by a human or computer. Classification is a step which assigns a class label to a set of features extracted from the signal. In other words it is about assigning a command to a specific pattern of the EEG. The two process are usually integrated in one "processing program" in normal BCI systems [START_REF] Proverbio | Observation of Static Pictures of Dynamic Actions Enhances the Activity of Movement-Related Brain Areas[END_REF] Visualization and command representation is the step of exhibiting the command. It is the step which understood to be revealing the intention of the brain. The step has always an on-screen visualization in normal BCI systems.

THE PHYSICAL FEEDBACK TECHNIQUE

In the seek of achieving a physical visualization system, Our system undertake a modifications on the normal system steps. Firstly, and as hinted in the step of Acquisition, we work on mu/beta rhythm while subject in motor imagery activity. Here, we adopt two different positions to detect signals from, namely the C3 (Left hemisphere) and C4 (Right hemisphere) in the 10/20 system. Although the normal use is a 32-channel of positions, the promise of the only two positions mentioned and the differ in the following steps made our modification reliable and achieving a simplicity would contribute to the final outcome in terms of speed and accuracy. Fig 5 shows the notable attenuation while in motor imagery .

In the following, mention our novel system to be integrated with the normal BCS systems (or with the mentioned modification. To achieve the reliable physical visualization, 3 major concepts in electronics, physics and chemistry was integrated in an intellectual unification paradigm. Those are: A) Classification Software This attenuation in mu rhythm which is our feature to be extracted was induced by a simulation of a software program named BCI 2000 as there were two electrodes in the C3 position in the two hemispheres. the classification process begins when this attenuation is detected and then sampled by a MATLAB program. It works as follows: within the range of attenuation the higher of the two will be sampled in a two bit sampled as 1 or 0 (this is because even if the activity is centered in one hemisphere , the other will have otherbut much lower -one ) . Those 4 outcomes which established from the 2 bit sampling was used to produce the current and the then the movement based upon to achieve the simulation of the imagined movement.

B) Arduino Board and Electromagnet

The 0/1 binaries gathered from the BCI simulation system was transformed to an Arduino controlling system. The Arduino was used to induce current flowing through a solenoid or electromagnet. That was based upon Ampère's circuital law that an induced electric field could establish a magnetic field and vice versa. The electromagnet dimensions was estimated as follows: it will be like a solenoid having the length of an iron nail (10cm) with radius of 1cm. According to Elementary physics equation which describe the intensity of the magnetic field around the solenoid, The movement of the ferrofluid movement is basedas mentioned before-upon the induced current in electromagnet. The idea is that this electromagnet is movable in a circular path based upon a motor and gear . The angle of rotation depends on the integration of the data at the Arduino. Algorithm was used to achieve the following table paradigm.

So, now if patient thought about his right hand movement, his left hemisphere gives higher attenuation and is then 1 in the binary code and left hemisphere is 0. this makes the electromagnet rotates 180 and we know that he thought to move his right hand. Similar approach, achieving 360 rotation is done for left hand imagining movement TESTS AND RESULTS

1.Velocity

Velocity is measured by calculating the distance and dividing it by time taken by the droplet to move. Although a force affects the droplet and acceleration should be calculated, High Stroke force affect the motion decreasing the magnitude of the acceleration making it negligible. Data is Figure .. shows this relation such that velocity of the droplet is nearly negligible. For precise calculation for the time the droplet taken to move three trials for each distance was experimented and taken to average. The experiment was made for each of the three different volumes of the droplet ranging from 5 to 20 μL. The following data table no.2 show each trial corresponds to what distance and volume. There was a random error by ± 0.335 ( 1%). Measurement tool of distance was a roller and measurement tool of time was a stop watch

2.Accuracy

Accuracy is defined as the how much success does the droplet achieved. A simple ruler was used to determine the actual distance the droplet moved and then compared with the distance the droplet should move. The experiment is considered precise as even the friction is calculated in the stroke force as it defined by the viscosity of the droplet. The distance measurement error by ± 0.2 (20.5%). The measurement tool of distances was the roller. Table 4 shows the average of the many trial tests on accuracy Formation of the Ferrofluid: (Fig 7)

.0 mL of 2M FeCl2 solution and 4.0 mL of 1M FeCl3 were added to ammonia solution with a concentration of 50 mL of 0.7 M. An exothermic reaction was observed that preaches for the formation of Magnetite, a black precipitate. At the end reaction, heat energy was added to the solution to get rid of excess ammonia. The heating of the solution persisted for 3 hours. A clear black solution was observed that sign for the successfulness of the chemical synthesis. The balanced equation of magnetite formation is as follows: 2FeCl3+ FeCl2 + 8NH3 + 4H2O → Fe3O4 + 8NH4Cl

The surfactant used in this synthesis is citric acid with a concentration of 2 mL (25%). The Ferrofluid was then heated again. After the second thermal accumulation, Kerosene was added to the solution as a carrier to the nanoparticles. Extending the system use can be achieved also after increasing the number of options that the interface can manage to exhibit. This can be done by adding more option for the classification and their corresponding command of different angle of rotation. Idea demonstration is shown in figure 11 .(where different colors correspond to different commands)

The tool can also be used for improvement of BCIgames as the tool is suitable to be used ,if other electromagnets were added and a study on the impact of each of which on the other, in games as it would be possible to reconstruct abstract 2D object as shown in figure 12.

This multi effect of different magnetic fields can be understood through Maxwell's equations. In our case, we are changing magnetic fields slowly (compared to radio frequencies) thus the magneto-static equations are appropriate. These are where B⃑ is the magnetic field [in Tesla], H⃗ is the magnetic intensity [Amperes/meter], j⃗ is the current density [A/m2], M⃗ is the material magnetization [A/m], χ is the magnetic susceptibility, and μo = 4π × 10-7 N/A2 is the permeability of a vacuum. The force on a single superparamagnetic particle is then When a magnetic force is applied, a single particle will accelerate in the direction of that force until it sees an equal and opposite fluid (Stokes) drag force. Since the Stokes force is

Figure 1 :Figure 2 :

 12 Figure 1 : EEG BCI paradigm steps

Figure 3 :

 3 Figure 3: mu rythem attenuation

Figure 4 :Figure 5 :

 45 Figure 4: 10/20 positioning system

  Constructing the controlling system (figure 8,9): As shown in Fig 1 , The interface was designed at AutoCAD andas shown in Fig 2-then printed by Ultimaker 2+ machine in STEM FabLab. The glass sheet was also shaped to a circular sphere. The radius of the internal circle was 6 cm. There was also another side-connected circle With radius of 7 cm.

Figure 8 :Figure 9 :

 89 Figure 8 : AutoCAD design

Table 1

 1 Figure 6: schematic of our device

Table 2 Graph 1 Graph 2 Table 3 MATERIALS

 2123 AND METHOD

816898 0.765278 0.613889 Time Trial 1 Trial 2 Trail 3 Average Volume in μL

	Distance 5 μL		10 μL		20 μL
	3 cm			80%	82%	69%
	6 cm		81.67%	78%	59%
	9 cm		81.48%	70%	62%
	12 cm	83.61%	76%	55.00%
	2.4 /3 80% 82% 69% Average 0.90.98 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 3 cm	2.5 81.67% 78% 59% 2.3 6 cm	2.43 81.48% 70% 62% 9 cm	5 55.00% 83.61% 76% 12 cm
	181.95	5.1 /6		4.7	4.9		4.91	5
	278.81	7.3 9 /		7.6	7.1		7.33	5
	365.65	9.8 12 /	10.1	10.2		10.03	5
	41.98	2.2 3 /		2.3	2.9 2.4666667	10
	83.79	4.4 /6		4.5	5.1 4.6666667	10
	126.93	6.3 /9		6.7	5.9		6.3	10
	167.82	8.9 /12		9.1	9.4		9.13	10
	27.13	1.7 /3		2.1	2.4 2.0666667	20
	57.65	3.3 /6		3.5	3.9 3.5666667	20
	83.47	5.5 /9		5.1	6.2		5.60	20
	110.77	6.4 /12		6.1	7.3		6.62	20

Accuracy in Percentage Distance Travelled in cm 5 microL 10 micro L 20 micro L