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hấp phụ Arsenic vào trong môi trường đất
Analytica Chimica Acta 512 (2004) 1–10 Blank values, adsorption, pre-concentration, and sample preservation for arsenic speciation of environmental water samples Jen-How Huang a,∗ , Gunter Ilgen b a Department of Soil Ecology, Bayreuth Institute for Terrestrial Ecosystem Research (BITÖK), University of Bayreuth, D-95440 Bayreuth, Germany b Central Analytics, Bayreuth Institute for Terrestrial Ecosystem Research (BITÖK), University of Bayreuth, D-95440 Bayreuth, Germany Received 15 December 2003; received in revised form 26 January 2004; accepted 2 February 2004 Available online 14 April 2004 Abstract Arsenic is the focus of public attention because of its toxicity. Arsenic analysis, its toxicity, and its fate in the environment have been broadly studied, still its blank values, adsorption to sampling materials and pre-concentration of water samples as well as stabilization of arsenic compounds in water samples under field conditions have been very little investigated. In this study, we investigate the blank values and adsorption of arsenic compounds for different laboratory materials. We focused our work onto pre-concentration of water samples and how to stabilize arsenic compounds under field conditions. When using glassware for arsenic analysis, we suggest testing arsenic blank values due to the potential release of arsenic from the glass. Adsorption of arsenic compounds on different laboratory materials (<10%) showed little influence on the arsenic speciation. Pre-concentration of methanol–water solutions could result in potential overestimation of arsenic compounds concentrations. Successful pre-concentration of water samples by nitrogen-purge provides an analytical possibility for arsenic compounds with high recoveries (>80%) and low transformation of arsenic compounds. Thus, concentrations as low as 1ng As l −1 can be determined. Addition of ethylenediaminetetraacetic acid (EDTA) and storage in the dark can decrease the transformation among arsenic compounds in rainwater and soil-pore water for at least a week under field conditions. © 2004 Elsevier B.V. All rights reserved. Keywords: Arsenic; Pre-concentration; Blank value; Adsorption; Stability; Speciation 1. Introduction Today, arsenic is the focus of public attention. The WHO guidelines for arsenic in drinking water standard decreased with time, and in the last edition, it was 10 gl −1 (1993). Because of the limitations in analytical techniques, 10 gl −1 is regarded as a provisional guideline value. However, this value would be less than 10 gl −1 if based on health criteria [1]. In the past decades, analytical techniques for arsenic spe- ciation have developed [2]. Toxicity of arsenic compounds to human beings and creatures, and the fate and behaviors of arsenic compounds in the environment have been broadly studied [3,4]. Nevertheless, some basic and relevant infor- mation for arsenic analysis is rare, such as the arsenic blank values and the adsorption behaviors to sampling materials. Since arsenic in the form of As 2 O 3 is used in the glass in- dustry [5,6], release of arsenic from glassware during exper- ∗ Corresponding author. Tel.: +49-921-555761; fax: +49-921-555799. E-mail address: how.huang@bitoek.uni-bayreuth.de (J H. Huang). iments cannot be excluded. Use of different flasks and vials for arsenic compounds analysis is inevitable for sampling, sample storage, pre-concentration, and further analytical steps. Procedural blank values of arsenic released from con- tainer walls may leadto an overestimation of the arsenic con- centrations and a false impression of arsenic speciation. This effect is especially serious for less contaminated samples. Although arsenic in different waters has been much in- vestigated [3], the materials of containers for sampling were usually not well documented. Koch et al. [7] and Zheng et al. [8] used polypropylene bottles and Guerin et al. [9] used polycarbonate bottles for the water samples. Adsorp- tion of organotin compounds to the container walls leads to underestimates of their concentrations in water [10]. Similar to organotin compounds, organic arsenic compounds may have high affinity to polymer materials, but no information is available. In less contaminated water, the concentrations of arsenic compounds are usually low. The baseline concentrations of arsenic in rainwater (including snow) and river water are 0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.02.043 2 J H. Huang, G. Ilgen /Analytica Chimica Acta 512 (2004) 1–10 less than 0.03 and 0.1−0.8gl −1 , respectively [3]. Because the toxicity of arsenic compounds depends on their chem- ical forms, a false impression of speciation may lead to a false risk assessment of the environmental water sam- ples. Thus, a reliable pre-concentration method is a basis for a more precise speciation analysis for the less contam- inated samples and a sequential risk assessment. Hydride generation enhances the sensitivity normally up to 100-fold over the commonly used liquid sample nebulization pro- cedures. However, several organic arsenic compounds do not form volatile hydrides under the tetrahydroborate treat- ment. Methods have been developed to convert them to hydride-forming species, but are usually complicated and troublesome [2]. Elizalde-Gonzalez et al. [11] used natural zeolites for pre-concentration of arsenic species in the wa- ter samples, but this method was only tested with inorganic arsenic. Arsenic enrichment by solvent evaporation has of- ten been applied for methanol–water extracts of biological samples, but little is reported about the validity of these pre-concentration procedures. Many efforts have been made to preserve arsenic com- pounds in their original forms in the environmental samples [2]. Unlike other environmental waters, the sampling of rainwater and soil-pore water normally requires long-term collection in the field for a certain period or for sampling certain amounts. Although the storage of the samples in a frozen state is the most highly recommended procedure [12], these conditions are not very practical for preserving rain- water and soil-pore water in the field. Therefore, a practical stabilization of the arsenic compounds under field conditions for several days is essential for such samples. Acidification of samples with nitric acid, hydrochloric acid or acetic acid has been applied to decrease microbial activity [13,14]. However, interference with the arsenic speciation may oc- cur and Hall et al. [15] reported an immediate oxidation of As(III) after addition of either nitric acid or hydrochloric acid in spiked river water. Oxidation of As(III) by oxygen is increased by several order of magnitudes due to the presence of Fe(II) and UV radiation light [16]. Therefore, storage of the samples to protect against UV radiation does not reduce only the microbial activity, but also prevents photo-oxidation due to UV exposure [17]. Addition of ethylenediaminete- traacetic acid (EDTA) and the storage of the samples in opaque bottles can stabilize arsenate and arsenite in ground- water at 20 ◦ C for more than 3 months, and is superior to the addition of hydrochloric acid, nitric acid, and sulfuric acid [18]. The objectives of this study are: (1) to identify the poten- tial sources of the arsenic blank values as caused by arsenic released from different laboratory materials, (2) to investi- gate the adsorption behavior of different arsenic compounds on different laboratory materials, (3) to evaluate different pre-concentration methods for aqueous samples to provide a more precise speciation of arsenic compounds in less con- taminated water samples, (4) to evaluate the validation of the pre-concentration method for methanol–water solutions, and (5) to test the stabilization effect of arsenic compounds provided by EDTA in the dark using different rainwater and soil-pore water samples under conditions similar to those in the field. 2. Experimental 2.1. Instrumentation A liquid chromatograph (LC) (BIOTEK Instruments, USA), consisting of a gradient pump (System 525), cap- illary PEEK tubing (0.25 mm i.d.) and a 200-l injection loop (Stainless Steel), and a LC autosampler 465 (Kontron Instruments, Germany) was connected to an anion-exchange column (IonPak AG7 and AS7, both Dionex) and cou- pled to an inductively coupled plasma-mass spectrometer (ICP-MS) (Agilent 7500c, Japan), equipped with a concen- tric nebulizer (Glass Expansion, Australia) and a Scott-type glass spray chamber. The separation was performed at a flow rate of 1 ml min −1 , using a nitric acid gradient between pH 3.4 and 1.8. The dipotassium salt of benzene-1,2-disulfonic acid (0.05mM) was added to the eluent as an ion-pairing reagent. At the outlet of the separation column, an internal standard (10g Ge l −1 in 0.01M nitric acid) was added by means of a Y-connector. Determination of total arsenic in liquid samples was con- ducted directly with the ICP-MS, using germanium (10 g Ge l −1 ) as an internal standard. Detection limits for arsenic species and the total arsenic were calculated as three times the standard deviation from instrumental blank values. 2.2. Reagents Arsenate (As(V)), arsenite (As(III)), and dimethylarsinic acid (DMA) were purchased from Merck (Darmstadt, Germany). Arsenbetaine (AsB) was obtained from Fluka and monomethylarsonic acid (MMA) and arsenocholine (AsC) from Argus Chemicals, Italy. De-ionized water, used throughout the work, was purified in a Milli-Q system (Millipore Corp., Milford, MA). Individual stock solutions (50mg As l −1 ) of As(III), As(V), MMA, DMA, AsB, and AsC were prepared in Milli-Q water, and stored at 4 ◦ C in the dark. A multi-compound working solution with a total concentration of 5 gAsl −1 was prepared before each use by diluting the stock solutions with Milli-Q water. 2.3. Release of arsenic from different materials The tested bottles of different materials were filled with Milli-Q water or 10% nitric acid, and incubated in the dark at room temperature for 24 h. After incubation, 1 ml of the solution was analyzed for total arsenic using the ICP-MS. J H. Huang, G. Ilgen /Analytica Chimica Acta 512 (2004) 1–10 3 2.4. Adsorption of arsenic compounds to different materials To 50ml artificial rainwater (containing 11.6 mg l −1 NH 4 NO 3 , 7.85 mg l −1 K 2 SO 4 , 1.11 mg l −1 Na 2 SO 4 , 1.31 mg l −1 MgSO 4 ·7H 2 O, 4.32mgl −1 CaCl 2 ) was added 50 l the stock solution of mixed arsenic compounds solu- tion with each 5mg As l −1 to adjust the concentrations of each arsenic compounds to 5 gAsl −1 . Fifty milliliters of the artificial rainwater was used to fill the bottles of different materials. The bottles were shaken in the dark at 5 ± 1 ◦ C and 20±1 ◦ C for 24 h, and 1 ml of the solution was taken for the analysis of the arsenic compounds using LC–ICP-MS. High-density polyethylene (translucent, 250ml), polypro- pylene (translucent, 250 ml), Teflon FEP (transparent, 250 ml), and polycarbonate (transparent, 125 ml) bottles were purchased from Nalgene, USA. Glass bottles (trans- parent, 250ml) were purchased from Schott, Germany. Teflon PFA bottles (translucent, 250 ml) and polyethylene bottles (translucent, 250 ml) were bought from Vitlab, Ger- many and VWR, Germany, respectively. These bottles were either new or used very rarely. For the adsorption experi- ments, the bottles were kept incompletely filled to prevent the solution from making contact with the cap, which is made of different materials from the bottle, in the cases of glass, polycarbonate, and Teflon FEP bottles. All the bottles were cleaned with detergents and distilled water at 70 ◦ C. 2.5. Freeze-dry pre-concentration Five and ten milliliters of mixed arsenic compound solu- tions in the polyethylene tubes each with 5 gAsl −1 were frozen at −40 ◦ C in the dark. The frozen samples were then freeze-dried to dryness or until ca. 1 ml samples were left. For the dry samples, 1 ml of Milli-Q water was used to rinse the tubes, and then, for determining the arsenic compounds using LC–ICP-MS. For samples with ca. 1 ml remaining, the sample thawed in the dark at room temperature, and the arsenic compounds were determined using LC–ICP-MS. 2.6. Nitrogen-purge pre-concentration Nitrogen-purge pre-concentration was conducted using a Turbo Vap II (Zymark, USA). In principle, nitrogen-purging at 1bar evaporates solvents such as methanol and water either from the methanol–water solutions or as just aque- ous samples. A water bath was used to control the sample temperature during pre-concentration at maximum 60 ◦ C. For the recovery tests in methanol–water solutions, a 10ml methanol–water solution (90% methanol, v/v) of mixed arsenic compound solutions each with 5 gAsl −1 were pre-concentrated at 25, 30, 40, 50, and 60 ◦ C. For the re- covery tests in the aqueous samples, 5 and 10ml mixed arsenic compound solutions each with 5 gAsl −1 were pre-concentrated at 25, 30, and 40 ◦ C, preventing the degra- dation and transformation of the organic arsenic compounds at high temperatures. Pre-concentration was stopped auto- matically when the solution volume had reached 0.7 ml. An additional 0.3ml of Milli-Q water was added to rinse the tube wall to desorb residual arsenic compounds. After pre-concentration, the residual solutions were determined by LC–ICP-MS as mentioned above. 2.7. Speciation analysis of arsenic compounds in environmental water samples Rainwater, soil-pore water, and river water were collected in November 2003 from a remote site, the Lehstenbach catchment in NE Bavaria, Germany. Polyethylene samplers were placed 1m above the ground in opaque polyethylene tubes to exclude sunlight. Samplers were installed under the canopy for through-fall sampling and at an open site for bulk precipitation. Soil-pore water from the forest floor and the mineral soil were collected by lysimeters at 20 and 90cm depth, respectively. The water samples were afterwards fil- tered through a membrane filter and pre-concentrated imme- diately, stored at 4 ◦ C, and analyzed by the with LC–ICP-MS within 48 h. 2.8. Stability of arsenic compounds in rainwater and soil-pore water Rainwater (bulk precipitation and through-fall) and soil-pore water (20 and 90cm) were collected at the same event, but not filtered. Through-fall usually has a higher concentrations of cations, anions, and dissolved organic carbon (DOC) than bulk precipitation as a result of canopy washout. Soil-pore water usually has a higher ionic strength and is more strongly buffered than rainwater. Soil-pore water sampled from forest floors contains higher concen- trations of cations, anions, and DOC as compared to that sampled from mineral soil [19]. The sampled rainwater and soil-pore water were spiked with different arsenic compounds immediately after sam- pling. For this, we used an amount of each 5gAsl −1 and 1.25 mM EDTA, and incubated the samples in the dark at 20 ± 1 ◦ C for seven days. A set of control samples were spiked in parallel with the same amount of arsenic com- pounds, and incubated under the same conditions without addition of EDTA.After incubation, all samples were filtered with membrane filter and then, analyzedby the LC–ICP-MS. 3. Results and discussion 3.1. Some sources of blank values We found significant amounts of total arsenic in the tap water. In contrast, total arsenic in the Milli-Q water was below the detection limit (Table 1). Although the concen- tration of total arsenic in the tap water was low (0.13g As l −1 ) compared to the WHO limit of drinking water stan- dard (10 gAsl −1 ), using tap water for rinsing bottles and 4 J H. Huang, G. Ilgen /Analytica Chimica Acta 512 (2004) 1–10 Table 1 Blank values (ng As l −1 ) of total arsenic in Milli-Q water and tap water, and release of total arsenic from different laboratory materials Sample Milli-Q water <DL Tap water 130 ± 11 Polyethylene a <DL Polycarbonate a <DL Glass 1 a <DL Glass 2 a 16 000 ± 1300 Glass b <DL Glass HNO 3 c 14 ± 5.9 Mean values and S.Ds. of three replicates are shown; Detection limit (DL) of total arsenic: 10ng As l −1 . a Incubated with Milli-Q water for 24h. b Rinsing with Milli-Q water after 10% nitric acid bath, and incubated with Milli-Q water for 24h. c Incubated with 10% nitric acid for 24h. as a solvent for arsenic trace analysis should be avoided to prevent probable contamination. Release of arsenic from plastic bottles (e.g. polyethylene and polycarbonate) and from most of the glassware was not detected when Milli-Q water was used. However, a large amount of arsenic was released as As(V) (16 gAsl −1 )in the Milli-Q water in one particular case. Generally, when glassware was first incubated in a 10% nitric acid bath and then rinsed several times with Milli-Q water, no release of arsenic was found in the Milli-Q water from the glassware. Thus, although the use of As 2 O 3 in the glass industry [5,6] suggests the potential occurrence of arsenic in glassware, only one case of release of arsenic from glassware was iden- tified. For arsenic analysis, special care, such as testing ar- senic blank values, should be taken when using glassware. Small amounts of arsenic were detected when the glass- ware was incubated with 10% nitric acid. In this case, there are two possible sources to the arsenic; leaching from glass- ware by nitric acid or the blank value originated from nitric acid itself. Therefore, use of glassware and cleaning glass- ware with nitric acid is not recommended. 3.2. Adsorption of arsenic compounds on different materials Most of the spiked arsenic compounds showed an adsorp- tion of <5% on glass and the different polymer materials at Fig. 1. Recoveries of arsenic compounds from bottles of different materials at 5 ◦ C. Mean values and S.Ds. of three replicates are shown. () Polyethylene; ( ) high-density polyethylene; ( ) polypropylene; (ᮀ) glass; ( ) Teflon FEP; ( ) Teflon PFA; and ( ) polycarbonate. 0 50 100 150 200 As(III) MMA DMA As(V) AsB AsC Recoveries (%) Fig. 2. Recoveries of arsenic compounds from the bottle of polyethylene ( ) and glass (ᮀ)at20 ◦ C. Mean values and S.Ds. of three replicates are shown. 5 ◦ C (see Fig. 1). Only MMA had a slightly higher adsorp- tion on all materials, but it was still below 10%. We have also tested the adsorption of arsenic compounds on glass and polyethylene at 20 ◦ C, and obtained similar results as at 5 ◦ C (see Fig. 2). However, a transformation of As(V) to As(III) during the batch experiment was observed. The adsorption experiments indicated negligible adsorp- tion of both inorganic and organic arsenicals to different materials. The temperature variation seemed to have little influence on the adsorption of arsenic compounds to differ- ent materials. Underestimation of concentrations of arsenic compounds, caused by adsorption to container walls, should be low. 3.3. Pre-concentration of water samples by freeze-drying We tried to pre-concentrate the spiked water samples by freeze-drying. However, recoveries of arsenic compounds in the aqueous solutions after freeze-drying were low, and the loss of arsenic compounds related directly to the time used to freeze-dry the samples (Table 2). In all cases, re- coveries of the total arsenic in the solution decreased after freeze-drying, suggesting the loss of arsenic compounds in the freeze-drying process. As(III) always had the lowest recoveries and As(V) had the highest recoveries compared to the other arsenic compounds. Ellwood and Maher [20] reported that As(III) concentrations determined in freeze-dried and air-exposed sediments were much lower than in sediments that were not freeze-dried with minimum air exposure. Exposure of the J H. Huang, G. Ilgen /Analytica Chimica Acta 512 (2004) 1–10 5 Table 2 Recoveries (%) of arsenic compounds in aqueous solutions pre-concentrated by freeze-drying As(III) MMA DMA As(V) AsB AsC Freeze-dry 24 h, starting with 10 ml 26.1 ± 17.9 41.0 ± 32.9 57.7 ± 17.5 79.4 ± 11.9 57.2 ± 16.7 51.9 ± 16.0 Freeze-dry until ca. 1 ml solution left, starting with 10ml a 54.4 ± 1.10 67.4 ± 6.28 68.5 ± 6.60 71.0 ± 10.9 67.4 ± 6.09 68.3 ± 6.08 Freeze-dry until ca. 1 ml solution left, starting with 5ml b 74.1 ± 26.8 81.1 ± 17.9 82.8 ± 18.1 83.9 ± 19.5 80.3 ± 17.3 79.5 ± 16.4 Mean values and S.Ds. of three replicates are shown. a Listing for ca. 13h. b Listing for ca. 8h. Fig. 3. Recoveries of arsenic compounds after pre-concentration starting with methanol–water solution (90% methanol, v/v) using nitrogen-purge method. ( ) Arsenite; ( ) monomethylarsonic acid; ( ) dimethylarsinic acid; (ᮀ ) arsenate; ( ) arsenbetaine; and ( ) arsenocholine. Mean values and S.Ds. of three replicates are shown. samples to the air prior to analysis seems to oxidize As(III) into As(V). This effect may lead to a false impression of the true speciation within the sample. 3.4. Pre-concentration of methanol–water solutions by nitrogen-purge After pre-concentration of methanol–water solution (90% methanol, v/v) by nitrogen-purge, the arsenic compounds, especially As(III) and MMA, had recoveries of >100% ac- companied by large standard deviations (see Fig. 3). This phenomenon was more apparent when the pre-concentration was conducted at lower temperatures (25 and 30 ◦ C) than that at higher temperatures (50 and 60 ◦ C). It is well established that addition of carbon (as methanol) to aqueous solutions improves the ionization efficiency of arsenic in the plasma [21,22]. Kohlmeyer et al. [22] demonstrated that adding methanol could enhance the ar- senic signals in the LC–ICP-MS. We tested the influence of methanol concentration on signals for the different ar- senic compounds. The signals increased generally with the increase in methanol concentrations (see Fig. 4). However, As(III) and MMA signals were much more enhanced com- pared to the other arsenic compounds when the methanol concentrations were less than ca. 50% but leveled-off when the methanol concentrations were between 50 and 100%. Besides, the As(III) and MMA peaks were more close to each other, broadened, and then, overlapped as the methanol concentration in solution increased (see Fig. 5). Since the retention time of methanol in our LC program was very close to the As(III) and MMA peaks (see Fig. 6), especially As(III), the enhancement of arsenic signals by addition of methanol was thereby in the order: As(III) MMA the other arsenic compounds. We also suspected that large amounts of methanol in the eluent interfered with the separation of As(III) and MMA. There seems to be significant amounts of methanol left as a residue in the pre-concentrated methanol–water extracts. The effect was only slightly reduced with the increasing pre-concentration temperature. According to our results in this section, pre-concentration of methanol–water solutions at all temperatures may result in overestimating the concen- trations of As(III) and MMA in the samples. Therefore, we suggest either to avoid using methanol for extraction or spe- cial care should be taken when calibrating the arsenic com- pounds concentrations. 300 600 900 1200 020406080100 Methanol(%v/v) Intensity As (III) MMA DMA As(V) AsB AsC Fig. 4. Response of arsenic compounds as a function of methanol con- centration. Mean values and S.Ds. of three replicates are shown. 6 J H. Huang, G. Ilgen /Analytica Chimica Acta 512 (2004) 1–10 0 2000 4000 6000 8000 10000 12000 14000 0 100 200 300 400 500 600 700 800 900 Retention time (s) Intensity (cps) 0% 30% 60% 100% 0 2000 4000 6000 8000 80 100 120 140 160 Retention time (s) Intensity (cps) As(III) aMMA DMA As(V) AsC aAsB As(III) aMMA Fig. 5. Liquid chromatogram of arsenic compounds with each 5 gAsl −1 in methanol–water solutions (0, 30, 60, and 100% methanol, v/v). 0 5000 10000 15000 20000 25000 0 100 200 300 400 500 600 700 800 900 Retention time (s) Intensity (cps) 4000 0 8000 12000 16000 As75 (left axis) Ar40 C13 (right axis) As(V) DMA AsB MMA As(III) AsC Fig. 6. Liquid chromatogram of 5gAsl −1 arsenic standard and trace monitor of m/z 53. 3.5. Pre-concentration of water samples by nitrogen-purge Pre-concentration using a nitrogen-purge showed recov- eries of >80% of all arsenic compounds at different tem- peratures and different pre-concentration ratios (see Figs. 7 Fig. 7. Recoveries of arsenic compounds after pre-concentration using nitrogen-purge method. (a) Starting with 5 ml solution; and (b) starting with 10 ml. ( ) Arsenite; ( ) monomethylarsonic acid; ( ) dimethylarsinic acid; (ᮀ) arsenate; ( ) arsenbetaine; and ( ) arsenocholine. Mean values and S.Ds. of three replicates are shown. and 8). As(III) and AsC always had the lowest recover- ies among the arsenic compounds and under all conditions tested. In contrast, As(V) and AsB were enriched in most cases, reflecting the probable oxidation of As(III) to As(V) and AsC to AsB [12]. Even though the evaporation of water J H. Huang, G. Ilgen /Analytica Chimica Acta 512 (2004) 1–10 7 0 50000 100000 150000 200000 250000 0 100 200 300 400 500 600 700 800 900 Retention time (s) Intensity (cps) original 5 times concentrated 10 times concentrated As(III) aMMA aDMA AsB aAsC As(V) Fig. 8. Liquid chromatogram of arsenic compounds with each 5 gAsl −1 in an aqueous solution, after pre-concentration (5–1 ml and 10–1 ml, respectively) using nitrogen-purge at 30 ◦ C. was done by nitrogen-purge, we can not completely exclude the exposure of arsenic compounds to O 2 in the atmosphere during pre-concentration. Five times pre-concentration (see Fig. 7a) showed less transformation of As(III) to As(V) than 10 times pre-concentration (see Fig. 7b). This may support the exposure of arsenic compounds to air, because the 10 times pre-concentration procedure (ca. 9 h) lasts twice as long as that for the five times (ca. 4 h). Comparing the results from freeze-drying and nitrogen- purge, the use of the latter is concluded to be superior to freeze-drying to pre-concentrate water samples, because it is more convenient, has higher recoveries, and less arsenic transformation. Although the recoveries of arsenic com- pounds pre-concentrated with nitrogen-purging varied little with temperature, we recommend pre-concentrating water samples at low temperatures. Pre-concentration at higher temperatures may enhance the risk of transformation among arsenic compounds. Combining pre-concentration with nitrogen-purge and LC–ICP-MS allows the detection of arsenic compounds in the water samples at the 1ng Asl −1 level. It is particu- larly suitable for the environmental water samples with low arsenic concentrations. 0 200 400 600 800 1000 0 100 200 300 400 500 600 700 800 900 Retention time (s) Intensity (cps) 10 times concentrated original As(III) DMA As(V) AsC Fig. 9. Liquid chromatogram of arsenic compounds in the river water before and after 10 times pre-concentration using nitrogen-purge method. 3.6. Determination of arsenic compounds in less contaminated water samples We determined the concentrations of arsenic compounds in the less contaminated rainwater, soil-pore water, and river water. The concentrations of total arsenic in these water sam- ples were all below 1 gAsl −1 with the dominance of inor- ganic arsenic, As(III), and As(V). Remarkable amounts of organic arsenic were observed in through-fall and soil-pore water from the forest floor with MMA, DMA, AsB or AsC up to 290 ng Asl −1 (Table 3). Organic arsenic compounds were found only in trace amounts in bulk precipitation, min- eral soil-pore water, and river water. The pre-concentration was most effective in the case of mineral soil-pore water and river water. No organic arsenic compounds could be detected in mineral soil-pore water and river water without pre-concentration, due to the their ex- tremely low concentrations (see Fig. 9). The concentrations of arsenic compounds determined in pre-concentrated sam- ples were not far from those determined in the original sam- ples (recoveries >80%), demonstrating the validation of the pre-concentration; however, transformation of small amount among arsenic compounds seems inevitable. Except prob- 8 J H. Huang, G. Ilgen /Analytica Chimica Acta 512 (2004) 1–10 Table 3 Concentrations of arsenic compounds in rainwater, river water, and soil-pore water, and recoveries (%) of arsenic compounds as determined on original samples and on samples pre-concentrated 10 times As(III) MMA DMA As(V) AsB AsC Total As Organic As (%) Bulk precipitation a (ng As l −1 ) 82.8 <DL 13.9 242 <DL <DL 339 4.11 Bulk precipitation b (ng As l −1 ) 76.2 <DL 15.3 219 <DL <DL 312 4.92 Recovery (%) 94.0 – 110 83.5 −− 92.0 Through-fall a (ng As l −1 ) 90.7 <DL 291 452 34.9 58.5 928 41.4 Through-fall b (ng As l −1 ) 88.4 <DL 285 440 35.4 52.3 901 41.3 Recovery (%) 97.4 – 97.8 97.2 102 89.5 97.1 Soil-pore water 20cm a (ng As l −1 ) 88.3 20.9 130 387 35.2 41.5 702 32.4 Soil-pore water 20cm b (ng As l −1 ) 77.7 21.0 127 422 34.1 38.3 719 30.6 Recovery (%) 87.9 101 97.7 109 96.7 92.2 102 Soil-pore water 90cm a (ng As l −1 ) 11.4 <DL <DL 114 <DL <DL 125 0.00 Soil-pore water 90cm b (ng As l −1 ) 12.6 <DL 5.3 107 <DL <DL 124 4.28 Recovery (%) 110 – − 93.9 −− 99.7 River water a (ng As l −1 ) 176 <DL <DL 811 <DL <DL 987 0.00 River water b (ng As l −1 ) 181 <DL 5.6 850 <DL 5.4 1040 1.06 Recovery (%) 103 – − 105 −− 106 Mean values of three replicates are shown and all S.Ds. are <5%. (−): Recovery not applicable. a Determining on original sample, DL: 10ng As l −1 . b Determining on 10 times pre-concentration solution, DL: 1ng As l −1 . able transformation of the arsenic compounds during the pre-concentration process, transformation of arsenic com- pounds may occur before analysis due to the contact of the samples with air or reduction caused by DOC in the water samples [18]. Since the concentrations of arsenic compounds in these samples were very low, a slight transformation may already cause remarkable inaccuracy. Thus, storage of the treated samples at low temperature (e.g. 4 ◦ C) before anal- ysis or immediate analysis of the samples is required. Usually, DMA and MMA were the most common organic arsenic compounds in the rainwater and soil-pore water, but in small proportions [23,24]. Similarly, only trace amounts of DMA were found in the bulk precipitation. However, con- siderable amounts of organic arsenic compounds were ob- served in the through-fall and soil-pore water from the forest floor (40 and 30%, respectively), including trace amounts of the more complicated AsB and AsC. The microbial ac- tivity in the phyllosphere and forest floor is usually high [19,25], suggesting these organic arsenic compounds may 0 4000 8000 12000 16000 0 100 200 300 400 500 600 700 800 900 Retention time (s) Intensity (cps) original after incubation As(III) aMMA aAs(V) aAsB DMA a? a? aAsC Fig. 10. Liquid chromatogram of arsenic compounds spiked with each 5 gAsl −1 in through-fall before and after 7 days incubation without addition of EDTA. be formed by in situ methylation. The proportion of organic arsenic decreased to trace amounts in the mineral soil-pore water and inorganic arsenic compounds were enriched in the river water. Arsenic compounds may undergo different transformations and transport processes in the soils. How- ever, these can not be explained only using our results and available knowledge. More investigations about the trans- formation and transportation of arsenic in the forest ecosys- tems are necessary, to gain a more clear sight of the biogeo- chemical fate and the behavior of arsenic in the terrestrial environments. 3.7. Stability of arsenic compounds in rainwater and soil-pore water The chromatogram in Fig. 10 showed a typical transfor- mation of arsenic compounds in unfiltered rainwater and soil-pore water without any treatment. Large amounts of As(V) were converted into As(III), which might be caused J H. Huang, G. Ilgen /Analytica Chimica Acta 512 (2004) 1–10 9 Table 4 Recoveries (%) of spiked arsenic compounds in rainwater and soil-pore water with addition of 1.25mM EDTA, and incubated at 20 ◦ C in the dark for 7 days As(III) MMA DMA As(V) AsB AsC Bulk precipitation 108 ± 17.9 101 ± 2.93 88.8 ± 1.77 81.9 ± 26.8 85.9 ± 1.78 98.9 ± 3.10 Through-fall 105 ± 10.9 85.2 ± 0.88 89.4 ± 1.60 91.6 ± 7.10 92.1 ± 4.09 93.6 ± 1.08 Soil-pore water 20cm 101 ± 2.17 106 ± 3.15 94.1 ± 0.89 96.9 ± 5.32 101 ± 2.33 106 ± 4.21 Soil-pore water 90cm 106 ± 4.65 86.8 ± 6.31 94.5 ± 2.57 95.2 ± 2.49 95.2 ± 1.73 90.2 ± 0.79 Mean values and S.Ds. of three replicates are shown. by reduction by DOC [18]. The sum of inorganic arsenic compounds was higher than the original amount after 7 days incubation, suggesting the degradation of organic ar- senic species. Although MMA and DMA were suggested to be much more stable [26], we have also observe decreased amounts of MMA. Nevertheless, we found that DMA is also much more enriched after incubation. Since DMA was the degradation intermediate of AsB and AsC [12], the abun- dance of DMA after incubation was caused by the degrada- tion of AsB and AsC. The results from the control experi- ment indicate the essential steps to reduce transformation of arsenic compounds during storage in the field. With the addition of 1.25 mM EDTA to rainwater and to soil-pore water, transformation of arsenic compounds and degradation of organic arsenic compounds during the in- cubation was successfully reduced (Table 4). A loss of or- ganic arsenic compounds (<15%) and transformation be- tween As(V) and As(III) (<20%) were observed, especially in the case of rainwater. There may be some variation among rainwater and soil-pore water samples; of course, this we have not accounted for here. EDTA chelates metal cations, buffers the sample pH, and reduces the microbial activity [27]. The stabilization effect of EDTA was shown to be superior to that of mineral acids [18]. Our results demonstrated the stabilization effect of arsenic compounds provided by EDTA and in the dark in unfiltered rainwater and soil-pore water. 4. Conclusions The use of laboratory glassware for arsenic analysis should be careful because of potential release of arsenic. Adsorption of arsenic compounds on different laboratory materials had little influence on the arsenic speciation. Pre-concentration of methanol–water solutions could re- sult in potential overestimation of arsenic compounds concentrations. Pre-concentration of aqueous samples by nitrogen-purge provides an analytical possibility for arsenic compounds at the level of 1 ng As l −1 , and has high recov- ery and low transformation. Addition of EDTA and storage in the dark can reduce the transformation among arsenic compounds in rainwater and soil-pore water under field conditions. 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Acta 512 (2004) 1–10 Blank values, adsorption, pre-concentration, and sample preservation for arsenic speciation of environmental water samples Jen-How Huang a,∗ ,. techniques for arsenic spe- ciation have developed [2]. Toxicity of arsenic compounds to human beings and creatures, and the fate and behaviors of arsenic