Blank values, adsorption, pre concentration, and sample preservation for arsenic speciation of environme

hấp phụ Arsenic vào trong môi trường đất

Blank values, adsorption, pre-concentration, and sample preservation

for arsenic speciation of environmental water samples

Jen-How Huanga,∗, Gunter Ilgenb

aDepartment of Soil Ecology, Bayreuth Institute for Terrestrial Ecosystem Research (BITÖK), University of Bayreuth, D-95440 Bayreuth, Germany

bCentral 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 1 ng As l−1can 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␮g l−1 (1993).

Because of the limitations in analytical techniques, 10␮g l−1

is regarded as a provisional guideline value However, this

value would be less than 10␮g l−1if 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 As2O3is 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 tainer walls may lead to 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

less than 0.03 and 0.1−0.8 ␮g l−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 rain-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 coucou-pled 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 (0.05 mM) was added to the eluent as an ion-pairing reagent At the outlet of the separation column, an internal standard (10␮g

Ge l−1 in 0.01 M 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 (50 mg 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

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

2.4 Adsorption of arsenic compounds to different materials

To 50 ml artificial rainwater (containing 11.6 mg l−1

NH4NO3, 7.85 mg l−1 K

2SO4, 1.11 mg l−1 Na

2SO4, 1.31 mg l−1 MgSO

4·7H2O, 4.32 mg l−1 CaCl

2) was added

solu-tion with each 5 mg As l−1to adjust the concentrations of

each arsenic compounds to 5␮g As l−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, 250 ml),

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, 250 ml) 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, GerGer-many, 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␮g As l−1were

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 1 bar 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 10 ml

methanol–water solution (90% methanol, v/v) of mixed

arsenic compound solutions each with 5␮g As l−1 were

pre-concentrated at 25, 30, 40, 50, and 60◦C For the

re-covery tests in the aqueous samples, 5 and 10 ml mixed

arsenic compound solutions each with 5␮g As l−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 0.3 ml 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 1 m 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 90 cm 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 90 cm) 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 5␮g As l−1and 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, analyzed by 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 (0.13␮g

As l−1) compared to the WHO limit of drinking water stan-dard (10␮g As l−1), using tap water for rinsing bottles and

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

Mean values and S.Ds of three replicates are shown; Detection limit

(DL) of total arsenic: 10 ng As l −1.

a Incubated with Milli-Q water for 24 h.

b Rinsing with Milli-Q water after 10% nitric acid bath, and incubated

with Milli-Q water for 24 h.

c Incubated with 10% nitric acid for 24 h.

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␮g As l−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 As2O3in 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 (䊏) Polyethylene; ( ) high-density polyethylene; ( ) polypropylene; (䊐) glass; ( ) Teflon FEP; ( ) Teflon PFA; and ( ) polycarbonate.

0 50 100 150 200

Fig 2 Recoveries of arsenic compounds from the bottle of polyethylene (䊏) and glass (䊐) at 20 ◦C Mean values and S.Ds of three replicates are shown.

5◦C (seeFig 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 (seeFig 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 concdiffer-entrations 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

Table 2

Recoveries (%) of arsenic compounds in aqueous solutions pre-concentrated by freeze-drying

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 10 mla

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 5 mlb

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 13 h.

b Listing for ca 8 h.

Fig 3 Recoveries of arsenic compounds after pre-concentration starting with methanol–water solution (90% methanol, v/v) using nitrogen-purge method (䊏) Arsenite; ( ) monomethylarsonic acid; ( ) dimethylarsinic acid; (䊐) 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 (seeFig 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 (seeFig 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

Methanol(%v/v)

Fig 4 Response of arsenic compounds as a function of methanol con-centration Mean values and S.Ds of three replicates are shown.

0 2000 4000 6000 8000 10000

12000

14000

Retention time (s)

0%

30%

60%

100%

0 2000 4000 6000 8000

Retention time (s)

As(III)

a MMA

DMA

As(V)

AsC

a AsB

As(III)

a MMA

Fig 5 Liquid chromatogram of arsenic compounds with each 5 ␮g As l −1in methanol–water solutions (0, 30, 60, and 100% methanol, v/v).

0 5000 10000 15000 20000 25000

Retention time (s)

0

8000 12000

16000 As75 (left axis)

Ar40 C13 (right axis) As(V)

DMA

AsB MMA

Fig 6 Liquid chromatogram of 5 ␮g As l −1arsenic 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 (seeFigs 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 (䊏) Arsenite; ( ) monomethylarsonic acid; ( ) dimethylarsinic acid; (䊐) 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

0 50000 100000 150000 200000 250000

Retention time (s)

original

5 times concentrated

10 times concentrated

As(III)

a MMA

a DMA

AsB

a AsC As(V)

Fig 8 Liquid chromatogram of arsenic compounds with each 5 ␮g As l −1in 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 O2in 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 (seeFig 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 1 ng As l−1 level It is

particu-larly suitable for the environmental water samples with low

arsenic concentrations

0 200

400

600

800

1000

Retention time (s)

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␮g As l−1with 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 As l−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 (seeFig 9) The concentrations

of arsenic compounds determined in pre-concentrated 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-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

Bulk precipitation a (ng As l −1) 82.8 <DL 13.9 242 <DL <DL 339 4.11

Bulk precipitationb (ng As l −1) 76.2 <DL 15.3 219 <DL <DL 312 4.92

Soil-pore water 20 cm a (ng As l −1) 88.3 20.9 130 387 35.2 41.5 702 32.4

Soil-pore water 20 cm b (ng As l −1) 77.7 21.0 127 422 34.1 38.3 719 30.6

Soil-pore water 90 cm a (ng As l −1) 11.4 <DL <DL 114 <DL <DL 125 0.00

Soil-pore water 90 cm b (ng As l −1) 12.6 <DL 5.3 107 <DL <DL 124 4.28

Mean values of three replicates are shown and all S.Ds are<5% (−): Recovery not applicable.

a Determining on original sample, DL: 10 ng As l −1.

b Determining on 10 times pre-concentration solution, DL: 1 ng 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 analanal-ysis 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

120 00

160 00

Retention time (s)

original after incubation As(III)

a MMA

a As(V)

a AsB

DMA

a ?

a?

a AsC

Fig 10 Liquid chromatogram of arsenic compounds spiked with each 5 ␮g As l −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 inFig 10showed 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

Table 4

Recoveries (%) of spiked arsenic compounds in rainwater and soil-pore water with addition of 1.25 mM EDTA, and incubated at 20 ◦C in the dark for 7 days

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 20 cm 101 ± 2.17 106 ± 3.15 94.1 ± 0.89 96.9 ± 5.32 101 ± 2.33 106 ± 4.21

Soil-pore water 90 cm 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

Pconcentration 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

Acknowledgements

The authors would like to thank Björn Berg for helpful comments on this manuscript Financial support was given

by the German Academic Exchange Program (DAAD) and the German Ministry of Science and Education (BMBF, Grant No.: BEO 0339476D)

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