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Speciation of arsenic by hydride generation–atomic absorption spectrometry (HG–AAS) in hydrochloric acid reaction medium
Talanta 50 (1999) 1109 – 1127 www.elsevier.com/locate/talanta
Speciation of arsenic by hydride generation–atomic absorption spectrometry (HG–AAS) in hydrochloric acid reaction medium Amjad Shraim a,*, Barry Chiswell a, Henry Olszowy b a b
Department of Chemistry, The Uni6ersity of Queensland, St. Lucia, Qld 4072, Australia Queensland Health Scientific Ser6ices, 39 Kessels Rd., Coopers Plains, Qld 4108, Australia Received 31 March 1999; received in revised form 7 July 1999; accepted 15 July 1999
Abstract The effects on the absorbance signals obtained using HG – AAS of variations in concentrations of the reaction medium (hydrochloric acid), the reducing agent [sodium tetrahydroborate(III); NaBH4], the pre-reducing agent (L-cysteine), and the contact time (between L-cysteine and arsenic-containing solutions) for the arsines generated from solutions of arsenite, arsenate, monomethylarsonic acid (MMA), and dimethylarsenic acid (DMA), have been investigated to find a method for analysis of the four arsenic species in environmental samples. Signals were found to be greatly enhanced in low acid concentration in both the absence (0.03 – 0.60 M HCl) and the presence of L –cysteine (0.001–0.03 M HCl), however with L-cysteine present, higher signals were obtained. Total arsenic content and speciation of DMA, As(III), MMA, and As(V) in mixtures containing the four arsenic species, as well as some environmental samples have been obtained using the following conditions: (i) total arsenic: 0.01 M acid, 2% NaBH4, 5% L-cysteine, and contact time B 10 min; (ii) DMA: 1.0 M acid, 0.3 – 0.6% NaBH4, 4.0% L-cysteine, and contact time B5 min; (iii) As(III): 4–6 M acid and 0.05% NaBH4 in the absence of L-cysteine; (iv) MMA: 4.0 M acid, 0.03% NaBH4, 0.4% L-cysteine, and contact time of 30 min; (v) As(V): by difference. Detection limits (ppb) for analysis of total arsenic, DMA, As(III), and MMA were found to be 1.1 (n=7), 0.5 (n= 5), 0.6 (n = 7), and 1.8 (n =4), respectively. Good percentage recoveries (102– 114%) of added spikes were obtained for all analyses. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Arsenic; Arsenite; Arsenate; Monomethylarsonic acid; Dimethylarsenic acid; Speciation; L-cysteine; Hydride generation; Selective reduction
1. Introduction
* Corresponding author. Fax: +61-7-33653839. E-mail address: [email protected] (A. Shraim)
Arsenic, which is potentially toxic to humans, animals, and plants [1–4] and according to recent reports [5–8] may be carcinogenic to humans, occurs naturally in many chemical forms. Arsenic
0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 9 ) 0 0 2 2 1 - 0
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poisoning is a major public health problem, especially in countries such as Bangladesh, where the soil is high in arsenic compounds and the well water may contain as much as 300 – 4000 mg L − 1 of arsenic [9]. The toxicity of arsenic varies widely, ranging from highly hazardous inorganic arsenicals (arsine, arsenite, and arsenate) to relatively harmless organic species (monomethylarsonate and dimethylarsenate) [10 – 14]. Indeed some organoarsenicals, such as arsenobetaine and arsenocholine, are effectively non-toxic towards living organisms [15]. Therefore, determination of total arsenic content in a sample does not reflect the level of hazard of the element actually present, and it is increasingly important that the various forms of arsenic be determined in biological and environmental samples to provide a much clearer view of the risk associated with exposure to arsenic in the environment. Reported arsenic concentrations in some parts of Australia range up to 36 000 mg kg − 1 in surface soils and 300 mg kg − 1 in ground water [16]; the current ANZECC/ NHMRC arsenic guideline for soil is 100 mg kg − 1, while the NHMRC drinking water guideline has recently been reduced from 50 to 7 mg/l [16]. Speciation of arsenic in environmental samples usually involves several steps including derivitization, separation, and detection; hydride generation (HG), initially developed by Braman and Foreback in 1973 [17], coupled to one of several separation and detection systems has been found to be one of the most common techniques used for derivitization of arsenic. A number of detection systems have been used in arsenic analysis, of which the most popular and preferred one in terms of simplicity, sensitivity, precision, speed, and cost is AAS. Our aim in the work described here was to develop a simple, rapid, and inexpensive technique for the speciation of the most commonly occurring species in environmental samples, viz. As(III), As(V), monomethylarsonic acid (MMA), and dimethylarsenic acid (DMA) using hydride generation– atomic adsorption spectrometry (HG–AAS), i.e. a derivitization and detection technique only, without the need for a separation
step such as HPLC and cryogenic trapping, which we believe has caused many complications and resulted in lengthy analytical techniques. To compensate for the absence of a separation step in our method, a selective–reduction concept is employed to generate hydrides of each arsenic species. Critical evaluation of the existing literature, showed that some work has been done on the use of HG–AAS for the speciation of arsenic [18–21], and much of this work dealt with the use of thiol-containing ligands, such as L-cysteine, Lcystine, and thioglycerol, to obtain identical response from all four arsenic species. These thiols have been used as pre-reducing agents before the addition of NaBH4, and have been shown to enhance the arsine signals in low acid concentration and to reduce the effects of interferences [20,22–24]. In this study, the effect of HCl as a reaction medium on the arsine generation from the four arsenic species has been investigated. Control of the concentrations of the reaction medium (HCl), of the reducing and hydride generating agent (NaBH4), and of the pre-reducing agent (L-cysteine) when used, and the employment of HG– AAS has resulted in methods for the analysis and speciation of the four arsenic species in environmental samples.
2. Experimental
2.1. Equipment A Vapour Generation Accessory (VGA-76, Varian) connected to an Atomic Absorption Spectrometer (Spectra 300, Varian) was used in this study, and operated according to manufacturer’s instructions; instrument parameters used are summarised in Table 1. In this system, arsenic-containing solutions were pumped into a mixer and reacted with sodium tetrahydroborate(III) solution; generated arsines were swept to a gas–liquid separator using nitrogen gas and then to a heated T-shaped absorption cell.
A. Shraim et al. / Talanta 50 (1999) 1109–1127
2.2. Reagents and solutions 2.2.1. General All chemicals were of analytical-reagent grade unless otherwise specified. All glassware was soaked in 4 M HNO3 for a minimum of 12 h and washed with distilled water and finally rinsed with Milli-Q reagent water before use. All water used was obtained from a Milli-Q reagent system (Millipore), resistivity 18 MV cm. L-cysteine (minimum 98%, TLC) was obtained from Sigma (St. Louis, MO), sodium tetrahydroborate(III) form Merck and Alfa, As(III) atomic absorption standard solution (1 mg ml − 1) and As(V) (As2O5, 99.999%) from Acros, MMA (disodium methylarsenate, 99%) from Chem Service, and DMA (cacodylic acid, 98%) was obtained form Aldrich. Various concentrations of NaBH4 solution stabilised with NaOH were used; the concentration of NaOH was maintained at 0.5% in experiments where NaBH4 concentrations exceeded 0.6%. While both reagents were kept at the same concentration when NaBH4 was used at concentration levels B0.6%. 2.2.2. Arsenic stock solutions The arsenic stock solutions were prepared as follows: 1000 ppm As(III): arsenic(III) atomic absorption standard solution (1 mg ml − 1 As in 2% potassium hydroxide). Table 1 Operating conditions of the HG–AAS system Instrument mode
Absorbance
Calibration mode Measurement mode Slit width (nm) Slit height Wavelength (nm) Flame Sample introduction Delay time (s) Time constant Measurement time (s) Replicates Background correction Sample flow rate (ml min(1) NaBH4 flow rate (ml min(1)
Concentration Integration 0.5 Normal 193.7 Air–acetylene Normal 40 0.05 2.0 3 On 7 1
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1000 ppm As(V): 0.03835 g of arsenic(V) oxide (As2O5) was dissolved in a minimum volume of 4.0 M NaOH, neutralised by a same volume of 4.0 M HCl, and the final volume was adjusted to 25 ml with Milli-Q water. 1000 ppm MMA: 0.09844 g of disodium methylarsenate (CH3AsO(ONa)2 · 6H2O) was dissolved in 25 ml Milli-Q water. 1000 ppm DMA: 0.04699 g of cacodylic acid (CH3)2AsO(OH) was dissolved in 25 ml of Milli-Q water. Cacodylic acid is the commercial name for DMA, the acronym used in this work. The stock solutions of As(V), MMA, and DMA were prepared monthly and stored in glass volumetric flasks wrapped with aluminum foil and kept refrigerated at 4°C to prevent any change in speciation. The As(III) stock solution was also kept refrigerated at 4°C. All arsenic solutions were found to be stable under these conditions when tested after 1 month.
2.3. Analytical procedures Solutions were prepared as required by appropriate dilution of stock solutions and additions of the required volumes of hydrochloric acid and L-cysteine solutions to achieve the required concentrations. Solutions were then rapidly mixed. When using L-cysteine, measurement of contact time was commenced upon addition of L-cysteine and stopped at the beginning of the analysis. Throughout this study contact time refers to the time that has been allowed for L-cysteine to react with the arsenic-containing solutions before the commencement of the introduction of solutions to HG–AAS. 3. Results and discussion To understand the role of NaBH4 as a reducing and HG agent in the analysis of arsenic, the following mechanism has been proposed [25,26]: Rn As(O)(OH)3 − n + H+ + BH4− Rn As(OH)3 − n + H2O+ BH3 Rn As(OH)3 − n + (3− n)BH4− + (3− n)H+
(1)
Rn AsH3 − n + (3− n)BH3 + (3− n)H2O
(2)
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Table 2 Steps followed for the analysis of arsenic speciesa Steps Step Step Step Step Step a
1 2 3 4 5
HCl concentration
NaBH4 concentration
L-cysteine
V C V C C
C V C V C
0 0 C C C
concentration
Contact time NA NA C C V
V= varied; C = constant; 0 = zero; NA = not applicable.
BH3 +3H2O H3BO3 +3H2.
(3)
While in the presence of L-cysteine the proposed mechanism is shown below [23,25,27]: 2RSH +R*n AsO(OH)3 − n RS– SR +R*n As(OH)3 − n +H2O
(4)
R*n As(OH)3 − n +(3 −n) RSH R*n As(SR)3 − n +2H2O
(5)
R*n As(SR)3 − n +(3 −n)BH4− R*n AsH3 − n +(3 −n)BH3 +(3 −n)RS −
(6)
BH3 +3H2O H3BO3 +3H2.
(7)
We have used these proposed mechanisms to develop a method for the speciation of arsenic using the steps shown in the Table 2.
3.1. Effect of HCl concentration when using 0.6% NaBH4 solution in the absence of L – cysteine The effect of using 0.001 – 9.6 M HCl on the absorbance signals in the absence of L-cysteine is shown in Fig. 1. The absorption signals of As(III), MMA, and DMA sharply increase with increase in acid concentration over a range of 0.01 –0.10 M, while the increase in the As(V) absorption signal is much slower. Further increase in the acid concentration results in: (a) a sharp decrease in the DMA signal up to an acid concentration of 1.0 M, then a slow decrease to negligible values beyond 4.0 M, (b) a broad maximum for MMA over an acid concentration range of 0.2 – 1.0 M, and a slow decrease thereafter to a negligible signal at 9.60 M, (c) a slow increase in the As(III) signal up to an
acid concentration of 2.0 M and a very broad plateau afterwards, and (d) a much slower increase in the As(V) signal, when compared to the As(III) signal, up to an acid concentration of 4 M, and a broad plateau thereafter.Our results are similar to those obtained by workers who used similar instrumentation and analytical procedure. For example Anderson et al. [18] and Hakala and Pyy [28] used a HG–AAS and a HCl concentration of up to 5.0 M and reported similar results. However, changes in experimental conditions may cause major differences in results, as shown in the work of Rude and Puchelt [19], who used flow injection analysis (FIA)–HG–AAS and HCl concentrations of up to 5.0 M, and found the following: 1. at HCl concentration of \ 1.0 M, they obtained an increasing response with a linear function (r=0.995) for As(III) which disagrees with our and the findings of others [18,20,29], in which As(III) showed a constant response. This increasing linear response is most probably caused by incomplete generation of arsine, which is due to the use of a low KBH4 concentration (0.2%). As reported below, results obtained by us when using low NaBH4 concentrations are similar to those of Rude and Puchelt [19]; 2. the low KBH4 concentration used by Rude and Puchelt [34] yielded a negligible As(V) absorbance across the whole acid concentration range; 3. MMA has an almost identical response to DMA over the whole acid range (0.0–5.0 M) covered by the study of Rude and Puchelt [19], and both MMA and DMA showed a negligible response in 2.0 M and higher HCl concentrations.
A. Shraim et al. / Talanta 50 (1999) 1109–1127
Further examination of Fig. 1, shows the following: 1. the sharp increase in the absorption signals of the arsenic species, especially As(III), MMA, and DMA, with increase in the acid concentration over the low range (0.1 – 0.6 M), suggests that an investigation into the use of higher NaBH4 concentrations may be warranted to determine whether this condition might increase the signal of As(V) and produce identical responses from all four arsenic species and thus enable the determination of total arsenic; 2. at low acid concentration (0.01 – 0.1 M), DMA shows higher signals when compared to the other three species. An investigation into the use of NaBH4 concentrations seemed appropriate and suggested that a low acid and low NaBH4 concentrations could result in a condi-
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tion where all species signals, except that of DMA, are reduced to negligible values, thus leading to the speciation of DMA in the presence of the other three species, 3. the observation that the DMA and MMA response signals decrease to negligible levels at high HCl concentrations whilst As(III) and As(V) essentially provide a constant positive response prompted another investigation where an As(III) speciation might be possible, with the As(III) signal retained, by use of low NaBH4 concentration at high acid concentrations. To address the above-mentioned points the effect of NaBH4 concentration over the range 0.02– 2.0% when using low (0.005–0.1 M) and moderate to high acid concentrations (2.0–8.0 M) has been investigated.
Fig. 1. Effect of HCl concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.6% NaBH4, in the absence of L-cysteine.
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Fig. 2. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.06 M HCl (A) and 0.10 M HCl (B) in the absence of L-cysteine.
3.2. Effect of NaBH4 concentration in the absence of L -cysteine 3.2.1. Use of low concentrations of HCl The effect of using 0.1 – 2.0% NaBH4 and low acid concentrations (0.06 and 0.1 M) on the absorption signals of the four arsenic species is shown in Fig. 2. The use of NaBH4 concentrations up to 2.0% failed to produce identical signals from the four arsenic species, and thus under these conditions, determination of a total arsenic signal is not possible. Further increase in NaBH4 concentrations up to 4.0% was found to cause large instability in the signals, and could not be used to check possible increases in signal responses. This instability may be caused by either or both of the following two reasons: 1. it was observed that the preparation of NaBH4 solutions with concentrations \ 1.0% results in cloudy solutions (an indication of undissolved particles) which settle down with time and become clear. The settled particles tend to dissolve when mixed with acid solution inside reaction tubes, producing a non-homogeneous
reaction mixture with localised cells of NaBH4 concentrations thus causing the signal instability. Filtering the solution, or leaving it to settle and withdrawing clear portions partially reduced this instability; 2. The use of high concentrations of NaBH4 may results in a vigorous production of arsines, as well as the production of large quantities of H2 gas, where both can cause large signal instability. Several droplets of solution were observed at the top of the gas-liquid separator when using high NaBH4 concentrations, which support this assumption. It is also clear from Fig. 2 that the use of NaBH4 concentrations of B 0.6% has resulted in a decrease in the signals of the four arsenic species. Fig. 2A and B show that DMA still yields a relatively large signal, whereas the signals of the other three species are reduced to lower values. Nevertheless the signals of As(III), As(V), and MMA effectively result in a total interference of 60–70% in the DMA signal. In order to reduce this interference, the use of lower acid concentrations has been investigated; results are shown in Fig. 3.
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Fig. 3A and B show that the use of low NaBH4 concentrations, when using 0.03 and 0.01 M acid, resulted in a high DMA signal with low interference from As(III) and negligible signals for both As(V) and MMA. On the other hand, at a NaBH4 concentration of 2.0% in 0.01 M HCl (Fig. 3B), DMA showed a reduced response, but with no interference from the other three species. Further decrease in acid concentration to 0.005 M eliminated all the inter-
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ferences and produced significant signals from DMA at NaBH4 concentrations of B0.6% (Fig. 3C). To eliminate the interference of As(III) in the DMA signal when using 0.01 M acid (see Fig. 3B), the use of NaBH4 concentrations of lower than 0.1% has been investigated; as shown in Fig. 3D, this approach has eliminated the interference of As(III) in the DMA signal when NaBH4 concentrations of 0.02–0.075% are used.
Fig. 3. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using A: 0.03; B: 0.01; C: 0.005 and D: 0.01 M HCl in the absence of L-cysteine.
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Fig. 4. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using A: 2; B: 4; C: 6; and D: 8 M HCl in the absence of L-cysteine.
The use of 0.005 M HCl and 0.1% NaBH4 in the absence of L-cysteine has produced excellent results for the speciation of DMA in a mixture containing all four arsenic species (Table 3A). On the other hand the use of 0.07 and 2.0% NaBH4 with 0.01 M HCl yields very large errors. Although the other three arsenic species show no signals when analysed separately (see Fig. 3B and D), they yielded a combined interference of 27.3
and 56.3% when NaBH4 concentrations of 0.07 and 2.0%, respectively were used.
3.2.2. Use of moderate to high concentrations of HCl The effect of using 0.02–2.0% NaBH4 and moderate to high acid concentrations (2, 4, 6, and 8 M) on the absorption signals of the four arsenic species is shown in Fig. 4; the results indicate that the use of NaBH4 concentrations of 50.1% at any acid concentration produces high signals from
Table 3 Experimental conditions for speciation of arsenic in mixtures containing the four arsenic species Speciesa
A B
DMA As(III)
C
TAs
D E
DMA MMA a
[HCl], M 0.005 4.0 6.0 0.01 0.01 1.0 4.0
% L-cysteine
Tb
%NaBH4
As usedc
Nd
Avg
0.0 0.0 0.0 4.0 5.0 4.0 0.4
N/A N/A N/A 10 5 4 30
0.1 0.05 0.05 1.0 2.0 0.6 0.03
80 80 80 40 40 80 80
7 7 8 3 7 5 4
20.2 20.0 19.7 39.6 40.8 21.7 20.4
e
%Errf
dg
Equationh
R 2i
LWRj
DLk
0.8 0.1 –1.6 –1.0 1.9 8.7 2.2
0.46 0.59 0.36 0.82 0.72 0.56 0.56
y= 0.005408x y= 0.005587x y= 0.009332x y= 0.030760x y= 0.032440x y= 0.012853x y= 0.005693x
0.9987 0.9928 0.9963 0.9986 0.9913 0.9961 0.9952
0–40 0–80 0–40 0–20 0–10 0–20 0–80
0.9 0.6 0.8 1.0 1.1 0.5 1.8
Species for which analysis was undertaken. Contact time after which the first reading was taken. c Total arsenic in the mixture solution, equal concentrations of each of the four arsenic species were added (ppb). d Number of readings taken for each speciation analysis. e Average concentration of arsenic species found (ppb). f Percentage error from the average. g S.D. (ppb). h Equation of calibration curve. i R 2 value of calibration curve. j Linear working range of calibration curve (ppb). k Detection limit (ppb) calculated from three times the S.D. of the replicated blanks. b
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As(III) only and negligible signals from the other three species. The increase in acid concentration from 2 to 8, when using NaBH4 concentrations of 0.02–2.0%, slightly increased the As(V) signal, but has little effect on the As(III) signal, while the signals of the organic arsenicals decrease with increase in the acid concentration. From the results shown in Fig. 4, it appears likely that a signal for As(III) only can be obtained when acid concentrations of 2 – 8 M and low concentrations of NaBH4 are used. To check this, the analysis of arsenic solution mixtures using 4 and 6 M HCl and 0.05% NaBH4 has been carried out and found to yield excellent results (see Table 3B). Controlling the concentrations of acid and NaBH4 in the absence of L-cysteine has resulted in methods for the speciation of DMA and As(III). To find methods for the speciation of the other species and the analysis of total arsenic, the use of L-cysteine has been introduced.
3.3. Effect of HCl concentration when using 0.4% L -cysteine, 0.6% NaBH4 solutions, and constant contact time The effect of HCl concentration (0.001–9.2 M) and 0.6% NaBH4, in the presence of 0.4% L-cysteine, on the absorption signals of the four arsenic species is shown in Fig. 5. The presence of L-cysteine produces a very rapid increase in the signals for the four arsenic species with increase in acid concentration over a lower and narrower range of 0.001–0.03 M HCl compared to the signals obtained in the absence of L-cysteine (see Fig. 1). The four species showed similar maxima at an acid concentration 0.03–0.06 M with very similar absorption signals for As(III), As(V), and DMA and a lower signal for MMA (Fig. 5B). As discussed later, improvements in the MMA signal and production of similar signals from all four species have been achieved by using more concentrated solutions of L-cysteine and NaBH4 allowing the determination of total arsenic content.
Fig. 5. Effect of HCl concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.6% NaBH4 and 0.4% L-cysteine after a contact time of 2 h.
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Chen et al. [23] used a HG – DCP –AES to study the effect of low concentrations of HCl and HNO3 (0.002 – 0.1 M) on the arsine generation from As(III) and As(V) only, in the absence and the presence of L-cysteine. In agreement with our findings, they reported that the As(III) signal, in the presence of L-cysteine, was increased by more than 75% when using an acid concentration of :0.02 M. They also reported that the reduction of As(V) to As(III) in the presence of L-cysteine is slow and time-dependent. Our results showed identical responses from both As(III) and As(V) over the entire HCl concentration range (0.001– 9.2 M) in the presence of 0.4% L-cysteine. Le et al. [20] have also studied the effect of HCl concentration (\0.0 – 4.0 M) on the arsine generation from As(III), As(V), MMA, and DMA in the presence and the absence of many pre-reducing agents, such as L-cysteine and methionine, using a FIA – HG – AAS with 2.5% NaBH4. They obtained maximum and identical responses from the four arsenic species when using 0.3–0.7 M HCl after 10 – 20 min of contact with 2.5% L-cysteine. These results are different to ours and to the work of Anderson et al. [18] in which very similar responses from the four arsenic species were obtained when using much lower and narrower acid concentration ranges of 0.01 – 0.03 M HCl and 0.06–0.1 M mercaptoacetic acid, respectively. Reasons behind these differences are unclear, but the use of high concentrations of L-cysteine and NaBH4 may be responsible. Also, the reported results for the determination of MMA and DMA in acid concentration above 1 M are inconsistent; thus Le et al. [20] found that DMA gave a higher signal than MMA, whereas our results show the opposite. The increase in the As(III) and As(V) signals at \ 2 M acid was also much slower than that found by us. The use of high concentrations of L-cysteine and NaBH4 by Le et al. [20] may be responsible for these differences. Further examination of Fig. 5 shows the following: 1. the use of high acid concentrations of ]6.0 M produces similar signals from all species, except DMA, which showed a lower signal; we have been unsuccessful in our attempts to increase the DMA signal,
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2. obtaining a signal for only DMA at an acid concentration of : 0.6 M is possible if the interferences of MMA, As(III), and As(V) signals are eliminated, 3. obtaining an MMA signal at an acid concentration of 2–3 M is also possible, if the interference of the other three species is also eliminated, 4. the presence of L-cysteine produces similar signals from both As(III) and As(V) over the whole acid concentration range, supporting the proposed mechanism shown in Eqs. (4)– (7). Based on these observations, we proceeded to study the effect of using NaBH4 and L-cysteine concentrations other than 0.6 and 0.4%, respectively in 0.01, 0.6, and \ 2.0 M HCl.
3.4. Effect of NaBH4 concentration 3.4.1. Effect of NaBH4 concentration on obtaining a total arsenic signal when using 0.01 M HCl, 2.5% L -cysteine, and constant contact time The effect of NaBH4 concentration when using 0.01 M HCl, 2.5% L-cysteine over a contact time of 50–70 min is shown in Fig. 6. The MMA signal has been enhanced and similar signals from the four arsenic species have been produced, when a minimum NaBH4 concentration of 0.6% is employed; this allows for determination of a total arsenic signal. A long contact time has been used to make sure that the reduction of arsenic(V) species to arsenic(III) analogues, and subsequent generation of arsine is complete. Fig. 6 also indicates that increase in NaBH4 concentration up to 0.6% results in sharp increase in absorption signals of all four arsenic species; no more increase is observed with increase in NaBH4 concentration beyond 0.6%. 3.4.2. Effect of NaBH4 concentration on obtaining a sole signal for DMA when using 0.6 M HCl, 0.4% L -cysteine, and constant contact time Fig. 5 indicates that at an acid concentration of : 0.6 M, obtaining a signal for only DMA may be possible if the signals for the other three spe-
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Fig. 6. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.01 M HCl and 2.5% L-cysteine after a contact time of 50 – 70 min.
cies are reduced to negligible values. Based on this, the effect of NaBH4 concentrations of 0.02– 2.0% and 0.4% L-cysteine at a contact time of about 2 h has been investigated as shown in Fig. 7. It is evident that a DMA signal, although low, can be obtained if NaBH4 concentrations between 0.05 and 0.1% are employed. The use of NaBH4 concentrations of \0.6% will significantly increase the signals of the other species and therefore increase their interference with the DMA signal.
3.4.3. Effect of NaBH4 concentration on obtaining a sole MMA signal when using low to medium HCl concentrations, 0.4% L -cysteine, and constant contact time The effect of NaBH4 concentrations of 0.02– 2.0% and 0.4% L-cysteine at a contact time of about two h when using 1.5 – 6.0 M acid, has been investigated as shown in Fig. 8. The use of low concentrations of NaBH4 ( B0.03%) when using 3–4 M acid (Fig. 8B and C) produces a high
signal only from MMA with negligible interference from the other three species. The use of an acid concentration of B 3 M (Fig. 8A) has increased the interference caused by DMA, while the use of acid concentrations of \ 4 M (Fig. 8D) has increased the As(III) and As(V) interferences with the MMA signal.
3.5. Effect of contact time 3.5.1. Effect of contact time on obtaining a total arsenic signal when using 0.01 M HCl, 4.0% L -cysteine, and 1.0% NaBH4 In an attempt to find a short analysis time for determination of total arsenic signal when using the experimental conditions described in Section 3.4.1, but with an L-cysteine concentration increased to 4%, the effect of contact time of 1–20 min on the absorption signals of solutions containing single arsenic species was investigated. It was found that a minimum of 5–10 min was needed before obtaining similar signals from all
A. Shraim et al. / Talanta 50 (1999) 1109–1127
four species. It was also noticed that signals of all species were largely increased with increase in contact time from 1 to 5 min, except for As(III), where the signal reached a maximum from the beginning and remained so until the end of the analysis time. To confirm these results, the analysis of total arsenic in a solution mixture containing 40 ppb as total arsenic (10 ppb each of the four species) using the above-mentioned experimental conditions was investigated. The reduction of arsenic species was found to be complete after 8 – 10 min under these experimental conditions, and very good results for total arsenic were achieved. However the use of higher concentrations of L-cysteine (5%) and NaBH4 (2.0%) reduces the reduction time to 5 min; very good results for total arsenic were also obtained (Table 3C). Therefore the use of 0.01 M HCl under these experimental conditions can be used for the determination of total arsenic.
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Low concentrations of HCl and HNO3 in the presence L-cysteine, have been used in literature for the determination of total arsenic content [20,22–24], and two techniques similar to ours have been employed in these studies e.g. FIA– HG–AAS [20,22] and HG–Direct Current Plasma–AES [23,24]. Le et al. [20] allowed 10–20 min as a contact time between the sample and L-cysteine when using FIA–HG–AAS for the determination of total arsenic in urine samples. For each 10 ml sample in 2% L-cysteine, the concentration and the flow rate of the HCl and the NaBH4 were 0.5 M and 3.4 ml min − 1, and 0.65 M and 3.4 ml min − 1, respectively. In comparison, our results have shorter contact time (5–10 min) when using lower acid concentration (0.01 M), and NaBH4 concentrations of 1.0–2.0% at a higher L-cysteine concentration (4.0%). In the study by Yin et al. [22], the contact time allowed for the determination of total inorganic arsenic when using FIA–HG–AAS was much longer than ours; they found that the reduction of
Fig. 7. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.60 M HCl and 0.40% L-cysteine after a contact time of 2 h.
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Fig. 8. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using A: 1.5; B: 3; C: 4 and D: 6 M HCl and 0.4% L-cysteine after a contact time of 2 h.
As(V) (500 ml of 2 ppb As) to As(III) was completed within 60, 40, and 20 min when using 0.04, 0.08, and 1.6 M, respectively of L-cysteine in 0.024 M HCl (or 0.029 M HNO3) and 0.5% NaBH4. Flow rates of acid and NaBH4 were 7.8 and 5.4 ml min − 1, respectively. In the present study, the As(V) reduction was completed in B10 min compared to 40 min in their work. In the other two studies [23,24], the reduction of As(V) required a longer time to complete, but
when the sample and L-cysteine were boiled for a short time before the introduction of NaBH4, the reduction was immediate. In the work of Chen et al. [23], 0.01 M HCl (or HNO3) has been employed to determine the total inorganic arsenic using HG–DCP–AES. The As(V) content was determined as follows: 1 ml of 2% L-cysteine was injected into a 5 ml sample (the final L-cysteine concentration was 0.33%), followed by 0.5% NaBH4. No As(V) signal was
A. Shraim et al. / Talanta 50 (1999) 1109–1127
detected when the NaBH4 was added just after the addition of L-cysteine, but when the reaction was given 5 min before the addition of NaBH4, a small signal was detected, which increased with increase in contact time. The As(V) reduction was complete within 35, 60, and 135 min when 1.0, 0.5, and 0.25 g, respectively of L-cysteine were added to 100 ml of 50 ppb As(V) when using 0.02 M acid. But when the solution was heated in boiling water for 5 min, As(V) was completely reduced to As(III). We have been able to obtain complete reduction of As(V) at room temperature in a much shorter time ( B10 min) when using 4.0% L-cysteine. In the other study, Brindle et al. [24] have achieved an immediate and complete reduction of As(V) to As(III) after mixing the sample solution with L-cysteine and NaBH4 under the following conditions: in a continuous flow HG – DCP–AES the sample (10 ml min − 1), L-cysteine (0.7%, 1.6 ml min − 1), and HNO3 (0.02 M, 10 ml min − 1) were mixed and heated, online to 95–98°C,
1123
cooled to room temperature, and reacted with NaBH4 (0.5%, 1.6 ml min − 1). It is evident from the last two studies [23,24], that the introduction of a heating step has eliminated any delay and spontaneously and completely produced arsine from As(V). We will consider this in future work.
3.5.2. Effect of the contact time on obtaining a DMA signal when using 0.6 M HCl, 4.0% L -cysteine, and 0.1 and 0.6% NaBH4 Use of 0.6% NaBH4 and applying the same experimental conditions of Section 3.4.2 (0.6 M HCl, 0.4% L-cysteine), results in negligible signals from all species including DMA, even after the application of contact time of 40 min. As a result the L-cysteine concentration was increased to 4.0% to obtain a high DMA signal within reasonable contact times. The effect of contact time, under the new experimental conditions is shown in Fig. 9, which shows high DMA signals with negligible signals from the other three species up
Fig. 9. Effect of contact time on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.60 M HCl, 4.0% L-cysteine, and 0.60% NaBH4.
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Fig. 10. Effect of contact time on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 4.0 M HCl, 0.03% NaBH4, and A: 0.4% and B: 1% L-cysteine.
to a contact times B 18 min. It is also clear from Fig. 9 that a maximum of 12 min (the lowest time tried) can be safely used to obtain a signal for DMA only, in the presence of the other three species; shorter times can also be used. Even though no interferences from the other three species on the DMA signals were found when solutions of single arsenic species were analysed (see Fig. 9), the use of 0.6 M HCl, 0.6% NaBH4, and 4.0% L-cysteine for the analysis of a mixture of the four arsenic species has produced huge errors (70%) in the DMA analysis. The increase in HCl concentration to 1 M, under the above experimental conditions, has significantly reduced the interferences and produced very good results after a contact time of only 4 min (Table 3D). By comparison, the use of 0.005 M HCl, 0.1% NaBH4 in the absence of L-cysteine has produced better results with an error of only 0.81%. However, the analysis of actual environmental samples will decide which method is more appropriate for DMA speciation.
3.5.3. Effect of contact time on obtaining a sole MMA signal when using 4.0 M HCl, 0.2, 0.4, and 1.0% L -cysteine, and 0.03% NaBH4 Attempts to study the effect of contact time on speciation of MMA was undertaken using 4.0 M HCl, 0.4 and 1.0% L-cysteine, and 0.03% NaBH4, and results are shown in Fig. 10. Use of 0.2% L-cysteine after a contact time of 20 min was also examined and found to result in negligible signals from all species. Increasing the concentration of L-cysteine to 0.4% results in a linearly increasing signal for MMA with increase in contact time from 1.8 to 35 min, while the other three species exhibited negligible signals (Fig. 10A). Therefore, under these conditions, an MMA signal can be obtained with good intensity and little interference from the other three species after a contact time of around 20 min. However, the sole MMA signal may also be obtained in a shorter contact time, but with less intensity. On the other hand, the use of L-cysteine concentrations of \0.4% (1.0% as shown in Fig. 10B), under the abovementioned conditions, yields a higher MMA sig-
A. Shraim et al. / Talanta 50 (1999) 1109–1127
nals over shorter contact times; however the DMA signal starts to appear after 6 min, and consequently interferes with the MMA signal. Therefore, the best conditions, under these circumstances, for obtaining a sole signal for MMA with minimal interference from the other three species, are 4.0 M HCl, 0.03% NaBH4, and 0.4% L-cysteine after any contact time between 10 and 35 min. The speciation of MMA in a solution mixture containing all four arsenic species has been achieved by using 4 M HCl, 0.4% L-cysteine, and 0.03% NaBH4, but a minimum contact time of 30 min should be provided to obtain good reproducible results (see Table 3E).
3.6. Calibration cur6es Using appropriate conditions developed in this study, five different calibration curves were constructed for the speciation analysis. Arsenic species used to construct a calibration curve were the same ones for which the speciation analysis was
1125
undertaken. Although any species could be used for TAs analysis, As(III) was used in this study.
3.7. Analysis of en6ironmental samples Two environmental water samples were obtained from Coen dam, Queensland, Australia; a dam close to gold mining activities. The first sample was taken before water treatment purification (BWT), while the other one was taken after treatment (AWT). Using experimental conditions shown in Table 3, five sub-samples of each water sample were analysed; the first was used for the analysis of TAs in the presence of L-cysteine, the second two sub-samples were used for the speciation of DMA and As(III) in the absence of L-cysteine, and the last two sub-samples were used for the speciation of DMA and MMA in the presence of L-cysteine. Details of the results and experimental conditions applied to obtain the results are summarised in Table 4. The AWT water sample was found, as expected, to contain much less TAs when compared
Table 4 Results and details of experimental conditions used for the analysis of water samples Sample ID
BWTf AWTg BWTf AWTg BWTf AWTg BWTf AWTg BWTf AWTg BWTf AWTg a
Analysis
[HCl]
%L-cyst
T a(, min
%NaBH4 Arsenic (ppb)
TAs
0.01
5.0
5
2.0
DMA
0.005
0.0
N/A
0.1
As(III)
4.0
0.0
N/A
0.05
DMA
1.0
4.0
5
0.6
MMA
4.0
0.4
45
0.03
As(V)h
Foundb
Addedc
Totald
%Recoverye
38.9 2.8 0.8 4.9 0.2 0.0 0.3 0.0 1.6 0.1 36.8 2.7
8.0 16.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0
47.5 19.1 18.3 25.4 21.3 21.9 21.0 20.6 23.8 22.8
107.5 101.9 87.5 102.5 105.5 109.5 103.5 103.0 111.0 113.5
Contact time (min). Total arsenic concentration found (ppb) in unspiked sample. c Total arsenic concentration added (ppb) to spike the sample, equal quantities of each of the four arsenic species were added. d Total arsenic concentration found (ppb) in spiked sample. e %Recovery of added spike. f Water sample before water treatment. g Water sample after water treatment. h Calculated by difference, i.e. As(V) = [TAs−{As(III)+MMA+DMA}]. b
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to the BWT sample; TAs concentration of 2.8 ppb was found which represents the arsenic residue left after water treatment. Two methods for the speciation of DMA were applied; the first one in the absence of L-cysteine, while the second method was in the presence of L-cysteine. The first method yielded a DMA value of 0.8 ppb for the BWT sample, while a much larger value of 4.9 ppb was obtained for the AWT sample. The second method gave DMA values of 0.3 and 0.0 ppb for BWT and AWT samples respectively. The DMA value obtained for the AWT sample when using the first method appears to be unrealistically high and was rejected. As expected, As(V) was the main species found in both samples; its concentration ]95% of TAs. The pH value of each sample was found to be :6.5, and as the dominancy of the inorganic arsenic species in natural and ground waters is controlled by the pH values and the oxidising or reducing conditions of such waters, it would be expected that As(V) would be the most dominant species in oxygenated natural waters, as it is the most thermodynamically stable species under these conditions [30–33]. Results for other natural water samples, which have been analysed for their arsenic species concentration by various workers, confirm these predictions; As(V) was found to be the predominant species in these studies [30,34– 36] with values of greater than 90% of total arsenic. Initial attempts to assess the accuracy of the methods developed in this work were undertaken using the standard addition method. Table 4 indicates that very good recoveries were obtained (101.9 – 113.5%); equal concentrations from all four arsenic species were added to each of the two water samples, ie. for the speciation of As(III), 20 ppb of each of the four species was added to give a total arsenic concentration of 80 ppb. From these samples 21.1 and 21.9 ppb As(III) for the BWT and AWT samples, respectively were recovered; these results represent percentage recoveries of 105.5 and 109.5% for the BWT and AWT samples, respectively.
4. Conclusions The results reported in this paper, for the analyses and speciation of arsenic using methods developed in this work employing the selective-reduction–HG–AAS technique, show that these analyses can be quickly and accurately undertaken using this simple and inexpensive technique. The only exception is the analysis of MMA, which needs a minimum of 30–45 min to provide reliable results. The reduction of contact time in case of MMA may be achieved if different suitable experimental conditions such as a heating step are introduced to this system. There is no doubt that a much more comprehensive analysis program of environmental samples from a wide variety of sources has to be undertaken before the usefulness of these methods can be accurately assessed. Acknowledgements The authors would like to thank Claire Moore and Ron Sumner of the Queensland Health Scientific Services for their help in operating the HGAAS. References [1] J.P. Gustafsson, G. Jacks, Appl. Geochem. 10 (1995) 307. [2] A.R. Marin, P.H. Masscheleyn, W.H. Patrick, Plant Soil 152 (1993) 245. [3] J.R. Abernathy, Role of Arsenical chemicals in agriculture, in: W.H. Lederer, R.J. Fentsterheim (Eds.), Arsenic: Industrial, Biomedical, Environmental Perspective’s, VNR, New York, 1983, pp. 57 – 62. [4] K. Ringwood, Arsenic in the Gold and Base-Metal Mining Industry, Australian Minerals and Energy Environment Foundation (AMEEF), Melbourne, Australia, 1995, pp. 1 – 33. [5] M. Piscator, Life Sci. Res. Rep 33 (1986) 59. [6] M. Buat-Menard, P.J. Peterson, M. Havas, E. Steinnes, D. Turner, Group Report: Arsenic, in: T.C. Hutchinson, K.M. Meema (Ed.), Lead, Mercury, Cadmium Arsenic in the Environment, SCOPE 31, Pub. John Wiley and Sons, Chichester, England, 1987, pp. 43 – 48. [7] G. Stohrer, Arch. Toxicol. 65 (1991) 525. [8] C. Hopenhayn-Rich, M.L. Biggs, A.H. Smith, D.A. Kalman, L.E. Moore, Environ. Health Perspect. 104 (1996) 620.
A. Shraim et al. / Talanta 50 (1999) 1109–1127 [9] J. Beard, L. Cruces, New Sci. 28 (1998) 10. [10] R.W. Whitcare, C.S. Pearse, Miner. Ind. Bull. 17 (1974) 1. [11] G.M.P. Morrison, G.E. Batley, T.M. Florence, Chem. Brit. 25 (1989) 791. [12] B. Amran, F. Lagrade, M.J.F. Leroy, A. Lamotte, M. Olle, M. Albert, G. Rauret, J.F. Lopez-Sanchez, Tech. Instr. Anal. Chem. 17 (1995) 285. [13] W.R. Cullen, K.J. Reimer, Chem. Rev. 89 (1989) 713. [14] J.M. Wood, Science 183 (1974) 1049. [15] S. Caroli, F.L. Torre, F. Petrucci N. Violante, Arsenic speciation and Health Aspects, in: S. Caroli (Ed.), Element Speciation in Bioinorganic Chemistry, John Wiley and Sons, New York, 1996, pp. 445–463. [16] A. Hinwood, R. Bannister, A. Shugg, M. Sim, Water 25 (4) (1998) 34. [17] R.S. Braman, C.C. Foreback, Science 182 (1973) 1247. [18] R.K. Anderson, M. Thompson, E. Culbard, Analyst 111 (1986) 1143. [19] T.R. Rude, H. Puchelt, Fresenius Z. Anal. Chem. 350 (1994) 44. [20] X.C. Le, W.R. Cullen, K.J. Reimer, Anal. Chim. Acta 285 (1994) 277. [21] I.D. Brindle, X.C. Le, Anal. Chem. 61 (1989) 1175.
.
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[22] X. Yin, E. Hoffmann, C. Ludke, Fresenius Z. Anal. Chem. 355 (1996) 324. [23] H. Chen, I.D. Brindle, X.C. Le, Anal. Chem. 64 (1992) 667. [24] I.D. Brindle, H. Alarabi, S. Karshman, X.C. Le, S. Zheng, H. Chen, Analyst 117 (1992) 407. [25] A.G. Howard, J. Anal. At. Spectrom. 12 (1997) 267. [26] J. Aggett, A.C. Aspell, Analyst 101 (1976) 341. [27] A.G. Howard, C. Salou, Anal. Chim. Acta 333 (1996) 89. [28] E. Hakala, P. Lauri, J. Anal. At. Spectrom. 7 (1992) 191. [29] P.H. Masscheleyn, R.D. Delune, W.H. Patrick, J. Envir. Qual. 20 (1991) 96. [30] Battelle, A Report: Speciation of Selenium and Arsenic in Natural Waters and Sediments. Vol. 2: Arsenic Speciation, No. EPRI EA-4641, Pacific Northwest Laboratories, Sequim, Washington, 1986. [31] L.E. Hunt, A.G. Howard, Mar. Pollut. Bull. 28 (1994) 33. [32] H. Hasegawa, Y. Sohrin, M. Matsul, M. Hojo, M. Kawashima, Anal. Chem. 66 (1994) 3247. [33] J. Stummeyer, B. Harazim, T. Wippermann, Fresenius Z. Anal. Chem. 354 (1996) 344. [34] P. Thomas, K. Sniatecki, J. Anal. At. Spectrom. 10 (1995) 616. [35] C.J. Hwang, S.J. Jiang, Anal. Chim. Acta 289 (1994) 205. [36] R.J.A. Van Cleuvenbergen, W.E. Van Mol, F.C. Adams, J. Anal. At. Spectrom. 3 (1988) 169.
Speciation of arsenic by hydride generation–atomic absorption spectrometry (HG–AAS) in hydrochloric acid reaction medium Amjad Shraim a,*, Barry Chiswell a, Henry Olszowy b a b
Department of Chemistry, The Uni6ersity of Queensland, St. Lucia, Qld 4072, Australia Queensland Health Scientific Ser6ices, 39 Kessels Rd., Coopers Plains, Qld 4108, Australia Received 31 March 1999; received in revised form 7 July 1999; accepted 15 July 1999
Abstract The effects on the absorbance signals obtained using HG – AAS of variations in concentrations of the reaction medium (hydrochloric acid), the reducing agent [sodium tetrahydroborate(III); NaBH4], the pre-reducing agent (L-cysteine), and the contact time (between L-cysteine and arsenic-containing solutions) for the arsines generated from solutions of arsenite, arsenate, monomethylarsonic acid (MMA), and dimethylarsenic acid (DMA), have been investigated to find a method for analysis of the four arsenic species in environmental samples. Signals were found to be greatly enhanced in low acid concentration in both the absence (0.03 – 0.60 M HCl) and the presence of L –cysteine (0.001–0.03 M HCl), however with L-cysteine present, higher signals were obtained. Total arsenic content and speciation of DMA, As(III), MMA, and As(V) in mixtures containing the four arsenic species, as well as some environmental samples have been obtained using the following conditions: (i) total arsenic: 0.01 M acid, 2% NaBH4, 5% L-cysteine, and contact time B 10 min; (ii) DMA: 1.0 M acid, 0.3 – 0.6% NaBH4, 4.0% L-cysteine, and contact time B5 min; (iii) As(III): 4–6 M acid and 0.05% NaBH4 in the absence of L-cysteine; (iv) MMA: 4.0 M acid, 0.03% NaBH4, 0.4% L-cysteine, and contact time of 30 min; (v) As(V): by difference. Detection limits (ppb) for analysis of total arsenic, DMA, As(III), and MMA were found to be 1.1 (n=7), 0.5 (n= 5), 0.6 (n = 7), and 1.8 (n =4), respectively. Good percentage recoveries (102– 114%) of added spikes were obtained for all analyses. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Arsenic; Arsenite; Arsenate; Monomethylarsonic acid; Dimethylarsenic acid; Speciation; L-cysteine; Hydride generation; Selective reduction
1. Introduction
* Corresponding author. Fax: +61-7-33653839. E-mail address: [email protected] (A. Shraim)
Arsenic, which is potentially toxic to humans, animals, and plants [1–4] and according to recent reports [5–8] may be carcinogenic to humans, occurs naturally in many chemical forms. Arsenic
0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 9 ) 0 0 2 2 1 - 0
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poisoning is a major public health problem, especially in countries such as Bangladesh, where the soil is high in arsenic compounds and the well water may contain as much as 300 – 4000 mg L − 1 of arsenic [9]. The toxicity of arsenic varies widely, ranging from highly hazardous inorganic arsenicals (arsine, arsenite, and arsenate) to relatively harmless organic species (monomethylarsonate and dimethylarsenate) [10 – 14]. Indeed some organoarsenicals, such as arsenobetaine and arsenocholine, are effectively non-toxic towards living organisms [15]. Therefore, determination of total arsenic content in a sample does not reflect the level of hazard of the element actually present, and it is increasingly important that the various forms of arsenic be determined in biological and environmental samples to provide a much clearer view of the risk associated with exposure to arsenic in the environment. Reported arsenic concentrations in some parts of Australia range up to 36 000 mg kg − 1 in surface soils and 300 mg kg − 1 in ground water [16]; the current ANZECC/ NHMRC arsenic guideline for soil is 100 mg kg − 1, while the NHMRC drinking water guideline has recently been reduced from 50 to 7 mg/l [16]. Speciation of arsenic in environmental samples usually involves several steps including derivitization, separation, and detection; hydride generation (HG), initially developed by Braman and Foreback in 1973 [17], coupled to one of several separation and detection systems has been found to be one of the most common techniques used for derivitization of arsenic. A number of detection systems have been used in arsenic analysis, of which the most popular and preferred one in terms of simplicity, sensitivity, precision, speed, and cost is AAS. Our aim in the work described here was to develop a simple, rapid, and inexpensive technique for the speciation of the most commonly occurring species in environmental samples, viz. As(III), As(V), monomethylarsonic acid (MMA), and dimethylarsenic acid (DMA) using hydride generation– atomic adsorption spectrometry (HG–AAS), i.e. a derivitization and detection technique only, without the need for a separation
step such as HPLC and cryogenic trapping, which we believe has caused many complications and resulted in lengthy analytical techniques. To compensate for the absence of a separation step in our method, a selective–reduction concept is employed to generate hydrides of each arsenic species. Critical evaluation of the existing literature, showed that some work has been done on the use of HG–AAS for the speciation of arsenic [18–21], and much of this work dealt with the use of thiol-containing ligands, such as L-cysteine, Lcystine, and thioglycerol, to obtain identical response from all four arsenic species. These thiols have been used as pre-reducing agents before the addition of NaBH4, and have been shown to enhance the arsine signals in low acid concentration and to reduce the effects of interferences [20,22–24]. In this study, the effect of HCl as a reaction medium on the arsine generation from the four arsenic species has been investigated. Control of the concentrations of the reaction medium (HCl), of the reducing and hydride generating agent (NaBH4), and of the pre-reducing agent (L-cysteine) when used, and the employment of HG– AAS has resulted in methods for the analysis and speciation of the four arsenic species in environmental samples.
2. Experimental
2.1. Equipment A Vapour Generation Accessory (VGA-76, Varian) connected to an Atomic Absorption Spectrometer (Spectra 300, Varian) was used in this study, and operated according to manufacturer’s instructions; instrument parameters used are summarised in Table 1. In this system, arsenic-containing solutions were pumped into a mixer and reacted with sodium tetrahydroborate(III) solution; generated arsines were swept to a gas–liquid separator using nitrogen gas and then to a heated T-shaped absorption cell.
A. Shraim et al. / Talanta 50 (1999) 1109–1127
2.2. Reagents and solutions 2.2.1. General All chemicals were of analytical-reagent grade unless otherwise specified. All glassware was soaked in 4 M HNO3 for a minimum of 12 h and washed with distilled water and finally rinsed with Milli-Q reagent water before use. All water used was obtained from a Milli-Q reagent system (Millipore), resistivity 18 MV cm. L-cysteine (minimum 98%, TLC) was obtained from Sigma (St. Louis, MO), sodium tetrahydroborate(III) form Merck and Alfa, As(III) atomic absorption standard solution (1 mg ml − 1) and As(V) (As2O5, 99.999%) from Acros, MMA (disodium methylarsenate, 99%) from Chem Service, and DMA (cacodylic acid, 98%) was obtained form Aldrich. Various concentrations of NaBH4 solution stabilised with NaOH were used; the concentration of NaOH was maintained at 0.5% in experiments where NaBH4 concentrations exceeded 0.6%. While both reagents were kept at the same concentration when NaBH4 was used at concentration levels B0.6%. 2.2.2. Arsenic stock solutions The arsenic stock solutions were prepared as follows: 1000 ppm As(III): arsenic(III) atomic absorption standard solution (1 mg ml − 1 As in 2% potassium hydroxide). Table 1 Operating conditions of the HG–AAS system Instrument mode
Absorbance
Calibration mode Measurement mode Slit width (nm) Slit height Wavelength (nm) Flame Sample introduction Delay time (s) Time constant Measurement time (s) Replicates Background correction Sample flow rate (ml min(1) NaBH4 flow rate (ml min(1)
Concentration Integration 0.5 Normal 193.7 Air–acetylene Normal 40 0.05 2.0 3 On 7 1
1111
1000 ppm As(V): 0.03835 g of arsenic(V) oxide (As2O5) was dissolved in a minimum volume of 4.0 M NaOH, neutralised by a same volume of 4.0 M HCl, and the final volume was adjusted to 25 ml with Milli-Q water. 1000 ppm MMA: 0.09844 g of disodium methylarsenate (CH3AsO(ONa)2 · 6H2O) was dissolved in 25 ml Milli-Q water. 1000 ppm DMA: 0.04699 g of cacodylic acid (CH3)2AsO(OH) was dissolved in 25 ml of Milli-Q water. Cacodylic acid is the commercial name for DMA, the acronym used in this work. The stock solutions of As(V), MMA, and DMA were prepared monthly and stored in glass volumetric flasks wrapped with aluminum foil and kept refrigerated at 4°C to prevent any change in speciation. The As(III) stock solution was also kept refrigerated at 4°C. All arsenic solutions were found to be stable under these conditions when tested after 1 month.
2.3. Analytical procedures Solutions were prepared as required by appropriate dilution of stock solutions and additions of the required volumes of hydrochloric acid and L-cysteine solutions to achieve the required concentrations. Solutions were then rapidly mixed. When using L-cysteine, measurement of contact time was commenced upon addition of L-cysteine and stopped at the beginning of the analysis. Throughout this study contact time refers to the time that has been allowed for L-cysteine to react with the arsenic-containing solutions before the commencement of the introduction of solutions to HG–AAS. 3. Results and discussion To understand the role of NaBH4 as a reducing and HG agent in the analysis of arsenic, the following mechanism has been proposed [25,26]: Rn As(O)(OH)3 − n + H+ + BH4− Rn As(OH)3 − n + H2O+ BH3 Rn As(OH)3 − n + (3− n)BH4− + (3− n)H+
(1)
Rn AsH3 − n + (3− n)BH3 + (3− n)H2O
(2)
A. Shraim et al. / Talanta 50 (1999) 1109–1127
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Table 2 Steps followed for the analysis of arsenic speciesa Steps Step Step Step Step Step a
1 2 3 4 5
HCl concentration
NaBH4 concentration
L-cysteine
V C V C C
C V C V C
0 0 C C C
concentration
Contact time NA NA C C V
V= varied; C = constant; 0 = zero; NA = not applicable.
BH3 +3H2O H3BO3 +3H2.
(3)
While in the presence of L-cysteine the proposed mechanism is shown below [23,25,27]: 2RSH +R*n AsO(OH)3 − n RS– SR +R*n As(OH)3 − n +H2O
(4)
R*n As(OH)3 − n +(3 −n) RSH R*n As(SR)3 − n +2H2O
(5)
R*n As(SR)3 − n +(3 −n)BH4− R*n AsH3 − n +(3 −n)BH3 +(3 −n)RS −
(6)
BH3 +3H2O H3BO3 +3H2.
(7)
We have used these proposed mechanisms to develop a method for the speciation of arsenic using the steps shown in the Table 2.
3.1. Effect of HCl concentration when using 0.6% NaBH4 solution in the absence of L – cysteine The effect of using 0.001 – 9.6 M HCl on the absorbance signals in the absence of L-cysteine is shown in Fig. 1. The absorption signals of As(III), MMA, and DMA sharply increase with increase in acid concentration over a range of 0.01 –0.10 M, while the increase in the As(V) absorption signal is much slower. Further increase in the acid concentration results in: (a) a sharp decrease in the DMA signal up to an acid concentration of 1.0 M, then a slow decrease to negligible values beyond 4.0 M, (b) a broad maximum for MMA over an acid concentration range of 0.2 – 1.0 M, and a slow decrease thereafter to a negligible signal at 9.60 M, (c) a slow increase in the As(III) signal up to an
acid concentration of 2.0 M and a very broad plateau afterwards, and (d) a much slower increase in the As(V) signal, when compared to the As(III) signal, up to an acid concentration of 4 M, and a broad plateau thereafter.Our results are similar to those obtained by workers who used similar instrumentation and analytical procedure. For example Anderson et al. [18] and Hakala and Pyy [28] used a HG–AAS and a HCl concentration of up to 5.0 M and reported similar results. However, changes in experimental conditions may cause major differences in results, as shown in the work of Rude and Puchelt [19], who used flow injection analysis (FIA)–HG–AAS and HCl concentrations of up to 5.0 M, and found the following: 1. at HCl concentration of \ 1.0 M, they obtained an increasing response with a linear function (r=0.995) for As(III) which disagrees with our and the findings of others [18,20,29], in which As(III) showed a constant response. This increasing linear response is most probably caused by incomplete generation of arsine, which is due to the use of a low KBH4 concentration (0.2%). As reported below, results obtained by us when using low NaBH4 concentrations are similar to those of Rude and Puchelt [19]; 2. the low KBH4 concentration used by Rude and Puchelt [34] yielded a negligible As(V) absorbance across the whole acid concentration range; 3. MMA has an almost identical response to DMA over the whole acid range (0.0–5.0 M) covered by the study of Rude and Puchelt [19], and both MMA and DMA showed a negligible response in 2.0 M and higher HCl concentrations.
A. Shraim et al. / Talanta 50 (1999) 1109–1127
Further examination of Fig. 1, shows the following: 1. the sharp increase in the absorption signals of the arsenic species, especially As(III), MMA, and DMA, with increase in the acid concentration over the low range (0.1 – 0.6 M), suggests that an investigation into the use of higher NaBH4 concentrations may be warranted to determine whether this condition might increase the signal of As(V) and produce identical responses from all four arsenic species and thus enable the determination of total arsenic; 2. at low acid concentration (0.01 – 0.1 M), DMA shows higher signals when compared to the other three species. An investigation into the use of NaBH4 concentrations seemed appropriate and suggested that a low acid and low NaBH4 concentrations could result in a condi-
1113
tion where all species signals, except that of DMA, are reduced to negligible values, thus leading to the speciation of DMA in the presence of the other three species, 3. the observation that the DMA and MMA response signals decrease to negligible levels at high HCl concentrations whilst As(III) and As(V) essentially provide a constant positive response prompted another investigation where an As(III) speciation might be possible, with the As(III) signal retained, by use of low NaBH4 concentration at high acid concentrations. To address the above-mentioned points the effect of NaBH4 concentration over the range 0.02– 2.0% when using low (0.005–0.1 M) and moderate to high acid concentrations (2.0–8.0 M) has been investigated.
Fig. 1. Effect of HCl concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.6% NaBH4, in the absence of L-cysteine.
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A. Shraim et al. / Talanta 50 (1999) 1109–1127
Fig. 2. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.06 M HCl (A) and 0.10 M HCl (B) in the absence of L-cysteine.
3.2. Effect of NaBH4 concentration in the absence of L -cysteine 3.2.1. Use of low concentrations of HCl The effect of using 0.1 – 2.0% NaBH4 and low acid concentrations (0.06 and 0.1 M) on the absorption signals of the four arsenic species is shown in Fig. 2. The use of NaBH4 concentrations up to 2.0% failed to produce identical signals from the four arsenic species, and thus under these conditions, determination of a total arsenic signal is not possible. Further increase in NaBH4 concentrations up to 4.0% was found to cause large instability in the signals, and could not be used to check possible increases in signal responses. This instability may be caused by either or both of the following two reasons: 1. it was observed that the preparation of NaBH4 solutions with concentrations \ 1.0% results in cloudy solutions (an indication of undissolved particles) which settle down with time and become clear. The settled particles tend to dissolve when mixed with acid solution inside reaction tubes, producing a non-homogeneous
reaction mixture with localised cells of NaBH4 concentrations thus causing the signal instability. Filtering the solution, or leaving it to settle and withdrawing clear portions partially reduced this instability; 2. The use of high concentrations of NaBH4 may results in a vigorous production of arsines, as well as the production of large quantities of H2 gas, where both can cause large signal instability. Several droplets of solution were observed at the top of the gas-liquid separator when using high NaBH4 concentrations, which support this assumption. It is also clear from Fig. 2 that the use of NaBH4 concentrations of B 0.6% has resulted in a decrease in the signals of the four arsenic species. Fig. 2A and B show that DMA still yields a relatively large signal, whereas the signals of the other three species are reduced to lower values. Nevertheless the signals of As(III), As(V), and MMA effectively result in a total interference of 60–70% in the DMA signal. In order to reduce this interference, the use of lower acid concentrations has been investigated; results are shown in Fig. 3.
A. Shraim et al. / Talanta 50 (1999) 1109–1127
Fig. 3A and B show that the use of low NaBH4 concentrations, when using 0.03 and 0.01 M acid, resulted in a high DMA signal with low interference from As(III) and negligible signals for both As(V) and MMA. On the other hand, at a NaBH4 concentration of 2.0% in 0.01 M HCl (Fig. 3B), DMA showed a reduced response, but with no interference from the other three species. Further decrease in acid concentration to 0.005 M eliminated all the inter-
1115
ferences and produced significant signals from DMA at NaBH4 concentrations of B0.6% (Fig. 3C). To eliminate the interference of As(III) in the DMA signal when using 0.01 M acid (see Fig. 3B), the use of NaBH4 concentrations of lower than 0.1% has been investigated; as shown in Fig. 3D, this approach has eliminated the interference of As(III) in the DMA signal when NaBH4 concentrations of 0.02–0.075% are used.
Fig. 3. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using A: 0.03; B: 0.01; C: 0.005 and D: 0.01 M HCl in the absence of L-cysteine.
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Fig. 4. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using A: 2; B: 4; C: 6; and D: 8 M HCl in the absence of L-cysteine.
The use of 0.005 M HCl and 0.1% NaBH4 in the absence of L-cysteine has produced excellent results for the speciation of DMA in a mixture containing all four arsenic species (Table 3A). On the other hand the use of 0.07 and 2.0% NaBH4 with 0.01 M HCl yields very large errors. Although the other three arsenic species show no signals when analysed separately (see Fig. 3B and D), they yielded a combined interference of 27.3
and 56.3% when NaBH4 concentrations of 0.07 and 2.0%, respectively were used.
3.2.2. Use of moderate to high concentrations of HCl The effect of using 0.02–2.0% NaBH4 and moderate to high acid concentrations (2, 4, 6, and 8 M) on the absorption signals of the four arsenic species is shown in Fig. 4; the results indicate that the use of NaBH4 concentrations of 50.1% at any acid concentration produces high signals from
Table 3 Experimental conditions for speciation of arsenic in mixtures containing the four arsenic species Speciesa
A B
DMA As(III)
C
TAs
D E
DMA MMA a
[HCl], M 0.005 4.0 6.0 0.01 0.01 1.0 4.0
% L-cysteine
Tb
%NaBH4
As usedc
Nd
Avg
0.0 0.0 0.0 4.0 5.0 4.0 0.4
N/A N/A N/A 10 5 4 30
0.1 0.05 0.05 1.0 2.0 0.6 0.03
80 80 80 40 40 80 80
7 7 8 3 7 5 4
20.2 20.0 19.7 39.6 40.8 21.7 20.4
e
%Errf
dg
Equationh
R 2i
LWRj
DLk
0.8 0.1 –1.6 –1.0 1.9 8.7 2.2
0.46 0.59 0.36 0.82 0.72 0.56 0.56
y= 0.005408x y= 0.005587x y= 0.009332x y= 0.030760x y= 0.032440x y= 0.012853x y= 0.005693x
0.9987 0.9928 0.9963 0.9986 0.9913 0.9961 0.9952
0–40 0–80 0–40 0–20 0–10 0–20 0–80
0.9 0.6 0.8 1.0 1.1 0.5 1.8
Species for which analysis was undertaken. Contact time after which the first reading was taken. c Total arsenic in the mixture solution, equal concentrations of each of the four arsenic species were added (ppb). d Number of readings taken for each speciation analysis. e Average concentration of arsenic species found (ppb). f Percentage error from the average. g S.D. (ppb). h Equation of calibration curve. i R 2 value of calibration curve. j Linear working range of calibration curve (ppb). k Detection limit (ppb) calculated from three times the S.D. of the replicated blanks. b
A. Shraim et al. / Talanta 50 (1999) 1109–1127
No.
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As(III) only and negligible signals from the other three species. The increase in acid concentration from 2 to 8, when using NaBH4 concentrations of 0.02–2.0%, slightly increased the As(V) signal, but has little effect on the As(III) signal, while the signals of the organic arsenicals decrease with increase in the acid concentration. From the results shown in Fig. 4, it appears likely that a signal for As(III) only can be obtained when acid concentrations of 2 – 8 M and low concentrations of NaBH4 are used. To check this, the analysis of arsenic solution mixtures using 4 and 6 M HCl and 0.05% NaBH4 has been carried out and found to yield excellent results (see Table 3B). Controlling the concentrations of acid and NaBH4 in the absence of L-cysteine has resulted in methods for the speciation of DMA and As(III). To find methods for the speciation of the other species and the analysis of total arsenic, the use of L-cysteine has been introduced.
3.3. Effect of HCl concentration when using 0.4% L -cysteine, 0.6% NaBH4 solutions, and constant contact time The effect of HCl concentration (0.001–9.2 M) and 0.6% NaBH4, in the presence of 0.4% L-cysteine, on the absorption signals of the four arsenic species is shown in Fig. 5. The presence of L-cysteine produces a very rapid increase in the signals for the four arsenic species with increase in acid concentration over a lower and narrower range of 0.001–0.03 M HCl compared to the signals obtained in the absence of L-cysteine (see Fig. 1). The four species showed similar maxima at an acid concentration 0.03–0.06 M with very similar absorption signals for As(III), As(V), and DMA and a lower signal for MMA (Fig. 5B). As discussed later, improvements in the MMA signal and production of similar signals from all four species have been achieved by using more concentrated solutions of L-cysteine and NaBH4 allowing the determination of total arsenic content.
Fig. 5. Effect of HCl concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.6% NaBH4 and 0.4% L-cysteine after a contact time of 2 h.
A. Shraim et al. / Talanta 50 (1999) 1109–1127
Chen et al. [23] used a HG – DCP –AES to study the effect of low concentrations of HCl and HNO3 (0.002 – 0.1 M) on the arsine generation from As(III) and As(V) only, in the absence and the presence of L-cysteine. In agreement with our findings, they reported that the As(III) signal, in the presence of L-cysteine, was increased by more than 75% when using an acid concentration of :0.02 M. They also reported that the reduction of As(V) to As(III) in the presence of L-cysteine is slow and time-dependent. Our results showed identical responses from both As(III) and As(V) over the entire HCl concentration range (0.001– 9.2 M) in the presence of 0.4% L-cysteine. Le et al. [20] have also studied the effect of HCl concentration (\0.0 – 4.0 M) on the arsine generation from As(III), As(V), MMA, and DMA in the presence and the absence of many pre-reducing agents, such as L-cysteine and methionine, using a FIA – HG – AAS with 2.5% NaBH4. They obtained maximum and identical responses from the four arsenic species when using 0.3–0.7 M HCl after 10 – 20 min of contact with 2.5% L-cysteine. These results are different to ours and to the work of Anderson et al. [18] in which very similar responses from the four arsenic species were obtained when using much lower and narrower acid concentration ranges of 0.01 – 0.03 M HCl and 0.06–0.1 M mercaptoacetic acid, respectively. Reasons behind these differences are unclear, but the use of high concentrations of L-cysteine and NaBH4 may be responsible. Also, the reported results for the determination of MMA and DMA in acid concentration above 1 M are inconsistent; thus Le et al. [20] found that DMA gave a higher signal than MMA, whereas our results show the opposite. The increase in the As(III) and As(V) signals at \ 2 M acid was also much slower than that found by us. The use of high concentrations of L-cysteine and NaBH4 by Le et al. [20] may be responsible for these differences. Further examination of Fig. 5 shows the following: 1. the use of high acid concentrations of ]6.0 M produces similar signals from all species, except DMA, which showed a lower signal; we have been unsuccessful in our attempts to increase the DMA signal,
1119
2. obtaining a signal for only DMA at an acid concentration of : 0.6 M is possible if the interferences of MMA, As(III), and As(V) signals are eliminated, 3. obtaining an MMA signal at an acid concentration of 2–3 M is also possible, if the interference of the other three species is also eliminated, 4. the presence of L-cysteine produces similar signals from both As(III) and As(V) over the whole acid concentration range, supporting the proposed mechanism shown in Eqs. (4)– (7). Based on these observations, we proceeded to study the effect of using NaBH4 and L-cysteine concentrations other than 0.6 and 0.4%, respectively in 0.01, 0.6, and \ 2.0 M HCl.
3.4. Effect of NaBH4 concentration 3.4.1. Effect of NaBH4 concentration on obtaining a total arsenic signal when using 0.01 M HCl, 2.5% L -cysteine, and constant contact time The effect of NaBH4 concentration when using 0.01 M HCl, 2.5% L-cysteine over a contact time of 50–70 min is shown in Fig. 6. The MMA signal has been enhanced and similar signals from the four arsenic species have been produced, when a minimum NaBH4 concentration of 0.6% is employed; this allows for determination of a total arsenic signal. A long contact time has been used to make sure that the reduction of arsenic(V) species to arsenic(III) analogues, and subsequent generation of arsine is complete. Fig. 6 also indicates that increase in NaBH4 concentration up to 0.6% results in sharp increase in absorption signals of all four arsenic species; no more increase is observed with increase in NaBH4 concentration beyond 0.6%. 3.4.2. Effect of NaBH4 concentration on obtaining a sole signal for DMA when using 0.6 M HCl, 0.4% L -cysteine, and constant contact time Fig. 5 indicates that at an acid concentration of : 0.6 M, obtaining a signal for only DMA may be possible if the signals for the other three spe-
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A. Shraim et al. / Talanta 50 (1999) 1109–1127
Fig. 6. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.01 M HCl and 2.5% L-cysteine after a contact time of 50 – 70 min.
cies are reduced to negligible values. Based on this, the effect of NaBH4 concentrations of 0.02– 2.0% and 0.4% L-cysteine at a contact time of about 2 h has been investigated as shown in Fig. 7. It is evident that a DMA signal, although low, can be obtained if NaBH4 concentrations between 0.05 and 0.1% are employed. The use of NaBH4 concentrations of \0.6% will significantly increase the signals of the other species and therefore increase their interference with the DMA signal.
3.4.3. Effect of NaBH4 concentration on obtaining a sole MMA signal when using low to medium HCl concentrations, 0.4% L -cysteine, and constant contact time The effect of NaBH4 concentrations of 0.02– 2.0% and 0.4% L-cysteine at a contact time of about two h when using 1.5 – 6.0 M acid, has been investigated as shown in Fig. 8. The use of low concentrations of NaBH4 ( B0.03%) when using 3–4 M acid (Fig. 8B and C) produces a high
signal only from MMA with negligible interference from the other three species. The use of an acid concentration of B 3 M (Fig. 8A) has increased the interference caused by DMA, while the use of acid concentrations of \ 4 M (Fig. 8D) has increased the As(III) and As(V) interferences with the MMA signal.
3.5. Effect of contact time 3.5.1. Effect of contact time on obtaining a total arsenic signal when using 0.01 M HCl, 4.0% L -cysteine, and 1.0% NaBH4 In an attempt to find a short analysis time for determination of total arsenic signal when using the experimental conditions described in Section 3.4.1, but with an L-cysteine concentration increased to 4%, the effect of contact time of 1–20 min on the absorption signals of solutions containing single arsenic species was investigated. It was found that a minimum of 5–10 min was needed before obtaining similar signals from all
A. Shraim et al. / Talanta 50 (1999) 1109–1127
four species. It was also noticed that signals of all species were largely increased with increase in contact time from 1 to 5 min, except for As(III), where the signal reached a maximum from the beginning and remained so until the end of the analysis time. To confirm these results, the analysis of total arsenic in a solution mixture containing 40 ppb as total arsenic (10 ppb each of the four species) using the above-mentioned experimental conditions was investigated. The reduction of arsenic species was found to be complete after 8 – 10 min under these experimental conditions, and very good results for total arsenic were achieved. However the use of higher concentrations of L-cysteine (5%) and NaBH4 (2.0%) reduces the reduction time to 5 min; very good results for total arsenic were also obtained (Table 3C). Therefore the use of 0.01 M HCl under these experimental conditions can be used for the determination of total arsenic.
1121
Low concentrations of HCl and HNO3 in the presence L-cysteine, have been used in literature for the determination of total arsenic content [20,22–24], and two techniques similar to ours have been employed in these studies e.g. FIA– HG–AAS [20,22] and HG–Direct Current Plasma–AES [23,24]. Le et al. [20] allowed 10–20 min as a contact time between the sample and L-cysteine when using FIA–HG–AAS for the determination of total arsenic in urine samples. For each 10 ml sample in 2% L-cysteine, the concentration and the flow rate of the HCl and the NaBH4 were 0.5 M and 3.4 ml min − 1, and 0.65 M and 3.4 ml min − 1, respectively. In comparison, our results have shorter contact time (5–10 min) when using lower acid concentration (0.01 M), and NaBH4 concentrations of 1.0–2.0% at a higher L-cysteine concentration (4.0%). In the study by Yin et al. [22], the contact time allowed for the determination of total inorganic arsenic when using FIA–HG–AAS was much longer than ours; they found that the reduction of
Fig. 7. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.60 M HCl and 0.40% L-cysteine after a contact time of 2 h.
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Fig. 8. Effect of NaBH4 concentration on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using A: 1.5; B: 3; C: 4 and D: 6 M HCl and 0.4% L-cysteine after a contact time of 2 h.
As(V) (500 ml of 2 ppb As) to As(III) was completed within 60, 40, and 20 min when using 0.04, 0.08, and 1.6 M, respectively of L-cysteine in 0.024 M HCl (or 0.029 M HNO3) and 0.5% NaBH4. Flow rates of acid and NaBH4 were 7.8 and 5.4 ml min − 1, respectively. In the present study, the As(V) reduction was completed in B10 min compared to 40 min in their work. In the other two studies [23,24], the reduction of As(V) required a longer time to complete, but
when the sample and L-cysteine were boiled for a short time before the introduction of NaBH4, the reduction was immediate. In the work of Chen et al. [23], 0.01 M HCl (or HNO3) has been employed to determine the total inorganic arsenic using HG–DCP–AES. The As(V) content was determined as follows: 1 ml of 2% L-cysteine was injected into a 5 ml sample (the final L-cysteine concentration was 0.33%), followed by 0.5% NaBH4. No As(V) signal was
A. Shraim et al. / Talanta 50 (1999) 1109–1127
detected when the NaBH4 was added just after the addition of L-cysteine, but when the reaction was given 5 min before the addition of NaBH4, a small signal was detected, which increased with increase in contact time. The As(V) reduction was complete within 35, 60, and 135 min when 1.0, 0.5, and 0.25 g, respectively of L-cysteine were added to 100 ml of 50 ppb As(V) when using 0.02 M acid. But when the solution was heated in boiling water for 5 min, As(V) was completely reduced to As(III). We have been able to obtain complete reduction of As(V) at room temperature in a much shorter time ( B10 min) when using 4.0% L-cysteine. In the other study, Brindle et al. [24] have achieved an immediate and complete reduction of As(V) to As(III) after mixing the sample solution with L-cysteine and NaBH4 under the following conditions: in a continuous flow HG – DCP–AES the sample (10 ml min − 1), L-cysteine (0.7%, 1.6 ml min − 1), and HNO3 (0.02 M, 10 ml min − 1) were mixed and heated, online to 95–98°C,
1123
cooled to room temperature, and reacted with NaBH4 (0.5%, 1.6 ml min − 1). It is evident from the last two studies [23,24], that the introduction of a heating step has eliminated any delay and spontaneously and completely produced arsine from As(V). We will consider this in future work.
3.5.2. Effect of the contact time on obtaining a DMA signal when using 0.6 M HCl, 4.0% L -cysteine, and 0.1 and 0.6% NaBH4 Use of 0.6% NaBH4 and applying the same experimental conditions of Section 3.4.2 (0.6 M HCl, 0.4% L-cysteine), results in negligible signals from all species including DMA, even after the application of contact time of 40 min. As a result the L-cysteine concentration was increased to 4.0% to obtain a high DMA signal within reasonable contact times. The effect of contact time, under the new experimental conditions is shown in Fig. 9, which shows high DMA signals with negligible signals from the other three species up
Fig. 9. Effect of contact time on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 0.60 M HCl, 4.0% L-cysteine, and 0.60% NaBH4.
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A. Shraim et al. / Talanta 50 (1999) 1109–1127
Fig. 10. Effect of contact time on the absorption signals of As(III), As(V), MMA, and DMA (40 ppb As each) when using 4.0 M HCl, 0.03% NaBH4, and A: 0.4% and B: 1% L-cysteine.
to a contact times B 18 min. It is also clear from Fig. 9 that a maximum of 12 min (the lowest time tried) can be safely used to obtain a signal for DMA only, in the presence of the other three species; shorter times can also be used. Even though no interferences from the other three species on the DMA signals were found when solutions of single arsenic species were analysed (see Fig. 9), the use of 0.6 M HCl, 0.6% NaBH4, and 4.0% L-cysteine for the analysis of a mixture of the four arsenic species has produced huge errors (70%) in the DMA analysis. The increase in HCl concentration to 1 M, under the above experimental conditions, has significantly reduced the interferences and produced very good results after a contact time of only 4 min (Table 3D). By comparison, the use of 0.005 M HCl, 0.1% NaBH4 in the absence of L-cysteine has produced better results with an error of only 0.81%. However, the analysis of actual environmental samples will decide which method is more appropriate for DMA speciation.
3.5.3. Effect of contact time on obtaining a sole MMA signal when using 4.0 M HCl, 0.2, 0.4, and 1.0% L -cysteine, and 0.03% NaBH4 Attempts to study the effect of contact time on speciation of MMA was undertaken using 4.0 M HCl, 0.4 and 1.0% L-cysteine, and 0.03% NaBH4, and results are shown in Fig. 10. Use of 0.2% L-cysteine after a contact time of 20 min was also examined and found to result in negligible signals from all species. Increasing the concentration of L-cysteine to 0.4% results in a linearly increasing signal for MMA with increase in contact time from 1.8 to 35 min, while the other three species exhibited negligible signals (Fig. 10A). Therefore, under these conditions, an MMA signal can be obtained with good intensity and little interference from the other three species after a contact time of around 20 min. However, the sole MMA signal may also be obtained in a shorter contact time, but with less intensity. On the other hand, the use of L-cysteine concentrations of \0.4% (1.0% as shown in Fig. 10B), under the abovementioned conditions, yields a higher MMA sig-
A. Shraim et al. / Talanta 50 (1999) 1109–1127
nals over shorter contact times; however the DMA signal starts to appear after 6 min, and consequently interferes with the MMA signal. Therefore, the best conditions, under these circumstances, for obtaining a sole signal for MMA with minimal interference from the other three species, are 4.0 M HCl, 0.03% NaBH4, and 0.4% L-cysteine after any contact time between 10 and 35 min. The speciation of MMA in a solution mixture containing all four arsenic species has been achieved by using 4 M HCl, 0.4% L-cysteine, and 0.03% NaBH4, but a minimum contact time of 30 min should be provided to obtain good reproducible results (see Table 3E).
3.6. Calibration cur6es Using appropriate conditions developed in this study, five different calibration curves were constructed for the speciation analysis. Arsenic species used to construct a calibration curve were the same ones for which the speciation analysis was
1125
undertaken. Although any species could be used for TAs analysis, As(III) was used in this study.
3.7. Analysis of en6ironmental samples Two environmental water samples were obtained from Coen dam, Queensland, Australia; a dam close to gold mining activities. The first sample was taken before water treatment purification (BWT), while the other one was taken after treatment (AWT). Using experimental conditions shown in Table 3, five sub-samples of each water sample were analysed; the first was used for the analysis of TAs in the presence of L-cysteine, the second two sub-samples were used for the speciation of DMA and As(III) in the absence of L-cysteine, and the last two sub-samples were used for the speciation of DMA and MMA in the presence of L-cysteine. Details of the results and experimental conditions applied to obtain the results are summarised in Table 4. The AWT water sample was found, as expected, to contain much less TAs when compared
Table 4 Results and details of experimental conditions used for the analysis of water samples Sample ID
BWTf AWTg BWTf AWTg BWTf AWTg BWTf AWTg BWTf AWTg BWTf AWTg a
Analysis
[HCl]
%L-cyst
T a(, min
%NaBH4 Arsenic (ppb)
TAs
0.01
5.0
5
2.0
DMA
0.005
0.0
N/A
0.1
As(III)
4.0
0.0
N/A
0.05
DMA
1.0
4.0
5
0.6
MMA
4.0
0.4
45
0.03
As(V)h
Foundb
Addedc
Totald
%Recoverye
38.9 2.8 0.8 4.9 0.2 0.0 0.3 0.0 1.6 0.1 36.8 2.7
8.0 16.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0
47.5 19.1 18.3 25.4 21.3 21.9 21.0 20.6 23.8 22.8
107.5 101.9 87.5 102.5 105.5 109.5 103.5 103.0 111.0 113.5
Contact time (min). Total arsenic concentration found (ppb) in unspiked sample. c Total arsenic concentration added (ppb) to spike the sample, equal quantities of each of the four arsenic species were added. d Total arsenic concentration found (ppb) in spiked sample. e %Recovery of added spike. f Water sample before water treatment. g Water sample after water treatment. h Calculated by difference, i.e. As(V) = [TAs−{As(III)+MMA+DMA}]. b
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to the BWT sample; TAs concentration of 2.8 ppb was found which represents the arsenic residue left after water treatment. Two methods for the speciation of DMA were applied; the first one in the absence of L-cysteine, while the second method was in the presence of L-cysteine. The first method yielded a DMA value of 0.8 ppb for the BWT sample, while a much larger value of 4.9 ppb was obtained for the AWT sample. The second method gave DMA values of 0.3 and 0.0 ppb for BWT and AWT samples respectively. The DMA value obtained for the AWT sample when using the first method appears to be unrealistically high and was rejected. As expected, As(V) was the main species found in both samples; its concentration ]95% of TAs. The pH value of each sample was found to be :6.5, and as the dominancy of the inorganic arsenic species in natural and ground waters is controlled by the pH values and the oxidising or reducing conditions of such waters, it would be expected that As(V) would be the most dominant species in oxygenated natural waters, as it is the most thermodynamically stable species under these conditions [30–33]. Results for other natural water samples, which have been analysed for their arsenic species concentration by various workers, confirm these predictions; As(V) was found to be the predominant species in these studies [30,34– 36] with values of greater than 90% of total arsenic. Initial attempts to assess the accuracy of the methods developed in this work were undertaken using the standard addition method. Table 4 indicates that very good recoveries were obtained (101.9 – 113.5%); equal concentrations from all four arsenic species were added to each of the two water samples, ie. for the speciation of As(III), 20 ppb of each of the four species was added to give a total arsenic concentration of 80 ppb. From these samples 21.1 and 21.9 ppb As(III) for the BWT and AWT samples, respectively were recovered; these results represent percentage recoveries of 105.5 and 109.5% for the BWT and AWT samples, respectively.
4. Conclusions The results reported in this paper, for the analyses and speciation of arsenic using methods developed in this work employing the selective-reduction–HG–AAS technique, show that these analyses can be quickly and accurately undertaken using this simple and inexpensive technique. The only exception is the analysis of MMA, which needs a minimum of 30–45 min to provide reliable results. The reduction of contact time in case of MMA may be achieved if different suitable experimental conditions such as a heating step are introduced to this system. There is no doubt that a much more comprehensive analysis program of environmental samples from a wide variety of sources has to be undertaken before the usefulness of these methods can be accurately assessed. Acknowledgements The authors would like to thank Claire Moore and Ron Sumner of the Queensland Health Scientific Services for their help in operating the HGAAS. References [1] J.P. Gustafsson, G. Jacks, Appl. Geochem. 10 (1995) 307. [2] A.R. Marin, P.H. Masscheleyn, W.H. Patrick, Plant Soil 152 (1993) 245. [3] J.R. Abernathy, Role of Arsenical chemicals in agriculture, in: W.H. Lederer, R.J. Fentsterheim (Eds.), Arsenic: Industrial, Biomedical, Environmental Perspective’s, VNR, New York, 1983, pp. 57 – 62. [4] K. Ringwood, Arsenic in the Gold and Base-Metal Mining Industry, Australian Minerals and Energy Environment Foundation (AMEEF), Melbourne, Australia, 1995, pp. 1 – 33. [5] M. Piscator, Life Sci. Res. Rep 33 (1986) 59. [6] M. Buat-Menard, P.J. Peterson, M. Havas, E. Steinnes, D. Turner, Group Report: Arsenic, in: T.C. Hutchinson, K.M. Meema (Ed.), Lead, Mercury, Cadmium Arsenic in the Environment, SCOPE 31, Pub. John Wiley and Sons, Chichester, England, 1987, pp. 43 – 48. [7] G. Stohrer, Arch. Toxicol. 65 (1991) 525. [8] C. Hopenhayn-Rich, M.L. Biggs, A.H. Smith, D.A. Kalman, L.E. Moore, Environ. Health Perspect. 104 (1996) 620.
A. Shraim et al. / Talanta 50 (1999) 1109–1127 [9] J. Beard, L. Cruces, New Sci. 28 (1998) 10. [10] R.W. Whitcare, C.S. Pearse, Miner. Ind. Bull. 17 (1974) 1. [11] G.M.P. Morrison, G.E. Batley, T.M. Florence, Chem. Brit. 25 (1989) 791. [12] B. Amran, F. Lagrade, M.J.F. Leroy, A. Lamotte, M. Olle, M. Albert, G. Rauret, J.F. Lopez-Sanchez, Tech. Instr. Anal. Chem. 17 (1995) 285. [13] W.R. Cullen, K.J. Reimer, Chem. Rev. 89 (1989) 713. [14] J.M. Wood, Science 183 (1974) 1049. [15] S. Caroli, F.L. Torre, F. Petrucci N. Violante, Arsenic speciation and Health Aspects, in: S. Caroli (Ed.), Element Speciation in Bioinorganic Chemistry, John Wiley and Sons, New York, 1996, pp. 445–463. [16] A. Hinwood, R. Bannister, A. Shugg, M. Sim, Water 25 (4) (1998) 34. [17] R.S. Braman, C.C. Foreback, Science 182 (1973) 1247. [18] R.K. Anderson, M. Thompson, E. Culbard, Analyst 111 (1986) 1143. [19] T.R. Rude, H. Puchelt, Fresenius Z. Anal. Chem. 350 (1994) 44. [20] X.C. Le, W.R. Cullen, K.J. Reimer, Anal. Chim. Acta 285 (1994) 277. [21] I.D. Brindle, X.C. Le, Anal. Chem. 61 (1989) 1175.
.
1127
[22] X. Yin, E. Hoffmann, C. Ludke, Fresenius Z. Anal. Chem. 355 (1996) 324. [23] H. Chen, I.D. Brindle, X.C. Le, Anal. Chem. 64 (1992) 667. [24] I.D. Brindle, H. Alarabi, S. Karshman, X.C. Le, S. Zheng, H. Chen, Analyst 117 (1992) 407. [25] A.G. Howard, J. Anal. At. Spectrom. 12 (1997) 267. [26] J. Aggett, A.C. Aspell, Analyst 101 (1976) 341. [27] A.G. Howard, C. Salou, Anal. Chim. Acta 333 (1996) 89. [28] E. Hakala, P. Lauri, J. Anal. At. Spectrom. 7 (1992) 191. [29] P.H. Masscheleyn, R.D. Delune, W.H. Patrick, J. Envir. Qual. 20 (1991) 96. [30] Battelle, A Report: Speciation of Selenium and Arsenic in Natural Waters and Sediments. Vol. 2: Arsenic Speciation, No. EPRI EA-4641, Pacific Northwest Laboratories, Sequim, Washington, 1986. [31] L.E. Hunt, A.G. Howard, Mar. Pollut. Bull. 28 (1994) 33. [32] H. Hasegawa, Y. Sohrin, M. Matsul, M. Hojo, M. Kawashima, Anal. Chem. 66 (1994) 3247. [33] J. Stummeyer, B. Harazim, T. Wippermann, Fresenius Z. Anal. Chem. 354 (1996) 344. [34] P. Thomas, K. Sniatecki, J. Anal. At. Spectrom. 10 (1995) 616. [35] C.J. Hwang, S.J. Jiang, Anal. Chim. Acta 289 (1994) 205. [36] R.J.A. Van Cleuvenbergen, W.E. Van Mol, F.C. Adams, J. Anal. At. Spectrom. 3 (1988) 169.