Anal. Chem. 2002, 74, 1281-1287
Porous Graphitic Carbon as Stationary Phase for LC-ICPMS Separation of Arsenic Compounds in Water S. Mazan,* G. Cre´tier, N. Gilon, J.-M. Mermet, and J.-L. Rocca
Laboratoire des Sciences et Strate´ gies Analytiques, Universite´ Claude Bernard-Lyon I, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France
A new liquid chromatographic separation method was developed for the speciation of the four main arsenic compounds present in water. Arsenite (As(III)), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA) and arsenate (As(V)) were separated on a recently introduced stationary phase: porous graphitic carbon (PGC). The separation was first obtained under formic acid gradient conditions, but an adsorption phenomenon of As(V) on PGC was observed. To overcome this problem, As(V) was backflushed, and an efficient separation of the four solutes was achieved within 10 min. Extremely low detection limits (ranging from 10 to 70 ng‚L-1) were obtained by coupling LC with an ICPMS. The method was successfully applied to different spiked mineral waters and a naturally arsenic-containing freshwater. Arsenic is a widely distributed element in the environment. It occurs as inorganic and organic compounds and its toxicity is strongly related to its chemical form. Consequently it is essential to perform the speciation of this element in aqueous, geological, and biological matrixes. Several recent reviews1-4 show that the hyphenation of liquid chromatography (LC) on anion-exchanger or alkyl-bonded phases with an element-specific detector, such as inductively coupled plasma atomic emission spectrometry (ICPAES) or inductively coupled plasma mass spectrometry (ICPMS), is the most usual analytical methodology for quantification of anionic species of arsenic. The major drawbacks lie in the use of mobile phases with high salt content (such as phosphates, carbonates, or various ionpairing agents) that are more or less tolerated by the detection system. This study focuses on the four forms of arsenic that are likely to be present in natural waters: arsenite (As(III), pKa ) 9.29) and arsenate (As(V), pKa1 ) 2.24, pKa2 ) 6.96, pKa3 ) 11.5), which are the most toxic forms, and two methylated forms, monomethylarsonic acid (MMA, pKa1 ) 4.19, pKa2 ) 8.77) and dimethylarsinic acid (DMA, pKa1 ) 1.78, pKa2 ) 6.14), the toxicity of which is much more limited. Although a great number of * E-mail:
[email protected]. (1) Burguera, M.; Burguera, J. L. Talanta 1997, 44, 1581-1604. (2) Sutton, K. L.; Caruso, J. A. J. Chromatogr. A 1999, 856, 243-258. (3) Guerin, T.; Astruc, A.; Astruc, M. Talanta 1999, 50, 1-24. (4) Szpunar, J. Analyst 2000, 125, 963-988. 10.1021/ac010823m CCC: $22.00 Published on Web 02/08/2002
© 2002 American Chemical Society
articles has been published on arsenic speciation, only a few of these present water as the sample of interest. Thomas et al.5 identified As(V) as a major arsenic species in spring waters and could also identify low amounts of As(V) in bottled mineral waters. Recovery efficiency in natural water samples was studied by Gettar et al.6 Several authors also realized the simultaneous speciation of arsenic and chromium in potable water7 or surface water.8 With the aim of studying real-world samples, it may be necessary to implement several separation mechanisms to confirm the identity of species at such low levels that molecular mass spectrometry, which would be more suited for this purpose, cannot be used. Therefore, there is a need for new separative methods based on retention mechanisms different from those of the other published methods. In this work, we present the use of porous graphitic carbon (PGC) as the stationary phase for the liquid chromatographic separation of As(III), As(V), MMA, and DMA. PGC was introduced by Knox9 and is composed of large and flat sheets of hexagonally arranged carbon atoms. It was first considered as an ideal reversed-phase chromatography packing, able to generate only strong hydrophobic interactions. However, there is now unquestionable evidence10-14 that PGC also shows unusual retentive behavior for polar solutes and purely inorganic compounds. Lim10 proposed an electronic interaction mechanism to explain retention of pertechnetate and perrhenate anions on PGC. More recently, the polar retention effect on graphite (PREG) was discussed in detail:15 it illustrates the ability of molecules having lone-pair or aromatic-ring electrons to apparently interact with graphite by some kind of electron-transfer mechanism to the electronic cloud of graphite. Most separations of anionic solutes (5) Thomas, P.; Sniatecki, K. J. Anal. At. Spectrom. 1995, 10, 615-618. (6) Gettar, R. T.; Garavaglia, R. N.; Gautier, E. A.; Batistoni, D. A. J. Chromatogr. A 2000, 884, 211-221. (7) Pantsar-kallio, M.; Manninen, P. K. G. J. Chromatogr. A 1997, 779, 139146. (8) Roig-Navarro, A. F.; Martinez-Bravo, Y.; Lopez, F. J.; Hernandez, F. J. Chromatogr. A 2001, 912, 319-327. (9) Knox, J. H.; Kaur, B.; Millward, G. R. J. Chromatogr. A 1986, 352, 3-26. (10) Lim, C. K. Biomed. Chromatogr. 1989, 3, 92-93. (11) Elfakir, C.; Chaimbault, P.; Dreux, M. J. Chromatogr. A 1998, 829, 193199. (12) Emery, M. F.; Lim, C. K. J. Chromatogr. A 1989, 479, 212-215. (13) Mercier, J. P.; Morin, P.; Dreux, M.; Tambute´, A. J. Chromatogr. A 1999, 849, 197-207. (14) Chaimbault, P.; Petritis, K.; Elfakir, C.; Dreux, M. J. Chromatogr. A 2000, 870, 245-254. (15) Knox, J. H.; Ross, P. Advances in Chromatography 37; Brown, P. R., Grushka, E., Eds.; Marcel Dekker: New York, 1997; pp 73-162.
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Table 1. Detection Conditions parameters forward power plasma gas flow rate auxiliary gas flow rate nebulizer gas flow rate nebulizer spray chamber wavelength for arsenic forward power plasma gas flow rate auxiliary gas flow rate nebulizer gas flow rate nebulizer spray chamber dwell time cone material monitored mass other parameters were optimized daily
Figure 1. Schematic diagram of the chromatograph used in arsenic speciation studies. Switching valve positions: (a) normal elution, (b) backflush.
on PGC10-14 were performed using aqueous carboxylic acid as eluent. Changing the acid concentration affects the solute retention by modifying, on one hand, the pH of the mobile phase and consequently the degree of ionization of the analytes, and on the other hand, the concentration of the carboxylate anions, that is, the concentration of electronic competitor in the mobile phase. In this work, we selected formic acid as the mobile phase modifier in order to facilitate detection by ICPAES or ICPMS: use of the shortest carboxylic acid limits the amount of carbon brought to the plasma and, hence, the sooting of the torch and the sampler cone of the ICPMS spectrometer. EXPERIMENTAL SECTION Instrumentation. The liquid chromatograph (Figure 1) is composed of a LC-10AD high-pressure binary gradient system (Shimadzu, Tokyo, Japan), a 7725i sampling valve equipped with a 500-µL loop (Rheodyne, Cotati, CA), a 7010 switching valve (Rheodyne), and a 100 × 4.6 mm i.d. column packed with Hypercarb 5 µm (Hypersil, Runcorn, U.K.). According to the switching valve position, the column was normally eluted (valve position N) or backflushed (valve position B). The column outlet was connected to the pneumatic nebulizer of the detection system by 150 mm × 50 µm i.d. stainless steel tubing. The elution flow rate was 0.8 mL‚min-1, and the delay time was 0.6 min. Two different detection systems were successively used: a JY 138 Ultrace ICPAES instrument (Jobin Yvon, Longjumeau, France) and a PQ ExCell ICPMS instrument (VG Elemental, Cheshire, U.K.). ICPAES was first selected because this technique can easily 1282 Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
settings ICPAES 1200 W 12 L‚min-1 0.15 L‚min-1 0.9 L‚min-1 glass expansion concentric cyclonic 193.7 nm ICPMS 1350 W 13.5 L‚min-1 0.55 L‚min-1 0.94 L‚min-1 Glass Expansion concentric Impact bed, cooled at 4 °C 200 ms Ni m/z 75
tolerate a high acid or salt concentration. The ICPAES detector was modified to allow axial viewing and, therefore, to increase the sensitivity;16 its Czerny-Turner dispersive system was equipped with a 2400 line‚mm-1 grating (working in the second order below 300 nm) and with entrance and exit slits of 18 and 16 µm, respectively. The ICPMS detector included a rf-only hexapole ion guide (collision cell) that operated in an unpressurized mode in this work. The working conditions of the two detection systems are summarized in Table 1. Reagents. Stock standard solutions (As concentration, 1 g‚L-1) of As(III), As(V), DMA ,and MMA were prepared from arsenic trioxide (Aldrich, Saint Quentin Fallavier, France), sodium arsenate (Aldrich), dimethylarsinic sodium salt (Sigma, Saint Quentin Fallavier, France), and monomethylarsonic acid (Carlo Erba, Milano, Italy), respectively. Water was deionized and purified using an Elgastat UHQ II system (Elga, Buckinghamshire, U.K.). Formic acid was purchased from Riedel-de Haen (Saint Quentin Fallavier, France). The natural samples were filtered on 0.45-µm nylon membrane (Touzart & Matignon, Les Ulis, France) and on styrenedivinylbenzene extraction disks (3M Empore, St Paul, MN). Safety Considerations. Formic acid (R 35, S 23-26-45) causes severe burns. Breathing the vapors must be avoided. In case of contact with eyes, rinse immediately with plenty of water. Arsenite (R 45-28-34-50/53, S 53-45-60-61) and arsenate (R 45-23/25-50/ 53, S 53-45-60-61) are highly toxic (LD50 for rats, 14 and 20 mg/ kg, respectively17) and are suspected to be carcinogens. They are harmful if inhaled or swallowed. The methylated species, monomethylarsonic acid (S 1/2-20/21-28-44) and dimethylarsinic acid (R 26/27/28-40, S 45-36/37/39-22) are very toxic by inhalation, in contact with skin, and if swallowed. DMA is a possible carcinogen. The LD50 is 1350 mg/kg for DMA18 and ranges between 700 and 1800 mg/kg for MMA.17 (16) Chausseau, M.; Roussel, C.; Gilon, N.; Mermet, J. M. Fresenius’ J. Anal. Chem. 2000, 366, 476-480. (17) Le, X. C.; Ma. M. J. Chromatogr. A 1997, 764, 55-64. (18) The Merck Index,11th ed.; Merck & Co., Inc: Rahway, NJ, 1989; 244.
Figure 2. Retention factor k of arsenic compounds as a function of formic acid concentration in the mobile phase [HCOOH]0 on PGC column (injected volume, 40 µL; injected solute concentration, 2 mg‚L-1; solute solvent, pure water; switching valve position, N (see Figure 1a); detection system, ICPAES): 9, As(III); 0, DMA; b, MMA; O, As(V).
RESULTS AND DISCUSSION The effect of the formic acid concentration in the mobile phase [HCOOH]0 on the retention factor k of the four arsenic species on PGC column was investigated in the range 0-0.01 mol‚L-1 using ICPAES detection (Figure 2). The retention factor k was calculated as k ) Vr/Vm - 1, where Vr is the solute retention volume and Vm is the column void volume (from the supplier data, the total porosity of PGC column is equal to 0.75 and the corresponding Vm value is 1.25 mL). The retention behavior of As(III) is different from those of DMA, MMA, and As(V). When using pure water as the mobile phase ([HCOOH]0 ) 0), As(III) is eluted with a k value of 0.6, while DMA, MMA, and As(V) are strongly retained (k f ∝). With increasing acid concentration in the mobile phase, the retention factor of As(III) is kept constant, whereas the retention factors of DMA, MMA, and As(V) are decreased. However, this effect is less marked for DMA. To explain these differences in retention behavior, the pH of the mobile phase and the consequent apparent charge Qapp on the arsenical solutes as a function of [HCOOH]0 are presented in Figure 3. Qapp is given by Qapp ) (∑nδ)/(∑δ) where δ is the concentration of the form of the arsenical solute with a charge n (δ values were calculated from the pKa of the arsenic compounds obtained from refs 19 and 20). In the acid concentration range investigated (0-0.01 mol‚L-1) and the corresponding pH range (7.0-3.0), As(III) (pKa ) 9.3) is present as an uncharged species (Qapp ) 0), which could explain its very low retention time on PGC. For MMA (pKa1 ) 4.2) and As(V) (pKa1 ) 2.3), which are present as anions, the retention factor varies in the reverse order of apparent charge: the more negatively charged analyte is the more retained. Regarding DMA retention behavior, the situation (19) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York, 1977; Vol. 3, p 180. (20) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York, 1982; Vol. 5, pp 409-410, 445.
Table 2. Elution Conditions gradient elution mode column equilibration step
elution steps time (min) [HCOOH]0 (mol‚L-1) switching valve position
0 0 N
1 0.01 N
3 0.1 N
5 1 N
5.1 0 N
20 0 N
backflush elution mode elution steps time (min) [HCOOH]0 (mol‚L-1) switching valve position
0 0.05 N
3.25 0.05 B
8 0.05 B
column equilibration step 8.1 0 B
10 0 B
is more complicated. For very low concentration of formic acid, DMA is present as an uncharged species with retention higher than As(III) retention. This effect could be attributed to the two methylated groups that interact with the graphitized carbon. With increasing acid concentration, the apparent charge of DMA rapidly becomes positive, and its retention begins to decrease: DMA is probably excluded from the PGC pores, as previously related for cationic compounds on PGC.11 In summary, the four arsenic species show large differences in retention, and gradient elution conditions (Table 2) were used to improve resolution for the early eluting species and, at the same time, to get an acceptable analysis time. The gradient program started with pure water. Under these conditions, As(III) was poorly retained, but DMA, MMA, and As(V) were more strongly retained; their elution was obtained within 6 min by gradually adding formic acid in the mobile phase. Figure 4a shows the chromatogram obtained with ICPAES detection for injection of 40 µL with 6 mg‚L-1 (as arsenic) of each arsenic species. The reliability of the Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
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Figure 3. pH of the mobile phase and apparent charge Qapp of the arsenical solutes as a function of formic acid concentration in the mobile phase [HCOOH]0: 9, As(III); 0, DMA; b, MMA; O, As(V).
Figure 4. Chromatograms of the four arsenic compounds obtained under gradient elution mode (injected volume, 40 µL; solute solvent, pure water; for elution conditions, see Table 2). (a) Injected solute concentration, 6 mg‚L-1; detection system, ICPAES. (b) injected solute concentration, 50 µg‚L-1; detection system, ICPMS.
method was estimated from injections of pure water spiked with 400 and 600 µg‚L-1 and 4 and 6 mg‚L-1 (as arsenic) of each species, each concentration level being injected five times. The linear regressions of the calibration curves based on the peak heights are presented in Table 3. The validity of the selected linear regression model chosen was studied in two ways: by performing a linearity test based on analysis of variance (ANOVA)21 and by verifying that the zero was included in the 95% confidence interval of the intercept. F1 and F2 tests were similar to those described 1284 Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
in ref 21. Both of these tests showed that the linear regression model is an adequate description of the true relationship between peak height and injected solute concentration for As(III), DMA, and MMA. The corresponding limits of detection (LOD), determined as arsenic concentration giving a signal three times higher than the standard deviation of the baseline, were 18 µg‚L-1 for As(III), 16 µg‚L-1 for DMA, and 22 µg‚L-1 for MMA. In contrast, the two tests suggested a marked departure from the linear model for As(V). In an effort to understand the origin of the abnormal behavior of As(V), we explored a lower injected concentration range by using ICPMS detection. Figure 4b shows the chromatogram obtained under gradient elution mode with ICPMS detection for injection of 40 µL with 50 µg‚L-1 (as arsenic) of each species. The peak of As(V) clearly exhibited some tailing that could result from strong solute adsorption on PGC. This explanation was confirmed by carrying out a blank experiment following the analysis, which consisted of a gradient run without injecting. Some arsenate was eluted from the PGC column, which revealed an incomplete elution of As(V) during the previous run. Attempts to increase the elution strength of the mobile phase by increasing the formic acid concentration were made to improve desorption of As(V) from PGC, but no positive result was obtained. To limit the adsorption phenomenon of As(V) on PGC, the length of the stationary phase traveled by the solute was reduced by using a backflush elution mode (Table 2). After a 2-min equilibration time with pure water, a step gradient with 0.05 mol‚L-1 formic acid was used to elute As(III), DMA, and MMA. Just after the elution of MMA, the flow rate through the column was reversed by switching the valve, and As(V) was backflushed from the column. The chromatogram obtained with ICPMS detection for injection of 40 µL with 5 µg‚L-1 (as arsenic) of each species (Figure 5a) shows that As(V) was eluted with a normal peak shape under backflush conditions. Blank experiments confirmed that arsenate was totally eluted during the first run. In comparison (21) Massart, D. L.; Vandeginste, B. G. M.; Deming, S. N.; Michotte, Y.; Kaufman, L. Chemometrics: A Textbook; Elsevier Science Publishers B. V.: Amsterdam, 1988.
Table 3. Calibration Curves and Limits of Detection (LOD) Obtained under Gradient Elution Mode with ICPAES Detection
calibration curve characteristics slope b1 intercept b0 det coeff R2 linearity test (ANOVA) 95% 95% confidence interval of intercept LOD (µ‚L-1)
As(III)
DMA
MMA
As(V)
490 -10 867 0.999 994 F1 ) 0.025 < 3.89 F2 ) 11 168 > 4.6 (-23 169,1435) 16
998 21 846 0.9995 F1 ) 1.455 < 3.89 F2 ) 6231 > 4.6 (-209 663,253 355) 18
705 -57 197 0.9997 F1 ) 1.422 < 3.89 F2 ) 12 534 > 4.6 (-171 579,57 184) 22
558 -136 534 0.999 91 F1 ) 8.22 should be < 3.89 F2 ) 7184 > 4.6 (-195 579,-77 489)
Figure 6. Variation of peak height versus peak area for increasing injected volumes (20, 40, 100, and 300 µL) of each arsenic compound (injected solute concentration, 50 µg‚L-1; solute solvent, pure water; elution conditions, backflush elution mode (see Table 2); detection system, ICPMS): 9, As(III); 0, DMA; b, MMA; O, As(V). Figure 5. Chromatogram of the four arsenic compounds obtained under backflush elution mode (injected volume, 40 µL; injected solute concentration, 50 µg‚L-1; for elution conditions, see Table 2; detection system, ICPMS). (a) Solute solvent, pure water; (b) solute solvent, NaCl 1 g‚L-1.
with the gradient elution mode, the backflush elution mode also allowed a decrease in the total analysis time (column equilibration included) from 20 min down to 10 min (Table 2). Then the maximum injection volume in backflush elution mode was determined by plotting the variation of peak height versus peak area for injection of increasing volumes (20, 40, 100, and 300 µL) of each arsenic compound dissolved in pure water (Figure 6). For DMA, MMA, and As(V) (Figures 6b), peak height increased proportionally to peak area, which meant that there was no peak broadening up to 300 µL injected volume. It was the on-column focusing effect: the solute solvent (pure water) is a weak solvent which migrates faster than the solutes; but it acts as a local mobile phase when the solutes enter the column, and because of their very large retention in that solvent, the solutes are retained and enriched at the top of the column until the solvent penetrates the rest of the column. In contrast, the retention of As(III) on PGC with pure water as the mobile phase is very weak, and the oncolumn focusing effect is significantly less pronounced. Figure 6a shows that the maximum injection volume for As(III) was 40 µL. For a larger injected volume, peak height no longer increased
proportionally to peak area; it was the classical band broadening effect resulting from the large injection volume. Figures of merit (linearity, limits of detection and repeatability) of the new method based on backflush elution mode on PGC column and ICPMS detection are presented in Table 4. The characteristics of the calibration curves calculated for the peak heights were estimated from five consecutive 40-µL injections of six standard solutions containing, respectively, 0.1, 0.2, 0.5, 1, 5, and 50 µg‚L-1 (as arsenic) of each compound. The results of both the linearity test based on ANOVA and the comparison of the zero position in relation to the 95% confidence interval of the intercept confirmed the suitability of the linear regression model for the calibration curve of the four arsenic compound in the concentration range 0.1-50 µg‚L-1. The limits of detection, corresponding to a signal equal to three time the standard deviation of the baseline, are promising: 40 ng‚L-1 for As(III), 10 ng‚L-1 for DMA, 20 ng‚L-1 for MMA, and 70 ng‚L-1 for As(V). The relative differences observed in the limits of detection arise from the changes in the peak shape caused by the different elution conditions of individual species. For DMA and MMA, the oncolumn focusing effect is large, and peak dispersion is very low. Because of the delay time, As(III) is eluted under essentially isocratic conditions, and its peak undergoes no compression effect. As(V) is eluted under backflush conditions, that is, the flow rate is reversed during its migration inside the column, which results Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
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Table 4. Calibration Curves and Limits of Detection (LOD) Obtained under Backflush Elution Mode with ICPMS Detection
calibration curve characteristics slope b1 intercept b0 det coeff R2 linearity test (ANOVA) 95% 95% confidence interval of intercept LOD (ng‚L-1) RSD for concn of 200 ng‚L-1 (%)
As(III)
DMA
MMA
As(V)
1.36 461 0.999 91 F1 ) 0.822 < 3.1 F2 ) 29 457 > 4.28 (-49,973) 40 10.7
3.46 190 0.999 991 F1 ) 0.178 < 3.1 F2 ) 64 777 > 4.28 (-239,620) 10 9.4
1.88 83 0.999 999 0 F1 ) 0.005 < 3.1 F2 ) 19 223 > 4.28 (-20,187) 20 7.5
0.53 -29 0.999 93 F1 ) 0.524 < 3.1 F2 ) 27 316 > 4.28 (-203,144) 70 20
Table 5. Composition of Water Samplesa bottled mineral water A
bottled mineral water B
0.2 67.6 2 4 3.5 18 204
3.2 486 84 8.6 2.7 1187 403
K+ Ca2+ Mg2+ ClNO3SO42CO32a
Concentration in mg‚L-1.
Table 6. Recovery Percent for the Four Arsenic Speciesa Added to Different Water Samplesb Figure 7. Variation of normalized peak heights versus time (injected, 40 µL; injected solute concentration, 50 µg‚L-1; solute solvent, pure water; elution conditions, backflush elution mode (see Table 2); detection system, ICPMS; for normalization of peak heights, see the text): 9, As(III); 0, DMA; b, MMA; O, As(V).
in a larger peak dispersion. Table 4 also presents the repeatability, expressed as the relative standard deviation of a five-replicate injection of a 200 ng (As) L-1 solution of each species. For As(III), DMA, and MMA, the values found were below 10%. For As(V), the slightly higher value obtained (20%) is a consequence of the worse LOD obtained for this species. To ensure the reliability of the method, a long-term stability experiment was performed. The four arsenic species were injected twice every hour, each at a concentration of 5 µg‚L-1. For each of the species, the mean peak height was calculated from the data collected over 10 h. The signal for each hour was then normalized against this calculated mean value. As it can be seen on Figure 7, the peak heights were kept constant within 5% over 10 hours, which shows that the method does not suffer from carbon deposition effects that would modify the signal. To check the matrix effects, especially those resulting from common cations and anions contained in natural waters, and because a certified reference material regarding the studied species was not available, speciation of arsenic by the method previously defined (PGC column + backflush elution mode + ICPMS detection) was evaluated through a standard addition procedure. The method was first applied to two bottled mineral waters (A and B) purchased from local supermarkets and spiked with 5 µg‚L-1 (as arsenic) of each arsenic compound (As(III), DMA, MMA, and As(V)). The composition of these water samples, A and B, regarding common cations and anions is characteristic of low- and high-salt content respectively (Table 5). In the two bottled mineral waters, satisfactory recoveries were obtained for 1286 Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
bottled mineral water A
bottled mineral water B
NaCl 1 g‚L-1
recovery % RSD % recovery % RSD % recovery % RSD % As(III) DMA MMA As(V) a
93 101 104 113
1.2 1.3 0.6 2.0
97 100 103 112
1.8 3.0 3.2 2.5
101 100 102 105
1.6 2.2 3.5 7.8
5 µg‚L-1 As each. b n ) 5.
each arsenic species (Table 6); the difference between the determined concentration and the spiked concentration never exceeded 15%. Another major problem that may occur in arsenic speciation analysis using ICPMS detection, is the formation of 40Ar35Cl+, producing an isobaric interference on 75As+ with injection of chlorides. Because the Cl- content of the bottled mineral water A and B were low (4 an 8 mg‚L-1, respectively), the method was also applied to an aqueous solution containing 1 g‚L-1 of NaCl and spiked at 5 µg‚L-1 of each arsenic species. The chromatogram obtained under these conditions of solute solvent (Figure 5b) is rigorously identical to that obtained when the arsenical solutes were dissolved in pure water (Figure 5a), and recoveries, varying from 100 to 105%, were quantitative for all arsenic species (Table 6). Even if the coelution of Cl- and As(V) was shown by monitoring the 35Cl16O+ signal (m/z ) 51), there are minor matrix effects in 1 g‚L-1 NaCl samples. The slope obtained for the calibration curves of each arsenic species in the 0.1-50 mg‚L-1 concentration range was similar to that of pure water samples. In 10 g‚L-1 NaCl samples, As(III), DMA, MMA, and As(V) could no longer be satisfactorily analyzed; the separation of arsenic species was totally deteriorated.
Figure 8. Chromatogram of the real surface water sample obtained under backflush elution mode on PGC column. (injected volume, 40 µL; detection conditions, ICPMS).
Finally, the proposed method (PGC column + backflush elution mode + ICPMS detection) was applied to the determination of arsenic species in a real surface water sample from a lake of the Forez region (France). Sample collection and analysis were achieved within the same day. During transport, the samples were stored in polypropylene flasks, without any treatment. DMA and As(V) were found to be the main arsenic species at concentrations of 3.7 (RSDn)5 ) 4.5%) and 3.95 (RSDn)5 ) 4.8%) µg‚L-1, respectively (Figure 8). Trace amounts of As(III) and MMA acid were also detected with concentrations of 330 and 270 ng‚L-1 (RSDn)5 ) 10%). A fifth peak was present just before the void volume. Further experiments would be necessary to identify this compound. Prior to the speciation study, a total concentration of 8.85 µg‚L-1 (RSDn)5 ) 2.2%) in arsenic was found by flow injection analysis; hence, 93% of the arsenic contained in the real surface water sample was identified. CONCLUSION The liquid chromatographic method proposed for arsenic speciation is based on the use of PGC as stationary phase and aqueous solution of formic acid as mobile phase. It was demonstrated to be appropriate for the detection and quantification of
As(III), DMA, MMA, and As(V) in mineral and natural waters. The chromatographic procedure described enables good resolutions and a short analysis time without perceptible matrix effects. The large on-column focusing effect for anionic compounds on PGC and the direct coupling with ICPMS detection without signal suppression effect (no carbon deposition was observed and addition of oxygen to the plasma was not necessary) lead to very low limits of detection. Consequently, this method should allow the determination of the arsenic species at their naturally occurring concentration levels in any aquatic environment. ACKNOWLEDGMENT The authors thank VG Elemental (Thermo Elemental) company for the loan of the PQ ExCell ICPMS instrument and their technical support.
Received for review July 20, 2001. Accepted December 10, 2001. AC010823M
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