ANALYST, SEPTEMBER 1986, VOL. 111 1029 Determination of Arsenic(V) in Aqueous Solutions by D.c. Argon Plasma Emission Spectrometry. Interference Studies Kirnrno Srnolander” and Matti Kauppinen Department of Chemistry, University of Joensuu, P.O. Box 1 1 I , SF-80101 Joensuu 10, Finland The linear dynamic range, detection limits, accuracy and precision of the determination of As(V) in aqueous solutions by d.c. argon plasma emission spectrometry were studied at four emission lines of arsenic(\/). The interference and matrix effects of eight common acids and 12 cations [(Na(l), K(I), Mg(ll), Ca(ll), AI(III), Cr(lll), Cr(VI), Fe(lll), Co(ll), Ni(ll), Cu(ll) and Zn(ll)] on the determination of arsenic(\/) in aqueous solutions were investigated at the arsenic emission line at 193.696 nm.The linear range covered concentrations from 0.05 to 100 pg ml-1 of As(V). The RSD increased from 0.5 to 5% when the concentration of As(V) decreased from 10 to 0.75 pg ml-1 and from 15 to 50% in the range 0.5-0.05 pg ml-1 of As(V). The minimum detection limit was 0.063 pg ml-I, calculated to 30 of ten measurements of blank, as recommended by IUPAC. Keywords: Arsenic(V) determination; plasma emission spectrometry; interference studies; matrix effects Arsenic and its compounds are extremely toxic and concern about their levels in the environment has stimulated great interest in the development of analytical methods for the determination of arsenic in a variety of sample matrices. Several different plasma emission methods for the determi- nation of arsenic have been investigated by various workers.These include the direct determination of arsenic by direct current plasma emission spectrometry (DCP-AES)l.2.3 and inductively coupled plasma emission spectrometry (ICP- AES) ,4-7 and determination by ICP-AESG10 with hydride generation systems (HY-ICP-AES), microwave-induced plasma emission spectrometry and DCP-AES with hydride generation systems (HY-MIP-AESI’ 3 1 2 and HY-DCP-AES3). Extensive investigation of hydride-forming elements have been carried out by Thompson et al.879 using HY-ICP-AES. Simultaneous multi-element determinations including arsenic have been made by ICP-AES476.7 and by HY-ICP-AES.839J3 In comparison with the ICP-AES technique and the methods relying on hydride generation, the direct determination of arsenic by DCP-AES has been relatively little studied.Experimental Instrumentation A Spectrametric Spectraspan 111 single-channel plasma emission spectrometer with a d.c. argon plasma source and an Cchelle monochromator with an average resolution of 0.003 nm was used for the emission measurements. Data output was via a Hewlett-Packard 85A processor. The slit openings, as recommended by the manufxturer, had an entrance of 50 x 300 pm and an exit of 100 x 300 pm. The photomultiplier voltage used was 750 V. The argon gas flowed through the electrode sleeves and nebuliser at 50 and 30 lb in-2, respectively. The solution uptake rate was main- tained at 1.6 ml min-1 by a peristaltic pump. An integration period of 3 s was used, with three integrations per sample and standard.The spatial profile of the plasma was peaked horizontally and vertically. The optimum position in the plasma source, where the maximum net signal to background ratio is obtained, was at approximately 0.2 mm below the intersection of the three plasma legs when the plasma was at about the “-1” position specified in the manual.14 Reagents All reagents were of analytical-reagent grade. A standard stock solution of arsenic (2000 pg ml-1) was prepared by diluting an ampoule of As205 with water (Merck, Titrisol). Except for HBr, the strong acids (Merck) were diluted to 10 M solutions before being used to prepare the solutions used in the investigation. The water was distilled and de-ionised immediately before use (Milli Q system). All glassware was acid-washed before use.Results and Discussion Instrumental Parameters The most common emission line for the determination of arsenic by plasma emission spectrometry is 193.696 nm.473 This is also the strongest line in ICP-AES,16 although many other lines can and have been used.16J7 For example, Thompson et aZ.9 used the arsenic lines at 228.812 and 234.984 nm for their investigation of hydride-forming elements by HY-ICP-AES. Barnett et a1.12 selected the line at 234.984 nm for their measurement with a miniature HY-MIP-AES. Degners investigated seven emission lines of arsenic, but he reports only the detection limits, not the intensities. Urasa2 determined arsenite and arsenate by DCP-AES using the line at 197.197 nm, and Panaro and KrulP used the line at 228.812 nm with HY-DCP-AES.Important emission wavelengths and their relative intensities are tabulated in Table 1. We selected four emission lines for the investigation of arsenic(V), namely, 193.696, 197.197, 199.048 and 228.812 nm (Fig. 1). All of these were expected to be relatively free from direct spectral overlap by foreign ele- ments. Table 1. Relative intensities of common analytical wavelengths for arsenic with different plasma emission methods Relative intensity Wavelength/ nm ICP HY-ICP HY-MIP DCP 193.696 I 197.197 I 198.970 I 199.048 I 200.334 I 228.812 I 234.984 I 245.653 I Reference 83 145 108 - - 111 43 67 77 100 100 100 100 100 51 186 - - 23 - - - - - - - - - - - - - 16 9 12 Thispaper * To whom correspondence should be addressed.1030 ANALYST, SEPTEMBER 1986, VOL.111 I 0.2 nm - H Fig. 1. Spectral profiles of selected arsenic emission lines. (-), As(V) in water (2.5 pg ml-1); (---), water. (a) h = 193.696 nm; ( b ) h = 197.197 nm; (c) h = 199.048 nm; and ( d ) h = 228.812 nm Table 2. Detection limits (DL), calculated to 3a of ten measurements of blank as recommended by IUPAC, l9 background equivalent concentrations (BEC) and relative sensitivity for arsenic emission lines measured by DCP-AES. Wavelength/ DL/ BEC/ Relative 193.696 0.063 3.9 83 197.197 0.295 12.1 111 199.048 0.441 21.9 67 228.812 0.117 10.1 100 nm pg ml-1 pg ml-1 sensi tivity Detection Limit The detection limits (DL) of different wavelengths and methods differ appreciably, depending on the analytical line selected, the instrument used, the method of analysis and the form of arsenic determined. Urasazobtained DL of 20 ng ml-1 for As(II1) and 40 ng ml-1 for As(V) using DCP-AES, whereas Panaro and Krull3 found about 300 ng ml-l for both As(II1) and As(V) using the same method.The hydride generation system is much more sensitive than conventional plasma spectrometry. Oliveira et al. 13 have reported DL as low as 1 ng ml-1 for As(II1) and As(V) by HY-ICP-AES. Very low DL have been obtained by hydride generation system AAS (HY-AAS), e.g., 0.35 ng ml-l.l8 We determined the detection limit for aqueous solutions of As(V) at the line 193.696 nm using three different methods. The first method was to calculate three times the standard deviation of ten measurements of blanks, as recommended by IUPAC.19 The second method involved the measurement of the method detection limit for the 99% confidence level1 and the third was based on the equation DL = 3(RSD) (I,,/Za)Ca, where Ib and I, are the mean background analyte intensities and RSD the relative standard deviation of n measurements of the analyte at a concentration Ca.20 The DL values for the different methods were 0.063, 0.088 and 0.079 yg ml-l, respectively.The reproducibility of the detection limits varied from day to day by k30%, as calculated by the first method. Calibration Graphs and Precision The linearity and the analyte concentration range of the As(V) determination was investigated at the four emission lines with standards of 0, 1, 5 , 10, 50 and 100 pg ml-1. Not surprisingly, the 197.197 nm line gave the best intensity and sensitivity.16,17 According to the “Tables of Spectral Line Intensities, Part 1,”17 the line at 199.048 nm should have had better intensity I I Fig. 2. Spectral profiles of arsenic emission lines at 193.696 nm. (-), As(V) in water (2.5 pg ml-l); (. * .), HOAc (a) or HF (b), both 5 M; and (---), water and sensitivity than those recorded here. However, the values in the Tables are based on arc emission spectrometry, and in the corresponding table of Boumans,l6 where the sensitivities are corrected for ICP-AES, the line is one of the weakest. The calibration graphs of the three most sensitive lines (197.197, 228.812 and 193.696 nm, respectively) are linear from 0 to 100 pg ml-1. The weakest line (199.048 nm) consistently curves upwards. The background equivalent concentrations (BEC) calculated for different concentrations are given in Table 2 together with the sensitivities of the four lines.The emission line at 193.696 nm was selected for more detailed study in this investigation because it had the lowest detection limit and good sensitivity. The linearity, precision and accuracy of the selected line were studied with three standard series (0.05-2.5, 5-75 and 100-400 pg ml-1). The linear range covered 0.05 to 100 pg ml-1. Above 100 pg ml-1, the deviations from linearity began to increase, although the relative standard deviation values (RSD) were only about 0.5%. The deviations were less than 0.02 and 0.08 pg ml-1 in the first and second standard series, respectively. The precision, however, was significantly decreased at the lower concentration levels: the RSD values increased from 0.5 to 5% when the concentration decreased from 10.0 to 0.75 pg ml-1 and from 15 to 50% in the range 0.5-0.05 pg ml-1.Effects of Acid Concentration In order to avoid errors in the determination of As(V), it is necessary to match the total acid content of the samples and standards as closely as possible. Viscosity changes produced by variations in the total acid content affect sample transport properties and therefore the analytical sensitivities? Other factors, such as aerosol transport losses and changes in the excitation conditions in the plasma, also play a role.15 The effects of eight acids (HF, HC1, HBr, HN03, HC104, H2SO4, H3P04 and HOAc) on determinations of 2.5 yg ml-1 As(V) solutions were studied with pure acids at concen- trations of up to 5 M.Only two of the acids has a significant background effect. HF showed an emission line at the same wavelength as As(V) [Fig. 2(b)], and HOAc gave many lines, as can be seen in Fig. 2(a), and the background level was higher than with other acids. These two acids caused the largest signal enhancements of As(V); they increased the intensity of the As line in an almost linear manner in up to 5 M acid solutions, in which the intensities were +60 and +110% higher, respectively, than in an aqueous solution. A linear correlation between intensity and acid concentration was also observed for HBr, the increase being +30% in 5 M acid solutions (Fig. 3). In 0.1 M H2SO4 and 0.5 M HC104 solutions the intensity was decreased by about -4 and -lo%, respectively, from where it decreased linearly with increasing acid concentration to -30% for H2SO4 and remained approximately constant for HC104.The H3P04 solutions initially increased the intensity slightly, with a maximum at about 1.5 M, and then there was a linear decrease to - 18%. The deviations in HCl and HN03 solutions remained approximately constant at about - 2 to -5% lower than in water solutions.ANALYST, SEPTEMBER 1986, VOL. 111 1031 80 - - 40 0 1 2 3 4 5 Acid concentration/mol I-’ Fig. 3. Effect of different acids on 2.5 vg ml-1 of As(V) in water at the emission line at 193.696 nm. (a) A, HOAc; B, HF; C, HBr; and D, HC1. (b) E, H3P04; F, HNO,; G, H,SO,; and H, HC104 Our results are in agreement with the slight suppressing effect of HN03 and HC1 and the very strong suppressing effect of H2SO4 reported for the determination of As by ICP- AES,15,21 and also with the acid effect of HC104 reported in a multi-element determination by ICP-AES.7 A h I I I I 1 0 1 2 3 4 Log(Cr concentration/vg ml-1) Fig.4. at the emission line at 193.696 nm. A, Cr(IIfi; and B, Cr(V1) Effect of Cr(II1) and Cr(V1) on 2.5 p ml-1 of As(V) in water Effect of Foreign Elements The 2.5 pg ml-l As(V) solution was spiked with 10-5000 pg ml-1 of different nitrate salts [except Cr, which was added as Cr(II1) chloride and Cr(V1) oxide]. The cations Mg(II), Ca(II), Al(III), Fe(III), Co(II), Ni(II), Cu(I1) and Zn(I1) increased the intensity of the As(V) line oniy slightly: deviations in 1000 pg ml-1 solutions were less than 4%, whereas in solutions of 5000 pg ml-1, Na and K caused deviations of less than 5%, Mg, Ca, Ni, Cu and Zn deviations of about 10% and Al, Fe and Co deviations of about 24,19 and 14y0, respectively.The greatest effect was caused by Cr(II1) chloride and Cr(V1) oxide, both of which enhanced the intensity linearly up to 500 pg ml-1, where the deviations were about 14 and 8%, respectively. In 1000 and 5000 pg ml-1 solutions the intensities were 23, 81 and 15, 61% stronger, respectively (Figs. 4 and 5). Literature values for the spectral interference effects of 1000 gg ml-1 of A1 as determined by ICP-AES are larger than the values in this study.6.7 according to Tao et LzZ.,~ 1000 pg ml-l of A1 are equivalent to 23.9 pg ml-1 of arsenic at the emission line 193.696 nm, however, we observed only a slight increase in deviation with 1000 yg ml-1 and a +24% deviation with 5000 yg ml-1 of A1 at the same wavelength.Other elements reported to cause an interference effect are Fe, Mg and Ti. Solutions of 1000 pg ml-1 of these elements were found to be equivalent to 2.92,0.206 and 0.400 pg ml-1 of As, respectively.6 Degners investigated the interference effects of Fe, Cr and Cu at seven different emission lines of arsenic by ICP-AES. The maximum concentrations of these elements that caused no interference effects were 10, 100 and 1000 pg ml-1 for Fe, Cr and Cu, respectively. In our study, the deviations of arsenic with 1000 pg ml-1 of Fe, Cr(II1) and Cr(V1) added were about +4, +23 and +15 pg ml-1, whereas no effect was found with Cu addition.Elements such as Ca, Mg, Na and K often cause a stray light effect in ICP-AES6 and DCP-AES.1.22.23 We observed a Fig. 5. Spectral profiles at 193.696 nm of 2.5 p ml-1 As(V) in water (---) and of 5000 pg ml-* Cr(V1) in water (* . .! after subtracting the signal due to water deviation of only about 3% for 1000 pg ml-1 solutions, and of about +10 and +5% deviation for 5000 pg ml-1 solutions of Ca, Mg and Na, K, respectively. The foreign elements have less interference effects in our study by DCP-AES than in the reported studies by ICP-AES. This is due to the very good resolution of the Cchelle monochromator that provides a two-dimensional spectral pattern with an average resolution of 0.003 nm.14 Conclusions The As(V) emission line of 193.696 nm gave the lowest background and detection limit and good sensitivity.The acids HOAc and HF caused strong spectral interference and increased intensities the most; HCl and HN03 had the least effect. Among the cations, Cr(II1) and Cr(V1) had the greatest enhancing effects. The stray light effects of Ca, Mg, Na and K, and the matrix effects of Al, Fe, Co, Ni, Cu and Zn, were almost non-existent at 1000 pg ml-1 concentrations. The precision decreased significantly below As( V) concen- trations of 0.75 pg ml-1, which limits the usefulness of the direct method. Sensitivities are considerably better for envi- ronmental samples using the hydride generation system in AAS and AES. 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