Determination of Arsenic in Environmental and Biological Samples by Flow Injection Inductively Coupled Plasma Mass Spectrometry MENG-FEN HUANG SHIUH-JEN JIANG* AND CHORNG-JEV HWANG Department of Chemistry National Sun Yat-Sen University Kaohsiung Taiwan 804 Republic of China A simple and very inexpensive in situ nebulizer-hydride generator was used with inductively coupled plasma mass spectrometry (ICP-MS) for the determination of arsenic in environmental and biological samples. The application of hydride generation (HG)-ICP-MS alleviated the spectral interferences and sensitivity problems of arsenic determinations encountered when conventional pneumatic nebulization is used for sample introduction. The sample was introduced by flow injection to minimize deposition of solids on the sampling orifice.The arsenic in the sample was reduced to AS(III) with L-cysteine before being injected into the HG system. A detection limit of 0.003 ng ml-' was obtained for arsenic. The method has been successfully applied to the determination of arsenic in National Research Council of Canada reference materials CASS-2 (Nearshore Seawater Reference Material for Trace Metals) NASS-3 (Open Ocean Reference Material for Trace Metals) and SLRS-2 (Riverine Water Reference Material for Trace Metals) and in National Institute of Standards and Technology Standard Reference Material 2670 Toxic Metals in Freeze-Dried Urine. Precision was less than 5% and analysis results were within 6% of the certified values for all determinations. Keywords Inductively coupled plasma mass spectrometer; arsenic; flow injection; hydride generation; biological and environmental samples Inductively coupled plasma mass spectrometry (ICP-MS) is a relatively new technique for trace multielement and isotopic analysis'.' and still has some limitations.A highly saline sample can cause both spectral interferences and matrix effects. Spectral overlaps are seen when the polyatomic ions from the matrix such as ArNa' C10' or ArCl' overlap with analyte ions such as 63Cu+ 51V' or 75A~+ respectively. Changes in analyte counting rates are observed with high levels of salts particularly heavy matrix The determination of arsenic in environmental and biological systems is gaining increasing importance mainly because of the ubiquitous nature of this element and the toxic character- istics of some arsenic species.Interference by ArCl' occurs in the determination of arsenic in high chloride content samples by ICP-MS. Generally chloride interference is either removed by correction procedures7 or chemical separation procedures such as column separation or hydride generation (HG) Sample introduction by HG has been applied in several ICP-MS methods for the determination of ar~enic.~-'~ However one of the major drawbacks of these analyses is that the acid HCl used for HG could form the molecular ion ArCl' which interferes with arsenic determinati~n.~ In the * To whom correspondence should be addressed. Journal of Analytical Atomic Spectrometry present work a simple continuous-flow HG system without conventional gas-liquid phase separation has been employed as a sample introduction device for flow injection (F1)-ICP-MS ana1y~is.l~~'~ With this system only a minimal and inexpensive modification of existing standard equipment is required.Furthermore L-cysteine was employed as the pre-reductant with this reagent only a mild nitric acid condition is required for ~G.13315-19 These combinations reduced Arc1 + molecular interference significantly. Finally the sample was introduced by FI to minimize deposition of solids on the sampling orifice. The sampler would clog in a few minutes if the difficult urine and seawater matrices were introduced continually with the sample introduction system used in the present work. Results obtained for the determination of arsenic by FI-ICP-MS with in situ HG-nebulization are presented in this paper.EXPERIMENTAL ICP-MS Device and Conditions An Elan 5000 ICP-MS instrument (Perkin-Elmer SCIEX Thornhill Ontario Canada) was used. Samples after passing through the HG system were introduced with a crossflow pneumatic nebulizer with a spray chamber of Scott type. The ICP conditions were selected to maximize ion signals while a solution containing 10 ng m1-l arsenic in 0.1 moll-' HNO was continuously introduced into the hydride generator. The sensitivity of the instrument varied slightly from day-to-day. The operating conditions used throughout this work are summarized in Table 1. Table 1 Instrumentation and conditions Plasma conditions Rf power/W Plasma gas flow/] min-' Intermediate gas flow/l min Aerosol gas flow/l min-' Mass spectrometer settings Bessel box lens/V Bessel box plate lens/V Photon stop lens/V Einzel lenses 1 & 3/V Resolution Baseline time/ms Points per spectral peak Number of replicates Reading per replicate Sweeps per reading Dwell time/ms Replicate time/ms* Transfer frequency 1 1100 14 0.9 1.03 10.95 - 65.90 - 10.05 2.97 Normal 5000 1 1 103 20 20 41200 Replicate * Replicate time = (dwell time) x (sweeps per reading) x (readings per replicate).Journal of Analytical Atomic Spectrometry January 1995 Vol. 10 31Data acquisition parameters used for this study are listed in Table 1. The element selected FI peaks were recorded in real time and stored on the hard disk with the 'graphic' software. Under the combination of dwell time sweeps per reading and readings per replicate a data point could usually be obtained in 1 s.Either peak height or peak area of the flow injection peak can be used for data handling. Flow Injection System A simple FI system was used throughout this study. It was assembled from a six port injection valve (Rheodyne Type 50) with a 200 pl sample loop. The carrier solution (0.05 mol 1-' HNO,) was delivered with a peristaltic pump. The schematic diagram of the FI system is shown in Fig. 1. Hydride Generation System and Conditions In this study a continuous-flow in situ HG-nebulizer sample introduction system was coupled with ICP-MS for arsenic determination with FI analysis. With this sample introduction system the entire injected sample was nebulized. The nebuliz- ation process in which the liquid is shattered into fine droplets in an Ar stream is a very effective way to purge AsH3 from the liquid probably more so than bubbling Ar through a static reservoir of bulk liquid as in a conventional gas-liquid separ- ator.Almost all the arsenic is liberated from the droplets as ASH and then goes to the plasma. The schematic diagram of the in situ HG-nebulizer and the experimental facility is presented in Fig. 1. The operating conditions for HG were optimized by FI analysis using 10 ng ml-' arsenic(Ir1) as the model solution. This solution was loaded into the injection loop and injected into the HG system. Operating parameters which could affect the efficiency of hydride formation concentration of HNO volume of mixing coil and the concentration of NaBH4 were studied in order to optimize the conditions. Interferences Studies Since a conventional pneumatic nebulizer instead of a gas- liquid separator was used in this study the chloride still reached the plasma and the ArCl' interference was still present.In order to demonstrate the effectiveness of this FI-HG system for alleviating the interference of ArCl' on Asf a solution of 10 ng ml-' arsenic was spiked with increasing concentrations of chloride (0 100 1000 10000 pg ml-') as NH,Cl. These solutions were injected into the HG-ICP-MS instrument and the arsenic signals were determined and compared with the results obtained when a conventional pneumatic nebulizer was used. Transition metal interference (Cu Ni and Co) in the determi- nation of arsenic by HG-ICP-MS was also carefully investi- gated.This was done by spiking 10ngml-' arsenic with various concentrations of Cu Ni and Co. These solutions were analysed and the arsenic signals were obtained and compared with the signals obtained for the interferent-free solution. Reagents and Sample Preparation Analytical-reagent grade chemicals were used without further purification. The applicability of the method to real samples was demonstrated by the analysis of National Research Council of Canada (NRCC) seawater reference materials CASS-2 ( Nearshore Seawater Reference Material for Trace Metals) NASS-3 (Open Ocean Reference Material for Trace Metals) and SLRS-2 ( Riverine Water Reference Material for Trace Metals). As AS(III) shows better sensitivity in the arsine gener- ation process it is preferred to reduce other arsenic species to AS (111) with pre-reducing agents before arsine generation.20*21 The sample pre-treatment procedure used is as follows an 80 ml portion of the water sample was transferred into a 100ml volumetric flask followed the addition of a suitable amount of 1 mol 1-' HNO and L-cysteine and dilution to the mark with distilled de-ionized water.The final solution contained 1% L-cysteine and 0.1 mol 1-' HNO and this solution was heated in a water bath at 100 "C for 30 min. In a separate experiment it was found that As(v) monomethylar- sonic acid and dimethylarsenic acid were quantitatively reduced to AS(III) by the pre-reduction procedure described above. Another reference material of high salinity [National Institute of Standards and Technology Standard Reference Material (SRM) 2670 Toxic Metals in Freeze-Dried Urine] was also analysed.The solid provided was digested by the following procedure the contents of one sample bottle were reconstituted with 20ml of water and poured into a Teflon PFA vessel followed by the addition of 5 ml of nitric acid. The vessel was sealed and then heated inside a microwave oven (CEM MDS-2000). After cooling the digest was diluted 50 times and treated with the same pre-reductant procedure described above. RESULTS AND DISCUSSION Selection of FI Operating Conditions Acid concentration is critical in the determination of arsenic by HG. Thus various concentrations of HNO solution were tested as the carrier solution for the FI system. The result is shown in Fig.2. Since the injected samples contained 0.1 moll-' HNO and 1% L-cysteine the concentration of HNO in the carrier did not affect the arsenic signal signifi- cantly. In fact the arsenic ion signal decreased when the HN03 concentration was greater than 0.05 moll-'. Thus 0.05 mol 1-' HNO was selected as the carrier in the following experiments. Selection of HG Conditions Fig. 3 shows the peak area of the FI peak as a function of sodium tetrahydraborate. The optimum concentration is 0.3% as shown in Fig. 3. Compared with conventional pneumatic 200 pI Injector 200 pi Mixing coil (1 mi min-') .... .... 0.3 % NaBH in 0.02 mol I-' NaOH (1 rnl min-') Per i sta I t ic pump Fig. 1 Schematic diagram of FI-HG-ICP-MS 32 Journal of Analytical Atomic Spectrometry January 1995 Vol.101.5 1.6 3 1.4 C 0 .- 1.2 > .- c - 0) 1.0 fE 0 0.1 0.2 [HNO,l/mol I - ' ' ' ' Fig. 2 Effect of carrier solution composition on arsenic signal. Concentration of NaBH was 0.3% in 0.02 moll-' NaOH. All the solution flow rates were set to 1.0 ml min-l. All the data were measured relative to the first point loo 1 0 0.2 0.4 0.6 0.8 1.0 [NaBH,] (%) Fig.3 Effect of NaBH concentration on arsenic signal. Carrier solution of FI system was 0.05 moll-' HNO,. Injected arsenic concen- tration was 10 ng ml-I in 0.1 moll-' HNO and 1% L-cysteine. All the solution flow rates were set to l.Omlmin-'. All the data were measured relative to the first point nebulization a 75 times improvement in the arsenic ion signal was obtained when 0.3% NaBH was used as reductant in the HG system.This concentration is much lower than the concen- tration used in conventional methods.12.22 One possible expla- nation for this is that the use of L-cysteine could improve arsine generation at a low concentration of NaBH,. Furthermore as the concentration increases the amount of hydrogen generated increases as well. The increased hydrogen production appears to have a detrimental effect on the ICP-MS system. Thus the optimum NaBH concentration is a compro- mise between the increase in the amount of arsenic introduced and the decrease in the ionization efficiency of the plasma. No obvious sampler blocking was observed over four hours of analysis when 0.3% NaBH was used as reductant. Since a suitable amount of L-cysteine had been added during sample pretreatment no extra L-cysteine was needed in the HG system.In separate experiments it was found that the volume of the mixing coil did affect the ion signal significantly a 200 pl mixing coil was used in the following experiments. In summary the optimum operating conditions of the FI-HG system are presented in Fig. 1. Selection of ICP Operation Conditions The performance of an ICP-MS instrument is strongly depen- dent on operating conditions.'3J3 The two key parameters are the aerosol gas flow rate and the plasma forward power. The dependence of the arsenic ion signal on the aerosol gas flow rate is depicted in Fig. 4. Although not illustrated here the dependence of the arsenic ion signal on the plasma forward power is similar to that reported previously.12*22 0.8 1 I 0.95 1.00 1.05 1.10 Aerosol g a s flow rate/l min-' Fig.4 Effect of aerosol gas flow rate on arsenic ion signal.Plasma forward power was 1.1 kW. All the data were measured relative to the first point Flow Injection Peaks and Detection Limit Typical FI peaks obtained for 200 pl injections of two solutions containing 1 ng ml-' arsenic and 10000 pg ml-1 chloride respectively are shown in Fig. 5. As shown in Fig. 5 10000pgml-' C1 will produce a signal equivalent to 0.3 ng ml-' arsenic at m/z 75 with this HG sample introduction method. Repeatability was determined using seven injections of a 1 ng ml-1 arsenic test solution. The relative standard deviation of the peak heights for these seven injections was less than 2% for arsenic. Calibration curves based on peak heights were linear for arsenic in the range tested (0.1-10 ng ml-I).Sensitivity for arsenic was 5700 countsml s-' ng-' and the background was about 400 counts s-' at m/z 75. The detection limit was estimated from these calibration curves based on the usual definition as the amount (or concentration) necessary to yield a net signal equal to three times the standard deviation of background noise. The absolute detection limit was 0.6 pg corresponding to a relative value 3 pg ml-I. The detection limit obtained in this work is comparable to or better than previous results with similar technique^.^-^' The reagent blank was found to contain 20pg of arsenic which could be a result of contaminants in the L-cysteine and HN03 used in the sample pre-treatment ( pre-reductant).Interference Studies The relative arsenic signal with and without HG of 10 ng ml-1 arsenic solution is shown in Fig. 6. It can be seen that the ArCl' interference is removed at up to 10000pgml-' of 10000 r 1 c I v1 v) 3 *-' 8 -. c 2 4- C 0 0 A I 4000 ::I 2000 0 20 40 60 80 100 120 E I u t io n ti m e/s Fig. 5 Typical flow-injection peaks of A 1 ng ml-l As and B 10000 pg ml-' C1. Reagent blank was subtracted in both elution peaks. Operating conditions of FI hydride generation are given in Fig. 1 Journal of Analytical Atomic Spectrometry January 1995 Vol. 10 334 l - I - .P I 3 PI 0 ' I I I 10 100 1000 10000 [Chloridel/pg ml ' Fig. 6 Effect of increasing chloride concentration on relative As signal in ICP-MS with @ hydride generation sample introduction and 0 conventional nebulization. Actual concentration of arsenic was 10 ng ml-'.Operating conditions of FI hydride generation are given in Fig. 1 120 I 1 100 p=+4 2o t I u 0' I I l l I 1 0.1 1 10 100 1000 10000 [Transition metal ionl/pg ml-' Fig.7 Effect of increasing concentration of Ni and Cu on arsine generation 0 10 ng ml-' As in 0.1 moll-' HN03 and presence of Cu; @ 10 ng ml-' As in 1% L-cysteine and 0.1 moll-' HNO and presence of Cu; 0 10 ng ml-' As in 0.1 moll-' and presence of Ni; B 10ngml-' As in 1% L-cysteine and 0.1 moll-1 HNO and presence of Ni. Operating conditions of FI hydride generation are given in Fig. 1 chloride when the sample was introduced with HG. The data with conventional nebulization show a larger positive bias thus demonstrating the value of HG for alleviating ArCl' interference. L-cysteine has proved to be very efficient in reducing inter- ferences in the determination of arsenic by HG and atomic spe~trometry.'~-'~ The efficiency of L-cysteine at reducing interferences from transition metal ions is illustrated in Fig.7. In Fig. 7 the interference effects are compared of Ni(r1) and Cu(n) in the determination of arsenic with and without the addition of L-cysteine as a releasing agent. Recoveries were calculated by comparison with the arsenic standard in the absence of the interfering ion. In the absence of L-cysteine 10 pg ml-' of Ni(r1) and 10 pg ml-' of CU(II) reduce the arsenic signal severely. However with the addition of 1% L-cysteine there was no significant interference from 100 pg ml-' of Ni(I1) or 1000 pg ml-' of CU(II).Although not illustrated here other transition metals showed similar results. These results are in agreement with those reported previously with similar atomic- spectroscopic technique^.'^-'^ Determination of Arsenic in Riverine Water and Urine In order to prove the suitability of the system in real sample analysis several reference samples were analysed. A 200 pl portion of the low C1 content SLRS-2 was analysed for arsenic using the FI-HG system. The concentration of arsenic present in the solution was quantified by the standard additions method and the result is presented in Table2. This result compares satisfactorily with the certified value. Concentrations of arsenic in high C1 content NIST SRM 2670 were also determined by FI-HG-ICP-MS.The results are presented in Table 2. This experiment indicated that arsenic in urine could be readily determined by HG-ICP-MS using the FI procedure. No obvious molecular ion overlap interference was observed. Determination of Arsenic in Seawater Samples A sample with a large concentration of chloride forms the molecular ion 40Ar35C1+ that interferes with the determination of 75A~+. The extent of interference is such that a direct determination of arsenic in a concentrated chloride sample is impossible. When the interfering signal (ArC1') is stable and not much larger than the analyte signal (As') the ArCl' contribution to the ion signal can be subtracted from the total ion signal according to the following equation i7'As=i(75)-3.08 x i(77)+0.993 x i(78) where i7'As is the ion signal of arsenic at m/z 75 and i(75) i( 77) and i( 78) are the total ion signals at m/z 75 77 and 78 respectively.As shown in Fig. 5 10000 pg ml-' C1 only pro- duces a signal equivalent to the signal produced by 0.3 ng ml-' arsenic at m/z when HG was used. The correction equation described above should be suitable for subtracting ArCl' contribution at m/z 75. In order to demonstrate the effectiveness of this method for alleviating the ArC1' interference. Two high chloride content samples (NRCC NASS-3 and CASS-2) were analysed for arsenic. The amount of arsenic present in each sample was determined by the standard additions method. The ArC1' interference was removed by the correction equation described above.The results are given in Table2. These results agree with the certified value. Since FI sample introduction was used and only a small volume of sample was injected the sensitivity of arsenic did not change too much even when seawater was analysed. The value of HG with ICP-MS for the quantification of arsenic in matrices containing high concentration of C1- has been demonstrated convincingly. This research was supported by a grant from the National Science Council of the Republic of China. Table 2 FI-HG-ICP-MS analysis of arsenic in selected reference materials. 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