Analysis of Arsenic, Selenium and Antimony in Seawater by Continuous-Flow Hydride ICP-MS with ISIS

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1 ICP-MS Environmental Analysis of Arsenic, Selenium and Antimony in Seawater by Continuous-Flow Hydride ICP-MS with ISIS Application Note Steve Wilbur Analysis of arsenic and selenium in seawater at trace levels presents a number of challenges. While ICP- MS is generally considered to be a highly sensitive, interference free technique for analysis of trace metals in environmental samples, matrix effects can result in unacceptably high detection limits for these two elements. These matrix effects are based on two phenomena; 1) ionization suppression in the plasma of high ionization energy elements such as As (9.81 EV) and Se (9.75 EV) in the presence of a significant excess of easily ionizable elements such as Na (5.14 EV) and 2) spectral interferences by argon based polyatomic species such as ArCl and ArAr. For example, ArCl interferes with the only isotope of arsenic and all the significant selenium isotopes suffer from polyatomic interferences of Ar, Cl, or Br. Optimum sensitivity therefore requires some mechanism for separating the analyte from the matrix and reducing or eliminating the argon based polyatomic species. Since arsenic, selenium and a number of other elements (Sb, Te, Bi, Ge, Sn, Pb) are known to form gaseous hydrides under specific reducing conditions, these elements can be removed from the matrix (for example the Na and Ca) and analyzed as gasses in a flowing stream of argon. Reduction or elimination of argon polyatomics can be achieved in the Agilent 4500 or 7500 ICP-MS systems through the use of the ShieldTorch and cooler plasma conditions. As a result, by combining hydride generation with cool plasma/shieldtorch, it is possible to lower the background equivalent concentrations for all As and Se isotopes to low ppb to mid ppt levels in seawater samples. Figure 1. Full-scan Mass Spectrum of Selenium (20 ppb) Showing Excellent Agreement with Expected Isotope Ratios.

2 2 Table 1. Common Interferences on As and Se by Normal Plasma, Direct Nebulization ICP-MS Selenium Isotopes % Abundance Major Interferent(s) % Abundance of Interferent Mass(s) Ge ArAr ArCl ArAr ArAr, BrH , Kr, BrH 11.6, Arsenic Isotope ArCl, CaCl 75.48, Reaction Chemistry: For optimum sensitivity, accuracy and precision, both As and Se must be prereduced to the most efficient oxidation state for hydride formation. This is achieved through the use of a prereduction step. In the case of Se, prereduction to the +IV state can be achieved by the use of HCl plus heat. Arsenic requires a stronger reducing environment, in this case a solution of KI plus ascorbic acid is used to reduce As to the desired +III state. Standards and Reagents: Tune solution: 20 ppb solution of Se or As prereduced as follows. Pre-reductant Stock for As and Sb (KI + ascorbic acid) Dissolve 5 grams each KI and ascorbic acid in 100 ml DI water in a polyethylene bottle. Cap and shake to dissolve solids. Reductant (NaBH 4 solution) Weigh 0.5 g high-purity NaBH4 and g NaOH into a 250 ml polyethylene bottle. Bring to volume (250 ml), cap and shake to dissolve solids. Prepare fresh daily. Calibration standards: While plasma matrix effects are all but eliminated by using hydride generation, the efficiency of the prereduction and reduction steps can be affected by matrix. Therefore, best results will be obtained using matrixmatched standards. In the case of seawater, calibration by method of standard additions gives good results. The standard addition calibration can then be converted to an external standard calibration for analysis of subsequent seawater samples. Replicate 10 ml aliquots of CASS 3 or NASS 5 1 were spiked with a multielement calibration stock containing selenium and pre-reduced as described below. Spike levels were 0, 0.01, 0.05, 0.1, 0.5, 1.0 and 5 ppb. Pre-reduction of samples and standards: - Arsenic (Antimony and Bismuth) 10 ml of sample (seawater) is added to a 50 ml polypropylene centrifuge tube. 1 ml of the KI/ascorbic acid prereduction reagent is added with swirling. 3 ml of concentrated tracemetal grade HCl is added with swirling. The tube is capped loosely and allowed to set for 15 minutes after which it is brought to a final volume of 25 ml with 18 MOhm deionized water. - Selenium (and Telurium) 10 ml of sample is added to a 50 ml polypropylene centrifuge tube. 10 ml of concentrated, tracemetal grade HCl is slowly added with swirling. The tube is loosely capped and heated in a heat block or boiling water bath at 100 degrees C. for 10 minutes. After allowing to cool, the sample is brought to 25 ml final volume with 18 Mohm DI water. 1 National Research Council, Canada

3 3 Tuning Optimized tuning involves maximizing the analyte signal(s) while minimizing Basically, a combination of forward power, sample gas flow (carrier plus makeup), and sample depth which minimizes m/z 78 and 80 in the blank and maximizes those masses in the tune solution (20 ppb Se, pre-reduced as described) is desired. background may be due to trace Se in the reagents used for pre-reduction or hydride formation. Now aspirate the 20 ppb Se tuning solution. Set the acquisition masses at 78, 80 and 82. Set the displayed ion ratio to calculate the ratio of 82 to 80. The natural isotope ratio of Se82 to Se80 is 8.73/49.61 or 17.59%. Therefore as the displayed ratio approaches 17.6%, the background at m/z 80 due to ArAr is minimized. Try to maximize the signal at 78 while maintaining as close to 17.6% for 82/80 as possible. 2 Figure 3. Se spike in CASS 3 Showing Interferences at m/z 80 and 82 from BrH Figure 2. Tune Screens, 20 ppb Se Standard and Prep Blank the interferences. Since the interferences are primarily due to argon-based polyatomics, the use of ShieldTorch with slightly reduced forward power to minimize the ionization of Ar is quite effective. The following conditions were set to allow the measurement of Se at m/z 78 or 80 and also work quite well for As. As a first step, while aspirating a prep blank under hydride generating conditions, try to reduce the background at m/z = 80 to 10-20,000 counts per 0.1 sec. This is accomplished by cooling the plasma using a combination of RF power and carrier/makeup gas. See figure 2 for sample tune condiitions. Some of the 2 Note: This must be done on a clean Tune solution, not a sea water spike since BrH from the sea water can cause high background at both 80 and 82. Occasionally, krypton (m/z 82) in the argon supply can be sufficiently high to adversely affect the ratio as well.

4 4 ISIS Program The ISIS program builder was used to create the ISIS program below. Typical values for ISIS parameters for hydride generation are shown. As a rule, sensitivity increases with sample flow at the expense of sample consumption. The ratio of sample to reductant is important and should be optimized as well. Normal rinseout times are very fast due to the high sample flows utilized. Figure 4. ISIS Program Builder and Method ISIS Parameters Showing Prerun, Startrun and Postrun Programs and Setpoints for ISIS Pumps

5 5 Figure 5. Selenium Standard Addition Calibration in DI Water at 0, 10, 50, 100 and 500 ppt. Figure 6. Standard Addition Calibration in CASS 3 Standard Reference Seawater. Calibration Levels; 0, 10, 50, 100, and 500 ppt.

6 6 Analysis of Certified Reference Materials Seawater certified reference materials CASS 3 and NASS 5 were analyzed for As, Se and Sb. Standard addition calibrations were prepared by spiking 10 aliquots of sample with a mixed calibration solution containing the elements of interest. Standard addition calibrations were prepared and then converted to external calibrations for subsequent sample analysis. 3 sigma MDLs were calculated from seven replicate analyses of the unspiked seawater samples using the converted external calibrations. Spike recoveries were also calculated for samples spiked at 0.05 and 2.5 ppb for both elements. Figure 7. Arsenic in CASS 3 by Standard Addition Figure 8. Antimony in CASS 3 by Standard Addition

7 7 Summary The use of online, continuous-flow hydride generation coupled to the Agilent 4500 or 7500 ICP-MS offers a fast, sensitive, routine analytical technique for the analysis of the hydride forming elements such as As, Se and Sb in difficult matrices such seawater. The process can be fully automated for multiple samples using the Cetac ASX-500 autosampler and the Agilent Integrated Sample Introduction system (ISIS). 3- sigma detection limits are typically ppt for these elements, which is below ambient levels for all three. However, slightly elevated background equivalent concentration for Se can make ambient-level Se analysis borderline at best. The use of purified reagents may help to reduce the BEC for Se to levels closer to the calculated detection limit. When compared to direct nebulization ICP-MS analysis of 10X diluted seawater, detection limits are improved from 10 to 50 times with no long-term matrix effects on the ICP-MS interface or ion lenses. Table 2. Results of Analysis of Certified Reference Seawater Materials Sample Element /Isotope Measured Value Certified Value Spike Amount Blank Measurement % Recovery (3) CASS 3 As/ ppb 1.09 N/A CASS 3 Se/ (1) (2) CASS 3 Sb/ not certified NASS 5 Sb/ not certified NASS 5 As/ ppb 1.27 N/A MDL (4) (1) raw measured concentration, not corrected for prep(reagent) blank (2) total selenium is listed but not certified in CASS 3 (3) recovery calculated against certified value where available and against matrix spike recovery where certified value is not available. (4) 3-sigma using seven replicates Agilent Technologies shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing, performance or use of this material. Information, descriptions and specifications in this publication are subject to change without notice. Visit our website at Copyright ã 2000 Agilent Technologies, Inc. Printed in Japan (03/00) E