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Final Validation of Arsenic, Selenium and a Full Suite of Metals in Rainwater Using ICP-DRC-MS

Monica Garcia-Strickland
Frontier Geosciences Inc.,
Seattle, WA USA

The National Atmospheric Deposition Program (NADP) Mercury Deposition Network (MDN) has been operating since 1994 and is well established with over 110 sites collecting wet deposition for total mercury with a subset of sites measuring methyl mercury. With mercury as a well established part of the NADP, the MDN Hg Analytical Lab (HAL) began an initiative to accurately measure a series of priority trace metals including but not limited to Ag, As, Be, Cd, Cr, Cu, Pb, Ni, Se, Zn.

Since 1998, the HAL has conducted trace metals wet deposition studies at a total of 20 MDN sites. At present, the HAL has made many advances to support this initiative for the NADP MDN: (1) Modified collector for trace metals collection (2) Designed and field tested trace metals sample train (3) Developed routine trace metals sample train cleaning protocols (4) Developed standard operating procedures for trace metals field sampling. The final step of this initiative involves the development of an ultra low-level detection limit technique for quantifying a full suite of metals in rainwater.

Due to ionic interferences in the measurement of low-level concentrations of selenium and arsenic via standard ICP-MS, these samples have been analyzed via hydride generation atomic fluorescence spectrometry (HG-AFS). In order to measure the full suite of metals in rainwater, the HAL has had to split each rainwater sample into 3 sub-samples (1) As by HG-AFS (2) Se by HG-AFS and (3) Cd, Cu, Cr, Ni, Pb, Zn, and Be are measured by standard ICP-MS. This labor intensive approach has made it cost prohibitive to measure the full suite of metals in rainwater as well as cumbersome for data management.

The HAL has overcome these inefficiencies through the design and validation of an ICP-MS based technique using Dynamic Reaction Cell (DRC) technology. DRC removes certain polyatomic interferences for a variety of metals; specifically As, Se and Ni for these samples. The HAL has successfully optimized operating parameters such that only one preparation and analytical run is required for all metals of interest. Analyzing all metals in one run has allowed for decreased operating costs, simplified data processing requirements and a reduction in the amount of sample required, an important benefit for volume-limited wet deposition samples.

Full validation of this method for analysis of trace metals in wet deposition will be demonstrated by

1. DRC optimization for As, Se and Ni

2. MDL/PQL studies for As, Se and Ni by ICP-DRC-MS and all metals of interest by standard ICP-MS

3. Comparison of twenty wet deposition samples analyzed for As and Se by ICP-MS, ICP-DRC-MS and HG-AFS.