Determination of Arsenic and Selenium in Chloro-matrix Rainwater

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					                                     NATIONAL HIGH MAGNETIC FIELD LABORATORY
                                               2009 RESEARCH REPORT

Determination of Arsenic and Selenium in Chloro-matrix Rainwater Collected near a CFPP by
Quadrupole-ICP-MS with an Octopole Reaction System

John Rolison; Sambuddha Misra; P. N. Froelich; William Landing; Katie Gosnell (NHMFL &
Oceanography Department, FSU)

Coal-fired power plants (CFPPs) are responsible for the release of many toxic elements and compounds to the atmosphere,
which are then removed from the atmosphere through wet and dry deposition. Of particular interest is the fate of gas-phase
mercury due to the inherent toxicity of mercury and the very toxic organo form, methylmercury. Essential to understanding
the fate of atmospheric mercury is the ability to trace the path of CFPP emissions through the atmosphere. This can be
accomplished by measuring a suite of elements that are expected to be present in CFPP emissions at elevated concentrations
relative to background atmospheric values. Two such elements of significance are arsenic and selenium, which are
volatilized and emitted in a similar manner to mercury, and are therefore important tracers for understanding emissions,
atmospheric reactivity and transport.

Measuring trace (< 1 ppb) concentrations of arsenic and selenium using ICP-MS presents a considerable challenge. Analyses
of arsenic, a monoisotopic element (75As), suffer from severe interferences due to the polyatomic ions 40Ar35Cl+ and 38Ar37Cl+.
These interferences are substantial even at low chloride levels. The rainwater samples in this study were collected near the
Crist CFPP in Pensacola, Fl, and contain marine salts. Also, the rainwater samples are spiked with 0.045N HCl to preserve
methylmercury, and then with 0.048N HNO3 to preserve total mercury. Selenium isotopes 76Se, 77Se, 78Se, and 80Se suffer
from interferences due to plasma dimers, most notably 38Ar40Ar+ and 40Ar40Ar+. To eliminate these interferences we have
developed a method that utilizes a Q-ICP-MS with an octopole reaction system (ORS).

Rainwater samples were analyzed for arsenic and selenium under two distinct conditions with an Agilent 7500cs. 75As was
measured while the ORS was in collision mode, which uses non-reactive helium to reduce interference due to 40Ar35Cl+
through kinetic energy discrimination. Operating the ORS in reaction mode allows for the reduction of interferences on
selenium isotopes by reacting hydrogen (H2) with plasma dimers. Each sample analysis requires approximately 90 seconds
and <0.5 mL of rainwater.

Results and Discussion
Figure: The use of the ORS drastically reduces the
interferences of interest. The matrix blank, consisting
of DI water and the mixed acid spike, has an average
signal intensity of ~105 cps for 75As before activation
of the ORS.         Optimized ORS collision mode
conditions reduces the average 75As signal intensity in
the matrix blank to ~200 cps with an average
sensitivity of ~3,000 cps/ppb. Optimization of the
ORS in reaction mode reduces average 78Se signal
intensities in the matrix blank from ~6x104 cps to ~75
cps with an average sensitivity of ~3,500 cps/ppb-Se
(not shown). Due to better signal stability and a higher
signal to noise ratio, 78Se was chosen as the selenium
isotope to be measured over the more abundant 80Se.

This analytical method is a viable option for analyzing
chloro-matrix samples for arsenic and selenium.

Froelich, P. N., S. Misra, K. Gosnell, J. Rolison, W.
Landing, 2010. J. Anal. Atom. Spec., to be submitted.

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Tags: Selenium
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