Evaluation of approaches to the abatement of nitrate interference with photochemical vapor generation

Abstract

Photochemical vapor generation (PVG) provides an alternative to chemical vapor generation techniques but suffers severe interference in the presence of oxidizing species, notably nitrate and nitrite. Although the mechanism is not completely understood, scavenging of photogenerated free electrons and hydrogen radicals by NO₂⁻ and NO₃⁻ likely plays a role in terminating analyte reduction reactions which lead to hydridization, carbonylation or alkylation of analytes. In this study, several strategies for the elimination of nitrate and nitrite from samples prior to PVG were examined. Since the Se(IV)-acetic acid PVG system proved to be extremely sensitive to low levels of these oxidants, wherein 50% loss of generation efficiency is realized at [NO₃⁻] > 2.5 mM, it was used as a test case. To this end, a number of practical approaches to the reduction of nitrate and nitrite were evaluated, including: utilizing sulfamic acid, hydrazine sulfate, sodium thiosulfate and V(III), as well as removal by photo-reduction and quantitative precipitation by nitron. Whereas none of these approaches could be implemented with impunity, a methodology based on UV photolysis of a basic solution containing 2% hydrazine sulfate provided high NO₃⁻ removal efficiencies. Residual NO₂⁻ was quantitatively eliminated by addition of sulfamic acid to the acidified solution, allowing efficient generation of volatile Se(CH₃)₂ species. The methodology was tested by application to Certified Reference Material SLRS-3 (river water, acidified to pH 1.6 with nitric acid), achieving 83 ± 2% Se(IV) spike recoveries using PVG sample introduction coupled with ICP-MS for detection, illustrating effective control over interferences and recovery of Se(IV).