Assessment of matrix-dependent analyte stability and volatility during open-vessel sample dissolution for arsenic, cadmium, mercury and selenium.

The effects of calcium and magnesium (as nitrates) and phosphorous (as hydrogen phosphate) were investigated on the stability of As, Cd, Hg, and Se during open-vessel dissolution in Teflon vessels. Samples of mainly inorganic and biological matrices were dissolved in screw-capped Teflon tubes in HNO(3) only or in a mixture of HNO(3)-HF. The caps were then removed and the solutions were simultaneously evaporated at 120 °C to near dryness without drying the contents (Method I) or to complete dryness with extended heating for 20 min at dryness (Method II). ICP-MS analysis indicated that the stabilities of Se and Hg were highly influenced by Ca, Mg and PO(4) content in the sample. Arsenic (As) and Cd did not show any significant instability or volatility. Selenium was lost in Method II from biological samples containing trace levels of Ca, Mg and PO(4). Mercury was unstable during heating in all samples, except bone ash for which no significant loss was detected in Method I. Losses observed for Hg and Se were consistent with Ca, Mg and PO(4) deficiency in the samples and hence indicated that nitrate and hydrogen phosphate salts of these matrix elements do improve stability of the relatively volatile elements during open-vessel dissolution in teflon vessels. While Se was effectively stabilized with sub-per cent levels of Ca, Mg and PO(4), Hg due its high volatility required significantly higher levels of Ca and PO(4) in the bone ash.