Cysteine enhancement of the cryogenic trap hydride AAS determination of dissolved arsenic species

Pre-derivatization of arsenic species with l-cysteine prior to their determination by a modified cryogenic trap hydride AAS procedure leads to more uniform sensitivities to arsenite, arsenate, methylarsonate and dimethylarsinate and improved resistance to interferences from metal ions. This pre-treatment can be carried out at room temperature, or more rapidly by microwave heating. The conditions required for the reduction of the resulting arsenic-cysteine complexes differ from those required for the generation of hydrides from the parent compounds. In particular, lower levels of acid are required, resulting in a reduction in the acid-derived blank contributions to the inorganic arsenic response. The pre-treatment procedure has been found to improve the performance of the hydride trap procedure when measurements are made in the presence of metals such as iron(II), nickel(II), cobalt(II), manganese(II) and copper(II) which interfere in the conventional cryogenic trap HG-AAS speciation procedures. The detection limits (3 sigma) of the four species were in the range 30–99 pg with relative standard deviations of 3.1–7.9%, giving with 1 ml samples, detection limits of ca 50 ng 1−1.