Interference by volatile nitrogen oxides and transition-metal catalysis in the preconcentration of arsenic and selenium as hydrides

When tested individually In a clean system, NO3- and HNOa are shown not to be serious Inhibitors of AsH3 and H2Se evolution. The real inhibitors of AsH3 and H2Se evolution (encountered when solid samples are oxidatively dissolved In nitric acid) are shown to be correspondingly reduced nitrogen oxides (NO2-, etc.) produced In sample digestion. Volatile NO, Interferants can then result upon acidic disproportionation. These volatile NOx species are found to transport with the AsH3 and H2Se analyte. The N2O3 species Is detected In a cryogenic trap. When dissolved In downstream H2S04 drying agents, the NOx can give rise to powerful oxidants such as NO+ which can accumulate In the H2S04 and thereby account for “memory Interferences” encountered with subsequent samples. The use of sulfamic acid Is Investigated In the preliminary removal of reduced nitrogen oxides such as NO2-. Transition metal cation interferences are also studied. Several metal masking procedures previously reported to work well for AsH3 are shown here to fall for the evolution of H2Se. BH3CN” Is shown to be an effective new reductant to evolve AsH3 and H2Se from As(III,V) and Se(IV) In acid media. BH3CN” Is also found to prevent the formation of reduced nickel and copper and to eliminate or reduce the magnitude of these metal Interferences. Reduced copper Is shown to catalyze the BH4- reduction of NO3- to give NO2- (a more severe Inhibitor of AsH3 and H2Se evolution). Inhibition by a combination of copper and NO3” Is therefore far more deleterious than can be accounted for by the sum of the Individual inhibitory behavior of these species taken separately. In this case, as little as 0.5 ppm Cu2+ produces a severe catalytic Inhibition In the presence of NO3-.