Chemical vapor generation for atomic spectrometry. A contribution to the comprehension of reaction mechanisms in the generation of volatile hydrides using borane complexes

A systematic study has been developed in order to clarify the mechanism of hydride generation using different borane complexes [sodium tetrahydroborate(III), NaBH4 (THB); borane–ammonia complex, H3B–NH3 (AB); borane–tert-Butylamine complex, H3B–NH2C(CH3)3 (TBAB)], as derivatizing reagents. Stannane, stibine and bismuthine were generated in a continuous flow reaction system at different acidities in the pH range of 1.38–12.7. The pH of sample solution was pre-equilibrated on-line in a mixing loop by the addition of appropriate solution before the reaction with the derivatizing reagent in a reaction loop. The generated hydrides were delivered to a miniature argon hydrogen flame atomizer and free atoms detected by atomic absorption spectrometry (AAS). The effect of pH on the relative sensitivity has been investigated by varying both the mixing loop volume (4, 15 and 50 μl) and reaction loop volume (100 and 500 μl). The mixing rates of the solutions have been also tested to avoid any undesired effect arising from the incomplete mixing of the solution in the flow reaction system. The generation of hydrides using on-line pre-equilibration of pH can be observed also in alkaline or neutral conditions, while the generation of the same hydrides is observed only in acidic solution if the equilibration of pH was performed off-line. Stannane generation using amineboranes has never been reported before. Kinetic calculations were performed in order to estimate the concentration of nascent hydrogen arising from the decomposition of the derivatizing agents in the flow reaction system. It has been found that in many cases, the mechanism of nascent hydrogen failed to explain the generation of the hydrides. The direct action of BH4− and H3B-X species (X=ammonia or amino group) on the analyte element, present in solution in a suitable chemical form, is the only possible mechanism of hydride formation in a wide range of solution acidities, from pH 4.5 up to pH 12.7. The possible role that pH and different acid species, HA, could play in the modification of derivatization reaction is discussed.