Development of a Simple System for the Determination of Arsenic after Hydride Generation Atomic Absorption Spectrophotometry

摘要

A simple and inexpensive hydride generation system that uses a pyrex boiling tube as a reduction vessel to hold the acidified sample into which 2cm3 of 100-300mg sodium tetrahydroborate (III) solution is injected such that the excess molecular hydrogen evolved carries the generated arsine directly through the atomic absorption spectrophotometer nebuliser to a nitrogen-hydrogen entrained air flame for atomic absorption measurement is described. The average absorption by arsenic atoms generated from the total arsine evolved was measured by integrating the absorbance for 16 seconds. The use of the absorption signal integration mode was found to free the technique from most kinetic interferences, while the stripping of the arsine from solution reduces the potential for chemical interferences in the solution phase. Gas flow rates affected sensitivity markedly, but the optimal flow rates for nitrogen and hydrogen were standardised on 90cm3s-1 and 30cm3s-1 respectively as a compromise between lower flow-rates giving longer arsenic residence times and higher sensitivity, and higher flow-rates giving a stiffer, less draught-sensitive flame. An optimum flame height of 6mm above the burner produced sufficient atomization and low background absorption. Generally, sample volumes between 5-20cm3 could be used in different acidic media within the concentration range 10% (v/v) - 50% (v/v). The proposed system produced a significantly improved atomic absorption sensitivity of 25ng and a detection limit of (95% confidence) of 5.2ng for arsenic. Both performance characteristics were found to be better by a factor ≥ 12 and ≥ 27 respectively than achieved with air-acetylene flames. The method is highly reproducible at trace levels having an overall precision of 1% for 0.5μg of arsenic. The upper limit of linearity of the response for atomic absorption measurements is about 0.7μg. In use, the apparatus is fast and convenient to operate, enabling about forty determinations per hour. @ JASEM