On-line cryogenic trapping with microwave heating for the determination and speciation of arsenic by flow injection/hydride generation/atomic absorption spectrometry.

An on-line flow injection-hydride generation/atomic absorption spectrometry method was developed for the preconcentration and selective determination of inorganic arsenic [As(III) and As(V)] and its methylated species. The separation of the arsenic species was performed by an automated pH-selective arsines generation technique, using sodium tetrahydroborate(III) as reductant. Each arsine was cryogenically trapped in a PTFE coil, knotted and sealed inside another wider diameter tube, through which liquid nitrogen was suctioned by negative pressure. Then, based on their different boiling points, the arsine species were selectively liberated by using a heating cycle of microwave radiation, followed by atomic absorption detection. A sample solution aliquot mixed with 1% citric acid was used for the determination of As(III) alone, while a second sample aliquot mixed with 2 mol l(-1) nitric acid was used for the quantitative determination of total inorganic arsenic, monomethylarsonic acid and dimethylarsinic acid. Based on 10 ml sample, the detection limits lie within the range 20-60 ng As l(-1), which are sufficiently low to detect the arsines-forming species in natural waters. These values are negatively affected by the reagents purity and background noise due to flame flickering, but the sensitivity can substantially be improved by increasing sample size or running several consecutive reactions.