Spectrophotometric detection of arsenic using flow-injection hydride generation following sorbent extraction preconcentration

An automated system with a C18 bonded silica gel packed minicolumn is proposed for spectrophotometric detection of arsenic using flow-injection hydride generation following sorbent extraction preconcentration. Complexes formed between arsenic(III) and ammonium diethyl dithiophosphate (ADDP) are retained on a C18 sorbent. The eluted As-DDP complexes are merged with a 1.5% (w/v) NaBH4 and the resulting solution is thereafter injected into the hydride generator/gas–liquid separator. The arsine generated is carried out by a stream of N2 and trapped in an alkaline iodine solution in which the analyte is determined by the arsenomolybdenum blue method. With preconcentration time of 120 s, calibration in the 5.00–50.0 μg As l−1 range and sampling rate of about 20 samples h−1 are achieved, corresponding to 36 mg ADDP plus 36 mg ammonium heptamolybdate plus 7 mg hydrazine sulfate plus 0.7 mg stannous chloride and about 7 ml sample consumed per determination. The detection limit is 0.06 μg l−1 and the relative standard deviation (n=12) for a typical 17.0 μg As l−1 sample is ca. 6%. The accuracy was checked for arsenic determination in plant materials from the NIST (1572 citrus leaves; 1573 tomato leaves) and the results were in agreement with the certified values at 95% confidence level. Good recoveries (94–104%) of spiked tap waters, sugars and synthetic mixtures of trivalent and pentavalent arsenic were also found.