Fracdonal determination of arsenite, arsenate, monometliylarsonic acid, and dimethylarsinic acid in natural waters by hydride generation with atomic absorption spectrophotometry combined with a liquid nitrogen trap

A method was developed for the determination of arsenite, arsenate, monomethylarsonic acid (MMAA), and dimethylarsinic acid (DMAA) in natural waters at sub-ppb levels. This technique is based on the fractional vaporization of arsines from a liquid nitrogen trap. Arsenite, arsenate, MMAA, and DMAA in a sample were reduced to arsine, methylarsine, and dimethylarsine, respectively, by an addition of 5 ml of 5 % NaBH4 after adjusting pH to 0.5 by 6N HCl in a reaction vessel. The arsines generated from the sample were collected in a liquid nitrogen trap, and the separation of arsines was accomplished by the sequential vaporization due to their different boiling points. Arsines vaporized from the trap were carried to a quartz furnace by helium carrier gas and were analyzed by atomic absorption spectrophotometry. The reduction of arsenic compounds depended on pH of the solution. Arsenite was reduced at pH 4, while arsenate was not reduced above this pH. Therefore, arsenite was determined at pH 4 by an addition of 5 % potassium biphthalate buffer, and total inorganic arsenic (arsenite+ arsenate) was determined at pH 0.5. Detection limits were 4 ppt for arsenate at pH 0.5, 4ppt for arsenite at pH 4, 14 ppt for MMAA at pH 0.5, and 26 ppt for DMAA at pH 4, when a 50 ml sample was used. Reproducibility was tested by the replicate analysis of the standard solutions. The precision (C. V.) were 4.3% for arsenate of 5 ng, 5.1 % for arsenite of 5ng, 4.9% for MMAA of 20 ng, and 3.7% for DMAA of 30 ng. This method was highly sensitive and reproducible for arsenic determination at trace levels. Therefore, this method can be satisfactrily applied for the determination of arsenic compounds in natural waters.