Determination of hexavalent and total chromium in sea water by isotope dilution mass spectrometry

Hexavalent chromium - The sample is acidified to pH 2 immediately after collection and filtration through a 0.45-μ membrane filter. A known amount of 50Cr(VI) spike and 2 ml of 1% ammonium pyrrolidine dithiocarbamate solution are added to the sample solution (100 ml) in a separatory funnel. Hexavalent chromium is extracted into two 10 ml portions of chloroform. The organic phase is washed with 10 ml of water, and transfered into a 100-ml quartz beaker. The solution is evaporated to dryness, and 2 ml of nitric acid and a drop of sulfuric acid are added and evaporated to near dryness. The residue is transferred to a 1-ml quartz beaker with 1 ml of nitric acid and evaporated to dryness. The quartz beaker is heated to about 8000C in a silicon carbide furnace. The residue is transfered on a rhenium filament with small amount of water by a quartz capillary. The isotope ratios are measured by a surface ionization mass spectrometer. Total chromium - A known amount of 50Cr spike, 1 ml of 10 N sulfuric acid and 2 ml of 0.02 N ammonium ceric nitrate are added to the sample solution (100 ml) in a teflon bottle. The solution is kept at 500C for 30 min. Sodium azide solution is added to the cooled solution to destroy the yellow color of the excess cerium (IV). The pH of solution is adjusted to about 2 with dilute ammonia water. Total chromium in the solution is extracted and transfered on a filament by the same procedure as hexavalent chromium. The lower limit of quantitative determination of chromium by this method is around 0.001 μg Cr/l. The standard deviations of the results obtained by this method are estimated to be both within 5% for hexavalent and total chromium. (auth.)