ICP-MS Combined with Ionic Liquid Microextraction and Back-Extraction of Nitric Acid for Speciation Analysis of Chromium

The sample (200 mL) was digested with 0.1 mL of HNO₃ solution (3+97) for 15 min. An aliquot (5 mL) of the sample solution was taken and diluted to 10 mL with HNO₃ solution (3+97), and total chromium was determined in this solution by ICP-MS. Another aliquot of 5 mL was taken from the sample solution, and 1 mL of pH 5 acetate buffer solution, 0.5 mL of 30 g·L^-1 sodium diethyldithiocarbamate solution and 100 μL of 1-hexyl-3-methylimidazolium hexafluorophosphate were added and the mixture was treated ultrasonically for 15 min at 30℃. After centrifuging, the lower organic phase was separated, and 2 mL of 2 mmol·L^-1 HNO₃ solution were added. The mixture was treated ultrasonically for 15 min. After centrifuging, the upper aqueous layer was separated and diluted to 10 mL with HNO₃ solution (3+97). Content of Cr(Ⅵ) was determined in this solution by ICP-MS. Content of Cr(Ⅲ) of the water sample was found by substracting value of Cr(Ⅵ) content from the total chromium. Linearity ranges of Cr(Ⅵ) and total chromium were same within 80 μg·L^-1 and values of D.L. (3s/k) were 0.061, 0.034 μg·L^-1 respectively. Recovery rates of Cr(Ⅵ) obtained by standard addition method were in the range of 96.4%-105% and the RSDs (n=6) were in the range of 2.1%-5.9%.