Optimization and validation of strategies for quantifying chromium species in soil based on speciated isotope dilution mass spectrometry with mass balance

Abstract

Strategies were designed to quantify hexavalent (Cr(VI)), soluble trivalent (Cr(III)) and insoluble chromium (Cr) species in soil by integrating existing methods of Cr(VI) and total Cr determination with speciated isotope dilution mass spectrometry (SIDMS, EPA Method 6800). Two different extraction methods that utilize a NaOH–Na₂CO₃ solution (EPA Method 3060A) and alkaline solution of ethylenediaminetetraacetic acid (EDTA) were used to extract Cr(VI) (along with soluble Cr(III) in the latter case). The extracted Cr was speciated by ion chromatography-inductively coupled plasma mass spectrometry (IC-ICP-MS), and the separated species were quantified using the mathematical relationships in SIDMS with simultaneous correction for their method-induced transformations. The Cr species that fell out as insoluble solid during extraction were determined by isotope dilution mass spectrometry (IDMS) after decomposing the extraction residues in a mixture of mineral acids according to EPA Method 3052. Several certified reference materials and a soil sample were analyzed using the proposed strategies. The measured mass fractions of Cr(VI) and total Cr in the reference materials statistically agreed with the certified values at 95% CL. The insoluble fraction of Cr accounted for 64–107% of the total Cr in the samples. The validity of the strategies was proved using mass balance by comparing the sum of the mass fractions of Cr(VI), soluble Cr(III) and insoluble Cr with the total Cr measured in the corresponding samples; the latter was determined by IDMS after decomposing the soils using EPA Method 3052 with single spiking.