NOM and alkalinity interference in trace-level hexavalent chromium analysis

Three analytical methods were evaluated for hexavalent and trivalent chromium analyses in the presence of natural organic matter (NOM) and alkalinity. Each method was tested using a simulated tap water with 1 μg L−1 Cr(VI) and 0.8 μg L−1 Cr(III) and several concentrations of NOM and/or alkalinity. An ion chromatograph with post column reaction cell conforming to USEPA Method 218.7 could accurately quantify Cr(VI) in the presence of up to 8 mg C L−1 NOM and up to 170 mg L−1 as CaCO3 alkalinity, and no oxidation of chromium was observed when 0.8 μg L−1 Cr(III) was also present. A high-performance liquid chromatography coupled with inductively coupled plasma (HPLC–ICPMS) method and a field speciation method were also evaluated. Each of these methods was unaffected by the presence of alkalinity; however, the presence of NOM created issues. For the HPLC–ICPMS method, as the concentration of NOM increased the recovery of Cr(VI) decreased, resulting in a ‘false negative’ for Cr(VI). However, for the field speciation method, Cr(III) was complexed by NOM and carried through the ion exchange column, resulting in a ‘false positive’ for Cr(VI).