NOM and alkalinity interference in trace-level hexavalent chromium analysis

Abstract

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 1μgL−1 Cr(VI) and 0.8μgL−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 8mgCL−1 NOM and up to 170mgL−1 as CaCO3 alkalinity, and no oxidation of chromium was observed when 0.8μgL−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).