NOM and alkalinity interference in trace-level hexavalent chromium analysis

• Three analytical methods for hexavalent chromium evaluated.
• Alkalinity up to 170 mg L−1 as CaCO3 did not affect recovery of Cr(VI).
• NOM up to 8 mg C L−1 did not affect Cr(VI) measurement using the EPA Method 218.7.
• NOM caused a ‘false positive’ in Cr(VI) using field ion exchange method.

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).

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