Rapid, high-sensitivity analysis of oxyhalides by non-suppressed ion chromatography-electrospray ionization-mass spectrometry: application to ClO4−, ClO3−, ClO2−, and BrO3− quantification during sunlight/chlorine advanced oxidation

A rapid and sensitive method is described for measuring perchlorate (ClO4−), chlorate (ClO3−), chlorite (ClO2−), bromate (BrO3−), and iodate (IO3−) ions in natural and treated waters using non-suppressed ion chromatography with electrospray ionization and tandem mass spectrometry (NS-IC-MS/MS). Major benefits of the NS-IC-MS/MS method include a short analysis time (12 minutes), low limits of quantification for BrO3− (0.10 μg L−1), ClO4− (0.06 μg L−1), ClO3− (0.80 μg L−1), and ClO2− (0.40 μg L−1), and compatibility with conventional LC-MS/MS instrumentation. Chromatographic separations were generally performed under isocratic conditions with a Thermo Scientific Dionex AS16 column, using a mobile phase of 20% 1 M aqueous methylamine and 80% acetonitrile. The isocratic method can also be optimized for IO3− analysis by including a gradient from the isocratic mobile phase to 100% 1 M aqueous methylamine. Four common anions (Cl−, Br−, SO42−, and HCO3−/CO32−), a natural organic matter isolate (Suwannee River NOM), and several real water samples were tested to examine influences of natural water constituents on oxyhalide detection. Only ClO2− quantification was significantly affected – by elevated chloride concentrations (>2 mM) and NOM. The method was successfully applied to quantify oxyhalides in natural waters, chlorinated tap water, and waters subjected to advanced oxidation by sunlight-driven photolysis of free available chlorine (sunlight/FAC). Sunlight/FAC treatment of NOM-free waters containing 200 μg L−1 Br− resulted in formation of up to 263 ± 35 μg L−1 and 764 ± 54 μg L−1 ClO3−, and up to 20.1 ± 1.0 μg L−1 and 33.8 ± 1.0 μg L−1 BrO3− (at pH 6 and 8, respectively). NOM strongly inhibited ClO3− and BrO3− formation, likely by scavenging reactive oxygen or halogen species. As prior work shows that the greatest benefits in applying the sunlight/FAC process for purposes of improving disinfection of chlorine-resistant microorganisms are realized in waters with lower DOC levels and higher pH, it may therefore be desirable to limit potential applications to waters containing moderate DOC concentrations (e.g., ∼1–2 mgC L−1), low Br− concentrations (e.g., <50 μg L−1), and circumneutral to moderately alkaline pH (e.g., pH 7–8) to strike a balance between maximizing microbial inactivation while minimizing formation of oxyhalides and other disinfection byproducts.