Bromate peak distortion in ion chromatography in samples containing high chloride concentrations

• Concept of competitive Langmuir isotherm is explained for multi-component systems.
• Overload chloride peaks on hydroxide and carbonate selective columns showed fronting.
• Chloride matrix overload effect on the peak shapes of trace bromate was investigated.
• Chloride matrix overload resulted in pull-back effect on trace bromate peak.
• Pull-back effect can reduce the recovery of trace bromate and affect quantitation.

In this study, the effect of column overload of the matrix ion, chloride, on the elution peak profiles of trace bromate is investigated. The resultant peak profiles of chloride and bromate are explained on the basis of competitive Langmuir isotherms. The Thermo IonPac AS9-HC, AS19 and AS23 columns are recommended by the manufacturer for bromate (a carcinogen) analysis. Under trace conditions, these columns provide baseline resolution of bromate from matrix ions such as chloride (Rs = 2.9, 3.3 and 3.2, respectively for the three columns). Injection of 10–300 mM chloride with both hydroxide and carbonate eluents resulted in overload on these columns. On the basis of competitive Langmuir isotherms, a deficiency in the local concentration of the more retained eluent in addition to analyte overload leads to fronting of the overloaded analyte peak. The peak asymmetries (B/A10%) for chloride changed from 1.0 (Gaussian) under trace conditions to 0.7 (fronting) at 300 mM Cl− for IonPac AS9-HC, 0.9–0.6 for AS19 and 0.8–0.5, for AS23, respectively. The 10 mM bromate peak is initially near Gaussian (B/A10% = 0.9) but becomes increasingly distorted and pulled back into the chloride peak as the concentration of chloride increased. Increasing the eluent strength reduced the pull-back effect on bromate and fronting in chloride in all cases.