Simultaneous determination of monochloramine and common anions in drinking water by mixed-mode ion chromatography with sequential UV absorbance and suppressed conductivity detection

A sensitive and rapid mixed-mode ion chromatography-based method for the simultaneous quantification of the disinfectant monochloramine and common inorganic anions within treated drinking waters is presented. The separation was achieved on a high-capacity anion-exchange column (Dionex IonPac AS15, 4 mm x 250 mm) using a mixed-mode approach, which combined ion-exchange and ion-exclusion-based retention for the inorganic anions and monochloramine, respectively. An electrolytically produced potassium hydroxide eluent gradient was employed, with the method combining suppressed conductivity detection with in-house assembled 50-mm flow cell high-sensitivity UV absorbance detection. This configuration provides a five-fold enhancement in UV absorbance signal relative to a conventional 10-mm flow cell, substantially improving sensitivity for UV-absorbing species. Monochloramine is detected directly and without derivatisation, with an absorbance maximum at 240 nm. The optimised method was able to separate and quantify eight target analytes commonly monitored in treated drinking waters, namely fluoride, chloride, nitrite, bromide, nitrate, phosphate, sulphate, and monochloramine, all within a 13-minute runtime. The method was validated, demonstrating excellent linearity (r≥0.996) across a 7-point calibration range and high precision with %RSD consistently at or below 1.8 %. The LOD for monochloramine is 0.11 mg/L, while those for the other anions ranged from 1.5 µg/L for fluoride to 0.17 mg/L for nitrate. The method's limits of detection were well below the maximum levels recommended in World Health Organization (WHO) guidelines for drinking water. Notably, the system was selective for monochloramine detection in the presence of dichloramine.