Simultaneous separation and detection of monochloramine, nitrite, and nitrate by step-gradient mixed-mode ion chromatography: Translation from benchtop to portable ion chromatograph

Abstract

Background: Nitrite (NO₂^-) and nitrate (NO₃^-) can be produced in the distribution systems of chloraminated drinking water due to the nitrification of ammonia. The most applied inorganic chloramine for this purpose, namely monochloramine (NH₂Cl), is also released into aquatic environments from water treatment plants' effluent and within industrial waste streams. Within the treatment process, the continuous monitoring of disinfectant levels is necessary to limit the harmful disinfectant by-product (DBP) formation. Currently, NH₂Cl can interfere with nutrient analysis in water samples, and there are no analytical techniques available for the simultaneous analysis of NH₂Cl, NO₂^-, and NO₃^-.

Results: A green analytical method based on mixed-mode ion chromatography, specifically ion exchange and ion exclusion modes, was developed for the simultaneous separation and detection of NH₂Cl, NO₂^-, and NO₃^-. The separation was achieved using a Dionex IonPac AG15 column guard column and a step gradient elution involving deionized water and 120.0 mM NaCl. The method was developed using a benchtop HPLC with a custom-made multi-wavelength UV absorbance detector with a 50-mm flow cell to enable the sensitive detection of NH₂Cl, NO₂^-, and NO₃^- at 240 nm, 220 nm, and 215 nm, respectively. The developed method was then transferred to a portable ion chromatography (IC) system, the Aquamonitrix analyser. The total run time was less than 10 min for both systems. The benchtop HPLC method had a limit of detection (LOD) of 0.07 μg mL^-1 as Cl₂ for NH₂Cl, 0.01 μg mL^-1 for NO₂^-, and 0.03 μg mL^-1 for NO₃^-. The LODs obtained using the portable Aquamonitrix analyser were found to be 0.36 μg mL^-1 as Cl₂, 0.02 μg mL^-1, and 0.11 μg mL^-1 for NH₂Cl, NO₂^-, and NO₃^-, respectively. Excellent linearity (r ≥ 0.9999) was achieved using the portable analyser over the studied concentration ranges. The developed system was applied to the analysis of spiked municipal drinking water samples and showed excellent repeatability for the three analytes at three different concentration levels (RSD of triplicate recovery experiments ≤ 1.9 %). Moreover, the variation in retention time was negligible for the three target analytes with RSD ≤ 0.8 % over 12 runs.

Significance: We are reporting the first ion chromatographic method for the simultaneous separation and detection of NH₂Cl, NO₂^-, and NO₃^- in water samples. The monitoring of NH₂Cl, NO₂^-, and NO₃^- is critical for the determination of disinfectant dosing, water quality, and nitrification status. The developed method can be applied using a benchtop HPLC or via the portable automated IC system to monitor for the three target compounds analysis in water treatment plants.

Keywords: Ion exchange chromatography; Ion exclusion chromatography; Monochloramine; Nitrate; Nitrite.