Ion chromatography high-resolution mass spectrometry-quality assurance by co-injection of internal standards

Highly polar and mobile organic substances challenge water analysis due to their persistence, high solubility, and incomplete removal during conventional drinking-water treatment. Their ionic or ionisable nature makes them suitable for ion chromatography coupled to high-resolution mass spectrometry (IC-HRMS), enabling direct injection of aqueous samples and retention of small anions. In this study, an IC-HRMS method was developed and validated for 23 highly polar contaminants, including haloacetic acids, pesticide transformation products, and other mobile and persistent compounds. Limits of quantification ranged from 0.03 to 0.37 µg/L in a linear range of 0.01-100 µg/L. Autosampler-based co-injection of stable isotope-labelled internal standards enabled continuous quality control of retention time, signal intensity, and mass accuracy. Retention time deviations over 4 months of measuring remained below 12 s and mass deviations below 2 ppm, demonstrating robust performance. Application to samples along the drinking-water treatment train confirmed the presence of persistent and mobile substances in the low µg/L range, including trifluoroacetic acid, sulfamic acid, cyanuric acid, and chlorothalonil transformation products. These results demonstrate that IC-HRMS with automated co-injection is a reliable tool for routine monitoring of highly polar anionic contaminants in water.