Analysis of p-chlorobenzoic acid in water by liquid chromatography–tandem mass spectrometry

para-Chlorobenzoic acid (p-CBA) is typically used as a probe compound to indirectly quantify hydroxyl radicals formed during advanced oxidation processes used in drinking water and wastewater treatment. A method has been developed for the sensitive analysis of p-CBA in water using liquid chromatography–tandem mass spectrometry (LC–MS/MS). A reporting limit in water of 100 ng/L was determined for the method, which is 40-fold lower than the 4.0 μg/L reporting limit of the widely used liquid chromatography with UV detection (LC-UV) method. The method was found to be robust in difficult matrices such as wastewater and highly selective, unlike LC-UV which relies on non-specific detection at 234 nm. The detection of p-CBA below 1 μg/L during bench-scale ozonation of wastewater after hydrogen peroxide addition was demonstrated. Duplicate samples were analyzed by LC–MS/MS and LC-UV and results were found to be comparable at concentrations quantifiable by both methods.