Trace analysis of isothiazolinones in water samples by large-volume direct injection liquid chromatography tandem mass spectrometry

Isothiazolinones are used as preservatives, biocides and disinfectants in a variety of industrial and domestic applications. Some of the isothiazolinones are difficult to isolate from water due their high polarity. A sensitive and selective analytical method was developed and optimized for the determination of sub-μg/L levels of three isothiazolinones in water samples. Three isothiazolinones are described in this paper: 2-methyl-3-isothiazolinone, 5-chloro-2-methyl-3-isothiazolinone and 4,5-dichloro-2-methyl-3-isothiazolinone. The analytical method involves a robust large-volume direct on-column loop injection of 2 mL on an Aqua ODS HPLC column and tandem MS detection (HPLC–MS/MS). After filtration, samples are directly injected without further pretreatment. Detection limits of the individual target compounds were between 0.03 and 0.1 μg/L employing Multi-Reaction Monitoring (MRM) mass spectrometry. Based on the constant ratio of two selected product-ions together with the retention time, the identification is very selective and quantification is reliable. The method was successfully applied to real samples of membrane flushings, drinking water, surface waters and waste water. Additional investigations showed the instability of the isothiazolinones in river- and waste water. Preservation of river water and waste water samples with sodium azide (NaN₃) promotes the stability of the isothiazolines in solution. In membrane flushings, waste water, surface waters and drinking water, levels of the three isothiazolinones were all below the limit of detection. In effluents of households containing washings from normal shampoo use, isothiazolinones could be detected.