In 2001, two potential disinfection by-products (DBPs) were tentatively identified as 1-aminoxy-1-chlorobutan-2-ol (DBP-A) and its bromo analogue (DBP-B) (Taguchi 2001). Subsequently it became clear, by consulting an updated version of the NIST database, that their mass spectra are close to those of the halohydrins 4-chloro-2-methylbutan-2-ol and 3-bromo-2-methylbutan-2-ol. To establish the structures of these DBPs, additional mass spectrometric experiments, including Fourier transform ion cyclotron resonance (FTICR), were performed on treated drinking water samples and authentic halohydrin standards. It appears that DBP-A is 3-chloro-2-methylbutan-2-ol and that DBP-B is its bromo analogue. DBP-B has been detected in ozonated waters containing bromide. Our study also shows that these DBPs can be laboratory artefacts, generated by the reaction of residual chlorine in the sample with 2-methyl-2-butene, the stabilizer in the CH2Cl2 used for extraction. This was shown by experiments using CH2Cl2 stabilized with deuterium labelled 2-methyl-2-butene. Quenching any residual chlorine in the drinking water sample with sodium thiosulfate minimizes the formation of these artefacts. 1. Introduction Since its inception in the late 19th century, drinking water disinfection has been one of the most important advancements for public health. While disinfectants such as chlorine, ozone, chloramines, and chlorine dioxide are used to kill harmful microorganisms, an unintended consequence is the formation of the so-called disinfection by-products (DBPs), which arise from the degradation of natural organic matter by the disinfectants [1, 2]. It is now widely known that DBPs, such as the two most common classes of DBPs, the trihalomethanes (THMs) and the haloacetic acids (HAAs), are associated with long-term health risks [3, 4]. The Ontario Ministry of the Environment (MOE) has been monitoring raw and treated drinking water as part of the Drinking Water Surveillance Program (DWSP) since 1986. Target compound analyses include THMs and HAAs. To complement these target compound analyses, gas chromatography-mass spectrometry (GC-MS) is routinely used to characterize a broad range of organic compounds, including DBPs, which may also be present in drinking water but whose identity has not yet been established. More than ten years ago, two unexpected disinfection by-products, a chloro compound labelled DBP-A, and a bromo analogue DBP-B, were being detected. Their electron ionization (EI) mass spectra were not available in the NIST98 database. On the basis of various mass
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