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.
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 [
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 spectrometric experiments [
Tentative structure proposals from [
Following the study of [
The halohydrins of this study.
A major challenge associated with the interpretation of the EI mass spectra of DBP-A and DBP-B is that the spectra do not display a molecular ion. In the study of [
In this study, we aimed to determine whether the above DBPs have an “aminoxy” or a “halohydrin” structure by performing additional GC-MS experiments on treated water samples and by analyzing authentic samples of the halohydrins prepared via unambiguous synthetic procedures based upon the early study of [
Each 800 mL aliquot of water was spiked with the internal standard, d10-phenanthrene, and the surrogates, d6-
The EI and CI GC-MS analysis of the water samples was performed using an Agilent 6890 Series GC coupled to a Micromass GCT Time-of-Flight Mass Spectrometer at McMaster University. For the CI experiments, ammonia was used as the reagent gas. Selected EI experiments were also performed using a Hewlett Packard (HP) 6890 GC coupled to an HP 5973 mass selective detector (MSD) at the Ontario Ministry of the Environment (MOE). The column was a 60 m Restek Rtx-5, 0.25 mm i.d., 0.25
Authentic samples of the halohydrins 3- and 4-chloro-2-methylbutan-2-ol, 3- and 4-bromo-2-methylbutan-2-ol, and 1-chloro-2-methylbutan-2-ol were analyzed on the above instruments and also on the Varian 920FT GC-FTICR mass spectrometer at the MOE and the McMaster University ZAB-R instrument [
The halohydrins [
In general, the Grignard reactions were performed by adding an ethereal solution of the ketone or ester to a solution of the relevant alkylmagnesium halide under a nitrogen atmosphere at such a rate as to induce gentle reflux. After stirring overnight, sufficient saturated aqueous ammonium chloride (150 mL/mol of alkylmagnesium halide) was added dropwise to cause a clear solution to develop. The resultant solution was decanted, and the residual organic solids were extracted with diethyl ether (200 mL/mol alkylmagnesium halide). The combined organic ethereal solution was dried with MgSO4 and then evaporated at reduced pressure to give the crude halohydrin as an amber oil. The halohydrins were purified by distillation at reduced pressure (~35 mmHg). 1H NMR indicated that the above materials were essentially pure.
The total ion chromatogram (TIC) of the treated drinking water sample (Figure
Total ion chromatogram (TIC) of a treated drinking water extract.
The EI mass spectrum of (DBP-A) see Figure
Accurate mass measurements (EI mode) of the DBPs using GCT and FTICR instruments.
DBP | Formula | Mass (Calc.) | Mass (GCT)[a] | Dev.[c] | Mass (FTICR)[b] | Dev.[c] |
---|---|---|---|---|---|---|
A | C2H3O | 43.01784 | 43.01795 | 2.6 | — | — |
C3H7O | 59.04914 | 59.04895 | −3.2 | — | — | |
C5H9 | 69.06988 | 69.07005 | 2.5 | 69.06977 | −1.5 | |
C4H7O | 71.04914 | 71.04915 | 0.1 | 71.04912 | −0.3 | |
C5H11O | 87.08044 | 87.08015 | −3.3 | — | — | |
C4H8O35Cl | 107.02582 | 107.02595 | 1.2 | 107.02577 | −0.5 | |
C4H8O37Cl | 109.02287 | 109.02305 | 1.7 | 109.02290 | 0.3 | |
B | C2H3O | 43.01784 | 43.01815 | 7.2 | — | — |
C2H5O | 45.03349 | 45.03405 | 12.4 | — | — | |
C3H7O | 59.04914 | 59.04925 | 1.9 | — | — | |
C5H9 | 69.06988 | 69.06985 | −0.4 | 69.06985 | −0.4 | |
C4H7O | 71.04914 | 71.04945 | 4.4 | 71.04921 | 1.0 | |
C5H11O | 87.08044 | 87.08085 | 4.7 | — | — | |
C4H8O79Br | 150.97530 | 150.97695 | 10.9 | 150.97542 | 0.7 | |
C4H8O81Br | 152.97326 | 152.97375 | 3.2 | 152.97340 | 0.9 | |
C | C2H3O | 43.01784 | 43.01815 | 7.2 | — | — |
C2H5O | 45.03349 | 45.03385 | 8.0 | — | — | |
C4H7 | 55.05423 | 55.05455 | 5.8 | — | — | |
C3H5O | 57.03349 | 57.03395 | 8.1 | — | — | |
C4H9O | 73.06479 | 73.06455 | −3.3 | 73.06467 | −1.6 | |
C3H6O35Cl | 93.01017 | 93.01035 | 1.9 | 93.01013 | −0.4 | |
C4H8O35Cl | 107.02582 | 107.02645 | 5.9 | 107.02572 | −1.0 | |
C4H8O37Cl | 109.02287 | 109.02355 | 6.2 | 109.02279 | −0.7 |
EI mass spectra of (a) DBP-A, (b) DBP-B, (c) DBP-C, and (d) the CI mass spectrum of DBP-A obtained on the Micromass GCT instrument.
The conclusion that DBP-A is a halohydrin, rather than an aminoxy compound, follows from the ammonia CI mass spectrum of Figure
The elemental compositions of the ammoniated halohydrin and the protonated aminoxy compound, that is
As shown in Table
Accurate mass measurements (NH3 CI mode) of the DBPs using GCT and FTICR instruments.
DBP | Formula | Mass (calculated) | Mass (GCT)[a] | Dev.[c] | Mass (FTICR)[b] | Dev.[c] |
---|---|---|---|---|---|---|
A | C5H13N35Cl | 122.07310 | 122.07355 | 3.7 | 122.07323 | 1.0 |
C4H10NO2Cl | 140.04783 | 140.08465 | 263 | 140.08374 | ||
C5H15NO35Cl | 140.08367 | 140.08465 | 7.0 | 140.08374 | 0.5 | |
C5H15NO37Cl l | 142.08072 | 142.08125 | 3.7 | 142.08079 | 0.5 | |
B | C5H13N79Br | 166.02259 | 166.02425 | 10.0 | 166.02249 | 0.6 |
C5H13N81Br | 168.02054 | 168.02205 | 9.0 | 168.02041 | 0.8 | |
C5H15NO79Br | 184.03315 | — | — | 184.03311 | 0.2 | |
C5H15NO81Br | 186.03111 | — | — | 186.03102 | 0.4 | |
C | C5H15NO35Cl | 140.08367 | 140.08485 | 8.4 | 140.08367 | 0.0 |
C5H15NO37Cl | 142.08072 | 142.08105 | 2.3 | 142.08081 | 0.6 |
The EI mass spectrum of Figure
The other C5H11OCl isomer expected to show similar dissociation characteristics is the halohydrin 3-chloro-2-methylbutan-2-ol of Scheme
The halohydrin 3-bromo-2-methylbutan-2-ol yields the best library match to the EI spectrum of DBP-B shown in Figure
The CI mass spectrum of DBP-C displays signals at m/z 140 and 142. The accurate mass measurements of Table
Recognizing the need for accurate mass measurements for the structure analysis of unknown compounds, the Ministry of the Environment has acquired a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer capable of a mass resolution of 100,000 on the capillary GC time scale. Tables
An important clue that the halohydrins reported in the study of [
As shown in Scheme
The halohydrins are generated in the reaction of residual chlorine in the water sample with the stabilizer (2-methyl-2-butene) of the CH2Cl2.
To support this proposal, a control experiment was performed with a stock solution of CH2Cl2 spiked with 50 ppm of the labelled 2-methyl-2-butene (CD3)2C=CHCH3. Analysis of a water sample extracted with this spiked solvent indeed showed GC-MS signals of comparable intensity for labelled and unlabelled DBP-A as well as labelled and unlabelled DBP-B.
DBP-C is proposed to be generated from 2-methyl-1-butene (see Scheme
A complementary experiment, in which 5 ppm of NaOCl was added to the water sample, showed a 100-fold increase in yield of the chlorohydrins DBP-A and DBP-C. In contrast, the yield of DBPs was reduced 100-fold when 200 ppm of sodium thiosulfate was added to the treated drinking water sample to reduce any residual chlorine. This shows that the better part of the halohydrins in our samples are laboratory artefacts, but trace quantities may be genuine DBPs [
The present study leaves little doubt that DBP-A does not have the previously proposed “aminoxy” structure. It shows that DBP-A is the halohydrin 3-chloro-2-methylbutan-2-ol and that DBP-B is its bromo analogue. The EI mass spectra of the 3-chloro and 4-chloro-isomers and their bromo analogues are closely similar, but their retention times are very different. We propose that DBP-C is the isomeric halohydrin 1-chloro-2-methylbutan-2-ol. This study also shows that halohydrins are not necessarily genuine disinfection by-products. They can also be laboratory artefacts generated by the reaction of residual chlorine in the water with the 2-methyl-2-butene, a stabilizer in the CH2Cl2 extraction solvent. The interference of these artefacts can be minimized by adding sodium thiosulfate to the aliquots of drinking-water that are being investigated in the monitoring and testing process.
J. K. Terlouw and K. J. Jobst gratefully acknowledge financial support from the Ontario Ministry of the Environment for the collaborative research described in this paper. The assistance of Miss V. H. Coulthard and Miss K. E. Mercer in preparing the labelled halohydrins and 2-methyl-2-butene is gratefully acknowledged.