Article pubs.acs.org/est
Electrospray Ionization-Tandem Mass Spectrometry Method for Differentiating Chlorine Substitution in Disinfection Byproduct Formation Zhuo Deng,† Xin Yang,‡ Chii Shang,*,† and Xiangru Zhang† †
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ‡ School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China S Supporting Information *
ABSTRACT: An electrospray ionization-tandem mass spectrometry (ESItqMS) method was developed to identify the location of chlorine substitution during the chlorination of model organic compounds. The chlorine substitution in the aliphatic part and that in the benzene ring of an organic molecule can be differentiated by their corresponding ranges of optimum collision energies, 5−7 eV and over 15 eV, respectively, in the precursor ion scan of m/z 35. The method was applied to predict the structures of intermediates and reveal the transformation pathways during the chlorination of 4-amino-2-chlorobenzoic acid and phenylalanine as a function of reaction time and the chlorine-to-precursor ratio. In the case of phenylalanine, chlorine was found to replace one hydrogen atom attached to the aliphatic nitrogen; in the case of 4-amino-2-chlorobenzoic acid, chlorine was found to replace the hydrogen atoms attached to the aromatic rings.
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the R-carbon of enolizable carbonyl compounds.3,4 The predominant oxidation reactions of chlorine with model compounds lead to the formation of oxygenated functional groups such as α-diketone-type moieties. The activated carbon in the moieties is quickly fully substituted by chlorine. Hydrolysis then occurs rapidly to form ketone-type intermediates with a dichlorinated methyl and an R group. Dichloroacetic acid is generated when the R group is OH or OR. A generalized conceptual model describing the formation of both THMs and HAAs has thus been proposed. The reactions between chlorine and organic nitrogenous compounds further complicate the chemistry. In aqueous solutions, free chlorine reacts with organic nitrogenous compounds (such as amino acids, proteins and amino sugars) to form organic chloramines, the degradation of which leads to the formation of THMs, HAAs, haloacetonitriles (HANs), cyanogen halides, and halonitromethanes.5−7 The reaction pathways during the chlorination of some simple amino acids or amines have been proposed based on the precursor structures and the identified products. For example, Nchloroglycine was observed to form rapidly through the reaction of glycine with aqueous chlorine.8 The product N-
INTRODUCTION Chlorination is the most widely adopted disinfection process in the world. However, the chlorination of drinking water, swimming pool water, and municipal wastewater is wellknown to produce disinfection byproducts (DBPs), some of whichsuch as trihalomethanes (THMs) and haloacetic acids (HAAs)pose a certain risk to human health. The maximum contaminant level (MCL) set by the United States Environmental Protection Agency for total THMs in drinking water is 80 μg/L, whereas the MCL for five of the nine HAAs is 60 μg/ L.1 New DBPs continue to be identified through research. So far over 600 DBPs have been reported in the literature,2 and a large portion of them are halogenated organic DBPs produced from chlorination or chloramination. Understanding the precursor-disinfectant relationships or interactions that lead to DBP formation is the first step toward facilitating DBP control, although it is easier said than done. One major challenge lies in the complexity of natural organic matter (NOM) and its interactions with chlorine. Model organic compounds that represent functional groups or moieties in NOM or other DBP precursors are often used to elucidate DBP formation mechanisms. For example, Reckhow and Singer3 studied the chlorination of a large number of aromatic and aliphatic model compounds and found that chloroform, trichloroacetic acid, and dichloroacetic acid were frequently produced. These byproducts were formed through the “haloform reaction” involving an electrophilic addition to © 2014 American Chemical Society
Received: Revised: Accepted: Published: 4877
December 26, 2013 March 21, 2014 April 4, 2014 April 4, 2014 dx.doi.org/10.1021/es405758b | Environ. Sci. Technol. 2014, 48, 4877−4884
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Table 1. Structures and m/z Values of Aliphatic Chloramines, Chlorinated Aliphatic Carbonaceous Compounds, and Chlorinated Aromatic Compounds
compound(s) containing chloride but is unable to identify the location of the chlorine substitution. On the basis of the different bond dissociation energies among chemical structures (Table S1), we propose a new ESItqMS method for probing the location of chlorine substitution in a chlorinated compound to illustrate the transformation pathways during the chlorination of model organic compounds. Our hypothesis is that the optimum collision energydefined here as the level of collision energy that gives the highest abundance of chloride (m/z 35)in the precursor ion scan mode during ESI-tqMS analysis gives an indication of the bond dissociation energy. The different levels of optimum collision energy among chemical structures shed light on the locations of chlorine substitution, either in the aliphatic part or the benzene ring of the compound. With the additional information on the fragments of the selected molecular ion from the product ion scan,16 the structures of the chlorinated intermediates can be proposed. The objectives of this study were to (1) develop an ESItqMS method for determining the location of chlorine substitution based on the optimum collision energy and (2) demonstrate its utility for predicting the structures of intermediates and revealing the transformation pathways during the chlorination of two model nitrogenous organic compounds4-amino-2-chlorobenzoic acid and phenylalanine.
chloroglycine was then either hydrolyzed or oxidized, which yielded nitriles and aldehydes.9 Further chlorination of nitriles and aldehydes led to the formation of commonly known DBPs such as dichloroacetonitrile and cyanogen chloride (CNCl).10,11 To detect the intermediates and products during chlorination, the commonly used analytical methods were gas chromatography methods such as gas chromatography/mass spectrometry (GC/MS), gas chromatography-electron capture detection (GC-ECD), and membrane-introduction mass spectrometry (MIMS).2,5 However, these approaches have their limitations. GC and MIMS can only detect volatile and semivolatile compounds. Nuclear magnetic resonance (NMR) spectrometry can reveal the distribution of various functional groups present in complex materials, but the limits of detection are rather high, and an overlap of resonance could occur in the NMR spectrum,12 making the technique undesirable for identifying the structures of chlorinated intermediates. Liquid chromatography (LC) has been employed to separate polar and ionic DBPs, and electrospray ionization MS (ESIMS) has been used to selectively detect halogenated DBPs.13 ESI is a soft ionization technique that can be performed without appreciably altering the structures of pseudomolecular ions. The ESI-triple quadrupole-MS (ESI-tqMS) method uses the precursor ion scans of m/z 35 and 37 to selectively detect polar chlorinated compounds that produce chloride (35Cl− or 37 − Cl ) via collision with argon gas in the collision-induced dissociation chamber.14,15 The method is able to detect the
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EXPERIMENTAL SECTION Chemicals and Solution Preparation. Solutions were prepared from reagent-grade chemicals or stock solutions. They
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collection mode was set to Multi-Channel Analysis. This mode greatly enhances the precursor ion abundance by accumulating multiple scans and eliminating possible ion intensity fluctuation in a single scan.18
were diluted to target concentrations using pure water (18.2 MΩ·cm) obtained from the NANOpure Diamond purifier system (Barnstead). All chemicals and stock solutions used were purchased from Sigma-Aldrich. Both acetonitrile (ACN) of HPLC grade and methyl tert-butyl ether (MtBE) of analytical grade were purchased from Aldrich. The six chlorine-containing compounds obtained commercially were monochloroacetic acid, (chloromethylamino)methanol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and 4-chlororesorcinol (see Table 1). These compounds were dissolved in pure water to form stock solutions. The five organic nitrogen precursors we used were glycine, glycylglycine, phenylalanine, urea, and 4-amino-2chlorobenzoic acid. These precursors were dissolved in pure water to a concentration of 10 mM as N. A free chlorine stock solution was prepared from 5% sodium hypochlorite (NaOCl), diluted to 1000−2000 mg/L as Cl2, and stored in the dark at 4 °C in a flask fitted with a glass stopper. The stock solution was standardized periodically by DPD colorimetric titration.17 Chlorination Procedures. To develop and optimize the ESI-tqMS method, chlorinated glycine, glycylglycine, phenylalanine and urea solutions were prepared by adding the hypochlorite stock solution to the organic nitrogen solutions at an initial chlorine-to-precursor (Cl/P) molar ratio of 0.8 at pH 7 (5 mM phosphate buffer) and 20 °C. After 30 min of chlorination with rapid stirring, the remaining free chlorine was quenched by ammonium chloride18 at an ammonium chlorideto-initial chlorine molar ratio of 1.2:1. The chloro-organic nitrogen solutions were subjected to sample pretreatment before ESI-tqMS analysis. To demonstrate the utility of the ESI-tqMS method, 1 mM 4-amino-2-chlorobenzoic acid and 1 mM phenylalanine were chlorinated at pH 7 (5 mM phosphate buffer) and 20 °C. Samples were taken after 30 min of contact with chlorine at initial Cl/P molar ratios of 0−4 for 4-amino-2-chlorobenzoic acid and after different periods of contact ranging from 1 min to 4 h with chlorine at an initial Cl/P molar ratio of 1.5 for phenylalanine. Residual free chlorine in the samples was quenched immediately by ammonium chloride at an ammonium chloride-to-initial chlorine molar ratio of 1.2, and the aqueous solutions were subjected to sample pretreatment before ESI-tqMS analysis. Sample Pretreatment. All samples were pretreated in accordance with the procedure for analysis of polar halogenated DBPs described by Zhai and Zhang.16 Briefly, an aqueous sample was acidified to pH 0.5 with sulfuric acid and dosed with sodium sulfate at 100 g/L. The products were extracted from the aqueous sample with MtBE at an MtBE-to-water volume ratio of 0.1. After extraction, the MtBE layer was transferred to a rotary evaporator for concentration to 1 mL. The 1 mL MtBE extract was mixed with 15 mL of ACN, and the mixture was rotoevaporated to a 1 mL sample in ACN. Then it was stored at 4 °C and diluted with 1 mL of pure water prior to ESI-tqMS analysis. ESI-tqMS Analysis. The pretreated samples were analyzed with a Waters Acquity ESI-tqMS device. The device was operated in the negative mode, and the following conditions were set: a sample flow rate of 10 μL/min via an infusion pump, a source temperature of 110 °C, a desolvation temperature of 350 °C, a desolvation gas flow of 650 L/h, a cone gas flow of 50 L/h, a collision gas (Argon) flow of 0.25 mL/min, a low mass resolution of 15, a high mass resolution of 15, and a capillary voltage of 2.9 kV. For all scans, the data
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RESULTS AND DISCUSSION System Optimization. Of all the ESI-tqMS parameters, the cone voltage and the collision energy affect the signal abundance of the electrospray-ionized compound the most. These two parameters were first optimized using the 10 model chlorine-containing nitrogenous and carbonaceous compounds, in which the chlorine atoms were attached to the aliphatic nitrogen or carbon atoms or to the benzene rings (see Table 1). Figure 1 shows the variation in the abundances of the molecular ions relative to their corresponding maximum
Figure 1. Relative abundances of chlorine-containing compounds as a function of the cone voltage. The collision energy was 2 eV. Chlorinated nitrogenous compounds were generated after 30 min chlorination of the precursors at Cl/P of 0.8 at 20 °C and pH 7. Open and filled symbols represent aliphatic and aromatic compounds, respectively.
molecular ion abundances as a function of the cone voltage at collision energy of 2 eV in the precursor ion scan. The relative abundance of each molecular ion increased with increasing cone voltage until the latter exceeded the threshold at which point the compound became fragmented at the sample cone and the relative abundance decreased.16,19 The optimum cone voltage for generating the peak abundances of the six aliphatic chloramines and chlorinated aliphatic carbonaceous compounds at the given collision energy was around 5−20 V; that for generating the peak abundances of the four chlorinated aromatic ring compounds was around 30−40 V. Considering that the concentrations of nitrogenous DBPs are often found to be lower than those of carbonaceous DBPs, the cone voltage should be selected with the detection of chlorinated nitrogenous compounds in mind. Therefore, a cone voltage of 20 V was chosen, at which the abundances of the chlorinated nitrogenous compounds were close to the peaks and the abundances of the chlorinated carbonaceous compounds were approximately 60−80% of the peaks. At a cone voltage of 20 V, the abundances of precursors as a function of the collision energy in the precursor ion scan (of m/ 4879
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z 35) were evaluated. As shown in Figure 2, the relative abundances of the molecular ions had a sharp hump-shaped
to determine the structures of the intermediates and the chlorination pathways. Chlorination of 4-Amino-2-chlorobenzoic Acid as a Function of the Cl/P Molar Ratio. 4-Amino-2-chlorobenzoic acid, which contains one amino group and one benzene ring, has been reported to react with chlorine to produce DBPs, including THMs, HAAs, and HANs.23 During the chlorination of 4-amino-2-chlorobenzoic acid, either the hydrogen atoms attached to the aromatic ring or the hydrogen atoms attached to the amino group are substituted by chlorine. In the case of the former, the substitution generates intermediates with a chlorinated benzene ring.24 In the case of the latter, the substitution creates an organic chloramine as the intermediate.25 Both cases are important because they lead to completely different reaction pathways and may result in different intermediates and final products after a prolonged period. 4Amino-2-chlorobenzoic acid was thus chlorinated at different Cl/P molar ratios and the mixtures analyzed using the new ESItqMS method. Figure S1 shows the ESI-tqMS precursor ion scan spectra of m/z 35 after chlorination of aqueous 4-amino-2-chlorobenzoic acid for 30 min at Cl/P molar ratios of 0, 0.8, 1.5, and 4.0 at pH 7. A large number of polar chlorinated intermediates were selectively detected in the spectra obtained at the different Cl/P ratios, among which the four major ones produced molecular ion peak clusters at m/z 170, 204/206, 240/242/246, and 213/ 215/217. The representative product ion scan spectra of the peak clusters at m/z 170, 204, 240, and 213 are shown in Figure S2. Figure 3 displays the relative abundances of the peak ion clusters of 4-amino-2-chlorobenzoic acid. In the spectrum for 4-
Figure 2. Relative abundances of chlorine-containing compounds as a function of the collision energy. The cone voltage was 20 V. Chlorinated nitrogenous compounds were generated after 30 min chlorination of the precursors at Cl/P of 0.8 at 20 °C and pH 7. Open and filled symbols represent aliphatic and aromatic compounds, respectively.
relationship with collision energy. Moreover, the optimum collision energy of the 10 tested compounds was either within the range of 5−7 eV or over 15 eV in accordance with the location of the chlorine attachment. For those compounds in which chlorine was attached to the aliphatic carbon or nitrogen, i.e., N-chloroglycine, N-chloroglyglycine, N-chlorophenylalanine, N-chlorourea, (chloromethylamino)-methanol, and monochloroacetic acid, the optimum collision energy was 5− 7 eV. For those compounds in which chlorine was attached to the aromatic ring, i.e., 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and 4-chlororesorcinol, the optimum collision energy was 15 eV or above. The difference in the optimum collision energy is manifested in the difference in the bond dissociation energy of the chlorine atom in these model compounds. As shown in Table S1, the bond dissociation energy of the chlorine atom attached to aliphatic carbon or nitrogen was around 240−310 kJ/mol, much lower than that of the chlorine atom attached directly to the aromatic ring (400 kJ/mol).20−22 The bonds with lower dissociation energy demanded less collision energy to break in the collision chamber during the precursor ion scan of m/z 35. The fact that the optimum collision energy fell within one of two ranges in the ESI-tqMS precursor ion scan of m/z 35 makes it possible to differentiate chlorine substitution in the aliphatic part from chlorine substitution in the benzene ring of a well-defined organic molecule. Taking advantage of this simple method to uncover the location of the chlorine substitution, the structures of intermediates during chlorination of more complicated model organic compounds can be predicted, and the transformation pathways of such compounds during their chlorination can be proposed, even if no standards of the intermediates are available and/or there exist two distinct chlorine-reactive sites in the DBP precursors. Two compounds4-amino-2-chlorobenzoic acid and phenylalaninewere chlorinated, and the intermediates or products were traced using the new ESI-tqMS method
Figure 3. Relative abundances of major chlorinated intermediates after 30 min chlorination of 4-amino-2-chlorobenzoic acid at different Cl/P ratios at 20 °C and pH 7.
amino-2-chlorobenzoic acid without chlorination (Cl/P of 0), the molecular ion peak was located at m/z 170, which corresponds to the M-1 peak of 4-amino-2-chlorobenzoic acid after the loss of a hydrogen atom. At a Cl/P molar ratio of 0.8, a peak cluster at m/z 204/206 with an isotopic peak ratio of 3:1 appeared in the precursor ion scan of m/z 35 in Figure S1b (equivalent to the isotopic ratio of 9:6:1 in the full scan), indicating the presence of a dichloro compound23 as a result of adding one chlorine atom to the 4-amino-2-chlorobenzoic acid molecule. As the Cl/P molar ratio increased from 0.8 to 1.5, the 4880
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abundance of the peak cluster located at m/z 204/206 also increased, and another peak cluster at m/z 240/242/244 became much more evident (Figure S1c). The peak cluster at m/z 240/242/244 with an isotopic ratio of 9:6:1 in the precursor ion scan of m/z 35 in Figure S1c (equivalent to the isotopic ratio of 27:27:9:1 in the full scan) suggests a trichloro compound.23 The trichloro compound was formed from the substitution of hydrogen atoms by two additional chlorine atoms in the 4-amino-2-chlorobenzoic acid molecule. As the Cl/P molar ratio increased further to 4, the abundances of ion clusters of the precursor and the intermediates observed at Cl/ P ratios of 0, 0.8, and 1.5 reduced to their minimum levels, while a strong peak cluster became evident at m/z 213/215/ 217 with an isotopic ratio of 9:6:1 in the precursor ion scan of m/z 35 in Figure S1d (equivalent to the isotopic ratio of 27:27:9:1 in the full scan). This peak cluster, based on the isotopic ratio, is attributed to a trichloro product. The representative ions of m/z 170, m/z 204, and m/z 240 all displayed the highest abundance at the optimum collision energy of 15 eV (Figure 4), which is within the range of
Scheme 1. Chlorination of 4-Amino-2-chlorobenzoic Acid with Different Cl/P Molar Ratios
compound is suggested to have structure III. The compound having a peak cluster at m/z 240/242/244 is proposed to have either structure IV or structure V (as shown in Scheme 1). The results suggest that chlorine substitution of one of the hydrogen atoms attached to the benzene ring of the precursor is more likely at low Cl/P ratios. She et al.24 arrived at a similar conclusion from GC/MS and NMR measurements after chlorinating 4-aminobenzoic acid and 4-aminosalicylic acid. Based on the analysis of the intermediates in the current study, the pathway of chlorine transfer in the chlorination of 4-amino2-chlorobenzoic acid as a function of the Cl/P molar ratio was hypothesized (Scheme 1). This example demonstrates that insights into the chlorine substitution during chlorination can be derived simply by checking whether the optimum collision energy of the molecular ions falls within the range of optimum collision energies of chlorinated aliphatics or within that of benzene rings. The insights in turn help to elucidate the influences of the Cl/P ratio on the transformation of 4-amino2-chlorobenzoic acid during chlorination. Time-dependent Transformation of Phenylalanine during Chlorination. Phenylalanine is an α-amino acid consisting of an aliphatic amine and an aromatic ring. Generally, chlorine reacts rapidly with α-amino acids to form N-chloro-amino acids such as N-chlorophenylalanine.27 This is followed by decarboxylation, which leads to the formation of phenylacetonitrile and phenylacetaldehyde.28,29 Panzella et al.30 demonstrated by HPLC and NMR analysis that the reaction of phenylalanine with chlorine formed a mixture of pyridines through a Chichibabin-like pyridine synthesis pathway, i.e. by forming N-chloroimine intermediates. In the example of the current study, the ESI-tqMS method showed the sequential
Figure 4. The optimum collision energy of major chlorinated intermediates after 30 min chlorination of 4-amino-2-chlorobenzoic acid at different Cl/P ratios at 20 °C and pH 7.
optimum collision energies of chlorinated benzene rings. The representative ion of m/z 240 displayed two peaks in its relative abundance at 5 and 15 eV, which indicates that the trichloro product contains chlorine atoms attached to both the aliphatic amine and the benzene ring. It should be noted that the ESItqMS method was incapable of identifying the exact location of the chlorine substitution in the benzene ring. Any pre-existing substitutes in the precursor will affect the chlorine substitution.26 4-Amino-2-chlorobenzoic acid consists of three preexisting substitutesa chlorine atom, the amino group, and the carboxyl group. The chlorine atom and the amine group enhance chlorine substitution in the ortho and para positions of the benzene ring, while the carboxyl group promotes chlorine substitution in the meta position as shown in Table S2.26 Therefore, two possible structures (structure I and structure II) corresponding to the peak cluster at m/z 204/206 are proposed at the Cl/P molar ratio of 0.8 as shown in Scheme 1. The compound having a peak cluster at m/z 213/215/217 has three chlorine atoms on the benzene ring. Its product ion scan suggests the presence of two hydroxyl groups. Thus, this 4881
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substitutions of phenylalanine by chlorine as a function of chlorination time and demonstrated the pathway of the timedependent transformation. Figure S3 shows the ESI-tqMS precursor ion scan spectra of m/z 35 after the chlorination of phenylalanine at a Cl/P molar ratio of 1.5 and pH 7 as a function of time. The major molecular ions obtained within the 4 h of chlorination are located at m/z 200/202, 241/243/245, 300/302, and 336/338/ 340. The representative product ion scan spectra of m/z 200, 241, 300, and 336 are shown in Figure S4. Among the four major ion peak clusters, the compound showing the ion of m/z 200 formed extremely quickly, and the ion cluster reached its maximum abundance within 1 min. The abundance then decreased continuously for the next 50 min as shown in Figure 5 and Figure S3. As the reaction time increased from 0 to 30
Figure 6. The optimal collision energy of major chlorinated intermediates at Cl/P of 1.5 of chlorination of phenylalanine at different reaction times at 20 °C and pH 7.
the peak at m/z 169 is 15 eV, which is within the range of optimum collision energies of chlorinated benzene rings and suggests chlorine substitution in the benzene ring. The fragments of these molecular ions in product ion scans are shown in Figure S4. Based on the numbers of chlorine atoms, the fragments obtained from their product ion scans, and information reported in the literature studying the reaction pathways of the chlorination of amino acids,27−29 the compounds of m/z 200/202, 241/243/245, 300/302, 336/ 338/340, and 169/171 are proposed to have structure VI, structure VII, structure VIII, structure IX, and structure X, respectively (see Scheme 2). The complete time-dependent transformation pathway during the chlorination of phenylalanine (Scheme 2) is thus hypothesized. Deuterated phenylalanine may be used to confirm the structures. This example thus demonstrates the utility of the ESI-tqMS method for predicting the structures of intermediates and revealing the transformation pathway of phenylalanine during chlorination as a function of time by identifying the ranges of optimum collision energies of the molecular ions in the precursor ion scan at each time step. Applicability and Limitation. The new ESI-tqMS method was demonstrated to be useful for studying the chlorination of model organic compounds where the chlorine substitution may occur in one of two locations. The method not only gives additional information to help identify the structures of intermediates but also provides direct evidence of the transformation scheme. It does, however, have its limitations. First, although the method can determine whether the chlorine atom is located in the aliphatic part or the benzene ring of the compound according to the optimum collision energy, it cannot pinpoint the exact location of the chlorine substitution when there are two or more possible positions of substitution in either the benzene ring or the aliphatic part of the compound. Thus, this method is unable to reveal precisely the nature of carbon−chlorine bonds in chlorination of complex molecules such as NOM. Another limitation of the ESI-tqMS method is related to the potential interference from chlorinated carbon−carbon unsaturated bonds. Our supplementary test showed that the optimum collision energy of the chlorinated carbon−carbon double bond (i.e., trichloroethene)
Figure 5. Relative abundances of major chlorinated intermediates at Cl/P molar ratio of 1.5 of chlorination of phenylalanine at different reaction times at 20 °C and pH 7.
min, the abundances of the peak clusters at m/z 241/243/245, 300/302, and 336/338/340 also increased. The peak cluster at m/z 241/243/245 with an isotopic ratio of 9:6:1 in the precursor ion scan of m/z 35 in Figure S3b (equivalent to the isotopic ratio of 27:27:9:1 in the full scan) indicates that a trichloro intermediate was generated. The peak cluster at m/z 336/338 with an isotopic ratio of 3:1 in the precursor ion scan of m/z 35 in Figure S3b (equivalent to the isotopic ratio of 9:6:1 in the full scan) can be seen clearly. Based on the isotopic ratio, the peak cluster at 336/338 should belong to a dichloro intermediate. As the reaction time further increased from 30 to 60 min, the abundances of ion clusters at m/z 241/243/245, 300/302, and 336/338/340 decreased to their minimum levels while a strong peak cluster emerged at m/z 169/171. The peak cluster at m/z 169/171 with an isotopic ratio of 3:1 in the precursor ion scan of m/z 35 in Figure S3c (equivalent to the isotopic ratio of 9:6:1 in the full scan) increased continuously for 60 min of the reaction. After that, the abundances of all peak clusters of the precursor and the intermediates decreased. The representative optimum collision energy of the ions of m/z 200, 241, and 300 is 7 eV (Figure 6), which is within the range of optimum collision energies of chlorinated aliphatic carbon/ nitrogen. The ion of m/z 336 displayed two peaks in its relative abundance at 5 and 15 eV, indicating that this dichloro compound contains chlorine atoms attached to both the amine group and the benzene ring. The optimum collision energy of 4882
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Scheme 2. Chlorination of Phenylalanine with Different Reaction Times
(3) Reckhow, D. A.; Singer, P. C. Mechanisms of organic halide formation during fulvic acid chlorination and implications with respect to pre-ozonation. In Water Chlorination: Chemistry, Environmental Impact and Health Effects; Jolley, R. L., Bull, R. J., Davis, W. P., Katz, S., Roberts Jr., M. H., Jacobs, V. A., Eds.; Lewis Publishers: Chelsea, MI, 1985; Vol. 5, pp 1229−1257. (4) Morris, J. C. The chemistry of aqueous chlorine in relation to water chlorination. Jolleys, R. L., Ed.; Water Chlorination: Environmental Impact and Health Effects; Ann Arbor Science: Ann Arbor, MI, 1978; Vol. 1, pp 21−35. (5) Shang, C.; Gong, W. L.; Blatchley, E. R., III. Breakpoint chemistry and volatile byproduct formation resulting from chlorination of model organic-N compounds. Environ. Sci. Technol. 2000, 34, 1721−1728. (6) Joo, S. H.; Mitch, W. A. Nitrile, aldehyde, and halonitroalkane formation during chlorination/chloramination of primary amines. Environ. Sci. Technol. 2007, 41, 1288−1296. (7) Shah, A. D.; Mitch, W. A. Halonitroalkanes, halonitriles, haloamides, and N-nitrosamines: A Critical review of nitrogenous disinfection byproduct formation pathways. Environ. Sci. Technol. 2012, 46, 119−131. (8) Scully, F. E.; White, W. N.; Boethling, R. S. Reactions of drinking water contaminants with aqueous chlorine and monochloramine. In Water Contamination and Health: Integration of Exposure Assessment, Toxicology, and Risk Assessment; Wang, R. G. M., Ed.; Dekker: New York, 1994; pp 45−65. (9) Alouini, Z.; Seux, R. Kinetics and mechanisms of hypochlorite oxidation of α-amino acids at the time of water disinfection. Water Res. 1987, 21 (3), 335−343. (10) Trehy, M. L.; Yost, R. A.; Miles, C. J. Chlorination byproducts of amino acids in natural waters. Environ. Sci. Technol. 1986, 20, 1117− 1122. (11) Yang, X.; Shang, C. Chlorination byproduct formation in the presence of humic acid, model nitrogenous organic compounds, ammonia, and bromide. Environ. Sci. Technol. 2004, 38 (19), 4995− 5001.
was also around 15 eV, which is within the range of optimum collision energies of chlorinated benzene rings. Nevertheless, this would not interfere with the study of DBP formation during water disinfection with chlorine or chloramine, because compounds containing chlorinated unsaturated carbon−carbon bonds are unlikely to form under normal water treatment conditions.25
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ASSOCIATED CONTENT
S Supporting Information *
One supporting table and four supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (852)2358 7885. Fax: (852)2358 1534. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by Hong Kong’s Research Grants Council under grant number 619108. REFERENCES
(1) National Primary Drinking Water Regulations: Stage 2 Disinfectants and Disinfection Byproducts Rule. Final Rule, Fed. Reg. Part II, 40 CFR Part 9, 141 and 142, 71:2:388, U.S. EPA: Washington, DC, 2006. (2) Krasner, S. W.; Weinberg, H. S.; Richardson, S. D.; Pastor, S. J.; Chinn, R.; Sclimenti, M. J.; Onstad, G. D.; Thruston, A. D., Jr. Occurrence of a new generation of disinfection byproducts. Environ. Sci. Technol. 2006, 40, 7175−7185. 4883
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dx.doi.org/10.1021/es405758b | Environ. Sci. Technol. 2014, 48, 4877−4884
