Determination of 14 Nitrosamines at Nanogram per Liter Levels in

Ocean College, Zhejiang University, Hangzhou 310058, China. ∥ Metropolitan Water District of Southern California, 700 Moreno Avenue, La Verne, Calif...
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Determination of 14 Nitrosamines at Nanogram per Liter Levels in Drinking Water Yichao Qian,† Minghuo Wu,† Wei Wang,† Beibei Chen,†,‡ Hao Zheng,§ Stuart W. Krasner,∥ Steve E. Hrudey,† and Xing-Fang Li*,† †

Division of Analytical and Environmental Toxicology, Department of Laboratory Medicine and Pathology, University of Alberta, Edmonton, Alberta T6G 2G3, Canada ‡ Key Laboratory of Analytical Chemistry for Biology and Medicine, Department of Chemistry, Wuhan University, Wuhan 430072, China § Ocean College, Zhejiang University, Hangzhou 310058, China ∥ Metropolitan Water District of Southern California, 700 Moreno Avenue, La Verne, California 91750-3399, United States S Supporting Information *

ABSTRACT: N-Nitrosamines, probable human carcinogens, are a group of disinfection byproducts under consideration for drinking water regulation. Currently, no method can determine trace levels of alkyl and tobacco-specific nitrosamines (TSNAs) of varying physical and chemical properties in water by a single analysis. To tackle this difficulty, we developed a single solid-phase extraction (SPE) method with highperformance liquid chromatography−tandem mass spectrometry (HPLC−MS/MS) for the determination of 14 nitrosamines of health concern with widely differing properties. We made a cartridge composed of a vinyl/divinylbenzene polymer that efficiently concentrated the 14 nitrosamines in 100 mL of water (in contrast to 500 mL in other methods). This single SPE−HPLC−MS/MS technique provided calculated method detection limits of 0.01−2.7 ng/L and recoveries of 53−93% for the 14 nitrosamines. We have successfully demonstrated that this method can determine the presence or absence of the 14 nitrosamines in drinking water systems (eight were evaluated in Canada and the U.S.), with occurrence similar to that in other surveys. N-Nitrosodimethylamine (NDMA), Nnitrosodiphenylamine, and the TSNA 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol were identified and quantified in authentic drinking water. Formation potential (FP) tests demonstrated that NDMA and TSNA precursors were present in (1) water samples in which tobacco was leached and (2) wastewater-impacted drinking water. Our results showed that prechlorination or ozonation destroyed most of the nitrosamine precursors in water. Our new single method determination of alkylnitrosamines and TSNAs significantly reduced the time and resource demands of analysis and will enable other studies to more efficiently study precursor sources, formation mechanisms, and removal techniques. It will be useful for human exposure and health risk assessments of nitrosamines in drinking water. the estimated total nitrosamines,11 suggesting that the majority of nitrosamines remain unknown. Epidemiological studies have suggested associations for a magnitude of cancer risk that cannot be explained by currently identified DBPs.12,13 A possible group of putative DBPs that may be bladder carcinogens are nitrosamines derived from alkaloids.13 We recently identified and confirmed two tobaccospecific nitrosamines (TSNAs), 4-(methylnitrosamino)-1-(3pyridyl)-1-butanol (NNAL) and 4-(methylnitrosamino)-1-(3pyridyl)-1-butanone (NNK), as DBPs.14 That study indicated that other nitrosamines may exist in wastewater-impacted

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hloramines are increasingly used by drinking water treatment plants (DWTPs) to reduce the formation of trihalomethanes, haloacetic acids, and other halogenated disinfection byproducts (DBPs) as a cost-effective alternative disinfectant to chlorine.1 However, chloramination is associated with the formation of N-nitrosamines, a group of nonhalogenated DBPs, classified as probable human carcinogens.2−4 After the initial discovery of N-nitrosodimethylamine (NDMA) in drinking water in Ontario, Canada,5 different types of N-nitrosamines have been detected and characterized in several other locations in North America, the United Kingdom, Australia, and Asia.2,6−10 In these studies, NDMA was the most commonly detected N-nitrosamine. However, a recent study has demonstrated that NDMA may only account for ∼5% of © XXXX American Chemical Society

Received: November 3, 2014 Accepted: December 19, 2014

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DOI: 10.1021/ac504104k Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry Table 1. Target Nitrosamine Analytes: Formula, Molecular Weight, and Internal Standard Nitrosamine target compounds

abbrev

formula

mol wt

log Kow

internal standard

N-nitrosodimethylamine N-nitrosomethylethylamine N-nitrosopyrrolidine N-nitrosodiethylamine N-nitrosopiperidine N-nitrosomorpholine N-nitrosodi-n-propylamine N-nitrosodi-n-butylamine N-nitrosodiphenylamine N-nitrosonornicotine N-nitrosoanatabine N-nitrosoanabasine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol

NDMA NMEA NPyr NDEA NPip NMor NDPA NDBA NDPhA NNN NAT NAB NNK NNAL

C2H6N2O C3H8N2O C4H8N2O C4H10N2O C5H10N2O C4H8N2O2 C6H14N2O C8H18N2O C12H10N2O C9H11N3O C10H11N3O C10H13N3O C10H13N3O2 C10H15N3O2

74 88 100 102 114 116 130 158 198 177 189 191 207 209

−0.5715 0.0815 −0.1915 0.4815 0.6315 −0.4415 1.3615 1.9215 3.1315 0.3236 0.6036 0.8136 −0.0036 −0.1836

NDMA-d6 NDMA-d6 NDMA-d6 NDEA-d10 NDEA-d10 NDMA-d6 NDPA-d14 NDPA-d14 NDPhA-d6 NNN-13C6 NNK-d4 NNK-d4 NNK-d4 NNN-13C6

levels. However, this method was specific for solid samples, and it did not detect thermally unstable nitrosamines (e.g., NDPhA). No method is available, in a single analysis, to determine all 14 nitrosamines of interest at nanogram per liter levels in water. The previous studies have shown the occurrence of 14 nitrosamines at low or sub nanogram per liter levels in water;14,15 therefore, the analytical methods should have detection limits at sub to low nanogram per liter levels for these compounds. Preconcentration becomes an essential part of the method to detect such low levels of nitrosamines. Recent progress in preconcentration methods has been summarized in reviews, with solid-phase extraction (SPE) being the most useful method.10,15,31,32 Activated carbon is widely used as a SPE sorbent because of its capability of retaining NDMA and other alkylnitrosamines.33,34 A two-layer cartridge, containing Lichrolut EN and Ambersorb 572, was used to increase the recovery of nitrosamines in water.35 A Waters Oasis HLB cartridge was used to extract five TSNAs from drinking water.14 Because the structures and hydrophobicities (Table 1) of all 14 nitrosamines vary significantly, no SPE method has been developed for extraction of the 14 nitrosamines at trace levels in water. This is an analytical challenge that is important to address for current issues surrounding regulatory considerations and practical monitoring of a full range of nitrosamines in drinking water. The objective of this study was to develop a sensitive single method for the determination of all 14 N-nitrosamines (Table 1) in water at nanogram per liter levels. We report here the development and demonstration of a SPE−HPLC−MS/MS method to measure, in one procedure, the 14 N-nitrosamines in eight drinking water systems (DWSs) in Canada and the U.S. The application of the method was further demonstrated to examine formation potential (FP) samples from one DWTP in Canada as well as two tobacco-leaching samples.

drinking water, where wastewater discharges were a source of TSNA precursors. N-Nitrosamines are typically analyzed using gas chromatography−mass spectrometry (GC−MS) or high-performance liquid chromatography−tandem mass spectrometry (HPLCMS/MS) methods.5,15−18 GC−MS methods [e.g., Environmental Protection Agency (EPA) method 521, Standard Methods19] are the most widely used for routine analysis of up to eight nitrosamines because of the high sensitivity and selectivity that these methods offer.16,18 However, GC−MS methods fail to detect the thermally unstable nitrosamines. For example, N-nitrosodiphenylamine (NDPhA) in drinking water cannot be detected using EPA method 521 because of thermal decomposition in the injection port. HPLC−MS techniques can overcome this problem, as demonstrated in the study of HPLC−MS/MS determination of GC-detectable nitrosamines, 17 including NDMA, N-nitrosomethylethylamine (NMEA), N-nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA), N-nitrosomorpholine (NMor), N-nitrosopyrrolidine (NPyr), N-nitrosopiperidine (NPip), N-nitrosodi-n-butylamine (NDBA), and a GC-nondetectable nitrosamine, NDPhA. These nine nitrosamines are referred to as the alkylnitrosamines hereafter. Moreover, several non-MS analytical methods are available, but these lack sensitivity and selectivity. They include ultraviolet (UV) photolysis,20 electrochemical electrodes,21 denitrosation agents,22 and other different detectors.23 TSNAs have been analyzed as a distinct group of nitrosamines in tobacco smoke, as well as in urine and other human biological samples.24−28 GC coupled with thermal energy analysis and LC−MS have been the main techniques for the measurement of TSNAs.26,28 TSNAs are more toxic than NDMA,29 and they may exist in drinking water, especially in those supplies that are impacted by wastewater.14 Determination of the alkylnitrosamines and TSNAs is required to evaluate which nitrosamines may occur in drinking water and to what extent. However, multiple methods (one for alkylnitrosamines and another for TSNAs) for sample analysis are resource-intensive, time-consuming, and possibly impractical for routine monitoring. Ramı ́rez et al.30 developed a GC method coupled with a nitrogen chemiluminescence detector to determine nicotine, seven volatile N-nitrosamines, and five TSNAs in house dust. Pressurized liquid extraction was used for extraction of these analytes, and the limits of detection (LODs) for the target compounds could reach low nanogram per gram

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MATERIALS AND METHODS

Reagents and Chemicals. Nine alkylnitrosamine standards (EPA 8210 Appendix IX Nitrosamine Mix; NDMA, NMEA, NPyr, NDEA, NPip, NMor, NDPA, NDBA, and NDPhA) were purchased from Sigma-Aldrich (St. Louis, MO). Five TSNA standards [N-nitrosonornicotine (NNN), Nnitrosoanabasine (NAB), N-nitrosoanatabine (NAT), NAK, and NNAL] and two internal standards (NNN-13C6 and NNKd4) were purchased from Toronto Research Chemicals B

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Analytical Chemistry Table 2. Performance of the SPE−HPLC−MS/MS Method for the 14 Nitrosamines nitrosamine

retention time (min)

NDMA NMEA NPyr NDEA NPip NMor NDPA NDBA NDPhA NNN NAT NAB NNK NNAL

2.46 2.72 2.62 3.21 3.36 2.47 5.70 7.50 7.58 2.78 3.54 3.81 2.93 2.64

recoverya (%, n = 3)

LODb (ng/mL)

low standardc (ng/mL)

MDLd (ng/L)

MQLe (ng/L)

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.25 0.27 0.72 2.06 0.78 0.17 0.26 0.14 0.002 0.001 0.002 0.001 0.003 0.006

0.5 0.5 1.0 3.0 0.5 0.5 0.5 0.5 0.01 0.01 0.01 0.01 0.01 0.01

0.45 0.25 0.33 2.67 0.45 0.36 0.31 0.92 0.04 0.01 0.01 0.01 0.01 0.04

1.47 0.84 1.09 8.78 1.50 1.20 1.03 3.03 0.12 0.03 0.03 0.03 0.03 0.12

69 70 90 75 77 53 65 76 75 93 84 90 89 72

3 4 7 3 4 4 2 7 1 4 4 2 2 3

a

The recoveries of the 14 nitrosamines were obtained from triplicate extraction, when spiked with 20 ng/L each of the 14 standards in 100 mL of Optima water. bLOD is the limit of detection for HPLC−MS/MS (without SPE). It was calculated from triplicate HPLC−MS/MS analyses (direct injection) of water samples containing 0.01−10 ng/mL of each nitrosamine. The LOD was determined as the concentration of each analyte at a signal-to-noise ratio (S/N) of 3:1. cLow-level standard for the linear calibration curve from direct injection. dMethod detection limit (MDL) was obtained from triplicate SPE−HPLC−MS/MS analyses of water samples containing 0.1, 5, and 40 ng/L of each nitrosamine. The MDL was determined as the concentration of each analyte at a S/N of 3:1. eMethod quantification limit (MQL) was obtained from triplicate SPE−HPLC− MS/MS analyses of water samples containing 0.1, 5, and 40 ng/L of each nitrosamine. The MQL was determined as the concentration of each analyte at a S/N of 10:1.

set at 0.3 mL/min, and the sample injection volume was 30 μL. The gradient elution was performed as follows: linearly increased B from 35% to 90% in 5 min; kept at 90% B for 3 min; returned to 35% B for column equilibration at 8.1−17 min. Analysis of the 14 nitrosamines was performed by positive electrospray ionization combined with multiple-reaction monitoring (MRM). Table S1 in the Supporting Information (SI) lists the MRM transitions and optimized MRM parameters for each analyte and internal standard. Optimized MS conditions were set as follows: curtain gas, 30.0 psi; collision gas, 6.0 psi; gas 1, 60.0 psi; gas 2, 20.0 psi; ion spray voltage, 5000 V; temperature, 200 °C. The quantification method for the 14 nitrosamines is described in the SI. Field Sample Collection and FP Tests. Drinking water samples were collected from eight DWSs in Canada and the U.S. in 2013 and 2014. Plant influent, plant effluent, and distribution samples (average and maximum detention times from the plant) were collected and stored in precleaned 2 L amber bottles. A total of 2 g of ascorbic acid was added into the samples immediately after collection to eliminate residual chlorine, which would otherwise continue reacting with precursors to produce oxidation byproducts. A travel-blank sample (500 mL of Optima water and 0.5 g of ascorbic acid) was prepared and transported with all of the samples. Plant influent samples were filtered with a glass microfiber filter (47 mm × 1.5 μm, Waterman) and a nylon membrane disk filter (47 mm × 0.45 μm, Pall Corp.). All samples were stored at 4 °C and analyzed within 5 days. One wastewater-impacted DWTP was sampled for nitrosamine precursors in the plant influent, after coagulation, after ozonation, and after biofiltration. FP tests were conducted to determine precursor levels using a previously reported procedure.14 The FP test is briefly described in the SI. In addition, tobacco-leaching samples were prepared by soaking 0.67 g of two different brands of cigarettes in 100 mL of Optima water and stirring for 24 h at room temperature. After filtration with a glass microfiber filter (47 mm × 1.5 μm, Waterman) and a nylon membrane disk filter (47 mm × 0.45

(Toronto, Ontario, Canada). The other four internal standards (NDMA-d6, NDEA-d10, NDPA-d14, and NDPhA-d6) were obtained from Cambridge Isotope Laboratories (Andover, MA). Water, methanol, hexane, and acetonitrile used in this study were Optima LC/MS grade and were purchased from Fisher Scientific (Fair Lawn, NJ). ACS-grade dichloromethane was obtained from BDH (Radnor, PA). The SPE absorbents LiChrolut EN (vinyl/divinyl polymer) and Ambersorb 572 (activated carbon) were purchased from EMD Millipore (Darmstadt, Germany) and Sigma-Aldrich, respectively. SPE Procedure for 14 Nitrosamines. The SPE cartridges developed in this study were packed in a glass tube with 500 mg of LiChrolut EN or Ambersorb 572 (either sorbent was the bottom layer) and glass wool (top layer; Sigma-Aldrich). These cartridges were first rinsed with 15 mL of hexane and 15 mL of dichloromethane to remove possible contaminants in the packing materials. After the residual organic solvent was removed under a full vacuum, the cartridges were preconditioned with 10 mL of methanol and 10 mL of water. A 100 mL water sample was passed through the SPE cartridge under vacuum at a flow rate of 3−4 mL/min. The loaded cartridges were dried for 30 min, and the analytes were eluted sequentially with 10 mL of dichloromethane and 5 mL of an acetonitrile/ water mixture (95:5, v/v). The two elutions (dichloromethane and acetonitrile/water) were collected and separately condensed to less than 0.05 mL with nitrogen. After concentration, the two fractions of the extracts were mixed and six internal standards (NDMA-d6, NDEA-d10, NDPA-d14, NDPhA-d6, NNN-13C6, and NNK-d4) were added. The final extracts were reconstituted to 0.2 mL with water. These concentrated samples were stored at 4 °C and analyzed within 24 h. LC−MS/MS Analysis of 14 Nitrosamines. An Agilent 1100 high-performance liquid chromatograph (Waldbronn, Germany) coupled with a Kinetex C8 column (100 × 3.0 mm i.d., 2.6 μm; Phenomenex, Torrance, CA) was employed to separate the 14 nitrosamines. The mobile phase was composed of solvent A (10 mM ammonium acetate and 0.01% acetic acid in water) and solvent B (100% methanol). The flow rate was C

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Analytical Chemistry μm, Pall Corp.), the tobacco-leaching samples were diluted with water (200000-fold), and the diluted samples were evaluated with FP tests (see the SI) to determine the levels of nitrosamine precursors. Also, samples were prechlorinated to determine the impact of preoxidation on nitrosamine precursors. Quality Assurance/Quality Control. All glassware and containers used in this study were rinsed with dichloromethane and baked at 180 °C overnight prior to use. An SPE blank was included in each batch of SPE samples. Method blank samples containing 65% water and 35% methanol were also analyzed to detect any contamination during LC−MS/MS analysis. The accuracy (recovery) was determined on spiked water samples (Table 2). Experiments were performed in triplicate or duplicate, and the mean values and standard deviations were reported.

to develop a single SPE method for concentrating trace levels of the 14 nitrosamines in water. SPE of 14 Nitrosamines. The determination of sub or low nanogram per liter levels of nitrosamines in drinking water requires sample preconcentration and removal of certain interferences prior to HPLC−MS/MS analysis. Extraction of the 14 nitrosamines, which have widely varying hydrophilic and hydrophobic properties, presented a challenge using a single SPE. Therefore, we first systematically examined different SPE cartridges and evaluated the efficiency of each step of the process, including sample loading (adsorption), elution (desorption), and solvent evaporation. First, we examined the adsorption (loading) efficiency of the different SPE cartridges for the 14 compounds. Lichrolut EN and Ambersorb 572 cartridges captured most of the target compounds with >92% efficiency (Table S3 in the SI). NDEA showed a relatively lower loading efficiency (>79.9%) because of its relatively high LOD (Table 2). The Oasis HLB cartridge “only” absorbed 78% of NDMA, where “as much as” 22% was detected in the cartridge eluent (Table S3 in the SI). Second, we investigated the recovery of the 14 nitrosamines after blowdown with nitrogen in different organic solvents. As shown in Table S4 in the SI, a recovery of ≥75% for all of the 14 nitrosamines was obtained when they were dissolved in dichloromethane. Similar recoveries were obtained when using acetonitrile as the solvent, except for NDMA (65%) and NMor (63%). Methanol and toluene were also evaluated, but the recoveries for these solvents were quite low for certain nitrosamines (Table S4 in the SI). Therefore, dichloromethane and acetonitrile were selected to evaluate the elution recovery. Finally, the recoveries from the elution of the 14 nitrosamines with dichloromethane and acetonitrile from our laboratory-made LiChrolut EN and Ambersorb 572 cartridges were investigated. The Ambersorb 572 cartridge was shown to be unfit for concentration of the 14 nitrosamines because of its poor elution efficiency for the five TSNAs (Table S5 in the SI), whereas 15 mL of dichloromethane eluted the nine alkylnitrosamines from the Ambersorb 572 cartridge with recoveries greater than 47%. The LiChrolut EN cartridge, using 10 mL of dichloromethane to elute the nine alkylnitrosamines, was efficient, but the five TSNAs were not recovered (Table S6 in the SI). Compared with acetonitrile, an acetonitrile/water (95:5, v/v, 5 mL) mixture significantly improved the recoveries of the five TSNAs, ranging from 81% to 94% (Table S6 in the SI). However, the recovery of NDMA was only 33% when 5 mL of the acetonitrile/water mixture was used, but this increased to 47% with the addition of 15 mL of the mixture (Table S6 in the SI). Detailed information on the development of the SPE procedure is described in the SI. Because dichloromethane efficiently eluted the nine alkylnitrosamines, whereas the acetonitrile/water (95:5, v/v) mixture eluted the five TSNAs from the LiChrolut EN cartridge, we designed a single-layer SPE cartridge (composed of the vinyl/ divinylbenzene polymer) with a two-step elution procedure to efficiently recover all 14 nitrosamines in one overall method after sample loading. First, dichloromethane (10 mL) was applied to elute NDMA, NMEA, NPyr, NDEA, NPip, NMor, NDPA, NDBA, and NDPhA; second, a volume of 5 mL of acetonitrile/water (95:5, v/v) was used to elute NNN, NAB, NAT, NNK, NNAL, and any residual of the nine alkylnitrosamines. The two fractions of dichloromethane and acetonitrile/ water eluents were separately collected and condensed to approximately 0.05 mL under nitrogen. This strategy effectively

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RESULTS AND DISCUSSION HPLC−MS/MS Analysis for 14 Nitrosamines. Figure 1 shows the typical HPLC−MS/MS chromatograms of 14

Figure 1. MRM chromatograms of 5 ng/L of the 14 nitrosamines by the HPLC−MS/MS method.

nitrosamine standards (5 ng/mL each), demonstrating the separation of the 14 nitrosamines in 8 min based on the ion transitions (Table S1 in the SI) and retention times (Table 2). The MRM method of the 14 analytes included 37 ion pairs. To ensure adequate acquisition time over the peaks, the 37 MRM transitions were divided into two periods (0−5 and 5−8 min). NDMA, NMEA, NPyr, and NDEA showed one ion-transition pair because the intensities of the second transitions of these compounds were too low to be quantified when they were at trace concentrations. The total analysis time for an authentic sample was 17 min, including the column washing and equilibration time. The linear response range for direct injection standards was 0.5−40 ng/mL for NDMA, NMEA, NPip, NMor, NDPA, and NDBA, 1.0−40 ng/mL for NPyr, 3.0−40 ng/mL for NDEA, and 0.01−40 ng/mL for NDPhA, NAB, NNN, NAT, NNK, and NNAL, with R2 greater than 0.99. The LODs (Table 2) of the HPLC−MS/MS determination of 14 nitrosamines without SPE ranged from 0.001 to 2.06 ng/mL. Note that most of the calculated LODs were below the lowest level standards in the calibration curve. Nonetheless, this method can determine TSNAs and NDPhA at low levels without SPE. Because of the low LODs provided by the HPLC−MS/MS method, we used smaller volumes of water samples (100 mL instead of 500 mL) D

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Analytical Chemistry

than those reported in the previously published LC−MS/MS method (10.6 and 3.1 ng/L, respectively).17 Moreover, these two nitrosamines have been rarely detected in other studies.8 For NDPhA, the MDL is now lower than that previously reported (0.1 ng/L).17 For the TSNAs, the MDLs in the new method are similar to those reported in the TSNA-only method (0.02−0.05 ng/L each14). Thus, this single SPE−HPLC−MS/ MS method simplifies analysis of the 14 nitrosamines at low or sub nanogram per liter levels in drinking water and achieves comparably good sensitivities while using a lower sample volume. The resource demands of sampling, analytical materials, labor, and instrumentation time are reduced by approximately 25−50% compared with analysis of the two fractions separately. These savings improve the feasibility of performing routine surveys of all 14 nitrosamines for a large number of samples or water systems. The LC−MS/MS accuracy can be influenced by matrix components that are coconcentrated through the SPE procedure. Matrix effects in drinking water originate from source water and treatment process chemicals. In this study, two representative drinking water samples (with different source water and treatment processes, as described in Table S7 in the SI) were selected to investigate the extraction efficiency of 14 nitrosamines using the SPE−HPLC−MS/MS method described above. As expected, the recoveries of the 14 nitrosamines from the tap waters were lower than those from the Optima water (Tables 2 and S8 in the SI), but the recoveries did not show considerable differences between the two tap water samples with different matrixes. The average recoveries for the 20 ng/L spiked samples were in the range of 53−86% except for NMor (43%) (drinking water sample 1; Table S8 in the SI) and 52−85% (drinking water sample 2; Table S8 in the SI) except for NDEA (49%), with an average decrease of only about 10% compared to the Optima water. For the 5 ng/L spiked sample, NDEA cannot be quantified in either of the two drinking water matrixes, where this analyte had a MQL of 8.8 ng/L in reagent water. For the 0.5 ng/L spiked sample (Table S8 in the SI), NDPhA and the five TSNAs had recoveries of 65−80%, but the alkylnitrosamines were not recovered at this spike level. The matrix effects observed highlights the need for using isotopic standards to quantify the 14 nitrosamines. Alternatively, a standard addition

minimized the loss of analytes. Consequently, the new SPE method using sequential elution with dichloromethane and acetonitrile/water (Figure 2) provided an efficient recovery of all 14 nitrosamines.

Figure 2. Sequence of the SPE method.

Method Performance. The method performance, including SPE and HPLC−MS/MS, was evaluated. Table 2 shows the overall method recoveries and detection limits for the 14 nitrosamines. In general, the recoveries of all of the analytes were better than 70% except one (NMor, 53%). The average recoveries of the 13 other nitrosamines ranged from 70% to 93% from analysis of the triplicate samples containing the 14 nitrosamines at a concentration of 20 ng/L each in Optima water. These results were better or comparable with those obtained using other SPE−HPLC−MS/MS methods that cannot determine all 14 nitrosamines simultaneously.14,17 The calculated method detection limits (MDLs) ranged from 0.01 to 2.7 ng/L, and the calculated method quantification levels (MQLs) ranged from 0.03 to 8.8 ng/L. The MDLs obtained in this method were better than those from non-MS methods.20,23 In addition, the minimum reporting level for each of the eight GC/MS-amenable alkylnitrosamines by the Ambersorb standard method19 was 2.0 ng/L. Thus, the MQLs in the new method were similar or better in most cases for these analytes, except for NDEA and NDBA. However, the MDLs for the latter two nitrosamines in the combined method were lower

Table 3. Occurrence, Frequency, and Concentrations of the 14 Nitrosamines in the Samples Collected from Eight DWSs range (median) values of the detects (ng/L)

a

nitrosamine

occurrence frequency

NDMA NMEA NPyr NDEA NPip NMor NDPA NDBA NDPhA NNN NAT NAB NNK NNAL

6/8 0/8 0/8 0/8 0/8 0/8 0/8 0/8 4/8 0/8 0/8 0/8 0/8 1/8

a

plant influent

plant effluent

distribution 1 (average detention)

distribution 2 (maximum detention)