Anal. Chem. 2001, 73, 997-1006
Determination of Carbamate, Urea, and Thiourea Pesticides and Herbicides in Water Nan Wang†
Oak Ridge Institute for Science and Education, 26 West Martin L. King Drive, Cincinnati, Ohio 45268 William L. Budde*
Office of Research and Development, National Exposure Research Laboratory, U.S. Environmental Protection Agency, 26 West Martin L. King Drive, Cincinnati, Ohio 45268
Microbore liquid chromatography and positive ion electrospray mass spectrometry are applied to the determination of 16 carbamate, urea, and thiourea pesticides and herbicides in water. The electrospray mass spectra of the analytes were measured and are discussed and mobilephase matrix effects were evaluated. Analyte positive ion abundances are generally inversely related to the concentration of acetic acid in the acetonitrile-water mobile phase in the range of 0.001-0.1% (v/v) acetic acid. Using an internal standard for quantitative analyses and no acid in the mobile phase, retention time precision, peak width precision, concentration measurement precision, mean recoveries, and instrument detection limits were determined in reagent water. The 16 analytes were also measured in fortified environmental water samples from a recreational lake, a groundwater well, a cistern, a farm pond, and drinking water. These measurements were at 5 ng/mL of each analyte, which is within the range expected for environmental pesticide and herbicide contaminants. The analytes were separated from the environmental water matrixes with an on-line extraction and concentration to provide rapid sample analyses without a slow off-line liquid-liquid or liquid-solid-liquid extraction and extract concentration. Recoveries of 12 of the analytes from 4 environmental water samples were in the range of 75-124% with relative standard deviations in the range of 11-16%. Some widely used agricultural pesticides and herbicides are usually not target analytes in environmental monitoring of air, groundwaters, rivers, and lakes. For example, of the N-methylcarbamate pesticides shown in Chart 1 and the urea- and thioureaderived herbicides in Chart 2, only the carbamates carbaryl and carbofuran and the ureas linuron and fluometuron were targeted analytes in recent surveys.1-5 These surveys were of the Missis* Corresponding author: (voice) 513-569-7309; (fax) 513-569-7757; (e-mail)
[email protected]. † Present address: Cognis Corp., 4900 Este Ave., Cincinnati, OH 45232. (1) Pereira, W. E.; Hostettler, F. D. Environ. Sci. Technol. 1993, 27, 15421552. (2) Kolpin, D. W.; Thurman, E. M.; Goolsby, D. A. Environ. Sci. Technol. 1996, 30, 335-340. 10.1021/ac0010734 CCC: $20.00 Published on Web 01/25/2001
© 2001 American Chemical Society
sippi River and its tributaries, Midwestern near-surface aquifers, shallow groundwater in 20 hydrologic basins, three streams of the Mississippi River Delta, and the air along the Mississippi River.1-5 The likely reason for omitting most or all of the compounds in Charts 1 and 2, and many analogous toxic substances, is that they are generally not amenable to separation by gas chromatography (GC). Therefore they cannot be determined by GC/mass spectrometry (MS), which is commonly used for reliable identifications and measurements of pesticides and herbicides in environmental samples.1-5 Several of the cited surveys monitored a significant number of pesticides, that is 46 in one 3 and 42 plus 3 transformation products in another,5 but essentially all of the targeted analytes are amenable to GC/MS. Only one of these reports recognized typical GC performance problems with the N-methylcarbamates carbaryl and carbofuran (footnote g to Table 1 in ref 5). The compounds in Charts 1 and 2 are subject to thermal decomposition and/or have inadequate vapor pressures at injection port and column oven temperatures required for separation by GC. The compounds in Charts 1 and 2 are amenable to separation with reversed-phase high-performance liquid chromatography (HPLC), but they are not readily determined with standard HPLC detectors. These substances generally do not contain a strong ultraviolet or visible light absorbing chromophore or other structural features that allow selective and high-sensitivity detection. Furthermore, chemical derivatives with properties that facilitate detection are not readily prepared. Most of the Nmethylcarbamates in Chart 1, including two with promulgated drinking water maximum contaminant levels in the United States, are monitored using a fairly complex procedure that is subject to serious interferences. In this method, a 0.4-mL aliquot of sample water is injected into a wide-bore (3.9-4.6-mm inside diameter (i.d.)) HPLC column and after separation the N-methylcarbamates are hydrolyzed on-line with dilute sodium hydroxide at 95 °C to release methylamine. The methylamine reacts with o-phthalalde(3) Kolpin, D. W.; Barbash, J. E.; Gilliom, R. J. Environ. Sci. Technol. 1998, 32, 558-566. (4) Coupe, R. H.; Thurman, E. M.; Zimmerman, L. R. Environ. Sci. Technol. 1998, 32, 3673-3680. (5) Majewski, M. S.; Foreman, W. T.; Goolsby, D. A.; Nakagaki, N. Environ. Sci. Technol. 1998, 32, 3689-3698.
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Chart 1
Chart 2
hyde and 2-mercaptoethanol to form a fluorescent compound that is detected with high sensitivity but with little or no selectivity.6 (6) Method 531.1, Revision 3.1. In Methods for the Determination of Organic Compounds in Drinking Water, Supplement III, USEPA Report EPA/600/ R-95/131, August, 1995; URL http://www.epa.gov/nerlcwww/methmans. html.
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All analytes must be separated because all are detected with the identical degradation product and its derivative. Other coeluting N-methylcarbamates must not be present because they would also produce methylamine and the same derivative. Other compounds that could produce methylamine under the separation or derivatizing conditions must not be present. This method is not suitable for samples that may have many potential interferences including river water, lake water, and groundwater containing agricultural runoff, industrial discharges, or hazardous waste leachate. The purpose of this research was to investigate a more suitable, highly selective, and high-sensitivity analytical method for the identification and measurement of the compounds in Charts 1 and 2 in ambient waters. Packed reversed-phase HPLC columns with inner diameters of 1 mm or less, that is, microbore columns, have been known for many years to provide superior efficiency and high sensitivity7 compared to conventional wider bore HPLC columns, but they are rarely used for environmental analyses. These more efficient columns are now commercially available from several suppliers along with specialized pumps, injectors, and (7) Novotny, M. Anal. Chem. 1981, 53, 1294A-1308A.
Table 1. Major Positive Ions and Their Relative Abundances in the Electrospray Spectra of the Analytes at a CapEX of 85 Va analyte MW m/z ADC ASX ASX ASN ASN MTH MTH MTH OXM OXM CBR CBR CBF CBF HCF HCF HCF BGN BGN MTC MTC MXC DIU LIN FMT SID-a SID-a SID-b ANT
190 206 222 162 219 201 221 237 209 225 222 232 248 232 232 202
116 207 229 223 245 163 106 88 237 242 202 145 222 244 238 255 220 210 168 226 169 223 233 249 233 233 465 233 203
compositionb M + H - MIC - H2O M+H M + Na M+H M + Na M+H M + H - MIC M + H - MIC - H2O M + (NH4) M + Na M+H M + H - MIC M+H M + Na M+H M + (NH4) M + H - (H2O) M+H M + H - (C3H6) M+H M + H - MIC M+H M+H M+H M+H M+H 2M + H M+H M+H
rel abundc rel abundd 29.5 4.2 4.9 16.1 10.3 18.3 12.0 19.2 6.6 9.5 31.2 33.2 75.6 92.9 13.7 11.0 13.6 51.5 32.9 43.1 23.6 90.7 56.3 27.5 98.5 100 21.4 28.8 16.6
100 86 100 100 64 96 63 100 70 100 94 100 81 100 100 80 38 100 64 100 55 100 100 100 100 100 21 100 100
a After separation of 0.625 ng of each analyte on a 1 mm i.d. C-18 column with an acetonitrile-water gradient elution. bMIC, methylisocyanate (MW 57). cRelative to the absolute abundance of the m/z 233 ion of SID-a, which gave the largest absolute abundance measured with 0.625 ng of each analyte. dRelative to the most abundant ion in the spectrum of each analyte.
other accessories that permit the convenient utilization of this technology. In addition, microbore columns utilize significantly lower mobile-phase flow rates, that is tens of microliters per minute instead of one or more milliliters per minute, which reduces both the cost of organic solvents and the cost and potential environmental contamination from waste disposal. Electrospray (ES) is an effective interface for HPLC and MS and mass spectrometers, which are universal detectors, are commercially available as relatively low-cost benchtop models. The low mobilephase flow rates used with microbore HPLC columns are eminently compatible with the ES interface and MS. Therefore, the technique of narrow-bore or microbore HPLC combined with ES-MS was selected for this investigation of the compounds in Charts 1 and 2 in environmental water. Some of the compounds in Charts 1 and 2 have been studied previously with HPLC/ES-MS and several limited analytical approaches have been described. An early investigation used a conventional 4.6-mm-i.d. HPLC column with a 1 mL/min mobilephase flow rate, but the column effluent had to be split 20:1 before the ES interface, which effectively discarded 95% of the analytes.8 This study did show that eight of the N-methyl carbamates in Chart 1 gave predominately (M + H)+ ions and one or several fragment ions with an acidic mobile phase and pneumatically (8) Pleasance, S.; Anacleto, J. F.; Bailey, M. R.; North, D. H. J. Am. Soc. Mass Spectrom. 1992, 3, 378-397.
assisted ES. Although seven of the eight carbamates were well separated, the HPLC peaks were rather broad (∼10-20 s) and detection limits relatively high. An ion trap mass spectrometer was used with ES to study the collision induced dissociation (CID) of aldicarb sulfone ions.9 This compound, carbofuran, and another N-methyl carbamate, propoxur, were slightly separated by on-line HPLC with a 1-mm-i.d. microbore column and a flow rate of 10 µL/min.9 An analytical method for carbofuran in water was developed using on-line immunoaffinity chromatography followed by a 2.1-mm-i.d. HPLC column and pneumatically assisted ESMS.10 In a study that included several of the compounds in Chart 2, and many analogous analytes, a 2-mm-i.d. HPLC column was used with a flow rate of 0.3 mL/min.11 Diuron and linuron (Chart 2) and eight other urea herbicides gave predominately (M + H)+ ions and one or several fragment ions with ES-MS. While these results are encouraging, a systematic quantitative study of the high-resolution separation and high-sensitivity ES-MS detection of a variety of the compounds in Charts 1 and 2 is required to develop this analytical approach and permit application to environmental monitoring. EXPERIMENTAL SECTION Materials. Twelve of the pesticides and herbicides were crystalline reference standards obtained from the U.S. Environmental Protection Agency (USEPA) pesticide repository with stated purities of 99% or better except 1-naphthylthiourea, which had a stated purity of 93.9%. Ten analytes and the internal standard were obtained as standard solutions in methanol for USEPA method 531.16 from Absolute Standards, Inc. (Hamden, CT). Acetonitrile, Optima grade with a 99.9+% purity, glacial acetic acid, Optima grade with a 99.5% purity, and ammonium acetate, HPLC grade with a 98.2% purity, were obtained from Fisher Scientific (Pittsburgh, PA). Ammonium formate, ammonium citrate (dibasic), and sodium acetate are certified ACS reagent grade and were also obtained from Fisher Scientific. Formic acid is from Sigma Chemical Co. (St. Louis, MO) with a stated purity of 95.8% (4.2% water for stabilization). Water was obtained from a Milli-Q water system (Millipore, Bedford, MA). Standard Solutions. About 20 mg of each crystalline analyte was weighed into a separate 10-mL volumetric flask and acetonitrile added to prepare a series of solutions containing one analyte each at a concentration of ∼2 mg/mL. Aliquots of these solutions were diluted with acetonitrile or Milli-Q water to prepare a series of solutions containing one analyte each at various concentrations in several solvent matrixes. Solutions containing multiple analytes were prepared by mixing aliquots of the individual analyte solutions and diluting with acetonitrile or water. The dilute solutions used in most experiments typically contained less than 2% (v/v) acetonitrile. Commercial standard solutions of single analytes in methanol were similarly diluted and aliquots mixed and diluted with water to prepare multianalyte standard solutions containing less than 2% (v/v) methanol. Liquid Chromatograph. Separations were conducted with a Hewlett-Packard model 1090 Series II liquid chromatograph equipped with a model 79847A autosampler, a 79846A autoinjector, a 79835A solvent delivery system, and a 25-µL syringe. The mobile(9) Lin, H.-Y.; Voyksner, R. D. Anal. Chem. 1993, 65, 451-456. (10) Rule, G. S.; Mordehai, A. V.; Henion, J. Anal. Chem. 1994, 66, 230-235. (11) Volmer, D.; Levsen, K. J. Am. Soc. Mass Spectrom. 1994, 5, 655-675.
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Figure 1. Separations of 11 representative analytes with a 2.1 i.d. × 150 mm C-18 column (a) and a 1 mm i.d. × 150 mm C-18 column (b) using an acetonitrile-water gradient elution with 0.1% acetic acid in the mobile phase. The peak numbers are identified with the names and structures of the analytes in Charts 1 and 2.
phase flow from the solvent delivery system was split to obtain reliable flow rates less than 100 µL/min. A stainless steel “T” with a 0.25-mm bore size (Valco Instruments, Houston, TX) was placed between the solvent delivery system and the injector. The other outlet from the “T” was directed to waste using a length of 50µm-i.d. fused-silica tubing. The length of the fused-silica tubing and the flow rate from the solvent delivery system were adjusted to obtain the desired flow rate through the analytical column. The HPLC instrument autosampler and autoinjector were used for all experiments except the analyses of environmental water samples, which were injected manually using the internal Rheodyne (Rohnert Park, CA) model 7010 sample injector which was reconfigured as described under Fortified U.S. Environmental Protection Agency, Environmental Water Samples and in Results and Discussion. Mass Spectrometer. The mass spectrometer was a HewlettPackard (Palo Alto, CA) model 5989B single-quadrupole instrument equipped with a Hewlett-Packard 59987A electrospray ion source containing some components (model 103313 serial number 1066) manufactured by Analytica of Branford (Branford, CT). The mass spectrometer was also equipped with a high energy conversion dynode detector, a G1034C MS ChemStation data system, and G1047A LC/MS software. The electrospray interface has a design similar to that described by Fenn et al.,12 in which the electrospray needle is at ground potential and high voltages are applied to the cylindrical and end plate electrodes and to the entrance of a dielectric capillary leading to the mass spectrometer. The interface voltages were optimized for maximum signal/noise ratios and were -3.05 kV on the cylindrical electrode, -3.3 kV on the endplate electrode, and -3.8 kV on the capillary entrance. (12) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64-71.
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A low voltage of opposite polarity was applied to the exit of the dielectric capillary (CapEX) and this was 85 V unless otherwise stated. The ends of the dielectric capillary were coated with a thin layer of Pt to facilitate connection of the electrical leads. The original optics of the ion source were replaced with an Iris hexapole ion guide (Analytica of Branford) to improve transmission of low-m/z species. All experiments were conducted using positive ions with either full spectrum data acquisition or selected ion monitoring. The ES-MS was tuned with a Hewlett-Packard small-molecule positive ion electrospray tuning solution which contained a mixture of peptides. In the full spectrum mode, the spectrometer was scanned from 115 to 280 m/z at a rate of 1.1 scans/s or from 50 to 550 m/z at a rate of 0.39 scans/s. Nitrogen at 70-80 psi flowed through the outer concentric stainless steel tube of the ES needle assembly to provide pneumatic assistance for the electrospray nebulization and promote the formation of a fine spray. A flow of heated dry nitrogen (heater temperature 200 °C) was maintained counter to the electrospray flow to promote desolvation and declustering of the positive ions. Conditions for Figure 1a. The column was a 2.1-mm (i.d.) × 150 mm Zorbax RX-C18 (MAC-MOD Analytical, Inc., Chadds Ford, PA). The flow rate was ∼50 µL/min and both solvent A (water) and solvent B (acetonitrile) contained 0.1% (v/v) acetic acid. The gradient elution was hold at 35% B for 2 min, a linear gradient to 50% B at 4 min, and 100% B at 21 min. The injection was 25 µL of a 100 ng/mL solution of each of 11 analytes (2.5 ng of each). The CapEX was 90 V, and the spectrometer was repetitively scanned from 115 to 280 m/z. Conditions for Figure 1b. The column was a 1.0 mm (i.d.) × 150 mm Zorbax 3.5-µm SB-C18 (Micro-Tech Scientific, Sunnyvale, CA). The flow rate was ∼35 µL/min, and both solvent A (water) and solvent B (acetonitrile) contained 0.1% (v/v) acetic
Figure 2. Total ion abundance of 11 representative analytes after separation on a 1 mm i.d. × 150 mm C-18 column with an acetonitrilewater gradient elution with varying amounts of acetic acid in the mobile phase. The analyte abbreviations are identified with the names and structures of the analytes in Charts 1 and 2.
Figure 3. Separation of 16 analytes with a 1 mm i.d. × 150 mm C-18 column and an acetonitrile-water gradient elution without acetic acid. The peak numbers are identified with the names and structures of the analytes in Charts 1 and 2 and in Table 2.
acid. The gradient elution was hold at 35% B for 2 min and a linear gradient to 100% B at 17 min. The injection was 25 µL of a 100 ng/mL solution of each of 11 analytes (2.5 ng of each). The CapEX was 85 V, and the spectrometer was repetitively scanned from 115 to 280 m/z. Conditions for Figure 2. These were the same as Figure 1b except the concentrations of acetic acid in the mobile phase were 0.001 (v/v), 0.0015, 0.006, 0.01, and 0.1% and the injection was 10 µL of a 100 ng/mL solution of 11 analytes (1 ng of each). Conditions for Table 1. The column was a 1.0 mm (i.d.) × 150 mm LUNA 3-µm C18 (Phenomenex, Torrance, CA). The flow rate was ∼35 µL/min, and the gradient elution was hold at 35% B (acetonitrile) for 2 min and a linear gradient to 98% B at 17 min. No acetic acid was used in the mobile phase. The injection was 25 µL of a 25 ng/mL solution of 16 analytes (0.625 ng of each).
The CapEX were 50, 70, 85, 90, 110, and 130 V, but only the 85-V data are shown in Table 1, and the spectrometer was repetitively scanned from 50 to 550 m/z. Conditions for Figure 3. These were the same as Table 1 except the injection was 25 µL of a 5 ng/mL solution of 16 analytes (125 pg of each), the CapEX was 85 V, and data were acquired by SIM using the ions in Table 2. Conditions for Tables 2-4. These were the same as Figure 3 except the environmental water samples in Table 4 were processed as described under Fortified Environmental Water Samples. Fortified Environmental Water Samples. One-liter water samples were collected from the Sharon Woods and Winton Woods lakes in Hamilton County, Ohio; the Ohio River near the public landing in Cincinnati; a private groundwater well in a rural Analytical Chemistry, Vol. 73, No. 5, March 1, 2001
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Table 2. Analytes, SIM Data Acquisition Ions, Retention Times (RT), and Peak Widths at Half-Height (W1/2) for the Carbamate and Urea Pesticides and the Internal Standard analytes (Charts 1 and 2)
no. in Figure 3
SIM data acquisition ions
mean RT (min)
RSDa
mean W1/2 (min)
RSDa
ADC ASX ASN MTH OXM CBR CBF HCF BGN MTC MXC DIU LIN FMT SID ANT
7 1 3 4 2 11 9 5 8 14 16 12 15 10 13 6
213, 116 207, 229 223, 240, 245 163, 185, 106 220, 237, 242 202, 145, 224 222, 165, 244 238, 220, 260 210, 168, 232 226, 169, 232 166, 223 233, 235, 255, 465 249, 251, 271 233, 255, 465 233, 255, 465 203, 225
9.75 4.09 4.47 4.93 4.30 12.80 12.17 5.83 12.00 15.54 16.45 13.25 15.96 12.68 15.14 8.59
1.04 2.03 1.85 1.61 1.94 0.77 0.71 1.54 0.70 0.68 0.62 0.77 0.67 0.75 0.70 1.21
0.122 0.041 0.069 0.080 0.065 0.107 0.109 0.110 0.108 0.107 0.106 0.109 0.110 0.105 0.108 0.132
4.2 13 8.1 8.4 8.9 8.6 5.2 6.6 5.2 6.1 9.4 6.6 9.8 4.4 3.3 6.6
258, 260, 280, 282
15.89
0.69
0.108
6.1
BDCb a
Relative standard deviation from seven replicate measurements. b Internal standard.
Table 3. Measurement Precision, Mean Recoveries, and Instrument Detection Limits for Carbamate and Urea Pesticides and Herbicides in Reagent Water
analytes
mean measd concna (ng/mL)
RSDa (%)
measd/true concn (%)b
IDLc (pg/mL)
IDLc (pg)
ADC ASX ASN MTH OXM CBR CBF HCF BGN MTC MXC DIU LIN FMT SID ANT
4.92 4.54 4.81 4.96 5.11 5.65 5.29 5.21 5.17 5.73 5.42 5.1 5.45 5.05 5.61 4.68
11 13 5.7 9.6 6.8 13 7.7 8.2 8.0 4.1 10 11 8.3 13 6.5 6.9
99 91 96 99 102 113 106 104 103 115 108 102 109 101 112 94
163 57 91 171 122 92 75 120 210 78 41 116 144 64 53 957
4.1 1.4 2.3 4.3 3.1 2.3 1.9 3.0 5.3 2.0 1.0 2.9 3.6 1.6 1.3 24
a Relative standard deviations from seven replicate analyses of a 5 ng/mL solution. bThe measured concentration as a percentage of the fortified concentration. cInstrument detection limit (IDL).
area of Butler County, Ohio; a private farm pond in Boone County, Kentucky; a cistern at a private residence in Hamilton County, Ohio; and a laboratory tap dispensing Cincinnati drinking water. The pH of the samples were in the 7.1-8.4 range and 0.1-0.3 mL of glacial acetic acid was added to each to reduce the pH to just below 3. The samples were filtered through a Whatman (Fisher Scientific, Pittsburgh, PA) 25-cm-diameter membrane filter with 0.2-µm pore size and stored in a refrigerator until used. Before the samples were analyzed, aliquots of aqueous standard solutions of the 16 analytes in Charts 1 and 2 and the internal standard 4-bromo-3,5-dimethylphenyl-N-methyl carbamate (BDC-17) were added to the water samples to give analyte concentrations of 5 ng/mL and an internal standard concentration of 2.5 ng/mL. 1002 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001
A 1 mm (i.d.) × 10 mm Zorbax 3.5-µm SB-C18 guard column (Micro-Tech Scientific) was used to extract the analytes from the water. The analyte capture column was slowly flushed with 100 µL of water, a 25-µL aliquot of water sample was slowly injected, and the column was then slowly purged with 70 µL of water to remove ionic and other weakly retained nonanalyte material from the capture column. The Rheodyne model 7010 injector was then switched to allow mobile phase to elute the analytes from the analyte capture column, separate them on the analytical column, and pass the effluent into the electrospray interface. The other HPLC conditions and calibration techniques used for these analyses are the same as those used for the determination of retention time, peak width, measurement precision, and recovery figures of merit (Figure 3 and Tables 2 and 3). RESULTS AND DISCUSSION Figure 1a shows the separation of 11 analytes with a 2.1 mm i.d. × 150 mm C-18 reversed-phase column, and Figure 1b shows the separation with a 1 mm i.d. × 150 mm microbore C-18 reversed-phase column. Both separations were conducted with the same concentrations and injected amounts (2.5 ng) of the analytes, very similar acetonitrile/water gradient elutions, the same ES-MS full spectrum data acquisition parameters, but slightly different mobile-phase flow rates. The resolution of the analytes and the signal/noise ratios are much better with the 1-mm-i.d. microbore column than the 2.1-mm-i.d. column. The peaks are generally broader with the larger diameter column, but all analytes are clearly separated into narrow peaks with the microbore column. Some analytes, for example, ASX-1, ASN-3, and MTH-4 (the numbers refer to the peak identifiers in the figures), are difficult to detect in the total ion chromatogram obtained with the 2.1-mm-i.d. column (Figure 1a). Two siduron isomers (SID-a and SID-b) are evident with the 2.1-mm-i.d. column and are clearly resolved with the 1-mm-i.d. column. The major isomer is identified as 13 in Figure 1a and b, and the minor isomer elutes immediately after it but is not numbered. All subsequent experiments reported in this paper were conducted with a 1-mm-i.d. microbore column.
Table 4. Concentrations Measured and Percent Recoveries of the Analytes in Five Water Matrixes at 5 ng/mL measured concns (ng/mL) and % recoveriesa
a
analytes
Winton Lake
well water
cistern water
farm pond
tap water
ADC ASX ASN MTH OXM CBR CBF HCF BGN MTC MXC DIU LIN FMT SID ANT
4.53 (91) 3.5 (70) 5.43 (109) 3.72 (74) 3.74 (75) 5.67 (113) 5.34 (107) 5.7 (114) 5.43 (109) 5.98 (120) 5.79 (116) 4.26 (85) 4.82 (96) 4.43 (89) 5.07 (101) 0.83 (17)
4.69 (94) 2.45 (49) 4.17 (83) 2.87 (57) 1.78 (36) 5.56 (111) 5.62 (112) 5.88 (118) 5.19 (104) 5.20 (104) .25 (105) 4.25 (85) 4.56 (91) 4.47 (89) 4.34 (87) 1.13 (23)
4.59 (92) 2.05 (41) 4.81 (96) 3.68 (74) 2.51 (50) 5.31 (106) 5.70 (114) 6.19 (124) 5.34 (107) 4.68 (94) 4.62 (92) 4.11 (82) 3.77 (75) 4.24 (85) 3.75 (75) 2.04 (41)
4.91 (98) 2.42 (48) 4.55 (91) 3.75 (75) 5.08 (102) 5.38 (108) 5.65 (113) 5.96 (119) 5.20 (104) 5.24 (105) 4.49 (90) 4.40 (88) 4.17 (83) 4.30 (86) 4.61 (92) 0.61 (12)
3.57 (71) 1.58 (32) 2.77 (55) 2.51 (50) 1.46 (29) 4.15 (83) 4.06 (81) 4.69 (94) 4.14 (83) 4.37 (87) 4.26 (85) 3.78 (76) 3.92 (78) 3.10 (62) 3.56 (71) 1.21 (24)
Mean of two determinations of the indicated source water adjusted with acetic acid to pH 3 and fortified with 5 ng/mL of each analyte.
Mass Spectra. The ES mass spectra of the analytes in Charts 1 and 2 were measured after separation of 0.625 ng of each on a 1-mm-i.d. C-18 column with an acetonitrile/water gradient elution. An operational parameter of the ES interface used in this work is the capillary exit voltage (CapEX). This potential affects the collision energy of ions as they pass from a region in the interface of ∼1 Torr pressure into the high vacuum of the mass spectrometer. The ES spectra of the analytes were measured at CapEX voltages of 50, 70, 85, 90, 110, and 130 V, and the major ions observed at 85 V are listed in Table 1. An ion was considered major if its abundance was at least 10% of the m/z 233 ion of the first-eluting isomer of SID (SID-a) which was the most abundant analyte ion observed. Several ions with less than 10% relative abundance are also shown in Table 1 because they were the most abundant ions observed with the analytes ASX and OXM. Some (M + 18)+ adduct ions of the carbamates were observed and these are likely caused by traces of ammonium ion, perhaps from atmospheric ammonia, in the mobile phase. Generally, the abundances of the carbamate (M + 18)+ ions are higher at the lower CapEX voltages and decrease significantly in the 85-90-V range. These adduct ions are decomposed by collisions with vaporphase molecules in the interface at the higher CapEX voltages. Only two relatively low abundance (M + 18)+ ions from OXM and HCF are observed at a CapEX of 85 V. Sodium ion adducts of the carbamates also form, probably from traces of Na+ in the mobile phase or column, and they are more resistant to decomposition and persist at the higher CapEX voltages. Four carbamates (ASX, ASN, OXM, CBF) gave sodium ion adducts, but only the CBF ion is among the most abundant of all the ions observed. Ammonium ion adducts of the urea and thiourea derivatives were not observed and sodium ion adducts were either not observed or had low abundances. Nearly all the analytes in Charts 1 and 2 give major (M + H)+ ions (Table 1). The abundances of the carbamate (M + H)+ ions often maximize in the 70-90 CapEX voltage range, but the urea derivatives generally give abundant or very abundant (M + H)+ ions over a broader CapEX voltage range. At most CapEX voltages, some fragmentation of the carbamate (M + H)+ ions is observed but these reactions are generally more important at CapEX in the
Chart 3
range of 90-130 V. Carbamate (M + H)+ ions often lose methyl isocyanate (MesNdCdO) as shown in reactions 1 and 2 in Chart 3. The N-methyl oxime carbamates ADC, ASX, and ASN have a hydrogen on the carbon of the oxime CdN bond and water is eliminated from the (M + H - 57)+ ions to form the protonated nitriles (reaction 1 in Chart 3). This reaction gives the most abundant ion with ADC, an abundant ion with ASX, and a lowabundance ion with ASN. The (M + H - 57)+ ion of MTH does not have a hydrogen on the carbon of the oxime CdN bond, but it can lose the elements of water to form an abundant (M + H 57 - 18)+ ion at m/z 88. The oxime carbamate OXM must form stable Na+ and NH4+ adducts because the loss of methyl isocyanate is not observed with this analyte even at the highest Analytical Chemistry, Vol. 73, No. 5, March 1, 2001
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CapEX voltages. All the N-methyl O-arylcarbamates shown in reaction 2 of Chart 3 give (M + H - 57)+ ions, and these are major ions at 85 V with CBR and MTC. The carbofuran oxidation product 3-hydroxycarbofuran (Chart 1) is protonated at the hydroxy group and expels a water molecule to give an abundant (M + H - 18)+ ion at m/z 220. Similarly baygon (Chart 1) is protonated at the ether oxygen and eliminates propene to produce an abundant ion at m/z 168 (reaction 3 in Chart 3). The (M + H)+ and (M + Na)+ ions from the substituted ureas and thiourea (Chart 2) are generally stable at CapEX voltages in the 85-90 range and give either no fragment ions or low-abundance fragment ions. However, three of the ureas, DIU, FMT, and SID, give moderately abundant proton-bound dimers at m/z 465. A CapEX of 85 V was selected as appropriate for most analytes and was used for all subsequent experiments. Mobile-Phase Matrix Effects. Since nearly all the analytes give major (M + H)+ ions, increasing the concentration or strength of an acid in the mobile phase should increase the abundances of these and related fragment ions in the gas phase that are measured in the mass spectrometer. The effect of acetic acid concentration on ion abundances was determined using a group of 11 representative analytes separated on a 1-mm-i.d. reversed-phase column. The stronger formic acid was also used but this experiment was inconclusive because the relative standard deviations (RSDs) of repetitive ion-abundance measurements were generally large and often >15% whereas RSDs of measurements in the lower concentration acetic acid solutions averaged
