Seven sulfonylurea herbicides were studied using negative-ion fast atom bombardment mass spectrometry (FAB-MS). The spectra were characteristic of the ...
W. Winnik, W.C. Brumley and L.D. Betowski, Eur. Mass Spectrom. 2, 43–47 (1996)
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Negative-Ion FAB Mass Spectrometry of Sulfonylureas W. Winnik, W.C. Brumley and L.D. Betowski, Eur. Mass Spectrom. 2, 43–47 (1996)
Negative-ion fast atom bombardment mass spectrometry of sulfonylurea herbicides
Witold Winnik, a William C. Brumley* and Leon D. Betowski United States Environmental Protection Agency, National Exposure Research Laboratory, Characterization Research Division, Las Vegas, Nevada 89193, USA.
Seven sulfonylurea herbicides were studied using negative-ion fast atom bombardment mass spectrometry (FAB-MS). The spectra were characteristic of the structure of the examined herbicides with fragmentation at the sulfonylurea bridge representing both aromatic moieties. The fragmentation pattern was established based on the FAB-B/E mass spectrometry/mass spectrometry linked-scan spectra. Background-subtracted negative ion FAB-MS spectra of the herbicides exhibit [M–1] – peaks and their fragmentation products. Application of triethylenetetramine as a FAB matrix resulted in enhanced fragmentation. The 3-nitrobenzyl alcohol FAB spectra typically exhibited high-abundance [M–1] – peaks and fewer fragment peaks. Keywords: negative ionization, tandem mass spectrometry, sulfonylurea herbicides, fast atom bombardment, FAB, negative-ion FAB-MS, linked-scan, triethylenetetramine, 3-nitrobenzyl alcohol.
Introduction One of the goals of the Analytical Chemistry Research Program at the US Environmental Protection Agency (Characterization Research Division, Las Vegas) is to ensure that mass spectral techniques, such as fast atom bombardment mass spectrometry (FAB-MS), become available for assessing exposure and for routine environmental monitoring. Sulfonylurea herbicides constitute a relatively new class of herbicides. Their very high plant toxicity, in addition to high crop selectivity, allows for low field application rates. A typical half-life of these compounds in soil is less than two months. These features, coupled with low acute toxicity to mammals (oral LD50 greater than 4,000 mg kg–1 in male rats) have made these herbicides increasingly common. 1 Very low application rates of sulfonylurea herbicides (several grams per hectare) and their fast degradation in the environment demand use of sensitive and selective detection techniques. Sulfonylureas have been investigated by several mass spectral techniques using a positive-ion mode of operation. For example, gas chromatography–mass spectrometry (GC/MS) was investigated as a tool for the analysis of
a
National Research Council Associate at the US EPA, Las Vegas, Nevada, USA.
methylated chlorsulfuron derivatives.2 Sulfonylurea herbicide determinations by thermospray liquid chromatography–mass spectrometry (LC/MS) have been described.3–5 A continuous-flow positive-ion (CFFAB/MS) technique was used to detect eight sulfonylurea herbicides separated from their mixture by capillary electrophoresis;6 full-scan collisionally activated decomposition (CAD) spectra were recorded for each component of the mixture. Nanoscale capillary liquid chromatography and capillary zone electrophoresis have been interfaced with a quadrupole mass spectrometer equipped with electrospray ionization for the structurally related sulfonamide antibiotics.7a–b The present work is a continuation of our investigation of gas-phase chemistry of sulfonylurea herbicides.8,9 Previously, we reported the results of desorption–chemical ionization negative-ion mass spectrometry/mass spectrometry (MS/MS) experiments involving sulfonylureas. We now extend this to negative-ion (NI) FAB-MS and MS/MS spectra for seven sulfonylurea herbicides. Experimental Sulfonylurea herbicide standards were obtained from ChemService (West Chester, PA, USA). The samples were mixed with a FAB matrix. Triethylenetetramine (TETA), 3-nitrobenzyl alcohol (NBA) and O-deuterated glycerol-d3 were purchased from Aldrich Chemical Com-
44
Negative-Ion FAB Mass Spectrometry of Sulfonylureas
pany, Inc. The NI/FAB-MS and NI/FAB linked-scan B/E CAD experiments were performed using a VG 7070EQ mass spectrometer. FAB conditions: current 1 mA; accelerating voltage 6 kV (xenon gas); in the B/E CAD experiments helium gas was introduced into the first field-free region to attenuate the ion beam by 50%. Typically, 5 to 15 scans were averaged to obtain a spectrum. Results and discussion The NI/FAB-MS spectrum of sulfometuron methyl (1; Figure 1), obtained using the TETA matrix, displayed an abundant (100% RA) peak corresponding to the [M – H]– ion (Table 1). Peaks appearing at m/z 122 and 182 were assigned to the fragmentation products of this ion. The structural assignment was facilitated by the results of the B/E CAD experiment involving this precursor ion, where the spectrum of the [M – H]– ion showed peaks at m/z 122, 182 and 214 (Table 2). These ions were assigned structures of 4,6-dimethylpyrimidine, saccharine and methyl 2-(aminosulfonyl)benzoate. In the NI/FAB-MS spectrum of 1, obtained using the NBA matrix, the peak at m/z 122 was obscured by the interfering matrix peak at m/z 122. The NI/FAB-MS spectrum and the B/E CAD spectrum of the [M – H]– ion of metsulfuron methyl (2), obtained using the TETA matrix, exhibited a [M – H] – peak (m/z 380) and peaks m/z 139, m/z 182 and m/z 214, which
Figure 1. Structural formulas of the investigated sulfonylurea herbicides 1–7.
Table 1. Most significant FAB-MS peaks, scan range: m/z 100–501. Compound 1
2
3
4
5
6
7
m/z (% relative abundance) 1.
in TETA:
122 (42), 182 (17), 199 (19), 228 (9), 363 (100)
2.
in NBA:
363 (100) [peaks 122 and 199 overlap with the matrix peaks]
1.
in TETA:
107 (31), 125 (54), 139 (100), 182 (70), 214 (18), 390 (98)
2.
in NBA:
139 (45), 182 (27), 214 (30), 380 (100)
1.
in TETA:
139 (100), [peak 190 overlaps with the matrix peak], 356 (69), 358 (30)
2
in NBA:
139 (69), 190 (72), 192 (30), 356 (100), 358 (43)
1.
in TETA:
107 (24), 125 (35), 139 (100), 162 (54), 246 (18), 334 (94), 360 (21), 386 (40), 500 (51)
2.
in NBA:
139 (81), 220 (24), 386 (100)
1.
in TETA:
122 (29), 127 (31), 140 (88), 154 (100), 196 (40), 254 (20)
2.
in NBA:
[peaks 122 and 154 overlap with the matrix peaks, 166 (26), 249 (26)]
1.
in TETA:
122 (56), 158 (100), 160 (43), 182 (52), 228 (8), 413 (77), 415 (31)
2.
in NBA:
[peak 122 overlaps with the matrix peak], 158 (49), 160 (17), 182 (79), 228 (49), 413 (100), 415 (42)
1.
in TETA:
106 (46), 122 (32), 140 (78), 154 (100), 228 (62), 254 (19), 261 (28), 395 (24), 409 (100)
2.
in NBA:
[peak 122 overlaps with the matrix peak], 140 (19), 154 (52), 228 (35), 409 (100)
3.
in Gly-d3:
154 (48), 155 (93), 228 (45), 229 (98), 409 (80), 410 (80)
W. Winnik, W.C. Brumley and L.D. Betowski, Eur. Mass Spectrom. 2, 43–47 (1996)
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Table 2. Most important FAB (TETA) B/E-scan CAD product ions of [M – H]– precursor ions. (Structures of ions a–f are presented in Figure 2.) Compound
m/z (% relative abundance, normalized to the most abundant fragment ion) (M – H)
1
2
3
4
5
6
7
–
a
b
c
d
e
f
363
122 (100)
214 (11)
182 (11)
—
—
—
380
139 (100)
214 (26)
182 (15)
—
—
—
356
139 (100)
190 (14)
—
—
—
—
386
139 (100)
220 (22)
—
246 (16)
—
—
409
154 (47)
228 (11)
196 (13)
—
254 (100)
190 (25)
413
158 (100)
228 (8)
182 (17)
—
—
—
409
154 (100)
228 (35)
—
254 (41)
—
—
corresponded to the dissociation product ions a, c and b of the m/z 380 ion (Table 1 and 2). Some peaks (m/z 107, 125) displayed in the TETA NI/FAB-MS spectrum were not observed in the NBA NI/FAB-MS spectrum of 2. However, we do not possess enough evidence that would allow for an unambiguous elucidation of the structure of these ions. Chlorsulfuron (3) fragmented under the TETA NI/FAB-MS conditions to give the amine ion a (100% RA) and a less abundant sulfonamide ion b. The linkedscan B/E CAD spectrum confirmed this fragmentation pattern. In the spectrum, the peak associated with a was over seven times as abundant as the peak corresponding to the ion b. Similarly, the NBA NI/FAB-MS spectrum of compound 3 also displayed peaks associated with a (m/z 139) and b (m/z 190). The TETA NI/FAB-MS spectrum of thifensulfuron methyl (4) exhibited, in addition to the typically observed peaks associated with the ions [M – H] –, a, b and c, peaks at m/z 162, 334, 360 and 500, which correspond to the condensation product of 4 with TETA and to its fragment ions (Table 1). Thus, the m/z 500 ion was identified as the condensation product, which then fragmented by elimination of a heterocyclic amine molecule to give the m/z 360 anion, and of aryl isocyanate to give the m/z 334 ion. An aliphatic isocyanate loss from m/z 334 ion gave rise to the sulfamide ion m/z 162. This fragmentation pattern was supported by the linked-scan B/E CAD experiments involving ions m/z 500 and 334 (Table 3, Figure 2). Consequently, the peaks at m/z 162, 334, 360 and 500 did not appear in the NBA NI/FAB-MS spectrum of 4.
The TETA NI/FAB-MS spectrum of bensulfuron methyl (5) displayed peaks assigned to moderately abundant ions [M – H]– and c and to a base peak a (Figure 3). The additional peak (m/z 254) that appeared in the spectrum was assigned the structure of arylidenesulfonylisocyanate e (m/z 254). The B/E CAD spectrum of the [M – H]– ion exhibited peaks resulting from the ions a, b, c, e and from the ion f (m/z 190) of arylideneisocy-
Figure 2. FAB B/E collisionally induced decomposition of precursor ions m/z 500 and m/z 334.
46
Negative-Ion FAB Mass Spectrometry of Sulfonylureas
Table 3. Additional FAB B/E-MS/MS spectra. Compound (matrix)
Product ions
Parent ion (m/z)
m/z (%RA, normalized to the most abundant fragment ion) 4 (TETA)
7 (glycerol-d3)
334
162 (100)
500
334 (100), 360 (10)
409, 410
154 (100), 155 (82), 228 (19), 229 (10), 254 (25), 255 (5)
anate. The NBA FAB-MS spectrum of compound 5 displayed a base peak corresponding to the [M – H]– ion and several small peaks associated with its fragment ions. The TETA and NBA NI/FAB-MS spectra of chlorimuron ethyl (6) were similar. They exhibited peaks corresponding to the ions [M – H]–, a and c; additionally, a peak at m/z 122 was observed, which corresponded to the loss of hydrogen chloride from the heterocyclic amine ion a. The peak corresponding to the ion b (m/z 228) was much higher in the FAB-NBA spectrum compared with the TETA FAB-MS spectrum (49 vs 8% RA). The structural assignment was supported by the B/E CAD experiment involving the [M – H]– ion, which displayed fragment ions a, b, c and m/z 122. The TETA NI/FAB B/E CAD spectrum of the [M – H]– ion of nicosulfuron (7) resulted in three major fragment ions: m/z 154 (a), 228 (b) and 254 (d). The peaks corresponding to the ions a and b also appeared in the regular (TETA and NBA) NI/FAB-MS spectra of 7. To elucidate the number of acidic protons present in the ion d (m/z 254), which were exchangeable by deuterons under FAB conditions, O-deuterated glycerol-d 3 was used as a FAB
matrix. Under exchange conditions, a mixture of nondeuterated and mono-deuterated ions, a (m/z 154 and 155) and b (m/z 228 and 229), respectively, was observed; however, the m/z 254 peak was too small to be analytically useful. Therefore, a glycerol-d 3 FAB B/E CAD experiment was carried out. In this experiment, both the non-deuterated and mono-deuterated [M – 1] – ions (m/z 409 and 410) were subjected to B/E CAD (Table 3). This process resulted in spectra possessing double peaks associated with the ions a (m/z 154 and 155) and b (m/z 228 and 229). The peak m/z 254 assigned to the ion d, however, was accompanied by the adjacent m/z 255 peak of only negliglible abundance. This result indicated that ions a and b, unlike the m/z 254 ion, retained a relatively acidic amine proton of the [M – H]– precursor ion. Conclusions The FAB/MS spectra of sulfonylureas in the TETA matrix displayed greater diversity of fragmentation than the corresponding spectra of sulfonylureas in NBA. Typically, the abundances of the peaks corresponding to the
Figure 3. Major FAB and FAB B/E fragment ions of sulfonylurea herbicides, shown with bensulfuron methyl as an example.
W. Winnik, W.C. Brumley and L.D. Betowski, Eur. Mass Spectrom. 2, 43–47 (1996)
fragment ions were higher in the TETA spectra, with the exception of peaks related to the ions b in the spectra of compounds 1–4 and 6. Compound 4 underwent a nucleophilic substitution reaction during which the methoxy group of the ester moiety was substituted by TETA. Peaks corresponding to the ions a–c, d and e in the B/E CAD spectra are also displayed by the desorption chemical ionization MS/MS spectra.9 FAB/MS and FAB B/E spectra of compound 7 mixed with O-deuterated glycerol-d 3 indicated that the unique ion m/z 254 did not possess the proton that was initially situated on one of the nitrogen atoms of 7. This proton, however, was shown to be present in the product ions a and b. Interestingly, the neutral compounds corresponding to the ions a and b were reported to be sulfonylurea hydrolysis products.10 Thus, the stability of the anions a and b in the gas phase is paralleled by the stability of the corresponding neutral species in solution. Because negative-ion FAB spectra exhibit peaks associated with ions indicative of both aromatic moieties in the molecules, they are potentially useful for structure confirmation.
Acknowledgment Witold Winnik gratefully acknowledges the US National Research Council for the postdoctoral research award during the tenure of which this work was completed.
Notice The US Environmental Protection Agency (EPA), through its Office of Research and Development (ORD), funded and performed the research described here. It has been subjected to the Agency’s peer review and has been approved as an EPA publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
47
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