Mass Spectrometry - American Chemical

Downloaded by UNIV OF MARYLAND BALTIMORE COUNTY on February 3, 2015 | http://pubs.acs.org Publication Date: February 22, 1990 | doi: 10.1021/bk-1990-0420.ch004

Chapter 4

Use of Solvent Adduct Ions To Confirm Structure of Selected Herbicides with Thermospray Liquid Chromatography/Mass Spectrometry Damià Barceló Environmental Chemistry Department, CID-CSIC, c/Jordi Girona, 18 08034 Barcelona, Spain

Conventional positive-ion and negative-ion modes (PI and NI ,respectively), the use of ammonium acetate and ammonium formate and the addition of 2% chloroacetonitrile in the liquid chromatographic eluent using filament-on thermospray LC-MS have been applied for the determination of selected herbicides. By using acetonitrile-water and 0.05 M ammonium acetate mixtures, the chlorotriazine herbicides showed [M + (CH CN)H] and +

3

-

[M - H] ions as base peaks in PI and NI modes, respectively. When ammonium formate and ammonium acetate were added to methanol-water eluent, the phenylurea herbicides exhibited in PI mode a relevant peak between 50-70% relative intensity corresponding to [M + (CH OH)NH ]

+

3

4

and [M + (CHCOOH)NH]+, respectively, whereas the base peak was [M + NH ] in both cases. The formation of [M + HCOOHNH ] was incidently observed in PI when ammonium formate was used. With this ionizing additive and NI the chlorinated phenoxyacid herbicides exhibited [M + HCOO] as base peak, thus allowing a complementary structural information to [M + CH COO] , a typical ion obtained when ammonium acetate was added as ionizing additive. The addition of 2% chloroacetonitrile to acetonitrile-water eluent showed in NI mode a [M + Cl] ion as base peak instead of [M + CHCOO] or [M + HCCO] ions in combination 3

4

+

4

+

4

-

-

3

-

-

-

3

-

with other main ions which were assigned to [M - H] and [M + H] . Applications are reported for the determination of phenylurea and chlorinated phenoxyacid herbicides in spiked water samples by using PI and NI modes, respectively. -

0097-6156/90A)420-0048$06.00A) © 1990 American Chemical Society In Liquid Chromatography/Mass Spectrometry; Brown, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


4. BARCELO

Solvent Adduct Ions To Confirm Structure ofSelected Herbicides 49

The on-line combination of liquid chromâtography-mass spectrometry (LC-MS) plays an important role in environmental organic analysis and compared with gas chromâtogrpahy-mass spectrometry (GC-MS) offers major advantages for analyzing polar pesticides and herbicides such as organophosphorus pesticides (1-5), chlorinated and phenylurea herbicides ( 6 - 8 ) , carbamates (9) and pyrethroid pesticides (10). An excellent book (11) and review articles (1,12) have been recently published reporting LC-MS applications in environmental pesticide analysis. Among the different LC-MS approaches, the thermospray (TSP) interfacing system is probably the most widely used and typically involves the use of reversed-phase columns and volatile buffers with or without a filament. In the work described here the u t i l i t y of solvent adduct ions in TSP LC-MS which consist in the use of novel additives in the chromatographic eluent, such as ammonium formate or chloroacetonitrile, w i l l be demonstrated for confirmation of structure of a variety of herbicides including triazines, phenylurea and chlorinated phenoxyacids. Complementary adduct ion information to the conventional TSP LC-MS mode of operation w i l l be obtained. Because TSP LC-MS involves mainly a chemical ionization process where the vaporized eluent acts as chemical ionization gas, i t w i l l be of interest to compare the different adduct ions obtained here with those using other interfacing systems such as direct liquid introduction (DLI) (13-18).

EXPERIMENTAL Chemicals HPLC-grade water (Farmitalia Carlo Erba, Milano, Italy), methanol (Scharlau, Barcelona, Spain) and acetonitrile (Romil, Shepshed, Leics, UK) were passed through a 0.45 μα f i l t e r (Scharlau .Barcelona, Spain) before use. Analytical-reagent grade ammonium acetate was obtained from Panreac, (Barcelona, Spain), ammonium formate from Farmitalia Carlo Erba, (Milano, Italy), atrazine, diuron, monuron, 2,4,-D, 2,4,5-T, and silvex from Polyscience (Niles, II, USA), lmuron from Riedel-de-Haen (Seelze-Hannover, FRG) and desethylatrazine and desisopropylatrazine from Dr. Su.I. Ehrenstorfer (Augsburg, FRG) Sample pyepayaUon For extraction of phenylurea and chlorinated phenoxy acid herbicides in water samples, sample pre-treatment was performed by the procedures described in ref. 19 and 20, respectively. After both sample pre-treatments were finished, methanol was added to yield a final solution volume of 0.5 mL and injected into the LC-MS system.

In Liquid Chromatography/Mass Spectrometry; Brown, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

50

LIQUID CHROMATOGRAPHY/MASS


Chromai ogrfrptup

SPECTROMETRY

oondiupng

Eluent delivery was provided by two model 510 high-pressure pumps coupled with automated gradient controller model 680 (Waters Chromatography Division, Millipore, MA, USA) and a model 7125 infection valve with a 20-μΙ loop from Rheodyne (Cotati, CA, USA). Stainless-steel columns (30 χ 0.40 cm I.D.) packed from Tracer Analltica (Barcelona, Spam) with 10 μχη particle diameter Spherisorb ODS-2 (Merck, Darmstadt, FRG) were used. Four different LC mobile phase compositions were tested: acetonitrile-water (50:50) + 0.05 M ammonium acetate, acetonitrile-water-chloroacetonitrile (49:49.2) + 0.05 M ammonium acetate, methanol.water (50:50) + 0.1M ammonium acetate and methanol : water (50:50) + 0.1M ammonium formate at flow rates between 1-1.2 ml/min. Mass spectrometric analysis A Hewlett-Packard (Palo Alto, CA, USA) Model 5988A TSP LC-MS quadrupole mass spectrometer and a Hewlett-Packard Model 59970C instrument for data acquisition and processing were employed. The TSP temperatures were: stem: 100 C , tip: 178 C, vapour: 194 C and ion source 296 C with the filament on. In a l l the experiments the filament-on mode (ionization by an electron beam emitted from a heated filament) was used. In this mode of operation conventional positive and negative chemical ionization can be carried out by using the vaporised mobile phase as the chemical ionization reagent gas (4). a

fi

fi

ft

RESULTS ΑΙΦ DISCUSSION

Conventional PI and NI TSP LC-MS Atrazine and two major degradation products, desethylatrazme and desisopropylatrazine have been analysed by using acetonitrile-water (50:50) and 0.05 M ammonium acetate. As regards to the chloroatrazines, i t should be commented that in PI mode TSP LC-MS a common feature in the adduct ion formation has been observed, with the formation of [M + H • O^CN]* as base peak with the exception of desisopropylatrazine which exhibits [M + CH CN] as base peak. This fact can be attributed to i t s lower number of substituents and consequently lower proton affinity in comparison with the other two triazines. In contrast, i t has been reported that when filament-off or thermospray ionization is employed [M + H] is the base peak for different chloroatrazines (21,6) similarly as when DLI LC-MS was used (13). Such a difference in the relative abundance of the different adducts in the mass spectrum between filament-on and filament-off has been previously observed for other groups of pesticides (22,23). For chlorotriazines a [M + 6 0 ] i o n was the base peak using an eluent of methanol-water and ammonium +

3

+

+



4. BARCELO


acetate (7/8). Differences in the base peak between acetonitrile-water and methanol-water mixtures can be attributed to the proton affinity values of acetic acid and acetonitrile, 797 and 798 KJ/mol, respectively, which are much higher than of methanol with a value of 773 KJ/mol (24). Table I l i s t s the different ions obtained for atrazine by using three LC solvents in TSP LC-MS thus illustrating the dependence on adduct ion formation with the solvent used. In the Fig. 1, the PI mode TSP mass spectra of the three chloroatrazines studied are shown. When NI mode TSP LC-MS was used for the characterization of the chloroatrazines [M - H]~ was the base peak, similarly as when methanol-water mixtures in TSP for cyanazine (8) and when acetonitrile-water mixtures in DLI for atrazine (13) were employed. Sensitivities in this mode of operation were about 1 order of magnitude lower than in PI mode, in contrast to previous results with greater differences between PI and NI modes (25). Other ionizing additives in PI TSP LC-MS One limitation in TSP LC-MS is the need for a volatile buffer in the eluent in order to provide a soft ionization process. The most common volatile buffer used is ammonium acetate although ammonium formate has been incidently used (7,21). In this regard, a comparison between the use of both ionizing additves has been published elsewhere for the determination of a variety of herbicides(7). For phenylurea herbicides, methanol was preferred over acetonitrile as LC eluent owing to a gain (11) in sensitivity for such compounds containg aryl halide rings The mass spectra of phenylurea herbicides monuron, diuron and linuron usually exhibit [M + NH ] as the base peak in the PI mode when ammonium formate or ammonium acetate are used as ionizing additives (7, 8,11), although the [M + H] ion was the base peak for several phenylurea herbicides when filament-off TSP ionization was employed (6). The formation of [M +H] with relative intensity values close to 20% with both ionizing additives indicates that such compounds have proton affinities below that of ammonia (858 kJ/mol) (24). When other LC-MS systems not containing ionizing additive in the eluent have been employed, such as DLI with a micro-LC (14), s p l i t ( 16) or moving belt (27), [M + H] was always the base peak. In the case of using ammonium formate, the formation of adduct ions with the methanol such as [M + (CH3OH) NH ]+ and [M • (CH OH) NH ] is obtained (7). The formation of adducts with the ionizing additive, such as [M + CH COOH] and [M + (CH3COOH) NH ] when ammonium +

4

+

+

+

+

4

3

+

2

4

+

3

4

+

acetate is used (22) or [M + (HCOOH)NH ] 4

in the case of ammonium +

formate has been also observed. The dimers [2M + H ] and [2M + NH ] are obtained with both ionizing additives.As an example, the different adduct ions obtained for diuron in several eluents are shown in Table I. The sensitivity was always much better in PI mode than in the NI mode of operation (about 30% lower). In contrast, other authors have found greater differences in sensitivities between both ionization modes because filament-off ionization was used (26). +

4


52

LIQUID

CHROMATOGRAPHY/MASS SPECTROMETRY

TABU) I MAIN IONS AND RELATIVE ABUNDANCES OF THREE HERBICIDES IN THERMOSPRAY LIQUID CHROMATOGRAPHY-MASS SPECTROMETRY USING DIFFERENT ELUENT MIXTURES AND FILAMENT ON PI, and NI, positive and negative ion modes, respectively MeCN-H 0: acetonitrile-water ; MeOH-H 0: methanol-water NH^AcO : ammonium acetate, NH4F0: ammonium formate NI-C1: 2% chloroacetonitrile


2

Sol wt.

2

Pesticide

2

2

4

m/z 215

MeCN-H 0 (50:50) MeOH-H 0 (50:50] + 0.05M ΝΗ ΑσΟ +0.1M ΝΗ ΑσΟ +0.1M NH Fo PI HI NI-C1 E l HI El HI

ion

Atrazine 214 [M - H]" 215 [Μ] · 216 [M + H] 248 [M + MeOH + H ] 257 [M • MeCN + H] 275 [M 4 60]

100 n . i .

+

+

232

n.d. 30 2

10 100

n.d.

30

n.d.

2 100

100 10

100 70

10 10

7

10

n.i. n.i. n.i.

+

10 40 100

+

4

+ CI]" + Fo]" • MeOH NH ] + AcO]" + AcOH] + AcONH ] 314 [M • (MeOH) NH ]

10 20 100 100 80

+

4

100

+

45 70

+

4

2

323 351 465 482

100 50

100

[2M - H]" - ΗΓ «· H] + NH ]

100

100

2.4-P 219 [ Μ - Η Γ 255 [M + CI]" 265 [M + Fo] 279 [M + AcO]" 311 [M + FoHFo]" 315 [M • AcOHCl]"

Diuron 231 [M 233 [M 250 [M 267 [M 277 [M 282 [M 291 [M 292 [M 309 [M

100 50 10

4

+

439

4

60

+

220

4

+

20

4

[M 4 HF0F0]" [M + AcOHAcO]" [2M • H]* [2M • NH ]

70 80 50

+

4

n.i.= not investigated; n.d.= not detected.


4. BARCELO


214[M+CH CN]+

(PI)

3

100 Downloaded by UNIV OF MARYLAND BALTIMORE COUNTY on February 3, 2015 | http://pubs.acs.org Publication Date: February 22, 1990 | doi: 10.1021/bk-1990-0420.ch004

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Mass Spectrometry - American Chemical

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