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Anal. Chem. 1982, 5 4 , 1244-1245
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Flgure 1. Typical chromatogram of chlorosuifinates containing thionyl chloride and hydrogen chloride.
detection. Thermal conductivity detectors were rapidly corroded. Simple, quantitative detection was achieved by bubbling the gas chromatographic effluents through a stirred volume of distilled water monitored with a chloride-sensitiveelectrode. The samples were separated on a 1-m glass column containing 10% SP-2100on 100/120 Supelcoport. The injection port temperature was 75 "C and the column was kept at 50 "C for 5 min and then programmed upward at 16 OC/min to elute the chlorosulfinate components. A helium flow rate of 20 mL/min was used. Sample volumes of 10 pL were injected. A short length of in. diameter Teflon tubing connected to the end of the column allowed column effluent to bubble through 10 mL of stirred water, into which chloride-sensitive and reference electrodes were dipped. These were previously calibrated with aqueous sodium chloride standards in the 0-500 ppm chloride range. Hydrogen chloride eluted sharply before 1 min, followed by thionyl chloride. Thionyl chloride hydrolyzed quickly so that its theoretical chloride content was obtained. For best accuracy the procedure was calibrated by injection of known quantities of standards. A typical chromatogram is shown, in Figure 1.
RECEIVED for review February 1,1982. Accepted March 18, 1982.
Elimination of False Integration Counts in Automated Pesticide Residue Analysis Richard A. Slmonaitis" and James M. Zehner Stored-Product Insects Research and Development Laboratory, Agricultural Research Service, USDA, Savannah, Georgia 3 1403
Determination of organophosphate pesticide residues by gas chromatography with a Tracor flame photometric detector (1)is well documented. Such analyses are done routinely by measuring peak heights. The number of samples analyzed per day could be greatly increased by the use of automatic sample injection and integration with an integrator such as the Hewlett-Packard Model 3371B. However, automatic sampling is not usually attempted because the detector response passes through a minimum after the solvent elutes and then the detector response gradually rises to the base line. This phenomenon occurs because in the hydrogen-rich cool flame, the detector temperature increases as a flammable solvent such as acetone, hexane, benzene, or ethyl acetate burns. Then the detector response gradually returns to the base line as the detector temperature returns to equilibrium. Because the detector response is rising continuously after the solvent has eluted, the integrator will continue to integrate when the organophosphate peak elutes (Figure 1,right). The integrator, therefore, records an erroneous area because the entire signal from the time of the minimum to the time after the pesticide peak has eluted is included in the printout. Nonflammable solvents such as chloroform or methylene chloride cause the detector response to rise when the solvent elutes and then gradually return to the original base line after about 1 h. Thus the reponse of the detector to the eluting organophosphate pesticide cannot be detected in the much _ . . larger solvent peak. We took advantage of this phenomenon. By addition of a small amount of nonflammable solvent to each sample, the negative detector response caused by flammable solvent is
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Flgure 1. Typical chromatogram of 10 ng of organophosphate pestlcide residue analysis in a flammable solvent observed with a Tracor flame photometric detector (right) and with the addition of 2% methylene chloride (left).
This article not subject to U.S. Copyright. Published 1982 by the American Chemical Society
Anal. Chem. 1982, 5 4 , 1245-1247
eliminated and does not cause a great increase in the tailing of the solvent peak (Figure 1, left). The chromatogram is 10 ng of dichlorvos (2,2-dichlorovinyl dimethyl phosphate) in acetone containing 2% methylene chloride. Similar chromatograms were obtained for 10 ng of either methacriphos (trans-phosphorothioic acid 0,O-dimethyl 0-(2-methoxycarbonyl-2-methylvinyl) ester) or M9580 (0-methyl 0-ethyl 0-(l-methyl-3-chloro-5-pyrazolyl) thiophosphate) in acetone containing 2% methylene chloride. With the presence of 2% methylene chloridie in acetone, the detector response decreases continuously after the combined solvents elute as the temperature of the detector returns to equilibrium. Thus, when the organophosphlate pesticide begins to elute, the integrator will printout the ciolvents' integration count and initiate the integration of the pesticide peak at the correct time. This is a simple solution to the problem of automating the analysis of organophosphate pesticides.
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Scientific). Shake in wrist action shaker for 3 h. Filter through Whatman 2V filter paper. Gas Chromatographic Conditions. Instrument. HewlettPackard 5750B equipped with a flame photometric detector (FPD 100 AT, Melpar, Inc.), Hewlett-Packard 3371B integrator, and Hewlett-Packard 7670A automatic sampler. Column. 122 cm glass, 6 mm o.d., 4 mm id., packed with 8% OV-101 (methyl silicone) and 2% HI-EFF-BAP (cyclohexanedimethanol adipate) (Applied Science Laboratories) on Gas-Chrom Q 80/100 mesh (Applied Science Laboratories). Temperatures. Column, 180 "C; injection port, 230 "C; detector, 220 "C. Gas Flows. Nitrogen carrier, 75 mL/min; hydrogen, 150 mL/min; oxygen, 30 mL/min. Sample injection volume, 10 pL. Calculation. Calculate the pesticide concentration by comparison of integrator counts obtained for the sample with those obtained from known concentrations of analytical grade standards obtained from the manufacturer.
LITERATURE CITED EXPERIMENTAL SECTION The procedure used is as follows: Extraction. Weigh 40 g of representative samples and add 120 m& of acetone containing 2% methylene chloride (v/v) (Fisher
(1) Brady, S. S.; Chaney, J. E. J . Gas Chromatogr. IB66, 4 , 42.
RECEIVED for review ?January 13, 1982. Accepted March 2, 1982.
Methyl Nitrite as a Low Pressure Chemical Ionization Reagent W. D. Reents, Jr.,"' R. C. Burnler, Robert B. Cody, and Ben S. Freiser Department of ChernMy, Purdue University, West La fayette, Indiana 47907
Chemical ionization (CI) mass spectrometry ( I , 2) has provided a means for obtaining the molecular weight of an unknown compound. This is especially valuable when the molecular ion is absent from the normal electron impact mass spectrum. However, obtaining the molecular weight may be difficult if only a single mass ion is formed since its relationship to the molecular ion, e.g., one mass unit greater, is unknown. Chemical ionization with methane often produces the protonated molecular ion (M l),but in some circumstances another matss ion, e.g., the M - 17 ion from dehydration of an alcohol, will result. We wish to report on the use of methyl nitrite as a positive CI reagent gas. In addition to producing quasi-molecular ions (M + 30, M + 1, IM,M - 1) which may easily be related to the molecular ion, it is also usable as a CI reagent gas under bimolecular conditions Ruth as are present in a Fourier transform mass spectrometer (FTMS).
+
a few microseconds so high pressure ( 1torr) of reagent gas is required for chemical ionization spectra. With Fourier transform mass spectrometry (FTMS) ion lifetimes may be varied from s to >.1s thus reducing the required reagent gas pressure to ~ ~ torr.1 0 ~ Since we may vary the ion trapping time a variation of product ions with time may be observed. This capability may be exploited if there is a reagent gas whose chemical ionization properties differ as a function of time. Methyl nitrite exhibits this property in the positive ion mode. Its mass spectrum prior to ion/molecule reactions is dominated by the fragment ion NO+ (4). (CH20+has been shown to be of minor importance by deuterium labeling (5).) The ion/molecule reactions of NO+ have been studied by several groups (6-12). The major reactions observed have been hydride abstraction (reaction l), hydroxide abstraction (reaction 2), halide abstraction (reaction 3), dehydrohalogenation (reaction 4), and charge transfer (reaction 5). In addition, protonation by HCO+ and N
EXP'ERIMENTAL SECTION Mass spectrometric studies were accomplished with either a Nicolet prototype lFT/MS-1000 Fourier transform mass spectrometer or a Varian V-5900 ion cyclotron resonance mass spectrometer. Typical sample pressures were as follows: methyl nitrate reagent gas, 10-6-10-6 torr; organic base, 10-8-10-7 torr. Trapping times of 1-5 s yielded (M + NO)+ as the base peak in most instances. All chemicals were commercially available except methyl nitrite which was prepared by the literature method (3).
RESIJLTS AND DISCUSSION In conventional mass spectrometry, ion lifetimes are only
Permanent address: Bell Laboratories, Murray Hill, NJ 07974.
NO'
+ CH3CHOHCHZCHzCH3
4
CH3CHCH2CH2CH3'+ HONO (2)
+ (C:H3)2CHClNO' + (CH3),CHC1NO' + C&&3
NO'
+
(CHJ2CH'
+ ClNO
+
(3)
(C3H6)NO+ HCl
(4)
+ NO
(5)
C&6'
the other fragment ions is possible. The extent of these reactions is highly variable. Charge exchange with aromatics occurs readily whereas protonation of oxygen bases is less likely to occur.
0003-2700/82/0354-1245$01.25/00 1982 American Chemical Society
