Anal. Chem. 1982, 54, 1245-1247 1245

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 chro- matograms were obtained for 10 ng of either methacriphos (trans-phosphorothioic acid 0,O-dimethyl 0-(2-methoxy- carbonyl-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 tem- perature 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.

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

Scientific). Shake in wrist action shaker for 3 h. Filter through Whatman 2V filter paper.

Gas Chromatographic Conditions. Instrument. Hewlett- Packard 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 (cyclohexane- dimethanol adipate) (Applied Science Laboratories) on Gas-Chrom Q 80/100 mesh (Applied Science Laboratories).

Temperatures. Column, 180 "C; injection port, 230 "C; de- tector, 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 com- parison of integrator counts obtained for the sample with those obtained from known concentrations of analytical grade standards obtained from the manufacturer.

LITERATURE CITED (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 rela- tionship 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 circum- stances 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).

EXP'ERIMENTAL SECTION Mass spectrometric studies were accomplished with either a

Nicolet prototype lFT/MS-1000 Fourier transform mass spec- trometer 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.

a few microseconds so high pressure ( N 1 torr) of reagent gas is required for chemical ionization spectra. With Fourier transform mass spectrometry (FTMS) ion lifetimes may be varied from s to >.1 s thus reducing the required reagent gas pressure to ~ ~ 1 0 ~ torr.

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

NO' + CH3CHOHCHZCHzCH3 4

CH3CHCH2CH2CH3' + HONO (2)

NO' + (C:H3)2CHCl- (CHJ2CH' + ClNO (3)

NO' + (CH3),CHC1- (C3H6)NO+ + HCl (4)

NO' + C&&3 + C&6' + NO (5)

the other fragment ions is possible. The extent of these re- actions is highly variable. Charge exchange with aromatics occurs readily whereas protonation of oxygen bases is less likely to occur.

0003-2700/82/0354-1245$01.25/0 0 1982 American Chemical Society

1246 ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982

Table I. Methyl Nitrite Chemical Ionization Product Ions for Various Bases

product ions

compound

CH ,OH C,H,OH n-C,H,OH i-C, H,OH t-C,H,OH

CH,CHO C,H,CHO n-C,H,CHO n-C,H,CHO

CH , CO CH, CH,COC, H, C, H ,COC, H (i-C,H,),CO

furan tetrahydrofuran

CH,CO,CH, CH,CO,C,H, CH,CO,CH, CH, CH,

M - M t M + 1 M l a 30

Alcohols X X X (X) x X (XI x X (XI ; Aldehydes

X X X

X

X X X X

X X X X

X X X X

X X X

other compound

[M- 21 + 30 [M- 21 t 30 [M- 21 t 30 [M- 171

cyclohep tatriene C 6 H 6

C 6 H 5 C H 3

C 6 H 5 C 2 H 5 n-C6H,C,H, i-C6H,C3H,

1,3,5-C6H3( CH3)3 p-C6H4(CH3)2

C,H,Cl C,H,F

p-FC6H,CH, C6H5CF3

p - C 6 H 4 F 2 C,H,CN C,H,CHO

C,H,OCH, C6H5N02

pyridine

n-C,H,NH, i- C, H, NH

(C,H,),"

( C 2 H 5 ) 3 N

" 3

CH,CN

(CH,),S tetrahydro-

thiophene 2-methyl-

thiophene

product ions M t M t

1 M la 30 Hydrocarbons x x X

M -

X X x x X x x X x x X x x X x x X x x X

X X X X

X x x X

X X X

X X X

X X

Amines x x x

x x x x x x x x

Nitriles X

Thio Ethers X X X X

X X

other

[M- 11 t 30

[M- 191 t 30 [M- 11 t 30

a Values in parentheses indicate that formation of the protonated molecular ion occurs only at high relative concentration of the compound.

At long trapping times the sole ion/molecule product is CH30(NO)NO+, which is envisioned as having both NO groups bonding to the methoxy oxygen via their nitrogens (5). This ion readily transfers NO+ to numerous bases (13 ,14) to form the (M + 30)' quasi-molecular ion as shown in Table I.

Several consistencies are found in Table I among the various classes of compounds. Firstly, a hydride is abstracted when a stable ion is formed, e.g., from alcohols to form protonated ketones or aldehydes, from aldehydes to form RCO+, from alkyl aromatics to form benzyl (or tropylium) derivatives, etc.

Secondly, when the ionization potential of the molecule is less than or approximates the ionization potential of NO, then charge exchange to form the molecular ion occurs. The charge exchange reaction with aromatics has been studied in some detail by Ausloos and Lias (11, 12).

Thirdly, the M + 1 ion may be formed for most oxygen bases and amines. However the protonated oxygen bases will react away as shown (14)

to be a loosely bound complex between NO+ and aromatic and oxygen bases (4 ) and propene (6).

Many of the product ions observed for the oxygen bases have also been observed by Hunt and Ryan (7) using nitric oxide as the CI reagent gas in a conventional mass spectrom- eter. Using nitric oxide is not feasible under low-pressure bimolecular conditions because an M + 30 ion will not gen- erally form (13, 14 ) .

Methyl nitrite, however, enables formation of quasi-mo- lecular (M + 30) ions at long trapping times as well as other ions indicative of the molecular species (low ionization po- tential, abstractable hydride, or high proton affinity). Thus methyl nitrite is a viable alternate bimolecular CI reagent gas which may also be used to check a molecular weight obtained by using methane (and similar) reagent gases. I t may be used in a GC/FTMS (15-17) system where a reagent gas pressure of torr would permit a total CI spectrum within 1 s.

LITERATURE CITED

The protonated amines do not react by this mechanism and so are present even a t long trapping times.

Fourthly, the M + 30 ion is produced in every case, except with triethylamine, and often is the sole ion at long times. The failure of triethylamine to form an M + 30 ion is speculated to be due to a steric effet but there is no evidence to support or refute this conjecture. The M + 30 ions has been found

Munson, B. Anal. Chem. 1977, 49, 772 A-778 A. Hunt, Donald F.; Sethl, Satinder K. "ACS Symposium Series"; Amerl- can Chemical Society: Washirgton, DC, 1978; Vol. 70, pp 150-178. Hartung, W. H.; Crossley, F. Organic Syntheses", Collect. Vol. 11; Wlley: New York 1943; pp 363-364. Stenhagen, Elnar, Abrahamsson, Sixteh McLafferty, Fred W. "Registry of Mass Spectral Data"; Wlley: New York, 1974. Farld, R.; McMahon, T. B. Int. J . Mass Spectrom. Ion Phys. 1978, 27, 163-183. Williamson, Ashley D.; Beauchamp, J. L. J . Am. Chem. Soc. 1975, 97,5714-57ia.

(7) Hunt. Donald F.: Rvan. James F. J. Chem. Soc.. Chem Commun. . . 1972, 620-621.' .

(8) Hunt, Donald F.; Harvey, T. Michael Anal. Chem. 1975, 47 1965-1 969.

(9) Hunt Donald F.; Harvey T. Michael Anal. Chem. 1975, 4 7 , 2 136-2 141.

(10) Hunt, Donald F. Adv. Mass Spectrom. 1974, 6 , 517-522. (11) Ausloos, P.; Lias, S. G. Adv. Mass Spectrom. 1978, 7A, 321-325. (12) Lias, Sharon G. Chem. Phys. Lett. 1978, 54, 147-153. (13) Reents, W. D., Jr.; Freiser, B. S. J. Am. Chem. Soc. 1980, 102,

271-276. (14) Reents, W. D., Jr.; Freiser, B. S. J. Am. Chem. Soc. W81, 103,

2791-2797. (15) Ledford, Edward B., Jr.; White, Robert L.; Ghaderi, Sahba; Wilkins,

Charles L.; Groiss, Michael L. "Proceedings of the 28th Annual Confer-

ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982 * 1247

ence on Mass Spectrometry and Allied Topics", New York, 1980; pp

(16) White, Robert L.; Ledford, E. B., Jr.; Wilkins, C. L.; Gross M. L. Proceedings of the 29th Annual Conference on Mass Spectrometry

and Allied Topics", Minneapolis, MN, 1981; pp 5-6. (17) Ghaderi, Sahba; Kulkarni, P. S.; Ledford, Edward B., Jr.; Wilkins,

Charles L.; Gross, Michael L. Anal. Chem. 1981, 53, 428-437.

662-663.

RECENED for review December 31,1981. Accepted March 15, 1982. R.C.B., R.B.C., and B.S.F. wish to thank the National Science Foundation for the funds to purchase the FTMS instrument.

CORRECTIONS

Laser-Induced Thermal Diffraction for Calorimetric Absorption Measurements

M. J. Pelletier, H. R. Thorsheim, and J. M. Harris (Anal. Chem. 1982, 54, 239-242).

There is an unfortunate omission in eq 8. A superscript 2 should be placed after the brackets so that the equation reads

l2 G.908P(dn /dT) X,2a

32a2krX, sin2 (0/2) sin 8

Determination of Dissolved Organic Carbon in Water

Ronald A. van Steenderen and Jiunn-Shyh Lin (Anal. Chem. 1981,53, 2157-2158).

Please change the numbering in Figures 1 and 3 to match the captions and text. In Figures 1 and 3 (on the graphs themselves) number (2) becomes (3) and number (3) becomes (4).