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mended by the Commission on Electroanalytical Chemistry of the International Union of Pure and Applied Chemistry for the removal of various other impurities from methanol and ethanol will be published (IO). ACKNOWLEDGMENT The authors acknowledge the help of C. W. Gardner, Jr., who did the gas chromatographic work. Registry No. Methanol, 67-56-1; ethanol, 64-17-5;2-propanol, 67-63-0. LITERATURE CITED (1) Coetzee, J. F.; Deshmukh, B. K.; Liao, C., unpublished work, Universlty of Pittsburgh, 1983. (2) Coetzee, J. F., Ed. “Recommended Methods for Purification of Solvents and Tests for Impurities”; Pergamon Press: New York, 1982. (3) Amerlcan Chemical Society ”Reagent Chemlcals”, 5th ed.; American Chemical Society: Washington, DC, 1974. (4) Coetzee, J. F.; Istone, W. K. Anal. Chem. 1980, 52, 53. (5) Coetzee, J. F.; Martin, M. W. Anal. Chem. 1980, 52, 2412.
(8) Coetzee, J. F.; Gardner, C. W., Jr., unpublished work, University of Plttsburoh. 1983. (7) Bell, R.-P.’ “The Proton in Chemistry”; Cornel1 University Press: Ithaca, NY, 1959. ( 8 ) Coetzee, J. F.; Simon, J. M.; Bertozzi, R. J . Anal. Chem. 1969, 4 1 , 788. (9) Collins, B. M.; Kitchen, D.; Rees, T. D. Chem. Ind. (London) 1972, 173. (IO) Marcus, Y. Pure Appl. Chem., in press.
J. F. Coetzee* B. K. Deshmukh Department of Chemistry University of Pittsburgh Pittsburgh, Pennsylvania 15260
RECEIVED for review July 11,1983. Accepted August 29, 1983. This work was supported by the National Science Foundation under Grant No. CHE-8106778 (to J.F.C.) and by the Government of India through a National Scholarship for Study Abroad (to B.K.D.).
Negative Ion Laser Mass Spectrometry of Aromatic Nitro Compounds and Their Use as Solid-State Chemical Ionization Reagents Sir: Laser mass spectra of organic compounds normally show even-electron quasi-molecular ions (M + H, M - H, etc.); formation of odd-electron molecular ions is not common. It has been well documented that organic compounds containing suitable electron-withdrawing groups yield odd-electron molecular anions (M-.) in conventional electron capture negative ion mass spectrometry (ECNIMS) (I). Aromatic nitro compounds are a particularly good example of negative ion formation in ECNIMS (2). As part of an ongoing study to characterize negative ions of organic compounds using laser mass spectrometry, we have studied a series of aromatic nitro compounds. The results are somewhat surprising in light of the behavior of similar compounds in ENCIMS. EXPERIMENTAL SECTION Laser mass spectra were obtained on a LAMMA-500 laser microprobe mass analyzer (Leybold-Heraeus). The output of a frequency quadrupled Q-switched Nd-YAG laser (265 nm, 15 ns pulse duration) was focused onto the sample with a 32X objective. Optimum power density to obtain the mass spectra ( 108 W/cm2) was achieved by using a set of neutral density filters. All samples of the aromatic nitro compounds, except dinitrobenzenes, were prepared by dissolving the compounds in methanol [ 1% (wt/wt) solution] and a few drops of the solution were evaporated onto a Formvar filmed copper grid. As the dinitrobenzenes (ortho, meta, and para) sublime at room temperature under the vacuum conditions used in LAMMA-500,these samples were prepared by dissolving the compound in 0.4% (wt/wt) Formvar solution (in CH2C1,) [overallsample concentration 1‘70 (wt/wt)] and evaporating a few drops on a copper grid. The samples that involved mixing 1,3,5-trinitrobenzene with aromatic hydrocarbons were prepared by dissolving both substances in toluene [ 1% solution (wt/wt) each] and a few drops of the solution were evaporated onto a Formvar filmed copper grid. The Formvar, coated on copper grids in these experiments, is sufficiently thin that it did not contribute significantly to the spectra; only very weak peaks were seen and only in the region below m / z 100. N
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ion LMS of a series of aromatic nitro compounds. The series includes 0-,m-, and p-dinitrobenzenes, 1,8-dinitro-, 1,5-dinitro-, and 2,3,5-trinitronaphthalenes, 2,3- and 3,4-dinitrotoluenes, and 1,3,5-trinitrobenzene. Common peaks in the negative ion laser mass spectra (LMS) of these compounds were (Table I) CN- (m/z 26), OCN- (m/z 42), NOz- (m/z 46), C3N- (m/z 50),and (M - NO)- (where M is molecular weight). In addition to the above fragment ions, an intense peak at (M + 15)- was observed for all of the aromatic nitro compounds except m- and p-dinitrobenzenes. Figure 1 shows the negative ion LMS of o-dinitrobenzene as an example. The peak at m / z 183 corresponds to (M + 15)- and the peak at m/z 138 corresponds to (M - NO)-. The purity of the o-dinitrobenzene used has been checked by TLC, E1 mass spectrometry, proton NMR, and HPLC. None of these techniques showed any detectable impurities (i.e., 1 X 1014s2 were employed for all of the measurements. All of the work was conducted at 95 “C. RESULTS AND DISCUSSION Data comparing the response of a zirconia sensor with that
of a GP glass electrode in the presence of 1 m sodium chloride are shown in Figure 1. They were obtained by slowly titrating an added increment of hydrochloride acid with 5 m sodium hydroxide over a period of 15 min with a motor driven syringe. The smaller response of the glass electrode undoubtedly results from the well-known “alkaline error” associated with the general purpose type of electrode. Additional comparisonsof the voltage responses of zirconia sensors and a glass electrode (this time the HA type) were made over the pH range bracketed by 0.025 m HC1 and 1.0 m NaOH. Representative data are summarized in Figure 2 in which the pH values of the solution were calculated by using a value of 12.32 for pKw at 95 “C as derived from data of Naumov et al. (4). Required activity coefficients were calculated with the extended Debye-Huckel equation log y = - A d , / ( l +
Bad0
(1)
where I is the ionic strength and where values of a = 9.0 8, for the hydrogen ion and 3.0 A for the hydroxide ion (5) were employed. Values of A = 0.59385 and B = 0.34255 were derived by interpolation from tabulations in ref 4. It is seen that even the HA type glass electrode deviates significantly from a linear response at high pH, while the response of the zirconia sensor is linear over the entire range. In the worst case at 1 m NaOH the glass electrode shows a deviation of about 0.8 pH unit from the calculated value. It was also found that in the absence of large excesses of sodium ion the correlation between the zirconia sensor and the HA glass electrode was excellent over the range of pH from 1.2 to 11.2 (0.0875 m NaOH), and no hysteresis was observed in
0003-2700/83/0355-2426$01.50/00 1963 American Chemical Society
