Measurement of Negative Ions Formed by Electron Impact. VII. Negative Ion Mass Spectra from Alkyl Cyanides

The negative ion mass spectra from alkyl (methyl, ethyl, n- and iso-propyl) cyanides with electron energies of 80 eV, 40 eV and 9.5 eV were measured by a Hitachi RMU-6D mass spectrometer with emphasis on the relative abundance of negative ions to positive ions with electron energies of 80 eV, 40 eV and 15 eV. In every sample, the electron impacts with 80 eV and 40 eV gave almost the same distribution of m⁄e for negative ion mass spectra. Then, the peak of m⁄e 26(CN ) ions was the most intense and also the relatively strong peak of 25(C2H−) ions was found through all samples. On the other hand, the formations of 38(C2N−), 39(CHCN−) and 41(CH3CN−) ions from methyl cyanide, of 50 (C3N−) ions from ethyl cyanide, n- and iso-propyl cyanides, of 40(CH2CN−) ion from n-propyl cyanide were characteristic of each sample. The ratio of the yield of negative ions to positive ions was respectively of the order of ∼104 for CH3CN+/CN− (CH3CN), C2H4+/CN− (C2H5CN), C3H5+/CN− (n-C3H7CN) and C3H6+/CN− (iso-C3H7CN) at 80 eV and 40 eV electron energies. At 9.5 eV, m⁄e 26 (CN−) ions predominated over other negative ions for all the samples and a much larger peak of 40 (CH2CN−) ions in n-C3H7CN than in iso-C3H7CN was also observed. Then, we got the values of 19∼65 for CH3CN+/CN−, C2H4+/CN−, C3H5+/CN− and C3H6+/CN− (15 eV for positive ions). This work also gave linear relationship of the yield of each negative ion to the pressure in the range used for an usual chemical analysis.