The ionization potentials of methane, and of the four deuterated methanes, have been determined by electron impact using two 90° mass spectrometers with different types of sources. In addition, the ionization potentials of acetylene, deuterated acetylene, ethylene, and deuterated ethylene have been measured. A simple empirical method is used to obtain values which are reproducible to ±0.02 volt. For the methanes the ionization potential is found to increase with the number of deuterium atoms in the molecule, with a difference of 0.18 volt between the value for CH4 and that for CD4. Although this difference is found to be the same on each of the mass spectrometers, there is a difference of 0.11 volt between the absolute values of the ionization potential as determined for CH4 on the two instruments. In order to explain the difference between the ionization potentials of CH4 and CD4 on the basis of zero point energy differences, it is estimated that the force constants in the ionic state would need to have only about ¼ of their value in the ground state. Since this seems unreasonably low, it is suggested that the difference between the ionization potentials of the methanes, as measured by electron impact, may be caused, in part, by excess energy required for a ``vertical'' transition.

The Ionization Potentials of the Deuterated Methanes

A new approach to mass spectrometric measurement and analysis has been used to study the electron impact ionization of the rare gases in the low electron energy regime (<180 eV). Critical considerations are given to the problems of ion extraction from the ion source (Nier‐type) and the transmission of the extracted ions through the mass spectrometer system (double focusing sector field Varian MAT CH5). Accurate relative partial cross section functions were obtained for production of He+ and He2+ from He; Ne+, Ne2+, and Ne3+ from Ne; Ar+, Ar2+, Ar3+ from Ar, and Kr+, Kr2+, Kr3+ and Kr4+ from Kr. In addition, the cross section ratios for the partial ionization cross sections of each individual gas were determined. These ratios were used to sum up the relative partial cross section functions for each gas, hence obtaining relative total ionization cross section functions by weighting each partial cross section function with the respective charge number. The shape of the relative total ionization cross section...

Mass spectrometric determination of partial electron impact ionization cross sections of He, Ne, Ar and Kr from threshold up to 180 eV

Qualitative Analysis of Organic Compounds by Mass Spectrometry

A method has been developed for measuring the partial pressure of methyl radicals in thermally decomposing gases, using a modified mass spectrometer. The sensitivity of the instrument to methyl radicals has been determined by decomposing known quantities of mercury dimethyl and di‐t‐butyl peroxide in a stream of carrier gas and measuring the amounts of the stable products and the net height of the mass 15 peak after correction for contributions from these stable products. Assuming a 100 percent carbon balance, the sensitivity of the instrument to methyl radicals was found to be 0.47±0.07 of the sensitivity to methane under the conditions used. With the present instrument this means that an ion current of 3×10−14 ampere at mass 15 (a peak 1 cm in height) is given by a partial pressure of methyl of 0.02 micron Hg above the leak in the reactor. The rate constants for the decomposition of di‐t‐butyl peroxide have been measured with this instrument. Combining the values obtained with values from the literature...

Free Radicals by Mass Spectrometry. I. The Measurement of Methyl Radical Concentrations

Sulphur isotope fractionation in nature and geological and biological time scales

A mass spectrometer of the Nier type has been designed and constructed for the identification of isotopes and measurement of their relative abundances. This instrument employs a universal gas-type ion source which may readily be replaced with a furnace or filament-type ion source for use with solids. The instrument is compact and portable with the exception of service connexions and a galvanometer shelf. The electronic units designed for this instrument are described in detail. These units operate satisfactorily from unstabilized a.c. and do not require batteries. The mass spectrometer is capable of detecting isotopic components present to the extent of 1 part in 100,000 of a sample. With the present slit dimensions, the instrument was found to have a resolving power better than 1 in 130, which agrees with that calculated from the slit dimensions and the radius of curvature.

https://iopscience.iop.org/article/10.1088/0950-7671/24/5/302/pdf

An a.c. Operated Mass Spectrometer for Isotope Abundance Measurements

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An a.c. Operated Mass Spectrometer for Isotope Abundance Measurements