Publication costs assisted by Extranuclear Laboratories Incorporated The realization that terminal ion spectra in a weak plasma are often dominated by ions characteristic of minute trace impurities has led to the development of atmospheric pressure weak plasma ion sources for mass spectrometry (API-MS). In addition to their application in analytical tasks, these ion sources may prove useful in the study of ion-molecule reactions at high pressure. The quiescent ion spectrum in an API-MS employing a 63Ni /3-ray primary ionization source interfaced to a positive/negative ion mass spectrometer system is discussed for the cases of N2, Ar, and air as carrier gases. The effects of carrier gas impurities, especially H2O, are discussed and demonstrated experimentally. The API source is modeled in terms of engineering parameters and the rate constants for charge transferring processes, electron attachment, and recombination. The model is shown to predict quiescent spectra in agreement with experimental results. Response as an analytical detector for trace sample species is modeled, and the model found to agree with experimental results with respect to sensitivity and dynamic range. Illustrative data are presented and discussed for response to SFe, Freons 11,12,13, 21, 22, and 23, vinyl chloride (HC=CCI), NO, and SO2. By the exponential dilution technique calibrated sensitivities to the 10 ppt level are demonstrated for appropriately responding species. This limit is shown to be set by wall retention and desorption in the dilution vessel, and sub partper-trillion sensitivity may be extrapolated in the absence of these effects. Preliminary data obtained with a gas chromatograph-atmospheric pressure ionizer-mass spectrometer system are presented and discussed.
The cross section for ionization of the hydrogen atom on electron impact has been measured as a function of electron energy. A modulated beam of atoms and molecules in varying proportions, taken from a furnace, is crossed by a dc electron beam, and the positive ions formed are taken into a mass spectrometer. By using such modulation techniques, the ions formed by ionization of the beam are distinguished from the much larger number of ions formed by collisions of electrons with the residual gas in the vacuum chamber. From the study of the mass-spectrometer peak strengths as a function of temperature with constant gas flow, the ratio of the cross sections for ionization of the atom and the molecule is directly determined. The absolute atomic cross sections, determined by multiplying this ratio by the known molecular cross sections, are cross-checked by taking relative cross sections for the atom. Complete agreement with the first Born approximation is found only above about 250 ev. Deviations of experiment from theory at lower energies are as predicted qualitatively by theory.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.