Journal of the Mass Spectrometry Society of Japan

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Because noble gases are chemically inert, scarce in the earth and meteorite (except ⁴⁰Ar in the earth’s atmosphere), and highly volatile, their isotope ratios have provided important insights in research fields of earth, planetary, and environmental sciences. The progress of noble gas geochemistry and cosmochemistry has been paced by the rate of developments in mass spectrometry. In this contribution, the history and basic principles of noble gas mass spectrometry and general techniques used in most modern laboratories are overviewed. The noble gas mass spectrometry has developed with inventions of various techniques—Nier type electron ionization (EI) source, static mode operation under ultrahigh-vacuum, adequately high resolution to distinguish very minor noble gas isotopes from interferences, and numerous small tips accumulated in laboratories—to attain increasingly greater precision to distinguish the often subtle variations in isotopic compositions, higher sensitivity to measure the low abundances found in many materials, and lower blanks to remove interference from atmospheric gases. New technologies recently exploited, such as simultaneous detection of noble gas isotopes with multicollector detection system, high-transmission EI source, resonance ionization, compression EI source, post-ionization of sputtered noble gases by focused ion beam, which enable us to detect quite small amount of noble gases down to several thousands of atoms, are opening new era of noble gas mass spectrometry. The highly-sensitive noble gas mass spectrometry can be applied to continuous monitoring of activity of volcanoes or active faults and to detect trace amount of noble-gas producing elements upon neutron irradiation, such as halogens, potassium, calcium, uranium, etc.

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Typical modes of bond cleavages of organic compounds in mass spectrometry are briefly summarized. Although these fragmentation rules can be quite useful for interpreting mass spectra of simple compounds, application to structurally complex molecules that contain multiple hetero atoms such as nitrogen or oxygen becomes increasingly difficult, because the exact location of an unpaired electron or positive or negative charges becomes obscure in precursor ions. About a decade ago, we proposed “a rule of mass shift,” which correctly predicts the m/z for observed peaks corresponding to singly charged even-electron fragment ions. The basis of the rule postulates that ions observed as peaks in an ordinary mass spectrum should be sufficiently stable to survive during the flight path in a mass spectrometer. The important recognition is that each atom in a stable ion should be in an ordinary valence state, and no free valence should be allowed. Therefore, if the cleavage of a bond leads to an ion with an unstable structure, some structural changes must take place in order for the ion to be observed in the mass spectrum. Such structural changes can be the addition of hydrogen atom(s) and/or a proton for positive ions, and the addition of a hydrogen atom and/or the elimination of two hydrogen atoms in the case of negative ions. These required structural changes in each case are schematically depicted and discussed in detail. Two typical examples are shown, in which m/z’s of the observed peaks are correctly predicted. The scope and limitations, as well as the significance of the rule for analyzing fragmentations in organic mass spectrometry are also discussed.

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