Advisory Panel Jonathan W. Amy Richard A. Durst G. Phillip Hicks
INSTRUMENTATION Donald R. Johnson Charles E. Klopfenstein Marvin Margoshes
Harry L. Pardue Howard J. Sloane Ralph E. Thiers
Ionization Sources in Mass Spectrometry E. M. CHAIT, Du Pont Instruments 1500 South Shamrock Avenue, Monrovia, Calif. 91016
Traditional electron impact sources are giving way to alternative means of ionization for many fascinating special applications.
Field ionization, chemical ionization, spark
source ionization, surface ionization, and photo ionization are finding a place in mass spectrometry techniques in such diverse areas as forensics, toxicology, structure determination, polymer analysis, and plasma chromatography
rriHE MASS SPECTROMETER is an ana-
J- lytical laboratory for charged particles. Mass analyzers commonly in use (magnetic, time of flight, quadrupole, cycloidal, etc.) all derive their utility from their ability to separate ions which reflect the nature of the sample introduced into the spectrometer. The source of these ions is of primary importance and must be considered as the "heart of the mass spectrometer." Often the success of a mass spectrometric analysis depends upon the type of ion source chosen. The mode of production of ions often determines if they will indeed be representative of the sample in question. Analyzed ion signals, positive or negative ions, originating from a variety of ion sources may be interpreted by the chemist to yield such vital information as molecular structure, analysis of a gas mixture, identity of a gc effluent, trace dopants in a semiconductor, isotopic composition of a lunar rock—in fact, information on any form of matter that can be ionized in the mass spectrometer vacuum. Ion Source as a Chemical Reactor
It is attractive for the chemist to think of the mass spectrometer's ion source as a chemical reactor, for in some way a chemical change occurs in the sample when it becomes an ionized
plasma prior to injection into the mass analyzer. This analogy is easily understood in the case of the commonly used electron ionization source. Here the reagent is the stream of 70 eV electrons creating excited molecular ions of a complex molecule via a Franck-Condon process. The ensuing chemistry of unimolecular ion decompositions produces the fragment ions, rearrangement ions, and metastables we have learned to interpret to obtain molecular structure. In the chemical ionization source, a different kind of chemistry may be exploited. Ions resulting from the electron bombardment of methane at high pressures (1 torr) may be used as a reagent for reaction with a neutral sample gas in bimolecular chemistry. The products of these ion-molecule reactions provide additional information on the nature of the sample. Each ion source is a different kind of chemical reaction vessel and must be chosen as appropriate to the sample—be it a steroid or a semiconductor. Complex instrumentation technology has resulted from the development of multifarious ion sources (Figure 1), while at the same time ionization of materials for mass spectral analysis remains a challenge for future development of instrumentation. This discussion will elaborate on both the present
technology (1, 2) and future challenge of ionization sources in mass spectrometry. Requirements for Ion Sources
Monoenergetic Ion Beam. Ion sources not only have the function of producing ions but must also accelerate the ions into the mass analyzer for eventual detection. To insure maximum resolving power of the spectrometer, precautions are taken that the ions have a small spread of kinetic energies prior to acceleration. All ion sources are ion optical devices and contain lens elements which limit the energy spread of the ion beam. Ion sources should produce all masses without discrimination so that there is equal probability for both high and low masses being injected into the analyzer. Even so, some ion sources, as in the case of spark sources for analysis of solids, produce a large spread of energies in the ion beam and are usually used with double focusing mass analyzers which filter the ions according to energy prior to mass analysis so that high resolution may still be obtained. Ionization Efficiency. The ionization efficiency of a source must be high so that a large proportion of the neutral sample particles presented will become ions to be analyzed and detected
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
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Instrumentation
ION SOURCE INSTRUMENTATION
ELECTRO» IMPACT
Sample
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FIELD I0WZATION
SURFACE IONIZATION
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I Gases, liquids Ssolidsinorgame which can be made volatile
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