Methods of Ions Generation in Mass Spectrometry

Almost all compounds that we want to identify using mass spectrometric techniques are not charged. Moreover, analytes are usually present in liquid, solid, or gas phases at atmospheric pressure while mass spectrometers operate under high vacuum. Thus, we have to choose such sample introduction and ionization technique which preserves an analyte from decomposition during its transfer from a common environment (condensed or gas phase) to an ion source. The sample, which is initially at atmospheric pressure (~760 Torr) must be introduced into the instrument in such a way that the vacuum inside the instrument remains relatively unchanged (from ~10-5 up to ~10-11 torr in ICR and Orbitrap mass analyzers). The most common methods of sample introduction are direct infusion or LC injection into an atmospheric pressure ionization ion source such as ESI or APCI, sample introduction via capillary GC, and direct insertion with a probe or plate (the last is used with MALDI MS).

• Electrospray Ionization

• Electron Ionization

• Chemical Ionization

Recommended Literature

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J.B. Fenn, J. Rosell, Ch. Kai Meng, In Electrospray Ionization, How Much Pull Does an Ion Need to Escape Its Droplet Prison?, J. Am. Soc. Mass Spectrom., 1997, 8, 1147.

J.B. Fenn, Ion Formation from Charged Droplets: Roles of Geometry, Energy, and Time, J. Am. Soc. Mass Spectrom., 1993, 4, 524.

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M. Karas, M. Gluckmann, J. Schafer, Ionization in matrix-assisted laser desorption/ionization: singly charged molecular ions are the lucky survivors, J. Mass Spectrom., 2000, 35, 1.

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R. Zenobi, R. Knochenmuss, Ion Formation in MALDI Mass Spectrometry, Mass Spectr. Rev., 1998, 17, 337.

D.F. Hunt, G.C. Stafford, Jr.F.W. Crow, J.W. Russell, Pulsed Positive Negative Ion Chemical Ionization Mass Spectrometry, Anal. Chem., 1976, 48, 14, 2098.

P. Kebarle, U.H. Verkerk, Electrospray: From Ions in Solution to Ions in the Gas Phase, What We Know Now, Mass Spectr. Rev., 2009, 6, 898.T.R. Covey, B.A. Thomson, B.B Schneider, Atmospheric Pressure Ion Sources, Mass Spectr. Rev., 2009, 6, 870.