METHODS OF IONIZATION

This chapter discusses various methods of ionizing molecules in the gas phase and directly from the liquid or solid state. Historically, in the development and application of mass spectrometry to the analysis of organic molecules, during the 1950s and early 1960s, electron ionization (EI) was the only practical ionization method available. The development of chemical ionization (CI) in the late 1960s provided a complementary method for ionizing gaseous molecules. A disadvantage of both methods is that the sample of interest must be present in the gas phase at a pressure of 10-5 to 10-4 torr. With the extension of mass spectrometry to large, involatile, and, often, thermally fragile biomolecules, research during the 1970s and 1980s has been directed toward the development of ionization methods capable of ionizing such molecules directly from the solid or solution state; this work has led to the development of a variety of desorption ionization techniques. Decomposition of a molecular ion occurs whenever sufficient energy accumulates in the appropriate vibrational mode or modes to cause bond rupture. The fragment ions formed may have sufficient internal energy to fragment further, and rearrangement of the molecular framework may occur at any time. © 1990, Elsevier Inc. All rights reserved.