Abstract
Atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), and electrospray ionization (ESI) have gained widespread popularity to couple liquid chromatography to mass spectrometry (MS). APCI-MS and APPI-MS produce similar mass spectra, whereas ESI-MS produces mostly adduct ions useful for further MS/MS, unless some energy is provided to induce up-front collision-induced dissociation. Because of the complementary nature of these atmospheric pressure ionization (API) techniques, there has been a dramatic increase in the number of publications describing either sequential analyses using two or more API techniques, combination sources allowing more than one API technique, or multiple API techniques used simultaneously in parallel, which is the topic of this chapter. Benefits, drawbacks, and special considerations for each API technique are discussed, and examples of quadruple parallel MS (LC1/MS4) are discussed. Finally, an example is provided to demonstrate use of four mass spectrometers as detectors in a comprehensive LC×LC experiment, for LC2/MS4 or LC1/MS2×LC1/MS2.
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