Applied Electrospray Mass Spectrometry (Practical by by Birendra N. Pramanik (Editor), A.K. Ganguly (Editor),

By by Birendra N. Pramanik (Editor), A.K. Ganguly (Editor), Michael L. Gross (Editor)

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Because the pressure is high, the chemical ionization reactions leading to the formation of reagent ions can be complex. The resulting spectra depend strongly on the nature of the solvent and additives. A mixture of adducts such as [M ϩ H]ϩ, [M ϩ Na]ϩ, [M ϩ K]ϩ, and [M ϩ NH4]ϩ can often be observed in APCI. This can make interpretation difficult, and it can be useful to add a buffer or dopant such as ammonium acetate to favor the production of a specific adduct species such as [M ϩ NH4]ϩ. Because APCI is a chemical ionization process, fragmentation is much more common than in ESI.

Multiply charged ions from large biomolecules [such as ionized duplex DNA (416) and noncovalently bound heme–myoglobin complexes (417)] are stable in an ion trap for relatively long periods, permitting the study of ion–molecule reactions [such as gas-phase H/D exchange (418)] and ion–ion reactions (419). Both cations and anions can be easily trapped in a quadrupole ion trap. This is not generally possible with FTICR because the polarity of the trapping plate potentials determines the polarity of the ions that can be stored.

Precharged species such as quaternary ammonium salts or sulfates are also readily detected by ESI, and cation attachment can also be observed for compounds such as polyethers and carbohydrates, which contain heteroatoms. Multiply charged species such as dications or dianions or high molecular weight compounds are candidates for ESI. Atmospheric pressure chemical ionization is usually the first choice for smaller, less polar compounds such as steroids or carotenoids, which are uncharged in solution and have relatively low proton affinities.

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