Chemical Ionization Techniques

Since some molecular masses at EI at 70 eV are not always visible, various workers used either recording mass spectra at 20 eV or gentler ionization methods. The theory and application of these latter techniques were developed in Harrison's book (1983) and in several papers (e.g., Bruins, 1987).

In a chemical ionization chamber, the pressure is higher (0.07 to 1.5 torr) than in a source under EI. In many cases, it is possible to obtain quasimolecular ions both in positive chemical ionization (PCI) using isobutane or ammonia as gaseous reagents (M + H)+, and in negative mode (NCI) using the hydroxyl OH— or NH2— ion as a reagent giving (M — H)—. Numerous other reagents are used for this purpose, and their applications in analysis of essential oils and aromas have been summarized by Vernin et al. (1998). PCI (NH3) gives better results with carbonyl compounds (terpenes, aliphatics, alicyclics, and aromatics) than in the NCI mode. Another important fragment at m/z = (M + NH4)+ is observed. Upon PCI/i-C4H10 sesquiterpene hydrocarbons give an intense quasimolecular ion (M + 1)+. Under the same conditions, mass spectra of sesquiterpene alcohols show intense fragment ions (M + H — H2O)+ at m/z = 205. Upon PCI(NH3)+, a typical set of ions is formed: (M + H — H2O)+, (M + NH4 —

Table 3.10 Example of separated compounds obtained by the Selected Ion Monitoring (SIM) technique*

Base Peak (m/z)

Separated compounds"

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