A complement to the use of KI is the selection of a certain number of characteristic fragments of a particular compound or a homologous series.This technique is called selected ion monitoring (SIM) and can be used both with electron impact (EI) and chemical ionization (CI).
Some of these ions for different groups of products have been reported by Vernin et al. (1998). For example, sesquiterpene hydrocarbons in an essential oil can be vizualized qualitatively and quantitatively by selection of ions at m/z = 93, 121,161, 204, and monoterpene hydrocarbons (with the exception of limonene, which gives upon EI a base peak at m/z = 68) by selection of ions at m/z = 93, 136, and so on. Thus, it is of particular importance to compare these two fractions obtained from different extraction methods and countries. This method was applied to the study of a "kintoki" Japanese ginger extract (Tanabe et al., 1991). Selected ions or base peaks (BP) of some compounds are reported in Table 3.10. Each compound has been quantified using naphthalene as the internal standard.
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