Adduct formation of steroids in APCI and its relation to structure identification

adducts could well be explained with the dissociation energies (DE) of the complexes [14]. The DE is related to the proton affinity (PA) of the constituent base by DE = a-b ΔPA where a and b are constants and ΔPA is the absolute PA difference [10]. Therefore it can be argued that the DE hypothesis used for the thermospray data can also be used for the explanation of the adduct formation in APCI. As the PA, and consequently the DE, of a molecule is dependent on its structure, the tendency of a molecule to form adduct ions can be used for structural identification purposes. A major problem with this theory is the presence of the collisionally induced dissociation processes occurring in the sampling region of the atmospheric pressure source (somewhat like the effect of the repeller in the thermospray interface) which are, ironically enough, meant to de-cluster the adduct ions. The following, limited study, discusses the data from the acetate adduct ions with steroid molecules (see Fig. 1 for a general structure and the numbering of the C atoms) in the negative ion mode. Abundant adduct ions for these type of molecules have been reported both with the thermospray [7] as with the HN [8,9] interface. These types of compounds are chosen because the MSMS data are considered hard to interpret [15]. That is hydrogen transfer