APCI process

FIGURE 5 Schematic presentation of APCI process (courtesy of Thermo Finnigan). 

Pseudomolecular Ions. In contrast to the traditional MS, the highest mass peaks in ESI/APCI spectra are not always the molecular ion of interest. Instead, pseudomolecular ions, or noncovalent complex ions, are commonly observed. The pseudomolecular ions are generally formed by the analyte-adduct interaction in the solution system that is preserved as a result of the soft ionization of the ESI/APCI process. These ions are also formed by analyte-adduct gas-phase collisions in the spray chamber [49]. The exact mechanisms of how the analyte adducts are formed in ESI/APCI still remain unresolved at this point. More often than not, the adduct ion formation is a major cause for the low detection limit for ESEAPCI MS. However, these associative processes have also created interest in the study of drug-protein/ drug-oligonucleotide gas-phase complexes that benefit from the ability of ESI/APCI MS analysis. 

Figure 13-2. A simplified schematic of the APCI process. (Courtesy of Dr. P. Tiller.)...

APCI is another very soft ionization technique and has many similarities to ESI. Ionization takes place at atmospheric pressure and the ions are extracted into the mass detector in the same way as in ESI. Similarly, [M - - H] and [M — H] ions are usually formed to give molecular weight information, and, when using a single quadrupole, fragmentation of the precursor ions can be induced in the source by increasing the cone voltage. Yet, the APCI process differs from the ESI one mainly in that ... 

Note The significant enhancement of primary ion formation by a corona discharge as compared to a Ni source has been implemented in early API sources, a context in which a basic APCI process has already been described (Eqs. 12.1-12.6 in Chap. 12.1.1) [174,175]. The corona needle employed to expand the acceptance range of TSP basically represents an APCI m e for these sources, too [28]. 

---