APPI spectra

The appearance of APPI spectra strongly depends on the actual combination of UV lamp, solvent, analyte, and eventually dopant. It is not always straightforward to predict whether protonation or molecular ion formation will dominate a positive-ion APPI spectrum, and analogously, whether deprotonation or radical anion formation will be observed in negative-ion mode. As a rule of thumb, basic molecules tend to appear as [M+H] ions while nonpolar analytes preferably form ions. Vice versa, acidic molecules easily deliver [M-H] ions in negative-ion mode while those of high electron affinity may undergo electron capture to yield IVT ions. Unfortunately, APPI tends towards mixed ion formation, e.g., IVU beside [M+H] ions, a property it has in common with FAB (Chap. 10). Some typical APPI spectra are compiled below (Fig. 12.45) [181,185,187,190]. 

Mass Spectrometry in Drug Discovery Rossi, D.T. Sinz, M.W. (eds.) Marcel Dekker New York, 2002. 

Modem Mass Spectrometry Schalley, C.A. (ed.) Springer New York, 2003. 

Ardrey, R.E. Liquid Chromatography-Mass Spectrometry — An Introduction Wiley Chichester, 2003. 

Siuzdak, G. The Expanding Role of Mass Spectrometry in Biotechnology, 2nd ed. MCC Press San Diego, 2006. 

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Fig. 12.45. APPI spectra of (a) acridine, (b) diphenyl sulfide, (c) naphthalene, (d) 5-fluoro-uracil, and (e) carbamazepine. Adapted from Ref. [181] with permission. American Chemical Society, 2000.

Figure 6.1 Mass spectrum of an impurity run in the positive APPI mode.

The real breakthrough in LC/MS development was achieved with the broad introduction in the 1990s of atmospheric pressure ionization (API) techniques, such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), which enable the analysis of a wide variety of molecular species. The spectrum of available API techniques has been amended meanwhile by the introduction of sonic spray ionization (SSI) and atmospheric pressure photoionization (APPI). 

Fig. 5.9 APPI FT-ICR mass spectrum of white fraction of hydrogenated C. (Talyzin et al. 2006a)...

The optimal condition is to use a HR instrument able to give an accurate MS/MS product ion spectrum in order to obtain the elemental composition of both precursor and product ions. In this section these analyzers will be taken into account, highlighting the strengths and weaknesses. However, due to the use of different atmospheric pressure ionization sources and consequently different optimal conditions, the comparison between distinct LC-MS systems could give a non realistic comparison between mass analyzers (Soler et al. 2005, 2006, 2007). Among the possible ionization techniques, electrospray (ESI) is by far the most widely used as compared with atmospheric-pressure chemical ionization (APCI) or the more recent atmospheric-pressure photoionization (APPI) (Krauss et al. 2010). 

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