Chapter 5: Ambient Analysis by Thermal Desorption Atmospheric-Pressure Photoionization

Among ambient ionization methods, thermal desorption-ionization stands out because of two attributes: (1) simplicity, rendering the technique suitable for in-field applications, and (2) ability to couple with a variety of gas-phase ionization methods, thereby broadening the range of molecules that can be analyzed with this method. In this chapter we will describe the use of atmospheric-pressure photoionization (APPI) for ambient analysis. The focus will be on uses as standalone sources, such as desorption-APPI (DAPPI), and combination with other ambient sources such as direct analysis in real time-APPI (DART-APPI). Generally, the direct photoionization and charge-transfer mechanisms offered by APPI in addition to a proton-transfer mechanism lead to a higher coverage of small molecules and better sensitivities using APPI and the combined sources. APPI has the advantage of ionizing over a wide range of polarities, in particular extending the range to classes of nonpolar compounds (e.g., PAHs, hydrocarbons, steroids, pesticides, etc.).