What types of compounds do dopants improve ionization efficiency for in APPI?

Dopants significantly enhance the ionization efficiency of compounds that are difficult to ionize in Atmospheric Pressure Photoionization (APPI) techniques. In APPI, the primary ionization mechanism relies on the absorption of ultraviolet (UV) light by the dopant, which generates energetic radicals or ions that can subsequently interact with the analyte, resulting in its ionization.

Compounds that are hard to ionize typically have high ionization potentials or require significant energy input to detach electrons, making traditional ionization methods less effective. By introducing a dopant, the energy from the UV light increases the likelihood of effective ionization for these challenging compounds. The dopant effectively lowers the energy barrier, facilitating the ionization process and resulting in increased sensitivity and improved detection of these analytes.

In contrast, other compounds listed—such as highly volatile compounds or non-volatile analytes—may be ionized more easily by traditional means or might not benefit as significantly from dopants. Radioactive materials are generally not relevant in this context, as their ionization mechanisms differ from those utilized in APPI. Thus, the role of dopants stands out prominently for enhancing ionization efficiency for compounds that were previously resistant to easy ionization.