What compound acts as a dopant to increase ionization efficiency in APPI?

The compound that acts as a dopant to increase ionization efficiency in Atmospheric Pressure Photoionization (APPI) is toluene. APPI is a technique used in mass spectrometry that relies on the interaction of photons with sample molecules to facilitate ionization. Toluene, being an aromatic compound, has favorable properties that enhance the ionization process.

When toluene is introduced into the ionization source, it can absorb ultraviolet (UV) light and generate a stable excited state. This excited state can collisively transfer energy to the analyte molecules, effectively increasing their ionization efficiency. This characteristic makes toluene a valuable dopant in APPI, as it helps to produce a higher yield of ions from the target molecules, thus improving overall detection sensitivity.

The other compounds mentioned—water, acetic acid, and propylene glycol—do play roles in various ionization processes but do not serve the same purpose as toluene in the context of enhancing the ionization efficiency specifically in APPI. Water, while a common solvent, may not provide the necessary energy transfer like toluene. Acetic acid can participate in ionization processes, but it does not primarily function as a dopant for increasing APPI efficiency.