What is the primary mobile phase used in Atmospheric Pressure Photoionization (APPI)?

In Atmospheric Pressure Photoionization (APPI), the primary mobile phase typically consists of organic solvents that can effectively facilitate the ionization of analytes. Water and acetonitrile is a common combination used as it offers a good balance of polar and non-polar interactions, allowing for the efficient ionization of a wide range of molecules.

Acetonitrile is especially favored because it has a relatively low ionization energy and works well with the UV light utilized in APPI, helping to generate ions from the analyte molecules. The presence of water can help in solubilizing more polar compounds, thus broadening the range of analytes that can be effectively ionized.

This combination maximizes the efficiency of the ionization process by allowing both polar and non-polar compounds to be effectively introduced into the ionization source, making it an ideal choice in various analytical applications utilizing APPI.