Which of the following is NOT a chemical additive used in APPI?

In the context of Atmospheric Pressure Photoionization (APPI) used in mass spectrometry, the role of chemical additives is crucial for optimizing ionization efficiency. Acetone, toluene, and anisole are all solvents or compounds that can enhance the ionization process in APPI due to their volatile nature and ability to provide a better environment for ion formation.

Water, on the other hand, is generally not used as a chemical additive in APPI. While it is a common solvent in many analytical procedures, it does not possess the same ionization properties and can actually lead to lower ionization efficiencies in the APPI process. Since the focus of APPI is on generating ions from gas-phase samples, the presence of water can introduce complications in the ionization process, making it less effective in this specific analytical technique.

Thus, the identification of water as the option that is not a chemical additive used in APPI is based on its lack of compatibility with the ionization methodology that APPI employs compared to the other choices presented.