Atmospheric pressure photoionization-mass spectrometry and atmospheric pressure chemical ionization-mass spectrometry of neurotransmitters.

A group of five neurotransmitters with different properties was analyzed using atmospheric pressure photoionization (APPI) and atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). The sensitivity of the techniques for the analytes was tested in six solvents and in positive and negative ion modes. APPI was found to be superior in sensitivity for all the compounds in both positive and negative ion modes. In positive ion mode, water/methanol/formic acid was found to be the best solvent, whereas in negative ion mode, water/methanol/ammonium hydroxide performed best. Detection limits using APPI were between 2.5-250 fmol, depending on the compound. The sensitivity was best for the neurosteroids dehydroepiandrosterone and beta-estradiol, and acetylcholine (LOD 2.5-10 fmol).