DART Analysis of Aspirin: Correcting a Misapprehension
In a recently published comparison of the ambient ionization techniques direct analysis in real time (DART™) and DESI, it was reported that a protonated molecule was not observed for DART, whereas the protonated molecule could be observed for DESI and DAPCI. This is an incorrect observation, resulting from the use of different experimental conditions for DART than were used for the other two techniques. Mass spectra of aspirin measured on a JEOL AccuTOF-DART™ mass spectrometer under the correct operating conditions are shown here. All assignments for the mass spectral peaks were confirmed by exact mass measurements.
Because aspirin (acetylsalicylic acid, Figure 1) is an organic acid, it has a relatively low proton affinity. Acidic compounds tend to lose protons rather than gain them. That is, proton transfer from protonated water clusters is not favored.