Abstract
Ionization of aspirin (A) in positive and negative modes of a corona discharge (CD) ion source was studied by ion mobility spectrometry (IMS). The IMS spectra showed that aspirin can be both protonated, AH+, and deprotonated, (A-H)-, in positive and negative modes of CD, respectively. Using DFT calculations, it was found that the carbonyl oxygen of COOH group is the most preferred site of protonation with proton affinity of 867.8 kJ mol-1. Also, the calculated ΔG values for deprotonation of aspirin by O2- and O3- were -71.2 and -28.5 kJ mol-1, respectively, indicating its deprotonation is thermodynamically possible. Hydration of AH+ and (A-H)- was studied theoretically and it was found that AH+ and AH+(H2O) are the most abundant species in the ionization region in the presence of 40 ppm of water vapor at 100–200 °C, while (A-H)- is not hydrated in the same condition. The results of measurements in urine showed that urea content of urine interferes with detection of aspirin in positive CD, while urea was not ionized in the negative CD so that aspirin was determined in urine with recovery of 94% in negative CD.
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