A Novel Application of Dopants in Ion Mobility Spectrometry: Suppression of Fragment Ions of Citric Acid.

Ion mobility spectra of citric acid (CA) are complex, and several peaks are observed for CA and its fragments in both the positive and negative modes. Using DFT calculations, we found that the fragments are both less acidic and less basic than CA in gas phase. Hence, we used a strong base, NH3, in positive mode to produce NH4+ as an alternative reactant ion (RI) and prevent protonation of the fragments. In the presence of NH4+, only one peak for CA was observed because of its higher proton affinity (873 kJ mol-1) compared to NH3 (854 kJ mol-1). In the negative mode, CHCl3, CHBr3, and CHI3 were used as dopant gases to produce Cl-, Br-, and I- as RIs. These halides have less basicity than the common RIs in negative mode (NO2-, NO3-, O2-) and selectively deprotonated CA in the presence of its fragments. Hence, using dopants with appropriate basicity, we could suppress the fragment peaks and obtain a plain IMS spectrum for CA containing only one peak in both the positive and negative modes. Using NH3 and CHCl3 dopants, the amount of CA in fresh lemon juice was determined as 39.5-42 g L-1 by direct injection without any purification. The effect of hydration of the reactant and product ions on the ionization mechanism in both negative and positive modes was investigated theoretically.