Study of Atmospheric Pressure Chemical Ionization Mechanism in Corona Discharge Ion Source with and without NH Dopant by Ion Mobility Spectrometry combined with Mass Spectrometry: A Theoretical and Experimental Study.

Ionization of 2-nonanone, cyclopentanone, acetophenone, pyridine, and di- tert-butylpyridine (DTBP) in a corona discharge (CD) atmospheric pressure chemical ionization (APCI) ion source was studied using ion mobility (IMS) and time-of-flight mass spectrometry (TOF-MS). The IMS and MS spectra were recorded in the absence and presence of ammonia dopant. Without NH dopant, the reactant ion (RI) was H(HO) , n = 3,4, and the MH(HO) clusters were produced as product ions. Modeling of hydration shows that the amount of hydration ( x) depends on basicity of M, temperature and water concentration of drift tube. In the presence of ammonia (NH(HO) as RI) two kinds of product ions, MH(HO) and MNH(HO) , were produced, depending on the basicity of M. With NH(HO) as RI, the product ions of pyridine and DTBP with higher basicity were MH(HO) while cyclopentanone, 2-nonanone, and acetophenone with lower basicity produce MNH(HO) . To interpret the formation of product ions, the interaction energies of M-H, H-NH, and H-OH in the M-H-NH and M-H-OH and M-H-M complexes were computed by B3LYP/6-311++G(d,p) method. It was found that for a molecule M with high basicity, the M-H interaction is strong leading in weakening of the H-NH, and H-OH interactions in the M-H-NH and M-H-OH complexes.