Study of Atmospheric Pressure Chemical Ionization Mechanism in Corona Discharge Ion Source with and without NH₃ Dopant by Ion Mobility Spectrometry combined with Mass Spectrometry: A Theoretical and Experimental Study

Abstract

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 NH₃ dopant, the reactant ion (RI) was H⁺(H₂O) _n, n = 3,4, and the MH⁺(H₂O) _x 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 (NH₄⁺(H₂O) _n as RI) two kinds of product ions, MH⁺(H₂O) _x and MNH₄⁺(H₂O) _x, were produced, depending on the basicity of M. With NH₄⁺(H₂O) _n as RI, the product ions of pyridine and DTBP with higher basicity were MH⁺(H₂O) _x while cyclopentanone, 2-nonanone, and acetophenone with lower basicity produce MNH₄⁺(H₂O) _x. To interpret the formation of product ions, the interaction energies of M-H⁺, H⁺-NH₃, and H⁺-OH₂ in the M-H⁺-NH₃ and M-H⁺-OH₂ 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⁺-NH₃, and H⁺-OH₂ interactions in the M-H⁺-NH₃ and M-H⁺-OH₂ complexes.