Evidence of neutral radical induced analyte ion transformations in APPI and Near-VUV APLI

Abstract

We report on the reactions of neutral radical species [OH, Cl, O(3P), H], generated in a typical atmospheric pressure ionization (API) source upon irradiation of the sample gases with either 193 nm laser radiation or 124 nm VUV light, the latter commonly used in atmospheric pressure photoionization (APPI). The present investigations focus on the polycyclic aromatic hydrocarbon pyrene as representative of the aromatic compound class. Experimental results are supported by computational methods: simple kinetic models are used to estimate the temporal evolution of the concentrations of reactants, intermediates, and final products, whereas density functional theory (DFT) energy calculations are carried out to further elucidate the proposed reaction pathways. The neutral radicals are generated upon photolysis of background water and oxygen always present in appreciable mixing ratios in typical API sources. Substantial amounts of oxygenated analyte product ions are observed using both techniques. In contrast, upon atmospheric pressure laser ionization (APLI) with 248 nm radiation, oxygenated products are virtually absent. In addition, kinetic data evaluation yielded a bimolecular rate constant of k = (1.9 +/- 0.9) x 10(-9) cm3 molecule(-1) s(-1) for the reaction of the pyrene radical cation with OH radicals.

Charge Exchange Reaction in Dopant-Assisted Atmospheric Pressure Chemical Ionization and Atmospheric Pressure Photoionization

“…Formation of neutral radicals in APPI has been shown previously for O 2 and H 2 O [41]. Different solvent ion compositions in DA-APPI and DA-APCI are thus suggested to reflect also neutral radical compositions of the ion sources and derive from the different energy distributions of primary ionizing species (photons versus corona discharge).…”

The Ionization Mechanisms in Direct and Dopant-Assisted Atmospheric Pressure Photoionization and Atmospheric Pressure Laser Ionization

“…This way APPI is a more selective method to generate ions of common analytical interest, compared with for instance atmospheric pressure chemical ionization (APCI). However, in the provided wavelength range, several species such as water and oxygen photodissociate with significant absorption cross-sections [6]. Consequently, open systems are prone to extensive neutral radical-induced ion transformation processes and radiative loss into non-charged species.…”

The role of ion‐bound cluster formation in negative ion mass spectrometry

“…The reported Gibbs free energy and enthalpy‐corrected total energies were obtained from geometry optimizations and subsequent frequency calculations using the density functional theory (DFT) with the Becke‐3‐Parameter‐Lee‐Yang‐Parr functional (B3LYP) and the 6‐31++G(d,p) doubly‐diffuse and doubly‐polarized split‐valence basis set. The combination of this functional and basis set type provided the optimal cost‐to‐benefit ratio with respect to CPU time and was thus used for a large number of comparable reaction systems …”