On the formation of negative ions in atmospheric pressure photoionization conditions

Abstract

Atmospheric pressure photoionization (APPI) 1 is a new technique, highly interesting for its features, mainly selectivity and sensitivity, somehow overlapping the performances of the well-established electrospray (ESI) 2 and atmospheric pressure chemical ionization (APCI) 3 techniques. The basic principle of this method is, at first sight, very simple: a molecule M can be ionized by interaction with a photon beam only if its ionization energy (IE) is lower, or equal to, the photon energy h :Hence the irradiation by a Kr discharge lamp (exhibiting a dish-topped emission band between 8 and 10 eV 4 ) of a sample solution vaporized at atmospheric pressure by a heated insertion line would lead to the formation of odd-electron molecular ions (M Cž ) of substances exhibiting IE values lower than 10 eV, without practically any interference from the solvents usually employed in liquid chromatography (LC) conditions. In fact, for example, H 2 O, CH 3 CN, CH 3 OH all exhibit IE values higher than 10 eV. 5 However, this view is not always correct. It has been shown 6,8 that photon irradiation can activate processes different from simple ionization: in some cases, the electronically excited molecule can isomerize, leading to species with ionization energies lower than that of the original molecule. This is the case of acetonitrile (IE D 12.2 eV), 4 which, under 10 eV photon irradiation, isomerizes to ketene imine (IE D 9.5 eV), which can be photoionized, becoming an active intermediate in ionization processes of analyte molecules in CH 3 CN solutions: 6,7 CH 3 CN C h ! [CH 3 CN]

Atmospheric pressure photoionization proton transfer for complex organic mixtures investigated by fourier transform ion cyclotron resonance mass spectrometry

“…Kostiainen et al studied negative ion formation mechanisms by investigating ionization efficiency and ion type (even-or odd-electron ion) for analytes of different polarity in various solvents [3,7]. Traldi et al reported negative ion formation by resonant electron capture from thermal electrons originating from metal surfaces and dopant [8].…”

Electron capture atmospheric pressure photoionization mass spectrometry: analysis of fullerenes, perfluorinated compounds, and pentafluorobenzyl derivatives

“…35 42,43 Negative-ion APPI has been studied through the investigation of its ionization mechanism by Kostiainen and co-workers 5,44 and Traldi and co-workers. 45 Although both Kostiainen and co-workers 5,44 and Traldi and co-workers 45 demonstrated the feasibility of ionization of less polar compounds by negative-ion APPI, only the analysis of polybrominated diphenyl ethers containing more than five bromine atoms proved to be superior to other API modes. 7,8 In a recent review on APPI, 2 Karst and co-workers stated that ''the ionization mechanism is probably the main issue here'' and that ''the solvent plays a much larger role in the analyte ionization, whereas the electron-ejection and electron-capture theory is less applicable than first expected, making the method less promising for low polarity analytes''.…”

Negative ion-atmospheric pressure photoionization: Electron capture, dissociative electron capture, proton transfer, and anion attachment

“…In dopant assisted negative ion-APPI, the ionization process is initiated by thermal electrons. Currently, subsequent ionization mechanisms including EC, dissociative EC, and proton transfer are suggested to be responsible for the formation of negative ions [8,10,11]. In this study, nitroaromatics were preliminarily chosen as analytes because previous studies demonstrated multiple ionization products by both EC-NICI [12,13] and EC-APCI [14 -19].…”