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

Abstract

A novel, gas-tight API interface for gas chromatography-mass spectrometry was used to study the ionization mechanism in direct and dopant-assisted atmospheric pressure photoionization (APPI) and atmospheric pressure laser ionization (APLI). Eight analytes (ethylbenzene, bromobenzene, naphthalene, anthracene, benzaldehyde, pyridine, quinolone, and acridine) with varying ionization energies (IEs) and proton affinities (PAs), and four common APPI dopants (toluene, acetone, anisole, and chlorobenzene) were chosen. All the studied compounds were ionized by direct APPI, forming mainly molecular ions. Addition of dopants suppressed the signal of the analytes with IEs above the IE of the dopant. For compounds with suitable IEs or Pas, the dopants increased the ionization efficiency as the analytes could be ionized through dopant-mediated gas-phase reactions, such as charge exchange, proton transfer, and other rather unexpected reactions, such as formation of [M + 77](+) in the presence of chlorobenzene. Experiments with deuterated toluene as the dopant verified that in case of proton transfer, the proton originated from the dopant instead of proton-bound solvent clusters, as in conventional open or non-tight APPI sources. In direct APLI using a 266 nm laser, a narrower range of compounds was ionized than in direct APPI, because of exceedingly high IEs or unfavorable two-photon absorption cross-sections. Introduction of dopants in the APLI system changed the ionization mechanism to similar dopant-mediated gas-phase reactions with the dopant as in APPI, which produced mainly ions of the same form as in APPI, and ionized a wider range of analytes than direct APLI.

Ionization of EPA Contaminants in Direct and Dopant-Assisted Atmospheric Pressure Photoionization and Atmospheric Pressure Laser Ionization

“…The mass spectrometer was a Thermo Scientific (Bremen, Germany) Exactive Orbitrap, equipped with a custom-made API interface, which has been described in detail before [15]. Shortly, a commercial transfer line (Thermo Scientific) was used to guide the sample flow from the GC column to an air-tight, conically-shaped ion source, which was heated to 325°C.…”

“…Recently, a novel API interface for GC-MS tailored towards APPI operation was introduced [15]. In this design, the crucial parameters for efficient direct ionization and chromatographic performance have been carefully considered.…”

“…In a previous study, the ionization mechanisms in direct and dopant-assisted APPI and direct and dopant-assisted APLI with the above-mentioned API interface was investigated using eight compounds [15]. It was shown that (1) in direct APPI and direct APLI, mainly molecular ions were formed; (2) direct APPI was shown to be more universal than direct APLI, whereas APLI was highly selective towards compounds that have favorable twophoton ionization cross sections; (3) introduction of dopants hindered direct photoionization in APPI and two-photon ionization in APLI and, instead, dopant-mediated reactions took place that led to more variety in the type of ions formed; (4) the dopantmediated reactions were mainly the same in APPI and APLI; and (5) introduction of dopants can enhance the ionization efficiency; (6) although in APPI, the presence of dopants in many cases decreased the number of compounds that were ionized; (7) whereas in APLI many compounds that were not ionized by resonance enhanced (1+1) multiphoton ionization (REMPI) were ionized with one or several of the dopants.…”

Theory and Practice of UHPLC and UHPLC–MS

“…Reviews of APPI-MS, including recent advances in this field, have been provided by, among others, Marotta et al (2003), Raffaelli and Saba (2003), Tubaro et al (2003), Syage et al (Syage, 2004;Syage et al, 2004Syage et al, , 2008, Hanold et al (2004), Robb and Blades (2005, 2006, Kauppila et al (2005Kauppila et al ( , 2014Kauppila et al ( , 2017, Bos et al (2006), Purcell et al (2006), , and Marchi et al (2009).…”

Quantitative Analysis of Bisphenol A in Recycled Paper with a Novel Direct Inlet Probe-Atmospheric Pressure Photoionization–IonTrap-MS

“…For non-polar compounds atmospheric pressure photoionization (APPI) gives often better results [15]. This is due to the difference in the ionization mechanism.…”