Determination of Relative Ionization Cross Sections for Resonance Enhanced Multiphoton Ionization of Polycyclic Aromatic Hydrocarbons

Abstract

Resonance enhanced multiphoton ionization (REMPI) is a powerful method for the sensitive determination of polycyclic aromatic hydrocarbons (PAHs) in gaseous mixtures via mass spectrometry (MS). In REMPI, ions are produced by the absorption of at least two photons including defined electronic intermediate states. As a result—unlike other laser-based ionization techniques—spectroscopic selectivity is involved into the ionization process. Nevertheless, these wavelength-dependent ionization rates impede the quantification using REMPI. For this purpose, relative photoionization cross sections (relPICS) give an easy-to-use approach to quantify REMPI-MS measurements. Hereby, the ionization behavior of a single compound was compared to that of a reference substance of a given concentration. In this study, relPICS of selected single-core aromatics and PAHs at wavelengths of 266 nm and 248 nm were determined using two different time-of-flight mass spectrometric systems (TOFMS). For PAHs, relPICS were obtained which showed a strong dependence on the applied laser intensity. In contrast, for single-core aromatics, constant values of relPICS were determined. Deviations of relPICS between both TOFMS systems were found for small aromatics (e.g., benzene), which can be assigned to the differences in UV generation in the particular system. However, the relPICS of this study were found to be in good agreement with previous results and can be used for system-independent quantification.

External trap‐and‐release membrane inlet for photoionization mass spectrometry: Towards fast direct analysis of aromatic pollutants in aquatic systems

“…Nevertheless, both compounds show similar MIMS behavior, due to similarities in their vapor pressures. The detected differences in signal intensities are in good agreement with differences in the ionization yields of both substances with REMPI at 266 nm presented by Gehm et al 50 For both compounds rapid heating of the membrane leads to an increase in signal intensity by a factor of 3 compared with steady‐state flux due to enhancing evaporation rates from the membrane. Thus, the OEC approach is already capable of improving S/N ratios compared with fully continuous measurements.…”

Composition of carbonaceous fine particulate emissions of a flexible fuel DISI engine under high velocity and municipal conditions

“…However, the applied method cannot distinguish between different isomers with the same degree of alkylation because with growing PAH core structure the position of alkylation has a progressively smaller effect on the cross section at the specific wavelength. In addition, the absolute peak heights do not reflect the absolute sample composition because cross sections of different species may differ up to two orders of magnitude [21,31]. As described in the previous section, oxygenates such as ethanol exhibit a soot reducing effect in combustion processes due to the limitation of aromatic precursors [30].…”

“…Single species identification made here is only possible due to different REMPI ionisation cross sections of isobaric compounds at the used photon energy. At 266 nm, the cross section of the phenanthrene isobaric compound anthracene is negligible, hence m/z 178 was solely assigned to phenanthrene [19,31]. However, the applied method cannot distinguish between different isomers with the same degree of alkylation because with growing PAH core structure the position of alkylation has a progressively smaller effect on the cross section at the specific wavelength.…”

Atmospheric Pressure Single Photon Laser Ionization (APSPLI) Mass Spectrometry Using a 157 nm Fluorine Excimer Laser for Sensitive and Selective Detection of Non- to Semipolar Hydrocarbons

“…In classical REMPI, the ionization cross-section and, thus, mass spectrometric response varies drastically with the isomeric structure, e.g., at 266 nm phenanthrene is considerably better ionized than anthracene. 42,36 For the presented APSPLI approach the behavior is more comparable to vacuum single photoionization, where ionization cross-sections were reported to vary within one order of magnitude. [43][44][45] As noticeable from the TIC in Figure 2, APSPLI provides a very low and nearly absent chemical background noise as a clear advantage.…”