Vacuum-Ultraviolet-Excited and CH₂Cl₂/H₂O-Amplified Ionization-Coupled Mass Spectrometry for Oxygenated Organics Analysis

Abstract

The mass spectrometry analysis of oxygenated volatile organic compounds (OVOCs) remains challenging due to their limited ionization efficiencies. In this study, we surprisingly found that, under vacuum-UV (VUV) excitation, a gaseous mixture of CHCl/HO/analyte (OVOCs) in N buffer generated large amounts of HO and protonated analyte even when the photon energy was lower than the ionization energy of the neutral species involved. In contrast to those obtained with VUV photoionization alone, the signal intensities of oxygenated organics can be amplified by more than 3 orders of magnitude. The isotope tracing experiment revealed that the proton donor is water, and the dependence of the signal intensities on the VUV photon intensities verified that the reaction was a single-photon process. The observed ionization process is assigned as an undocumented chemi-ionization reaction in which a complex formed from the ion-pair state CHCl*, HO, and analyte and then autoionized to produce the protonated analyte with the aid of the reorganization energy released from the formation of CHO and HCl. Essentially, here we present an efficient chemi-ionization method for the direct protonation of oxygenated organics. By the method, the mass spectrometric sensitivities toward acetic acid, ethanol, aldehyde, diethyl ether, and acetone were determined to be 224 ± 17, 245 ± 5, 477 ± 14, 679 ± 11, and 684 ± 6 counts pptv, respectively, in 10 s acquisition time. In addition, the present ionization process provides a new method for the generation of a high-intensity HO source (∼10 ions s, measured by ion current) by which general organics can be indirectly protonated via a conventional proton-transfer reaction. These results open new aspects of chemi-ionization reactions and offer new technological applications that have the potential to greatly improve mass spectrometry sensitivity for detecting trace gaseous organics.

Implementation and study of dopant‐assisted photoionization with a miniature capillary inlet ion trap mass spectrometer

“…The ionization efficiency of analytes was improved using dopants with lower IEs, because these compounds are generally more easily ionized under VUV irradiation. Of course, the in‐trap dopant‐assisted PI described in this study was actually a complex process, which should also be affected by many other factors besides dopant IE, such as the absorption cross‐section of the analyte, the capture efficiency of the ion trap for different ions, the collision energy, and thus the rate constants for dopant analyte reactions driven by the trap RF . In general, each dopant was suitable for the detection of some specific compounds, and the doping conditions should be optimized in accordance with the target analytes.…”

“…Of course, the in-trap dopant-assisted PI described in this study was actually a complex process, which should also be affected by many other factors besides dopant IE, such as the absorption cross-section of the analyte, the capture efficiency of the ion trap for different ions, the collision energy, and thus the rate constants for dopant analyte reactions driven by the trap RF. 3,4,13,[29][30][31] In general, each dopant was suitable for the detection of some specific compounds, and the doping conditions should be optimized in accordance with the target analytes. In addition to parameter optimization, the switching of dopants could also be applied to other measurement scenarios.…”

“…[1][2][3][4] Mass spectrometry (MS) and ion mobility spectrometry (IMS) devices coupled with an atmospheric-pressure PI source allow the rapid detection of compounds with different polarities. [5][6][7][8] In recent years, the emergence of some commercialized vacuum ultraviolet (VUV) lamps has promoted the applications of PI-based detection technology, 9,10 such as the sensitive detection of explosives 11 and volatile organic compounds, [12][13][14] online breath analysis, 15 and rapid validation of illegal drugs. 16 The direct PI of analytes is generally inefficient in practical applications because most photons emitted by a VUV lamp tend to be absorbed by abundant air or solvent molecules.…”

“…Mass spectrometry (MS) and ion mobility spectrometry (IMS) devices coupled with an atmospheric‐pressure PI source allow the rapid detection of compounds with different polarities . In recent years, the emergence of some commercialized vacuum ultraviolet (VUV) lamps has promoted the applications of PI‐based detection technology, such as the sensitive detection of explosives and volatile organic compounds, online breath analysis, and rapid validation of illegal drugs …”

Emerging non‑invasive detection methodologies for lung cancer (Review)

“…In order to validate the efficacy of VOC profiles in the exhaled breath as a means of diagnosing LC, advancements in the following technologies are critically required: i) Standardized clinical utility; and ii) advanced mass spectrometry with ultra-high sensitivity. Recently, the excited state proton transfer (ESPT) ionization technology was established by LPPI-MS, discovering new mechanisms of chemi-ionization reactions and offering new technological applications that have the potential to greatly improve mass spectrometry sensitivity for detecting trace gaseous organics (124,125). VUV excitation is applied to produce a chemi-ionization reaction, which yields substantial H 3 O + ions, and the protonated analyte, an equal amount of Cl -, may be produced with the aid of the reorganization energy released from the formation of CH 2 O and HCl.…”