Ionization of Dichloromethane by a Vacuum Ultraviolet Krypton Lamp: Competition Between Photoinduced Ion-Pair and Photodissociation-Assisted Photoionization

Self Cite

The ion conversion processes in CH 2 Cl 2 -enhanced vacuum ultraviolet photoionization of oxygenated volatile organic compounds (OVOCs) have been systematically studied by regulating the pressure, humidity, and reaction time in the ionization source of a time-of-flight mass spectrometer. As the ionization source pressure increased from 100 to 1100 Pa, the main characteristic ions changed from CH 2 Cl + to CH 2 Cl + (H 2 O), CH 2 OH + , and C 2 H 4 OH + and then to the hydrated hydronium ions H 3 O + (H 2 O) n (n = 1, 2, 3). The total ion current (TIC) almost remained unchanged even if the humidity increased from 44 to 3120 ppmv, indicating interconversion between ions through ionmolecule reactions. The intensity of protonated methanol/ethanol (sample S) ion was almost linearly correlated with the intensity of H 3 O + (H 2 O) n , which pointed to the proton transfer reaction (PTR) mechanism. The reaction time was regulated by the electric field strength in the ionization region. The intensity variation trends of different ions with the reaction time indicated that a series of stepby-step ion-molecule reactions occurred in the ionization source, i.e., the primary ion CH 2 Cl + reacted with H 2 O and converted to the intermediate product ions CH 2 OH + and C 2 H 4 OH + , which then further reacted with H 2 O and led to the production of H 3 O + , and finally, the protonated sample ion SH + was obtained through PTR with H 3 O + , as the ion-molecule reactions progressed. This study provides valuable insights into understanding the formation mechanism of some unexpected intermediate product ions and hydrated hydronium ions in dopant-enhanced VUV photoionization and also helps to optimize experimental conditions to enhance the sensitivity of OVOCs.

Section: Resultsmentioning

confidence: 99%

“…Yu et al found the primary photoionization product ions of CH 2 Cl 2 were CH 2 Cl + and CHCl 2 + at relatively low pressure of 25–150 Pa and proposed the ion-pair or photodissociation-assisted photoionization mechanism. There are several questions to be addressed before we can understand the difference between the two studies.…”

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confidence: 99%

Manipulation of Ion Conversion in Dichloromethane-Enhanced Vacuum Ultraviolet Photoionization Mass Spectrometry of Oxygenated Volatile Organic Compounds

Yu,

Jiang,

Hua

et al. 2023

Self Cite

“…30−33 Radical ions could also result from the photoionization of photodissociation products during chlorinated methanes' VUV photoionization. 34 Sensitivities and LODs of EUV-PI-TOFMS. Figure 4 shows the sensitivity curves of CH 3 Cl (black), CH 2 Cl 2 (red), CHCl 3 (blue), and CCl 4 (green) in a concentration range of 1 to 100 ppbv.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…The mass peak at m / z 116 is attributed to unknown matter from synthetic air, speculatively. CH 2 Cl + , CHCl 2 + , and CCl 3 + formations have been observed in the VUV photodissociative photoionization of chlorinated methanes. − Radical ions could also result from the photoionization of photodissociation products during chlorinated methanes’ VUV photoionization …”

Section: Resultsmentioning

confidence: 99%

Novel MCP-Windowed EUV Light Source and Its Mass Spectrometric Application for Detecting Chlorinated Methanes

Cheng,

Yan,

Shan

et al. 2023

Various vacuum ultraviolet (VUV) lamps are simple and convenient VUV light sources for mass spectrometry and other research fields. However, the strong absorption of high-energy photons by window materials limits the application of an extreme ultraviolet (EUV) light. In this study, a novel high-flux EUV light source is developed using a microchannel plate (MCP) window to transmit 73.6 nm (16.9 eV) EUV light generated via the radio frequency (RF) inductive discharge of neon. The MCP used is a 0.5 mm thick glass plate with a regular array of microtubes (12 μm i.d.). The photon fluxes of the EUV light source with the MCP window (12 mm i.d.) and an aperture (1.8 mm i.d.) are ∼1.31 × 10 14 and ∼9.80 × 10 12 photons s −1 , respectively, while their corresponding leakage flow rates of the discharge gas are 0.062 and 0.046 cm 3 atom s −1 , according to the contrast experiments. The transmission efficiency of the MCP to the EUV light is 30.2%, with a 1.2% deviation. An EUV photoionization time-of-flight mass spectrometer (EUV-PI-TOFMS) is built to validate the practicality of the MCP-windowed EUV light source further. The detection sensitivities in 30 s measurements for methyl chloride (CH 3 Cl), methylene chloride (CH 2 Cl 2 ), trichloromethane (CHCl 3 ), and carbon tetrachloride (CCl 4 ) in synthetic air are 4366, 4120, 5854, and 4095 counts ppbv −1 , respectively. The corresponding 3σ limits of detection (LODs) are 42, 34, 24, and 15 pptv. This study develops a new feasible method for efficiently utilizing highenergy EUV light, with many application prospects in scientific research.

“…At first, researchers improve the light intensity of the VUV beam by developing high-power VUV lamps. , Second, since PI is initiated by collisions between photons and gas analyte molecules, increasing the source pressure can also promote analyte ionization through more frequent photon–molecule collisions. − Another way to enhance PI efficiency is to introduce a dopant to the source to induce the dopant-assisted chemical ionization. ,, For this strategy, the dopant molecules absorb the photons and form sufficient reagent ions that can further react with the analyte molecules to induce their ionization. Volatile compounds such as benzene, toluene, hydrogen sulfide, and CH 2 Cl 2 are commonly used dopants. ,, Despite being effective in enhancing ion yields, these solvents are not suitable for widespread use as they may cause some health and environmental issues. − Some green dopants such as O 2 have also been developed for PI analysis. The reactive O 2 + ions are first generated as precursor ions and then used to initiate selective analyte ionization .…”

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confidence: 99%

Hydrogen-Assisted Photoionization and Its Use in Promoting Mass Spectrometry Analysis of VOCs

Zhu,

Aliang,

Wang

et al. 2023

As a simple soft ionization method, photoionization (PI) is often coupled with mass spectrometry (MS) for the direct analysis of volatile organic compounds (VOCs). PI enables selective ionization of analytes, but the ion yield is generally not high due to the limited light intensity of the ultraviolet lamp. Here, a hydrogenassisted photoionization (HAPI) strategy was developed and integrated into a miniature ion trap mass spectrometer. In particular, hydrogen was introduced as a versatile buffer gas to facilitate both photoionization and ion trap operation. This can increase the ion yields by up to 2 orders of magnitude compared to conventional PI-MS, with a low hydrogen consumption (less than 100 μL) for each analysis. The generation of protonated ions indicates a specific photochemical process in HAPI, which has also been studied and initially revealed. The detection of various VOCs and plant volatile gases confirmed the versatility and practicality of the HAPI technology.