Direct Detection of Small <i>n</i>‑Alkanes at Sub-ppbv Level by Photoelectron-Induced O₂⁺ Cation Chemical Ionization Mass Spectrometry at kPa Pressure

Direct mass spectrometric measurements of saturated hydrocarbons, especially small <i>n</i>-alkanes, remains a great challenge because of low basicity and lack of ionizable functional groups. In this work, a novel high-pressure photoelectron-induced O₂⁺ cation chemical ionization source (HPPI-OCI) at kPa based on a 10.6 eV krypton lamp was developed for a time-of-flight mass spectrometer (TOFMS). High-intensity O₂⁺ reactant ions were generated by photoelectron ionization of air molecules in the double electric field ionization region. The quasi-molecular ions, [M–H]⁺, of C3–C6 <i>n</i>-alkanes, gradually dominated in the mass spectra when the ion source pressure was elevated from 88 to 1080 Pa, with more than 3 orders of magnitude improvement in signal intensity. As a result, the achieved limits of detection were lowered to 0.14, 0.11, 0.07, and 0.1 ppbv for propane, <i>n</i>-butane, <i>n</i>-pentane, and <i>n</i>-hexane, respectively. The performance of the HPPI-OCI TOFMS was first demonstrated by analysis of exhaled small <i>n</i>-alkanes from healthy smokers and nonsmokers. Then the concentration variations of exhaled small <i>n</i>-alkanes of four healthy volunteers were analyzed after alcohol consumption to explore the alcohol-hepatoxicity-related oxidative stress. In summary, this work provides new insights for controlling the O₂⁺-participating chemical ionization by adjusting the ion source pressure and develops a novel direct mass spectrometric method for sensitive measurements of mall <i>n</i>-alkanes

Long-Term Real-Time Monitoring Catalytic Synthesis of Ammonia in a Microreactor by VUV-Lamp-Based Charge-Transfer Ionization Time-of-Flight Mass Spectrometry

With respect to massive consumption of ammonia and rigorous industrial synthesis conditions, many studies have been devoted to investigating more environmentally benign catalysts for ammonia synthesis under moderate conditions. However, traditional methods for analysis of synthesized ammonia (e.g., off-line ion chromatography (IC) and chemical titration) suffer from poor sensitivity, low time resolution, and sample manipulations. In this work, charge-transfer ionization (CTI) with O₂⁺ as the reagent ion based on a vacuum ultraviolet (VUV) lamp in a time-of-flight mass spectrometer (CTI-TOFMS) has been applied for real-time monitoring of the ammonia synthesis in a microreactor. For the necessity of long-term stable monitoring, a self-adjustment algorithm for stabilizing O₂⁺ ion intensity was developed to automatically compensate the attenuation of the O₂⁺ ion yield in the ion source as a result of the oxidation of the photoelectric electrode and contamination on the MgF₂ window of the VUV lamp. A wide linear calibration curve in the concentration range of 0.2–1000 ppmv with a correlation coefficient (<i>R</i>²) of 0.9986 was achieved, and the limit of quantification (LOQ) for NH₃ was in ppbv. Microcatalytic synthesis of ammonia with three catalysts prepared by transition-metal/carbon nanotubes was tested, and the rapid changes of NH₃ conversion rates with the reaction temperatures were quantitatively measured with a time resolution of 30 s. The high-time-resolution CTI-TOFMS could not only achieve the equilibrium conversion rates of NH₃ rapidly but also monitor the activity variations with respect to investigated catalysts during ammonia synthesis reactions

Photoionization-Generated Dibromomethane Cation Chemical Ionization Source for Time-of-Flight Mass Spectrometry and Its Application on Sensitive Detection of Volatile Sulfur Compounds

Soft ionization mass spectrometry is one of the key techniques for rapid detection of trace volatile organic compounds. In this work, a novel photoionization-generated dibromomethane cation chemical ionization (PDCI) source has been developed for time-of-flight mass spectrometry (TOFMS). Using a commercial VUV lamp, a stable flux of CH₂Br₂⁺ was generated with 1000 ppmv dibromomethane (CH₂Br₂) as the reagent gas, and the analytes were further ionized by reaction with CH₂Br₂⁺ cation via charge transfer and ion association. Five typical volatile sulfur compounds (VSCs) were chosen to evaluate the performance of the new ion source. The limits of detection (LOD), 0.01 ppbv for dimethyl sulfide and allyl methyl sulfide, 0.05 ppbv for carbon disulfide and methanthiol, and 0.2 ppbv for hydrogen sulfide were obtained. Compared to direct single photon ionization (SPI), the PDCI has two distinctive advantages: first, the signal intensities were greatly enhanced, for example more than 10-fold for CH₃SH and CS₂; second, H₂S could be measured in PDCI by formation [H₂S + CH₂Br₂]⁺ adduct ion and easy to recognize. Moreover, the rapid analytical capacity of this ion source was demonstrated by analysis of trace VSCs in breath gases of healthy volunteers and sewer gases

Dopant-Assisted Negative Photoionization Ion Mobility Spectrometry for Sensitive Detection of Explosives

Ion mobility spectrometry (IMS) is a key trace detection technique for explosives and the development of a simple, stable, and efficient nonradioactive ionization source is highly demanded. A dopant-assisted negative photoionization (DANP) source has been developed for IMS, which uses a commercial VUV krypton lamp to ionize acetone as the source of electrons to produce negative reactant ions in air. With 20 ppm of acetone as the dopant, a stable current of reactant ions of 1.35 nA was achieved. The reactant ions were identified to be CO₃^–(H₂O)_<i>n</i> (<i>K</i>₀ = 2.44 cm² V^–1 s^–1) by atmospheric pressure time-of-flight mass spectrometry, while the reactant ions in ⁶³Ni source were O₂^–(H₂O)_<i>n</i> (<i>K</i>₀ = 2.30 cm² V^–1 s^–1). Finally, its capabilities for detection of common explosives including ammonium nitrate fuel oil (ANFO), 2,4,6-trinitrotoluene (TNT), <i>N</i>-nitrobis­(2-hydroxyethyl)­amine dinitrate (DINA), and pentaerythritol tetranitrate (PETN) were evaluated, and the limits of detection of 10 pg (ANFO), 80 pg (TNT), and 100 pg (DINA) with a linear range of 2 orders of magnitude were achieved. The time-of-flight mass spectra obtained with use of DANP source clearly indicated that PETN and DINA can be directly ionized by the ion-association reaction of CO₃^– to form PETN·CO₃^– and DINA·CO₃^– adduct ions, which result in good sensitivity for the DANP source. The excellent stability, good sensitivity, and especially the better separation between the reactant and product ion peaks make the DANP a potential nonradioactive ionization source for IMS

Fast Switching of CO₃^–(H₂O)_<i>n</i> and O₂^–(H₂O)_<i>n</i> Reactant Ions in Dopant-Assisted Negative Photoionization Ion Mobility Spectrometry for Explosives Detection

Ion mobility spectrometry (IMS) has become the most deployed technique for on-site detection of trace explosives, and the reactant ions generated in the ionization source are tightly related to the performances of IMS. Combination of multiform reactant ions would provide more information and is in favor of correct identification of explosives. Fast switchable CO₃^–(H₂O)_<i>n</i> and O₂^–(H₂O)_<i>n</i> reactant ions were realized in a dopant-assisted negative photoionization ion mobility spectrometer (DANP-IMS). The switching could be achieved in less than 2 s by simply changing the gas flow direction. Up to 88% of the total reactant ions were CO₃^–(H₂O)_<i>n</i> in the bidirectional mode, and 89% of that were O₂^–(H₂O)_<i>n</i> in the unidirectional mode. The characteristics of combination of CO₃^–(H₂O)_<i>n</i> and O₂^–(H₂O)_<i>n</i> were demonstrated by the detection of explosives, including 2,4,6-trinitrotoluene (TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), ammonium nitrate fuel oil (ANFO), and black powder (BP). For TNT, RDX, and BP, product ions with different reduced mobility values (<i>K</i>₀) were observed with CO₃^–(H₂O)_<i>n</i> and O₂^–(H₂O)_<i>n</i>, respectively, which is a benefit for the accurate identification. For ANFO, the same product ions with <i>K</i>₀ of 2.07 cm² V^–1 s^–1 were generated, but improved peak-to-peak resolution as well as sensitivity were achieved with CO₃^–(H₂O)_<i>n</i>. Moreover, an improved peak-to-peak resolution was also obtained for BP with CO₃^–(H₂O)_<i>n</i>, while the better sensitivity was obtained with O₂^–(H₂O)_<i>n</i>

High-Pressure Photon Ionization Source for TOFMS and Its Application for Online Breath Analysis

Photon ionization mass spectrometry (PI-MS) is a widely used technique for the online detection of trace substances in complex matrices. In this work, a new high-pressure photon ionization (HPPI) ion source based on a vacuum ultraviolet (VUV) Kr lamp was developed for time-of-flight mass spectrometry (TOFMS). The detection sensitivity was improved by elevating the ion source pressure to about 700 Pa. A radio frequency (RF)-only quadrupole was employed as the ion guide system following the HPPI source to achieve high ion transmission efficiency. In-source collision induced dissociation (CID) was conducted for accurate chemical identification by varying the voltage between the ion source and the ion guide. The high humidity of the breath air can promote the detection of some compounds with higher ionization potentials (IPs) that could not be well detected by single photon ionization (SPI) at low pressure. Under 100% relative humidity (37 °C), the limits of detection down to 0.015 ppbv (parts per billion by volume) for aliphatic and aromatic hydrocarbons were obtained. This HPPI-TOFMS system was preliminarily applied for online investigations of the exhaled breath from both healthy nonsmoker and smoker subjects, demonstrating its analytical capacity for complicated gases analysis. Subsequently, several frequently reported VOCs in the breath of healthy volunteers, i.e., acetone, isoprene, 2-butanone, ethanol, acetic acid, and isopropanol, were successfully identified and quantified

Sensitive Detection of Black Powder by a Stand-Alone Ion Mobility Spectrometer with an Embedded Titration Region

Sensitive detection of black powder (BP) by stand-alone ion mobility spectrometry (IMS) is full of challenges. In conventional air-based IMS, overlap between the reactant ion O₂^–(H₂O)_<i>n</i> peak and the sulfur ion peak occurs severely; and common doping methods, providing alternative reactant ion Cl^–(H₂O)_<i>n</i>, would hinder the formation of ionic sulfur allotropes. In this work, an ion mobility spectrometer embedded with a titration region (TR-IMS) downstream from the ionization region was developed for selective and sensitive detection of sulfur in BP with CH₂Cl₂ as the titration reagent. Sulfur ions were produced via reactions between sulfur molecules and O₂^–(H₂O)_<i>n</i> ions in the ionization region, and the remaining O₂^–(H₂O)_<i>n</i> ions that entered the titration region were converted to Cl^–(H₂O)_<i>n</i> ions, which avoided the peak overlap as well as the negative effect of CH₂Cl₂ on sulfur ions. The limit of detection for sulfur was measured to be 5 pg. Furthermore, it was demonstrated that this TR-IMS was qualified for detecting less than 5 ng of BP and other nitro-organic explosives