Ultrasensitive detection of trace chemical warfare agent-related compounds by thermal desorption associative ionization time-of-flight mass spectrometry

Real-time detection of gaseous isotopic water molecules via photoinduced associative ionization mass spectrometry: Direct identification realized by distinctive mass spectrum patterns

The real-time detection of gaseous H2O and its typical isotopic molecules, e.g., H218O, D2O, HDO, and HTO, is highly desirable in many fundamental scientific studies and practical monitoring, such as mechanistic studies of H2O-involved chemical reactions and radiation risk warning of abnormal HTO emissions. However, ionization methods for mass spectrometry (MS) to directly measure isotopic water molecules are limited, and the discrimination of those with similar molecular weights (MWs) usually requires the use of high-resolution mass spectrometers. In this study, we present a highly efficient ionization method for water vapor based on a photoinduced associative ionization (PAI) reaction, where one excited-state CH2Cl2∗ reacts with two H2O molecules to transfer a proton from one H2O molecule to another (i.e., the analyte). Benefit to the distinctive ionization pathway, different featured ions of H2O, D2O, and H218O were observed. Surprisingly, each isotopic water molecule has its own unique PAI mass spectrum pattern according to the same ionization principle, which offers a convenient method for distinguishing isotopic water molecules with similar MWs, such as D2O, H218O, and HTO (20 Da), without the need for high-resolution mass spectrometers. The measured detection sensitivities of a laboratory-built PAI time-of-flight mass spectrometer towards D2O and H218O were 1465.1 ± 37.0 and 1324.4 ± 86.4 counts ppbv-1, respectively, in a detection time of 10 s. The corresponding 3σ LODs were 0.10 and 0.11 ppbv, respectively. This study provides a novel ionization method for the direct detection and discrimination of isotopic water molecules, which has potential to combine with portable MS for on-site measurements.

Rapid measurement of ethyl carbamate in Chinese liquor by fast gas chromatography photoionization-induced chemical ionization mass spectrometry

As a common by-product during the production of alcoholic beverages, such as Chinese liquor, ethyl carbamate (EC) poses potential genotoxicity and is associated with the risk of various cancers. Hence, rapidly and accurately measuring the content of EC in liquor is critical to assess the product quality and risks of mass samples during the production process. In this study, a feasible method based on fast gas chromatography photoionization-induced chemical ionization mass spectrometry (FastGC-PICI-TOFMS) was developed for the analysis of EC in Chinese liquor. The rapid separation of EC in Chinese liquor was conducted using FastGC based on a thermostatic column set at 150 °C to eliminate the interferences of matrix effects. The PICI-TOFMS could realize accurate quantification of EC without any sample pre-treatment due to the efficient ionization of EC by the PICI source. As a result, the total analysis time for EC in Chinese liquor was less than 4 min. The limit of detection (LOD) for EC was 4.4 μg L-1. And the intra-day and inter-day precision were 3.2%-3.7 % and 1.6 %, respectively. Finally, the ability of the proposed method was preliminarily proved by high-throughput and accurate measurement of EC in four different flavors of Chinese liquors.

A new photoionization-induced substitution reaction chemical ionization time-of-flight mass spectrometry for highly sensitive detection of trace exhaled ethylene

Highly sensitive and rapid detection of ethylene, the smallest alkene of great significance in human physiological metabolism remains a great challenge. In this study, we developed a new photoionization-induced substitution reaction chemical ionization time-of-flight mass spectrometry (PSCI-TOFMS) for trace exhaled ethylene detection. An intriguing ionization phenomenon involving a substitution reaction between the CH2Br2+ reactant ion and ethylene molecule was discovered and studied for the first time. The formation of readily identifiable [CH2Br·C2H4]+ product ion greatly enhanced the ionization efficiency of ethylene, which led to approximately 800-fold improvement of signal intensity over that in single photon ionization mode. The CH2Br2+ reactant ion intensity and ion-molecule reaction time were optimized, and a Nafion tube was employed to eliminate the influence of humidity on the ionization of ethylene. Consequently, a limit of detection (LOD) as low as 0.1 ppbv for ethylene was attained within 30 s at 100 % relative humidity. The application of PSCI-TOFMS on the rapid detection of trace amounts of exhaled ethylene from healthy smoker and non-smoker volunteers demonstrated the satisfactory performance and potential of this system for trace ethylene measurement in clinical diagnosis, atmospheric measurement, and process monitoring.

A Facile Determination of Herbicide Residues and Its Application in On-Site Analysis

Abuse of herbicides in food safety is a vital concern that has an influence on the sustainable development of the world. This work presents, a modified ionization method with separation of the sample and carrier gas inlets, which was utilized for efficient ionization and analyte transfer of herbicides in crops. The working parameters of voltage, injective distance, desorption temperature, and the carrier gas flow rate were optimized to achieve the high efficiency of the transfer and ionization of the analyte. When it was applied in the analysis of herbicides in laboratory, the method exhibited excellent performance in achieving the quantitative detection of herbicides in solutions and residues spiked in an actual matrix with a limit of quantification of 1-20 μg/kg and relative standard deviations of less than 15%. Although a simple QuEchERS process was used, the programmable heating platform ensured efficient gasification and transfer of the target analyte, with the advantages of high speed and selectivity, avoiding the noted matrix effect. The method exhibited a relatively acceptable performance by using air as the discharged gas (open air). It could be used to monitor herbicide residues in the growth stage via on-site non-destructive analysis, which obtained low LODs by dissociating the herbicides from the crops without any pretreatment. It showed great potential for the supervision of the food safety market by achieving non-destructive detection of crops anytime and anywhere. This finding may provide new insights into the determination of pesticide emergence and rice quality assessment.

A high-performance standalone planar FAIMS system for effective detection of chemical warfare agents via TSPSO algorithm

A high-performance standalone planar field asymmetric waveform ion mobility spectrometry (p-FAIMS) system with a deconvolution algorithm (two-step particle swarm optimization algorithm, TSPSO) for overlapping peaks was developed to effectively detect chemical warfare agents (CWAs). Four CWA simulants were applied in this study to systemically evaluate the performance of the standalone p-FAIMS system. The experimental results showed that each CWA simulant in the mixture can be positively identified by carefully comparing the compensation voltage (CV) value of each peak in the FAIMS spectra for the mixture to the ones in the spectra acquired by using the same FAIMS system for the pure CWA simulant standards. The FAIMS spectrum of the CWA simulant mixture might consist of multiple overlapping peaks, which would be difficult to accurately determine the CV value for each CWA simulant peak. This problem has been effectively resolved in this study by deconvoluting the overlapping peaks via the TSPSO algorithm. As the effective peak deconvolution via TSPSO requires the degree of overlap between each FAIMS peak to be lower than a specific value, the flow rate of FAIMS carrier gas was decreased to further improve the resolution of the p-FAIMS system. After the accurate deconvolution, the resolution of original FAIMS spectrum can also be enhanced to achieve baseline separation by using TSPSO algorithm to narrow the peak width of each peak. The experimental results in this study demonstrated the possibility of using TSPSO algorithm to achieve high-resolution on a typically low-resolution standalone FAIMS. The concept in this study can potentially be applied to any low-resolution instruments to achieve high-resolution results.

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

The ion conversion processes in CH2Cl2-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 CH2Cl+ to CH2Cl+(H2O), CH2OH+, and C2H4OH+ and then to the hydrated hydronium ions H3O+(H2O)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 ion-molecule reactions. The intensity of protonated methanol/ethanol (sample S) ion was almost linearly correlated with the intensity of H3O+(H2O)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 step-by-step ion-molecule reactions occurred in the ionization source, i.e., the primary ion CH2Cl+ reacted with H2O and converted to the intermediate product ions CH2OH+ and C2H4OH+, which then further reacted with H2O and led to the production of H3O+, and finally, the protonated sample ion SH+ was obtained through PTR with H3O+, 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.

A Simple High-Flux Switchable VUV Lamp Based on an Electrodeless Fluorescent Lamp for SPI/PAI Mass Spectrometry

Single-photon ionization (SPI) is a unique soft ionization technique for organic analysis. A convenient high-flux vacuum ultraviolet (VUV) light source is a key precondition for wide application of SPI techniques. In this study, we present a novel VUV lamp by simply modifying an ordinary electrodeless fluorescent lamp. By replacing the glass bulb with a stainless steel bulb and introducing 5% Kr/He (v/v) as the excitation gas, an excellent VUV photon flux over 4.0 × 1014 photons s-1 was obtained. Due to its rapid glow characteristics, the VUV lamp can be switched on and off instantly as required by detection, ensuring the stability and service life of the lamp. To demonstrate the performance of the new lamp, the switchable VUV lamp was coupled with an SPI-mass spectrometer, which could be changed to photoinduced associative ionization (PAI) mode by doping gaseous CH2Cl2 to initiate an associative ionization reaction. Two types of volatile organic compounds sensitive to SPI and PAI, typically benzene series and oxygenated organics, respectively, were selected as samples. The instrument exhibited a high detection sensitivity for the tested compounds. With a measurement time of 11 s, the 3σ limits of detection ranged from 0.33 to 0.75 pptv in SPI mode and from 0.03 to 0.12 pptv in PAI mode. This study provides an extremely simple method to assemble a VUV lamp with many merits, e.g., portability, robustness, durability, low cost, and high flux. The VUV lamp may contribute to the development of SPI-related highly sensitive detection technologies.

Direct detection of acetonitrile at the pptv level with photoinduced associative ionization time-of-flight mass spectrometry

Photoionization mass spectrometry (PI-MS) has become a versatile tool in the real-time analysis of volatile organic compounds (VOCs) from the atmosphere or exhaled breath. However, some key species, e.g., acetonitrile, are hard to measure due to their higher ionization energies than photon energy. In this study, the direct and sensitive detection of gaseous acetonitrile based on a photoinduced associative ionization (PAI) reaction was investigated with a laboratory-built PAI time-of-flight mass spectrometer (PAI-TOFMS). By doping CH₂Cl₂ in the photoionization ion source, the mass signal of acetonitrile that cannot be effectively obtained by photoionization appeared with an extremely high intensity through the PAI reaction between acetonitrile, CH₂Cl₂, and residual H₂O in the system. Though the moisture in the sample gas has an evident impact on the detection efficiency of acetonitrile, with a relative signal intensity decreasing from 100% under dry conditions to 60% at saturated relative humidity, excellent detection sensitivity was still obtained for gaseous acetonitrile in different matrixes. The sensitivity calibration experiment showed that the detection sensitivities of acetonitrile in N₂ buffer gas, exhaled gas, and outdoor air were 682.4 ± 5.2, 17.0 ± 0.7, and 23.9 ± 0.2 counts pptv^-1, respectively, with an analysis time of 10 s. The corresponding 3σ LODs reached 0.22, 8.82, and 6.28 pptv, which are equivalent to 0.40, 16.0, and 11.4 ng m^-3. The performance of the PAI-TOFMS was first demonstrated by analyzing exhaled acetonitrile from healthy non-smokers and smokers and continuous monitoring of acetonitrile in outdoor air. In summary, this study provides a new and highly sensitive method for the real-time detection of acetonitrile through mass spectrometry.

Atmospheric pressure thermal desorption chemical ionization mass spectrometry for ultra-sensitive explosive detection

Illegal explosives are a threat to aviation, transport sector, critical infrastructure and generally to public safety. Their detection requires extremely sensitive instruments with efficient workflows that allow large throughput of items. In this study, we built a trace explosives detection instrument that requires minimal sample treatment and reaches ultra-low picogram level detection limits for many common explosives. The instrument is based on thermal desorption of filters, which allows analysis of liquid and solid phase samples, and subsequent selective atmospheric pressure chemical ionization and detection with a mass spectrometer. We performed experiments to scope the optimal ionization chemistry for the system and selected Br^- as the reagent ion, and measured the limit of detection for 14 different explosives that were generally in the picogram range. Finally, we demonstrate the usability of the system by sampling air to a filter from a storage room known to contain explosives, from which we detect four different explosives.

Nebulization Swab Assisted Photoionization Tandem Miniaturized Ion Trap Mass Spectrometry for On-Site Analysis of Nonvolatile Compounds

Nonvolatile compounds usually have a high molecular weight and exhibit a high boiling point, which poses great challenges to the ionization method of MS. Ambient ionization sources can efficiently analyze the nonvolatile compounds without complex pretreatment, but they generally require special media such as heating devices, laser optical devices, or corona needles. Acoustic nebulization assisted photoionization (ANPI) is a potential method for the analysis of nonvolatile compounds that uses nebulization as a prerequisite for photoionization and introduces many advantages of PI, including excellent ionization efficiency, a high yield of molecular ions, and simplified spectrum interpretation. However, the ANPI source can be limited in on-site applications by the complexity of the analytical devices and the high cost of the nebulization chip. To address this issue, in this paper, we explored cheap and commercially piezoelectric materials used in a mist sprayer and fabricated a nebulization swab assisted photoionization (NSAP) as an ambient ionization source. Some useful results are presented: numerical simulation was introduced successfully for optimizing the aerosol distribution in the NSAP source; nonvolatile muscle relaxants, drugs of abuse, antibiotics, phthalates, and cholesterol were detected mostly as their protonated molecular ions while some special acetone/water cluster ions were detected. In addition, the LOD for most of the target analytes ranged from 10.0 to 50.0 pg with RSD ≤ 9%. Finally, this method is implemented for Chinese baijiu spiked with phthalates. The experimental data shows the capability of a NSAP source in high sensitivity and on-site analysis of the nonvolatile compounds.