Ultrasensitive detection of volatile aldehydes with chemi-ionization-coupled time-of-flight mass spectrometry

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.

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.

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.

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

A thermal desorption associative ionization time-of-flight mass spectrometer was developed for ultrasensitive detection of semi-volatile chemical warfare agents (CWAs). The excited-state CH₂Cl₂-induced associative ionization method presented a soft ionization characterization and an excellent sensitivity towards CWAs. The detection sensitivities of the investigated nine CWA-related substances were 2.56 × 10⁵-5.01 × 10⁶ counts ng^-1 in a detection cycle (30 s or 100 s). The corresponding 3σ limits of detection (LODs) were 0.08-3.90 pg. Compared with the best-documented LODs via the dielectric barrier discharge ionization (DBDI) and secondary electrospray ionization (SESI), the obtained LODs of the investigated compounds were improved by 2-76 times. Additionally, the measured sensitivity of 2-Chloroethyl ethyl, a proxy for mustard gas, is 550 counts pptv^-1, which exceeds the DBDI and SESI's corresponding values (4.4 counts pptv^-1 and 6.5 counts pptv^-1) nearly by two orders of magnitude. A field application simulation was conducted by putting a strip of PTFE film contaminated with the CWA-related agent into the thermal desorption unit. The simulation showed that the sensitivities of the instrument via swipe surveying could achieve 2.19 × 10⁵ to 5.23 × 10⁶ counts ng^-1. The experimental results demonstrate that the excited-state CH₂Cl₂-induced associative ionization is an ultrasensitive ionization method for CWAs and reveal a prospect for improving the detection of CWA species future.

A Cataluminescence Sensor Based on NiO Nanoparticles for Sensitive Detection of Acetaldehyde

Sensitive and selective detection of harmful gas is an important task in environmental monitoring. In this work, a gas sensor based on cataluminescence (CTL) for detection of acetaldehyde was designed by using nano-NiO as the sensing material. The sensor shows sensitive response to acetaldehyde at a relatively low working temperature of 200 °C. The linear range of CTL intensity versus acetaldehyde concentration is 0.02–2.5 mg/L, with a limit of detection of 0.006 mg/L at a signal-to-noise ratio of three. Mechanism study shows that electronically excited CO₂ is the excited intermediate for CTL emission during the catalytic oxidation of acetaldehyde on the NiO surface. The proposed sensor has promising application in monitoring acetaldehyde in residential buildings and in the workplace.

Array-Assisted SERS Microfluidic Chips for Highly Sensitive and Multiplex Gas Sensing

A novel kind of array-assisted surface-enhanced Raman spectroscopy (SERS) microfluidic chip (ArraySERS chip) is demonstrated for gas sensing, which has the advantages of both ultrahigh sensitivity and multiplex sensing ability. On the one hand, the introduction of a microstructured triangular array can greatly increase the multiple collision probability between gas molecules and sensing interfaces in the channel. Compared with traditional gas sensors using sealed boxes, where gaseous molecules move only by diffusion, the ArraySERS chip exhibits significantly improved sensitivity. On the other hand, a composite nanoparticle is fabricated as a SERS probe for reading out the fingerprint spectral data, which consists of metal-organic framework (MOF) materials [Zeolitic Imidazolate framework-8 (ZIF-8)] and Au@Ag nanocubes, as well as cysteamine (CA) that serves as the gas-capturing agent. The experimental results show that such a structure of the SERS probe can further increase the sensing ability because of better adsorption of ZIF-8 for gas and the lower SERS background of CA itself. In addition, the simultaneous detection of multiplex gases was easily performed according to their own intrinsic SERS signals. Taking aldehyde gas as a model of a typical air pollutant, trace and multicomponent detection was realized using the ArraySERS chip. The limit of detection value was as low as 1 ppb, which is 2 magnitudes lower than that obtained by traditional methods. This strategy can be well extended for the detection of universal gases and help unleash the potential of existing gas sensors, especially for samples at low concentrations in air.

Kinetic Understanding of the Ultrahigh Ionization Efficiencies (up to 28%) of Excited-State CH2Cl2-Induced Associative Ionization: A Case Study with Nitro Compounds

Excited-state CH₂Cl₂-induced associative ionization (AI) is a newly developed ionization method that is very effective for oxygenated organics. However, this method is not widely known. In this study, an unprecedented ionization efficiency and ultrafast reaction rate of AI toward nitro compounds were observed. The ionization efficiencies of o-nitrotoluene (o-NT), m-nitrotoluene (m-NT), and nitrobenzene (NB) were as high as (28 ± 3)%, (27 ± 2)%, and (13 ± 1)%, respectively (∼1-3 ions for every 10 molecules). The measured reaction rate coefficients of these nitroaromatics were (0.5-1.3) × 10^-7 molecule^-1 cm³ s^-1 (∼300 K). These unusual rate coefficients indicated strong long-range interactions between the two neutral reactants, which was regarded as a key factor leading to the ultrahigh ionization efficiency. The detection sensitivities of the nitroaromatics, (1.01-2.16) × 10⁴ counts pptv^-1 in 10 s acquisition time, were obtained by an AI time-of-flight mass spectrometer (AI-TOFMS). These experimental results not only provide new insight into the AI reaction but also reveal an excellent ionization method that can improve the detection sensitivity of nitroaromatics to an unprecedented degree.