Rapid and highly sensitive measurement of trimethylamine in seawater using dynamic purge-release and dopant-assisted atmospheric pressure photoionization mass spectrometry

Improving the Sensitivity and Linear Range of Photoionization Ion Mobility Spectrometry via Confining the Ion Recombination and Space Charge Effects Assisted by Theoretical Modeling

Photoionization (PI) is an efficient ionization source for ion mobility spectrometry (IMS) and mass spectrometry. Its hyphenation with IMS (PI-IMS) has been employed in various on-site analysis scenarios targeting a wide range of compounds. However, the signal intensity and linear dynamic range of PI-IMS at ambient pressure usually do not follow the Beer-Lambert law predictions, and the factors causing that negative deviation remain unclear. In this work, a variable pressure PI-IMS system was developed to examine the ion loss effects from factors like ion recombination and space charge by varying its working pressure from 1 to 0.1 bar. Assisted by theoretical modeling, it was found that ion recombination could contribute up to 90% of signal intensity loss for ambient pressure PI-IMS setups. Lowering the pressure and increasing the electric field in PI-IMS helped suppress the ion recombination process and thus an optimal pressure Poptimal appeared for best signal intensity, despite the decreased net ion number density and the increased space charge effect. A simplified theoretical equation taking ion recombination as the primary ion loss factor was derived to link Poptimal with analyte concentration and electric field in PI-IMS, enabling a swift optimization of the PI-IMS performance. For example, compared to ambient pressure, PI-IMS at a Poptimal of 0.4 bar provided a signal intensity increment of more than 400% for 0.716 ppmv toluene and also expanded the linear dynamic range by more than two times. Revealing factors influencing the PI-IMS response would also benefit the applications of other chemical ionization sources in IMS or mass spectrometry (MS).

Contribution of marine biological emissions to gaseous methylamines in the atmosphere: An emission inventory based on multi-source data sets

Methylamines are a class of highly reactive organic alkaline gases in the atmosphere. At present, the gridded emission inventories of amines used in the atmospheric numerical model is mostly based on the amine/ammonia ratio method and do not consider the air-sea exchange of methylamines, which oversimplifies the emission scenario. Marine biological emissions (MBE), an important source of methylamines, has been insufficiently investigated. These shortcomings in the inventories can limit the simulation of amines by numerical models in the context of compound pollution in China. To acquire a more complete gridded inventory of amines (monomethylamine (MMA), dimethylamines (DMA), and trimethylamines (TMA)), we established a more reasonable MBE inventory of amines by using multi-source data sets (Sea Surface Temperature (SST), Chlorophyll-a (Chla), Sea Surface Salinity (SSS), NH3 column concentration (NH3), and Wind Speed (WS)), and merged it with the anthropogenic emissions (AE) inventory (by adopting the amine/ammonia ratio method and the Multi-resolution Emission Inventory for China (MEIC)). The new methodology can reveal the air-sea exchange fluxes and direction of different amines. Oceans can act as a sink for DMA and source for TMA while it can be either a source or sink for MMA. The concentration of amines above the coastal area increased significantly when the MBE was merged to the AE inventory. TMA and MMA showed significant increases, TMA increased by 43,917.0 %, and 804.0 %, in July 2015 and December 2019, respectively; while MMA increased by 2635.4 % and 0.37 % during the same periods; however, only slight changes were observed in the DMA concentration (-3.9 % in July 2015, and 1.1 % in December 2019). WS, Chla, and the total dissolved concentration of amines ([C+(s)tot]) were found to be the dominant factors affecting MBE fluxes. In addition, the emission fluxes and spatial distribution of AE, and wet deposition also affect the simulation of amines concentration.

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.

Rapid Detection of Volatile Organic Metabolites in Urine by High-Pressure Photoionization Mass Spectrometry for Breast Cancer Screening: A Pilot Study

Despite surpassing lung cancer as the most frequently diagnosed cancer, female breast cancer (BC) still lacks rapid detection methods for screening that can be implemented on a large scale in practical clinical settings. However, urine is a readily available biofluid obtained non-invasively and contains numerous volatile organic metabolites (VOMs) that offer valuable metabolic information concerning the onset and progression of diseases. In this work, a rapid method for analysis of VOMs in urine by using high-pressure photon ionization time-of-flight mass spectrometry (HPPI-TOFMS) coupled with dynamic purge injection. A simple pretreatment process of urine samples by adding acid and salt was employed for efficient VOM sampling, and the numbers of metabolites increased and the detection sensitivity was improved after the acid (HCl) and salt (NaCl) addition. The established mass spectrometry detection method was applied to analyze a set of training samples collected from a local hospital, including 24 breast cancer patients and 27 healthy controls. Statistical analysis techniques such as principal component analysis, partial least squares discriminant analysis, and the Mann-Whitney U test were used, and nine VOMs were identified as differential metabolites. Finally, acrolein, 2-pentanone, and methyl allyl sulfide were selected to build a metabolite combination model for distinguishing breast cancer patients from the healthy group, and the achieved sensitivity and specificity were 92.6% and 91.7%, respectively, according to the receiver operating characteristic curve analysis. The results demonstrate that this technology has potential to become a rapid screening tool for breast cancer, with significant room for further development.

Spatiotemporal variations, surface inventory, and cross regional impact of current-use organoamine pesticides in Chinese Marginal Seas

Current-use Organoamine Pesticides (CUOAPs) are a growing concern as emerging pesticide pollutants that were initially discovered on a large scale in Chinese Marginal Seas (CMSs). The highest level was detected in the East China Sea in the late spring and decreased in the following order: East China Sea (early spring) > the South China Sea > the Bohai Sea. The crucial role played by the Yangtze and Yellow rivers as significant terrestrial sources were established. The fluctuations in the land application and riverine input led to variations in the source, distribution, and seasonal patterns of CUOAPs. Terrestrial-exported CUOAPs were transported and redistributed spatially by the surface ocean currents, resulting in significant regional disparities. The results displayed a pronounced terrestrial source signature and a cross-regional impact. By the ocean current transport, CMSs will likely become a secondary source region for the surrounding seas.

Real-time monitoring of atmospheric ammonia based on modifier-enhanced vacuum ultraviolet photoionization ion mobility spectrometry

Ammonia (NH₃) plays an important role in the atmospheric environment such as the formation of PM_2.5, the concentration monitoring of which could hence help in the air quality assessment. In this study, a method for quantitative monitoring of atmospheric NH₃ was developed based on modifier-enhanced selectivity detection using a homemade vacuum ultraviolet photoionization ion mobility spectrometry (VUV-PI-IMS). To enhance the resolution and sensitivity of measuring NH₃, 2-butanone as the gas modifier was introduced into the drift tube with the drift gas. Atmospheric NH₃ can be selectively detected, where the peak-to-peak resolution (R_P-P) of 7.69 was obtained. The product ions were identified to be [C₄H₈O]₂NH₄⁺ by using a homemade time-of-flight mass spectrometer. The calculated limit of detection (LOD) was 0.39 ppbv improving about 10 times. For the most common concentration variation of atmospheric NH₃ in the range of 10-100 ppbv, the linear curve was obtained with R² of 0.997. Lastly, the VUV-PI-IMS was used to monitor the evolution of atmospheric NH₃ near our laboratory and mounted on a car for monitoring the regional distribution of atmospheric NH₃ in Dalian, China. The results also showed that VUV-PI-IMS has a promising application prospect in monitoring the concentration of atmospheric NH₃ and supporting the air quality assessment.

Rapid Determination of Ethyl Carbamate in Chinese Liquor via a Direct Injection Mass Spectrometry with Time-Resolved Flash-Thermal-Vaporization and Acetone-Assisted High-Pressure Photoionization Strategy

Ethyl carbamate (EC), a carcinogenic compound, is naturally produced in fermented foods and alcoholic beverages. Rapid and accurate measurement of EC is necessary and important for quality control and safety evaluation of Chinese liquor, a traditionally distilled spirit with the highest consumption in China, but it remains a great challenge. In this work, a direct injection mass spectrometry (DIMS) with time-resolved flash-thermal-vaporization (TRFTV) and acetone-assisted high-pressure photoionization (HPPI) strategy has been developed. EC was rapidly separated from the main matrix components, ethyl acetate (EA) and ethanol, by the TRFTV sampling strategy due to the retention time difference of these three compounds with large boiling point differences on the inner wall of a poly(tetrafluoroethylene) (PTFE) tube. Therefore, the matrix effect of EA and ethanol was effectively eliminated. The acetone-assisted HPPI source was developed for efficient ionization of EC through a photoionization-induced proton transfer reaction between EC molecules and protonated acetone ions. The accurate quantitative analysis of EC in liquor was achieved by introducing an internal standard method (ISM) using deuterated EC (d₅-EC). As a result, the limit of detection (LOD) for EC was 8.88 μg/L with the analysis time of only 2 min, and the recoveries ranged from 92.3 to 113.1%. Finally, the prominent capability of the developed system was demonstrated by rapid determination of trace EC in Chinese liquors with different flavor types, exhibiting wide potential applications in online quality control and safety evaluation of not only Chinese liquors but also other liquor and alcoholic beverages.

The determination of volatile amines in aquatic marine systems: A review

This review provides a critical assessment of knowledge regarding the determination of volatile, low molecular weight amines, and particularly methylamines, in marine aquatic; systems. It provides context for the motivation to determine methylamines in the marine aquatic environment and the analytical challenges associated with their measurement.While sensitive analytical methods have been reported in recent decades, they have not been adopted by the oceanographic community to investigate methylamines' biogeochemistry and advance understanding of these analytes to the degree achieved for other marine volatiles. Gas chromatography, high performance liquid chromatography, ion chromatography and infusion-mass spectrometry techniques are discussed and critically determined, alongside offline and online preconcentration steps. Interest in the marine occurrence and cycling of methylamines has increased within the last 10-15 years, due to their potential role in climate regulation. As such, the need for robust, reproducible methods to elucidate biogeochemical cycles for nitrogen and populate marine models is apparent. Recommendations are made as to what equipment would be most suitable for future research in this area.

Hexapole-Assisted Continuous Atmospheric Pressure Interface for a High-Pressure Photoionization Miniature Ion Trap Mass Spectrometer

Miniature mass spectrometers are powerful tools for on-site chemical analysis in the fields of homeland security, personal healthcare, and environmental monitoring. This study presents a novel hexapole-assisted continuous atmospheric pressure interface for a high-pressure photoionization miniature ion trap mass spectrometer (HA-HPPI-IT). Efficient ion transmission was achieved by combining radial focusing by an RF electric field and axial driving by gas flow, which was demonstrated by SIMION simulation and experimental verification. The pressure in the ionization-transmission chamber and the inner diameter of the skimmer were optimized, which helped in determining the number density of product ions and affected the ion transmission in the hexapole, respectively. After systematic optimizations, about 16-fold increase in signal intensity was achieved as the RF amplitude was varied from 140 to 400 V_pp, and a limit of detection of 1 ppbv was obtained. In addition, the HA-HPPI-IT exhibited high stability and the relative standard deviation was as low as 5.47%. Finally, the apparatus was applied for discovering the simulated spot for illicit drug synthesis by detecting toluene and propiophenone released to air and monitoring the evolutions of perchloroethylene residues from dry-cleaned clothes.

Protonated acetone ion chemical ionization time-of-flight mass spectrometry for real-time measurement of atmospheric ammonia

Ammonia (NH₃) is ubiquitous in the atmosphere, it can affect the formation of secondary aerosols and particulate matter, and cause soil eutrophication through sedimentation. Currently, the use of radioactive primary reagent ion source and the humidity interference on the sensitivity and stability are the two major issues faced by chemical ionization mass spectrometer (CIMS) in the analysis of atmospheric ammonia. In this work, a vacuum ultraviolet (VUV) Kr lamp was used to replace the radioactive source, and acetone was ionized under atmospheric pressure to obtain protonated acetone reagent ions to ionize ammonia. The ionization source is designed as a separated three-zone structure, and even 90 vol.% high-humidity samples can still be directly analyzed with a sensitivity of sub-ppbv. A signal normalization processing method was designed, and with this new method, the quantitative relative standard deviation (RSD) of the instrument was decreased from 17.5% to 9.1%, and the coefficient of determination was increased from 0.8340 to 0.9856. The humidity correction parameters of the instrument were calculated from different humidity, and the ammonia concentrations obtained under different humidity were converted to its concentration under zero humidity condition with these correction parameters. The analytical time for a single sample is only 60 sec, and the limit of detection (LOD) was 8.59 pptv (signal-to-noise ratio S/N = 3). The ambient measurement made in Qingdao, China, in January 2021 with this newly designed CIMS, showed that the concentration of ammonia ranged from 1 to 130 ppbv.

Photoionization-induced NO+ chemical ionization time-of-flight mass spectrometry for rapid measurement of aldehydes and benzenes in vehicles

In-vehicle air pollution has become a major concern to public health in recent years. The traditional analytical methods for detection of volatile organic compounds (VOCs) pollutants in air are based on gas chromatography - mass spectrometry (GC-MS) or high-performance liquid chromatography (HPLC), including complicated pretreatment and separation procedures, which are not only time-consuming and labor-intensive, but also incapable of simultaneously measuring both aldehydes and benzenes. In this work, a new photoionization-induced NO⁺ chemical ionization time-of-flight mass spectrometry (PNCI-TOFMS) was developed for real-time and continuous measurement of aldehydes and benzenes in vehicles. High-intensity NO⁺ reactant ions could be generated by photoionization of NO reagent gas, and efficient chemical ionization between NO⁺ reactant ions and analyte molecules occurred to produce adduct ions M·NO⁺ at an elevated ion source pressure of 800 Pa. Consequently, the achieved LODs for aldehydes and benzenes were down to sub-ppbv within 60 s. The analytical capacity of this system was demonstrated by continuous and online monitoring of in-vehicle VOCs in a used car, exhibiting broad potential applications of the PNCI-TOFMS in air pollutants monitoring and in-vehicle air quality analysis.