Atmospheric Pressure Chemical Ionization Source. 1. Ionization of Compounds in the Gas Phase

Soft ionization mechanisms in flexible µ-tube plasma-elucidation of He-, Ar-, Kr-, and Xe-FµTP

The soft ionization mechanism of helium-based plasma seems to be understood while it still remains challenging in argon-based plasma, although many studies have used argon plasmas as a soft ionization source with good ionization efficiencies. In this study, helium, argon, krypton, and xenon were fed into the same discharge geometry, a flexible micro-tube plasma (FµTP), to determine the ionization mechanisms. The FµTPs operated with the named noble gases obtained comparable ionization efficiencies by MS measurements. The optical emission results showed that N2+ were the dominant ions within the helium-FµTP and noble gas ions were dominant for the other plasmas. These ions support the development of excitation and eventually stop at the end of the capillary. Therefore, Penning ionization and charge transfer between plasma and ambient air/analytes in the open atmosphere have been proven not to be the primary soft ionization mechanism. Furthermore, it was found that photoionization played a minor role in soft ionization. Using helium as a diagnosis gas in front of the discharge capillary nozzle of the FµTP, where the sample is usually positioned, shows that helium can be ignited by all of these FµTPs. This demonstrates that the excitation of a diagnosis gas as well as the ionization of analytes is independent of the type of the discharge gas. An alternative mechanism that a transient potential created by the ions is responsible for the soft ionization is subsequently proposed.

Profiling of carbonyl metabolic fingerprints in urine of Graves' disease patients based on atmospheric ionization mass spectrometry

Graves' disease (GD) is considered among the organ autoimmune diseases and is somewhat linked to other autoimmune and secondary diseases. Commonly used detection methods rely on identifying characteristic clinical features and abnormal biochemical markers, but they have certain limitations and may be affected by patient medication. In this study, a desorption separation ionization (DSI) device coupled with a linear ion trap mass spectrometer is introduced for effective detection and screening of urine from GD patients. To enhance the sensitivity of MS analysis, derivatization reagent is utilized as a labeling method. The MS signal is used for metabolic profiling, through which differential metabolites and pathways are identified. Subsequently, processing the acquired spectra with a machine learning algorithm enables successful differentiation of GD patients and healthy individuals. This method is believed to provide versatile and powerful technical support for effective detection on the scene. Notably, this method offers the advantage of achieving early and rapid diagnosis of thyroid-related diseases.

Characterization of arsenic species by liquid sampling-atmospheric pressure glow discharge ionization mass spectrometry

A liquid sampling-atmospheric pressure glow discharge (LS-APGD) ionization source operating at a nominal power of 30 W and solution flow rate of 30 µL min-1 and supported in a He sheath gas flow rate of 500 mL min-1 was interfaced to an Orbitrap mass spectrometer and assessed for use in rapid identification of inorganic and organic arsenic species, including As(III), As(V), monomethylarsonic acid, dimethylarsinic acid, and arsenobetaine in a 2% (v/v) nitric acid medium. Mass spectral acquisition in low-resolution mode, using only the ion trap analyzer, provided detection of protonated molecular ions for AsBet (m/z 179), DMA (m/z 139), MMA (m/z 141), and As(V) (m/z 143). As(III) is oxidized to As(V), likely due to in-source processes. Typical fragmentation of these compounds resulted in the loss of either water or methyl groups, as appropriate, i.e., introducing DMA also generated ions corresponding to MMA and As(V) as dissociation products. Structure assignments were also confirmed by high-resolution Orbitrap measurements. Spectral fingerprint assignments were based on the introduction of solutions containing 5 µg mL-1 of each arsenic compound.

Desorption Separation Ionization Mass Spectrometry (DSI-MS) for Rapid Analysis of COVID-19

During the coronavirus disease 2019 (COVID-19) pandemic, which has witnessed over 772 million confirmed cases and over 6 million deaths globally, the outbreak of COVID-19 has emerged as a significant medical challenge affecting both affluent and impoverished nations. Therefore, there is an urgent need to explore the disease mechanism and to implement rapid detection methods. To address this, we employed the desorption separation ionization (DSI) device in conjunction with a mass spectrometer for the efficient detection and screening of COVID-19 urine samples. The study encompassed patients with COVID-19, healthy controls (HC), and patients with other types of pneumonia (OP) to evaluate their urine metabolomic profiles. Subsequently, we identified the differentially expressed metabolites in the COVID-19 patients and recognized amino acid metabolism as the predominant metabolic pathway involved. Furthermore, multiple established machine learning algorithms validated the exceptional performance of the metabolites in discriminating the COVID-19 group from healthy subjects, with an area under the curve of 0.932 in the blind test set. This study collectively suggests that the small-molecule metabolites detected from urine using the DSI device allow for rapid screening of COVID-19, taking just three minutes per sample. This approach has the potential to expand our understanding of the pathophysiological mechanisms of COVID-19 and offers a way to rapidly screen patients with COVID-19 through the utilization of machine learning algorithms.

Mass spectrometry of polymers: A tutorial review

Ever since the inception of synthetic polymeric materials in the late 19th century, the number of studies on polymers as well as the complexity of their structures have only increased. The development and commercialization of new polymers with properties fine-tuned for specific technological, environmental, consumer, or biomedical applications requires powerful analytical techniques that permit the in-depth characterization of these materials. One such method with the ability to provide chemical composition and structure information with high sensitivity, selectivity, specificity, and speed is mass spectrometry (MS). This tutorial review presents and exemplifies the various MS techniques available for the elucidation of specific structural features in a synthetic polymer, including compositional complexity, primary structure, architecture, topology, and surface properties. Key to every MS analysis is sample conversion to gas-phase ions. This review describes the fundamentals of the most suitable ionization methods for synthetic materials and provides relevant sample preparation protocols. Most importantly, structural characterizations via one-step as well as hyphenated or multidimensional approaches are introduced and demonstrated with specific applications, including surface sensitive and imaging techniques. The aim of this tutorial review is to illustrate the capabilities of MS for the characterization of large, complex polymers and emphasize its potential as a powerful compositional and structural elucidation tool in polymer chemistry.

Gas-Phase Alcoholysis of Benzylic Halides in the Atmospheric Pressure Ionization Source

The present study investigates the gas-phase alcoholysis reaction of benzylic halides under atmospheric pressure chemical ionization (APCI) conditions. The APCI corona discharge is used to initiate the novel reaction, which is monitored by ion trap mass spectrometry (IT-MS). The model compound α,α,α-trifluorotoluene is applied to observe the cascade methoxylation reaction during the +APCI-MS analysis, resulting in the formation of [PhC(OCH3)2]+. Based on the results of isotopic labeling and substrate expansion experiments, an addition-elimination mechanism is proposed: initially, the reaction was initiated by the dissociation of fluorine from PhCF3 under APCI condition, leading to the formation of [PhCF2]+; subsequently, two methanol molecules nucleophilicly attack [PhCF2]+ stepwisely, accompanied by the elimination of HF, yielding the product ion [PhC(OCH3)2]+. The proposed mechanism was further corroborated by theoretical calculations. The results of substrate scope expansion experiments suggest that this in-source reaction has the potential to differentiate the positional isomers of alcohols and phenols.

Utilizing a Metal Inlet Coiled with Copper Wire as the Ion Source for Ambient Ionization Mass Spectrometry

In ambient ionization mass spectrometry (MS), a customized metal inlet is typically adapted to the orifice of the mass spectrometer for ease of introduction of the sample. We herein explore that the metal inlet coiled with a copper wire (∼50 μm) can be directly used as an ion source to induce corona discharge-like processes for ionization of analytes in the gas phase. When the metal inlet is subjected to a high voltage in the mass spectrometer, the electric field provided by the mass spectrometer enables the generation of corona discharge to ionize volatile/semivolatile analytes derived from the sample in the condensed phase. The limit of detection for azulene derived from the aqueous sample was as low as ∼1 pM. Moreover, we also demonstrated the feasibility of coupling ultraviolet-visible absorption spectroscopy with MS by using the metal inlet coiled with a thin copper wire as the interface. Integration of these two techniques enables the simultaneous acquisition of spectra from both instruments for quantitative and qualitative analysis of the sample. Furthermore, we showed that polar and nonpolar analytes in a mixture can be acquired in the same mass spectrum by simply depositing a sample droplet (∼20 μL) on a dielectric substrate near the copper wire-coiled metal inlet of the mass spectrometer. The ionization processes involved both electrospray ionization and corona discharge. To demonstrate the applicability of our method for detecting nonpolar and polar analytes in complex samples, we spiked a nonpolar analyte, benzo[a]pyrene, to a spice sample and successfully detected analytes with different polarities using our approach.

Improvement in the performance of focusing plasma desorption ionization by altering its counter electrode

RATIONALE

Plasma-based ionization sources play a vital role in rapidly analyzing diverse compounds without extensive sample pretreatment. In contrast to other sources, DC voltage-based ionizations are more advantageous due to their high analytical sensitivity and good tandem with commercially available mass spectrometers without extra power supplies. However, their performance is at the risk of high current DC voltage and helium flow rate, which poses significant challenges to practical operation and increased expense.

METHODS

In this work, we propose a novel focusing plasma desorption ionization (FPDI) in which a visible plasma beam is favorably generated between a conducting wire in a polymeric tube and a counter electrode composed of metal mesh and filter paper drilled with holes. A systematic investigation has been conducted on the influences of the geometry of drilled holes in filter paper, applied DC voltage, helium flow rate, and filter paper size. The optimized system is used to analyze various pesticides in fluid foodstuffs.

RESULTS

Compared to metal mesh and conducting paper as the counter electrode for FPDI-MS, combining metal mesh and filter paper drilled holes improved the analysis sensitivity by a factor of more than five. By applying the developed protocol for determining pesticides in complex matrixes such as orange juice and milk, a limit of detection as low as 1.3-3.0 ng mL-1 could be achieved.

CONCLUSIONS

A novel FPDI-MS technique has been developed by combining metal mesh and filter paper drilled with holes as the counter electrode and sample carrier. The corresponding improvement in analysis sensitivity facilitates the future expansion of FPDI-MS applications into different pesticides and other compounds in complex matrixes.

Applications of mass spectrometry in cosmetic analysis: An overview

Mass spectrometry (MS) is a crucial tool in cosmetic analysis. It is widely used for ingredient screening, quality control, risk monitoring, authenticity verification, and efficacy evaluation. However, due to the diversity of cosmetic products and the rapid development of MS-based analytical methods, the relevant literature needs a more systematic collation of information on this subject to unravel the true potential of MS in cosmetic analysis. Herein, an overview of the role of MS in cosmetic analysis over the past two decades is presented. The currently used sample preparation methods, ionization techniques, and types of mass analyzers are demonstrated in detail. In addition, a brief perspective on the future development of MS for cosmetic analysis is provided.

Colorimetric Signal Readout for the Detection of Volatile Organic Compounds Using a Printable Glass-Based Dielectric Barrier Discharge-Type Helium Plasma Detector

In this paper, we report on a printable glass-based manufacturing method and a new proof-of-concept colorimetric signal readout scheme for a dielectric barrier discharge (DBD)-type helium plasma photoionization detector. The sensor consists of a millimeter-sized glass chamber manufactured using a printable glass suspension. Plasma inside the chip is generated using a custom-built power supply (900 V and 83.6 kHz), and the detector uses ∼5 W of power. Our new detection scheme is based on detecting the change in the color of plasma after the introduction of target gases. The change in color is first captured by a smartphone camera as a video output. The recorded video is then processed and converted to an image light intensity vs retention time plot (gas chromatogram) using three standard color space models (red, green, blue (RGB), hue, saturation, lightness (HSL), and hue, saturation, value (HSV)) with RGB performing the best among the three models. We successfully detected three different categories of volatile organic compounds using our new detection scheme and a 30-m-long gas chromatography column: (1) straight-chain alkanes (n-pentane, n-hexane, n-heptane, n-octane, and n-nonane), (2) aromatics (benzene, toluene, and ethylbenzene), and (3) polar compounds (acetone, ethanol, and dichloromethane). The best limit of detection of 10 ng was achieved for benzene at room temperature. Additionally, the device showed excellent performance for different types of sample mixtures consisting of three and five compounds. Our new detector readout method combined with our ability to print complex glass structures provides a new research avenue to analyze complex gas mixtures and their components.

Chemical analysis of electronic cigarette liquids (e-liquids) and direct nicotine quantitation using surface-assisted flowing atmospheric-pressure afterglow desorption/ionization mass spectrometry (SA-FAPA-MS)

Ambient desorption/ionization mass spectrometry (ADI-MS) has been widely used for direct analysis of real samples without sample preparation or separation. Studies on the quantification of low molecular weight compounds in complex matrices with ADI-MS remain scarce. In this paper, we report the application of surface-assisted flowing atmospheric-pressure afterglow mass spectrometry (SA-FAPA-MS) for fast qualitative screening of electronic cigarette liquid (e-liquids) ingredients and direct quantification of nicotine. The quantification approach is rapid, uses a deuterated D4-nicotine standard spike, and does not require a preceding chromatography step or other methods to remove the complex sample matrix. Selected e-liquids were directly applied on thin-layer chromatography (TLC) plate surfaces (normal phase (NP) silica, reversed phase (RP) modified silica, cyano (CN) modified silica, and dimethyl (RP2) modified silica) after dilution and internal standard spiking. The plates served purely as sample carriers and no analyte separation was performed. Promising qualitative results were obtained, demonstrating the ability to detect nicotine alkaloids using this approach and the ability to differentiate e-liquids based on their flavor variations. In addition, dimethyl- (RP2-) and cyano-modified (CN-) silica surfaces were selected for quantification based on performance results of previous studies. It was shown that results were in high accordance with high-performance liquid chromatography (HPLC) experiments with lowest deviations <3% on dimethyl surfaces. Additional quantitative experiments including a certified reference material achieved equally satisfying results with lowest deviations of -1.1% from the certified nicotine content. For nicotine, detection limits down to the fmol range (96 fmol on CN and 20 fmol on RP2) were obtained. A detailed comparison of glass surfaces with functionalized surfaces showed that the functionalized surfaces were superior in terms of sample application reproducibility, mass spectra quality, sensitivity, and information density. Thus, functionalized thin-layer surfaces are considered promising tools for both qualitative and quantitative ADI-MS analysis of complex samples.

Effect of Sample Plates and Sample Matrix on the Quantification Capabilities of Surface-Assisted Flowing Atmospheric-Pressure Afterglow Mass Spectrometry (SA-FAPA-MS)

Ambient desorption/ionization mass spectrometry (ADI-MS) has been broadly applied to accomplish direct analysis without sample preparation or separation. However, quantification capabilities and analytical performance are sometimes limited. Here, we report signal enhancement effects and improved quantification capabilities in plasma-based ADI-MS, when a flowing atmospheric-pressure afterglow (FAPA) source is used to probe analytes on tailored thin-layer chromatography (TLC) plates. It was found that quantitative results could be achieved when the TLC plate merely served as a sampling plate without a preceding separation step. Specifically, the dynamic response of caffeine, nicotine, acetaminophen, and progesterone was investigated with FAPA-MS on a variety of different TLC surfaces (normal-phase silica, reversed-phase-modified silica, cyano [CN]-modified silica, and dimethyl [RP2]-modified silica). All analytes were studied as single-analyte standards and in a multianalyte mixture to evaluate the effect of sample plates and sample matrix on analytical performance and competitive ionization processes. Overall, dimethyl (RP2)- and CN-modified silica resulted in superior performance compared to other TLC materials. After careful optimization and without the use of internal standards, linear ranges of five orders of magnitude were accessible for caffeine and nicotine. Limits of detection down to femtomole amounts of analyte were achieved. Quantitation limits using RP2-TLC and FAPA-MS were 0.062, 0.062l, 0.31, and 14 pmol for caffeine, nicotine, progesterone, and acetaminophen, respectively. Interestingly, the presence of nicotine at relatively high amounts reduced the signal of the other analytes, an observation that was found to correlate with the differences in the enthalpy of vaporization (ΔHvap) and proton affinity. To prove the quantitative capabilities, nicotine quantification in a real matrix-heavy e-liquid sample was demonstrated using an isotopically labeled standard. The use of TLC-based surfaces with FAPA-MS can aid in the direct and quantitative mass spectrometric investigation of complex mixtures.

Plasma-based ambient mass spectrometry: Recent progress and applications

Ambient mass spectrometry (AMS) has grown as a group of advanced analytical techniques that allow for the direct sampling and ionization of the analytes in different statuses from their native environment without or with minimum sample pretreatments. As a significant category of AMS, plasma-based AMS has gained a lot of attention due to its features that allow rapid, real-time, high-throughput, in vivo, and in situ analysis in various fields, including bioanalysis, pharmaceuticals, forensics, food safety, and mass spectrometry imaging. Tens of new methods have been developed since the introduction of the first plasma-based AMS technique direct analysis in real-time. This review first provides a comprehensive overview of the established plasma-based AMS techniques from their ion source configurations, mechanisms, and developments. Then, the progress of the representative applications in various scientific fields in the past 4 years (January 2017 to January 2021) has been summarized. Finally, we discuss the current challenges and propose the future directions of plasma-based AMS from our perspective.

Focusing Plasma Desorption/Ionization Mass Spectrometry

A plasma-based source named focusing plasma desorption/ionization (FPDI) is described, which applies a high direct current voltage between a metal wire inside a polymeric hollow truncated cone and a piece of a one-sided coated conducting paper substrate. The conducting paper acts as both the counter electrode and the sample carrier. Upon the generation of a visible plasma beam, it would directly ionize the samples spotted on the conducting paper substrate or located around the plasma beam. The signal intensity of target analytes in mass spectrometric analysis is dependent highly on whether the conducting paper substrate is grounded or not, the type of conducting paper substrate, the inside diameter of the polymeric hollow truncated cone tip, the metal wire tip-to-polymer tip distance, the polymer tip-to-paper substrate distance, the applied voltage, and the helium flow rate. Based on the experimental observation, a plausible mechanism is proposed for the generation of the plasma beam from FPDI. Compared to the available low-temperature plasma, flowing atmospheric-pressure afterglow, and helium plasma ionization sources, FPDI has demonstrated higher sensitivity and better compatibility with commercial mass spectrometers without any extra power supplies. As a proof of concept, FPDI coupled with a mass spectrometer has also been applied for the discrimination of different brands of gasoline and determination of solid tablets and pesticides with limits of detection in the range of 2.2 to 30.7 ng mL^-1.

Proton Affinities of Alkanes

Chemical characterization of complex mixtures of large alkanes is critically important for many fields, including petroleomics and the development of renewable transportation fuels. Tandem mass spectrometry is the only analytical method that can be used to characterize such mixtures at the molecular level. Many ionization methods used in mass spectrometry involve proton transfer to the analyte. Unfortunately, very few proton affinity (PA) values are available for alkanes. Indeed, previous research has shown that most protonated alkanes (MH⁺) are not stable but fragment spontaneously via the elimination of a hydrogen molecule to form [M - H]⁺ ions. Here, the PAs of several n-alkanes and alkylcyclohexanes containing 5-8 carbon atoms, n-pentane, n-hexane, n-heptane, n-octane, cyclohexane, methylcyclohexane, and ethylcyclohexane, were determined via bracketing experiments by using a linear quadrupole ion trap mass spectrometer. Monitoring the formation of the [M - H]⁺ ions in reactions between the alkanes and protonated reference bases with known PAs revealed that the PAs of all the alkanes fell into the range 721 ± 20 kJ mol^-1. In order to obtain a more accurate estimate of the relative PAs of different alkanes, two alkanes were introduced simultaneously into the ion trap and allowed to react with the same protonated reference base. Based on these experiments, the longer the alkyl chain in an n-alkane or alkylcyclohexane the greater the PA. Further, when considering alkanes with the same number of carbon atoms, the PAs of those with a cyclohexane ring were found to be greater than those with no such ring.

First Novel Workflow for Semiquantification of Emerging Contaminants in Environmental Samples Analyzed by Gas Chromatography-Atmospheric Pressure Chemical Ionization-Quadrupole Time of Flight-Mass Spectrometry

The ionization efficiency of emerging contaminants was modeled for the first time in gas chromatography-high-resolution mass spectrometry (GC-HRMS) which is coupled to an atmospheric pressure chemical ionization source (APCI). The recent chemical space has been expanded in environmental samples such as soil, indoor dust, and sediments thanks to recent use of high-resolution mass spectrometric techniques; however, many of these chemicals have remained unquantified. Chemical exposure in dust can pose potential risk to human health, and semiquantitative analysis is potentially of need to semiquantify these newly identified substances and assist with their risk assessment and environmental fate. In this study, a rigorously tested semiquantification workflow was proposed based on GC-APCI-HRMS ionization efficiency measurements of 78 emerging contaminants. The mechanism of ionization of compounds in the APCI source was discussed via a simple connectivity index and topological structure. The quantitative structure–property relationship (QSPR)-based model was also built to predict the APCI ionization efficiencies of unknowns and later use it for their quantification analyses. The proposed semiquantification method could be transferred into the household indoor dust sample matrix, and it could include the effect of recovery and matrix in the predictions of actual concentrations of analytes. A suspect compound, which falls inside the application domain of the tool, can be semiquantified by an online web application, free of access at http://trams.chem.uoa.gr/semiquantification/.

Increasing DESI-MS Ion Signal by Plasma Treatment

Many studies are focused on using plasma in mass spectrometry as an ionization source or postionization method. In this study, the effect of plasma treatment in the sample preparation step of desorption electrospray ionization (DESI) has been investigated. The plasma treatment of polar samples, including morphine, codeine, captopril, theophylline, fructose, and amphiphilic compounds such as phosphatidylethanolamine (PE) in E. coli bacteria, as well as nonpolar compounds, including thebaine, papaverine, and noscapine, has been followed for ionization efficiency in DESI technique. An atmospheric-pressure glow discharge plasma (GDP) along with the electrospray ionization technique is examined. Plasma treatment before ambient ionization has a dramatic effect on polar and nonpolar sample signals in DESI-TOF mass spectrometry. The intensity of the mass spectrum shows an increase of 1.9-3.4 times for polar compounds, 2.1-2.5 times for nonpolar compounds, and 3.0 times for PE in E. coli bacteria (N = 4). Plasma is a source of reactive atoms, molecules, ions, radicals, and ultraviolet radiation. Plasma surface treatment before DESI analysis by energetic species through momentum/energy transfer yields higher energy surface molecules, leading to more/easier desorption. Under optimal treatment conditions, an improved ion signal intensity is observed without any fragmentation, decomposition, or chemical changes. Ion signals are increased possibly by both increased ionization through protonation of molecules and enhanced subsequent desorption during DESI analysis.

Contemporary Research Progress on the Detection of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are a class of the most common and widespread contaminants. The accumulation of PAHs has made a certain impact on the environment and is seriously threatening human health. Numerous general analytical methods suitable for PAHs were developed. With the development of economy, the environmental problems of PAHs in modern society are more extensive and prominent, and attract more attention from environmental scientists and analysts. Deeper understanding of the properties of PAHs depends on the advent of detection methods, which can also be more conducive to promoting the protection of the environment. Till now, more sensitive, more high-speed and more high-throughput analytical tools are being invented and have played important roles in the research of PAHs. In this short review article, we focused mainly on the contemporary analytical methods about PAHs. We started with a brief review on the hazards, migration, distribution and traditional analysis methods of PAHs in recent years, including liquid chromatography, gas chromatography, surface enhanced Raman spectroscopy and so on. We also presented the applications of the modern ambient mass spectrometry, especially microwave plasma torch mass spectrometry, in the detection of PAHs, as well as the far out novel results in our lab by using microwave plasma torch (MPT) mass spectrometry; for example, some new insights about Birch reduction, regular hydrogen addition and the robustness of molecular structure. These studies have demonstrated the versatility of MPT MS as a platform in the research of PAHs.

Three-Dimensional Mass Spectral Imaging of Polymers Via Laser-Assisted Micro-Pyrolysis Program with Flowing Atmospheric-Pressure Afterglow Ambient Mass Spectrometry

Herein, a novel diode laser-assisted micro-pyrolysis program (LAMP) technique is demonstrated and coupled with flowing atmospheric-pressure afterglow ambient mass spectrometry for instantaneously profiling polymers and polymer additives. Laser power modulation allows thermal separation of additives and different pyrolysis products, as shown through positive- and negative-mode high-resolution mass spectra and Kendrick mass defect plots of homopolymers, copolymers, polymer blends, and complex polymer samples. LAMP allows much faster temperature control through real-time duty cycle changes and gives significantly better spatial confinement compared to typical resistive heating pyrolysis approaches. Finally, MS imaging, with lateral and depth resolution, is demonstrated for a complex polymer pressure-sensitive adhesive tape sample.

Dual ambient plasma source ionization mass spectrometry for the rapid detection of trace sterols in urban water

A direct analytical method based on dual ambient plasma ion source tandem mass spectrometry was used for the simultaneous determination of four sterols in the environment. This technology has very high sensitivity and the method detects the four sterols in methanol-water (1:3) solutions with limits of detection (LOD) and limits of quantification (LOQ) ranging from 1.2 ng/L to 6.9 ng/L and 7.6 ng/L to 10.0 ng/L, respectively. The method was also used to test water quality at three locations within the city and successfully detected all four sterols at very low concentrations. The dual plasma source tandem mass spectrometry technique is extremely simple, rapid, sensitive and highly efficient compared to other traditional methods, providing a useful screening tool for sterols in water.

High-throughput and trace analysis of diazepam in plasma using DART-MS/MS and its pharmacokinetic application

A high-throughput quantitative analytical method based on Direct Analysis in Real Time tandem mass spectrometry (DART-MS/MS) has been developed and validated for the determination of diazepam in rat plasma, whereby analyzing of each sample needs merely 25 μL plasma, simple solid phase extraction sample preparation and 15 s acquisition time. The multiple reaction monitoring (MRM) transitions at m/z 285.2 → 193.1 and 316.0 → 270.0 were selected for the monitoring of diazepam and its internal standard clonazepam respectively. A good linearity within the range of 10-2000 ng/mL, an intra- and inter-day precisions within <7.78% as to an accuracy ranging from 1.04% to 7.92% have been achieved. The method has been successfully applied to the pharmacokinetic study of diazepam in rats' plasma after a single intragastric administration at a dose of 10 mg/kg. The results indicate that this method fulfills the requirements of the bioanalysis in sensitivity and accuracy. It shows considerable promise for application of DART-MS to the quantitative investigation of other drugs.

Plug-and-play laser ablation-mass spectrometry for molecular imaging by means of dielectric barrier discharge ionization

The plug-and-play hyphenation of UV-laser ablation (LA) and mass spectrometry is presented, using dielectric barrier discharge ionization (DBDI). The DBDI source employed here is characterized by its unique geometry, being directly mounted onto the inlet capillary of a mass spectrometer. In the literature, this particular kind of DBDI source is also referred to as active capillary plasma ionization. It has been commercialized as soft ionization by chemical reaction in transfer (SICRIT) and will be addressed as DBDI in this study. LA-DBDI-MS was used for the direct, molecule-specific and spatially resolved analysis of various solid samples, such as coffee beans and pain killer tablets without extensive sample preparation. The combination of fast washout UV-laser ablation and the principle of the DBDI source used here allowed for highly efficient soft ionization as well as high spatial resolution down to 10 μm for molecular imaging.

MODERN PLASMA-BASED DESORPTION/IONIZATION: FROM ATOMS AND MOLECULES TO CHEMICAL SYNTHESIS

Since the first mass spectrometry (MS) experiments were conducted by Thomson and Aston, plasmas have been used as ionization sources. Historically, plasma ion sources were used for these experiments because they were one of the few known sources of gas-phase ions at the time and they were relatively simple to setup and operate. Since then, developments in plasma ionization have continued to inform and motivate advances in other areas of MS. For example, plasma-desorption MS demonstrated ionization of large peptides and polymers more than 10 years before the first descriptions of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). As a result, significant effort was placed on development of ionization approaches, mass analysis, and detection approaches for very large molecules: even before the advent of ESI and MALDI. Since then, new analytical challenges and opportunities in plasma ionization have arisen. In this review, the emerging trends in plasma-based ionization for several areas of MS will be discussed, including molecular ionization, elemental ionization, hybrid elemental and molecular ion sources, and unique chemical transformations. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Laser assisted sampling vs direct desorption flowing atmospheric pressure afterglow mass spectrometry of complex polymer samples: Forensic implications for pressure sensitive tape chemical analysis

Flowing atmospheric pressure afterglow (FAPA) mass spectrometry (MS) is an easy-to-use, cost-effective, and potentially portable technique that allows direct desorption/ionization from samples with little-to-no sample preparation for real-time chemical analysis. However, it has limitations regarding analytes with low desorption efficiency, such as polymers. Here, laser assisted sampling (LAS) is developed and coupled to FAPA MS to allow access to a wider range of chemical information from polymer samples. This is achieved through laser-induced pyrolysis conditions that provide a much higher degree of spatio-temporal control compared to typical pyrolysis techniques. LAS FAPA MS, together with direct desorption FAPA MS, is implemented on pressure sensitive adhesive (PSA) tape samples, which are often found at crime scenes and recovered as forensic evidence. Comparative PSA tape examination is typically performed to assess any differences in the comparison of unknown and known samples and provide an evidentiary association between suspects and crime scenes in forensic applications. PSA tape samples from several manufacturers of duct, masking, and electrical tape were analyzed from the adhesive and backing side. Direct desorption FAPA provides top-surface selectivity and the tape mass spectra are dominated by more peaks at lower m/z, many of which correspond to polymer additives. LAS gives access to sampling from all of the tape layers and the FAPA mass spectra is extended to higher m/z, while polymer fragmentation patterns are evident. Principal components analysis (PCA) was implemented to assess the ability of each technique to distinguish and categorize identified tape classes within the sampled population. The complementary nature of the resulting mass spectra from direct desorption vs LAS FAPA was evident from the PCA as different tape brands sub-sets were discriminated by each technique. The differentiation obtained by combining both methods is already competitive, or better, than conventional techniques, with the additional benefits of AMS.

Parametric Sensitivity in a Generalized Model for Atmospheric Pressure Chemical Ionization Reactions

Gas phase reactions between hydrated protons H⁺(H₂O)_n and a substance M, as seen in atmospheric pressure chemical ionization (APCI) with mass spectrometry (MS) and ion mobility spectrometry (IMS), were modeled computationally using initial amounts of [M] and [H⁺(H₂O)_n], rate constants k₁ to form protonated monomer (MH⁺(H₂O)_x) and k₂ to form proton bound dimer (M₂H⁺(H₂O)_z), and diffusion constants. At 1 × 10¹⁰ cm^-3 (0.4 ppb) for [H⁺(H₂O)_n] and vapor concentrations for M from 10 ppb to 10 ppm, a maximum signal was reached at 4.5 μs to 4.6 ms for MH⁺(H₂O)_x and 7.8 μs to 46 ms for M₂H⁺(H₂O)_z. Maximum yield for protonated monomer for a reaction time of 1 ms was ∼40% for k₁ from 10^-11 to 10^-8 cm³·s^-1, for k₂/k₁ = 0.8, and specific values of [M]. This model demonstrates that ion distributions could be shifted from [M₂H⁺(H₂O)_z] to [MH⁺(H₂O)_x] using excessive levels of [H⁺(H₂O)_n], even for [M] > 10 ppb, as commonly found in APCI MS and IMS measurements. Ion losses by collisions on surfaces were insignificant with losses of <0.5% for protonated monomer and <0.1% for proton bound dimer of dimethyl methylphosphonate (DMMP) at 5 ms. In this model, ion production in an APCI environment is treated over ranges of parameters important in mass spectrometric measurements. The models establish a foundation for detailed computations on response with mixtures of neutral substances.

Instantaneous Differentiation of Functional Isomers via Reactive Flowing Atmospheric Pressure Afterglow Mass Spectrometry

Ambient mass spectrometry (AMS) allows direct desorption and ionization of analytes in real time with minimal-to-no sample preparation. However, it may present inadequate capabilities for differentiating isomers. Here, a reactive flowing atmospheric-pressure afterglow (reactive-FAPA) AMS source is developed for rapid isomer differentiation by derivatization of analytes in real time. The effects of the reactive-FAPA operating conditions on the reagent and product ions were studied and optimized for highly volatile and non-volatile model compounds with different carbonyl functional groups. In addition, two functional isomers of valproic acid (VPA) metabolites, 4-ene VPA and γ-valprolactone, are successfully differentiated for the first time by incorporating methylamine (MA) reagent vapor into the plasma effluent used for desorption/ionization. Reactive-FAPAMS for 4-ene VPA shows only detectable peaks of the protonated acylation product [M + MA-H₂O + H]⁺, while for γ-valprolactone, it shows detectable peaks for both protonated acylation product [M + MA-H₂O + H]⁺ and protonated intermediate [M + MA + H]⁺. A method for quantitative characterization of mixtures of 4-ene VPA and γ-valprolactone is also developed and validated. In addition, reactive-FAPAMS also shows better detection sensitivity compared to nonreactive-FAPAMS for some larger analyte types, such as UV filters and steroids. The limit of detection (LOD) of pregnenolone acetate in reactive-FAPAMS is 310 ng/mL, which is about 10 times better than its LOD in nonreactive-FAPA.

Flowing atmospheric-pressure afterglow drift tube ion mobility spectrometry

In this work, the flowing atmospheric-pressure afterglow (FAPA) ambient desorption/ionization source has been coupled with stand-alone Drift Tube Ion Mobility Spectrometry (DTIMS) for the first time. A tip repeller electrode, modified to allow higher bias potential still below the Townsend's breakdown, was implemented at the FAPA/DTIMS interface to overcome the opposing potentials and facilitate ion transmission. The effect of the lab-built DTIMS and FAPA's operating conditions (such as plasma voltage, current, gas flow rate, repeller's potential and positioning, FAPA orientation, etc.) on the signal of selected analytes was studied, for both gas-phase injection and desorption. The FAPA reactant ion peak (RIP) reduced mobility coefficient (K₀) corresponds to protonated water clusters (H₂O)_nH⁺. The FAPA-DTIMS spectra of several selected compounds showed that their K₀ agrees with literature values. Moreover, quantitative characterization of acetaminophen and 2,6-di-tert-butylpyridine (2,6-DTBP) based on desorption or gas-phase injection yield limits of detection (LODs) of 0.03 μg and 18 ppb, respectively.

Flowing Atmospheric Pressure Afterglow for Ambient Ionization: Reaction Pathways Revealed by Modeling

We describe the plasma chemistry in a helium flowing atmospheric pressure afterglow (FAPA) used for analytical spectrometry, by means of a quasi-one-dimensional (1D) plasma chemical kinetics model. We study the effect of typical impurities present in the feed gas, as well as the afterglow in ambient humid air. The model provides the species density profiles in the discharge and afterglow regions and the chemical pathways. We demonstrate that H, N, and O atoms are formed in the discharge region, while the dominant reactive neutral species in the afterglow are O₃ and NO. He* and He₂* are responsible for Penning ionization of O₂, N₂, H₂O, H₂, and N, and especially O and H atoms. Besides, He₂⁺ also contributes to ionization of N₂, O₂, H₂O, and O through charge transfer reactions. From the pool of ions created in the discharge, NO⁺ and (H₂O)₃H⁺ are the dominant ions in the afterglow. Moreover, negatively charged clusters, such as NO₃H₂O^- and NO₂H₂O^-, are formed and their pathway is discussed as well. Our model predictions are in line with earlier observations in the literature about the important reagent ions and provide a comprehensive overview of the underlying pathways. The model explains in detail why helium provides a high analytical sensitivity because of high reagent ion formation by both Penning ionization and charge transfer. Such insights are very valuable for improving the analytical performance of this (and other) ambient desorption/ionization source(s).

Ionization of organic molecules with metal ions formed in the laser plasma

A laser plasma ion source was used to ionize volatile organic compounds in a gas sample. The plasma was generated on a metal target in the intermediate vacuum region of ~0.3 Torr using a pulsed Nd:YAG laser with a wavelength of 1 μm. The resulting ions mass spectra were acquired using orthogonal time-of-flight mass spectrometer (O-TOF MS). When using a copper target, the ions formed are simple complexes (CuM⁺ ) of copper ions with organic molecules. The possibility of online identification of trace amounts of alkanes in nitrogen and air, with a detection limit of ~10 ppb, was demonstrated. The ionization efficiency of volatile organic compounds through the formation of clusters with metal ions is 10^-4 in terms of the quasimolecular complex ions. The rate constants of ion-molecular reactions of copper ions with octane and water molecules in nitrogen and air are estimated.

Optical and mass spectral characterization of the electrospray ionization/corona discharge ionization interface

The interchange between electrospray ionization (ESI) and corona discharge ionization (CDI) with respect to applied bias on the needle is customarily placed at the point where light production begins at the tip of the needle. If a liquid sample is flowing through a needle that is observed to produce light, the ionization process is assumed to be harsher and the term coronaspray ionization has been coined to describe this hybrid ionization mechanism. In this work, the transition between ESI and CDI is investigated with respect to applied bias through optical and mass spectrometric measurements. As a function of applied bias potential, the optical signal at the tip of the needle was recorded simultaneously with the resultant ionization products. In this effort, the production of ions from an electrospray ionization needle has been demonstrated to produce light regardless of bias if ions are also formed. With this understanding, an ESI/CDI needle was designed to allow the bias to be temporarily pulsed over the 'onset' voltage necessary for ionization and the rise and decay of the optical signal was measured. Positive mode CDI onset to a stable discharge state within 0.05 ms, while positive ESI required 1.9 ms to reach a stable condition. In the negative mode, the stability of the ionization process was highly variable in both ESI and CDI modes, though CDI was generally faster to reach the stable mode of operation. When the resultant ions were investigated, the effect of increased bias on an ESI needle was found to be species-dependent. Recognizing that the range of compounds probed was limited, for those examined, it appears that stable, non-labile species may be investigated via ESI under extremely high biases while labile species demonstrate a narrow range of stable biases before significant fragmentation occurs.

Comparative study on a kilowatt-MPT-MS-based method with two ion polarity modes for the inert palladium metal

Inert metals are of much importance and play a key role in modern industrial manufacturing. The analytical techniques of inert metals remain challenging. In particular, the mass spectrometry of inert metal elements is yet to be further developed, which also limits the contemporary conceptual in situ analysis of inert metals. As the representative element, the mass spectral detection of palladium is critical and of far-reaching significance. Herein, we developed a mass spectrometry method, which can be used for the high-speed and in situ analysis of palladium, and even for other inert metals. Combining the line ion trap mass spectrometer with the versatile ambient ionization source, a novel kilowatt microwave plasma torch (MPT) can be used to obtain the fully characteristic MPT mass spectra of palladium. Detailed multistage tandem mass spectra show that the general form of target ions is [M(O2)x(NO)mNy(NO2)n]- for the negative ion mode and [M(H2O)x(NO2)y(N2)m]+ for the positive ion mode. Moreover, the formation and evolution of these palladium complex ions were reasonably derived based on the analysis of MPT background mass spectra. This mass spectrometric technique is also suitable for the determination of the palladium-containing solution in the sub-trace level. Semi-quantitative results showed that the detecting ability for palladium in the negative mode is better than that of the positive mode. Under the negative ion mode, the limit of detection (LOD) for m/z 259 were evaluated to be 0.5 μg L-1 under the optimized conditions of the negative mode, with the linear range of 1-100 μg·L-1 (R2 ≥ 0.9985) and the relative standard deviation (RSD, n = 11) being in the range of 1.20%-5.98% (refer to Table S3). Our experimental data showed that MPT-MS was a promising technique for providing another alternative in the on-site analysis of liquid samples and other intimate relevant fields, as the supplement of ICP-MS for the detection of inert metal elements. On the other hand, this work will also certainly promote the more broad applications of platinum-group elements (PGE) in modern science and industry.

Ambient (desorption/ionization) mass spectrometry methods for pesticide testing in food: a review

Ambient mass spectrometry refers to the family of techniques that allows ions to be generated from condensed phase samples under ambient conditions and then, collected and analysed by mass spectrometry. One of their key advantages relies on their ability to allow the analysis of samples with minimal to no sample workup. This feature maps well to the requirements of food safety testing, in particular, those related to the fast determination of pesticide residues in foods. This review discusses the application of different ambient ionization methods for the qualitative and (semi)quantitative determination of pesticides in foods, with the focus on different specific methods used and their ionization mechanisms. More popular techniques used are those commercially available including desorption electrospray ionization (DESI-MS), direct analysis on real time (DART-MS), paper spray (PS-MS) and low-temperature plasma (LTP-MS). Several applications described with ambient MS have reported limits of quantitation approaching those of reference methods, typically based on LC-MS and generic sample extraction procedures. Some of them have been combined with portable mass spectrometers thus allowing "in situ" analysis. In addition, these techniques have the ability to map surfaces (ambient MS imaging) to unravel the distribution of agrochemicals on crops.

Formation of the exceptional [M - H]+ cation in atmospheric pressure ionization mass spectrometry analysis of 2-(diphenylsilyl) cyclopropanecarboxylate esters

RATIONALE

In general, ionization of analytes in atmospheric pressure ionization mass spectrometry (API-MS) in positive ion mode results in the formation of protonated molecules ([M + H]⁺ ) and/or cationized molecules (e.g., [M + Na]⁺ ). The formation of specific [M - H]⁺ cations in the API process is of significant interest for further investigation.

METHODS

The ionization processes of 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate esters were investigated using electrospray ionization (ESI)-MS and atmospheric pressure chemical ionization-MS in positive ion mode. Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311 + G(2d,p) level.

RESULTS

The anomalous [M - H]⁺ ion and the regular [M + Na]⁺ ion were both observed using ESI-MS. Interestingly, no [M + H]⁺ ion was obtained in the ESI-MS analysis, and acidification of the ESI solvent accelerated the formation of [M - H]⁺ rather than [M + H]⁺ ion. DFT calculations for the typical methyl 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate (1) indicated that the [1 + H]⁺ ion can thermodynamically and kinetically undergo facile H₂ elimination to generate [1 - H]⁺ .

CONCLUSIONS

The favorable formation of [M - H]⁺ ions in these compounds is attributed to the unique diphenylhydrosilyl group in their structure. The [M + H]⁺ ion formed easily underwent H₂ elimination to produce the [1 - H]⁺ ion in the API source, and thus, acidification of the ESI solvent apparently accelerates the formation of the [1 - H]⁺ ion.

The ionization process of chemical warfare agent simulants in low temperature plasma ionization

The application of low-temperature plasma ionization technology in the chemical warfare agent detection was mostly focused on the research of rapid detection methods. Limited studies are available on the ionization process of chemical warfare agents in low temperature plasma. Through the intensity of protonated molecules of dimethyl methylphosphonate (DMMP) in different solvents including methanol, deuterated methanol (methanol-D₄), pure water, and deuterium oxide (water-D₂), it was concluded that the water molecule in the air provides the hydrogen ion (H⁺) needed for ionization. The product ion spectra and the collision-induced dissociation processes of protonated molecules of nerve agent simulants, including DMMP, diethyl methanephosphonate (DEMP), trimethyl phosphate (TMP), triethyl phosphate (TEP), tripropyl phosphate (TPP), and tributyl phosphate (TBP) were analyzed. Results revealed that H⁺ mostly combined with phosphorus oxygen double bond (P = O) in the low-temperature plasma ionization. By analyzing the peak intensity distribution of product ions of protonated molecules, the presence of proton and charge migration in the low temperature plasma ionization and collision-induced dissociation were researched. This study could provide technical guidance for the rapid and accurate detection of chemical warfare agents through low temperature plasma ionization-mass spectrometry.

Study of Controlled Atmosphere Flexible Microtube Plasma Soft Ionization Mass Spectrometry for Detection of Volatile Organic Compounds as Potential Biomarkers in Saliva for Cancer

A new soft ionization device for mass spectrometry is presented using the flexible microtube plasma under controlled atmospheric conditions. The controlled atmosphere flexible microtube plasma consists of the plasma source itself connected to a gas chromatograph and a mass spectrometer using a borosilicate glass cross piece. Controlled atmosphere, for example, nitrogen and/or an oxygen mixture, is introduced to the system to create a clean ionization environment. Reproducibility issues are discussed, and solutions are presented manipulating the gas flow in the cross piece. A proof of concept is shown using a ketone mixture introduced to the mass spectrometer to optimize atmospheric conditions. Furthermore, application of the presented device for the sensitive and nonfragmenting ionization of volatile organic biomarkers relevant for cancer is carried out. Sample treatment for human saliva is described, and relevant candidate biomarkers are measured in the saliva matrix, showing a very good ionization efficiency and neglectable matrix effects with limits of detection below 80 ppt.

Modeling plasmas in analytical chemistry-an example of cross-fertilization

This paper gives an overview of the modeling work developed in our group in the last 25 years for various plasmas used in analytical spectrochemistry, i.e., glow discharges (GDs), inductively coupled plasmas (ICPs), and laser ablation (LA) for sample introduction in the ICP and for laser-induced breakdown spectroscopy (LIBS). The modeling approaches are briefly presented, which are different for each case, and some characteristic results are illustrated. These plasmas are used not only in analytical chemistry but also in other applications, and the insights obtained in these other fields were quite helpful for us to develop models for the analytical plasmas. Likewise, there is now a huge interest in plasma-liquid interaction, atmospheric pressure glow discharges (APGDs), and dielectric barrier discharges (DBDs) for environmental, medical, and materials applications of plasmas. The insights obtained in these fields are also very relevant for ambient desorption/ionization sources and for liquid sampling, which are nowadays very popular in analytical chemistry, and they could be very helpful in developing models for these sources as well. Graphical abstract.

A "Brick" Mass Spectrometer with Photoionization for Direct Analysis of Trace Volatile Compounds

With high portability and favorable performance, miniature mass spectrometers have become one of the most attractive tools for on-site analysis of trace volatile compounds. Based on the "Brick" mass spectrometer (BMS) developed previously, a hand-held BMS integrated with a photoionization source (PI-BMS) was developed in this study for volatile compound analysis. With compact dimensions of 30 cm × 18.5 cm × 27.6 cm (length × width × height), the PI-BMS was equipped with a 10.6 eV UV lamp and capable of generating molecular ions. The capabilities of qualitative and quantitative analyses for different volatile samples were demonstrated and characterized. Under optimized conditions, high detection sensitivity in open air was obtained for the PI-BMS with a limit of detection (LOD) of ∼10 ppbv. As demonstrations of mixture analysis, four different fresh fruits were directly analyzed using PI-BMS, observing characteristic mass spectra for each type of fruit.

Plasma-Based Ambient Desorption/Ionization Mass Spectrometry for the Analysis of Liquid Crystals Employed in Display Devices

Liquid-crystal displays (LCDs) are the most frequently used display technology worldwide these days. Due to the rather complex manufacturing process and purity requirements for the chemicals used, quality control and display failure analysis are important analytical tasks. Currently, the state-of-the-art techniques (e.g., high-performance liquid chromatography (HPLC), gas chromatography (GC) coupled to mass spectrometry (MS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), or high-resolution microscopy) are costly and time-consuming. Hence, a new pathway to precisely analyze liquid-crystalline materials and LCDs in their native state is reported. A new approach for direct analysis via plasma-based ambient desorption/ionization mass spectrometry (ADI-MS) offers an inexpensive and faster alternative. In this study, direct analysis in real time (DART), the low-temperature plasma (LTP) probe, and flowing atmospheric-pressure afterglow (FAPA) ADI sources coupled to high-resolution mass spectrometry (HR-MS) are compared based on their capabilities and performance for liquid-crystal analysis. These sources enable direct analyte desorption from a sample surface at ambient conditions and ionize the vaporized analyte molecules in a subsequent step. Primarily, the ionization capabilities of the three ADI sources are compared for individual liquid-crystal standards, mixtures of liquid crystals (LCs), and complex liquid crystal/additive mixtures applied in commercially available LCDs. Furthermore, direct surface analysis from a glass substrate is also performed with ADI-MS to compare their applicability to this type of sample matrix.

Automatic Analyte-Ion Recognition and Background Removal for Ambient Mass-Spectrometric Data Based on Cross-Correlation

Ambient mass spectrometry is a powerful approach for rapid, high-throughput, and direct sample analysis. Due to the open-air desorption and ionization processes, random fluctuations of ambient conditions can lead to large variances in mass-spectral signals over time. The mass-spectral data also can be further complicated due to multiple analytes present in the sample, background-ion signals stemming from the desorption/ionization source itself, and other laboratory-specific conditions (e.g., ambient laboratory air, nearby hardware). Thus, background removal and analyte-ion recognition can be quite difficult, particularly in non-targeted analyses. Here, we demonstrate the use of a cross-correlation-based approach to exploit chemical information encoded in the time domain to group/categorize mass-spectral peaks from a single analysis dataset. Ions that originate from ambient (or other) background species were readily flagged and removed from spectra; the result was a decrease in mass-spectral complexity by over 70% due to the removal of these background ions. Meanwhile, analyte ions were differentiated and categorized based on their time-domain profiles. With sufficient mass resolving-power and mass-spectral acquisition rate, isolated mass spectra containing ions from the same species in a sample could be extracted, leading to a reduction in mass-spectral complexity by more than 98% in some cases. The cross-correlation approach was tested with different ionization sources as well as reproducible and irreproducible sample introduction. Software built in-house enabled fully automated data processing, which can be performed within a few seconds. Ultimately, this approach provides an additional dimension of analyte separation in ambient mass-spectrometric analyses with information that is already recorded throughout the analysis.

In situ analysis of unsaturated fatty acids in human serum by negative-ion paper spray mass spectrometry

In situ identification and quantification of unsaturated fatty acid (FA) C=C positional isomers in human serum is herein performed by negative-ion paper spray (PS) mass spectrometry. Typically, by direct application of an alternating current (AC) voltage to the wet paper, the PS ionization could perform stably in the negative-ion mode without severe discharge. We suppose epoxidation reaction between unsaturated C=C bonds and reactive oxidative species might be initiated by a mild electrical discharge, which could be rapidly and controllably produced via a low amplitude AC voltage. Upon collision-induce dissociation (CID), the epoxide was fragmented to generate diagnostic ions indicating the C=C location. The intensity of the characteristic product ions could also be used for absolute quantification of the FA C=C positional isomers. The limits of detection (LODs) and limits of quantification (LOQs) were roughly in the range of 0.0178-0.0506 μM and 0.0218-0.3634 μM for standard FAs. Without the additional sample preparations or reactive chemical reagents, epoxidation of unsaturated FAs and ionization of the epoxide could be achieved in one-step by negative-ion mode PS, which enable a promising methodology for on-site clinical diagnosis.

Reactive Olfaction Ambient Mass Spectrometry

Chemical ionization of organic compounds with negligible vapor pressures (VP) is achieved at atmospheric pressure when the proximal sample is exposed to corona discharge. The vapor-phase analyte is produced through a reactive olfaction process, which is determined to include electrostatic charge induction in the proximal condensed-phase sample, resulting in the liberation of free particles. With no requirement for physical contact, a new contained nano-atmospheric pressure chemical ionization (nAPCI) source was developed that allowed direct mass spectrometry analysis of complex mixtures at a sample consumption rate less than nmol/min. The contained nAPCI source was applied to analyze a wide range of samples including the detection of 1 ng/mL cocaine in serum and 200 pg/mL caffeine in raw urine, as well as the differentiation of chemical composition of perfumes and beverages. Polar (e.g., carminic acid; estimated VP 5.1 × 10^-25 kPa) and nonpolar (e.g., vitamin D2; VP 8.5 × 10^-11 kPa) compounds were successfully ionized by the contained nAPCI ion source under ambient conditions, with the corresponding ion types of 78 other organic compounds characterized.

Petroleomic depth profiling of Staten Island salt marsh soil: 2ω detection FTICR MS offers a new solution for the analysis of environmental contaminants

Staten Island is located in one of the most densely populated regions of the US: the New York/New Jersey Estuary. Marine and industrial oil spills are commonplace in the area, causing the waterways and adjacent marshes to become polluted with a range of petroleum-related contaminants. Using Rock-Eval pyrolysis, the hydrocarbon impact on a salt marsh was assessed at regular intervals down to 90 cm, with several key sampling depths of interest identified for further analysis. Ultrahigh resolution data are obtained by direct infusion (DI) atmospheric pressure photoionization (APPI) on a 12 T solariX Fourier transform ion cyclotron resonance mass spectrometer (FTICR MS) allowing trends in the compositional profile with depth to be observed, such as changes in the relative hydrocarbon intensity and the relative contributions from oxygen- and sulfur-containing groups. These trends may correlate with the timing of major oil spills and leaks of petroleum and other industrial chemicals into the waterways. The use of gas chromatography (GC) coupled to a 7 T solariX 2XR FTICR MS equipped with an atmospheric pressure chemical ionization (APCI) ion source offers retention time resolved and extensive compositional information for the complex environmental samples complementary to that obtained by DI-APPI. The compositional profile observed using GC-APCI FTICR MS includes contributions from phosphorous-containing groups, which may be indicative of contamination from other anthropogenic sources.

Direct Analysis of Carbonyl Compounds by Mass Spectrometry with Double-Region Atmospheric Pressure Chemical Ionization

Direct analysis of highly reactive volatile species such as the aliphatic aldehydes as vital biomarkers remains a great challenge due to difficulties in the sample pretreatment. To address such a challenge, we herein report the development of a novel double-region atmospheric pressure chemical ionization mass spectrometry (DRAPCI-MS) method. The DRAPCI source implements a separated structural design that uses a focus electrode to divide the discharge and ionization region to reduce sample fragmentation in the ionization process. Counterflow introduction (CFI) configuration was adopted in the DRAPCI source to reduce background noise, while ion transmission efficiency was optimized through simulating the voltage of the focus electrode and the ion trajectory of the ion source. The limits of detection (LODs) of four carbonyl compounds cyclohexanone, hexanal, heptanal, and octanal by DRAPCI-MS were between 0.1 and 3 μg·m^-3, approximately two to eight times lower than those by atmospheric pressure chemical ionization mass spectrometry. Additionally, the DRAPCI-MS method carried out effective in situ analyses of the volatile components in expired milk and the exhaled breath of smokers, demonstrating the DRAPCI-MS as a practical tool to analyze complex mixtures. The DRAPCI-MS method provides a rapid, sensitive, and high-throughput technique in the real-time analysis of gaseous small-molecule compounds.