Analysis of drugs of abuse in biofluids by low temperature plasma (LTP) ionization mass spectrometry

Low-cost heat assisted ambient ionization source for mass spectrometry in food and pharmaceutical screening

Ambient Ionization Mass Spectrometry (AI-MS) techniques have revolutionized analytical chemistry by enabling rapid analysis of samples under atmospheric conditions with minimal to no preparation. In this study, the optimization of a cold atmospheric plasma for the analysis of food and pharmaceutical samples, liquid and solid, using a Heat-Assisted Dielectric Barrier Discharge Ionization (HA-DBDI) source is described. A significant enhancement in analyte signals was observed when a heating element was introduced into the design, potentially allowing for greater sensitivity. Furthermore, the synergy between the inlet temperature of the mass spectrometer and the heating element allows for precise control over the analytical process, leading to improved detection sensitivity and selectivity. Incorporating computational fluid dynamic (CFD) simulations into the study elucidated how heating modifications can influence gas transport properties, thereby facilitating enhanced analyte detection and increased signal intensity. These findings advance the understanding of HA-DBDI technology and provide valuable insights for optimizing AI-MS methodologies for a wide range of applications in food and pharmaceutical analysis.

Low-Temperature Plasma Pretreatment Enhanced Cholesterol Detection in Brain by Desorption Electrospray Ionization-Mass Spectrometry Imaging

Cholesterol is a primary lipid molecule in the brain that contains one-fourth of the total body cholesterol. Abnormal cholesterol homeostasis is associated with neurodegenerative disorders. Mass spectrometry imaging (MSI) technique is a powerful tool for studying lipidomics and metabolomics. Among the MSI techniques, desorption electrospray ionization-MSI (DESI-MSI) has been used advantageously to study brain lipidomics due to its soft and ambient ionization nature. However, brain cholesterol is poorly ionized. To this end, we have developed a new method for detecting brain cholesterol by DESI-MSI using low-temperature plasma (LTP) pretreatment as an ionization enhancement. In this method, the brain sections were treated with LTP for 1 and 2 min prior to DESI-MSI analyses. Interestingly, the MS signal intensity of cholesterol (at m/z 369.35 [M + H - H2O]+) was more than 2-fold higher in the 1 min LTP-treated brain section compared to the untreated section. In addition, we detected cholesterol, more specifically excluding isomers by targeted-DESI-MSI in multiple reaction monitoring (MRM) mode and similar results were observed: the signal intensity of each cholesterol transition (m/z 369.4 → 95.1, 109.1, 135.1, 147.1, and 161.1) was increased by more than 2-fold due to 1 min LTP treatment. Cholesterol showed characteristic distributions in the fiber tract region, including the corpus callosum and anterior commissure, anterior part of the brain where LTP markedly (p < 0.001) enhanced the cholesterol intensity. In addition, the distributions of some unknown analytes were exclusively detected in the LTP-treated section. Our study revealed LTP pretreatment as a potential strategy to ionize molecules that show poor ionization efficiency in the MSI technique.

Swab-in-Capillary Electrospray Ionization and a Miniature Mass Spectrometer for In Situ Drug Analysis

In situ analysis of drugs has been in increasing demand in many fields. As an updated version of capillary-in-capillary electrospray ionization (CC-ESI) developed previously, a disposable swab-in-capillary electrospray ionization (SC-ESI) source was designed in this study. With a micro medical swab for sampling and an integrated filter membrane for online filtration, SC-ESI was able to directly sample and MS analyze complex samples without the need for pretreatment. Coupled with a miniature mass spectrometer, SC-ESI was applied for direct analysis of effective ingredients in therapeutic drugs (in tablet, capsule, and liquid droplet) and drugs in saliva and quantitation of therapeutic drugs in blood. The limits of detection in absolute amounts were obtained as 1 ng for fentanyl and 0.5 ng for cocaine in saliva. Combining with an internal standard method, SC-ESI acquired linear quantitation ranges of 100-5000 ng/mL for imatinib in whole blood and 100-2000 ng/mL for clozapine in serum with high accuracies and precisions. The entire analysis process, from sampling to data acquisition, can be completed in less than 2 min. As demonstrated as a cheap, portable, and sampling-effective ionization source, SC-ESI has shown great potential for in situ drug analysis, especially in border drug screening and clinical therapeutic drug monitoring.

Non-Contact, Continuous Sampling of Porous Surfaces for the Detection of Particulate and Adsorbed Organic Contaminations by Low-Temperature Plasma Coupled to Ion Mobility Spectrometer

Chemical analysis of hazardous surface contaminations, such as hazardous substances, explosives or illicit drugs, is an essential task in security, environmental and safety applications. This task is mostly based on the collection of particles with swabs, followed by thermal desorption into a vapor analyzer, usually a detector based on ion mobility spectrometry (IMS). While this methodology is well established for several civil applications, such as border control, it is still not efficient enough for various conditions, as in sampling rough and porous surfaces. Additionally, the process of thermal desorption is energetically inefficient, requires bulky hardware and introduces device contamination memory effects. Low-temperature plasma (LTP) has been demonstrated as an ionization and desorption source for sample preparation-free analysis, mostly at the inlet of a mass spectrometer analyzer, and in rare cases in conjunction with an ion mobility spectrometer. Herein, we demonstrate, for the first time, the operation of a simple, low cost, home-built LTP apparatus for desorbing non-volatile analytes from various porous surfaces into the inlet of a handheld IMS vapor analyzer. We show ion mobility spectra that originate from operating the LTP jet on porous surfaces such as asphalt and shoes, contaminated with model amine-containing organic compounds. The spectra are in good correlation with spectra measured for thermally desorbed species. We verify through LC-MS analysis of the collected vapors that the sampled species are not fragmented, and can thus be identified by commercial IMS detectors.

Ambient ionization mass spectrometry applied to new psychoactive substance analysis

In the past decade a plethora of drugs with similar effects to controlled psychoactive drugs, like cannabis, amfetamine (amphetamine), or lysergic acid diethylamide, have been synthesized. These drugs can collectively be classified under the term new psychoactive substances (NPS) and are used for recreational purposes. The novelty of the substances, alongside the rapid rate of emergence and structural variability, makes their detection as well as their legal control highly challenging, increasing the demand for rapid and easy-to-use analytical techniques for their detection and identification. Therefore, interest in ambient ionization mass spectrometry applied to NPS has grown in recent years, which is largely because it is relatively fast and simple to use and has a low operating cost. This review aims to provide a critique of the suitability of current ambient ionization techniques for the analysis of NPS in the forensic and clinical toxicology fields. Consideration is given to analytical performance and ease of implementation, including ionization efficiency, selectivity, sensitivity, quantification, analyte chemistry, molecular coverage, validation, and practicality.

The development and application of matrix assisted laser desorption electrospray ionization: The teenage years

In the past 15 years, ambient ionization techniques have witnessed a significant incursion into the field of mass spectrometry imaging, demonstrating their ability to provide complementary information to matrix-assisted laser desorption ionization. Matrix-assisted laser desorption electrospray ionization is one such technique that has evolved since its first demonstrations with ultraviolet lasers coupled to Fourier transform-ion cyclotron resonance mass spectrometers to extensive use with infrared lasers coupled to orbitrap-based mass spectrometers. Concurrently, there have been transformative developments of this imaging platform due to the high level of control the principal group has retained over the laser technology, data acquisition software (RastirX), instrument communication, and image processing software (MSiReader). This review will discuss the developments of MALDESI since its first laboratory demonstration in 2005 to the most recent advances in 2021.

Heat Pulse Desorption of Low-Volatility Compounds by a Heated N2 Gas Pulse with Mass Spectrometry

For the thermal desorption of low-volatility compounds, rapid heating followed by instant cooling is desirable to suppress thermal decomposition. In this work, a new thermal desorption method, heat pulse desorption (HPD), was developed. A heated N₂ gas pulse (350 °C, 50 ms) was directed to the solid sample surface, and desorbed analytes were ionized by DC corona discharge and mass analyzed by an Orbitrap mass spectrometer. Because heat transfer from the heated N₂ gas to the solid surface is not very efficient, desorption of the solid sample occurs at a certain temperature before reaching 350 °C. In short, there is a self-controlling desorption depending on the volatility of each analyte. Because the exit of the copper tube for gas blowing is separated from the sample surface, no carryover occurs, enabling the repetitive analysis of samples. HPD was applied to various compounds such as narcotics, pharmaceutical tablets, and explosives. Because analysis is completed within a few seconds per sample, this method is highly useful for quick and consecutive analysis of real samples, having potential utility in food quality control, counterfeit drugs analysis, and the detection of explosives for safety and security.

Ambient Ionization Mass Spectrometry: Application and Prospective

Mass spectrometry (MS) is a formidable analytical tool for the analysis of non-polar to polar compounds individually and/or from mixtures, providing information on the molecular weights and chemical structures of the analytes. During the last more than one-decade, ambient ionization mass spectrometry (AIMS) has developed quickly, producing a wide range of platforms and proving scientific improvements in a variety of domains, from biological imaging to quick quality control. These methods have made it possible to detect target analytes in real time without sample preparation in an open environment, and they can be connected to any MS system with an atmospheric pressure interface. They also have the ability to analyze explosives, illicit drugs, disease diagnostics, drugs in biological samples, adulterants in food and agricultural products, reaction progress, and environmental monitoring. The development of novel ambient ionization techniques, such as probe electrospray ionization, paper spray ionization, and fiber spray ionization, employed even at picolitre to femtolitre solution levels to provide femtogram to attogram levels of the target analytes. The special characteristic of this ambient ion source, which has been extensively used, is the noninvasive property of PESI of examination of biological real samples. The results in the current review supports the idea that AIMS has emerged as a pioneer in MS-based approaches and that methods will continue to be developed along with improvements to existing ones in the near future.

Thermogravimetry combined with electrospray and atmospheric pressure chemical ionization mass spectrometry for characterization of synthetic polymers

RATIONALE

Thermogravimetry (TG) combined with electrospray and atmospheric chemical ionization (ESI+APCI) mass spectrometry (MS) was developed to rapidly characterize thermal decomposition products of synthetic polymers and plastic products. The ESI-based TG-MS method is useful for characterizing thermally labile, nonvolatile, and polar compounds over an extensive mass range; and the APCI-based TG-MS counterpart is useful for characterizing volatile and nonpolar compounds. Both polar and nonpolar compounds can be simultaneously detected by ESI+APCI-based TG-MS.

METHODS

Analytes with different volatility were produced from TG operated at different temperatures, which were delivered through a heated stainless-steel tube to the ESI+APCI source where they reacted with the primary charged species generated from electrospray and atmospheric pressure chemical ionization (ESI+APCI) of solvent and nitrogen. The analyte ions were then detected by an ion trap mass spectrometer.

RESULTS

A semi-volatile PEG 600 standard was used as the sample and protonated and sodiated molecular ions together with adduct ions including [(PEG)n + 15]⁺ , [(PEG)n + 18]⁺ , and [(PEG)n + 29]⁺ were detected by TG-ESI+APCI-MS. The technique was further utilized to characterize thermal decomposition products of nonvolatile polypropylene glycol (PPG) and polystyrene (PS) standards, as well as a PS-made water cup and coffee cup lid. The characteristic fragments of PPG and PS with mass differences of 58 and 104 between respective ion peaks were detected at the maximum decomposition temperature (T_max ).

CONCLUSIONS

The information obtained from the TG-ESI+APCI-MS analysis is useful in rapidly distinguishing different types of polymers and their products. In addition, the signals of the additives in the polymer products, including antioxidants and plasticizers, were also detected before the TG temperature reached T_max .

Trace residue identification, characterization, and longitudinal monitoring of the novel synthetic opioid β-U10, from discarded drug paraphernalia

Empirical data regarding dynamic alterations in illicit drug supply markets in response to the COVID-19 pandemic, including the potential for introduction of novel drug substances and/or increased poly-drug combination use at the "street" level, that is, directly proximal to the point of consumption, are currently lacking. Here, a high-throughput strategy employing ambient ionization-mass spectrometry is described for the trace residue identification, characterization, and longitudinal monitoring of illicit drug substances found within >6,600 discarded drug paraphernalia (DDP) samples collected during a pilot study of an early warning system for illicit drug use in Melbourne, Australia from August 2020 to February 2021, while significant COVID-19 lockdown conditions were imposed. The utility of this approach is demonstrated for the de novo identification and structural characterization of β-U10, a previously unreported naphthamide analog within the "U-series" of synthetic opioid drugs, including differentiation from its α-U10 isomer without need for sample preparation or chromatographic separation prior to analysis. Notably, β-U10 was observed with 23 other drug substances, most commonly in temporally distinct clusters with heroin, etizolam, and diphenhydramine, and in a total of 182 different poly-drug combinations. Longitudinal monitoring of the number and weekly "average signal intensity" (ASI) values of identified substances, developed here as a semi-quantitative proxy indicator of changes in availability, relative purity and compositions of street level drug samples, revealed that increases in the number of identifications and ASI for β-U10 and etizolam coincided with a 50% decrease in the number of positive detections and an order of magnitude decrease in the ASI for heroin.

Quantitative Analysis of Exhaled Breath Collected on Filter Substrates via Low-Temperature Plasma Desorption/Ionization Mass Spectrometry

Breath analysis has attracted increasing attention in recent years due to its great potential for disease diagnostics at early stages and for clinical drug monitoring. There are several recent examples of successful development of real-time, in vivo quantitative analysis of exhaled breath metabolites via mass spectrometry. On the other hand, current mass spectrometer accessibility limitations restrict point-of-care applications. Here now, an offline method is developed for quantitative analysis of exhaled breath collected on inexpensive filter substrates for direct desorption and ionization by using low-temperature plasma-mass spectrometry (LTP-MS). In particular, different operating conditions of the ionization source were systematically studied to optimize desorption/ionization by using glycerol, a low volatility compound. Applications with respect to propofol, γ-valprolactone, and nicotine analysis in exhaled breath are demonstrated in this study. The effects of several filter substrate properties, including filter material and pore size, on the analyte signal were characterized. Cellulose filter papers performed best with the present analytes. In addition, filters with smaller pores enabled a more efficient sample collection. Furthermore, sample-collection flow rate was determined to have a very significant effect, with slower flow rates yielding the best results. It was also found that filters loaded with sample can be successfully stored in glass vials with no observable sample loss even after 3 days. Limits of detection under optimized conditions are shown to be competitive or significantly better compared with relevant techniques and with additional benefits of cost-efficiency and sample storage capabilities.

Early Warning System for Illicit Drug Use at Large Public Events: Trace Residue Analysis of Discarded Drug Packaging Samples

Inspired by Locard's exchange principle, which states "every contact leaves a trace", a trace residue sampling strategy has been developed for the analysis of discarded drug packaging samples (DPS), as part of an early warning system for illicit drug use at large public events including music/dance festivals. Using direct analysis in real time/mass spectrometry and tandem mass spectrometry, rapid and high-throughput identification and characterization of a wide range of illicit drugs and adulterant substances was achieved, including in complex polydrug mixtures and at low relative ion abundances. A total of 1362 DPS were analyzed either off-site using laboratory-based instrumentation or on-site and in close to real time using a transportable mass spectrometer housed within a mobile analytical laboratory, with each analysis requiring less than 1 min per sample. Of the DPS analyzed, 92.2% yielded positive results for at least one of 15 different drugs and/or adulterants, including cocaine, MDMA, and ketamine, as well as numerous novel psychoactive substances (NPS). Also, 52.6% of positive DPS were found to contain polydrug mixtures, and a total of 42 different drug and polydrug combinations were observed throughout the study. For analyses performed on-site, reports to key stakeholders including event organizers, first aid and medical personnel, and peer-based harm reduction workers could be provided in as little as 5 min after sample collection. Following risk assessment of the potential harms associated with their use, drug advisories or alerts were then disseminated to event staff and patrons and subsequently to the general public when substances with particularly toxic properties were identified.

Integrating the MasSpec Pen with Sub-Atmospheric Pressure Chemical Ionization for Rapid Chemical Analysis and Forensic Applications

Analytical methods that allow rapid, sensitive, and specific chemical measurements are central to forensic analysis and essential to accelerating compound screening and confirmation. We have previously reported the development of the MasSpec Pen technology as an easy-to-use and disposable hand-held device integrated to a mass spectrometer for direct analysis and molecular profiling of biological samples. In this Technical Note, we describe a new apparatus that integrates the MasSpec Pen device with a subatmospheric pressure chemical ionization (sub-APCI) source and an ion trap mass spectrometer for detection and semiquantitative analysis of forensic-related compounds. Coupling the MasSpec Pen device to a sub-APCI source allowed semiquantitative analysis of the drugs cocaine and oxycodone, the agrochemicals atrazine and azoxystrobin, and the explosives trinitrotoluene and dinitroglycerin in under 20 s. Using chemical ionization, improved reproducibility and sensitivity for targeted chemical detection and compound identification was achieved while maintaining the user-friendly features of the hand-held MasSpec Pen device. Limits of detection in the high picogram to low nanogram range were obtained for the compounds analyzed, which are within the range of federal screening cutoffs and those reported for other ambient ionization MS techniques. Altogether, the MasSpec Pen sub-APCI system described enabled rapid and semiquantitative chemical analysis for forensic applications and could be further adapted and applied to other areas of chemical testing.

Mechanism behind the paper spray chemical ionization phenomenon and the choice of solvent

Paper spray chemical ionization (PSCI) combined with mass spectrometry has been proposed as a sensitive method for the analysis of nonpolar aromatic compounds; however, the mechanism behind PSCI is not well understood. In the present study, the evidence for the occurrence of corona discharge is provided and its mechanism is proposed. Photographs taken with a highly sensitive camera evidently demonstrate the occurrence of corona discharge at the end of the triangular shape tip when a nonpolar solvent such as hexane was used at an applied potential of 6-7 kV. Nevertheless, corona discharge was not observed in the presence of a polar solvent. The occurrence of the corona discharge was attributed to charge accumulation in the dielectric layer generated by the nonpolar solvent on the fibers of the paper tip. Specifically, corona discharge was generated at the tip end when the charge approached a critical threshold. In the presence of a polar solvent, however, the dielectric layer was not generated and, hence, corona discharge was not observed. Based on this information, three nonpolar solvents were selected and their sensitivity for analyzing the phenanthrene and maltene fractions of crude oil was evaluated. Chlorobenzene provided the highest signal abundance; therefore, it was suggested as the optimum solvent for PSCI. Notably, the fundamental understanding of corona discharge in PSCI acquired in this study provides a basis for further improvement of this technique by way of surface modification.

Applications and challenges of low temperature plasma in pharmaceutical field

Low temperature plasma (LTP) technology has shown an outstanding application value in the pharmaceutical filed in recent ten years. This paper reviews the research advances in LTP, including its effects on enhancing or inhibiting drug activity, its combined use with drugs to treat cancers, its effects on the improvement of drug delivery system, its use in preparation of new inactivated virus vaccines, its use with mass spectrometry for rapid detection of drug quality, and the anti-tumor and sterilization effects of plasma-activated liquids. The paper also analyzes the challenges of LTP in the pharmaceutical filed, hoping to promote related research.

Challenges and Strategies of Chemical Analysis of Drugs of Abuse and Explosives by Mass Spectrometry

In analytical science, mass spectrometry (MS) is known as a "gold analytical tool" because of its unique character of providing the direct molecular structural information of the relevant analyte molecules. Therefore, MS technique has widely been used in all branches of chemistry along with in proteomics, metabolomics, genomics, lipidomics, environmental monitoring etc. Mass spectrometry-based methods are very much needed for fast and reliable detection and quantification of drugs of abuse and explosives in order to provide fingerprint information for criminal investigation as well as for public security and safety at public places, respectively. Most of the compounds exist as their neutral form in nature except proteins, peptides, nucleic acids that are in ionic forms intrinsically. In MS, ion source is the heart of the MS that is used for ionizing the electrically neutral molecules. Performance of MS in terms of sensitivity and selectivity depends mainly on the efficiency of the ionization source. Accordingly, much attention has been paid to develop efficient ion sources for a wide range of compounds. Unfortunately, none of the commercial ion sources can be used for ionization of different types of compounds. Moreover, in MS, analyte molecules must be released into the gaseous phase and then ionize by using a suitable ion source for detection/quantification. Under these circumstances, fabrication of new ambient ion source and ultrasonic cutter blade-based non-thermal and thermal desorption methods have been taken into account. In this paper, challenges and strategies of mass spectrometry analysis of the drugs of abuse and explosives through fabrication of ambient ionization sources and new desorption methods for non-volatile compounds have been described. We will focus the literature progress mostly in the last decade and present our views for the future study.

Low-Temperature Plasma Probe Mass Spectrometry for Analytes Separated on Thin-Layer Chromatography Plates: Direct vs Laser Assisted Desorption

Thin-layer chromatography (TLC) is a widespread technique because it allows fast, simple, and inexpensive analyte separations. In addition, direct analysis of the compounds separated on TLC plates via mass spectrometry (MS) has been shown to provide high sensitivity and selectivity while avoiding time-consuming sample extraction protocols. Here, direct desorption low-temperature plasma-mass spectrometry (LTP-MS) as well as diode laser assisted desorption (LD) LTP-MS are studied for direct spatially resolved analysis of compounds from TLC plates. Qualitative and quantitative characterization of amino acids, pharmaceuticals, and structural isomers were performed. The nature of the TLC plate stationary phase was found to have a significant influence, together with the analyte's characteristics, on the desorption efficiency. Tandem MS is shown to greatly improve the limits of detection (LODs). Direct desorption LTP-MS, without external thermal assisted desorption, demonstrates its best performance with cellulose TLC plates (LODs, 0.01 ng/mm² to 2.55 ng/mm²) and restricted performance with normal-phase (NP) TLC plates (several analytes without observable signal). LD LTP-MS, with systematic optimization of irradiance and focal point diameter, is shown to overcome the direct-desorption limitations and reach significantly improved LODs with NP TLC plates (up to ×1000 better). In addition, a wide-ranging characterization of amino acid analytical figures of merit with LD LTP-MS shows that LODs from 84 pg/mm² down to 0.3 pg/mm² are achieved on NP TLC plates.

MASS SPECTROMETRY ANALYSIS OF DRUGS OF ABUSE: CHALLENGES AND EMERGING STRATEGIES

Mass spectrometry has been the "gold standard" for drugs of abuse (DoA) analysis for many decades because of the selectivity and sensitivity it affords. Recent progress in all aspects of mass spectrometry has seen significant developments in the field of DoA analysis. Mass spectrometry is particularly well suited to address the rapidly proliferating number of very high potency, novel psychoactive substances that are causing an alarming number of fatalities worldwide. This review surveys advancements in the areas of sample preparation, gas and liquid chromatography-mass spectrometry, as well as the rapidly emerging field of ambient ionization mass spectrometry. We have predominantly targeted literature progress over the past ten years and present our outlook for the future. © 2020 Periodicals, Inc. Mass Spec Rev.

A mechanism study of positive ionization processes in flowing atmospheric-pressure afterglow (FAPA) ambient ion source with controlled plasma and ambient conditions

Although plasma based ambient desorption/ionization (ADI) sources have been widely used for direct analysis of complex samples, mass spectrometric imaging, high throughput screening etc., the ionization mechanism of plasma-based ADI remains a mystery by now. In this report, a targeted study was conducted aiming at a better understanding of the ionization processes of plasma-based ambient desorption ionization source. As a representative of ambient desorption ionization source, an FAPA source was used and modified as a test platform to control the plasma discharge parameters and ambient ionization environment such as discharge gases, environmental gases and sampling conditions. Based on the ionization results from different ambient ionization conditions, a new mechanism was proposed to reveal the nature of regent ion production of FAPA. At the same time, the effect of buffer gas was investigated. For the first time, the multi-clustered hydronium ions formed by the massive water vapor in the air were explored to clarify reasons for the occurrence of selective ionization and the factors affecting ionization efficiency in such complex events. In addition, the formation of molecular ions and relevant reagent ions was speculated based on experimental observations.

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.

Analyte-Tailored Controlled Atmosphere Improves Dielectric Barrier Discharge Ionization Mass Spectrometry Performance

Plasma sources in atmospheric pressure soft-ionization mass spectrometry have gained significant interest in recent years. As many of these sources are used under ambient air conditions, their interaction with the surrounding atmosphere plays an important role in the ionization pathway. This study focuses on the interaction between the plasma source and the surrounding atmosphere by connecting the plasma source to the mass spectrometer using a 2 mm ID closed reactant capillary supplied by a reactant gas up to 500 mL per minute to gain a controlled atmosphere. Different reactant gases (Ar, He, O₂, and N₂) and reactant gas mixtures are tested with regard to the DBDI performance and then used to improve the ionization efficiency. Tailoring the controlled atmosphere for a certain analyte, for example, perfluorinated compounds, leads to significantly improved limits of detection up to 2 ppb.

Response in Ambient Low Temperature Plasma Ionization Compared to Electrospray and Atmospheric Pressure Chemical Ionization for Mass Spectrometry

Modern technical evolution made mass spectrometry (MS) an absolute must for analytical chemistry in terms of application range, detection limits and speed. When it comes to mass spectrometric detection, one of the critical steps is to ionize the analyte and bring it into the gas phase. Several ionization techniques were developed for this purpose among which electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) are two of the most frequently applied atmospheric pressure methods to ionize target compounds from liquid matrices or solutions. Moreover, recent efforts in the emerging field of "ambient" MS enable the applicability of newly developed atmospheric pressure techniques to solid matrices, greatly simplifying the analysis of samples with MS and anticipating, to ease the required or even leave out any sample preparation and enable analysis at ambient conditions, outside the instrument itself. These developments greatly extend the range of applications of modern mass spectrometry (MS). Ambient methods comprise many techniques; a particular prominent group is, however, the plasma-based methods. Although ambient MS is a rather new field of research, the interest in further developing the corresponding techniques and enhancing their performance is very strong due to their simplicity and often low cost of manufacturing. A precondition for improving the performance of such ion sources is a profound understanding how ionization works and which parameters determine signal response. Therefore, we review relevant compound characteristics for ionization with the two traditional methods ESI and APCI and compare those with one of the most frequently employed representatives of the plasma-based methods, i.e., low temperature plasma ionization. We present a detailed analysis in which compound characteristics are most beneficial for the response of aromatic nitrogen-containing compounds with these three methods and provide evidence that desorption characteristics appear to have the main common, general impact on signal response. In conclusion, our report provides a very useful resource to the optimization of instrumental conditions with respect to most important requirements of the three ionization techniques and, at the same time, for future developments in the field of ambient ionization.

Flexible Microtube Plasma (FμTP) as an Embedded Ionization Source for a Microchip Mass Spectrometer Interface

Dielectric barrier discharges are used as soft ionization sources for mass spectrometers or ion mobility spectrometers, enabling excellent possibilities for analytical applications. A new robust and small-footprint discharge design, flexible microtube plasma (FμTP), developed as a result of ongoing miniaturization and electrode design processes, is presented in this work. This design provides major safety benefits by fitting the electrode into an inert flexible fused silica capillary (tube). Notably, in this context, the small discharge dimensions enable very low gas flows in the range of <100 mL min^-1; portability; the use of hydrogen, nitrogen, and air in addition to noble gases such as helium and argon, including its mixtures with propane; and application in microchip environments. By coupling FμTP with gas chromatography/mass spectrometry, we show that the polarity principle of the new discharge design allows it to outperform established ionization sources such as dielectric barrier discharge for soft ionization (DBDI) and low-temperature plasma (LTP) at low concentrations of perfluoroalkanes in terms of sensitivity, ionization efficiency, chemical background, linear dynamic range, and limit of detection by a large margin. In negative ion mode, the limit of detection is improved by more than 3-fold compared with that of DBDI and by 8-fold compared with that of LTP. The protonation capability was evaluated by headspace measurements of diisopropyl methylphosphonate in positive ion mode, showing low fragmentation and high stability in comparison to DBDI and LTP.

New Advances in Toxicological Forensic Analysis Using Mass Spectrometry Techniques

This article reviews mass spectrometry methods in forensic toxicology for the identification and quantification of drugs of abuse in biological fluids, tissues, and synthetic samples, focusing on the methodologies most commonly used; it also discusses new methodologies in screening and target forensic analyses, as well as the evolution of instrumentation in mass spectrometry.

Recent advances in ambient mass spectrometry of trace explosives

Ambient mass spectrometry has evolved rapidly over the past decade, yielding a plethora of platforms and demonstrating scientific advancements across a range of fields from biological imaging to rapid quality control. These techniques have enabled real-time detection of target analytes in an open environment with no sample preparation and can be coupled to any mass analyzer with an atmospheric pressure interface; capabilities of clear interest to the defense, customs and border control, transportation security, and forensic science communities. This review aims to showcase and critically discuss advances in ambient mass spectrometry for the trace detection of explosives.

Integration of paper spray ionization high-field asymmetric waveform ion mobility spectrometry for forensic applications

RATIONALE

Paper spray ionization (PSI) is an attractive ambient ionization source for mass spectrometry (MS) since it allows the combination of surface sampling and ionization. The minimal sample preparation inherent in this approach greatly reduces the time needed for analysis. However, the ions generated from interfering compounds in the sample and the paper substrate may interfere with the analyte ions. Therefore, the integration of PSI with high-field asymmetric ion mobility spectrometry (FAIMS) is of significant interest since it should reduce the background ions entering the mass analyzer without complicating the analysis or increasing analysis time. Here we demonstrate the integration of PSI with FAIMS/MS and its potential for analysis of samples of forensic interest.

METHODS

In this work, the parameters that can influence the integration, including sampling and ionization by paper spray, the FAIMS separation of analytes from each other and background interferences, and the length of time that a usable signal can be observed for explosives on paper, were evaluated with the integrated system.

RESULTS

In the negative ion analysis of 2,4,6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), amounts as low as 1 ng on paper were readily observed. The successful positive ion separation of a set of illicit drugs including heroin, methamphetamine, and cocaine was also achieved. In addition, the positive ion analysis of the chemical warfare agent simulants dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP) was evaluated.

CONCLUSIONS

The integration of PSI-FAIMS/MS was demonstrated for the analyses of explosives in negative ion mode and for illicit drugs and CW simulants in positive mode. Paper background ions that could interfere with these analyses were separated by FAIMS. The compensation voltage of an ion obtained by FAIMS provided an additional identification parameter to be combined with the mass spectrum for each analyte.

Ambient Ionization and Miniature Mass Spectrometry Systems for Disease Diagnosis and Therapeutic Monitoring

Mass spectrometry has become a powerful tool in the field of biomedicine. The combination of ambient ionization and miniature mass spectrometry systems could most likely fulfill a significant need in medical diagnostics, providing highly specific molecular information in real time for clinical and even point-of-care analysis. In this review, we discuss the recent development of ambient ionization and miniature mass spectrometers as well as their potential in disease diagnosis and therapeutic monitoring, with an emphasis on their capability in analysis of biofluids and tissues. We also speculate the future development of the integrated, miniature MS systems and provide our perspectives on the challenges in technical development as well as possible solutions for path forward.

Real-Time Quantitative Analysis of Valproic Acid in Exhaled Breath by Low Temperature Plasma Ionization Mass Spectrometry

Real-time analysis of exhaled human breath is a rapidly growing field in analytical science and has great potential for rapid and noninvasive clinical diagnosis and drug monitoring. In the present study, an LTP-MS method was developed for real-time, in-vivo and quantitative analysis of γ-valprolactone, a metabolite of valproic acid (VPA), in exhaled breath without any sample pretreatment. In particular, the effect of working conditions and geometry of the LTP source on the ions of interest, protonated molecular ion at m/z 143 and ammonium adduct ion at m/z 160, were systematically characterized. Tandem mass spectrometry (MS/MS) with collision-induced dissociation (CID) was carried out in order to identify γ-valprolactone molecular ions (m/z 143), and the key fragment ion (m/z 97) was used for quantitation. In addition, the fragmentation of ammonium adduct ions to protonated molecular ions was performed in-source to improve the signal-to-noise ratio. At optimum conditions, signal reproducibility with an RSD of 8% was achieved. The concentration of γ-valprolactone in exhaled breath was determined for the first time to be 4.83 (±0.32) ng/L by using standard addition method. Also, a calibration curve was obtained with a linear range from 0.7 to 22.5 ng/L, and the limit of detection was 0.18 ng/L for γ-valprolactone in standard gas samples. Our results show that LTP-MS is a powerful analytical platform with high sensitivity for quantitative analysis of volatile organic compounds in human breath, and can have potential applications in pharmacokinetics or for patient monitoring and treatment. Graphical Abstract ᅟ.

The Effects of Added Hydrogen on Noble Gas Discharges Used as Ambient Desorption/Ionization Sources for Mass Spectrometry

We demonstrate the effectiveness of using hydrogen-doped argon as the support gas for the dielectric barrier discharge (DBD) ambient desorption/ionization (ADI) source in mass spectrometry. Also, we explore the chemistry responsible for the signal enhancement observed when using both hydrogen-doped argon and hydrogen-doped helium. The hydrogen-doped argon was tested for five analytes representing different classes of molecules. Addition of hydrogen to the argon plasma gas enhanced signals for gas-phase analytes and for analytes coated onto glass slides in positive and negative ion mode. The enhancements ranged from factors of 4 to 5 for gas-phase analytes and factors of 2 to 40 for coated slides. There was no significant increase in the background. The limit of detection for caffeine was lowered by a factor of 79 using H2/Ar and 2 using H2/He. Results are shown that help explain the fundamental differences between the pure-gas discharges and those that are hydrogen-doped for both argon and helium. Experiments with different discharge geometries and grounding schemes indicate that observed signal enhancements are strongly dependent on discharge configuration. Graphical Abstract ᅟ.

Low-pressure barrier discharge ion source using air as a carrier gas and its application to the analysis of drugs and explosives

In this work, a low-pressure air dielectric-barrier discharge (DBD) ion source using a capillary with the inner diameter of 0.115 and 12 mm long applicable to miniaturized mass spectrometers was developed. The analytes, trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), 1,3,5,7-tetranitroperhydro-1,3,5,7-tetrazocine (HMX), pentaerythritol tetranitrate (PETN), nitroglycerine (NG), hexamethylene triperoxide diamine (HMTD), caffeine, cocaine and morphine, introduced through the capillary, were ionized by a low-pressure air DBD. The ion source pressures were changed by using various sizes of the ion sampling orifice. The signal intensities of those analytes showed marked pressure dependence. TNT was detected with higher sensitivity at lower pressure but vice versa for other analytes. For all analytes, a marked signal enhancement was observed when a grounded cylindrical mesh electrode was installed in the DBD ion source. Among nine analytes, RDX, HMX, NG and PETN could be detected as cluster ions [analyte + NO3 ](-) even at low pressure and high temperature up to 180 °C. The detection indicates that these cluster ions are stable enough to survive under present experimental conditions. The unexpectedly high stabilities of these cluster ions were verified by density functional theory calculation.

Miniature Flow-Through Low-Temperature Plasma Ionization Source for Ambient Ionization of Gases and Aerosols

The design and operation of an inexpensive, miniature low-temperature plasma ion source is detailed. The miniature low-temperature plasma ion source is operated in a "flow-through" configuration, wherein the gaseous or aerosolized analyte, caffeine or pyrolyzed ethyl cellulose, in a carrier gas is used as the plasma gas. In this flow-through configuration, the sensitivity for the caffeine standard and the pyrolysis products of ethyl cellulose is maintained or increased and the reproducibility of the ion source is increased. Changes in the relative intensity of ions from the aerosol produced by pyrolysis of ethyl cellulose are observed in the mass spectrum when the low-temperature plasma ion source is used in the flow-through configuration. Experiments suggest this change in relative intensity is likely due to differences in ionization efficiency rather than increased fragmentation of ethyl cellulose pyrolysis products during ionization. Flow-through low-temperature plasma ionization with the miniature ion source is shown to be a promising technique for the ionization of compounds in gases or aerosol particles.

Fast screening of analytes for chemical reactions by reactive low-temperature plasma ionization mass spectrometry

RATIONALE

Approaches for analyte screening have been used to aid in the fine-tuning of chemical reactions. Herein, we present a simple and straightforward analyte screening method for chemical reactions via reactive low-temperature plasma ionization mass spectrometry (reactive LTP-MS).

METHODS

Solution-phase reagents deposited on sample substrates were desorbed into the vapor phase by action of the LTP and by thermal desorption. Treated with LTP, both reagents reacted through a vapor phase ion/molecule reaction to generate the product. Finally, protonated reagents and products were identified by LTP-MS.

RESULTS

Reaction products from imine formation reaction, Eschweiler-Clarke methylation and the Eberlin reaction were detected via reactive LTP-MS. Products from the imine formation reaction with reagents substituted with different functional groups (26 out of 28 trials) were successfully screened in a time of 30 s each. Besides, two short-lived reactive intermediates of Eschweiler-Clarke methylation were also detected.

CONCLUSIONS

LTP in this study serves both as an ambient ionization source for analyte identification (including reagents, intermediates and products) and as a means to produce reagent ions to assist gas-phase ion/molecule reactions. The present reactive LTP-MS method enables fast screening for several analytes from several chemical reactions, which possesses good reagent compatibility and the potential to perform high-throughput analyte screening. In addition, with the detection of various reactive intermediates (intermediates I and II of Eschweiler-Clarke methylation), the present method would also contribute to revealing and elucidating reaction mechanisms.

Regulated bioanalysis of conformers - A case study with ASP2151 in dog plasma and urine

We developed and validated bioanalytical methods for a potent helicase-primase inhibitor ASP2151 that has two conformers. These conformers elute as unseparated broad peaks under ordinary high-performance liquid chromatographic conditions, indicating discernable differences in hydrophobicity. We observed that column temperature and mobile phase pH have no effect on these peaks and that conformers form a single symmetrical peak when tetrahydrofuran is added to the mobile phase. In addition, we needed to develop semi-automated methods where inter-conversion of the conformers is unlikely to cause sample-to-sample extraction variability. Briefly, following the addition of deuterium-labeled ASP2151 as an internal standard (IS), dog plasma samples or acetonitrile-added urine samples were filtrated. The filtrates were then injected into a column-switching liquid chromatography-tandem mass spectrometry (LC-MS/MS) system and trapped onto an extraction column. Extracts were back-flushed onto an analytical C18 column (4.6×50mm, 3μm) with a mobile phase consisting of methanol, tetrahydrofuran, and 20mmol/L ammonium acetate (45:5:50, v/v/v). The eluent was monitored in the negative atmospheric pressure chemical ionization mode. The calibration curve was linear over a range of 5-1000ng/mL for plasma and 0.5-100μg/mL for urine. Validation data met the acceptance criteria in accordance with regulatory guidance and demonstrated that these methods were selective, accurate, and reproducible. In addition, the present methods were successfully applied to a pharmacokinetic study in dogs.

Ambient ionization MS for bioanalysis: recent developments and challenges

Ambient ionization MS has become very popular in analytical science and has now evolved as an effective analytical tool in metabolomics, biological tissue imaging, protein and small molecule drug analysis, where biological samples are probed in a rapid and direct fashion with minimal sample preparation at ambient conditions. However, certain inherent challenges continue to hinder the vibrant prospects of these methods for in situ analyses or to replace conventional methods in bioanalysis. This review provides an introduction to the field and its application in bioanalysis, with an emphasis on the most recent developments and applications. Furthermore, ongoing challenges or limitations related to quantitation, sensitivity, selectivity, instrumentation and mass range of these ambient methods will also be discussed.

Plasma-based ambient mass spectrometry techniques: The current status and future prospective

Plasma-based ambient mass spectrometry is emerging as a frontier technology for direct analysis of sample that employs low-energy plasma as the ionization reagent. The versatile sources of ambient mass spectrometry (MS) can be classified according to the plasma formation approaches; namely, corona discharge, glow discharge, dielectric barrier discharge, and microwave-induced discharge. These techniques allow pretreatment-free detection of samples, ranging from biological materials (e.g., flies, bacteria, plants, tissues, peptides, metabolites, and lipids) to pharmaceuticals, food-stuffs, polymers, chemical warfare reagents, and daily-use chemicals. In most cases, plasma-based ambient MS performs well as a qualitative tool and as an analyzer for semi-quantitation. Herein, we provide an overview of the key concepts, mechanisms, and applications of plasma-based ambient MS techniques, and discuss the challenges and outlook.

Low-temperature plasma for compositional depth profiling of crosslinking organic multilayers: comparison with C60 and giant argon gas cluster sources

RATIONALE

For organic electronics, device performance can be affected by interlayer diffusion across interfaces. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) can resolve buried structures with nanometer resolution, but instrument artifacts make this difficult. Low-temperature plasma (LTP) is suggested as a way to prepare artifact-free surfaces for accurate determination of chemical diffusion.

METHODS

A model organic layer system consisting of three 1 nm delta layers of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) separated by three 30 nm layers of tris(8-hydroxyquinolinato)aluminum (Alq3) was used to evaluate the effectiveness of LTP etching for the preparation of crater edge surfaces for subsequent compositional depth profile analysis. This was compared with depth profiles obtained using an instrument equipped with an argon cluster sputter source.

RESULTS

The quality of the depth profiles was determined by comparing the depth resolutions of the BCP delta layers. The full width at half maximum gave depth resolutions of 6.9 nm and 6.0 nm using LTP, and 6.2 nm and 5.8 nm using argon clusters. In comparison, the 1/e decay length of the trailing edge gave depth resolutions of 2.0 nm and 1.8 nm using LTP, and 3.5 nm and 3.4 nm using argon clusters.

CONCLUSIONS

The comparison of the 1/e decay lengths showed that LTP can determine the thickness and composition of the buried structures without instrument artifacts. Although it does suffer from contaminant deposition, LTP was shown to be a viable option for preparing crater edges for a more accurate determination of buried structures.