High-throughput screening of PAHs and polar trace contaminants in water matrices by direct solid-phase microextraction coupled to a dielectric barrier discharge ionization source

Concentric Hybrid Nanoelectrospray Ionization-Atmospheric Pressure Chemical Ionization Source for High-Coverage Mass Spectrometry Analysis of Single-Cell Metabolomics

High-coverage mass spectrometry analysis of single-cell metabolomics remains challenging due to the extremely low abundance and wide polarity of metabolites and ultra-small volume in single cells. Herein, a novel concentric hybrid ionization source, nanoelectrospray ionization-atmospheric pressure chemical ionization (nanoESI-APCI), is ingeniously designed to detect polar and nonpolar metabolites simultaneously in single cells. The source is constructed by inserting a pulled glass capillary coaxially into a glass tube that acts as a dielectric barrier layer. Benefitting from the integrated advantages of nanoESI and APCI, its limit of detection is improved by one order of magnitude to 10 pg mL-1. After the operational parameter optimization, 254 metabolites detected in nanoESI-APCI are tentatively identified from a single cell, and 82 more than those in nanoESI. The developed nanoESI-APCI is successively applied to study the metabolic heterogeneity of human hepatocellular carcinoma tissue microenvironment united with laser capture microdissection (LCM), the discrimination of cancer cell types and subtypes, the metabolic perturbations to glucose starvation in MCF7 cells and the metabolic regulation of cancer stem cells. These results demonstrated that the nanoESI-APCI not only opens a new avenue for high-coverage and high-sensitivity metabolomics analysis of single cell, but also facilitates spatially resolved metabolomics study coupled with LCM.

Perspective on SPME-MS: Green and high-performance methods for rapid screening

The direct coupling of solid-phase microextraction (SPME) with mass spectrometry (MS) offers rapid analysis with high sensitivity and low matrix effects by benefiting from the integration of sampling, high enrichment, and clean-up functions of SPME. Eliminating chromatographic separation reduces the amount of gas/solvent needed for analysis, while direct desorption in SPME-MS consumes none or few microliters of organic solvents per sample, further enhancing the greenness of the SPME technology. Over the past two decades, the rapid evolution of SPME-MS has given rise to numerous novel technologies that employ diverse ionization techniques and interfaces, several of which have already been commercialized. Drawing from an extensive review published earlier this year and our research experience, we provide perspectives on three aspects of these technologies: interface design and automation, integration with state-of-art MS instrumentation, and anticipated future developments.

Nanospray Laser-Induced Plasma Ionization Mass Spectrometry for Rapid and Sensitive Analysis of Polycyclic Aromatic Hydrocarbons and Halogenated Derivatives

Polycyclic aromatic hydrocarbons (PAHs) and halogenated derivatives are a series of environmental pollutants with potential toxicity and health risks on biosystems and the ecosystem. Rapid and sensitive analysis of trace PAHs and halogenated PAHs in complex environmental samples is a challenging topic for analytical science. Here we report the development of a nanospray laser-induced plasma ionization MS method for rapid and sensitive analysis of trace PAHs and halogenated PAHs under ambient and open-air conditions. A nanospray tip was applied for loading samples and placed pointing to the MS inlet, being a nanospray emitter with the application of a high voltage. A beam of laser was focused to induce energetic plasma between the nanospray emitter and the MS inlet for ionization of PAHs and halogenated PAHs for mass spectrometric analysis. Meanwhile, an inner-wall naphthyl-coated nanospray emitter was developed and applied as a solid-phase microextraction (SPME) probe for highly selective enrichment of trace PAHs and halogenated PAHs in complex environmental samples, and some organic solvent was applied to desorb the analytes for nanospray laser-induced plasma ionization MS analysis. Satisfactory linearity for each target PAH and halogenated PAH was obtained, with correlation coefficient values (r) no less than 0.9917. The method showed extremely high sensitivity for analysis of trace PAHs and halogenated PAHs in water, with limits of detection (LODs) and quantification (LOQs) of 0.0001-0.02 and 0.0003-0.08 μg/L, respectively. By using the inner-wall naphthyl-coated nanospray laser-induced plasma ionization MS method, sensitive detection of trace PAHs and halogenated PAHs in real sewage and wastewater samples was successfully achieved.

Spatio-Temporal Analysis of Anesthetics in Mice by Solid-Phase Microextraction: Dielectric Barrier Discharge Ionization Mass Spectrometry

Local anesthetics, drugs that only affect a restricted area of the body, are widely used in daily clinical practice. Less studied but equally important is the distribution of local anesthetics inside organisms. Here, we present a rapid in situ testing method of drug distribution in various organs. The temporal and spatial distribution of anesthetics in mice was measured by solid-phase microextraction (SPME), thermal desorption (TD), and dielectric barrier discharge ionization (DBDI) atmospheric pressure mass spectrometry. A coated SPME probe using a tungsten wire as the support covered with a carbonaceous material was prepared by a simple, low-cost flame method. An in-line structure of the inlet allows TD and DBDI to share the same capillary tube, which greatly improves the transmission efficiency. Nine kinds of anesthetics, such as lidocaine and dyclonine, were detected, and the limit of detection was determined to be as low as 13 pg/mL. In addition, the time-dependent distribution of drugs in mice organs was studied. We also found that macromolecules in organisms do not noticeably interfere with the detection. This method is convenient and efficient because it does not require tissue homogenates and allows direct in situ detection. Compared with the conventional analytical methods, this method is simple and rapid, works in situ, and allows microscale analysis of trace analytes in biological organisms with high sensitivity.

Direct Coupling of SPME to Mass Spectrometry

Solid-phase microextraction devices are normally analyzed by gas or liquid chromatography. Their use has become increasingly widespread since their introduction in 1990, and nowadays most analytical laboratories use or have used SPME as an efficient and green method to perform analyte extraction and sample clean-up in one step. The SPME technique is intrinsically flexible, and allows for a high degree of optimization with regard to the extracting phase, as well as the way sample is analyzed. Since its introduction, researchers have been trying different ways to transfer analytes extracted from the solid phase to a mass spectrometer, with the aim to increase throughput and reduce solvent, gas usage and costs associated with conventional chromatographic techniques. Furthermore, but not less important, for pure fun of developing new, more efficient and sensitive analytical strategies! This chapter aims at providing a comprehensive overview of the most relevant non-chromatographic mass spectrometric approaches developed for SPME. Technical aspects of each SPME-MS approach will be discussed, highlighting their advantages, disadvantages and future potential developments. Particular emphasis will be given on the most recent direct coupling approaches using novel ionization approaches, and a concise overview of the existing applications will also be provided.

Discriminating alkylbenzene isomers with tandem mass spectrometry using a dielectric barrier discharge ionization source

Soft ambient ionization sources generate reactive species that interact with analyte molecules to form intact molecular ions, which allows rapid, sensitive, and direct identification of the molecular mass. We used a dielectric barrier discharge ionization (DBDI) source with nitrogen at atmospheric pressure to detect alkylated aromatic hydrocarbon isomers (C₈ H₁₀ or C₉ H₁₂ ). Intact molecular ions [M]^•+ were detected at 2.4 kV_pp , but at increased voltage (3.4 kV_pp ), [M + N]⁺ ions were formed, which could be used to differentiate regioisomers by collision-induced dissociation (CID). At 2.4 kV_pp , alkylbenzene isomers with different alkyl-substituents could be identified by additional product ions: ethylbenzene and -toluene formed [M-2H]⁺ , isopropylbenzene formed abundant [M-H]⁺ , and propylbenzene formed abundant C₇ H₇ ⁺ . At an operating voltage of 3.4 kV_pp , fragmentation of [M + N]⁺ by CID led to neutral loss of HCN and CH₃ CN, which corresponded to steric hindrance for excited state N-atoms approaching the aromatic ring (C-H). The ratio of HCN to CH₃ N loss (interday relative standard deviation [RSD] < 20%) was distinct for ethylbenzene and ethyltoluene isomers. The greater the number of alkyl-substituents (C-CH₃ ) and the more sterically hindered (meta > para > ortho) the aromatic core, the greater the loss of CH₃ CN relative to HCN was.

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.

Dielectric Barrier Discharge Ionization Mechanisms: Polycyclic Aromatic Hydrocarbons as a Case of Study

Dielectric barrier discharge ionization (DBDI) is a versatile tool for small-molecule mass spectrometry applications, helping cover from polar to low polar molecules. However, the plasma gas-phase interactions are highly complex and have been scarcely investigated. The ionization mechanisms of plasmas have long been assumed to be somewhat similar to atmospheric pressure chemical ionization (APCI). Here, we evaluated the ionization mechanisms of a two-ring DBDI ion source, using different discharge gases to analyze vaporized liquid samples. Polycyclic aromatic hydrocarbons (PAHs) were used as model analytes to assess the mechanisms' dominance: protonation, [M + H]⁺, or radical ion species formation, [M]^·+. In the present work, two different ionization trends were observed for APCI and DBDI during the PAH analysis; the compounds with proton affinities (PA) over 856 kJ/mol were detected as [M + H]⁺ when APCI was used as ionization source. Meanwhile, independently of the PA, DBDI showed the prevalence of charge exchange reactions. The addition of dopants in the gas-phase region shifted the ionization mechanisms toward charge exchange reactions, facilitating the formation of [M]^·+ ion species, showing anisole a significant boost of the PAH radical ion species signals, over nine times for Ar-Prop-DBDI analysis. The presence of high-energy metastable atoms (e.g., He^M) with high ionization potentials (IE = 19.80 eV) did not show boosted PAH abundances or extensive molecule fragmentation. Moreover, other species in the plasma jet region with closer and more appropriate IE, such as N₂ B³Π_g excited molecules, are likely responsible for PAH Penning ionization.

Hybrid Ionization Source Combining Nanoelectrospray and Dielectric Barrier Discharge Ionization for the Simultaneous Detection of Polar and Nonpolar Compounds in Single Cells

Single-cell metabolomics is expected to deliver fast and dynamic information on cell function; therefore, it requires rapid analysis of a wide variety of very small quantities of metabolites in living cells. In this work, a hybrid ionization source that combines nanoelectrospray ionization (nanoESI) and dielectric barrier discharge ionization (DBDI) is proposed for single-cell analysis. A capillary with a 1 μm i.d. tip was inserted into cells for sampling and then directly used as the nanoESI source for ionization of polar metabolites. In addition, a DBDI source was employed as a post-ionization source to improve the ionization of apolar metabolites in cells that are not easily ionized by ESI. By increasing the voltage of the DBDI source from 0 to 3.2 kV, the classes of detected metabolites can be shifted from mostly polar to both polar and apolar to mainly apolar. Plant cells (onion) and human cells (PANC-1) were investigated in this study. After optimization, 50 compounds in onion cells and 40 compounds in PANC-1 cells were observed in ESI mode (3.5 kV) and an additional 49 compounds in onion cells and 73 compounds in PANC-1 cells were detected in ESI (3.5 kV)-DBDI (2.6 kV) hybrid mode. This hybrid ionization source improves the coverage, ionization efficiency, and limit of detection of metabolites with different polarities and could potentially contribute to the fast-growing field of single-cell metabolomics.

Aramid-wrapped CNT hybrid sol–gel sorbent for polycyclic aromatic hydrocarbons

This work describes the preparation of an analytical microextraction sorbent using a simple and versatile sol–gel hybrid composite, i.e., aramid oligomers wrapping multi-walled carbon nanotubes (CNTs) covalently bonded to a porous silica network.

High-Throughput Single-Cell Mass Spectrometry Reveals Abnormal Lipid Metabolism in Pancreatic Ductal Adenocarcinoma

Even populations of clonal cells are heterogeneous, which requires high-throughput analysis methods with single-cell sensitivity. Here, we propose a rapid, label-free single-cell analytical method based on active capillary dielectric barrier discharge ionization mass spectrometry, which can analyze multiple metabolites in single cells at a rate of 38 cells/minute. Multiple cell types (HEK-293T, PANC-1, CFPAC-1, H6c7, HeLa and iBAs) were discriminated successfully. We found evidence for abnormal lipid metabolism in pancreatic cancer cells. We also analyzed gene expression in a cancer genome atlas dataset and found that the mRNA level of a critical enzyme of lipid synthesis (ATP citrate lyase, ACLY) was upregulated in human pancreatic ductal adenocarcinoma (PDAC). Moreover, both an ACLY chemical inhibitor and a siRNA approach targeting ACLY could suppress the viability of PDAC cells. A significant reduction in lipid content in treated cells indicates that ACLY could be a potential target for treating pancreatic cancer.

Ambient electric arc ionization for versatile sample analysis using mass spectrometry

A novel, convenient ambient electric arc ionization (AEAI) device was developed as a mass spectrometry ion source for versatile sample analysis. AEAI could be considered as a soft ionization technique in which the protonated ion ([M + H]⁺) is the main ion species with little or no in-source fragmentation for most analytes. Coupled with a high-resolution Orbitrap mass spectrometer, AEAI could be applied to the analysis of a variety of organic compounds having a wide range of polarities, ranging from non-polar species such as polybenzenoid aromatic hydrocarbons (PAHs) to highly polar species such as amino acids. With its versatile capabilities in the mass spectrometric analysis of small molecules, AEAI has the potential to be an alternative to traditional ionization methods such as electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and electron impact (EI) ionization. The limitations of AEAI are also discussed.

Gas Chromatography-Atmospheric Pressure Inlet-Mass Spectrometer Utilizing Plasma-Based Soft Ionization for the Analysis of Saturated, Aliphatic Hydrocarbons

Soft ionization by a chemical reaction in transfer (SICRIT) is applied to couple gas chromatography (GC) to a high-resolution atmospheric pressure inlet mass spectrometer. These instruments are generally used in combination with liquid chromatography systems (LC-MS). Ionization of alkanes is not possible here with conventional electrospray ionization. Alternatively, separate GC-electron ionization (EI)-MS is employed for the analysis of nonpolar substances like alkanes, however, with the inherent challenge of strong fragmentation. In the case of alkanes, the determination of molecular masses becomes nearly impossible in complex hydrocarbon mixtures because of the wealth of similar fragment ions and the absence of the molecular ion signal. SICRIT, a soft ionization technique based on dielectric barrier discharge (DBDI), produces characteristic oxidized cations from alkanes that can be directly correlated to their molecular mass. Isotope labeling experiments reveal an ionization mechanism via hydride abstraction and reaction with water. Soft ionization can be achieved for iso- and n-alkanes, with very little fragmentation, enabling the determination of their molecular mass. Calibrations for n-alkanes from C10 to C30 were performed exhibiting high linearity, reproducibility, and sensitivity with an average LOD of 69 pg (on column). Measurements of diesel fuel samples are compared to traditional GC-EI-MS. The presented method combines sensitivity and easy handling of a GC-EI-MS with the determination of molecular mass commonly only achieved with field ionization (FI)-MS, while using existing and highly optimized mass spectrometers commonly coupled with LC. Additionally, many other analytes such as (alkylated-) PAHs could be detected simultaneously in the diesel sample.

Development of an inverse low-temperature plasma ionization source for liquid chromatography/mass spectrometry

RATIONALE

An argon inverse low-temperature plasma (iLTP) ionization source for liquid chromatography/tandem mass spectrometry was developed. The iLTP is constructed from simple chromatographic supply materials and is implemented into an atmospheric pressure chemical ionization (APCI) source replacing the APCI discharge needle electrode. The newly developed ion source was coupled to an ultra-high-performance liquid chromatography (UHPLC) system.

METHODS

The argon iLTP was characterized by optical emission spectroscopy. The soft ionization of selected standards was also demonstrated by direct infusion experiments. In addition to the use of argon as the discharge gas, helium, synthetic air, and oxygen were used, which were tested for their performance using testosterone and vitamin D₃ .

RESULTS

Spectroscopic measurements of the argon plasma were conducted, demonstrating the main emission band of argon metastables with corresponding energies of 11.53 eV and 11.72 eV. Infusion experiments indicate a gentle ionization by iLTP, e.g. caffeine, testosterone, reserpine, vitamin D₃ , and 25-hydroxyvitamin D₃ , which resulted in the corresponding protonated molecules. The splitless coupling with UHPLC (possible flow rates >1000 μL min^-1 ) shows promising results in interday repeatability (n = 10) for the substances with a relative standard deviation of less than 5% and limits of detection for caffeine, testosterone, reserpine, vitamin D₃ , and 25-hydroxyvitamin D₃ of 10 ng L^-1 , 50 ng L^-1 , 500 ng L^-1 , 5 μg L^-1 , and 5 μg L^-1 , respectively.

CONCLUSIONS

The argon iLTP ion source presented in this work shows promising approaches in the field of ionization of small organic molecules. The mechanism related to the discharge gas argon has not been elucidated so far and further investigations are needed. The iLTP ion source shows a very good performance with UHPLC coupling, even at increased flow rates. It could be shown that an argon iLTP can compete with the helium dielectric barrier discharge (DBD) preferred in the literature, making it a more economical choice.

Nebulization dielectric barrier discharge ionization mass spectrometry: Rapid and sensitive analysis of acenaphthene

A rapid, simple and sensitive method was proposed for low-polar acenaphthene analysis by coupling nebulization with dielectric barrier discharge ionization (N-DBDI). The sample solution was nebulized followed by heating and converted to be gas-phase analyte molecules prior to DBDI. This boosts the collision efficiency of analyte molecules with reactive species and thus the sensitivity, and the high-velocity gas from nebulization guides ions directed to the MS inlet without deflection. The dependence of sensitivity on the operation parameters was systematically investigated. The LOD and LOQ of acenaphthene were determined to be 0.61 ng/L and 2.05 ng/L, respectively, which were superior approximately 30 folds compared to those obtained by other methods. Parameters, including accuracy, precision, reproducibility and utility, were tested to further evaluate the performance of N-DBDI. Real environmental samples, including river water, initial rainwater and mineral water, were analyzed with good accuracy (93.61-103.50%) and satisfactory precision (RSD ≤ 8.92%). These findings suggest that the N-DBDI allows the determination of non/low-polar species at sub-pg/mL possible, and would benefit for the non/low-polar species analysis in real environmental samples.

Rapid screening and quantitation of PAHs in water and complex sample matrices by solid-phase microextraction coupled to capillary atmospheric pressure photoionization-mass spectrometry

A capillary atmospheric pressure photoionization (cAPPI) source was used to analyze polycyclic aromatic hydrocarbons (PAHs) in complex matrices like grilled meat extract and urban dust reference material, as well as screening for PAHs in aqueous samples such as tap and lake water. A high-throughput workflow was developed that allowed rapid screening of unknown samples by direct solid-phase microextraction (SPME) coupled with cAPPI-MS, with confirmatory gas chromatography performed only for samples containing trace amounts of PAHs. Extraction times were as low as 15 s, with a total analysis time of 2 min per sample for screening. Limits of detections were in the low pg/ml range and in the subpg/ml range for the direct and chromatographic approach, respectively, with a linear dynamic range between two and three orders of magnitude, as determined for 15 model PAHs. This rapid approach represents an attractive way to screen samples containing nonpolar compounds using an ambient ionization source.

A 3-in-1 Hand-Held Ambient Mass Spectrometry Interface for Identification and 2D Localization of Chemicals on Surfaces

Desorption electrospray ionization (DESI), easy ambient sonic-spray ionization (EASI) and low-temperature plasma (LTP) ionization are powerful ambient ionization techniques for mass spectrometry. However, every single method has its limitation in terms of polarity and molecular weight of analyte molecules. After the miniaturization of every possible component of the different ion sources, we finally were able to embed two emitters and an ion transfer tubing into a small, hand-held device. The pen-like interface is connected to the mass spectrometer and a separate control unit via a bundle of flexible tubing and cables. The novel device allows the user to ionize an extended range of chemicals by simple switching between DESI, voltage-free EASI, or LTP ionization as well as to freely move the interface over a surface of interest. A mini camera, which is mounted on the tip of the pen, magnifies the desorption area and enables a simple positioning of the pen. The interface was successfully tested using different types of chemicals, pharmaceuticals, and real life samples. Moreover, the combination of optical data from the camera module and chemical data obtained by mass analysis facilitates a novel type of imaging mass spectrometry, which we name “interactive mass spectrometry imaging (IMSI)”.

Hunting Molecules in Complex Matrices with SPME Arrows: A Review

Thirty years since the invention and public disclosure of solid phase microextraction (SPME), the technology continues evolving and inspiring several other green extraction technologies amenable for the collection of small molecules present in complex matrices. In this manuscript, we review the fundamental and operational aspects of a novel SPME geometry that can be used to “hunt” target molecules in complex matrices: the SPME Arrow. In addition, a series of applications in environmental, food, cannabis and forensic analysis are succinctly covered. Finally, special emphasis is placed on novel interfaces to analytical instrumentation, as well as recent developments in coating materials for the SPME Arrow.

Solid-phase microextraction Arrow for the volatile organic compounds in soy sauce

A novel solid-phase microextraction Arrow was used to separate volatile organic compounds from soy sauce, and the results were verified by using gas chromatography with mass spectrometry. Solid-phase microextraction Arrow was optimized in terms of three extraction conditions: type of fiber used (polydimethylsiloxane, polyacrylate, carbon wide range/polydimethylsiloxane, and divinylbenzene/polydimethylsiloxane), extraction temperature (40, 50, and 60°C), and extraction time (10, 30, and 60 min). The optimal solid-phase microextraction Arrow conditions were as follows: type of fiber = polyacrylate, extraction time = 60 min, and extraction temperature = 50°C. Under the optimized conditions, the solid-phase microextraction Arrow was compared with conventional solid-phase microextraction to determine extraction yields. The solid-phase microextraction Arrow yielded 6-42-fold higher levels than in solid-phase microextraction for all 21 volatile organic compounds detected in soy sauce due to the larger sorption phase volume. The findings of this study can provide practical guidelines for solid-phase microextraction Arrow applications in food matrixes by providing analytical methods for volatile organic compounds.

Fast screening of illicit drugs in beverages and biological fluids by direct coupling of thin film microextraction to dielectric barrier discharge ionization-mass spectrometry

A direct and fast method for quantification of illicit drugs in beverages and biological fluids was developed, using dielectric barrier discharge ionization in combination with high-resolution MS.