A study of the transient gas flow affected ion transmission in atmospheric pressure interfaces based on large eddy simulation for electrospray ionization mass spectrometry

Modern atmosphere pressure interface (API) enables high-efficiency coupling between mass analyzers in high vacuum and atmosphere ionization sources such as electrospray ionization (ESI) source. The transient gas flow entering API possesses strong compressibility and turbulent characteristics, which exerts a huge impact on ion transmission. However, the instantaneous nature and vortical morphology of the turbulence in API and its affection in ion transmission were hardly covered in the reported research. Here we conduct a transient turbulent flow-affected ion transmission evaluation for two typical APIs, the ion funnel and the S-lens, based on scale-resolving large eddy simulation and electro-hydrodynamical ion tracing simulation. In our simulation, the transient properties of the gas flow in the two APIs are illustrated and analyzed in-depth. After experimentally validated on a homemade ESI-TOF-MS platform, the results suggest that the ion funnel can achieve a higher droplet desolvation rate by introducing a unique droplet recirculation mechanism. Meanwhile, the less-dispersed gas flow in S-lens is beneficial in actuating ions axially. In conclusion, the application of the scale-resolving turbulence model helps us to understand the complicated fluid-ion interaction mechanism in APIs and is promising in the development of mass spectrometry instruments of higher performance.

Comment on the "Eco-friendly and efficient extraction of polyphenols from Ligustrum robustum by deep eutectic solvent assisted ultrasound"

Qin et al. have proposed an ecological and efficient method of extraction polyphenols from the leaves of Ligustrum robustum using eutectic solvents, assisted by ultrasounds. The authors tested a number of solvents, optimized the extraction parameters and explored the extraction mechanism. According to the results of biological activity experiments of the prepared Ligustrum robustum leaves extracts, they seem promising for practical applications. The issue which is disputable is the identification of the phenolic compounds by using electrospray ionization mass spectrometry, as described in details in this comment.

Identification of oxygenated triacylglycerols in pistachio nuts' lipids by reverse-phase high-resolution-liquid chromatography-ESI mass spectrometry

Hydroxy- and peroxy-triacylglycerols are common products of lipid peroxidation formed during oil storage or heating or as enzymatic oxidation product of arachidonic acid as signaling molecules in mammals. In this study, oxygenated triacylglycerides (TAG) were identified in pistachio oil based on reverse phase(RP), high-performance liquid chromatography coupled with electrospray ionization and mass spectrometry (HPLC- ESI -MS). 20 novel lipid plant metabolites, classified based on their fragment spectra into a hydroxy (TAG-OH), an epoxy (TAG-O), and hydroperoxide (TAG-OOH) groups. We believe that this class of compounds has been for the first time observed as genuine secondary plant metabolites in a natural source in this case pistachio lipids of dietary relevance.

Capillary self-aspirating electrospray ionization (CSESI) for convenient and versatile mass spectrometry analysis

Electrospray ionization (ESI) is one of the most widely used ionization techniques, and its simplification can benefit many interested users. In this study, sample introduction by capillary action was studied and used to develop a simple ESI source called capillary self-aspirating electrospray ionization (CSESI). A conventional CSESI source requires only a common capillary of appropriate diameter in addition to the support of high voltage (HV). No pumps and sample loading is needed because the solution can spontaneously climb across the capillary by capillary action. With the proper modification of the glass capillary, the operation of CSESI can be further simplified and efficient. Specifically, cold plasma processing of the capillary creates a more hydrophilic surface that can facilitate sample introduction. Moreover, sputtering a thin platinum layer on the capillary tip makes the application of HV more convenient, and it also eliminates the influence of air bubbles in the capillary to ensure a sustained and stable electrospray. Overall, CSESI exhibits multiple desirable features such as simple structure, self-aspiration ability, low sample consumption, and inherent physical filtration capability. Apart from the routine ESI-MS analysis, it has also been applied in real-time monitoring of the oxidative dimerization of 8-methyl-1,2,3,4-tetrahydroquinoline, as well as direct analysis of muddy soil solutions without pretreatment.

A 3D-printed sub-atmospheric pressure electrospray ionization source for robust, facile, and flexible mass spectrometry analysis

The operation and performance of electrospray ionization (ESI) is affected by the surrounding environment. In this study, a compact sub-atmospheric pressure ESI (SAP-ESI) source was designed and fabricated using the 3D printing method. This source has a simple structure and is easy to operate, as the sample solution and auxiliary gas are continuously sucked into the source through the pressure difference. The compact and enclosed ionization chamber can reduce the fluctuation of the surrounding gas flow to ensure a remarkably stable (< 3%) electrospray. Moreover, the source can offer variable SAP conditions for ESI analysis. The yield of analyte ions increases with decreasing pressure, while the production of background ions is suppressed under these conditions. In the analysis of protein samples, SAP-ESI can increase the yield and charge state of ions, which may be due to the reduction of proton transfer between charged proteins and surrounding gas. The SAP-ESI source was then used to continuously monitor the extract aqueous solution of tea leaves, and to detect the carbendazim residues on the apple surface by coupling with the liquid extraction surface analysis technique. Experimental results demonstrate that the developed SAP-ESI is a stable, practical, and versatile ionization technique.

Differentiation of free d-amino acids and amino acid isomers in solution using tandem mass spectrometry of hydrogen-bonded clusters

Free d-amino acids and amino acid isomers were differentiated using tandem mass spectrometry without chromatographic separation. Ultraviolet photodissociation and water adsorption of leucine (Leu) and isoleucine (Ile) enantiomers hydrogen-bonded with tryptophan (Trp) were investigated at 8 K in the gas phase. The enantiomer-selective Cα-Cβ bond cleavage of Trp was observed in the product ion spectra obtained by 285 nm photoexcitation, where the abundance of NH2CHCOOH-eliminated ion of heterochiral H+(d-Trp)(l-Leu) was higher than that of homochiral H+(l-Trp)(l-Leu). When comparing water adsorption on the surfaces of the heterochiral and homochiral clusters in a cold ion trap, the number of water molecules adsorbed on the heterochiral cluster was greater than that adsorbed on the homochiral cluster. These results indicate that the stronger intermolecular interactions within the homochiral H+(l-Trp)(l-Leu) compared to the heterochiral cluster inhibit enantiomer-selective photodissociation. Leu and Ile were differentiated by the isomer-selective Cα-Cβ bond cleavage of Trp in the clusters. Calibration curves for the differentiation of isomeric amino acids and their enantiomers were developed using monitoring isomer- and enantiomer-selective photodissociation, indicating that the molar fractions in solution could be determined from a single product ion spectrum.

Formation and Tandem Mass Spectrometry of Doubly Charged Lipid-Metal Ion Complexes

Phospholipids are major components of most eukaryotic cell membranes. Changes in metabolic states are often accompanied by phospholipid structure variations. The structural changes of phospholipids are the hallmark of disease states, or specific lipid structures have been associated with distinct organisms. Prime examples are microorganisms that synthesize phospholipids with, for example, different branched chain fatty acids. Assignment and relative quantitation of structural isomers of phospholipids that arise from attachment of different fatty acids to the glycerophospholipid backbone are difficult with routine tandem mass spectrometry or with liquid chromatography without authentic standards. In this work, we report on the observation that all investigated phospholipid classes form doubly charged lipid-metal ion complexes during electrospray ionization (ESI) and show that these complexes can be used to assign lipid classes and fatty acid moieties, distinguish isomers of branched chain fatty acids, and relatively quantify these isomers in positive-ion mode. Use of water free methanol and addition of divalent metal salts (100 mol %) to ESI spray solutions afford highly abundant doubly charged lipid-metal ion complexes (up to 70 times of protonated compounds). Higher-energy collisional dissociation and collision-induced dissociation of doubly charged complexes yield a diverse set of lipid class-dependent fragment ions. In common for all lipid classes is the liberation of fatty acid-metal adducts that yield fragment ions from the fatty acid hydrocarbon chain upon activation. This ability is used to pinpoint sites of branching in saturated fatty acids and is showcased for free fatty acids as well as glycerophospholipids. The analytical utility of doubly charged phospholipid-metal ion complexes is demonstrated by distinguishing fatty acid branching-site isomers in phospholipid mixtures and relatively quantifying the corresponding isomeric compounds.

Identification, structure elucidation and origin of a common pyridinium-thiocyanate intermediate in electrospray mass spectrometry among the benziamidazole-class proton pump inhibitors

During the analysis of benziamidazole-class irreversible proton pump inhibitors, an unusual mass spectral response with the mass-to-charge ratio at [M+10]⁺ intrigued us, as it couldn't be assigned to any literature known relevant structure, intermediate or adduct ion. Moreover, this mysterious mass pattern of [M+10]⁺ has been gradually observed by series of marketed proton pump inhibitors, viz. omeprazole, pantoprazole, lansoprazole and rabeprazole. All the previous attempts to isolate the corresponding component were unsuccessful. The investigation of present work addresses this kind of signal to a pyridinium thiocyanate mass spectral intermediate (10), which is the common fragment ion of series of labile aggregates. The origin of such aggregates can be traced to the reactive intermediates formed by acid-promoted degradation. These reactive intermediates tend to react with each other and give raise series of complicated aggregates systematically in a water/acetonitrile solution by electrospray ionization. The structure of the corresponding pyridinium thiocyanate species of omeprazole (10a) has been eventually characterized with the help of synthetic specimen (10a'). Our structural proposal as well as its origin was supported by in situ nuclear magnetic resonance, chemical derivatization and colorimetric experiments.

Minimally Invasive Liquid Microjunction Sampling to Identify Dye on a 17th Century Spanish Illuminated Manuscript

Common sampling methods for mass spectrometry such as sectioning are undesirably damaging to cultural heritage objects. A liquid microjunction sampling technique is developed that uses minimal solvent volume for analysis. Painted illustrations on a 17th century parchment manuscript from Spain were analyzed to identify the organic red pigment throughout its pages. By extracting with 0.1 μL solvent, the pigment was supplied for direct infusion electrospray MS, and the resulting disruption to the object surface was practically invisible to the naked eye.

Understanding mobile phase buffer composition and chemical structure effects on electrospray ionization mass spectrometry response

Mobile phase selection is of critical importance in liquid chromatography - mass spectrometry (LC-MS) based studies, since it affects retention, chromatographic selectivity, ionization, limits of detection and quantification, and linear dynamic range. Generalized LC-MS mobile phase selection criteria, suitable for a broad class of chemical compounds, do not exist thus far. Here we have performed a large-scale qualitative assessment of the effect of solvent composition used for reversed-phase LC separations on electrospray ionization (ESI) response for 240 small molecular weight drugs, representing various chemical compound classes. Of these 240 analytes 224 were detectable using ESI. The main chemical structural features affecting ESI response were found to all be surface area or surface charge-related. Mobile phase composition was found to be less differentiating, although for some compounds a pH effect was noted. Unsurprisingly, chemical structure was found to be the dominant factor for ESI response for the majority of the investigated analytes, representing about 85% of the replicating detectable complement of the sample data set. A weak correlation between ESI response and structure complexity was observed. Solvents based on isopropanol, and those containing phosphoric or di- and trifluoracetic acids, performed relatively poorly in terms of chromatographic or ESI response, whilst the best performing 'generic' LC solvents were based on methanol, acetonitrile using formic acid and ammonium acetate as buffer components, consistent with current practice in many laboratories.

Influence of mobile phase composition on the analytical sensitivity of LC-ESI-MS/MS for the concurrent analysis of bisphenols, parabens, chlorophenols, benzophenones, and alkylphenols

Phenols are significant environmental endocrine disruptors that can have adverse health effects on exposed individuals. Correlating phenol exposure to potential health implications requires the development of a comprehensive and sensitive analytical method capable of analyzing multiple phenols in a single sample preparation and analytical run. Currently, no such method is available for multiple classes of phenols due to electrospray ionization (ESI) limitations in concurrent ionization and lack of sensitivity to certain phenols, particularly alkylphenols. In this study, we investigated the influence of mobile phase compositions in ESI on concurrent ionization and analytical sensitivity of liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) during the analysis of multiple classes of phenols, and we propose a comprehensive and sensitive analytical method for various classes of phenols (i.e., bisphenols, parabens, benzophenones, chlorophenols, and alkylphenols). The proposed method was affected by 0.5 mM ammonium fluoride under methanol conditions. It enabled the concurrent ionization of all the phenols and significantly improved the analytical sensitivity for bisphenols and alkylphenols, which typically have poor ionization efficiency. This method, combined with a "dilute and shoot" approach, allowed us to simultaneously quantify 38 phenols with good chromatographic behavior and sensitivity. Furthermore, the method was successfully applied to the analysis of 61 urine samples collected from aquatic (swimming) and land (indoor volleyball and outdoor football) athletes.

Systematic analysis of in-source modifications of primary metabolites during flow-injection time-of-flight mass spectrometry

Flow-injection mass spectrometry (FI-MS) enables metabolomics studies with a very high sample-throughput. However, FI-MS is prone to in-source modifications of analytes because samples are directly injected into the electrospray ionization source of a mass spectrometer without prior chromatographic separation. Here, we spiked authentic standards of 160 primary metabolites individually into an Escherichia coli metabolite extract and measured the thus derived 160 spike-in samples by FI-MS. Our results demonstrate that FI-MS can capture a wide range of chemically diverse analytes within 30 s measurement time. However, the data also revealed extensive in-source modifications. Across all 160 spike-in samples, we identified significant increases of 11,013 ion peaks in positive and negative mode combined. To explain these unknown m/z features, we connected them to the m/z feature of the (de-)protonated metabolite using information about mass differences and MS2 spectra. This resulted in networks that explained on average 49 % of all significant features. The networks showed that a single metabolite undergoes compound specific and often sequential in-source modifications like adductions, chemical reactions, and fragmentations. Our results show that FI-MS generates complex MS1 spectra, which leads to an overestimation of significant features, but neutral losses and MS2 spectra explain many of these features.

Desorption electrospray ionization-mass spectrometry imaging of carnitine and imidazole dipeptides in pork chop tissues

Carnitine is essential for energy production and lipid metabolism in skeletal muscle. Carnosine and its methylated analogs anserine and balenine are histidine-containing imidazole dipeptides, which are antioxidative compounds. They are major health-related components in meat; however, analytical technique to investigate their distribution among tissues have not fully established. Here, we performed desorption electrospray ionization (DESI)-mass spectrometry imaging (MSI) of pork chop sections containing longissimus thoracis et lumborum muscle (loin), intermuscular fat tissue, transparent tissue, and spinalis muscle to investigate the distributions of carnitine and imidazole dipeptides. Liquid chromatography-MS revealed that the concentrations of carnitine, carnosine, anserine, and balenine were 11.0 ± 0.9, 330.1 ± 15.5, 21.2 ± 1.5, and 9.6 ± 0.5 mg/100 g, respectively. In the mass spectrum obtained by DESI-MSI, peaks corresponding to the chemical formulae of carnitine and imidazole dipeptides were detected. DESI-MSI provided definite identification of carnitine, while DESI-tandem MSI (MS/MSI) was necessary to accurately visualize carnosine, anserine, and balenine. Carnitine and these imidazole dipeptides were mainly distributed in the loin and spinalis muscle, while their distribution was not uniform in both muscle tissues. In addition, the balance between both tissues were different. The concentration of carnitine was higher in the spinalis muscle than that in the loin, while those of imidazole dipeptides were higher in the loin than those in the spinalis muscle. These results were consistent with those obtained by liquid chromatography-MS quantification, suggest that DESI-MSI analysis is useful for the distribution analysis of carnitine and imidazole dipeptides in meat.

Online protein unfolding characterized by ion mobility electron capture dissociation mass spectrometry: cytochrome C from neutral and acidic solutions

Electrospray ionization mass spectrometry (ESI-MS) experiments, including ion mobility spectrometry mass spectrometry (ESI-IMS-MS) and electron capture dissociation (ECD) of proteins ionized from aqueous solutions, have been used for the study of solution-like structures of intact proteins. By mixing aqueous proteins with denaturants online before ESI, the amount of protein unfolding can be precisely controlled and rapidly analyzed, permitting the characterization of protein folding intermediates in protein folding pathways. Herein, we mixed various pH solutions online with aqueous cytochrome C for unfolding and characterizing its unfolding intermediates with ESI-MS charge state distribution measurements, IMS, and ECD. The presence of folding intermediates and unfolded cytochrome c structures were detected from changes in charge states, arrival time distributions (ATDs), and ECD. We also compared structures from nondenaturing and denaturing solution mixtures measured under "gentle" (i.e., low energy) ion transmission conditions with structures measured under "harsh" (i.e., higher energy) transmission. This work confirms that when using "gentle" instrument conditions, the gas-phase cytochrome c ions reflect attributes of the various solution-phase structures. However, "harsh" conditions that maximize ion transmission produce extended structures that no longer correlate with changes in solution structure.

Development of a dual-electrospray ionization source with in-line absorbance-based voltage control

Emitter tip arrays for electrospray ionization have been used for a variety of MS sample introduction purposes, including detection of multiple sample eluent streams and improved accuracy through parallel infusion of an internal standard. User control is typically required for targeted application of high voltage to specific channels to maximize analyte signal and minimize other background signals. In this communication, an automated approach to applying electrospray voltage only when a detectable analyte is present is described. An in-line absorbance detector is used to identify the presence of an analyte in the fluidic path between the sample introduction valve and the mass spectrometer. A Raspberry Pi-controlled system is then used to apply high voltage to a downstream emitter tip at the MS inlet following a delay volume between the detectors. Demonstration of this technique on two parallel sample channels is reported, including a pulsed voltage application to maximize signal when analytes elute on each channel simultaneously.

Dinuclear complex-induced DNA melting

Dinuclear copper complexes have been designed for molecular recognition in order to selectively bind to two neighboring phosphate moieties in the backbone of double strand DNA. Associated biophysical, biochemical and cytotoxic effects on DNA were investigated in previous works, where atomic force microscopy (AFM) in ambient conditions turned out to be a particular valuable asset, since the complexes influence the macromechanical properties and configurations of the strands. To investigate and scrutinize these effects in more depth from a structural point of view, cutting-edge preparation methods and scanning force microscopy under ultra-high vacuum (UHV) conditions were employed to yield submolecular resolution images. DNA strand mechanics and interactions could be resolved on the single base pair level, including the amplified formation of melting bubbles. Even the interaction of singular complex molecules could be observed. To better assess the results, the appearance of treated DNA is also compared to the behavior of untreated DNA in UHV on different substrates. Finally, we present data from a statistical simulation reasoning about the nanomechanics of strand dissociation. This sort of quantitative experimental insights paralleled by statistical simulations impressively shade light on the rationale for strand dissociations of this novel DNA interaction process, that is an important nanomechanistic key and novel approach for the development of new chemotherapeutic agents.

Quantification of disaccharides in solution using isomer-selective ultraviolet photodissociation of hydrogen-bonded clusters in the gas phase

Chemical properties of gas-phase hydrogen-bonded clusters were investigated as a model for interstellar molecular clouds. Cold gas-phase hydrogen-bonded clusters of tryptophan (Trp) enantiomers and disaccharide isomers, including d-maltose and d-cellobiose, were generated by electrospray ionization and collisional cooling in an ion trap at 8 K. Product ion spectra in the 265-290 nm wavelength range were obtained using tandem mass spectrometry. NH₂CHCOOH loss via the C_α-C_β bond cleavage of Trp occurred frequently in homochiral H⁺(d-Trp)(d-maltose) compared with heterochiral H⁺(l-Trp)(d-maltose) at 278 nm, indicating that an enantiomeric excess of l-Trp was formed via the enantiomer-selective photodissociation. The photoreactivity differed between the enantiomers and isomers contained in the clusters at the photoexcitation of 278 nm. A calibration curve for the quantification of disaccharide isomers in solution was constructed by photoexcitation of the hydrogen-bonded clusters of disaccharide isomers with H⁺(l-Trp) at 278 nm. A linear relationship between the natural logarithm of the relative product ion abundance and the mole fraction of d-maltose to d-cellobiose ratio in the solution was obtained, indicating that the mole fraction could be determined from a single product ion spectrum. A calibration curve, for quantification of Trp enantiomers, was also obtained using d-maltose as a chiral auxiliary.

Geochemical matrix differently affects the response of internal standards and target analytes for pesticide transformation products measured in groundwater samples

Electrospray ionization (ESI) is the most common technique in liquid chromatography coupled to tandem mass spectrometry (LC-MS-MS) allowing for sensitive detection of polar compounds with online water concentration. The technique is popular in groundwater monitoring programs and has permitted great progress in the detection and quantification of polar pesticide transformation products (TP) in recent years. However, ESI is also known to be prone to matrix effects. The common solution to this potential bias is the use of labelled internal standards. Unfortunately, these are not available for all target compounds, which leads to the linkage of target compounds to non-homologue internal standards with unknown consequences for quantification in variable geochemical settings. We investigated these matrix effects for polar TP with a molecular mass range of 225-350 Da and logD_pH7 between -0.27 and -1.7 as well as for parent compounds with logD_pH3 between 0.84 and 3.22. The acquired internal standards were tested on a gradient of DOC, anions, conductivity and inorganic carbon with a set of ten carefully chosen groundwater samples. Internal standards that were measured in positive ionization mode proved to be insensitive to geochemical variations while those that were measured in negative ionization mode showed reduced response with increasing anion concentration. All pairs of internal standards and target analytes were investigated for deviating matrix effects using standard addition experiments. Positive ionization compounds and target compounds with deuterated homologues showed little deviation while non-homologue pairs in negative mode proved to be strongly biased. Although bias was up to factor five for some compounds it was remarkably stable over the entire gradient studied, suggesting an identical suppression mode at varying matrix levels for different compounds. We advocate the conduct of standard addition experiments if homologue internal standards are not available.

Detection of Biosignatures by Capillary Electrophoresis Mass Spectrometry in the Presence of Salts Relevant to Ocean Worlds Missions

Capillary electrophoresis (CE) is a promising liquid-based technique for in situ chemical analysis on ocean worlds that allows the detection of a wide range of organic molecules relevant to the search for life. CE coupled with mass spectrometry (MS) is particularly valuable as it also enables the discovery of unknown compounds. Here we demonstrate that CE coupled to MS via electrospray ionization (ESI) can readily analyze samples containing up to half the saturation levels of salts relevant to ocean worlds when using 5 M acetic acid as the separation media. A mixture containing amino acids, peptides, nucleobases, and nucleosides was analyzed in the presence of two salts, NaCl and MgSO₄, based on their relevance to Europa and Enceladus. We demonstrate here CE-MS limits of detection for these organics ranging from 0.05 to 1 μM (8 to 89 ppb) in the absence of salts. More importantly, we demonstrate here for the first time that organics in the low micromolar range (1-50 μM) are detected by CE-MS in the presence of 3 M NaCl without desalting, preconcentration, or derivatization. This demonstration highlights how CE-MS is uniquely suited for organic analysis on future missions to ocean worlds.

Supercharging reagents in LC-MS/MS hormone analyses: Enhancing ionization, not limit of quantification

One of the critical steps during LC-MS/MS hormone analyses that affects the sensitivity of the assay is the ionization process. Enhancing ionization efficiencies by the addition of supercharging reagents might be one way to improve sensitivity and reduce the limit of quantification (LOQ). Therefore, we investigated whether the addition of the supercharging reagents m-nitrobenzyl alcohol (m-NBA), sulfolane, propylene carbonate, and o-nitroanisole (o-NA) increased ionization efficiency and improved assay LOQ of insulin, oxytocin, sex steroids, and corticosteroids in test solutions. Additionally, the influence of the supercharging reagents was tested in serum samples after sample pretreatment to determine whether ionization would be enhanced similarly in routine analyses and, subsequently, lead to improved sensitivity. The screening experiments showed that the impact of the supercharging reagents varied for each hormone; although the addition of m-NBA increased the signal of all hormones, the other reagents only enhanced ionization efficiencies for some hormones. While the addition of 0.05 v/v% m-NBA and 0.05 v/v% o-NA did result in an increase in peak area in both test solutions and serum samples, it did not significantly improve the signal-to-noise ratio, as a simultaneous increase in noise was observed. In conclusion, even though supercharging reagents can enhance ionization efficiencies of hormones significantly, the addition of these reagents does not result in an improved LOQ for hormone measurements with LC-MS/MS.

Determination of tobramycin in eye drops with an open-source hardware ion mobility spectrometer

The analysis of tobramycin was demonstrated successfully as an example for electrospray ionization on an open-source hardware ion mobility spectrometer. This instrument was assembled inexpensively in-house, and required only very few purpose-made components. The quantitative determination of tobramycin required 20 s for a reading. The calibration curve for the range from 50 to 200 μM was found to be linear with a correlation coefficient of r = 0.9994. A good reproducibility was obtained (3% relative standard deviation) and the limit of detection was determined as 8 μM. As the concentration of the active ingredient in the eye drops (ophthalmic solutions) is too high for the sensitivity of the instrument, the samples had to be diluted appropriately.

Electrospray ionization mass spectrometry with wooden tips: A review

Electrospray ionization (ESI) is a powerful ionization technique in mass spectrometry (MS). There has been an increasing interest for the new development of ESI technique to extend its applications. ESI-MS with wooden tips (wooden-tip ESI-MS), an ESI technique invented in 2011, enabled not only new applications but also new insights into the ESI mechanism. In this review, the technical aspects of wooden-tip ESI-MS are described, the new features of wooden-tip ESI-MS for sampling and ionization of analytes are highlighted, and the important applications of wooden-tip ESI-MS in various fields in the past 10 years, including food safety, forensic investigation, environmental analysis, biomedical analysis and protein study, are summarized. The perspectives on the further development and applications of wooden-tip ESI-MS are also discussed.

Liquid extraction surface analysis-mass spectrometry: An advanced and environment-friendly analytical tool in modern analysis

The Liquid Extraction Surface Analysis technique is a new high-throughput instrument for ambient mass spectrometry. The benefits of the Liquid Extraction Surface Analysis-Mass Spectrometry approach are the high throughput screening of samples and the absence of sample preparation. Liquid Extraction Surface Analysis-Mass Spectrometry also consumes less solvent for extraction, making it more environmentally friendly and there is no substrate restriction. It utilizes advanced instrumentation like the use of robotic pipettes, nanoelectrospray systems, electronspray ionization chips which makes it highly efficient. In recent years, Liquid Extraction Surface Analysis-Mass Spectrometry has seen widespread use in a variety of analytical fields including drug metabolite analysis, mapping drug distribution in tissues, protein and lipid characterization etc. In this review, we have summarized the basic working principles of the Liquid Extraction Surface Analysis-Mass Spectrometry approach in detail along with a detailed description of the recently reported applications in the analysis of proteins, lipids, drugs and foods. The investigated analytes along with detection methodologies and significant outcomes of various research reports have been presented with the help of tables. This tool has also been utilized in clinical investigations of biological fluids, fingerprint analysis and authentication of agarwood. This article is protected by copyright. All rights reserved.

Differential Organic Contaminant Ionization Source Detection and Identification in Environmental Waters by Nontargeted Analysis

The development of nontargeted analysis (NTA) methods to assess environmental contaminants of emerging concern, which are not commonly monitored, is paramount, especially when no previous knowledge on the identity of the pollution source is available. We compared complementary ionization techniques, namely electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), in the detection and identification of organic contaminants in tap and surface waters from South Florida. Furthermore, the performance of a simple rationalized NTA method was assessed by analyzing 10 complex mixtures as part of the US Environmental Protection Agency's Non-targeted Analysis Collaborative Trial interlaboratory study, where limitations of the NTA approach have been identified (e.g., number of employed databases, false positives). Different water bodies displayed unique chemical features that can be used as chemical fingerprints for source tracking and discrimination. The APCI technique detected at least threefold as many chemical features as ESI in environmental water samples, corroborating the fact that APCI is more energetic and can ionize certain classes of compounds that are traditionally difficult to ionize by liquid chromatography-mass spectrometry. Kendrick mass defect plots and Van Krevelen diagrams were applied to elucidate unique patterns and theoretical chemical space regions of anthropogenic organic compounds belonging to homologous series or similar classes covered by ESI and APCI. Overall, APCI and ESI were established as complementary, expanding the detected NTA chemical space which would otherwise be underestimated by a single ionization source operated in a single polarity setting. Environ Toxicol Chem 2022;41:1154-1164. © 2021 SETAC.

A novel workflow for semi-quantification of emerging contaminants in environmental samples analyzed by LC-HRMS

There is an increasing need for developing a strategy to quantify the newly identified substances in environmental samples, where there are not always reference standards available. The semi-quantitative analysis can assist risk assessment of chemicals and their environmental fate. In this study, a rigorously tested and system-independent semi-quantification workflow is proposed based on ionization efficiency measurement of emerging contaminants analyzed in liquid chromatography-high-resolution mass spectrometry. The quantitative structure-property relationship (QSPR)-based model was built to predict the ionization efficiency of unknown compounds which can be later used for their semi-quantification. The proposed semi-quantification method was applied and tested in real environmental seawater samples. All semi-quantification-related calculations can be performed online and free of access at http://trams.chem.uoa.gr/semiquantification/ .

Sensitive and selective LC-Tandem mass spectrometry for determination of tolperisone and etodolac in human plasma and application to pharmacokinetic study

Tolperisone and etodolac were proven to have synergistic effect for patients of acute low back pain associated with musculoskeletal spasm. In this work, a specific, highly sensitive and reproducible analytical method was developed and validated for the simultaneous determination of tolperisone and etodolac in human plasma using liquid chromatography-tandem mass spectrometric technique. Liquid-liquid extraction was optimized for sample preparation. Zorbax C₈ column (3.5 μm, 50 × 4.6 mm) was used, carrying a mobile phase mixture of 10.0 mM ammonium formate: acetonitrile (40:60, v/v) pH 3.8, running in an isocratic mode. Chlorzoxazone acted as an internal standard. Sample volume of injection was 5.0 μL, analysis was achieved within 2.5 min. Detection and quantitation were performed by electrospray ionization mass spectrometry using the multiple-reaction monitoring mode. The proposed method could determine the analytes in the range of concentration 0.5 - 200.0 ng mL^-1 for tolperisone and 0.05 - 20.0 μg mL^-1 for etodolac. Findings of inter- and intra-day precisions were ≤ 12.3% with accuracy of ± 5.0%. Pharmacokinetics study for the two drugs after oral administration of healthy human volunteers was achieved with the aid of application of the developed study. This article is protected by copyright. All rights reserved.

Understanding the electrospray ionization response factors of per- and poly-fluoroalkyl substances (PFAS)

Per- and polyfluoroalkyl substances (PFAS) are used extensively in commercial products. Their unusual solubility properties make them an ideal class of compounds for various applications. However, these same properties have led to significant contamination and bioaccumulation given their persistence in the environment. Development of analytical techniques to detect and quantify these compounds must take into account the potential for these properties to perturb these measurements, specifically the potential to bias the electrospray ionization (ESI) process. Direct injection ESI analysis of 23 different PFAS species revealed that hydrophobicity and PFAS class can predict the ESI overall response factors. In this study, a method for predicting the behavior of individual PFAS compounds, including relative retention order in chromatography, is presented which is simply based on the number of fluorine atoms in the molecule as well as the class of the compound (e.g., perfluroalkylcarboxylic acids) vs. computational estimations (e.g., non-polar surface area and logP).

Deconvolving Native and Intact Protein Mass Spectra with UniDec

Intact protein, top-down, and native mass spectrometry (MS) generally requires the deconvolution of electrospray ionization (ESI) mass spectra to assign the mass of components from their charge state distribution. For small, well-resolved proteins, the charge can usually be assigned based on the isotope distribution. However, it can be challenging to determine charge states with larger proteins that lack isotopic resolution, in complex mass spectra with overlapping charge states, and in native spectra that show adduction. To overcome these challenges, UniDec uses Bayesian deconvolution to assign charge states and to create a zero-charge mass distribution. UniDec is fast, user-friendly, and includes a range of advanced tools to assist in intact protein, top-down, and native MS data analysis. This chapter provides a step-by-step protocol and an in-depth explanation of the UniDec algorithm, and highlights the parameters that affect the deconvolution. It also covers advanced data analysis tools, such as macromolecular mass defect analysis and tools for assigning potential PTMs and bound ligands. Overall, this chapter provides users with a deeper understanding of UniDec, which will enhance the quality of deconvolutions and allow for more intricate MS experiments.

Automated Multidimensional Nanoscale Chromatography for Ultrasensitive Targeted Mass Spectrometry

Recent advances in nanoscale separations and high-resolution mass spectrometry permit highly sensitive and accurate analyses of complex protein mixtures. Here, we describe improved methods for nanoscale multidimensional chromatography coupled to targeted mass spectrometry (tMS) to achieve ultrasensitive quantification of peptides in complex proteomes. The presented chromatographic system consists of capillary strong-cation exchange (SCX) chromatography column, from which peptides are eluted directly onto high-resolution reversed-phase (RP) analytical columns and nanoelectrospray ion source. SCX prefractionation is used to separate phosphorylated peptides, permitting their ultrasensitive quantification. Resolution and robustness of this chromatographic system, together with the orthogonality of SCX and RP separations, permit scheduling of large panels of targeted MS assays. This design also enables seamless scaling to three-dimensional separations, thereby enabling large-scale, ultrasensitive quantitative proteomics.

MALDI mass Spectrometry based proteomics for drug discovery & development

Matrix-assisted laser desorption/ ionization (MALDI) is a soft ionization technique for introducing wide range of analytes into a mass spectrometer (MS). MALDI MS is a powerful tool in drug discovery research and development, providing a high-throughput molecular analysis technique in both preclinical and clinical systems. In particular, MALDI MS is invaluable in the study of peptides and proteins that drive all biological functions. This technology is label-free, provides high specificity in molecular identification, and is high-throughput. MALDI MS has been used in biomarker discovery and quantitation in virtually all tissues, serum, plasma, CSF, and urine for diagnostics, patient stratification, and monitoring drug efficacy. Other applications include characterization of biological drugs, spatial mapping of biomarkers and drugs in tissues, drug screening, and toxicological assessment.

Microdroplet mass spectrometry: Accelerating reaction and application

Electrospray ionization (ESI) and desorption electrospray ionization (DESI) are common soft ionization method of mass spectrometry (MS). However, recent studies revealed that some chemical reactions can be induced or greatly accelerated in the sprayed microdroplets compared to the same reaction in the bulk. These open a new area in using microdroplet MS to explore new chemistry and develop new applications. This minireview will introduce microdroplet chemistries and explore various microdroplet techniques most of which are ESI- or DESI-based extensions by incorporating transfer tube, supersonic nebulizing gas, droplet fusion, spray extraction, laser irradiation, or laser ablation for online/offline MS analysis. Potential applications associated with new techniques, including real-time reaction monitoring, high-throughput reaction screening, protein identification, and protein characterization, are also described. Future outlook, such as coupling microdroplet MS with separation techniques, is proposed and discussed.

Observation of charged droplets from electrospray ionization (ESI) plumes in API mass spectrometers

Electrospray ionization (ESI) generates bare analyte ions from charged droplets, which result from spraying a liquid in a strong electric field. Experimental observations available in the literature suggest that at least a significant fraction of the initially generated droplets remain large, have long lifetimes, and can thus aspirate into the inlet system of an atmospheric pressure ionization mass spectrometer (API-MS). We report on the observation of fragment signatures from charged droplets penetrating deeply the vacuum stages of three commercial mass spectrometer systems with largely different ion source and spray configurations. Charged droplets can pass through the ion source and pressure reduction stages and even into the mass analyzer region. Since droplet signatures were found in all investigated instruments, the incorporation of charged droplets is considered a general phenomenon occurring with common spray conditions in ESI sources.

Fluoride-assisted liquid chromatography-tandem mass spectrometry method for simultaneous analysis of propofol and its metabolites without derivatization in urine

The misuse of propofol for recreational purposes has become a serious social issue. Accordingly, practical and sensitive analytical methods to investigate the chronic abuse and toxicity of propofol are required. However, current propofol determination methods using liquid chromatography-mass spectrometry (LC-MS/MS) suffer from problems associated with loss in sample preparation due to its volatility and its poor ionization efficiency and collision-induced dissociation in mass spectrometry. Herein, we have developed a sensitive and accurate fluoride-assisted LC-MS/MS method combined with direct-injection for propofol determination. Ionization via fluoride-ion attachment/induced deprotonation, effected by ammonium fluoride in the mobile phase, was found to dramatically improve the sensitivity of propofol without derivatization. Furthermore, direct injection without derivatization enables the simultaneous analysis of propofol and its phase II metabolites without analyte loss. The optimal concentration of ammonium fluoride in the mobile phase was found to be 1 mM under methanol conditions. The linearity is good (R² ≥ 0.999) and the intra- and inter-day precisions for propofol determination are between 1.9 and 8.7%. The accuracies range from 87.5% to 105.4% and the limits of detection and quantitation for propofol in urine are 0.15 and 0.44 ng mL^-1, respectively. The present method was successfully applied to human urine and showed a sufficient sensitivity to determine propofol and five phase II metabolites over 48 h in human urine after administration. Consequently, the fluoride-assisted LC-MS/MS method was demonstrated to be sensitive, accurate, and practical for the determination of propofol and its metabolites.

Investigating the position of the separation capillary and emitter tube tips in a nanoflow sheath-liquid CE-ESI-MS interface to decouple the ESI potential

Robust decoupling of the ESI potential from the separation potential in CE-ESI-MS interfaces is very important for the high performance of the CE-ESI-MS devices and their applications for highly sensitive analyses of ionogenic compounds. In this study, we utilize a nanoflow sheath-liquid CE-ESI-MS interface composed of a quartz emitter and a separation fused silica capillary treated by etching, which are threaded to cross coupling for sheath liquid and electrode connection. Specifically, we have tested the ability of the interface to decouple the ESI potential from the separation potential at different positions of the separation capillary and ESI emitter tube tips. The interface with the separation capillary tip protruding the emitter tip by 20 μm did not provide sufficient robustness. The real ESI potential (delivered as 2.0 kV from the independent high voltage power supply HV2) ranged from 2.1 kV to 4.5 kV depending on the applied separation voltage (12.0-20.0 kV, provided by the power supply HV1) and electric conductivity of the background electrolyte (BGE) used. The interface robustness was partially improved when the capillary tip was aligned with the emitter tip. However, the complete decoupling of the spray and separation potentials was achieved only when the capillary tip was retracted 20 μm inside the emitter. In this arrangement, the ESI potential was stable and independent of both the separation potential (voltage) and the BGE conductivity. Moreover, this setting provided better sensitivity for the CE-ESI-MS analysis of selected drugs and benzylpyridinium cations than the setup with the capillary tip aligned with or protruding the emitter tip.

Quantitative analysis of the relationship of derivatization reagents and detection sensitivity of electrospray ionization-triple quadrupole tandem mass spectrometry: Hydrazines as prototypes

Chemical derivatization-assisted electrospray ionization-triple quadrupole mass spectrometry (ESI-QqQ-MS) has become an efficient tool for the quantification of low-molecular-weight molecules. Many studies found that the derivatives of the same analytes derivatized by different derivatization reagents with the same reaction group had different detection sensitivity, even under the same conditions of electrospray ionization-mass spectrometry (ESI-MS). This phenomenon was suggested to be caused by the different modifying groups in the derivatization reagents. However, there is still a lack of systematic study on how modifying groups in the derivatization reagents affect the detection sensitivity of their corresponding derivatives of analytes, especially theoretical investigations. In this study, we employed a quantitative structure-activity relationship (QSAR) modeling approach to explore the relationship between modifying group structures and the detection sensitivity of derivatization reagents and their derivatives during ESI-MS detection. A total of 110 derivatization reagents of the hydrazine family and their hexanal derivatives (substituted hydrazones) were selected as the prototypes to construct QSAR models. The established models suggested that several molecular descriptors, related to hydrophobicity, electronegativity, and molecular shape, were related to the detection sensitivity of hexanal derivatives induced by different modifying groups in the derivatization reagents. Besides, we found that the detection sensitivity of compounds detected in selected ion mode (SIM) showed a positive correlation with that obtained in multiple reaction monitoring mode (MRM), and the ionization efficiency was the key factor on the detection sensitivity in both modes.

Quantification of vitamin D3 and 25-hydroxyvitamin D3 in food - The impact of eluent additives and labelled internal standards on matrix effects in LC-MS/MS analysis

Deuterated vitamin D standards are used commonly as internal standards in LC-MS/MS analysis of vitamin D3 and 25-hydroxyvitamin D3 in food. However, the use of various eluent additives, such as methylamine, formic acid and ammonium formate, also contributes to matrix effects and the performance of analysis by affecting accuracy and robustness. For the first time, continuous post-column infusion experiments of isotopically labelled vitamin D3-[d6] were performed to evaluate ion-suppression in a wide variety of food (salmon, cheese, pork fat, pork meat, and egg yolk). Furthermore, results collected using five analytical methods, employing DAD/UV and MS/MS-detectors, were evaluated with in-house and standardised reference materials. The matrix effect was significant when analysing vitamin D3 in most food matrices using the deuterium labelled internal standard. Even though the use of the 13C5-labelled internal standard reduced matrix effects, a standardised method is needed to agree on the true value of vitamin D in food.

Quantification of monosaccharide enantiomers using optical properties of hydrogen-bonded tryptophan

Chiral recognition between amino acids and monosaccharides in the gas phase was investigated as a model for chemical evolution in interstellar molecular clouds. Ultraviolet (UV) photodissociation spectra and product ion spectra of cold gas-phase hydrogen-bonded clusters of protonated tryptophan (Trp) and a pentose, including ribose and arabinose, were obtained using a tandem mass spectrometer equipped with an electrospray ionization source and a temperature-controlled ion trap. The relative intensity of the signal arising from the S₁-S₀ transition of protonated Trp observed at approximately 285 nm in the UV photodissociation spectrum of homochiral H⁺(d-Trp)(d-ribose) was significantly higher than that of heterochiral H⁺(l-Trp)(d-ribose), corresponding to the ππ* state of the Trp indole ring. Optical properties of Trp in the clusters induced by 285-nm photoexcitation were applied to the identification and quantification of pentose enantiomers in solution. Pentose enantiomeric excess in solution was determined from relative abundances observed in a single product ion spectrum of 285-nm photoexcited hydrogen-bonded clusters of H⁺(l-Trp) and pentose. A mixture of two pentoses could also be quantified by this method. The geometric and electronic structures of Trp enable recognition of biological molecules through hydrogen bonding.

Enhancing the power of liquid chromatography-Mass spectrometry for chemical fingerprinting of phytotoxins in the environment

Phytotoxins are plant secondary metabolites. They have recently been considered as chemicals of emerging concern (CECs) and there is a growing interest in their environmental fate and potential threat to public health. Dedicated target and non-target screening (NTS) analysis of phytotoxins in environmental samples are sparse, meanwhile phytotoxins are rarely detected in NTS-based analysis due to lack of an efficient methodology. Development of new analytical measurement methods is therefore highly needed. In this study, we for the first time investigated key parameters of reversed phase liquid chromatography-high resolution mass spectrometry (RPLC-HRMS) for five major classes of phytotoxins (alkaloids, steroids, terpenoids, flavonoids and aromatic polyketides) in environmental matrices; the investigation included analytical conditions which have not yet been explored by others, e.g. ionization at alkaline pH above 9. As the outcome we established a new analytical method for target/suspect screening and NTS of phytotoxins in the environment, which significantly improved the detection sensitivity with up to 40 times compared to previous methods, and enabled the discovery of over 30 phytotoxins in a NTS-based environmental study. We also observed that the negative ionization of phenols could be facilitated by the number of hydroxyl groups on the ring rather than their position of substitution. This study is of interest for a better fundamental understanding of the behavior of phytotoxins in LC-MS. Dedicated target/suspect screening and NTS methods will facilitate a better risk characterization of phytotoxins in the environment and stimulate implementation of new public regulation on phytotoxins.

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.

Molecular Adsorption on Cold Gas-Phase Hydrogen-Bonded Clusters of Chiral Molecules

Gas-phase molecular adsorption was investigated as a model for molecular cloud formation. Molecular adsorption on cold gas-phase hydrogen-bonded clusters containing protonated tryptophan (Trp) enantiomers and monosaccharides such as methyl-α-D-glucoside, D-ribose, and D-arabinose was detected using a tandem mass spectrometer equipped with an electrospray ionization source and cold ion trap. The adsorption sites on the surface of cold gas-phase hydrogen-bonded cluster ions were quantified using gas-phase N₂ adsorption-mass spectrometry. The gas-phase N₂ adsorption experiments indicated that the number of adsorption sites on the surface of the hydrogen-bonded heterochiral clusters containing L-Trp and D-monosaccharides exceeded the number of adsorption sites on the homochiral clusters containing D-Trp and D-monosaccharides. H₂O molecules were preferentially adsorbed on the heterochiral clusters, and larger water clusters were formed in the gas phase. Physical and chemical properties of cold gas-phase hydrogen-bonded clusters containing biological molecules were useful for investigating enantiomer selectivity and chemical evolution in interstellar molecular clouds.

Electrospray ionization with higher-energy collision dissociation tandem mass spectrometry toward characterization of ceramides as [M + Li]+ ions: Mechanisms of fragmentation and structural identification

Ceramide is a huge lipid family consisting of diversified structures in which various modifications are seen in the fatty acyl chain and the long chain base (LCB). In this contribution, a higher collision energy (HCD) linear ion-trap mass spectrometric method (LIT MSⁿ) was applied to study the mechanisms underlying the fragmentation processes of ceramide molecules in 12 subclasses, which were desorbed by ESI as the [M + Li]⁺ ions. Multiple sets of fragment ions reflecting the fatty acyl chain and LCB were observed in the HCD MS² spectra for all the ceramide classes, resulting in unambiguous definition of the ceramide structures, including the chain length and the modification (α-hydroxy-, β-hydroxy-, ω-hydroxy-FA) of the fatty acyl moiety, and the types of LCB (sphingosine, phytosphigosine, 6-hydroxy-sphingosine). Thereby, this approach permits differentiation of isomeric structures and ceramide species in the biological specimen can be unveiled in detail. By application of sequential MS³, the double bond position along the fatty acyl chain of the molecule can be located, and a complete structural characterization of ceramides can be achieved.

Capillary electrophoresis electrospray ionization-mass spectrometry for peptidomics-based processing site determination

Proteolytic cleavage at specific sites is a key event that modulates protein functions in biological processes. These cleavage sites are identified through mass spectrometry-based peptidomics of overlapping peptide sequences. Here, we assessed to what extent a recent capillary electrophoresis (CE) system interfaced with electrospray ionization-mass spectrometry (ESI-MS) contributes to identifying endogenous peptides present in a biological sample. Peptides released by a human endocrine cell line stimulated for secretion was analyzed for uncovering potential processing sites created by proprotein convertases (PCs) that cleave precursors in the secretory pathway. CE-ESI-MS was conducted, in comparison to a standard liquid chromatography (LC)-ESI-MS platform. LC and CE complemented each other in elucidating processing sites that match PC consensus sequences from known substrates. We suggest that the precursors BIGH3, STC1, LFNG, QSOX1 and CYTC are potential substrates for PCs, and that a CE-ESI system would come in handy and garner greater recognition as a robust tool in peptidomics.

Enhancing detection and characterization of lipids using charge manipulation in electrospray ionization-tandem mass spectrometry

Heightened awareness regarding the implication of disturbances in lipid metabolism with respect to prevalent human-related pathologies demands analytical techniques that provide unambiguous structural characterization and accurate quantitation of lipids in complex biological samples. The diversity in molecular structures of lipids along with their wide range of concentrations in biological matrices present formidable analytical challenges. Modern mass spectrometry (MS) offers an unprecedented level of analytical power in lipid analysis, as many advancements in the field of lipidomics have been facilitated through novel applications of and developments in electrospray ionization tandem mass spectrometry (ESI-MS/MS). ESI allows for the formation of intact lipid ions with little to no fragmentation and has become widely used in contemporary lipidomics experiments due to its sensitivity, reproducibility, and compatibility with condensed-phase modes of separation, such as liquid chromatography (LC). Owing to variations in lipid functional groups, ESI enables partial chemical separation of the lipidome, yet the preferred ion-type is not always formed, impacting lipid detection, characterization, and quantitation. Moreover, conventional ESI-MS/MS approaches often fail to expose diverse subtle structural features like the sites of unsaturation in fatty acyl constituents or acyl chain regiochemistry along the glycerol backbone, representing a significant challenge for ESI-MS/MS. To overcome these shortcomings, various charge manipulation strategies, including charge-switching, have been developed to transform ion-type and charge state, with aims of increasing sensitivity and selectivity of ESI-MS/MS approaches. Importantly, charge manipulation approaches afford enhanced ionization efficiency, improved mixture analysis performance, and access to informative fragmentation channels. Herein, we present a critical review of the current suite of solution-based and gas-phase strategies for the manipulation of lipid ion charge and type relevant to ESI-MS/MS.

A comparative study of UniSpray and electrospray sources for the ionization of neuropeptides in liquid chromatography tandem mass spectrometry

Despite the extensive use of electrospray ionization (ESI) for the quantification of neuropeptides by liquid chromatography-tandem mass spectrometry (LC-MS/MS), poor ionization and transmission efficiency are described for this ionization interface. A new atmospheric pressure ionization source, named UniSpray, was recently developed and commercialized. In this study, the LC-MS performance of this new ionization interface is evaluated and compared with ESI for the quantification of seven neuropeptides. Besides comparison of signal intensities and charge state distributions, also signal-to-noise (S/N) ratios and accuracy and precision were assessed. Additionally, matrix effects of human precipitated plasma and rat microdialysate were evaluated as well as the effect of three supercharging agents on the ionization of the seven neuropeptides. UniSpray ionization resulted in signal intensities four to eight times higher at the optimal capillary/impactor voltage for all seven neuropeptides. S/N values at the other hand only increased by not more than a twofold when the UniSpray source was used. Moreover, UniSpray ionization resulted in a shift towards lower charge states for some neuropeptides. Evaluation of the matrix effects by a post-column infusion set-up resulted in different infusion profiles between ESI and UniSpray. The charge state distributions of the neuropeptides obtained with UniSpray are highly comparable with ESI. Finally, the effect of the supercharging agents on the ionization of the neuropeptides tends to be peptide-dependent with both ionization sources.

Direct analysis of lateral flow immunoassays for deoxynivalenol using electrospray ionization mass spectrometry

Lateral flow immunoassays (LFIAs) are widely used for rapid food safety screening analysis. Thanks to simplified protocols and smartphone readouts, LFIAs are expected to be increasingly used on-site, even by non-experts. As a typical follow-up in EU regulatory settings, suspect samples are sent to laboratories for confirmatory analysis by liquid chromatography-tandem mass spectrometry (LC-MS/MS). However, re-analysis by LC-MS/MS is laborious and time-consuming. In this work, an identification LFIA (ID-LFIA) approach followed by quadrupole-orbitrap MS or triple quadrupole MS/MS analysis is presented. As a proof of concept, a dedicated ID-LFIA strip was developed for the mycotoxin deoxynivalenol (DON) following its initial screening by a commercial smartphone LFIA. The ID-LFIA strip can be simply immersed in the same sample extract used for the smartphone LFIA screening, and next, DON is retrieved from the monoclonal antibody with a dissociation solution consisting of methanol/ammonia. The solution thus obtained was analyzed directly in MS in order to rapidly confirm the presence of DON and any cross-reacting species. The protocol developed is capable of coping with severe ion suppression caused by LFIA buffers and nitrocellulose substrate residues. Initial analysis of blank, spiked, and incurred samples showed that the newly developed ID-LFIA-MS method was able to confirm the presence or absence of mycotoxins in the samples previously analyzed by LFIA and also differentiate between DON and DON 3-glucoside yielding the positive screening result. The concept and technique developed are envisaged to complement on-site screening and confirmation of any low molecular weight contaminant in future food control frameworks. Graphical abstract.

A robust sheath-flow CE-MS interface for hyphenation with Orbitrap MS

The hyphenation of capillary electrophoresis with high-resolution mass spectrometry, such as Orbitrap MS, is of broad interest for the unambiguous and exceptionally sensitive identification of compounds. However, the coupling of these techniques requires a robust ionization interface that does not influence the stability of the separation voltage while coping with oxidation of the emitter tip at large ionization voltages. Herein, we present the design of a sheath-flow CE-ESI-MS interface which combines a robust and easy to operate set-up with high-resolution Orbitrap MS detection. The sheath liquid interface is equipped with a gold coated electrospray emitter which increases the stability and overall lifetime of the system. For the characterization of the interface, the spray stability and durability were investigated in dependence of the sheath-flow rate, electrospray voltage, and additional gold coating. The optimized conditions were applied to a separation of angiotensin II and neurotensin resulting in LODs of 2.4 and 3.5 ng/mL.

The use of electrospray ionization mass spectrometry to monitor RNA-ligand interactions

RNAs are drawing increasing attention as potential therapeutic targets. A significant challenge in the RNA drug discovery process is identification of compounds that not only disrupt the natural functions of RNA by binding with high affinity, but also do so selectively. Assessing the binding mode of small molecules with RNA is important for understanding how they select their binding site and impart their mechanism of action. A number of complementary assays are often employed for analysis of the binding mode and to determine selectivity. One important technique that gives information about the binding affinity and stoichiometry is electrospray ionization mass spectrometry (ESI MS). More recent methods have also revealed the usefulness of ESI MS in determining the binding loci of small molecules on RNA.

Profiling of high molecular weight esters by flow injection analysis-high resolution mass spectrometry for the characterization of raw and archaeological beeswax and resinous substances

This work presents a method to characterize high molecular esters in beeswax and resinous substances based on the use of microwave-assisted extraction and flow injection analysis-high resolution mass spectrometry that combines the high efficiency of the extraction procedure with the advantages of high resolution mass spectrometry. This approach allows us to identify archaeological beeswax and plant resinous substances by the characterization of the survived intact high molecular weight components. By this way, several raw materials (beeswax, pine resin and pitch, and resin extracted from Euphorbia tirucalli) were studied and used as reference substances. The procedure was then tested on an adhesive dated 44-42 ka BP recovered from Border Cave (KwaZulu-Natal, Africa), allowing us to detect the high molecular weight species even after almost 50,000 years, and then used to chemically investigate unknown archaeological adhesives from Antinoopolis (Egypt), dated to the 4th-5th century AD. The results allowed us to extend our knowledge on the long-term behavior of beeswax and resinous substances.

Comment on the published data concerning the identification of biochanin A and prunetin by LC/ESI-MS

Biochanin A is one of the most common phytoestrogens, occurring in high concentrations in soy and red clover, for instance, which shows a wide spectrum of biological activity. Prunetin is an isomer of biochanin A, and even though it is not very common, its structural relationship to the latter makes it interesting, regarding its biological activity. Nowadays, LC/ESI-MS methods are widely used for identification of natural compounds, including biochanin A and prunetin. However, we found that the published data concerning the identification of biochanin A and prunetin are sometimes disputable. Namely, the identification is based on the product ions which cannot be regarded as characteristic of biochanin A or prunetin. The reported retention times sometimes may be also disputable.

Analyses of Cholesterol and Derivatives in Ocular Tissues Using LC-MS/MS Methods

Cholesterol is considered one of the most abundant sterols present in mammals, amphipathic in nature, and a key constituent of the cell membrane. Its unique chemical structure consisting of four linked hydrocarbon rings, with an aliphatic chain on one end and a hydroxyl group on the other, confers it the ability to form hydrogen bonds with other lipid classes, like phospholipids or sphingolipids head groups. However, due to its hydrophobic moiety, the fatty acid chain, free cholesterol (FC) it is difficult to be analyzed via electrospray ionization (ESI). The method presented in this chapter to identify and quantify free cholesterol and cholesteryl esters (CE) is based on the chemical derivatization of the sample, strategy devised to avoid the problematic of ESI. However, relevant mention should be made to an alternative separation protocol, which uses ultrahigh performance liquid chromatography and in-source collision-induced dissociation to achieve a simultaneous quantification of FC, CE, and triglycerides (TG).