Importance of gas-phase proton affinities in determining the electrospray ionization response for analytes and solvents

OptiMS: An Accessible Program for Automating Mass Spectrometry Parameter Optimization and Configuration

Mass spectrometers have an enormous number of user-changeable parameters that drastically affect the observed mass spectrum. Using optimal parameters can significantly improve mass spectrometric data by increasing signal stability and signal-to-noise ratio, which decreases the limit of detection, thus revealing previously unobservable species. However, ascertaining optimal parameters is time-consuming, tedious, and made further challenging by the fact that parameters can act dependently on each other. Consequently, suboptimal parameters are frequently used during characterization, reducing the quality of results. OptiMS, an open-source, cross-platform program, was developed to simplify, accelerate, and more accurately determine optimal mass spectrometer parameters for a given system. It addresses common difficulties associated with existing software such as slow performance, high costs, and limited functionality. OptiMS efficacy was demonstrated through its application to multiple systems, quickly and successfully optimizing instrument parameters unassisted to maximize a user-defined metric, such as the intensity of a particular analyte. Additionally, among other features, OptiMS allows running of a sequence of predefined parameter configurations, reducing the workload of users wishing to obtain mass spectra under multiple sets of conditions.

Predicting liquid chromatography-electrospray ionization/mass spectrometry signal from the structure of model compounds and experimental factors; case study of aripiprazole and its impurities

A priori estimation of analyte response is crucial for the efficient development of liquid chromatography-electrospray ionization/mass spectrometry (LC-ESI/MS) methods, but remains a demanding task given the lack of knowledge about the factors affecting the experimental outcome. In this research, we address the challenge of discovering the interactive relationship between signal response and structural properties, method parameters and solvent-related descriptors throughout an approach featuring quantitative structure-property relationship (QSPR) and design of experiments (DoE). To systematically investigate the experimental domain within which QSPR prediction should be undertaken, we varied LC and instrumental factors according to the Box-Behnken DoE scheme. Seven compounds, including aripiprazole and its impurities, were subjected to 57 different experimental conditions, resulting in 399 LC-ESI/MS data endpoints. To obtain a more standard distribution of the measured response, the peak areas were log-transformed before modeling. QSPR predictions were made using features selected by Genetic Algorithm (GA) and providing Gradient Boosted Trees (GBT) with training data. Proposed model showed satisfactory performance on test data with a RMSEP of 1.57 % and a of 96.48 %. This is the first QSPR study in LC-ESI/MS that provided a holistic overview of the analyte's response behavior across the experimental and chemical space. Since intramolecular electronic effects and molecular size were given great importance, the GA-GBT model improved the understanding of signal response generation of model compounds. It also highlighted the need to fine-tune the parameters affecting desolvation and droplet charging efficiency.

Supercritical Fluid Nanospray Mass Spectrometry: II. Effects on Ionization

Nanospraying supercritical fluids coupled to a mass spectrometer (nSF-MS) using a 90% supercritical fluid CO₂ carrier (sCO₂) has shown an enhanced desolvation compared to traditional liquid eluents. Capillaries of 25, 50, and 75 μm internal diameter (i.d.) with pulled emitter tips provided high MS detection sensitivity. Presented here is an evaluation of the effect of proton affinity, hydrophobicity, and nanoemitter tip size on the nSF-MS signal. This was done using a set of primary, secondary, tertiary, and quaternary amines with butyl, hexyl, octyl, and decyl chains as analytes. Each amine class was analyzed individually to evaluate hydrophobicity and proton affinity effects on signal intensity. The system has shown a mass sensitive detection on a linear dynamic range of 0.1-100 μM. Results indicate that hydrophobicity has a larger effect on the signal response than proton affinity. Nanospraying a mixture of all amine classes using the 75 μm emitter has shown a quaternary amine signal not suppressed by competing analytes. Competing ionization was observed for primary, secondary, and tertiary amines. The 75 and 50 μm emitters demonstrated increased signal with increasing hydrophobicity. Surprisingly, the 25 μm i.d. emitter yielded a signal decrease as the alkyl chain length increased, contrary to conventional understanding. Nanospraying the evaporative fluid in a sub-500 nm emitter likely resulted in differences in the ionization mechanism. Results suggest that 90% sCO₂ with 9.99% methanol and 0.01% formic acid yielded fast desolvation, high ionization efficiency, and low matrix effect, which could benefit complex biological matrix analysis.

Effect of Dopants and Gas-Phase Composition on Ionization Behavior and Efficiency in Dielectric Barrier Discharge Ionization

Cold plasma-based ionization techniques allow for soft ionization of a wide variety of chemical compounds. In this chemical ionization mechanism, the atmosphere plays a crucial role in ionization. Knowing its influence is critical for the optimization of analysis conditions and interpretation of resulting spectra. This study uses soft ionization by chemical reaction in transfer (SICRIT), a variant of dielectric barrier discharge ionization (DBDI), that allows for a controlled atmosphere to investigate atmosphere and dopant effects. The influence of eight makeup gas compositions (dry nitrogen, room air, and nitrogen-enriched with either water, HCl, MeOH, hexane, NH₃, and fluorobenzene) on the ionization with SICRIT was investigated. Fifteen compound classes, comprising alkanes, polyaromatic hydrocarbons (PAHs), terpenes, oxygen-containing terpenes, alkylphenols, chlorophenols, nitrophenols, trialkylamines, triazines, phthalates with or without ether groups, aldehydes, ketones, fatty acid methyl esters (FAMEs), and polyoxy-methylene ethers (OMEs) were measured via gas chromatography SICRIT high-resolution mass spectrometry (GC-SICRIT-HRMS). The different atmospheres were compared in terms of generated ions, ion intensities and fragmentation during ionization. Measurements of reactant ions were performed for a better understanding of the underlying mechanisms. All 15 compound classes were mostly softly ionized. For most compound classes and atmospheres, protonation is the dominant ionization mode. The highest number of compounds ionized via protonation was observed in dry nitrogen, followed by room air and humid nitrogen. The study should work as a guideline for the choice of atmosphere for specific compound classes and the interpretation of spectra generated under a specific atmosphere.

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.

Oxidation Products from the Neolignan Licarin A by Biomimetic Reactions and Assessment of in vivo Acute Toxicity

Licarin A, a dihydrobenzofuranic neolignan presents in several medicinal plants and seeds of nutmeg, exhibits strong activity against protozoans responsible for Chagas disease and leishmaniasis. From biomimetic reactions by metalloporphyrin and Jacobsen catalysts, seven products were determined: four isomeric products yielded by epoxidation from licarin A, besides a new product yielded by a vicinal diol, a benzylic aldehyde, and an unsaturated aldehyde in the structure of the licarin A. The incubation with rat and human liver microsomes partially reproduced the biomimetic reactions by the production of the same epoxidized product of m/z 343 [M + H]⁺. In vivo acute toxicity assays of licarin A suggested liver toxicity based on biomarker enzymatic changes. However, microscopic analysis of tissues sections did not show any tissue damage as indicative of toxicity after 14 days of exposure. New metabolic pathways of the licarin A were identified after in vitro biomimetic oxidation reaction and in vitro metabolism by rat or human liver microsomes.

Signal Suppression in LC-ESI-MS/MS from Concomitant Medications and Its Impact on Quantitative Studies: An Example Using Metformin and Glyburide

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been widely used in the quantitative analysis of drugs. The ubiquitous concomitant drug scenario in the clinic has spawned a large number of co-analyses based on this technique. However, signal suppression caused by concomitant drugs during electrospray ionization may affect the quantification accuracy of analytes, which has not received enough attention. In this study, metformin (MET) and glyburide (GLY) were co-eluted by the conventional optimization of chromatographic conditions to illustrate the effect of signal suppression caused by the combined drugs on the quantitative analysis. The response of MET was not affected by GLY over the investigated concentration range. However, the GLY signal could be suppressed by about 30% in the presence of MET, affecting its pharmacokinetic analysis in simulated samples. As an attempt to solve the suppression of GLY by co-eluting MET, dilution can alleviate the suppression. However, this method still has limitations due to the sacrifice of sensitivity. The stable isotope-labeled internal standard could play a role in correction and improve the quantitative accuracy of GLY, which was further confirmed in the pharmacokinetic study of simulated samples. This study provided an example model to illustrate the possible effect of clinical drug combination on LC-MS/MS drug quantitative analysis and investigated the effective methods to solve this problem.

Supercritical Fluid Nanospray Mass Spectrometry

Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO₂ with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5-1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15-25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO₂, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.

Comparing Selected-Ion Collision Induced Unfolding with All Ion Unfolding Methods for Comprehensive Protein Conformational Characterization

Structural analysis by native ion mobility-mass spectrometry provides a direct means to characterize protein interactions, stability, and other biophysical properties of disease-associated biomolecules. Such information is often extracted from collision-induced unfolding (CIU) experiments, performed by ramping a voltage used to accelerate ions entering a trap cell prior to an ion mobility separator. Traditionally, to simplify data analysis and achieve confident ion identification, precursor ion selection with a quadrupole is performed prior to collisional activation. Only one charge state can be selected at one time, leading to an imbalance between the total time required to survey CIU data across all protein charge states and the resulting structural analysis efficiency. Furthermore, the arbitrary selection of a single charge state can inherently bias CIU analyses. We herein aim to compare two conformation sampling methods for protein gas-phase unfolding: (1) traditional quadrupole selection-based CIU and (2) nontargeted, charge selection-free and shotgun workflow, all ion unfolding (AIU). Additionally, we provide a new data interpretation method that integrates across all charge states to project collisional cross section (CCS) data acquired over a range of activation voltages to produce a single unfolding fingerprint, regardless of charge state distributions. We find that AIU in combination with CCS accumulation across all charges offers an opportunity to maximize protein conformational information with minimal time cost, where additional benefits include (1) an improved signal-to-noise ratios for unfolding fingerprints and (2) a higher tolerance to charge state shifts induced by either operating parameters or other factors that affect protein ionization efficiency.

Fast Optimization of Supercritical Fluid Chromatography-Mass Spectrometry Interfacing Using Prediction Equations

The effect of makeup solvent composition in ultrahigh-performance supercritical fluid chromatography-triple quadrupole mass spectrometry using electrospray ionization was studied using a set of 91 compounds, 3 stationary phases, and 2 organic modifiers of the mobile phase. The 24 tested makeup solvents included pure alcohols and methanol in combination with commonly used additives such as water, formic and acetic acid, ammonia, and ammonia salts with varying molarity. The behavioral trends for different makeup solvent additives were established in the first step. Subsequently, the correlations between physicochemical properties and the MS responses were calculated using the Pearson correlation test and matrix plots. The regression analysis was performed using five descriptors: molecular weight, pK_a, log P, number of hydrogen donors/acceptors, and the MS responses obtained with methanol as the makeup solvent. The resulting regression equations had a high prediction rate calculated as R²-predicted coefficient, especially when 10 mmol/L ammonium in methanol was used as an organic modifier of the mobile phase in positive mode. The trueness of these equations was tested via the comparison between experimental and predicted responses expressed as R². Values of R² > 0.8 were found for 88% of the proposed equations. Thus, the MS response could be measured using only one makeup solvent and the responses of other makeup solvents could be easily estimated. The suitability and applicability of determined regression equations was confirmed by the analysis of 13 blind probes, i.e., compounds not included in the original set of analytes. Moreover, the predicted and experimental responses followed the same increasing/decreasing trend enabling one to predict makeup solvent compositions leading to the highest sensitivity.

Cutting-edge approach using dual-channel chromatography to overcome the sensitivity issues associated with polarity switching in pesticide residues analysis

A fast and sensitive method was validated for the analysis of pesticide residues in baby food. After an QuEChERS-based extraction, the samples were analysed with a dual-channel liquid chromatography instrument coupled to a triple quadrupole mass spectrometer. The method consisted of two independent injections per sample. In the first injection the sample was analysed with a mobile phase optimal for the positive polarity ionisation (water, methanol, formic acid, and ammonium formate), whereas in the second injection the mobile phase was optimised for the negative polarity ionisation (water, acetonitrile, acetic acid). The total number of pesticides was 264, out of which 238 were analysed with the methanol gradient and 26 with the acetonitrile gradient. The dual-channel instrument allowed for sample multiplexing. Thus, sample throughput was equivalent to that of a single-channel system. The limit of quantitation, determined according to the DG SANTE guidance document, was 0.003 mg/kg for 97% of the evaluated compounds. The validation study was followed by a real sample survey. In 42 samples, 16 positives were found.

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.

Determination of highly polar compounds in atmospheric aerosol particles at ultra-trace levels using ion chromatography Orbitrap mass spectrometry

A method using ion chromatography coupled to high-resolution Orbitrap mass spectrometry was developed to quantify highly-polar organic compounds in aqueous filter extracts of atmospheric particles. In total, 43 compounds, including short-chain carboxylic acids, terpene-derived acids, organosulfates, and inorganic anions were separated within 33 min by a KOH gradient. Ionization by electrospray was maximized by adding 100 µL min^-1 isopropanol as post-column solvent and optimizing the ion source settings. Detection limits (S/N ≥ 3) were in the range of 0.075-25 μg L^-1 and better than previously reported for 22 compounds. Recoveries of extraction typically range from 85 to 117%. The developed method was applied to three ambient samples, including two arctic flight samples, and one sample from Melpitz, a continental backround research site. A total of 32 different compounds were identified for all samples. From the arctic flight samples, organic tracers could be quantified for the first time with concentrations ranging from 0.1 to 17.8 ng m^-3 . Due to the minimal sample preparation, the beneficial figures of merit, and the broad range of accessible compounds, including very polar ones, the new method offers advantages over existing ones and enables a detailed analysis of organic marker compounds in atmospheric aerosol particles.

Analysis of Nucleosides and Nucleotides in Plants: An Update on Sample Preparation and LC-MS Techniques

Nucleotides fulfill many essential functions in plants. Compared to non-plant systems, these hydrophilic metabolites have not been adequately investigated in plants, especially the less abundant nucleotide species such as deoxyribonucleotides and modified or damaged nucleotides. Until recently, this was mainly due to a lack of adequate methods for in-depth analysis of nucleotides and nucleosides in plants. In this review, we focus on the current state-of-the-art of nucleotide analysis in plants with liquid chromatography coupled to mass spectrometry and describe recent major advances. Tissue disruption, quenching, liquid-liquid and solid-phase extraction, chromatographic strategies, and peculiarities of nucleotides and nucleosides in mass spectrometry are covered. We describe how the different steps of the analytical workflow influence each other, highlight the specific challenges of nucleotide analysis, and outline promising future developments. The metabolite matrix of plants is particularly complex. Therefore, it is likely that nucleotide analysis methods that work for plants can be applied to other organisms as well. Although this review focuses on plants, we also discuss advances in nucleotide analysis from non-plant systems to provide an overview of the analytical techniques available for this challenging class of metabolites.

Towards a new pseudo-quantitative approach to evaluate the ionization response of nitrogen compounds in complex matrices

Ultra high-resolution mass spectrometry (FT-ICR MS) coupled to electrospray ionization (ESI) provides unprecedented molecular characterization of complex matrices such as petroleum products. However, ESI faces major ionization competition phenomena that prevent the absolute quantification of the compounds of interest. On the other hand, comprehensive two-dimensional gas chromatography (GC × GC) coupled to specific detectors (HRMS or NCD) is able to quantify the main families identified in these complex matrices. In this paper, this innovative dual approach has been used to evaluate the ionization response of nitrogen compounds in gas oils as a case study. To this extent, a large gas oil dataset has been analyzed by GC × GC/HRMS, GC × GC-NCD and ESI(+/−)-FT-ICR MS. Then, the concentrations obtained from GC × GC-NCD have been compared to those obtained from FT-ICR MS hence proving that strong ionization competitions are taking place and also depending on the origin of the sample. Finally, multilinear regressions (MLR) have been used to quantitatively predict nitrogen families from FT-ICR MS measurements as well as start rationalizing the ionization competition phenomena taking place between them in different types of gas oils.

Post-Chromatographic Dicationic Ionic Liquid-Based Charge Complexation for Highly Sensitive Analysis of Anionic Compounds by Ultra-High-Performance Supercritical Fluid Chromatography Coupled with Electrospray Ionization Mass Spectrometry

A green analytical strategy has been developed for the analysis of 10 perfluorinated compounds (PFCs) incorporating supramolecular solvent (SUPRAS)-based extraction and ultra-high-performance supercritical fluid chromatography (UHPSFC)-tandem mass spectrometry. The SUPRAS was prepared through self-assembly of reverse micelles by mixing heptanol, tetrahydrofuran, and water at optimized volume ratios. An imidazolium-based germinal dicationic ionic liquid (DIL), 1,1-bis(3-methylimidazolium-1-yl) butylene difluoride ([C₄(MIM)₂]F₂), was dissolved in the make-up solvent of UHPSFC and introduced post-column but before the electrospray ionization source. After chromatographic separation on a Torus DIOL analytical column (100 mm × 2.1 mm, 1.7 μm), the PFC analytes associated with the DIL reagent through charge complexation. The formation of positively charged complexes resulted in improved ionization efficiency and analytical sensitivity. Enhancement in signal intensity by one to two magnitudes was achieved in the positive ionization mode compared to the negative ionization mode without using the dicationic ion-pairing reagent. The developed protocol was applied to 32 samples of real textiles and 6 samples of real food packaging materials, which exhibited great potential for the analysis of anionic compounds.

Study on the separation mechanism of solid-substrate electrospray ionization mass spectrometry

Solid-substrate electrospray ionization mass spectrometry is an important ambient ionization technology to simplify mass spectrometry analysis. Nowadays, its separation application has been reported increasingly, however, the detailed separation mechanism is still indistinct although the chromatographic effect was reported as a possible factor. In this study, substrate-filled capillary electrospray ionization mass spectrometry was developed as an ideal model to investigate the separation mechanism using over thirty small molecules (neutral, basic, and weakly acidic) as model compounds with C18-bonded silica gel and silica gel as the substrates. The chromatographic effect was validated by the negative t-value of oil-water distribution coefficient, and the electric field effect was verified by the paired t-test (p < 0.01) between the retention times at 5.5 and 4.0 kV. A differential equation was proposed to quantify the compound retention under electric field. The quantitative method was validated to rapidly quantify proline (31.88 μg/mL) and hydroxyproline (20.71 μg/mL) in plasma with acceptable selectivity and accuracy. In conclusion, the separation mechanism of solid-substrate electrospray ionization mass spectrometry was the combination of the chromatographic and electric field effects, which could provide theoretical guidance for the separation optimization, and also promote its applications in biological, pharmaceutical, forensic, food and environmental analyses, etc.

Three-dimensional aspects of formulation excipients in drug discovery: a critical assessment on orphan excipients, matrix effects and drug interactions

Formulation/pharmaceutical excipients play a major role in formulating drug candidates, with the objectives of ease of administration, targeted delivery and complete availability. Many excipients used in pharmaceutical formulations are orphanized in preclinical drug discovery. These orphan excipients could enhance formulatability of highly lipophilic compounds. Additionally, they are safe in preclinical species when used below the LD50 values. However, when the excipients are used in formulating compounds with diverse physico-chemical properties, they pose challenges by modulating study results through their bioanalytical matrix effects. Excipients invariably present in study samples and not in the calibration curve standards cause over-/under- estimation of exposures. Thus, the mechanism by which excipients cause matrix effects and strategies to nullify these effects needs to be revisited. Furthermore, formulation excipients cause drug interactions by moderating the pathways of drug metabolizing enzymes and drug transport proteins. Although it is not possible to get rid of excipient driven interactions, it is always advised to be aware of these interactions and apply the knowledge to draw meaningful conclusions from study results. In this review, we will comprehensively discuss a) orphan excipients that have wider applications in preclinical formulations, b) bioanalytical matrix effects and possible approaches to mitigating these effects, and c) excipient driven drug interactions and strategies to alleviate the impacts of drug interactions.

30 Years of research on ESI/MS response: Trends, contradictions and applications

The variation of ionization efficiency for different compounds has puzzled researchers since the invention of the electrospray mass spectrometry (ESI/MS). Ionization depends on the properties of the compound, eluent, matrix, and instrument. Despite significant research, some aspects have remained unclear. For example, research groups have reached contradicting conclusions regarding the ionization processes. One of the best-known is the significance of the logP value for predicting the ionization efficiency. In this tutorial review, we analyse the methodology used for ionization efficiency measurements as well as the most important trends observed in the data. Additionally, we give suggestions regarding the measurement methodology and modelling strategies to yield meaningful and consistent ionization efficiency data. Finally, we have collected a wide range of ionization efficiency values from the literature and evaluated the consistency of these data. We also make this collection available for everyone for downloading as well as for uploading additional and new ionization efficiency data. We hope this GitHub based ionization efficiency repository will allow a joined community effort to collect and unify the current knowledge about the ionization processes.

Advanced molecular-fingerprinting analysis of dissolved organic sulfur by electrospray ionization-Fourier transform ion cyclotron resonance mass spectrometry using optimal spray solvent

Molecular level characterization of dissolved organic sulfur (DOS) by electrospray ionization-Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR MS) is necessary for further understanding of the role of DOS in the environment. Here, ESI spray solvent, a key parameter for ion production during ESI process, was investigated for its effect on the molecular characterization of DOS by ESI-FTICR MS. 100% MeOH as spray solvent was found for the first time to remarkably enhance the ionization efficiency of the majority of CHOS-molecules in NOM, which facilitated a total of 1473 CHOS-molecular formulas with one sulfur atom to be detected. The number of CHOS-molecular formulas obtained using 100%MeOH as spray solvent increased notably over 740 in comparison with those using 50% MeOH aqueous solution (731) or 50% ACN aqueous solution (653). Moreover, due to the enhancement of ionization efficiency of DOS during ESI processes, the tandem mass spectra of the NOM CHOS-molecules could be easily obtained using 100% MeOH as spray solvent, which were hardly obtained using 50% MeOH aqueous solution as spray solvent. The results of the tandem mass spectra suggested the first discovery of organosulfates or sulfonic acids in Suwannee River NOM sample. A simple method based on 100% MeOH as ESI spray solvent for advanced molecular characterization of DOS by ESI-FTICR MS was proposed and applied, and the results revealed more molecular information of DOS in sea DOM samples.

Application of High-Resolution Mass Spectrometry and a Theoretical Model to the Quantification of Multifunctional Carbonyls and Organic Acids in e-Cigarette Aerosol

Electronic (e-) cigarette aerosol (particle and gas) is a complex mixture of chemicals, of which the profile is highly dependent on device operating parameters and e-liquid flavor formulation. The thermal degradation of the e-liquid solvents propylene glycol and glycerol often generates multifunctional carbonyls that are challenging to quantify because of unavailability of standards. We developed a theoretical method to calculate the relative electrospray ionization sensitivities of hydrazones of organic acids and carbonyls with 2,4-dinitrophenylhydrazine based on their gas-phase basicities (ΔG_{deprotonation}). This method enabled quantification by high-performance liquid chromatography-high-resolution mass spectrometry HPLC-HRMS in the absence of chemical standards. Accurate mass and tandem multistage MS (MSⁿ) were used for structure identification of vaping products. We quantified five simple carbonyls, six hydroxycarbonyls, four dicarbonyls, three acids, and one phenolic carbonyl in the e-cigarette aerosol with Classic Tobacco flavor. Our results suggest that hydroxycarbonyls, such as hydroxyacetone, lactaldehyde, and dihydroxyacetone can be significant components in e-cigarette aerosols but have received less attention in the literature and have poorly understood health effects. The data support the radical-mediated e-liquid thermal degradation scheme that has been previously proposed and emphasize the need for more research on the chemistry and toxicology of the complex product formation in e-cigarette aerosols.

Charge Retention/Charge Depletion in ESI-MS: Theoretical Rationale

Gas phase modification in ESI-MS can significantly alter the charge state distribution of small peptides and proteins. The preceding paper presented a systematic experimental study on this topic using Substance P and proposed a charge retention/charge depletion mechanism, explaining different gas- and liquid-phase modifications [Thinius et al. J. Am. Soc. Mass Spec. 2020, 10.1021/jasms.9b00044]. In this work, we aim to support this rational by theoretical investigations on the proton transfer processes from (multiply) charged analytes toward solvent clusters. As model systems we use small (di)amines as analytes and methanol (MeOH) and acetonitrile (ACN) as gas phase modifiers. The calculations are supported by a set of experiments using (di)amines, to bridge the gap between the present model system and Substance P used in the preceding study. Upon calculation of the thermochemical stability as well as the proton transfer pathways, we find that both ACN and MeOH form stable adduct clusters at the protonation site. MeOH can form large clusters through a chain of H-bridges, eventually lowering the barriers for proton transfer to an extent that charge transfer from the analyte to the MeOH cluster becomes feasible. ACN, however, cannot form H-bridged structures due to its aprotic nature. Hence, the charge is retained at the original protonation site, i.e., the analyte. The investigation confirms the proposed charge retention/charge depletion model. Thus, adding aprotic solvent vapors to the gas phase of an ESI source more likely yields higher charge states than using protic compounds.

Strategies for Drawing Quantitative Conclusions from Nontargeted Liquid Chromatography-High-Resolution Mass Spectrometry Analysis

This Feature aims at giving an overview of different possibilities for quantitatively comparing the results obtained from LC-HRMS-based nontargeted analysis. More specifically, quantification via structurally similar internal standards, different isotope labeling strategies, radiolabeling, and predicted ionization efficiencies are reviewed.

Structure, Anion, and Solvent Effects on Cation Response in ESI-MS

The abundance of an ion in an electrospray ionization mass spectrum is dependent on many factors beyond just solution concentration. Even in cases where the analytes of interest are permanently charged (under study here are ammonium and phosphonium ions) and do not rely on protonation or other chemical processes to acquire the necessary charge, factors such as cation structure, molecular weight, solvent, and the identity of the anion can affect results. Screening of a variety of combinations of cations, anions, and solvents provided insight into some of the more important factors. Rigid cations and anions that conferred high conductivity tended to provide the highest responses. The solvent that most closely reflected actual solution composition was acetonitrile, while methanol, acetonitrile/water, and dichloromethane produced a higher degree of discrimination between different ions. Functional groups that had affinity for the solvent tended to depress response. These observations will provide predictive power when accounting for analytes that for reasons of high reactivity can not be isolated.

Semi-quantitative non-target analysis of water with liquid chromatography/high-resolution mass spectrometry: How far are we?

Combining high-resolution mass spectrometry (HRMS) with liquid chromatography (LC) has considerably increased the capability of analytical chemistry. Among others, it has stimulated the growth of the non-target analysis, which aims at identifying compounds without their preceding selection. This approach is already widely applied in various fields, such as metabolomics, proteomics, etc. The applicability of LC/HRMS-based non-target analysis in environmental analyses, such as water studies, would be beneficial for understanding the environmental fate of polar pollutants and evaluating the health risks exposed by the new emerging contaminants. During the last five to seven years the use of LC/HRMS-based non-target analysis has grown rapidly. However, routine non-target analysis is still uncommon for most environmental monitoring agencies and environmental scientists. The main reasons are the complicated data processing and the inability to provide quantitative information about identified compounds. The latter shortcoming follows from the lack of standard substances, considered so far as the soul of each quantitative analysis for the newly discovered pollutants. To overcome this, non-target analyses could be combined with semi-quantitation. This Perspective aims at describing the current methods for non-target analysis, the possibilities and challenges of standard substance-free semi-quantitative analysis, and proposes tools to join these two fields together.

Influence of the amino acid composition on the ionization efficiencies of small peptides

Electrospray ionization is widely used to generate gas phase ions from a variety of molecules ranging from small ions to large proteins, while the ionization mechanism is claimed to depend on the size of the molecule. For small molecules, the ionization efficiency, the amount of gas phase ions produced in the electrospray process, depends on the properties of the compound. Here, we show that the amino acid composition also influences the ionization efficiency of the oligopeptide. Additionally, we show that the ionization efficiencies of oligopeptides consisting of more than five amino acid residues are very similar to one another, and assuming equal ionization efficiencies is feasible. Therefore, accurate ionization efficiency predictions are needed mainly for small oligopeptides. For these oligopeptides, the amino acid composition can be used to estimate the ionization efficiency in an easy to use manner.

A comparative study of the electrospray ionization response of β-O-4' lignin model compounds

Electrospray ionization mass spectrometry has recently become the technique of choice for rapid characterization of lignin degradation products. However, the fundamental question of the relationship between lignin structure and ionization efficiency has not been explored. In this work, we studied the electrospray ionization response of five structurally similar β-O-4' model lignin compounds using lithium cationization in the positive electrospray ionization mode. The studied compounds have the same β-O-4' backbone structure but differ at the α-position by increasing nonpolar side chains. Our results show a correlation between the ionization response and the length of the nonpolar side chain, with analytes having the longest side chain recording the highest ESI response in the full scan mode. Factors affecting the formation of analyte ions and analyte cluster ions were also studied. We have shown for the first time in this work that the introduction of a nonpolar group onto a β-O-4' lignin compound can increase the lithium cationization ESI response in the positive ion mode.

The chemical reactions in electrosprays of water do not always correspond to those at the pristine air-water interface

The recent application of electrosprays to characterize the air-water interface, along with the reports on dramatically accelerated chemical reactions in aqueous electrosprays, have sparked a broad interest. Herein, we report on complementary laboratory and in silico experiments tracking the oligomerization of isoprene, an important biogenic gas, in electrosprays and isoprene-water emulsions to differentiate the contributions of interfacial effects from those of high voltages leading to charge-separation and concentration of reactants in the electrosprays. To this end, we employed electrospray ionization mass spectrometry, proton nuclear magnetic resonance, ab initio calculations and molecular dynamics simulations. We found that the oligomerization of isoprene in aqueous electrosprays involved minimally hydrated and highly reactive hydronium ions. Those conditions, however, are non-existent at pristine air-water interfaces and oil-water emulsions under normal temperature and pressure. Thus, electrosprays should be complemented with surface-specific platforms and theoretical methods to reliably investigate chemistries at the pristine air-water interface.

Submicrometer Nanospray Emitters Provide New Insights into the Mechanism of Cation Adduction to Anionic Oligonucleotides

The electrospray-MS analysis of oligonucleotides is hampered by nonvolatile metal cations, which may produce adducts responsible for signal suppression and loss of resolution. Alternative to replacing metal cations with MS-friendly ammonium, we explored the utilization of nanospray emitters with submicrometer-diameter tips, which was shown to benefit the analysis of protein samples containing elevated salt concentrations. We demonstrated that such benefits are not limited to proteins, but extend also to oligonucleotide samples analyzed in the negative ion mode. At elevated Na⁺/Mg²⁺ concentrations, submicrometer tips produced significantly greater signal-to-noise ratios, as well as greatly reduced adducts and salt clusters, than observed when utilizing micrometer tips. These effects were marginally affected by emitter composition (i.e., borosilicate versus quartz), but varied according to salt concentration and number of oligonucleotide phosphates. The results confirmed that adduct formation is driven by the concentrating effects of the desolvation process, which leads to greatly increased solute concentrations as the volume of the droplet decreases. The process promotes cation-phosphate interactions that may not have necessarily existed in the initial sample, but nevertheless shape the observed adduct series. Therefore, such series may not accurately reflect the distribution of counterions surrounding the analyte in solution. No adverse effects were noted on specific metal interactions, such as those present in a model drug-DNA assembly. These observations indicate that the utilization of submicrometer tips represents an excellent alternative to traditional ammonium-replacement approaches, which enables the analysis of oligonucleotides in the presence of Na⁺/Mg²⁺ concentrations capable of preserving their structure and functional properties.

Buffer gas additives (modifiers/shift reagents) in ion mobility spectrometry: Applications, predictions of mobility shifts, and influence of interaction energy and structure

Ion mobility spectrometry (IMS) is an analytical technique used for fast and sensitive detection of illegal substances in customs and airports, diagnosis of diseases through detection of metabolites in breath, fundamental studies in physics and chemistry, space exploration, and many more applications. Ion mobility spectrometry separates ions in the gas-phase drifting under an electric field according to their size to charge ratio. Ion mobility spectrometry disadvantages are false positives that delay transportation, compromise patient's health and other negative issues when IMS is used for detection. To prevent false positives, IMS measures the ion mobilities in 2 different conditions, in pure buffer gas or when shift reagents (SRs) are introduced in this gas, providing 2 different characteristic properties of the ion and increasing the chances of right identification. Mobility shifts with the introduction of SRs in the buffer gas are due to clustering of analyte ions with SRs. Effective SRs are polar volatile compounds with free electron pairs with a tendency to form clusters with the analyte ion. Formation of clusters is favored by formation of stable analyte ion-SR hydrogen bonds, high analytes' proton affinity, and low steric hindrance in the ion charge while stabilization of ion charge by resonance may disfavor it. Inductive effects and the number of adduction sites also affect cluster formation. The prediction of IMS separations of overlapping peaks is important because it simplifies a trial and error procedure. Doping experiments to simplify IMS spectra by changing the ion-analyte reactions forming the so-called alternative reactant ions are not considered in this review and techniques other than drift tube IMS are marginally covered.

Gas-phase fragmentation of protonated piplartine and its fungal metabolites using tandem mass spectrometry and computational chemistry

Piplartine, an alkaloid produced by plants in the genus Piper, displays promising anticancer activity. Understanding the gas-phase fragmentation of piplartine by electrospray ionization tandem mass spectrometry can be a useful tool to characterize biotransformed compounds produced by in vitro and in vivo metabolism studies. As part of our efforts to understand natural product fragmentation in electrospray ionization tandem mass spectrometry, the gas-phase fragmentation of piplartine and its two metabolites 3,4-dihydropiplartine and 8,9-dihydropiplartine, produced by the endophytic fungus Penicillium crustosum VR4 biotransformation, were systematically investigated. Proposed fragmentation reactions were supported by ESI-MS/MS data and computational thermochemistry. Cleavage of the C-7 and N-amide bond, followed by the formation of an acylium ion, were characteristic fragmentation reactions of piplartine and its analogs. The production of the acylium ion was followed by three consecutive and competitive reactions that involved methyl and methoxyl radical eliminations and neutral CO elimination, followed by the formation of a four-member ring with a stabilized tertiary carbocation. The absence of a double bond between carbons C-8 and C-9 in 8,9-dihydropiplartine destabilized the acylium ion and resulted in a fragmentation pathway not observed for piplartine and 3,4-dihydropiplartine. These results contribute to the further understanding of alkaloid gas-phase fragmentation and the future identification of piplartine metabolites and analogs using tandem mass spectrometry techniques. Copyright © 2017 John Wiley & Sons, Ltd.

Ammonium Bicarbonate Addition Improves the Detection of Proteins by Desorption Electrospray Ionization Mass Spectrometry

The analysis of protein by desorption electrospray ionization mass spectrometry (DESI-MS) is considered impractical due to a mass-dependent loss in sensitivity with increase in protein molecular weights. With the addition of ammonium bicarbonate to the DESI-MS analysis the sensitivity towards proteins by DESI was improved. The signal to noise ratio (S/N) improvement for a variety of proteins increased between 2- to 3-fold relative to solvent systems containing formic acid and more than seven times relative to aqueous methanol spray solvents. Three methods for ammonium bicarbonate addition during DESI-MS were investigated. The additive delivered improvements in S/N whether it was mixed with the analyte prior to sample deposition, applied over pre-prepared samples, or simply added to the desorption spray solvent. The improvement correlated well with protein pI but not with protein size. Other ammonium or bicarbonate salts did not produce similar improvements in S/N, nor was this improvement in S/N observed for ESI of the same samples. As was previously described for ESI, DESI also caused extensive protein unfolding upon the addition of ammonium bicarbonate. Graphical Abstract ᅟ.

Predicting ESI/MS Signal Change for Anions in Different Solvents

LC/ESI/MS is a technique widely used for qualitative and quantitative analysis in various fields. However, quantification is currently possible only for compounds for which the standard substances are available, as the ionization efficiency of different compounds in ESI source differs by orders of magnitude. In this paper we present an approach for quantitative LC/ESI/MS analysis without standard substances. This approach relies on accurately predicting the ionization efficiencies in ESI source based on a model, which uses physicochemical parameters of analytes. Furthermore, the model has been made transferable between different mobile phases and instrument setups by using a suitable set of calibration compounds. This approach has been validated both in flow injection and chromatographic mode with gradient elution.

Prediction of Mass Spectral Response Factors from Predicted Chemometric Data for Druglike Molecules

A method is developed for the prediction of mass spectral ion counts of drug-like molecules using in silico calculated chemometric data. Various chemometric data, including polar and molecular surface areas, aqueous solvation free energies, and gas-phase and aqueous proton affinities were computed, and a statistically significant relationship between measured mass spectral ion counts and the combination of aqueous proton affinity and total molecular surface area was identified. In particular, through multilinear regression of ion counts on predicted chemometric data, we find that log₁₀(MS ion counts) = -4.824 + c ₁•PA + c ₂•SA, where PA is the aqueous proton affinity of the molecule computed at the SMD(aq)/M06-L/MIDI!//M06-L/MIDI! level of electronic structure theory, SA is the total surface area of the molecule in its conjugate base form, and c ₁ and c ₂ have values of -3.912 × 10^-2 mol kcal^-1 and 3.682 × 10^-3 Å^-2. On a 66-molecule training set, this regression exhibits a multiple R value of 0.791 with p values for the intercept, c ₁, and c ₂ of 1.4 × 10^-3, 4.3 × 10^-10, and 2.5 × 10^-6, respectively. Application of this regression to an 11-molecule test set provides a good correlation of prediction with experiment (R = 0.905) albeit with a systematic underestimation of about 0.2 log units. This method may prove useful for semiquantitative analysis of drug metabolites for which MS response factors or authentic standards are not readily available. Graphical Abstract ᅟ.

Studying Gas-Phase Interconversion of Tautomers Using Differential Mobility Spectrometry

In this study, we report on the use of differential mobility spectrometry (DMS) as a tool for studying tautomeric species, allowing a more in-depth interrogation of these elusive isomers using ion/molecule reactions and tandem mass spectrometry. As an example, we revisit a case study in which gas-phase hydrogen-deuterium exchange (HDX)-a probe of ion structure in mass spectrometry-actually altered analyte ion structure by tautomerization. For the N- and O-protonated tautomers of 4-aminobenzoic acid, when separated using DMS and subjected to subsequent HDX with trace levels of D2O, the anticipated difference between the exchange rates of the two tautomers is observed. However, when using higher levels of D2O or a more basic reagent, equivalent and almost complete exchange of all labile protons is observed. This second observation is a result of the interconversion of the N-protonated tautomer to the O-protonated form during HDX. We can monitor this transformation experimentally, with support from detailed molecular dynamics and electronic structure calculations. In fact, calculations suggest the onset of bulk solution phase properties for 4-aminobenzoic acid upon solvation with eight CH3OH molecules. These findings also underscore the need for choosing HDX reagents and conditions judiciously when separating interconvertible isomers using DMS. Graphical Abstract ᅟ.

Ionization Efficiency of Doubly Charged Ions Formed from Polyprotic Acids in Electrospray Negative Mode

The ability of polyprotic acids to give doubly charged ions in negative mode electrospray was studied and related to physicochemical properties of the acids via linear discriminant analysis (LDA). It was discovered that the compound has to be strongly acidic (low pK a1 and pK a2) and to have high hydrophobicity (logP ow) to become multiply charged. Ability to give multiply charged ions in ESI/MS cannot be directly predicted from the solution phase acidities. Therefore, for the first time, a quantitative model to predict the charge state of the analyte in ESI/MS is proposed and validated for small anions. Also, a model to predict ionization efficiencies of these analytes was developed. Results indicate that acidity of the analyte, its octanol-water partition coefficient, and charge delocalization are important factors that influence ionization efficiencies as well as charge states of the analytes. The pH of the solvent was also found to be an important factor influencing the ionization efficiency of doubly charged ions. Graphical Abstract ᅟ.

Transferability of the electrospray ionization efficiency scale between different instruments

For the first time, quantitative electrospray (ESI) ionization efficiencies (IE), expressed as logIE values, obtained on different mass-spectrometric setups (four mass analyzers and four ESI sources) are compared for 15 compounds of diverse properties. The general trends of change of IE with molecular structure are the same with all experimental setups. The obtained IE scales could be applied on different setups: there were no statistically significant changes in the order of ionization efficiency and the root mean of squared differences of the logIE values of compounds between the scales compiled on different instruments were found to be between 0.21 and 0.55 log units. The results show that orthogonal ESI source geometry gives better differentiating power and additional pneumatic assistance improves it even more. It is also shown that the ionization efficiency values are transferable between different mass-spectrometric setups by three anchoring points and a linear model. The root mean square error of logIE prediction ranged from 0.24 to 0.72 depending on the instrument. This work demonstrates for the first time the inter-instrument transferability of quantitative electrospray ionization efficiency data. Graphical Abstract ᅟ.

Solution vs. gas phase relative stability of the choline/acetylcholine cavitand complexes

How the information obtained from the gas phase experiments can reflect the processes in solution is a crucial question for analytical chemistry, and particularly the selective host-guest recognition mechanisms which are fundamental in biology. Here we combine ElectroSpray Ionization mass spectrometry (ESI-MS) and the Collision Induced Dissociation (CID) experiments to the density functional theory to investigate the interaction of acetylcholine and the choline cation with a triphosphonate cavitand. While the relative abundance of the cation complexes in the ESI mass spectrum reflects the preferential capture of the acetylcholine ion over the choline ion by the cavitand in the solution, the gas phase CID measurements indicate that after desolvation the choline cation is the most strongly bound to the host. The experimental results are interpreted by theory that underlines the role of the counterion in the stabilization of the complexes in solution and therefore in the selective recognition of substrates of biological interest.

Extremely supercharged proteins in mass spectrometry: profiling the pH of electrospray generated droplets, narrowing charge state distributions, and increasing ion fragmentation

High-performance solutions for supercharging proteins in electrospray ionization were optimized and the origin of the strong dependence of supercharging on acid strength was investigated.

Effect of mobile phase on electrospray ionization efficiency

Electrospray (ESI) ionization efficiencies (IE) of a set of 10 compounds differing by chemical nature, extent of ionization in solution (basicity), and by hydrophobicity (tetrapropylammonium and tetraethylammonium ion, triethylamine, 1-naphthylamine, N,N-dimethylaniline, diphenylphthalate, dimethylphtahalate, piperidine, pyrrolidine, pyridine) have been measured in seven mobile phases (three acetonitrile percentages 20%, 50%, and 80%, and three different pH-adjusting additives, 0.1% formic acid, 1 mM ammonia, pH 5.0 buffer combination) using the relative measurement method. MS parameters were optimized separately for each ion. The resulting relative IE data were converted into comparable logIE values by anchoring them to the logIE of tetrapropylammonium ion taking into account the differences of ionization in different solvents and thereby making the logIE values of the compounds comparable across solvents. The following conclusions were made from analysis of the data. The compounds with pK(a) values in the range of the solution pH values displayed higher IE at lower pH. The sensitivity of IE towards pH depends on hydrophobicity being very strong with pyridine, weaker with N,N-dimethylaniline, and weakest with 1-naphthylamine. IEs of tetraalkylammonium ions and triethylamine were expectedly insensitive towards solution pH. Surprisingly high IEs of phthalate esters were observed. The differences in solutions with different acetonitrile content and similar pH were smaller compared with the pH effects. These results highlight the importance of hydrophobicity in electrospray and demonstrate that high hydrophobicity can sometimes successfully compensate for low basicity.

DNA oligonucleotides: a model system with tunable binding strength to study monomer-dimer equilibria with electrospray ionization-mass spectrometry

Electrospray ionization (ESI) is increasingly used to measure binding strengths, but it is not always clear whether the ESI process introduces artifacts. Here we propose a model monomer-dimer equilibrium system based on DNA oligonucleotides to systematically explore biomolecular self-association with the ESI-mass spectrometry (MS) titration method. The oligonucleotides are designed to be self-complementary and have the same chemical composition and mass, allowing for equal ionization probability, ion transmission, and detection efficiency in ESI-MS. The only difference is the binding strength, which is determined by the nucleotide sequence and can be tuned to cover a range of dissociation constant values. This experimental design allows one to focus on the impact of ESI on the chemical equilibrium and to avoid the other typical sources of variation in ESI-MS signal responses, which yields a direct comparison of samples with different binding strengths. For a set of seven model DNA oligonucleotides, the monomer-dimer binding equilibrium was probed with the ESI-MS titration method in both positive and negative ion modes. A mathematical model describing the dependence of the monomer-to-dimer peak intensity ratio on the DNA concentration was proposed and used to extract apparent Kd values and the fraction of DNA duplex that irreversibly dissociates in the gas phase. The Kd values determined via ESI-MS titration were compared to those determined in solution with isothermal titration calorimetry and equilibrium thermal denaturation methods and were found to be significantly lower. The observed discrepancy was attributed to a greater electrospray response of dimers relative to that of monomers.