Latest Developments in Direct and Non-Direct LC-MS Methods Based on Liquid Electron Ionization (LEI).

Mass spectrometry (MS) enables precise identification and quantification of molecules, particularly when combined with chromatography. The advent of atmospheric pressure ionization (API) techniques allowed the efficient coupling of liquid chromatography with MS (LC-MS), extending analyses to nonvolatile and thermolabile compounds. API techniques present limitations such as low informative capacity and reproducibility of mass spectra, increasing instrument complexity and costs. Other challenges include analyzing poorly polar molecules and matrix effects (ME), which negatively impact quantitative analyses, necessitating extensive sample purification or using expensive labeled standards. These limitations prompted the exploration of alternative solutions, leading to the development of the Liquid Electron Ionization (LEI) interface. The system has demonstrated excellent robustness and reproducibility. LEI has been employed to analyze various compounds, including pesticides, drugs of abuse, phenols, polycyclic aromatic hydrocarbons (PAHs), phthalates, and many others. Its versatility has been validated with single quadrupole, triple quadrupole, and QToF detectors, operating in electron ionization (EI) or chemical ionization (CI) modes and with both reverse phase liquid chromatography (RPLC) and normal phase liquid chromatography (NPLC). LEI has also been successfully integrated with the Microfluidic Open Interface (MOI), Membrane Introduction Mass Spectrometry (MIMS), and Microfluidic Water-Assisted Trap Focusing (M-WATF), broadening its application scope and consistently demonstrating promising results in terms of sensitivity and identification power. The most recent advancement is the development of Extractive-Liquid Sampling Electron Ionization-Mass Spectrometry (E-LEI-MS), a surface sampling and real-time analysis technique based on the LEI concept. This review article offers a comprehensive and up-to-date picture of the potential of LEI.

Exploring Negative Chemical Ionization of Per- and Polyfluoroalkyl Substances via a Liquid Electron Ionization LC-MS Interface.

Per- and polyfluoroalkyl substances (PFASs) are a class of aliphatic manufactured compounds comprising fluoro-chemicals with varied functional groups and stable carbon-fluorine bonds. They are defined as "forever chemicals" due to their persistent and bioaccumulative character. These substances have been detected in various environmental samples, including water, air, soil, and human blood, posing significant health hazards. High-performance liquid chromatography coupled with electrospray ionization mass spectrometry (HPLC-ESI-MS) is typically employed for the analysis of PFASs. Negative chemical ionization (NCI) is generally coupled to gas chromatography (GC) and offers high selectivity and sensitivity for compounds containing electronegative atoms, such as PFASs. The liquid electron ionization (LEI) interface is an efficient mechanism developed to robustly couple a liquid flow rate from an LC system to an EI or a CI source. This interface has been successfully utilized for pesticide determination in UHPLC-LEI-CI in negative ion mode (NCI). This work aims to evaluate different parameters involved in the ionization of PFASs analyzed in LC-LEI-NCI and subsequently develop a method for their detection in real samples. The parameters considered for this study include (i) a comparison of different CI reagent gases (methane, isobutane, and argon); (ii) the use of acetonitrile as both the chromatographic solvent and CI reagent gas; (iii) the presence of water and formic acid as chromatographic mobile phase components; and (iv) the mobile phase flow rate. The optimal combination of these parameters led to promising results. Tentative fragmentation pathways of PFASs in NCI mode are proposed based on the dissociative electron capture mechanism.

Liquid electron ionization-mass spectrometry as a novel strategy for integrating normal-phase liquid chromatography with low and high-resolution mass spectrometry.

Normal-phase liquid chromatography (NPLC) plays a pivotal role in the rapid separation of non-polar compounds, facilitating isomer separation and finding applications in various crucial areas where aprotic solvents are necessary. Similar to reversed-phase liquid chromatography (RPLC), NPLC requires a robust and sensitive detector to unequivocally identify the analytes, such as a mass spectrometer. However, coupling NPLC with mass spectrometry (MS) poses challenges due to the incompatibility between the non-polar solvents used as the mobile phase and the primary ionization techniques employed in MS. Several analytical methods have been developed to combine NPLC with electrospray ionization (ESI), but these methods are restricted to the analysis of polar compounds. In most cases, atmospheric pressure chemical ionization (APCI) becomes necessary to expand the range of analysis applications. To overcome these limitations and fully realize the potential of NPLC-MS coupling, a technique termed liquid electron ionization-mass spectrometry (LEI-MS) can be used. LEI-MS offers a straightforward solution by enabling the effective coupling of NPLC with both low and high-resolution MS. LEI allows for the comprehensive analysis of non-polar compounds and provides a powerful tool for isomer separation and precise identification of analytes. Optimal separations, mass spectral qualities, and matches with the NIST library were obtained in both configurations, demonstrating the potential of the proposed approach.

Extractive-liquid sampling electron ionization-mass spectrometry (E-LEI-MS): a new powerful combination for direct analysis.

One of modern analytical chemistry main challenges is providing as fast as possible results in different application fields. In this view, real-time analysis techniques are experiencing ever-increasing success as they can provide data quickly, almost without sample preparation steps. Most of real-time approaches are based on direct mass spectrometry (DMS), a method of analyzing samples without the need for separation or pre-treatment steps. Instead, the sample is directly introduced into the mass spectrometer for analysis. In this context, ambient ionization mass spectrometry (AIMS) techniques are widely represented and successfully used. Extractive-liquid sampling electron ionization-mass spectrometry (E-LEI-MS) represents a different analytical strategy that allows coupling ambient sampling with electron ionization (EI), avoiding any sample preparation step and providing identification based on the comparison with the National Institute of Standards and Technology (NIST) library spectra. E-LEI-MS consists of a dispositive for solvent release and sampling at ambient conditions coupled with an EI source of a single quadrupole mass spectrometer. A micromanipulator allows fine (x,y,z) positioning of a sampling tip. MS can operate in scan or SIM modes depending on the application goals and requirements. Several preliminary successful results were already obtained due to the highly informative EI mass spectra generation. The system was applied to the analysis of active ingredients in pharmaceutical tablets, pesticides on fruit peel, a drug of abuse (cocaine) determination in banknotes, and analysis of unknown components on painting surfaces. Both forensic and artwork applications allowed determining the spatial distribution of the analytes. Here we present a proof-of-concept of E-LEI-MS for targeted/non-targeted analysis and semi-quantitative detection.

Enhanced microfluidic open interface for the direct coupling of solid phase microextraction with liquid electron ionization-tandem mass spectrometry.

Solid-phase microextraction (SPME) directly coupled to MS is a widespread technique for determining small molecules in different matrices in many application fields. Here we present a modified microfluidic open interface (MOI) connected to a passive-flow-splitter device (PFS) for the direct coupling of SPME to a liquid-electron ionization (LEI) interface in a tandem mass spectrometer for the analysis of complex biological samples. No chromatographic separation is involved. The new MOI-PFS configuration was designed to speed up the sample transfer to MS, improving the signal-to-noise ratio and peak shape and leading to fast and sensitive results. MOI-PFS-LEI-MS/MS experiments were conducted using fentanyl as a model compound in water and blood serum. The method uses a C18 Bio-SPME fiber by direct immersion (3 min) in 300 µL of the sample followed by rapid desorption (1 min) in a flow isolated volume (MOI chamber, 2.5 µL) filled with 100% acetonitrile. The PFS permits the rapid transfer of a fraction of the sample into the MS via the LEI interface. The optimal conditions were obtained at a flow rate of 10 µL·min-1 and a 1:20 split ratio. Altogether, extraction, desorption, and analysis require approximately 5 min. Good interday and intraday precision, excellent linearity and LOQs in the µg·L-1 range were obtained for fentanyl in water and serum. Greenness evaluation demonstrated a limited environmental impact of this technique.

Liquid Chromatography-Electron Capture Negative Ionization-Tandem Mass Spectrometry Detection of Pesticides in a Commercial Formulation.

Negative chemical ionization (NCI) and electron-capture negative ionization (ECNI) are gas chromatography-mass spectrometry (GC-MS) techniques that generate negative ions in the gas phase for compounds containing electronegative atoms or functional groups. In ECNI, gas-phase thermal electrons can be transferred to electrophilic substances to produce M-• ions and scarce fragmentation. As a result of the electrophilicity requirements, ECNI is characterized by high-specificity and low background noise, generally lower than EI, offering lower detection limits. The aim of this work is to explore the possibility of extending typical advantages of ECNI to liquid chromatography-mass spectrometry (LC-MS). The LC is combined with the novel liquid-EI (LEI) LC-EIMS interface, the eluent is vaporized and transferred inside a CI source, where it is mixed with methane as a buffer gas. As proof of concept, dicamba and tefluthrin, agrochemicals with herbicidal and insecticidal activity, respectively, were chosen as model compounds and detected together in a commercial formulation. The pesticides have different chemical properties, but both are suitable analytes for ECNI due to the presence of electronegative atoms in the molecules. The influence of the mobile phase and other LC- and MS-operative parameters were methodically evaluated. Part-per-trillion (ppt) detection limits were obtained. Ion abundances were found to be stable with quantitative linear detection, reliable, and reproducible, with no influence from coeluting interfering compounds from the sample matrix.

The history of electron ionization in LC-MS, from the early days to modern technologies: A review.

This review article traces the history of the use of liquid chromatography coupled with mass spectrometry (LC-MS) using electron ionization (EI) from the first attempts up to the present day. At the time of the first efforts to couple LC to MS, 70 eV EI was the most common ionization technique, typically used in gas chromatography-mass spectrometry (GC-MS) and providing highly reproducible mass spectra that could be collated in libraries. Therefore, it was obvious to transport this dominant approach to the early LC-MS coupling attempts. The use of LC coupled to EI-MS is challenging mainly due to restrictions related to high-vacuum and high-temperature conditions required for the operation of EI and the need to remove the eluent carrying the analyte before entering the ion source. The authors will take readers through a journey of about 50 years, showing how through the succession of different attempts it has been possible to successfully couple LC with EI-MS, which in principle appear to be incompatible.

Direct Coupling of Bio-SPME to Liquid Electron Ionization-MS/MS via a Modified Microfluidic Open Interface.

We present a modified microfluidic open interface (MOI) for the direct coupling of Bio-SPME to a liquid electron ionization-tandem mass spectrometry (LEI-MS/MS) system as a sensitive technique that can directly analyze biological samples without the need for sample cleanup or chromatographic separations as well as without measurable matrix effects (ME). We selected fentanyl as test compound. The method uses a C18 Bio-SPME fiber by direct immersion (DI) in urine and plasma and the subsequent quick desorption (1 min) in a flow-isolated volume (2.5 µL) filled with an internal standard-acetonitrile solution. The sample is then transferred to an EI source of a triple-quadrupole mass spectrometer via a LEI interface at a nanoscale flow rate. The desorption and analysis procedure requires less than 10 min. Up to 150 samples can be analyzed without observing a performance decline, with fentanyl quantitation at microgram-per-liter levels. The method workflow is extremely dependable, relatively fast, sustainable, and leads to reproducible results that enable the high-throughput screening of various biological samples.

Microfluidic water-assisted trap focusing method for ultra-large volume injection in reversed-phase nano-liquid chromatography coupled to electron ionization tandem-mass spectrometry.

Herein we present an efficient, column-switching method that relies on a custom-made T-union passive diffusion micromixer to assist water dilution and promote trap solute focusing of a high sample volume dissolved in pure organic solvent using a 0.075 mm i.d. nano-LC column. This method allows injecting 20 µL (or higher) of sample volume, speeding up the analysis time, with a 400-fold increase of the limits of quantitation for selected compounds. Five pesticides in different media were used as model compounds, and the analyses were carried out with a triple quadrupole mass spectrometer equipped with a Liquid Electron Ionization (LEI) LC-MS interface working in multiple reaction monitoring (MRM) mode. The system microfluidics were investigated using COMSOL modeling software. Robustness of the entire system was evaluated using a post-extraction addition soil extracts with limits of detection values spanning from 0.10 to 0.45 µg/L. Reproducible results in terms of peak area, peak shape, and retention times were achieved in soil matrix. Repeatability test on peak area variations were lower than 10%.

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

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

Mass Spectrometry Based Approach for Organic Synthesis Monitoring.

Current mass spectrometry-based methodologies for synthetic organic reaction monitoring largely use electrospray ionization (ESI), or other related atmospheric pressure ionization-based approaches. Monitoring of complex, heterogeneous systems may be problematic because of sampling hardware limitations, and many relevant analytes (neutrals) exhibit poor ESI performance. An alternative monitoring strategy addressing this significant impasse is condensed phase membrane introduction mass spectrometry using liquid electron ionization (CP-MIMS-LEI). In CP-MIMS, a semipermeable silicone membrane selects hydrophobic neutral analytes, rejecting particulates and charged chemical components. Analytes partition through the membrane, and are then transported to the LEI interface for sequential nebulization, vaporization, and ionization. CP-MIMS and LEI are both ideal for continuous monitoring applications of hydrophobic neutral molecules. We demonstrate quantitative reaction monitoring of harsh, complex reaction mixtures (alkaline, acidic, heterogeneous) in protic and aprotic organic solvents. Also presented are solvent-membrane compatibility investigations and, in situ quantitative monitoring of catalytic oxidation and alkylation reactions.

Rapid, hydrolysis-free, dilute-and-shoot method for the determination of buprenorphine, norbuprenorphine and their glucuronides in urine samples using UHPLC-MS/MS.

Buprenorphine and its metabolites are routinely monitored to assess patient compliance with drug detoxification programs or as pain killers. A rapid method for the simultaneous analysis of buprenorphine, norbuprenorphine, and glucuronides in urine using ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) was developed. Urine samples were diluted in water containing formic acid 0.1% and directly injected into the UHPLC-MS/MS system without any sample pretreatment. Quality control (QC) samples, prepared using 20 different urine matrices, fortified at 3 concentration levels, were quantified using four deuterated internal standards. The accuracy values obtained spanned from 90 to 114% with repeatability lower than 10% also in the inter-day between batch experiments. Matrix effects (ME), evaluated before correction with internal standards using Matuszewski procedure, mainly affected the analysis of buprenorphine glucuronide. The use of deuterated internal standards (IS) for each analyte was necessary to compensate for ME and was essential in the determination of glucuronides. The method was applied to 30 real urine samples from patients under a detoxification therapy. Duplicate analyses were performed with the presented method and compared with another method which involves a standard hydrolysis procedure. Real sample results were compared showing a good performance agreement, with differences between the two methods lower than ±20% in the quantification results.

Evaluation of a liquid electron ionization liquid chromatography-mass spectrometry interface.

Liquid Electron Ionization (LEI), is an innovative liquid chromatography-mass spectrometry (LC-MS) interface that converts liquid HPLC eluent to the gas-phase in a mass spectrometer equipped with an electron ionization (EI) source. LEI extends the electronic spectra libraries access to liquid chromatography, providing a powerful tool in the untargeted approacssh. Negligible matrix effects allow accurate quantitative information. The purpose of this research was to evaluate the main aspects concerning the interfacing process. These fundamental studies were necessary to understand the mechanism of LEI in details, and improve the interfacing process, especially regarding robustness and sensitivity. Hardware components were installed to prevent analytes precipitation, reduce thermal decomposition of sensitive compounds, and to stabilize the nano-flow delivery with different mobile-phase compositions. Particular attention was devoted to insulating the heated vaporization area from the LC part of the system. Experiments were performed to optimize the interface inner capillary dimensions, and other operative parameters, including temperature, gas and liquid flow rates. Test compounds of environmental interest were selected based on molecular weight, thermal stability, volatility, and polarity. Robustness was evaluated with a set of replicated injections and calibration experiments using a soil matrix as a test sample. MRM detection limits in the low-picogram range were obtained for five pesticides belonging to different classes in a soil sample. High-quality electron ionization mass spectra of a mixture of pesticides were also obtained.

MS-Based Analytical Techniques: Advances in Spray-Based Methods and EI-LC-MS Applications.

Mass spectrometry is the most powerful technique for the detection and identification of organic compounds. It can provide molecular weight information and a wealth of structural details that give a unique fingerprint for each analyte. Due to these characteristics, mass spectrometry-based analytical methods are showing an increasing interest in the scientific community, especially in food safety, environmental, and forensic investigation areas where the simultaneous detection of targeted and nontargeted compounds represents a key factor. In addition, safety risks can be identified at the early stage through online and real-time analytical methodologies. In this context, several efforts have been made to achieve analytical instrumentation able to perform real-time analysis in the native environment of samples and to generate highly informative spectra. This review article provides a survey of some instrumental innovations and their applications with particular attention to spray-based MS methods and food analysis issues. The survey will attempt to cover the state of the art from 2012 up to 2017.

Determination of benzodiazepines in beverages using green extraction methods and capillary HPLC-UV detection.

Dispersive liquid-liquid microextraction with and without ultrasound assistance (DLLME, UA-DLLME) and microextraction with packed sorbent (MEPS) methods for the extraction and determination of eight different benzodiazepines (BDZ) (chlordiazepoxide, flurazepam, bromazepam, oxazepam, lorazepam, clobazam, clonazepam, and flunitrazepam) in three commercial non-alcoholic and light alcoholic beverages were optimized and compared. Benzodiazepines are frequently used for their extensive diffusion and strong numbing effect in drug-facilitated crimes (DFC). The tiny small amount of sample required for DLLME and MEPS extraction makes them very suitable for specimens collected at the crime scene of DFCs. Microextraction techniques are of increasing interest thanks to their accordance to green analytical chemistry (GAC) guidelines providing good recovery values. Ultrasound assistance (UA-DLLME) was used to investigate whether this type of energy can improve the recoveries of the analytes. Analyses of the extracts were performed with reverse-phase capillary high-performance liquid chromatography with UV detection (HPLC - UV), thanks to low environmental impact, robustness, diffusion, and affordability. Recovery percentages at three different concentrations in the three beverages were between 14.30% and 103.28% with intraday and interday RSD lower than ±2.78%. The same samples were extracted using a MEPS protocol, and the results were compared with those obtained with DLLME. MEPS gave recoveries between 20.90% and 101.88% for all matrices showing a better performance than DLLME at higher concentrations, though lower recoveries were observed with diluted samples.

Direct Infusion Resonance-Enhanced Multiphoton Ionization Mass Spectrometry of Liquid Samples under Vacuum Conditions.

Direct infusion resonance-enhanced multiphoton ionization (DI-REMPI) was performed on liquid samples, which were introduced to the ion source via a direct liquid interface, to enable the investigation of dissolved aromatic compounds. Desolvation and nebulization of the samples were supported by a heated repeller using flow rates in the upper nL min-1 range. The obtained mass spectra of five pure polycyclic aromatic hydrocarbons as well as complex petroleum samples revealed predominantly molecular ions without evidence of solvent or dopant effects as observed in atmospheric pressure photoionization (APPI) and laser ionization (APLI) with limits of detection in the lower pmol range. Furthermore, it is demonstrated by the analysis of different complex oil samples that DI-REMPI covers a larger m/z range than external volatilization of the sample prior to introduction to the ion source by using thermogravimetry (TG) hyphenated to REMPI time-of-flight mass spectrometry (TOFMS). Analogous to reported setups with direct liquid interface and electron ionization, direct-REMPI may be an option for soft ionization in liquid chromatography.

Sol-gel coated ion sources for liquid chromatography-direct electron ionization mass spectrometry.

Advances in interfacing liquid chromatography and electron ionization mass spectrometry are presented. New ion source coatings synthesized by sol-gel technology were developed and tested as vaporization surfaces in terms of peak intensity, peak width and peak delay for the liquid chromatography-direct electron ionization mass spectrometry (Direct-EI) determination of environmental pollutants like polycyclic aromatic hydrocarbons and steroids. Silica-, titania-, and zirconia-based coatings were sprayed inside the stainless steel ion source and characterized in terms of thermal stability, film thickness and morphology. Negligible weight losses until 350-400 °C were observed for all the materials, with coating thicknesses in the 6 (±1)-11 (±2) µm range for optimal ionization process. The best performances in terms of both peak intensity and peak width were obtained by using the silica-based coating: the detection of the investigated compounds was feasible at low ng µl-1 levels with a good precision (RSD < 9% for polycyclic aromatic hydrocarbons and <11% for hormones).

Atmospheric Pressure Vaporization Mechanism for Coupling a Liquid Phase with Electron Ionization Mass Spectrometry.

A novel liquid chromatography-mass spectrometry (LC-MS) interfacing concept is presented and discussed. The new interface, called liquid-EI (LEI), is based on electron ionization (EI) but, differently from any previous attempt, the vaporization of solutes and mobile phase takes place at atmospheric pressure into a specifically designed region, called "vaporization microchannel", before entering the high-vacuum ion source. The interface is completely independent from the rest of the instrumentation and can be adapted to any gas chromatography/mass spectrometry (GC/MS) system, as an add-on for a rapid LC-MS conversion. Pressure drop and temperature gradient between LC and MS were considered to enhance the analyte response and reduce band broadening and/or solute carryovers. A fused silica liner, placed inside the vaporization microchannel, acts as an inert vaporization surface speeding up the gas-phase conversion of large molecules while lessening possible memory effects. The liner is easily replaceable for a quick and extremely simple interface maintenance. Proof of concept and detailed description of the interface are here presented.

Microextraction by packed sorbent (MEPS)-UHPLC-UV: A simple and efficient method for the determination of five benzodiazepines in an alcoholic beverage.

This article describes a nano-scale method for the determination and quantification of five benzodiazepines (BDZ) in an alcoholic grappa drink (chlordiazepoxide; lorazepam; diazepam; oxazepam; medazepam). BDZ are typically used in drug-facilitated crimes (DFC) for their accessibility and synergistic effects with alcohol. Specimens collected on the crime scene must be rapidly analyzed to prove the crime, though, in most cases, a very small amount is available. Off-line MEPS extraction of diluted grappa samples proved to be an efficient and reliable method for the recovery of the selected compounds. Requiring a very small amount of extraction solvents, MEPS is an environment-friendly technique. LC separation with UV detection was used as the analytical technique because it is simple, robust, relatively economic and easy-to-find in most laboratories. The method was validated in terms of precision, accuracy and recovery. Limits of detection and quantitation were in the range of 0.5-2ng/µL. Linearity (R(2)) spanned from 0.9994 and 1.0000. Intra- and inter-day repeatabilities were lower than 12% at any concentration. Recovery percentages of an equivalent-to-real sample at three different concentrations were between 70.7 and 74.1%.

Liquid chromatography-electron ionization tandem mass spectrometry with the Direct-EI interface in the fast determination of diazepam and flunitrazepam in alcoholic beverages.

This is the first application based on electron ionization (EI) using a Direct-EI LC interface and MS/MS to detect unequivocally target compounds in a very small real sample. The determination and quantification of benzodiazepines in very small residues of beverages, collected at the scene of drug-facilitated crimes are mandatory in legal procedures. A specific and sensitive analytical instrumentation is needed, involving little or no sample preparation. Here, a direct flow injection analysis of alcoholic beverages spiked with commercially available drugs containing diazepam and flunitrazepam is presented. The method proposed is very fast and requires neither sample preparation nor chromatographic separation. Linearity (R(2) ) was between 0.9977 and 0.9992; LOD and LOQ spanned from 0.01 to 0.02 ng/µL and from 0.1 to 0.5 ng/µL, respectively; intra- and interday repeatabilities were between 1 and 8%. No matrix effects were observed from the comparison of the linear regression curves obtained in real fortified samples and in pure ethanol. Vodka, whisky, and white wine specimens were fortified with commercial drugs, Valium(®) and Rohypnol(®) , at two different concentrations (20 and 50 ng/µL) to simulate the typical amounts found in adulterated real samples and analyzed to demonstrate the method applicability to forensic analyses.