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

A critical review of bioanalytical and clinical applications of solid phase microextraction

Studying the functions, mechanisms, and effects of drugs and other exogenous compounds on biological systems, together with investigations performed to understand biosystems better, comprises one of the most fascinating areas of research. Although classical sample preparation techniques are dominantly used to infer the relevant information from the investigated system, they fail to meet various imperative requirements, such as being environmentally friendly, applicable in-vivo, and compatible with online analysis. As a chameleon in the analytical toolbox, solid phase microextraction (SPME) is one of the best tools available for studying biological systems in unconventional ways. In this review, SPME is spotlighted, and its capability for bioanalytical applications, including drug analysis, untargeted and targeted metabolomics, in-vivo and clinical studies, is scrutinized based on studies reported in the past five years. In addition, novel extractive phases and instrumental coupling strategies developed to serve bioanalytical research are discussed to give the perspective for state-of-the-art and future developments. The literature assessment showed that SPME could act as a critical tool to investigate in-vivo biological systems and provide information about the elusive portion of the metabolome. Moreover, recently introduced miniaturized SPME probes further improved the low-invasive nature of the sampling and enabled sampling even from a single cell. The coupling of SPME directly to mass spectrometry significantly reduced the total analytical workflow and became one of the promising tools suitable for fast diagnostic purposes and drug analysis. The numerous applications and advancements reported in bioanalysis using SPME show that it will continue to be an indispensable technique in the future.

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.

Novel electrospun-based extractive probes for rapid determination of clinically important compounds in human plasma

BACKGROUND

Coated blade spray (CBS) represents an innovative approach that utilizes solid-phase microextraction principles for sampling and sample preparation. When combined with ambient mass spectrometry (MS), it can also serve as an electrospray ionization source. Therefore, it became a promising tool in analytical applications as it can significantly reduce the analysis time. However, the current CBS coatings are based on the immobilization of extractive particles in bulk polymeric glue, which constrains the diffusion of the analytes to reach the extractive phase; therefore, the full reward of the system cannot be taken at pre-equilibrium. This has sparked the notion of developing new CBS probes that exhibit enhanced kinetics.

RESULTS

With this aim, to generate a new extractive phase with improved extraction kinetics, poly(divinylbenzene) (PDVB) nanoparticles were synthesized by mini-emulsion polymerization and then immobilized into sub-micrometer (in diameter) sized polyacrylonitrile fibers which were obtained by electrospinning method. Following the optimization and characterization studies, the electrospun-coated blades were used to determine cholesterol, testosterone, and progesterone in plasma spots using the CBS-MS approach. For testosterone and progesterone, 10 ng mL-1 limits of quantification could be obtained, which was 200 ng mL-1 for cholesterol in spot-sized samples without including any pre-treatment steps to samples prior to extraction.

SIGNIFICANCE

The comparison of the initial kinetics for dip-coated and electrospun-coated CBS probes proved that the electrospinning process could enhance the extraction kinetics; therefore, it can be used for more sensitive analyses. The total analysis time with this method, from sample preparation to instrumental analysis, takes only 7 min, which suggests that the new probes are promising for fast diagnostic applications.

Recent advances in the use of SPME for drug analysis in clinical, toxicological, and forensic medicine studies

Solid-phase microextraction (SPME) has gained attention as a simple, fast, and non-exhaustive extraction technique, as its unique features enable its use for the extraction of many classes of drugs from biological matrices. This sample-preparation approach consolidates sampling and sample preparation into a single step, in addition to providing analyte preconcentration and sample clean-up. These features have helped SPME become an integral part of several analytical protocols for monitoring drug concentrations in human matrices in clinical, toxicological, and forensic medicine studies. Over the years, researchers have continued to develop the SPME technique, resulting in the introduction of novel sorbents and geometries, which have resulted in improved extraction efficiencies. This review summarizes developments and applications of SPME published between 2016 and 2022, specifically in relation to the analysis of central nervous system drugs, drugs used to treat cardiovascular disorders and bacterial infections, and drugs used in immunosuppressive and anticancer therapies.

Rapid Analysis of Fentanyl and Fentanyl Analogues from Whole Blood Using SPME Coupled to the Microfluidic Open Interface

Fentanyl and its analogues are potent opioids that pose a significant threat to society. Over the last several years, considerable focus has been on the concerning trend of increasing fentanyl usage among drug users. Fentanyl analogues are mainly synthesized to evade analytical detection or increase their potency; thus, very low concentrations are sufficient to achieve a therapeutic effect. In an effort to help combat the synthetic opioid epidemic, developing targeted mass spectrometric methods for quantifying fentanyl and its analogues at ultralow concentrations is incredibly important. Most methods used to analyze fentanyl and its analogues from whole blood require manual sample preparation protocols (solid-phase extraction or liquid-liquid extraction), followed by chromatographic separation and mass spectrometric detection. The main disadvantages of these methods are the tedious sample preparation workflows, resulting in lengthy analysis times. To mitigate these issues, we present a targeted method capable of analyzing 96 samples containing fentanyl, several fentanyl analogues, and a common fentanyl (analogue) precursor simultaneously in 2.4 min per sample. This is possible by using a high-throughput solid phase microextraction workflow on the Concept96 autosampler followed by manual coupling of solid-phase microextraction fibers to the microfluidic open interface for tandem mass spectrometry analysis. Our quantitative method is capable of extremely sensitive analysis, with limits of quantification ranging from 0.002 to 0.031 ng mL-1 and linearity ranging from 0.010 to 25.0 ng mL-1. The method shows very good reproducibility (1-18%), accuracy (81-100%) of calibration and validation points, and good interday reproducibility (6-15%).

The overshadowed role of electron ionization-mass spectrometry in analytical biotechnology

Target and untargeted analysis of several compounds are crucial methods in important areas such as omics sciences. Gas chromatography coupled to mass spectrometry (GC-MS) is widely used for volatile and thermally stable compounds. In this case, the electron ionization technique (EI) is preferable as it produces highly fragmented and reproducible spectra comparable to spectral libraries. However, only a fraction of target compounds is analyzable by GC without chemical derivatization. Therefore, liquid chromatography (LC) coupled with MS is the most used technique. Contrary to EI, electrospray ionization does not produce reproducible spectra. That is why researchers have been working on interfaces between LC and EI-MS to bridge the gap between those techniques. This short review will discuss advancements, applications, and perspectives on biotechnological analysis.

Coupling of Solid-Phase Microextraction Directly to Mass Spectrometry via an Improved Microfluidic Open Interface to Facilitate High-Throughput Determinations

Mass spectrometry analysis can be performed by introducing samples directly to mass spectrometry, allowing the increase of the analysis throughput; however, some disadvantages of direct-to-mass spectrometry analysis include susceptibility to matrix effects and risk of instrument contamination from inadequate sample preparation. Solid-phase microextraction is one of the most suitable sample preparation methods for direct-to-mass spectrometry analysis, as it offers matrix-compatible coatings which ensure analyte enrichment with minimal or no interference from matrix. One of the ways solid-phase microextraction can be coupled directly to mass spectrometry is via a microfluidic open interface. This manuscript reports improvements made to the initial microfluidic open interface design, where the system components have been simplified to mostly commercially available materials. In addition, the analysis of samples has been automated by implementing software that fully controls the analysis workflow, where the washing procedure is optimized to completely reduce the carryover. Herein, the extraction and desorption time profiles from thin and thick SPME devices was studied where the overall workflow consisted of high-throughput sample preparation of 1.3 min per 96 samples and <1 min per sample instrumental analysis.

Direct Coupling of SPME to Mass Spectrometry

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

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.

A Small Footprint and Robust Interface for Solid Phase Microextraction and Mass Spectrometry Based on Vibrating Sharp-Edge Spray Ionization

Combining solid phase microextraction (SPME) and mass spectrometry (MS) analysis has become increasingly important to many bioanalytical, environmental, and forensic applications due to its simplicity, rapid analysis, and capability of reducing matrix effects for complex samples. To further promote the adoption of SPME-MS based analysis and expand its application scope calls for efficient and convenient interfaces that couple the SPME sample handling with the efficient analyte ionization for MS. Here, we report a novel interface that integrates both the desorption and the ionization steps in one device based on the capillary vibrating sharp-edge spray ionization (cVSSI) method. We demonstrated that the cVSSI is capable of nebulizing liquid samples in a pulled-tip glass capillary with a battery powered function generator. The cVSSI device allows the insertion of a SPME probe into the spray capillary for desorption and then direct nebulization of the desorption solvent in situ. With the integrated interface, we have demonstrated rapid MS analysis of drug compounds from serum samples. Quantitative determination of various drug compounds including metoprolol, pindolol, acebutolol, oxprenolol, capecitabine, and irinotecan was achieved with good linearity (R2 = 0.97-0.99) and limit of detection ranging from 0.25 to 0.59 ng/mL without using a high voltage source. Only 3.5 μL of desorption solvent and 3 min desorption time were needed for the present method. Overall, we demonstrated a portable SPME-MS interface featuring high sensitivity, short analysis time, small footprint, and low cost, which makes it an attractive method for many applications requiring sample cleanup including drug compound monitoring, environmental sample analysis, and forensic sample analysis.

Tyrosol and Hydroxytyrosol Determination in Extra Virgin Olive Oil with Direct Liquid Electron Ionization-Tandem Mass Spectrometry

Extra virgin olive oil (EVOO) is one of the main ingredients of the Mediterranean diet. It is claimed as a functional food for its unique content of health-promoting compounds. Tyrosol (Tyr), Hydroxytyrosol (Htyr), and their phenolic derivatives present in EVOO show beneficial properties, and their identification and quantification, both in their free form and after the hydrolysis of more complex precursors, are important to certify its quality. An alternative method for quantifying free and total Tyr and Htyr in EVOO is presented using an LC–MS interface based on electron ionization (EI), called liquid electron ionization (LEI). This method requires neither sample preparation nor chromatography; the sample is diluted and injected. The selectivity and sensitivity were assessed in multiple reaction monitoring mode (MRM), obtaining confirmation and quantification in actual samples ranging from 5 to 11 mg/Kg for the free forms and from 32 to 80 mg/Kg for their total amount after hydrolysis. Two MS/MS transitions were acquired for both compounds using the Q/q ratios as confirmatory parameters. Standard addition calibration curves demonstrated optimal linearity and negligible matrix effects, allowing a correct quantification even without expensive and difficult to find labeled internal standards. After several weeks of operation, the system’s repeatability was excellent, with an intraday RSD (%) spanning from five to nine and an interday RSD (%) spanning from 9 to 11.

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.