Electrospray Ionization on Solid Substrates

Electrospray ionization mass spectrometry with wooden tips: A review

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

Immuno-Enriched Microspheres - Magnetic Blade Spray-Tandem Mass Spectrometry for Domoic Acid in Mussels

Paramagnetic microspheres can be used in planar array fluorescence immunoassays for single or multiplex screening of food contaminants. However, no confirmation of the molecular identity is obtained. Coated blade spray (CBS) is a direct ionization mass spectrometry (MS) technique, and when combined with triple quadrupole MS/MS, it allows for rapid confirmation of food contaminants. The lack of chromatography in CBS, though, compromises the specificity of the measurement for unequivocal identification of contaminants, based on the European Union (EU) regulation. Therefore, a rapid and easy-to-use immuno-magnetic blade spray (iMBS) method was developed in which immuno-enriched paramagnetic microspheres replace the coating of CBS. The iMBS-MS/MS method was fully optimized, validated in-house following the EU 2021/808 regulation, and benchmarked against a commercial lateral flow immunoassay (LFIA) for on-site screening of DA. The applicability of iMBS-MS/MS was further demonstrated by analyzing incurred mussel samples. The combination of immunorecognition and MS/MS detection in iMBS-MS/MS enhances the measurement's selectivity, which is demonstrated by the rapid differentiation between the marine toxin domoic acid (DA) and its structural analog kainic acid (KA), which cannot be achieved with the LFIA alone. Interestingly, this first-ever reported iMBS-MS/MS method is generic and can be adapted to include any other immuno-captured food contaminant, provided that monoclonal antibodies are available, thus offering a complementary confirmatory analysis approach to multiplex immunoassay screening methods. Moreover, thanks to its speed of analysis, iMBS-MS/MS can bridge the logistics gap between future large-scale on-site testings using LFIAs and classical time-consuming confirmatory MS analysis performed in official control laboratories.

Vibrating Sharp-edge Spray Ionization (VSSI) for voltage-free direct analysis of samples using mass spectrometry

RATIONALE

The development of miniaturized and field portable mass spectrometers could not succeed without a simple, compact, and robust ionization source. Here we present a voltage-free ionization method, Vibrating Sharp-edge Spray Ionization (VSSI), which can generate a spray of liquid samples using only one standard microscope glass slide to which a piezoelectric transducer is attached. Compared with existing ambient ionization methods, VSSI eliminates the need for a high electric field (~5000 V·cm-1 ) for spray generation, while sharing a similar level of simplicity and flexibility with the simplest direct ionization techniques currently available such as paper spray ionization (PSI) and other solid substrate-based electrospray ionization methods.

METHODS

The VSSI device was fabricated by attaching a piezoelectric transducer onto a standard glass microscope slide using epoxy glue. Liquid sample was aerosolized by either placing a droplet onto the vibrating edge of the glass slide or touching a wet surface with the glass edge. Mass spectrometric detection was achieved by placing the VSSI device 0.5-1 cm from the inlet of the mass spectrometer (Q-Exactive, ThermoScientific).

RESULTS

VSSI is demonstrated to ionize a diverse array of chemical species, including small organic molecules, carbohydrates, peptides, proteins, and nucleic acids. Preliminary sensitivity experiments show that high-quality mass spectra of acetaminophen can be obtained by consuming 100 femtomoles of the target. The dual spray of VSSI was also demonstrated by performing in-droplet denaturation of ubiquitin. Finally, due to the voltage-free nature and the direct-contact working mode of VSSI, it has been successfully applied for the detection of chemicals directly from human fingertips.

CONCLUSIONS

Overall, we report a compact ionization method based on vibrating sharp-edges. The simplicity and voltage-free nature of VSSI make it an attractive option for field portable applications or analyzing biological samples that are sensitive to high voltage or difficult to access by conventional ionization methods.

Rapid detection of pesticides in honey by solid-phase micro-extraction coupled with electrospray ionization mass spectrometry

Detection of pesticide residues in food samples is important for safeguarding food quality and safety. Conventional approaches for detection of pesticides in food samples typically involve labour-intensive and time-consuming sample pretreatment and chromatographic separation. In this study, solid phase micro-extraction fibres were used to rapidly extract and enrich pesticides in honey, a popular agricultural product with complex matrix, and then directly coupled with electrospray ionization mass spectrometry for qualitative and quantitative analysis. Three pesticides, ie, atrazine, benalaxyl, and pirimicarb, were investigated using the technique and their analytical performances were evaluated. The limits of detection and limits of quantitation of all the three pesticides could fulfil the cut-off values of the international standard. Linear calibration curves were constructed with good R² coefficients, and the accuracy and precision were in acceptable ranges for all the pesticides. The analysis time is much reduced, with only minimum sample preparation and no chromatographic separation involved. The technique is simple and easy to set up, and can be extended for analysis of other analytes and sample systems.

Rapid detection and quantitation of drugs-of-abuse by wooden-tip electrospray ionization mass spectrometry

Determination of drugs-of-abuse in body fluids of drug abusers is important for the law enforcement as well as the treatment and rehabilitation. In this study, wooden-tip electrospray ionization mass spectrometry (WT-ESI-MS), a simple and cost-effective technique, was developed for rapid detection and quantitation of common drugs-of-abuse, including methamphetamine, methylenedioxymethamphetamine (MDMA), cocaine, heroin and tetrahydrocannabinol (THC), in urine and oral fluid, following our previous successful demonstration for rapid and sensitive detection of ketamine and nor-ketamine in urine and oral fluid by this technique. It was found that the limit-of-detection for methamphetamine could fully fulfill the cut-off value requirements of the international standards, and those of MDMA and cocaine could fulfill some of the requirements. The linear range, accuracy and precision for quantitation of the drugs were generally satisfactory, except for THC for which the analytical performance still needs to be improved. Analysis of one sample could typically be completed within minutes. These results indicated that WT-ESI-MS could be used for rapid screening of drugs-of-abuse in urine, oral fluid as well as other body fluids.

Characterization and Analysis of Paper Spray Ionization of Organic Compounds

Paper spray ionization has arisen relatively recently as a complement and alternative to electro- and nanospray ionization with silica capillaries. A majority of the work in the present literature focuses on the chemical aspect of paper spray. In order to study the physical and phenomenological facet of its implementation, we measured current and voltage distributions of Taylor cones. To study transport phenomena on filter paper, we addressed the behavior of large, sparingly soluble tetraalkylammonium ions, which are usually used as mobility standards, in paper spray. The variation of intensity with time of monomers and dimers of these ions was measured with a differential mobility analyzer and compared with that produced by contamination in the paper. At the same time, we evaluated the proficiency of different paper spray techniques for protein analysis using nano spray as a reference. Experiments suggest that Taylor cones in paper spray are subject to hysteresis, whereas transport phenomena in the porous substrate notably affects the ionization of the sample. Additionally, we observed that paper spray tends to favor lower charge states in proteins. Graphical Abstract.

Polymer-spray mass spectrometric detection and quantitation of hydrophilic compounds and some narcotics

RATIONALE

High-throughput screening of biofluids is essential in monitoring concentration of a variety of drugs to determine their efficacy and toxicity. Organosiloxane polymers prepared by sol-gel chemistry as sample supports, and electrospray ionization emitters in a single material and as an alternative to paper substrates, is described in this study.

METHODS

Hydrophobic drugs and hydrophilic streptomycin were analyzed by polymer-spray mass spectrometry with an LTQ-Orbitrap mass spectrometer. Drug samples in urine (1-2 μL) were deposited on an OSX polymer, allowed to dry, then electrosprayed from the polymer tip into the mass spectrometer without sample pretreatment. The OSX polymers, whose polarity and porosity can be controlled, were prepared by sol-gel chemistry where methyl-substituted alkoxysilanes were hydrolyzed in the presence of a pore template and an acid catalyst.

RESULTS

Five nanograms each of seven narcotic drugs were detected in <1 min (relative standard deviation (RSD) of response <1% for each drug). Calibration curves of cocaine and streptomycin in urine were used to establish the performance of the polymer. For sample 1 (n = 2), the mean recovery for cocaine was 81% with paper and 90% with polymer. Streptomycin is detected with polymer, not with paper; for samples 1 and 2 (n = 3), mean recovery was 97% and 95%, respectively.

CONCLUSIONS

Organosiloxane polymers achieve more sensitive analysis than paper, allowing for more accurate quantitation of both hydrophobic and hydrophilic drug compounds. The ability to tailor the polymer polarity and porosity allows for the synthesis of a wide range of polymers, and thus opens many possibilities for further development and applications.

In situ analysis of soybeans and nuts by probe electrospray ionization mass spectrometry

The probe electrospray ionization (PESI) is an ESI-based ionization technique that generates electrospray from the tip of a solid metal needle. In the present work, we describe the PESI mass spectra obtained by in situ measurement of soybeans and several nuts (peanuts, walnuts, cashew nuts, macadamia nuts and almonds) using different solid needles as sampling probes. It was found that PESI-MS is a valuable approach for in situ lipid analysis of these seeds. The phospholipid and triacylglycerol PESI spectra of different nuts and soybean were compared by principal component analysis (PCA). PCA shows significant differences among the data of each family of seeds. Methanolic extracts of nuts and soybean were exposed to air and sunlight for several days. PESI mass spectra were recorded before and after the treatment. Along the aging of the oil (rancidification), the formation of oxidated species with variable number of hydroperoxide groups could be observed in the PESI spectra. The relative intensity of oxidated triacylglycerols signals increased with days of exposition. Monitoring sensitivity of PESI-MS was high. This method provides a fast, simple and sensitive technique for the analysis (detection and characterization) of lipids in seed tissue and degree of oxidation of the oil samples.