Detection of native proteins using solid-substrate electrospray ionization mass spectrometry with nonpolar solvents

COSMO-RS-based assessment of thermodynamic tools in predicting the polar and non-polar solvents efficiency in vegetable oil extraction

CONTEXT

One of the prevalent methods for evaluating separation performance is to predict the interactions of solvent and solute molecules. The infinite dilution activity coefficient, Gibbs free energy, and sigma profiles provided insights into the solubilization of a solute and revealed the intensity of the solution's molecular interactions. The effective thermodynamic tools (infinite dilution activity coefficient, Gibbs free energy) were evaluated for predicting the efficiency of 18 polar and non-polar organic solvents in rubber seed oil (RSO) extraction. An infinite dilution activity coefficient was computed to evaluate the solubility of the rubber seed oil model compound (linoleic acid) in the organic solvents. Gibbs free energy was evaluated to show the energy change associated with the molecules mixing process and forecast the miscibility of linoleic acid molecules in the solvents. Moreover, the study examined the sigma profiles and sigma surfaces of organic solvents and linoleic acid to acquire a deeper insight into their similarities and how they interact molecularly. According to the computational prediction and experimental verification, the thermodynamic properties of Gibbs free energy and activity coefficient proved to be highly effective tools for screening polar and moderately polar solvents, predicting the molecular interactions with solute. Whereas the sigma profile and sigma surface were found to be the most efficient tools for evaluating the efficacy of non-polar solvents. Solvents with moderate polarity, such as tetrahydrofuran and diethyl ether, as well as non-polar solvents like pentane, heptane, and n-hexane, proved to be effective and favorable for oil extraction, resulting in the highest oil yields of approximately 27.0%. Overall, the COSMO-RS method demonstrates its utility in estimating the solubility of RSO in organic solvents, enabling early identification of the most effective solvent.

METHODS

The initial geometry optimization of every component was conducted through density functional theory (DFT) using TmoleX software. A single-point density functional theory (DFT) computation using Becke Perdew 86 (BP86) and the Triple-Zeta Valence Potential (TZVPD) was performed to produce.cosmo files. COSMO-RS calculations were performed by applying the parameterization file BP_TZVPD_FINE_19.ctd using COSMOthermX software. The practical extraction of oil from plant seeds was performed using a sonicator bath to verify the accuracy of the COSMO-RS predictions.

Enhanced ultrasonic spray ionization for direct mass spectrometry analysis of aqueous solution and complex samples using a single-orifice piezoelectric atomizer

An efficient and superior soft ionization approach for direct mass spectrometry analysis of a variety of samples such as aqueous solution, raw biological sample and proteins, was developed based on commercially available piezoelectric atomizers. A single conical orifice (5 μm in diameter) was created on the atomizer, which resulted in generation of uniform fine droplets and long-duration of MS signal. The two electrodes of piezoelectric atomizer were connected to the two sides of ceramic ring which was insulated from the metallic substrate. The unique design allowed an additional high voltage input towards the spray reagents, which facilitated direct analysis of more complex samples without sample pre-treatment, such as biological samples (tomato tissue). The ionization was driven by an extremely low electrical power (3.5 V rechargeable battery) yet providing an efficient and superior soft ionization. The method displayed a better thermal and pH stability than nano electrospray ionization (nanoESI) and electrospray ionization (ESI) on direct analysis of Vitamin B and protein aqueous solutions. Quantitative analysis of Vitamin B and Rhodamine B aqueous solutions was also investigated, showing a good linearity (R² > 0.99). In addition, our results suggested that compared with ESI and nanoESI, the method not only could be used for direct analysis of intact protein, but also provide more information concerning the association between intact protein and the subunits.

Dissecting the structural heterogeneity of proteins by native mass spectrometry

A single gene yields many forms of proteins via combinations of post-transcriptional/post-translational modifications. Proteins also fold into higher-order structures and interact with other molecules. The combined molecular diversity leads to the heterogeneity of proteins that manifests as distinct phenotypes. Structural biology has generated vast amounts of data, effectively enabling accurate structural prediction by computational methods. However, structures are often obtained heterologously under homogeneous states in vitro. The lack of native heterogeneity under cellular context creates challenges in precisely connecting the structural data to phenotypes. Mass spectrometry (MS) based proteomics methods can profile proteome composition of complex biological samples. Most MS methods follow the "bottom-up" approach, which denatures and digests proteins into short peptide fragments for ease of detection. Coupled with chemical biology approaches, higher-order structures can be probed via incorporation of covalent labels on native proteins that are maintained at the peptide level. Alternatively, native MS follows the "top-down" approach and directly analyzes intact proteins under nondenaturing conditions. Various tandem MS activation methods can dissect the intact proteins for in-depth structural elucidation. Herein, we review recent native MS applications for characterizing heterogeneous samples, including proteins binding to mixtures of ligands, homo/hetero-complexes with varying stoichiometry, intrinsically disordered proteins with dynamic conformations, glycoprotein complexes with mixed modification states, and active membrane protein complexes in near-native membrane environments. We summarize the benefits, challenges, and ongoing developments in native MS, with the hope to demonstrate an emerging technology that complements other tools by filling the knowledge gaps in understanding molecular heterogeneity of proteins. This article is protected by copyright. All rights reserved.

Direct mass spectrometry analysis of biological tissue for diagnosis of thyroid cancer using wooden-tip electrospray ionization

Direct mass spectrometry (MS) analysis of human tissue at the molecular level could gain insight into biomarker discovery and disease diagnosis. Detecting metabolite profiles of tissue sample play an important role in understanding the pathological properties of disease development. Because the complex matrices in tissue samples, complicated and time-consuming sample preparation processes are usually required by conventional biological and clinical MS methods. Direct MS with ambient ionization technique is a new analytical strategy for direct sample analysis with little sample preparation, and has been proven to be a simple, rapid, and effective analytical tools for direct analysis of biological tissues. In this work, we applied a simple, low-cost, disposable wooden tip (WT) for loading tiny thyroid tissue, and then loading organic solvents to extract biomarkers under electrospray ionization (ESI) condition. Under such WT-ESI, the extract of thyroid was directly sprayed out from wooden tip to MS inlet. In this work, thyroid tissue from normal and cancer parts were analyzed by the established WT-ESI-MS, showing lipids were mainly detectable compounds in thyroid tissue. The MS data of lipids obtained from thyroid tissues were further analyzed with MS/MS experiment and multivariate variable analysis, and the biomarkers of thyroid cancer were also investigated.

Determination of nicotine in newborn meconium by high-Resolution ambient mass spectrometry using wooden-Tip spray

Prenatal exposure to nicotine that are mainly produced from tobacco smoke has been reported to affect infants. Therefore, nicotine exposure is one of important health concerns for newborn screening. Detecting nicotine and its metabolites such as cotinine in meconium were widely used to evaluate the tobacco exposure of pregnancy. In this study, disposable wooden tips were applied for touch sampling of meconium from newborn infants, and then were directly mounted on mass spectrometer (MS) to perform rapid screening of nicotine and cotinine. Choice of extraction/spray solvents was optimized. The limits of detection, reproducibility, linear response for direct analysis of meconium were also investigated. It is found the limits of detection (S/N = 3) to be as low as 0.36 ng/mg and 1.18 ng/mg for nicotine and cotinine, respectively, while the limits of quantitation (S/N = 10) to be 1.19 ng/mg and 3.94 ng/mg for nicotine and cotinine, respectively. The relative standard deviations (RSD) were found to be at 8.4%-19.8% (n = 6) for nicotine and cotinine, a good linear range from 5-500 ng/mL (R ² > 0.99). These analytical performances are well-accepted levels for ambient mass spectrometer analysis. In this study, evaluation of nicotine and cotinine in 22 puerpera volunteers were conducted by the established wooden-tip spray mass spectrometry (WTS-MS). These results showed that wooden-tip spray mass spectrometry would be useful for newborn screening of nicotine and cotinine in meconium with high reproducibility, speed, sensitivity, and specificity. Owing to the use of disposable wooden tips that involves no sample preparation and no chromatographic separation, our results show that wooden-tip spray mass spectrometry is a powerful tool for determination of nicotine in newborn meconium.

Wooden-Tip Electrospray Mass Spectrometry Characterization of Human Hemoglobin in Whole Blood Sample for Thalassemia Screening: A Pilot Study

Traditional analytical methods for thalassemia screening are needed to process complicated and time-consuming sample pretreatment. In recent decades, ambient mass spectrometry (MS) approaches have been proven to be an effective analytical strategy for direct sample analysis. In this work, we applied ambient MS with wooden-tip electrospray ionization (WT-ESI) for the direct analysis of raw human blood samples that were pre-identified by gene detection. A total of 319 whole blood samples were investigated in this work, including 100 α-thalassemia carriers, 67 β-thalassemia carriers, and 152 control healthy samples. Only one microliter of raw blood sample was directly loaded onto the surface of the wooden tip, and then five microliters of organic solvent and a high voltage of +3.0 kV were applied onto the wooden tip to generate spray ionization. Multiply charged ions of human hemoglobin (Hb) were directly observed by WT-ESI-MS from raw blood samples. The signal ratios of Hb chains were used to characterize two main types of thalassemia (α and β types) and healthy control blood samples. Our results suggested that the ratios of charged ions to Hb chains being at +13 would be an indicator for β-thalassemia screening.

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.

Pre-cleaned bare wooden toothpicks for the determination of drugs in oral fluid by mass spectrometry

This article deepens the potential of pre-cleaned bare wooden toothpicks (pb-WTs) for extracting drugs (antidepressants and acetaminophen) from oral fluid samples. The leaching of the intrinsic compounds from the wood matrix is identified as the main challenge for the final determination of the targets, even when a very selective instrumental technique, such as mass spectrometry, is employed. The pre-cleaning of the WTs is proposed for improving the analytical performance. The number of cleaning cycles depends on the injection mode (direct infusion or chromatography) into the mass spectrometer. The different variables affecting the extraction of selected antidepressant drugs were studied in detail, and the optimum procedure was validated using the two mentioned injection modes. The limits of detection were in the ranges 0.1–0.5 ng/mL and 0.1–0.3 ng/mL for direct infusion and liquid chromatography, respectively. The intra-day precision (expressed as relative standard deviation) was better than 12.1% and 8.6%, for direct infusion and liquid chromatography, respectively. Single-blind samples were used to study the applicability of the method. Finally, as a proof-of-concept, the potential of pb-WTs for in vivo sampling was outlined.

Wooden-based materials: Eco-friendly materials for direct mass spectrometric analysis and microextraction

Lignocellulosic materials have arisen as a sustainable alternative in microextraction techniques during the last 10 years. As they are natural materials, their use fits into some of the principles of Green Analytical Chemistry. Their inherent porosity, narrow shape, and rigidity permit their use in ambient ionization mass spectrometry techniques. In particular, the combination of wooden-based materials and direct analysis gives birth to the so-called wooden-tip electrospray ionization mass spectrometry technique. This approach has been used for the direct analysis of complex samples, and as a streamlined tool for fingerprint quality analysis. Also, wooden-based materials can be superficially modified to boost the interaction with target compounds, allowing their isolation from complex samples. This review describes the potential and applicability of direct analysis using lignocellulosic materials, as well as other alternatives related to their use in microextraction.

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.

Investigating distributions and changes of alkaloids in living Catharanthus roseus under low-phosphorus stress using wooden-tip electrospray ionisation mass spectrometry

INTRODUCTION

Vinca alkaloids are important sources for producing anticancer drugs from Catharanthus roseus. The phosphorus of soil is one of crucial factors for planting C. roseus.

OBJECTIVES

We aim to develop an in vivo sampling technique coupled with direct mass spectrometry with wooden tip for investigating distributions and changes of alkaloids in flowers, leaves, stems, veins and roots of living C. roseus under low-phosphorus stress.

MATERIALS AND METHODS

Living C. roseus were prepared under low-phosphorus stress (n = 10) and control conditions (n = 10). Wooden-tip electrospray ionisation mass spectrometry and conventional liquid chromatography-mass spectrometry were applied to analyse living C. roseus and extracts of C. roseus, respectively.

RESULTS

Distributions and changes of serpentine, vindoline, catharanthine, and anhydrovinblastine in living C. roseus under low-phosphorus stress and control conditions were successfully obtained.

CONCLUSION

Compared to control soil conditions, low-phosphorus soil was found to induce C. roseus to generate more serpentine but less catharanthine and vindoline in leaves, veins, stems and roots, and to generate more anhydrovinblastine in flowers, leaves, stems and roots. Overall, our results showed a simple, rapid, and effective method for in vivo sampling and direct analysis of living plants.

Analytical Chemistry in the 21st Century: Challenges, Solutions, and Future Perspectives of Complex Matrices Quantitative Analyses in Biological/Clinical Field

Nowadays, the challenges in analytical chemistry, and mostly in quantitative analysis, include the development and validation of new materials, strategies and procedures to meet the growing need for rapid, sensitive, selective and green methods. In this context, considering the constantly updated International Guidelines, constant innovation is mandatory both in the pre-treatment procedures and in the instrumental configurations to obtain reliable, true, and reproducible information. In this context, additionally to the classic plasma (or serum) matrices, biopsies, whole blood, and urine have seen an increase in the works that also consider non-conventional matrices. Obviously, all these studies have shown that there is a correlation between the blood levels and those found in the new matrix, in order to be able to correlate and compare the results in a robust way and reduce any bias problems. This review provides an update of the most recent developments currently in use in the sample pre-treatment and instrument configurations in the biological/clinical fields. Furthermore, the review concludes with a series of considerations regarding the role and future developments of Analytical Chemistry in light of the forthcoming challenges and new goals to be achieved.

Direct Detection of Lysozyme in Viscous Raw Hen Egg White Binding to Sodium Dodecyl Sulfonate by Reactive Wooden-tip Electrospray Ionization Mass Spectrometry

Direct characterization of native protein binding to ligands in raw biological samples is a challenging task, because the ligand solution might induce proteins to aggregation, flocculation and denaturation. In this work, we developed a reactive wooden-tip electrospray ionization mass spectrometry (ESI-MS) for formation and characterization of protein-ligand complexes upon rapid mixing in electrospray droplets. Raw viscous hen egg white (HEW) was directly loaded onto a wooden tip to induce spray ionization, and sodium dodecyl sulfonate (SDS) solution was directly loaded into the HEW spray by a pipette tip, and thus lysozyme-DS complexes were then formed in the electrospray droplets and were detected subsequently by mass spectrometry. The new approach was successfully applied to investigate interaction of SDS and native lysozyme in electrospray droplets of standard solution and raw egg white. Our results showed that wooden-tip ESI-MS is a promising method to form and characterize protein-ligand complexes.

Fast-switching high-voltage porous-tip electrospray ionization mass spectrometry for rapid detection of antirheumatic drugs in adulterated herbal dietary supplements

RATIONALE

Herbal dietary supplements (HDSs) adulterated with undeclared synthetic drugs can lead to serious health problems METHODS: A fast-switching positive/negative high-voltage (+/- HV) was developed to apply on electrospray ionization mass spectrometry (ESI-MS) with porous tips for rapid screening of five antirheumatic drugs in antirheumatic HDSs. The fast-switching (switch-time: 100 ms) negative and positive ions were alternately generated to perform full-MS and tandem-MS analysis, providing an effective method for rapid detection of analytes in whichever mode of detection was most suitable (negative or positive ion mode). The use of different tips and solvents was also optimized in this work.

RESULTS

The limits of detection of the five antirheumatic drugs were found to be less than 0.1 ng/g (S/N > 3). The reproducibility of the five drugs was measured to be 10.0-23.3% (n = 5). A single sample analysis could be completed within 1 min. Rapid screening of a total of 28 real HDS samples collected from the market was examined by the fast-switching HV substrate-tip ESI-MS method, and the screening result was further validated by conventional liquid chromatography/mass spectrometry.

CONCLUSIONS

Overall, our results demonstrated that fast-switching HV substrate-tip ESI-MS is a rapid, reliable, and effective method for simultaneous screening of various analytes in complex samples.

Returning to Nature for the Design of Sorptive Phases in Solid-Phase Microextraction

Green analytical chemistry principles aim to minimize the negative impact of analytical procedures in the environment, which can be considered both at close (to ensure the safety of the analysts) and global (to conserve our natural resources) levels. These principles suggest, among other guidelines, the reduction/minimization of the sample treatment and the use of renewable sources when possible. The first aspect is largely fulfilled by microextraction, which is considered to be among the greenest sample treatment techniques. The second consideration is attainable if natural products are used as raw materials for the preparation of new extraction phases. This strategy is in line with the change in our production system, which is being gradually moved from a linear model (take–make–dispose) to a circular one (including reusing and recycling as key terms). This article reviews the potential of natural products as sorbents in extraction and microextraction techniques from the synergic perspectives of two research groups working on the topic. The article covers the use of unmodified natural materials and the modified ones (although the latter has a less green character) to draw a general picture of the usefulness of the materials.

Direct coupling of solid phase microextraction with electrospray ionization mass spectrometry: A Case study for detection of ketamine in urine

Electrospray ionization mass spectrometry (ESI-MS) is a commonly used technique for analysis of various samples. Solid phase microextraction (SPME) is a simple and efficient technique that combines both sampling and sample preparation into one consolidated step, preconcentrating extracted analytes for ultra-sensitive analysis. Historically, SPME has been coupled with chromatography-based techniques for sample separation prior to analysis, however more recently, the chromatographic step has been omitted, with the SPME device directly coupled with the mass spectrometer. In this study, direct coupling of SPME with ESI-MS was developed, and extensively validated to quantitate ketamine from human urine, employing a practical experimental workflow and no extensive hardware modification to the equipment. Among the different fibers evaluated, SPME device coated with C₁₈/benzenesulfonic acid particles was selected for the analysis due to its good selectivity and signal response. Different approaches, including desorption spray, dripping, desorption ESI and nano-ESI were attempted for elution and ionization of the analytes extracted using the SPME fibers. The results showed that the desorption spray and nano-ESI methods offered better signal response and signal duration than the others that were evaluated. The analytical performance of the SPME-nano-ESI-MS setup was excellent, including limit of detection (LOD) of 0.027 ng/mL, limit of quantitation (LOQ) of 0.1 ng/mL, linear range of 0.1-500.0 ng/mL (R² = 0.9995) and recoveries of 90.8-109.4% with RSD 3.4-10.6% for three validation points at 4.0, 40.0 and 400.0 ng/mL, far better than the performance of conventional methods. The results herein presented, demonstrated that the direct coupling of SPME fibers with ESI-MS-based systems allowed for the simple and ultra-sensitive determination of analytes from raw samples such as human urine.

Direct Analysis of Aqueous Solutions and Untreated Biological Samples Using Nanoelectrospray Ionization Mass Spectrometry with Pipette Tip in Series with High-Ohmic Resistor as Ion Source

Commercially available disposable plastic pipette tip with the inner diameter of ca. 120 μm in series with a high-ohmic resistor (10 GΩ) was adapted as a low-cost alternative ion source for high-throughput nanoelectrospray mass spectrometry (nESI-MS) analysis of a variety of samples, especially aqueous solutions, without sample pretreatment. The use of high-ohmic resistor enabled the formation of stable electrospray of aqueous solutions at ambient conditions. In addition, corona discharge was avoided even with a high voltage applied. Quantitative analysis of vitamin B in water was successfully conducted by tip-ESI. The results exhibited a good linearity (R ˃ 0.9983), a low detection limit (0.25 ng/mL), and a wide dynamic response range (0.25-1000 ng/mL). Our study revealed that tip-ESI not only performed equally well to capillary nESI in terms of flow rate (˂ 100 nL/min), signal sensitivity, and sample consumption, but also offered a number of additional advantages, including better signal duration, tolerance to high analyte concentration (> 100 μg/mL) and high ionizing voltage (up to 6 kV), and obviation of tip clogging and corona discharge. High compatibility of tip-ESI with various kinds of samples (aqueous, viscous, solid, or bulk biological samples) makes it a promising tool for direct MS analysis.

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.

Detection and Seasonal Variations of Huanglongbing Disease in Navel Orange Trees Using Direct Ionization Mass Spectrometry

Citrus greening disease [Huanglongbing (HLB)] is the most destructive disease of citrus. In this work, we have established a metabolite-based mass spectrometry (MS) method for rapid detection of HLB in navel orange trees. Without sample pretreatment, characteristic mass spectra can be directly obtained from the raw plant samples using the direct MS method. The whole detection process can be accomplished within 1 min. By monitoring and comparisons of the healthy and infected plants throughout a whole year, characteristic MS peaks of metabolites are found to be specific responses from infected plants and, thus, could be used as biomarkers for detection of HLB. Therefore, HLB could be directly detected in the asymptomatic samples, such as stems, using this metabolite-based direct MS method. In addition, principal component analysis and partial least squares discriminant analysis modes of metabolites from healthy and infected trees were established for investigating differentiation and seasonal variations of HLB in leaves, veins, and stems, providing valuable information for understanding the HLB in different seasons.

High-throughput polymer tip-electrospray ionization mass spectrometry for enhanced detection of neopterin and biopterin in clinical urine samples

Urinary biopterin (Bio) and neopterin (Neo) are important markers for clinical diagnosis of hyperphenylalaninemia. Herein, we developed a high-throughput analysis method based on electrospray ionization mass spectrometry (ESI-MS) with polymer tips for the rapid quantitative detection of Bio and Neo in clinical urine samples. Different polymer tips were investigated. It is found that the best detection sensitivity was achieved with hydrophobic polymer tip, ie, polyethylene tips. The high-throughput polymer tip-ESI-MS method allowed a rapid analysis speed at ~40 seconds per sample. The limits of quantification (LOQ) (S/N ≥ 10) for the detection of Bio and Neo were improved to be 5.0 ng/mL. Acceptable relative standard deviation (RSD) values for Neo and Bio were measured to be 12.2% and 13.4% for direct measurement of Bio and Neo in raw urine samples, respectively. Furthermore, Bio and Neo were directly quantified from 18 clinical urine samples by presented method. The ratios of urinary Bio-to-Neo were analyzed for diagnosis of hyperphenylalaninemia. The results demonstrated that the present polymer tip-ESI-MS method is a promising strategy for the rapid analysis of clinical samples.

Electrostatic field-induced tip-electrospray ionization mass spectrometry for direct analysis of raw food materials

Rapid characterization of metabolites and risk compounds such as chemical residues and natural toxins in raw food materials such as vegetables, meats, and edible living plants and animals plays an important part in ensuing food quality and safety. To rapidly characterize the analytes in raw food materials, it is essential to develop in situ method for directly analyzing raw food materials. In this work, raw food materials including biological tissues and living samples were placed between an electrode and mass spectrometric (MS) inlet under a strong electrostatic field; analytes were rapidly induced to generate electrospray ionization (ESI) from the sample tip by adding a drop of solvent onto the sample. Therefore, the electrostatic field-induced tip-ESI-MS allows raw samples to avoid contacting high voltage, and thus this method has the advantage for in vivo analysis of food living plants and animals. Metabolite profiling, residues of pesticides and veterinary drugs, and natural toxins from raw food materials have been successfully detected. The analytical performances, including the linear ranges, sensitivity, and reproducibility, were investigated for direct sample analysis. The ionization mechanism of electrostatic field-induced tip-ESI was also discussed in this work.

Analyte-substrate interactions at functionalized tip electrospray ionization mass spectrometry: Molecular mechanisms and applications

Conventional electrospray ionization mass spectrometry (ESI-MS) commonly uses capillary tip for sample introduction and ionization. In recent years, ESI-MS using noncapillary substrate tips has attracted growing interest as it allows separation and enrichment of analytes from complex samples due to analytes-substrate interactions. In this work, model mixtures and functionalized tips were employed to investigate the molecular mechanism of the analyte-substrate interactions. The mixtures were directly loaded on substrate tips, and then temporal responses of analytes were investigated by monitoring selected ion chromatogram (SIC) responses of each analyte. It is found that all analytes are sprayed out together when bulk solution loaded substrate surface and then sequential ionization of analytes were observed. Sequential ionization of analytes was affected by the analytes-substrate interactions which caused analytes of weaker-interaction to be faster moved and the analytes of stronger-interactions to be retained on the substrate. The main molecular mechanisms of analyte-substrate interactions were revealed to be hydrophobic interactions and electrostatic interactions. Furthermore, based on the mechanistic insights, functionalized tips were further applied for rapid extractive sampling of target analytes from complex samples with good analytical performances. Overall, this study on the mechanism and applications of analyte-substrate interactions is useful for understanding the fundamental principles and further developments of functionalized tip electrospray ionization (TESI).

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.