New urea-modified paper substrate for enhanced analytical performance of negative ion mode paper spray mass spectrometry

Rapid fabrication of hydrophobic/hydrophilic patterns on paper substrates for paper spray mass spectrometry

A simple, rapid chemical coating and patterning method was developed and optimized for paper-based substrates for use in paper spray mass spectrometry (PS-MS). A variety of chlorosilanes were explored for coating paper substrates, and their effectiveness in forming hydrophobic surfaces was characterized via contact angle goniometry, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Trichloromethylsilane was selected as the primary coating agent because of the short time required to produce a hydrophobic surface (contact angle > 130°), as well as the ease of patterning. Patterning was performed using 3D-printed masks and an oxygen/plasma cleaner. Optimal mask thickness and oxygen/plasma cleaning parameters were determined to produce channels varying from 0.5 to 2.5 mm in width. The effectiveness of the patterned substrates for PS-MS was determined via analysis of four antiretrovirals: emtricitabine, lamivudine, efavirenz, and dolutegravir. Calibration curves were made for each antiretroviral at varying channel widths, and the limits of detection and limits of quantification for each drug were determined. These results show that this patterning method results in an average 7.2-fold improvement in sensitivity and an average 190-fold improvement in limits of detection over uncoated paper substrates in a neat matrix. In a proof-of-concept experiment, calibration curves were generated for each antiretroviral in urine. A patterned paper substrate with a 2-mm channel resulted in an average 7.4-fold improvement in sensitivity and an average 18-fold improvement in limits of detection over uncoated paper substrates.

Bifunctional Ti4+-modified paper for selective extraction or removal of phospholipids and paper spray mass spectrometry for bioanalysis in urine and plasma

BACKGROUND

Phospholipids (PLs) are major constituents of cell membranes, play important roles in cell proliferation and death, as well as in signal transduction, and therefore are relevant biomarkers for different pathologies. On the other hand, when the analysis of small compounds, such as therapeutics in blood is desired, then phospholipids are part of the matrix and cause serious interference during analysis. Currently, both the analysis and removal of PLs from biological samples are limited by extensive sample preparation and instrumental separation.

RESULTS

A fast and simple quantitative Ti4+-modified paper spray tandem mass spectrometric (TiPS-MS/MS) method was established in urine, where the enrichment of phospholipids was achieved, as well as reduction of matrix effects (primarily caused by high salt content) that ultimately led to improved sensitivity and selectivity. The method could achieve a physiologically relevant limit of detection (0.01-0.03 μg mL-1). Also, the usefulness of the Ti4+-modified paper was investigated in the opposite mode, namely for the selective removal of phospholipids from matrices such as plasma. Clonidine is used as model compound, as the detection of this compound is known to suffer from ion suppression by phospholipids. Compared with blank paper spray tandem mass spectrometry, the limit of detection could be improved from 0.3 μg mL-1 to 0.03 μg mL-1 by employing a Ti4+-modified paper on top of the spray tip to capture phospholipids from the sample.

SIGNIFICANCE AND NOVELTY

A novel Ti4+-modified paper was developed to allow for rapid solid-phase extraction of phospholipids from urine or selective removal from plasma, followed by direct paper spray mass spectrometric detection as a fast and convenient sample preparation and analysis combination. The paper properties are based on the Ti4+ metal ion, which can selectively bind phosphate-containing compounds under acidic conditions, and its applicability was demonstrated in relevant biological matrices.

On-Demand Portable Paper-Based Electrospray Ionization Mass Spectrometry for High-Sensitivity Analysis of Complex Samples

Paper spray ionization has been demonstrated to be the most promising substrate-based source, but this technique suffers from the low desorption efficiency of target compounds and poor portability. In the current study, we describe a portable paper-based electrospray ionization (PPESI) in which a piece of triangle paper and adsorbent are packed sequentially into a modified disposable micropipette tip. This source not only captures the feature of paper spray and adsorbent for highly efficient suppression of sample matrixes for target compound analysis but also takes advantage of a micropipette tip to prevent spray solvent from rapid evaporation. The performance of developed PPESI depends on the type and amount of packed adsorbent, paper substrate, and spray solvent and applied voltage. Moreover, by contrast to other related sources, the analytical sensitivity and the spray duration of PPESI in tandem with MS have been improved by factors of 2.8-32.3 and 2.0-13.3, respectively. Based on its high accuracy (>96%) and precision (less than 3% relative standard deviation), the PPESI coupled to a mass spectrometer has been used to determine diverse therapeutic drugs and pesticides in complex biological (e.g., whole blood, serum, and urine) and food (e.g., milk and orange juice) matrixes, and the limits of detection and quantification were 2-4 pg mL^-1 and 7-13 pg mL^-1, respectively. Taking the portability, high sensitivity, and repeatability, the technique may be a promising alternative for complex sample analysis.

Paper Spray Mass Spectrometry as an Effective Tool for Differentiating Coffees Based on Their Geographical Origins

With the rising trend of valuing flavor complexity of coffees, means to distinguish the properties of individual coffee sources is vital to the sustainable growth of the coffee industry. Herein, paper spray mass spectrometry (PS-MS), a simple technique with little sample preparation, was used to collect mass data from aqueous extracts of coffees from various sources. Thereafter, principal component analysis and linear discriminant analysis were used to successfully classify coffee samples (with 80-100 % accuracy) from various studies including the differentiations of Arabica and Robusta coffees, Arabica coffees from different countries, Robusta coffees from different geographical locations, and Arabica coffees from different locations within the same province in Thailand. With further insight from significant test via Fisher weight determination, this method was proved to be practical for differentiating coffees based on types and geographical origins, thus paving the way for broader applications.

h-FBN assisted negative ion paper spray for the sensitive detection of small molecules

Negative ion mode paper spray mass spectrometry (PS-MS) suffers from intense background noise and unstable MS signal. For the first time, we reported fluorinated boron nitride nanosheet (h-FBN) assisted negative ion PS-MS for the detection of a series of molecules. We demonstrated that the introduction of h-FBN can greatly improve the detection sensitivity and signal stability in the negative ion mode.

Ionization of glycans from alkali metal salt-impregnated paper

Ambient ionization of glycans is simply and efficiently achieved by spraying from an alkali metal salt-impregnated paper surface. Monosaccharides, oligosaccharides and ring glycans easily form abundant alkali metal adduct ions, and give simple and clean high-quality mass spectra. The enhancement is specific for glycans, compared to a wide variety of non-glycan compounds present in a matrix. In addition, molecular weight of unknown glycans can be further identified based on the ion mass difference of various alkali metal adduct ions from a certain compound when using a mixed salt-impregnated paper containing five cation salts. Successful determination of glycans and glycoconjugates in plant extracts, honey, blood and urine demonstrates the practicability of this approach to complicated matrices, especially biological matrices.

Rapid analysis of anionic and cationic surfactants in water by paper spray mass spectrometry

The discharge of wastewater containing surfactants and other industrial pollutants is contaminating the world's water resources which should be taken seriously. This paper describes a paper spray mass spectrometric (PS-MS) method for rapid and quantitative analysis of anionic and cationic surfactants in water samples. Eight cationic surfactants and four anionic surfactants can be simultaneously determined in positive or negative mode without sample pretreatment and chromatographic separation. The PS-MS analysis time was only 10 s. Under optimized conditions, the method presented a suitable linear range (1-80 μg mL^-1, linear regression coefficients (R²) higher than 0.995 for cationic surfactants, R² higher than 0.990 for anionic surfactants), a low limit of detection (0.05-0.35 μg mL^-1 for cationic surfactants and 0.20-0.35 μg mL^-1 for anionic surfactants), and satisfactory recovery values (97.3-106.1%). The method validation indicated that the method precision and accuracy were satisfactory. The results demonstrated that PS-MS is especially suitable for the high-throughput analysis of surfactants in water samples.

Reagent-Pencil and Paper Spray Mass Spectrometry: A Convenient Combination for Selective Analyses in Complex Matrixes

The recent developments on fieldable miniature mass spectrometers require efforts to produce easy-to-use and portable alternative tools to assist in point-of-care analysis. In this paper, the reagent-pencil (RP) technology, which has been used for solvent-free deposition of reagents in paper-based microfluidics, was combined with paper spray ionization mass spectrometry (PS-MS). In this approach, named RP-PS-MS, the PS triangular piece of paper was written with the reagent pencil, consisting of mixtures of graphite and bentonite (used as a support) and a reactive compound, and allowed to react with a given analyte from a sample matrix selectively. We conducted typical applications as proof-of-principles to verify the methodology's general usefulness in detecting small organic molecules in distinct samples. Hence, various aldehydes (2-furaldehyde, valeraldehyde, and benzaldehyde) in spiked cachaça samples (an alcoholic drink produced from fermentation/distillation of sugarcane juice) were promptly detected using a reagent pencil doped with 4-aminophenol (the reactive compound). Similarly, we recognized typical ginsenosides and triacylglycerols (TAGs) in ginseng aqueous infusions and soybean oil samples, respectively, using lithium chloride as the reactive compound. The results indicate that the reagent-pencil methodology is compatible with PS-MS and provides an easy and fast way to detect target analytes in complex samples. The advantage over the usual solution-based deposition of reagents lies in the lack of preparation or carrying different specific solutions for special applications, which can simplify operation, especially in point-of-care analysis with fieldable mass spectrometers.

Ambient (desorption/ionization) mass spectrometry methods for pesticide testing in food: a review

Ambient mass spectrometry refers to the family of techniques that allows ions to be generated from condensed phase samples under ambient conditions and then, collected and analysed by mass spectrometry. One of their key advantages relies on their ability to allow the analysis of samples with minimal to no sample workup. This feature maps well to the requirements of food safety testing, in particular, those related to the fast determination of pesticide residues in foods. This review discusses the application of different ambient ionization methods for the qualitative and (semi)quantitative determination of pesticides in foods, with the focus on different specific methods used and their ionization mechanisms. More popular techniques used are those commercially available including desorption electrospray ionization (DESI-MS), direct analysis on real time (DART-MS), paper spray (PS-MS) and low-temperature plasma (LTP-MS). Several applications described with ambient MS have reported limits of quantitation approaching those of reference methods, typically based on LC-MS and generic sample extraction procedures. Some of them have been combined with portable mass spectrometers thus allowing "in situ" analysis. In addition, these techniques have the ability to map surfaces (ambient MS imaging) to unravel the distribution of agrochemicals on crops.

Polypyrrole-coated needle as an electrospray emitter for ambient mass spectrometry

Polypyrrole (PPy) is a polymer widely used as an extraction phase due to its ability to perform intermolecular interactions with the analyte, such as acid-base, π-π, dipole-dipole, hydrophobic, and hydrogen bonding. In this manuscript, we report the coating of a stainless steel needle with a PPy film for analyte extraction and subsequent analysis by electrospray ionization mass spectrometry (ESI-MS) under ambient and open-air conditions. The method, named PPy-ESI-MS, was optimized for analysis of 3,4-methylenedioxyamphetamine (MDA) and 3,4-methylenedioxymethamphetamine (MDMA) in synthetic urine. Seven cycles of electrodeposition of the PPy film onto the needle surface, sample at pH 8, and 40 min of extraction of analytes were determined as the best analysis conditions. The analytical performance of PPy-ESI-MS was evaluated for MDA and MDMA compounds. Analytical curves were obtained with R2 > 0.98. Limits of detection (LODs) and limits of quantification (LOQs) were determined as 20 μg L-1 and 70 μg L-1 for MDA and as 25 μg L-1 and 80 μg L-1 for MDMA, respectively. Values of precision were below 17%, and values of accuracy below 5%. The apparent recoveries ranged between 84.5% and 111.3%. In addition, the PPy-ESI-MS method was applied for the analysis of sarcosine in synthetic urine in order to evaluate the performance of the method for another class of compounds. The calibration curve was obtained with R2 > 0.98, along with LOD and LOQ of 30 μg L-1 and 100 μg L-1, respectively. The precision and accuracy values were below 5% and 8%, respectively, and the apparent recoveries close to 100%. This work demonstrates the usefulness of combining an extraction phase with ESI-MS analysis under ambient conditions to determine different classes of small molecules in a complex sample.

Emerging trends in paper spray mass spectrometry: Microsampling, storage, direct analysis, and applications

Recent advancements in the sensitivity of chemical instrumentation have led to increased interest in the use of microsamples for translational and biomedical research. Paper substrates are by far the most widely used media for biofluid collection, and mass spectrometry is the preferred method of analysis of the resultant dried blood spot (DBS) samples. Although there have been a variety of review papers published on DBS, there has been no attempt to unify the century old DBS methodology with modern applications utilizing modified paper and paper-based microfluidics for sampling, storage, processing, and analysis. This critical review will discuss how mass spectrometry has expanded the utility of paper substrates from sample collection and storage, to direct complex mixture analysis to on-surface reaction monitoring.

Paper-based sorptive phases for microextraction and sensing

The simplification of the analytical procedures, including cost-effective materials and detectors, is a current research trend. In this context, paper has been identified as a useful material thanks to its low price and high availability in different compositions (office, filter, chromatographic). Its porosity, flexibility, and planar geometry permit the design of flow-through devices compatible with most instrumental techniques. This article provides a general overview of the potential of paper, as substrate, on the simplification of analytical chemistry methodologies. The design of paper-based sorptive phases is considered in-depth, and the different functionalization strategies are described. Considering our experience in sample preparation, special attention has been paid to the use of these phases under the classical microextraction-analysis workflow, which usually includes a chromatographic separation of the analytes before their determination. However, the interest of these materials extends beyond this field as they can be easily implemented into spectroscopic and electrochemical sensors. Finally, the direct analysis of paper substrates in mass spectrometry, in the so-called paper-spray technique is also discussed. This review is more focused on presenting ideas rather than the description of specific applications to draw a general picture of the potential of these materials.

Direct Analysis of Doping Agents in Raw Urine Using Hydrophobic Paper Spray Mass Spectrometry

In this study, the direct analysis of doping agents in urine samples with no sample preparation by a modified paper spray mass spectrometry (PS-MS) methodology has been demonstrated for the first time. We have described a paper surface treatment with trichloromethylsilane using a gas-phase reaction to increase the ionization of target compounds. This approach was applied for the analysis of two classes of banned substances in urine samples: anabolic agents (trenbolone and clenbuterol) and diuretics (furosemide and hydrochlorothiazide). Under optimized conditions, the developed methods presented satisfactory repeatability, and an analysis of variance showed linearity without lack-of-fit. Highly sensitive detections as low as sub-nanogram per milliliter levels, which is below the minimum required performance levels proposed by the World Anti-Doping Agency, have been reached using the hydrophobic PS-MS analysis without any preconcentration and cleanup step.

Teflon Spray Ionization Mass Spectrometry

Polytetrafluoroethene, commonly known as Teflon, is a plastic famous for its inertness, strength, and nonstick properties, allowing its repeated use in many applications. We report the use of a triangularly cut Teflon substrate to take the place of paper in a form of spray mass spectrometry. A conducting wire (gold) at high potential (positive or negative) makes contact with a drop of the liquid sample at the apex of the triangle, causing a spray of droplets to be directed toward the heated inlet of a mass spectrometer. Saccharides, drugs, illegal additives, peptides, proteins, bilirubin, and vancomycin give mass spectra with high signal-to-noise (S/N) ratios, allowing detection at the nanogram per milliliter (ng/mL) level. Examination of each of these analytes demonstrates that Teflon spray is several orders of magnitude more sensitive than paper spray under the same conditions. Teflon spray ionization mass spectrometry is applied to the metabolomic and lipidomic profiling of biological fluid samples. Detection of polycyclic aromatic hydrocarbons is achieved with Teflon spray at 10 μg/mL concentrations. These experiments show the advantage of using Teflon over a normal paper substrate in detecting many environmentally and biologically relevant systems with high sensitivity and S/N ratio.

Molecularly imprinted graphite spray ionization-ion mobility spectrometry: application to trace analysis of the pesticide propoxur

A porous graphite sheet modified by a molecularly imprinted polymer (MIP) was directly used as the spray ionization source for ion mobility spectrometry (IMS). Therefore, it was possible to selectively analyze samples extracted by the molecularly imprinted polymer. This obviates the need for the steps of elution, solvent evaporation, dissolution and injection. To prepare the sheet, the graphite surface was first modified by electrodeposition of a molecularly imprinted polypyrrole film. This polypyrrole film was fabricated in a three-electrode electrochemical system using cyclic voltammetry. The electropolymerization of the graphite sheet was carried out with LiClO₄ as a supporting electrolyte in the reaction solution. The effects of the amount of monomer, the level of template concentrations, and the time of polymerization on the extraction efficiency of the MIP film were evaluated. The extraction conditions including extraction time, the extraction temperature, the pH values, the salt concentrations, and the stirring rate were also studied. Methanol was selected as the most suitable solvent for both desorption and ionization which occur simultaneously. The pesticide propoxur (acting as a test compound) was extracted from water samples and directly analyzed using IMS. The analytical parameters (working range: 1.0 to 250 ng·mL^-1; detection limit: 0.3 ng·mL^-1) indicated that the direct coupling of MIP and IMS has a great potential in terms of reproducibility, and speed of the analysis, while maintaining acceptable sensitivity. Graphical abstract Schematic presentation of molecularly imprinted graphite spray ionization coupled with ion mobility spectrometry (IMS) for rapid/selective extraction and ionization: Application to the pre-concentration of propoxur prior to its quantification by IMS.

Evaluation of novel organosilane modifications of paper spray mass spectrometry substrates for analyzing polar compounds

Paper spray mass spectrometry (PSMS) is currently used in different analytical fields, but less effort has been made so far to use PSMS for highly polar compounds. Such analytes usually show poor performance in PSMS due to their high affinity for common paper substrates in addition to high matrix effects. In this study, strategies for hydrophobic modifications of commercially available paper substrates using ten different organosilanes were developed. The modified substrates were generated, characterized, and applied for PSMS analysis of polar toxins. By using the modified paper, PSMS performance of some of the toxins could be considerably increased, especially for orellanine, showing a more than 80-fold signal enhancement when substrates modified with chlorotrimethylsilane were used. For other toxins like ricinine, only small beneficial effects could be shown on PSMS performance when using modified substrates. Statistical equivalence tests showed sufficient ruggedness of the developed procedures also compared to conventional substrates. Thus, further systematic development of paper substrates modified with organosilane derivatives based on the presented study for application in PSMS should be encouraged.

On-substrate derivatization for detection of highly volatile G-series chemical warfare agents via paper spray mass spectrometry

RATIONALE

The analysis of chemical warfare agents (CWAs) from ambient atmosphere presents an analytical challenge due to their ease of degradation and volatility. Herein is described a method for derivatizing CWAs directly onto a paper spray substrate prior to analysis. This derivatization allows for much longer times of analysis without sample degradation and with little to no sample preparation.

METHODS

Derivatization was performed using 2-[(dimethylamino)methyl] phenol both in-vial and directly on paper spray cartridges. Solution studies were carried out over time and samples were analyzed via liquid chromatography/tandem mass spectrometry (LC/MS/MS) operated in positive ion mode. Paper spray substrates impregnated with the derivatizing agent prior to CWA vapor capture were also analyzed over time using a mass spectrometer operated in positive ion mode.

RESULTS

Use of 2-[(dimethylamino)methyl] phenol as a paper spray substrate dopant enables derivatization of G-series compounds into lower volatility complexes. The reaction occurs in solution and in the vapor phase. This new technique effectively traps and captures G-series agents for analysis while extending the time for which the compound remains absorbed. The complex is highly suitable for direct analysis via paper spray mass spectrometry.

CONCLUSIONS

Derivatization of paper spray substrates was shown to greatly increase the time for analysis of CWAs. This technique, combined with the vapor phase capture stage outlined previously, allows for rapid, quantitative CWA detection by paper spray ionization with little or no sample preparation.

Development of paper substrate for paper spray MS in high-sensitivity analysis of biological samples

Paper spray (PS) has demonstrated a promising future for direct mass spectrometric analysis. In the process of PS, paper substrate has been demonstrated as a crucial factor in determining the final performance of PS-MS, and therefore much attention is paid to modification of paper substrate. In this review, we systematically introduce the development of paper substrate for PS. Various commercial and modified papers are comprehensively reviewed, and much effort is focused on some physical and chemical approaches for modification of paper substrate. The application of modified paper substrates to biological sample analyses is discussed. The future promising directions of paper substrate for PS are highlighted.

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.