Portable Instrumentation for Ambient Ionization and Miniature Mass Spectrometers.

We critically evaluate the current status of portable mass spectrometry (pMS), particularly where this aligns with ambient ionization. Assessing the field of pMS can be quite subjective, especially in relation to the portable aspects of design, deployment, and operation. In this review, we discuss what it means to be portable and introduce a set of criteria by which pMS and ambient ionization sources can be assessed. Moreover, we consider the recent literature in terms of the most popular and significant advances in portable instrumentation for ambient ionization and miniature mass spectrometers. Finally, emerging trends and exciting future prospects are discussed and some recommendations are offered.

Development and Clinical Validation of a Hook Effect-Based Lateral Flow Immunoassay Sensor for Cerebrospinal Fluid Leak Detection.

BACKGROUND AND OBJECTIVES: Rapid detection of cerebrospinal fluid (CSF) leaks is vital for patient recovery after spinal surgery. However, distinguishing CSF-specific transferrin (TF) from serum TF using lateral flow immunoassays (LFI) is challenging due to their structural similarities. This study aims to develop a novel point-of-care diagnostic assay for precise CSF leak detection by quantifying total TF in both CSF and serum. METHODS: Capitalizing on the substantial 100-fold difference in TF concentrations between CSF and serum, we designed a diagnostic platform based on the well-known "hook effect" resulting from excessive analyte presence. Clinical samples from 37 patients were meticulously tested using the novel LFI sensor, alongside immunofixation as a reference standard. RESULTS: The hook effect-based LFI sensor exhibited outstanding performance, successfully discriminating positive clinical CSF samples from negative ones with remarkable statistical significance (positive vs negative t-test; P = 1.36E-05). This novel sensor achieved an impressive 100% sensitivity and 100% specificity in CSF leak detection, demonstrating its robust diagnostic capabilities. CONCLUSION: In conclusion, our study introduces a rapid, highly specific, and sensitive point-of-care test for CSF leak detection, harnessing the distinctive TF concentration profile in CSF compared with serum. This novel hook effect-based LFI sensor holds great promise for improving patient outcomes in the context of spinal surgery and postsurgical recovery. Its ease of use and reliability make it a valuable tool in clinical practice, ensuring timely and accurate CSF leak detection to enhance patient care.

Charge inversion under plasma-nanodroplet reaction conditions excludes Fischer esterification for unsaturated fatty acids: a chemical approach for type II isobaric overlap.

Direct infusion ionization methods provide the highest throughput strategy for mass spectrometry (MS) analysis of low-volume samples. But the trade-off includes matrix effects, which can significantly reduce analytical performance. Herein, we present a novel chemical approach to tackle a special type of matrix effect, namely type II isobaric overlap. We focus on detailed investigation of a nanodroplet-based esterification chemistry for differentiating isotopologue [M + 2] signal due to unsaturated fatty acid (FA) from the monoisotopic signal from a saturated FA. The method developed involves the online fusion of nonthermal plasma with charged nanodroplets, enabling selective esterification of saturated FAs. We discovered that unsaturated FAs undergo spontaneous intramolecular reaction via a novel mechanism based on a carbocation intermediate to afford a protonated lactone moiety (resonance stabilized cyclic carbonium ion), whose mass is the same as the original protonated unsaturated FA. Therefore, the monoisotopic signal from any saturated FA can be selectively shifted away from the mass-to-charge position where the isobaric interference occurs to enable effective characterization by MS. The mechanism governing the spontaneous intramolecular reactions for unsaturated FAs was validated with DFT calculations, experimentation with standards, and isotope labeling. This novel insight achieved via the ultrafast plasma-nanodroplet reaction environment provides a potentially useful synthetic pathway to achieve catalyst-free lactone preparation. Analytically, we believe the performance of direct infusion MS can be greatly enhanced by combining our approach with prior sample enrichment steps for applications in biomedicine and food safety. Also, combination with portable mass spectrometers can improve the efficiency of field studies since front-end separation is not possible under such conditions.

Emergency diagnosis made easy: matrix removal and analyte enrichment from raw saliva using paper-arrow mass spectrometry.

Paracetamol overdose is a leading cause of acute liver failure that can prove fatal. Establishing paracetamol concentration accurately and quickly is critical. Current detection methods are invasive, time-consuming and/or expensive. Non-invasive, rapid and cost-effective techniques are urgently required. To address this challenge, a novel approach, called Paper-Arrow Mass Spectrometry (PA-MS) has been developed. This technique combines sample collection, extraction, enrichment, separation and ionisation onto a single paper strip, and the entire analysis process, from sample to result, can be carried out in less than 10 min requiring only 2 µL of raw human saliva. PA-MS achieved a LOQ of 185 ng mL-1, mean recovery of 107 ± 7%, mean accuracy of 11 ± 8% and precision ≤5% using four concentrations, and had excellent linearity (r2 = 0.9988) in the range of 0.2-200 µg mL-1 covering the treatment concentration range, surpassing the best-in-class methods currently available for paracetamol analysis. Furthermore, from a panel of human saliva samples, inter-individual variability was found to be <10% using this approach. This technique represents a promising tool for rapid and accurate emergency diagnosis.

Focusing ion funnel-assisted ambient electrospray enables high-density and uniform deposition of non-spherical gold nanoparticles for highly sensitive surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS) is a powerful technique for detecting trace amounts of analytes. However, the performance of SERS substrates depends on many variables including the enhancement factor, morphology, consistency, and interaction with target analytes. In this study, we investigated, for the first time, the use of electrospray deposition (ESD) combined with a novel ambient focusing DC ion funnel to deposit a high density of gold nanoparticles (AuNPs) to generate large-area, uniform substrates for highly sensitive SERS analysis. We found that the combination of ambient ion focusing with ESD facilitated high-density and intact deposition of non-spherical NPs. This also allowed us to take advantage of a polydisperse colloidal solution of AuNPs (consisting of nanospheres and nanorods), as confirmed by finite-difference time domain (FDTD) simulations. Our SERS substrate exhibited excellent capture capacity for model analyte molecules, namely 4-aminothiophenol (4-ATP) and Rhodamine 6G (R6G), with detection limits in the region of 10-11 M and a relative standard deviation of <6% over a large area (∼500 × 500 µm2). Additionally, we assessed the quantitative performance of our SERS substrate using the R6G probe molecule. The results demonstrated excellent linearity (R2 > 0.99) over a wide concentration range (10-4 M to 10-10 M) with a detection limit of 80 pM.

Attaching protein-adsorbing silica particles to the surface of cotton substrates for bioaerosol capture including SARS-CoV-2.

The novel coronavirus pandemic (COVID-19) has necessitated a global increase in the use of face masks to limit the airborne spread of the virus. The global demand for personal protective equipment has at times led to shortages of face masks for the public, therefore makeshift masks have become commonplace. The severe acute respiratory syndrome caused by coronavirus-2 (SARS-CoV-2) has a spherical particle size of ~97 nm. However, the airborne transmission of this virus requires the expulsion of droplets, typically ~0.6-500 µm in diameter (by coughing, sneezing, breathing, and talking). In this paper, we propose a face covering that has been designed to effectively capture SARS-CoV-2 whilst providing uncompromised comfort and breathability for the wearer. Herein, we describe a material approach that uses amorphous silica microspheres attached to cotton fibres to capture bioaerosols, including SARS CoV-2. This has been demonstrated for the capture of aerosolised proteins (cytochrome c, myoglobin, ubiquitin, bovine serum albumin) and aerosolised inactivated SARS CoV-2, showing average filtration efficiencies of ~93% with minimal impact on breathability.

Cyanostilbene-based fluorescent paper array for monitoring fish and meat freshness via amino content detection.

The detection of biogenic amines released from degraded meats is an effective method for evaluating meat freshness. However, existing traditional methods like titration are deemed tedious, while the use of sophisticated analytical instruments is not amenable to field testing. Herein, a cyanostilbene-based fluorescent array was rapidly fabricated using macroarray synthesis on a cellulose paper surface to detect amines liberated from spoiled beef, fish, and chicken. The fluorescence changes of immobilized molecules from the interaction with gaseous amines were used to monitor changes in freshness levels. Thanks to the high-throughput nature of macroarray synthesis, a set of highly responsive molecules such as pyridinium and dicyanovinyl moieties were quickly revealed. Importantly, this method offers flexibility in sensing applications including (1) sensing by individual sensor molecules, where the fluorescence response correlated well with established titration methods, and (2) collective sensing whereby chemometric analysis was used to provide a cutoff of freshness with 73-100% accuracy depending on meat types. Overall, this study paves the way for a robust and cost-effective tool for monitoring meat freshness.

Resolving the paradox of unipolar induction: new experimental evidence on the influence of the test circuit.

A novel experiment has been devised shedding new light on the phenomenon of unipolar induction, also known as "Faraday's Paradox". This is a topic which continues to fascinate scientists and engineers with much debate continuing to this day. In particular, the question of the field co-rotating with the magnet or remaining stationary remains unsettled and supporting evidence exists for both positions. In this study, we present a novel experimental apparatus that includes, for the first time, the relative motion of the measurement circuit including the closing wires, as well as the magnet and disc respectively. The results show that the closing wire needs to be considered as part of the problem, which enables the apparent paradox associated with this phenomenon to be resolved. However, it remains impossible to tell if the field co-rotates with the magnet or if it remains stationary. Instead, direct electron interaction is considered as a viable alternative to resolve remaining paradoxes.

Simultaneous Multiplexed Quantification of Banned Sudan Dyes Using Surface Enhanced Raman Scattering and Chemometrics.

Azo compounds such as the Sudan dyes I-IV are frequently used illegally as colorants and added to a wide range of foods. These compounds have been linked to a number of food safety hazards. Several methods have been proposed to detect food contamination by azo compounds and most of these are laboratory based; however, the development of reliable and portable methods for the detection and quantification of food contaminated by these chemicals in low concentration is still needed due to their potentially carcinogenic properties. In this study, we investigated the ability of surface enhanced Raman scattering (SERS) combined with chemometrics to quantify Sudan I-IV dyes. SERS spectra were acquired using a portable Raman device and gold nanoparticles were employed as the SERS substrate. As these dyes are hydrophobic, they were first dissolved in water: acetonitrile (1:10, v/v) as single Sudan dyes (I-IV) at varying concentrations. SERS was performed at 785 nm and the spectra were analyzed by using partial least squares regression (PLS-R) with double cross-validations. The coefficient of determination (Q2) were 0.9286, 0.9206, 0.8676 and 0.9705 for Sudan I to IV, respectively; the corresponding limits of detection (LOD) for these dyes were estimated to be 6.27 × 10-6, 5.35 × 10-5, 9.40 × 10-6 and 1.84 × 10-6 M. Next, quadruplex mixtures were made containing all four Sudan dyes. As the number of possible combinations needed to cover the full concentration range at 5% intervals would have meant collecting SERS spectra from 194,481 samples (214 combinations) we used a sustainable solution based on Latin hypercubic sampling and reduced the number of mixtures to be analyzed to just 90. After collecting SERS spectra from these mixture PLS-R models with bootstrapping validations were employed. The results were slightly worse in which the Q2 for Sudan I to IV were 0.8593, 0.7255, 0.5207 and 0.5940 when PLS1 models (i.e., one model for one dye) was employed and they changed to 0.8329, 0.7288, 0.5032 and 0.5459 when PLS2 models were employed (i.e., four dyes were modelled simultaneously). These results showed the potential of SERS to be used as a high-throughput, low-cost, and reliable methods for detecting and quantifying multiple Sudan dyes in low concentration from illegally adulterated samples.

Assessment of creatinine concentration in whole blood spheroids using paper spray ionization-tandem mass spectrometry.

Accurate quantification of blood creatinine is important to estimate the glomerular filtration rate. Existing techniques using liquid chromatography tandem mass spectrometry (LC-MS/MS) have a high accuracy and eliminate most interferences encountered in routine enzymatic and Jaffé methods. However, they require laborious and time-consuming sample treatment and data acquisition. The aim of this study is to develop a fast and simple method to enable a direct analysis of whole blood creatinine with performance measures that are comparable to conventional LC-MS/MS. 5µL whole blood is formed as a three-dimensional spheroid on hydrophobic silanized paper substrates which then undergoes paper-spray ionization-tandem mass spectrometry (PSI-MS/MS). The method is validated using real human samples and compared with LC-MS/MS. PSI-MS/MS whole blood analysis exhibited a lower limit of quantification of 2.5 µg/mL, precision ≤ 6.3%, recovery in the range of 88-94% and excellent linearity (R2 > 0.99; 2.5-20 µg/mL) covering the normal range for creatinine levels. Creatinine levels were comparable to those measured by LC-MS/MS with small deviations of less than 0.3 µg/mL. This simple, fast and accurate microsampling technique for direct analysis of creatinine from whole blood shows promise for routine clinical screening and monitoring. This approach can be readily extended for other analytes of interest and, due to inherent advantages relating to cost, storability, speed, and simplicity, it can be especially advantageous for use in resource-limited settings.

Wearable Natural Rubber Latex Gloves with Curcumin for Torn Glove Detection in Clinical Settings.

Glove tear or perforation is a common occurrence during various activities that require gloves to be worn, posing a significant risk to the wearer and possibly others. This is vitally important in a clinical environment and particularly during surgical procedures. When a glove perforation occurs (and is noticed), the glove must be replaced as soon as possible; however, it is not always noticeable. The present article is focused on the design and development of a novel fluorescence-based sensing mechanism, which is integrated within the glove topology, to help alert the wearer of a perforation in situ. We hypothesized that natural rubber gloves with curcumin infused would yield fluorescence when the glove is damaged, particularly when torn or punctured. The glove design is based on double-dipping between Natural Rubber Latex (NRL) and an inner layer of latex mixed with curcumin, which results in a notable bright yellow-green emission when exposed to UV light. Curcumin (Cur) is a phenolic chemical found primarily in turmeric that fluoresces yellowish-green at 525 nm. The tear region on the glove will glow, indicating the presence of a Cur coating/dipping layer beneath. NRL film is modified by dipping it in a Cur dispersion solution mixed with NRL for the second dipping layer. Using Cur as a filler in NRL also has the distinct advantage of allowing the glove to be made stronger by evenly distributing it throughout the rubber phase. Herein, the optimized design is fully characterized, including physicochemical (fluorescence emission) and mechanical (tensile and tear tests) properties, highlighting the clear potential of this novel and low-cost approach for in situ torn glove detection.

Natural Rubber (NR) Latex Films with Antimicrobial Properties for Stethoscope Diaphragm Covers.

Systematic disinfection of the stethoscope diaphragm is required to ensure that it does not act as a vector for cross-transmission of health-related diseases. Thus, an antimicrobial latex film could be used as a cover to inhibit pathogenic bacteria from growing on its surface. The aim of this work is to determine the antimicrobial activity and mechanical properties of antimicrobial natural rubber (NR) latex films with different types of antimicrobial agents (mangosteen peel powder (MPP), zinc oxide nanoparticles (ZnO NP), and povidone-iodine (PVP-I)). The antimicrobial loading was varied from 0.5, to 1.0, and 2.0 phr to monitor the effective inhibition of Gram-negative bacteria and fungi growth. For MPP and PVP-I antimicrobial agents, a loading of 2.0 phr showed good antimicrobial efficacy with the largest zone of inhibition. Simultaneously, ZnO NP demonstrated excellent antimicrobial activity at low concentrations. The addition of antimicrobial agents shows a comparable effect on the mechanical properties of NR latex films. In comparison to control NR latex film (29.41 MPa, 48.49 N/mm), antimicrobial-filled films have significantly greater tensile and tear strengths (MPP (33.84 MPa, 65.21 N/mm), ZnO NP (31.79 MPa, 52.77 N/mm), and PVP-I (33.25 MPa, 50.75 N/mm). In conclusion, the addition of antimicrobial agents, particularly ZnO NP, can be a better choice for NR latex films because they will serve as both an activator and an antimicrobial. In a clinical context, with regard to frequently used medical equipment such as a stethoscope, such an approach offers significant promise to aid infection control.

Plasmon color-preserved gold nanoparticle clusters for high sensitivity detection of SARS-CoV-2 based on lateral flow immunoassay.

Lateral flow immunoassays (LFI) have shown great promise for point-of-care (POC) sensing applications, however, its clinical translation is often hindered by insufficient sensitivity for early detection of diseases, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This is mainly due to weak absorption signals of single gold nanoparticles (AuNPs). Here, we developed AuNP clusters that maintain the red color of isolated individual AuNPs, but increase the colorimetric readout to improve the detection sensitivity. The plasmon color-preserved (PLASCOP) AuNP clusters is simply made by mixing streptavidin-coated AuNP core with satellite AuNPs coated with biotinylated antibodies. The biotinylated antibody-streptavidin linker forms a gap size over 15 nm to avoid plasmon coupling between AuNPs, thus maintaining the plasmonic color while increasing the overall light absorption. LFI sensing using PLASCOP AuNP clusters composed of 40 nm AuNPs showed a high detection sensitivity for SARS-CoV-2 nucleocapsid proteins with a limit of detection (LOD) of 0.038 ng mL-1, which was 23.8- and 5.9-times lower value than that of single 15 nm and 40 nm AuNP conjugates, respectively. The PLASCOP AuNP clusters-based LFI sensing also shows good specificity for SARS-CoV-2 nucleocapsid proteins from other influenza and coronaviruses. In a clinical feasibility test, we demonstrated that SARS-CoV-2 particles spiked in human saliva could be detected with an LOD of 54 TCID50 mL-1. The developed PLASCOP AuNP clusters are promising colorimetric sensing reporters that present improved sensitivity in LFI sensing for broad POC sensing applications beyond SARS-CoV-2 detection.

Rapid geographical indication of peppercorn seeds using corona discharge mass spectrometry.

With increasing demands for more rapid and practical analyses, various techniques of ambient ionization mass spectrometry have gained significant interest due to the speed of analysis and abundance of information provided. Herein, an ambient ionization technique that utilizes corona discharge was applied, for the first time, to analyze and categorize whole seeds of black and white peppers from different origins. This setup requires no solvent application nor gas flow, thus resulting in a very simple and rapid analysis that can be applied directly to the sample without any prior workup or preparation. Combined with robust data pre-processing and subsequent chemometric analyses, this analytical method was capable of indicating the geographical origin of each pepper source with up to 98% accuracies in all sub-studies. The simplicity and speed of this approach open up the exciting opportunity for onsite analysis without the need for a highly trained operator. Furthermore, this methodology can be applied to a variety of spices and herbs, whose geographical indication or similar intellectual properties are economically important, hence it is capable of creating tremendous impact in the food and agricultural industries.

Accelerated nucleophilic substitution reactions of dansyl chloride with aniline under ambient conditions via dual-tip reactive paper spray.

Paper spray ionization (PSI) mass spectrometry (MS) is an emerging tool for ambient reaction monitoring via microdroplet reaction acceleration. PSI-MS was used to accelerate and monitor the time course of the reaction of dansyl chloride with aniline, in acetonitrile, to produce dansyl aniline. Three distinct PSI arrangements were explored in this study representing alternative approaches for sample loading and interaction; conventional single tip as well as two novel setups, a dual-tip and a co-axial arrangement were designed so as to limit any on-paper interaction between reagents. The effect on product abundance was investigated using these different paper configurations as it relates to the time course and distance of microdroplet travel. It was observed that product yield increases at a given distance and then decreases thereafter for all PSI configurations. The fluorescent property of the product (dansyl aniline) was used to visually inspect the reaction progress on the paper substrate during the spraying process. Amongst the variety of sample loading methods the novel dual-tip arrangement showed an increased product yield and microdroplet density, whilst avoiding any on-paper interaction between the reagents.

Analysis of non-conjugated steroids in water using paper spray mass spectrometry.

A novel strategy for the direct analysis of non-conjugated steroids in water using paper spray mass spectrometry (PS-MS) has been developed. PS-MS was used in the identification and quantification of non-conjugated (free) steroids in fish tank water samples. Data shown herein indicates that individual amounts of free steroids can be detected in aqua as low as; 0.17 ng/µL, 0.039 ng/µL, 0.43 ng/µL, 0.0076 ng/µL for aldosterone, corticosterone, cortisol, and ß-estrone, respectively, and with an average relative standard deviation of ca. < 10% in the positive ion mode using PS-MS/MS. Direct detection of free steroids in a raw water mixture, from aquaculture, without prior sample preparation is demonstrated. The presence of free steroids released in fish water samples was confirmed via tandem mass spectrometry using collision-induced dissociation. This approach shows promise for rapid and direct water quality monitoring to provide a holistic assessment of non-conjugated steroids in aqua.

Accurately predicting electron beam deflections in fringing fields of a solenoid.

Computer modelling is widely used in the design of scientific instrumentation for manipulating charged particles, for instance: to evaluate the behaviour of proposed designs, to determine the effects of manufacturing imperfections and to optimise the performance of apparatus. For solenoids, to predict charged particle trajectories, accurate values for the magnetic field through which charged species traverse are required, particularly at the end regions where fringe fields are most prevalent. In this paper, we describe a model that accurately predicts the deflection of an electron beam trajectory in the vicinity of the fringing field of a solenoid. The approach produces accurate beam deflection predictions in the fringe field region as well as in the centre of the solenoid. The model is based on a direct-line-of-action force between charges and is compared against field-based approaches including a commercially available package, with experimental verification (for three distinct cases). The direct-action model is shown to be more accurate than the other models relative to the experimental results obtained.

Flexible Drift Tube for High Resolution Ion Mobility Spectrometry (Flex-DT-IMS).

This paper describes, in detail, the development of a novel, low-cost, and flexible drift tube (DT) along with an associated ion mobility spectrometer system. The DT is constructed from a flexible printed circuit board (PCB), with a bespoke "dog-leg" track design, that can be rolled up for ease of assembly. This approach incorporates a shielding layer, as part of the flexible PCB design, and represents the minimum dimensional footprint conceivable for a DT. The low thermal mass of the polyimide substrate and overlapping electrodes, as afforded by the dog-leg design, allow for efficient heat management and high field linearity within the tube-achieved from a single PCB. This is further enhanced by a novel double-glazing configuration which provides a simple and effective means for gas management, minimizing thermal variation within the assembly. Herein, we provide a full experimental characterization of the flexible DT ion mobility spectrometer (Flex-DT-IMS) with corresponding electrodynamic (Simion 8.1) and fluid dynamic (SolidWorks) simulations. The Flex-DT-IMS is shown to have a resolution >80 and a detection limit of low nanograms for the analysis of common explosives (RDX, PETN, HMX, and TNT).

Furo[3,2-c]coumarin-derived Fe3+ Selective Fluorescence Sensor: Synthesis, Fluorescence Study and Application to Water Analysis.

Furocoumarin (furo[3,2-c]coumarin) derivatives have been synthesized from single step, high yielding (82-92%) chemistry involving a 4-hydroxycoumarin 4 + 1 cycloaddition reaction. They are characterized by FTIR, 1H-NMR, and, for the first time, a comprehensive UV-Vis and fluorescence spectroscopy study has been carried out to determine if these compounds can serve as useful sensors. Based on the fluorescence data, the most promising furocoumarin derivative (2-(cyclohexylamino)-3-phenyl-4H-furo[3,2-c]chromen-4-one, FH), exhibits strong fluorescence (ФF = 0.48) with long fluorescence lifetime (5.6 ns) and large Stokes' shift, suggesting FH could be used as a novel fluorescent chemosensor. FH exhibits a highly selective, sensitive and instant turn-off fluorescence response to Fe3+ over other metal ions which was attributed to a charge transfer mechanism. Selectivity was demonstrated against 13 other competing metal ions (Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Al3+, Ni2+, Cu2+, Zn2+, Co2+, Pb2+ and Ru3+) and aqueous compatibility was demonstrated in 10% MeOH-H2O solution. The FH sensor coordinates Fe3+ in a 1:2 stoichiometry with a binding constant, Ka = 5.25 × 103 M-1. This novel sensor has a limit of detection of 1.93 µM, below that of the US environmental protection agency guidelines (5.37 µM), with a linear dynamic range of ~28 (~2-30 µM) and an R2 value of 0.9975. As an exemplar application we demonstrate the potential of this sensor for the rapid measurement of Fe3+ in mineral and tap water samples demonstrating the real-world application of FH as a "turn off" fluorescence sensor.

Determining Surface Energy of Porous Substrates by Spray Ionization.

We have developed a new spray-based method for characterizing surface energies of planar, porous substrates. Distinct spray modes (electrospray versus electrostatic spray), from the porous substrates, occur in the presence of an applied DC potential after wetting with solvents of different surface tension. The ion current resulting from the spray process is maximized when the surface energy of the porous substrate approaches the surface tension of the wetting solvent. By monitoring the selected ion current (e.g., benzoylecgonine, m/z 290 → 168) with a mass spectrometer or the total ion current with an ammeter, we determined the solvent surface tension yielding the maximum ion current to indicate the surface energy of the solid. Detailed evaluations using polymeric substrates of known surface energies enabled effective calibration of the approach that resulted in the correct estimation of the surface energy of hydrophobic paper substrates prepared by gas-phase silanization. A three-parameter empirical model suggests that the experimentally observed ion current profile is governed by differential partitioning of analyte controlled by the interfacial forces between the wetting solvent and the porous substrate.