High Resolution Ambient MS Imaging of Biological Samples by Desorption Electro-Flow Focussing Ionization

In this study, we examine the suitability of desorption electro-flow focusing ionization (DEFFI) for mass spectrometry imaging (MSI) of biological tissue. We also compare the performance of desorption electrospray ionization (DESI) with and without the flow focusing setup. The main potential advantages of applying the flow focusing mechanism in DESI is its rotationally symmetric electrospray jet, higher intensity, more controllable parameters, and better portability due to the robustness of the sprayer. The parameters for DEFFI have therefore been thoroughly optimized, primarily for spatial resolution but also for intensity. Once the parameters have been optimized, DEFFI produces similar images to the existing DESI. MS images for mouse brain samples, acquired at a nominal pixel size of 50 μm, are comparable for both DESI setups, albeit the new sprayer design yields better sensitivity. Furthermore, the two methods are compared with regard to spectral intensity as well as the area of the desorbed crater on rhodamine-coated slides. Overall, the implementation of a flow focusing mechanism in DESI is shown to be highly suitable for imaging biological tissue and has potential to overcome some of the shortcomings experienced with the current geometrical design of DESI.

A review on the advancements in chemical examination of composition of latent fingerprint residues

Background

There are notable developments in the field of DNA analysis and recognition, still fingerprint analysis remains the most preferred approach for obtaining substantial forensic evidences. The identification of individuals through pattern comparison has been used through ages, but it becomes less effective when the pattern is blurred, partial in nature or not found in database. Thus, recent advances in analytical techniques over the last decade will provide additional information to the evidence. In the view of forensic investigations, the compilation of individual chemical profiles with the pattern would enhance the evidentiary value of the latent fingerprints.

Main body

The review has been divided into different parts, describing the various influencing factors which affect the chemical composition of a fingerprint, i.e., lifestyle and occupation, age of an individual, types of substrate on which fingerprint is deposited, environmental conditions, contaminants, and the various advanced instrumental techniques utilized till now in the detection of chemical constituents of fingerprint have been discussed.

Conclusion

The present work aims to enlighten the missing gaps of knowledge in elucidating the detailed chemical composition of fingerprints and highlight the various analytical techniques used till date. Though, there are several analytical techniques employed till date to explicate the constituents of fingerprints, detailed information is still lacking. Therefore, advanced future research is need of the hour for identification of the fingerprints and determining their aging kinetics.

Probing Folded Proteins and Intact Protein Complexes by Desorption Electrospray Ionization Mass Spectrometry

Native mass spectrometry (MS) enables the study of intact proteins as well as noncovalent protein-protein and protein-ligand complexes in their biological state. In this work, we present the application of a Waters desorption electrospray ionization (DESI) source with a prototype spray emitter for rapid surface measurements of folded and native protein structures. A comparison of DESI spray solvent shows that adding 50% methanol to 200 mM ammonium acetate solution does not reduce its performance in preserving folded protein structures. Instead, improved signal-to-noise (S/N) ratio is obtained, and less adducted peaks are detected by using this uncommon native MS solvent system. The standard DESI design with an inlet tube allows optimization of sampling temperature conditions to improve desolvation and therefore S/N ratio. Furthermore, tuning the inlet temperature enables the control and study of unfolding behavior of proteins from surface samples. The optimized condition for native DESI has been applied to several selected proteins and protein complexes with the molecular weight ranging from 8.6 to 66.4 kDa. Ions of folded proteins with narrow charge state distribution (CSD), or peaks showing noncovalent-bond-assembled intact protein complexes, are observed in the spectra. Evidence for the structural refolding of denatured proteins and protein complexes sampled with native solvent highlights the need for care when interpreting DESI native MS data, particularly for proteins with stable native structures.

Dripping, jetting and tip streaming

Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. First, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results of the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming in open systems. Second, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels.

Recent advances in the chemical imaging of human fingermarks (a review)

This review highlights the considerable advances in the chemical imaging of human fingermarks that provide more chemical information, including numerous endogenous and exogenous constituents. Despite remarkable development in DNA analysis and recognition, human fingermark analysis remains one of the priority approaches available for obtaining reliable forensic evidence. Additional information about the donor can be obtained from the chemical composition of latent fingermarks in addition to the ridge pattern, such as the age, gender, medical history, and possible drug habits. The analytical approaches reviewed here include spectroscopy, mass spectrometry, immuno-labelling and electrochemical methods. Each method has different capabilities with respect to sensitivity, reproducibility, selectivity, reliability and ultimately applicability, either for use in routine forensic practice or in academic research work. The advantages of spectroscopic techniques, including infrared, Raman and micro-X-ray fluorescence spectroscopy, are the capabilities of a rapid and non-destructive imaging of fingermarks by providing spectral information on chemical composition. In addition, mass spectrometry imaging can provide spatially specific information on fingermark chemical composition. Recently, the use of immuno-labelling in latent fingermark detection has attracted significant attention because it can overcome the sensitivity and selectivity problems experienced with other existing methods. The electrochemical method has also been employed to image latent fingermarks by measuring the electric current changes with the spatial chemical composition from the ridges and valleys at high resolution to provide a third level of detail, which is especially useful for multicoloured background surfaces or for surfaces contaminated with blood or other bodily fluids.

Broad spectrum infrared thermal desorption of wipe-based explosive and narcotic samples for trace mass spectrometric detection

Wipe collected analytes were thermally desorbed using broad spectrum near infrared heating for mass spectrometric detection. Employing a twin tube filament-based infrared emitter, rapid and efficiently powered thermal desorption and detection of nanogram levels of explosives and narcotics was demonstrated. The infrared thermal desorption (IRTD) platform developed here used multi-mode heating (direct radiation and secondary conduction from substrate and subsequent convection from air) and a temperature ramp to efficiently desorb analytes with vapor pressures across eight orders of magnitude. The wipe substrate experienced heating rates up to (85 ± 2) °C s^-1 with a time constant of (3.9 ± 0.2) s for 100% power emission. The detection of trace analytes was also demonstrated from complex mixtures, including plastic-bonded explosives and exogenous narcotics, explosives, and metabolites from collected artificial latent fingerprints. Manipulation of the emission power and duration directly controlled the heating rate and maximum temperature, enabling differential thermal desorption and a level of upstream separation for enhanced specificity. Transitioning from 100% power and 5 s emission duration to 25% power and 30 s emission enabled an order of magnitude increase in the temporal separation (single seconds to tens of seconds) of the desorption of volatile and semi-volatile species within a collected fingerprint. This mode of operation reduced local gas-phase concentrations, reducing matrix effects experienced with high concentration mixtures. IRTD provides a unique platform for the desorption of trace analytes from wipe collections, an area of importance to the security sector, transportation agencies, and customs and border protection.

Recent advances in ambient mass spectrometry of trace explosives

Ambient mass spectrometry has evolved rapidly over the past decade, yielding a plethora of platforms and demonstrating scientific advancements across a range of fields from biological imaging to rapid quality control. These techniques have enabled real-time detection of target analytes in an open environment with no sample preparation and can be coupled to any mass analyzer with an atmospheric pressure interface; capabilities of clear interest to the defense, customs and border control, transportation security, and forensic science communities. This review aims to showcase and critically discuss advances in ambient mass spectrometry for the trace detection of explosives.

Revealing Individual Lifestyles through Mass Spectrometry Imaging of Chemical Compounds in Fingerprints

Fingerprints, specifically the ridge details within the print, have long been used in forensic investigations for individual identification. Beyond the ridge detail, fingerprints contain useful chemical information. The study of fingerprint chemical information has become of interest, especially with mass spectrometry imaging technologies. Mass spectrometry imaging visualizes the spatial relationship of each compound detected, allowing ridge detail and chemical information in a single analysis. In this work, a range of exogenous fingerprint compounds that may reveal a personal lifestyle were studied using matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Studied chemical compounds include various brands of bug sprays and sunscreens, as well as food oils, alcohols, and citrus fruits. Brand differentiation and source determination were possible based on the active ingredients or exclusive compounds left in fingerprints. Tandem mass spectrometry was performed for the key compounds, so that these compounds could be confidently identified in a single multiplex mass spectrometry imaging data acquisition.

MSiReader v1.0: Evolving Open-Source Mass Spectrometry Imaging Software for Targeted and Untargeted Analyses

A major update to the mass spectrometry imaging (MSI) software MSiReader is presented, offering a multitude of newly added features critical to MSI analyses. MSiReader is a free, open-source, and vendor-neutral software written in the MATLAB platform and is capable of analyzing most common MSI data formats. A standalone version of the software, which does not require a MATLAB license, is also distributed. The newly incorporated data analysis features expand the utility of MSiReader beyond simple visualization of molecular distributions. The MSiQuantification tool allows researchers to calculate absolute concentrations from quantification MSI experiments exclusively through MSiReader software, significantly reducing data analysis time. An image overlay feature allows the incorporation of complementary imaging modalities to be displayed with the MSI data. A polarity filter has also been incorporated into the data loading step, allowing the facile analysis of polarity switching experiments without the need for data parsing prior to loading the data file into MSiReader. A quality assurance feature to generate a mass measurement accuracy (MMA) heatmap for an analyte of interest has also been added to allow for the investigation of MMA across the imaging experiment. Most importantly, as new features have been added performance has not degraded, in fact it has been dramatically improved. These new tools and the improvements to the performance in MSiReader v1.0 enable the MSI community to evaluate their data in greater depth and in less time. Graphical Abstract ᅟ.

Monosized dripping mode of axisymmetric flow focusing

We identify and analyze the perfectly regular dripping mode of flow focusing. This mode occurs within narrow intervals of injected flow rates and applied pressure drops and leads to homogeneous-size droplets with diameters similar to or smaller than that of the discharge orifice. The balance between the local acceleration of the fluid particle and the applied pressure drop yields the scaling law for the droplet diameter. This scaling law is validated experimentally with excellent accord.

Ion mobility spectrometry nuisance alarm threshold analysis for illicit narcotics based on environmental background and a ROC-curve approach

The discriminative potential of an ion mobility spectrometer (IMS) for trace detection of illicit narcotics relative to environmental background was investigated with a receiver operating characteristic (ROC) curve framework. The IMS response of cocaine, heroin, methamphetamine, 3,4-methylenedioxymethamphetamine (MDMA), and Δ(9)-tetrahydro-cannabinol (THC) was evaluated against environmental background levels derived from the screening of incoming delivery vehicles at a federal facility. Over 20 000 samples were collected over a multiyear period under two distinct sets of instrument operating conditions, a baseline mode and an increased desorption/drift tube temperature and sampling time mode. ROC curves provided a quantifiable representation of the interplay between sensitivity (true positive rate, TPR) and specificity (1 - false positive rate, FPR). A TPR of 90% and minimized FPR were targeted as the detection limits of IMS for the selected narcotics. MDMA, THC, and cocaine demonstrated single nanogram sensitivity at 90% TPR and <10% FPR, with improvements to both MDMA and cocaine in the elevated temperature/increased sampling mode. Detection limits in the tens of nanograms with poor specificity (FPR ≈ 20%) were observed for methamphetamine and heroin under baseline conditions. However, elevating the temperature reduced the background in the methamphetamine window, drastically improving its response (90% TPR and 3.8% FPR at 1 ng). On the contrary, the altered mode conditions increased the level of background for THC and heroin, partially offsetting observed enhancements to desorption. The presented framework demonstrated the significant effect environmental background distributions have on sensitivity and specificity.

Present and foreseeable future of metabolomics in forensic analysis

The revulsive publications during the last years on the precariousness of forensic sciences worldwide have promoted the move of major steps towards improvement of this science. One of the steps (viz. a higher involvement of metabolomics in the new era of forensic analysis) deserves to be discussed under different angles. Thus, the characteristics of metabolomics that make it a useful tool in forensic analysis, the aspects in which this omics is so far implicit, but not mentioned in forensic analyses, and how typical forensic parameters such as the post-mortem interval or fingerprints take benefits from metabolomics are critically discussed in this review. The way in which the metabolomics-forensic binomial succeeds when either conventional or less frequent samples are used is highlighted here. Finally, the pillars that should support future developments involving metabolomics and forensic analysis, and the research required for a fruitful in-depth involvement of metabolomics in forensic analysis are critically discussed.

Anti-theft device staining on banknotes detected by mass spectrometry imaging

We describe the identification and limits of detection of ink staining by mass spectrometry imaging (MSI), as used in anti-theft devices (ATDs). Such ink staining is applied to banknotes during automated teller machine (ATM) explosions. Desorption electrospray ionization (DESI) coupled with high-resolution and high-accuracy orbitrap mass spectrometry (MS) and a moving stage device were applied to obtain 2D molecular images of the major dyes used for staining, that is, 1-methylaminoanthraquinone (MAAQ), rhodamine B (RB) and rhodamine 6G (R6G). MAAQ could not be detected because of its inefficient desorption by DESI from the banknote cellulose surface. By contrast, ATD staining on banknotes is perceptible by the human naked eye only at concentrations higher than 0.2 μg cm(-2), whereas both RB and R6G at concentrations 200 times lower (as low as 0.001 μg cm(-2)) could be easily detected and imaged by DESI-MSI, with selective and specific identification of each analyte and their spatial distribution on samples from suspects. This technique is non-destructive, and no sample preparation is required, which ensures sample preservation for further forensic investigations.

Forensic Mass Spectrometry

Developments in forensic mass spectrometry tend to follow, rather than lead, the developments in other disciplines. Examples of techniques having forensic potential born independently of forensic applications include ambient ionization, imaging mass spectrometry, isotope ratio mass spectrometry, portable mass spectrometers, and hyphenated chromatography-mass spectrometry instruments, to name a few. Forensic science has the potential to benefit enormously from developments that are funded by other means, if only the infrastructure and personnel existed to adopt, validate, and implement the new technologies into casework. Perhaps one unique area in which forensic science is at the cutting edge is in the area of chemometrics and the determination of likelihood ratios for the evaluation of the weight of evidence. Such statistical techniques have been developed most extensively for ignitable-liquid residue analyses and isotope ratio analysis. This review attempts to capture the trends, motivating forces, and likely impact of developing areas of forensic mass spectrometry, with the caveat that none of this research is likely to have any real impact in the forensic community unless: (a) The instruments developed are turned into robust black boxes with red and green lights for positives and negatives, respectively, or (b) there are PhD graduates in the workforce who can help adopt these sophisticated techniques.

Rapid detection of sugar alcohol precursors and corresponding nitrate ester explosives using direct analysis in real time mass spectrometry

This work highlights the rapid detection of nitrate ester explosives and their sugar alcohol precursors by direct analysis in real time mass spectrometry (DART-MS) using an off-axis geometry. Demonstration of the effect of various parameters, such as ion polarity and in-source collision induced dissociation (CID) on the detection of these compounds is presented. Sensitivity of sugar alcohols and nitrate ester explosives was found to be greatest in negative ion mode with sensitivities ranging from hundreds of picograms to hundreds of nanograms, depending on the characteristics of the particular molecule. Altering the in-source CID potential allowed for acquisition of characteristic molecular ion spectra as well as fragmentation spectra. Additional studies were completed to identify the role of different experimental parameters on the sensitivity for these compounds. Variables that were examined included the DART gas stream temperature, the presence of a related compound (i.e., the effect of a precursor on the detection of a nitrate ester explosive), incorporation of dopant species and the role of the analysis surface. It was determined that each variable affected the response and detection of both sugar alcohols and the corresponding nitrate ester explosives. From this work, a rapid and sensitive method for the detection of individual sugar alcohols and corresponding nitrate ester explosives, or mixtures of the two, has been developed, providing a useful tool in the real-world identification of homemade explosives.

Rapid detection and isotopic measurement of discrete inorganic samples using acoustically actuated droplet ejection and extractive electrospray ionization mass spectrometry

RATIONALE

The rapid detection, screening, and isotopic signature analysis of inorganics provide invaluable information for a variety of applications including explosive device detection, nuclear forensics, and environmental monitoring. The coupling of ultrasonic nebulization and extractive electrospray ionization (EESI) enabled the mass spectrometric (MS) detection and analysis of inorganics from microliter sample solution aliquots.

METHODS

Ultrasonic nebulization and acoustic pressure wave focusing within an array of exponential horn structures were utilized for the efficient atomization of discrete liquid samples ranging in volume from 3 μL to 10 μL pipetted aliquots. In conjunction with an electro-flow focusing source for extractive electrospray ionization (EESI), in-source collision-induced dissociation (CID) was utilized to enhance inorganic detection through fragmentation of adducts and reduction in chemical noise from organic compounds.

RESULTS

The investigated system enhanced detection of the singly charged elemental cation species and provided accurate measurements of isotopic distributions for a number of metal ions. The extent of CID demonstrated the competition between ligand loss from hydrate clusters and charge reduction from the doubly charged to singly charged cations for the alkaline earth metal ions of strontium and barium. Inorganics were also detected from complex matrices, including synthetic fingerprint material and sediment, without detriment to device operation.

CONCLUSIONS

The described system provides a versatile tool for the rapid detection, speciation, and isotopic identification of inorganic compounds at nanogram to sub-nanogram levels from microliter aliquots. Published in 2014. This article is a U.S. Government work and is in the public domain in the USA.