Direct Tissue Mass Spectrometry Imaging by Atmospheric Pressure UV-Laser Desorption Plasma Postionization

Ambient mass spectrometry imaging of food natural products by angled direct analysis in real time high-resolution mass spectrometry

Direct surface analysis in ambient conditions provides information on the position and chemical composition of an object at the time of investigation. An angled sampling probe is developed in this work for direct analysis in real time (DART) ionization high-resolution mass spectrometry. The DART ion source and the interface were modified for improved surface resolution, increased ion transfer efficiency, as well as enabling two-dimensional surface scanning. The angled probe DART-MS system was used for investigating a variety of food samples including fruit peels, ginseng root, plant leaves and sections of radish. Abundant signals and distinct chemical profiles are obtained in seconds, and spatial distribution of different molecules across the sample surfaces can be observed. In addition, the developed system can quickly identify the chemical changes when the surfaces were treated. The method is capable of directly evaluating food sample surfaces with different shapes, hardness, and conditions, without any sample pretreatments.

Nanoparticle-based applications by atmospheric pressure matrix assisted desorption/ionization mass spectrometry

Recently, the development of atmospheric pressure matrix assisted desorption/ionization mass spectrometry (AP MALDI MS) has made contributions not only to biomolecule analysis but also to spatial distribution. This has positioned AP MALDI as a powerful tool in multiple domains, thanks to its comprehensive advantages compared to conventional MALDI MS. These developments have addressed challenges associated with previous AP MALDI analysis systems, such as optimization of apparatus settings, synthesis of novel matrices, preconcentration and isolation strategies before analysis. Herein, applications in different fields using AP MALDI MS were described, including peptide and protein analysis, metabolite analysis, pharmaceutical analysis, and mass spectrometry imaging.

Dielectric Barrier Discharge Ionization Mechanisms: Polycyclic Aromatic Hydrocarbons as a Case of Study

Dielectric barrier discharge ionization (DBDI) is a versatile tool for small-molecule mass spectrometry applications, helping cover from polar to low polar molecules. However, the plasma gas-phase interactions are highly complex and have been scarcely investigated. The ionization mechanisms of plasmas have long been assumed to be somewhat similar to atmospheric pressure chemical ionization (APCI). Here, we evaluated the ionization mechanisms of a two-ring DBDI ion source, using different discharge gases to analyze vaporized liquid samples. Polycyclic aromatic hydrocarbons (PAHs) were used as model analytes to assess the mechanisms' dominance: protonation, [M + H]+, or radical ion species formation, [M]·+. In the present work, two different ionization trends were observed for APCI and DBDI during the PAH analysis; the compounds with proton affinities (PA) over 856 kJ/mol were detected as [M + H]+ when APCI was used as ionization source. Meanwhile, independently of the PA, DBDI showed the prevalence of charge exchange reactions. The addition of dopants in the gas-phase region shifted the ionization mechanisms toward charge exchange reactions, facilitating the formation of [M]·+ ion species, showing anisole a significant boost of the PAH radical ion species signals, over nine times for Ar-Prop-DBDI analysis. The presence of high-energy metastable atoms (e.g., HeM) with high ionization potentials (IE = 19.80 eV) did not show boosted PAH abundances or extensive molecule fragmentation. Moreover, other species in the plasma jet region with closer and more appropriate IE, such as N2 B3Πg excited molecules, are likely responsible for PAH Penning ionization.

Quantitative Analysis of Exhaled Breath Collected on Filter Substrates via Low-Temperature Plasma Desorption/Ionization Mass Spectrometry

Breath analysis has attracted increasing attention in recent years due to its great potential for disease diagnostics at early stages and for clinical drug monitoring. There are several recent examples of successful development of real-time, in vivo quantitative analysis of exhaled breath metabolites via mass spectrometry. On the other hand, current mass spectrometer accessibility limitations restrict point-of-care applications. Here now, an offline method is developed for quantitative analysis of exhaled breath collected on inexpensive filter substrates for direct desorption and ionization by using low-temperature plasma-mass spectrometry (LTP-MS). In particular, different operating conditions of the ionization source were systematically studied to optimize desorption/ionization by using glycerol, a low volatility compound. Applications with respect to propofol, γ-valprolactone, and nicotine analysis in exhaled breath are demonstrated in this study. The effects of several filter substrate properties, including filter material and pore size, on the analyte signal were characterized. Cellulose filter papers performed best with the present analytes. In addition, filters with smaller pores enabled a more efficient sample collection. Furthermore, sample-collection flow rate was determined to have a very significant effect, with slower flow rates yielding the best results. It was also found that filters loaded with sample can be successfully stored in glass vials with no observable sample loss even after 3 days. Limits of detection under optimized conditions are shown to be competitive or significantly better compared with relevant techniques and with additional benefits of cost-efficiency and sample storage capabilities.

Atmospheric-Pressure Infrared Laser-Ablation Plasma-Postionization Mass Spectrometry Imaging of Formalin-Fixed Paraffin-Embedded (FFPE) and Fresh-Frozen Tissue Sections with No Sample Preparation

Mass spectrometry imaging (MSI) encompasses a powerful suit of techniques which provide spatially resolved atomic and molecular information from almost any sample type. MSI is now widely used in preclinical research to provide insight into metabolic phenotypes of disease. Typically, fresh-frozen tissue preparations are considered optimal for biological MSI and other traditional preservation methods such as formalin fixation, alone or with paraffin embedding (FFPE), are considered less optimal or even incompatible. Due to the prevalence of FFPE tissue storage, particularly for rare and therefore high-value tissue samples, there is substantial motivation for optimizing MSI methods for analysis of FFPE tissue. Here, we present a novel modality, atmospheric-pressure infrared laser-ablation plasma postionization (AP-IR-LA-PPI), with the first proof-of-concept examples of MSI for FFPE and fresh-frozen tissues, with no post-sectioning sample preparation. We present ion images from FFPE and fresh tissues in positive and negative ion modes. Molecular annotations (via the Metaspace annotation engine) and on-tissue MS/MS provide additional confidence that the detected ions arise from a broad range of metabolite and lipid classes from both FFPE and fresh-frozen tissues.

Sample preparation optimization of insects and zebrafish for whole-body mass spectrometry imaging

Appropriate sample preparation is one of the most critical steps in mass spectrometry imaging (MSI), which is closely associated with reproducible and reliable images. Despite that model insects and organisms have been widely used in various research fields, including toxicology, drug discovery, disease models, and neurobiology, a systematic investigation on sample preparation optimization for MSI analysis has been relatively rare. Unlike mammalian tissues with satisfactory homogeneity, freezing sectioning of the whole body of insects is still challenging because some insect tissues are hard on the outside and soft on the inside, especially for some small and fragile insects. Herein, we systematically investigated the sample preparation conditions of various insects and model organisms, including honeybees (Apis cerana), oriental fruit flies (Bactrocera dorsalis), zebrafish (Danio rerio), fall armyworms (Spodoptera frugiperda), and diamondback moths (Plutella xylostella), for MSI. Three cutting temperatures, four embedding agents, and seven thicknesses were comprehensively investigated to achieve optimal sample preparation protocols for MSI analysis. The results presented herein indicated that the optimal cutting temperature and embedding agent were -20 °C and gelatin, respectively, providing better tissue integrity and less mass spectral interference. However, the optimal thickness for different organisms can vary with each individual. Using this optimized protocol, we exploited the potential of MSI for visualizing the tissue-specific distribution of endogenous lipids in four insects and zebrafish. Taken together, this work provides guidelines for the optimized sample preparation of insects and model organisms, facilitating the expansion of the potential of MSI in the life sciences and environmental sciences.

Routine analysis of pesticides in foodstuffs: Emerging ambient ionization mass spectrometry as an alternative strategy to be on your radar

Pesticides residues in foodstuffs are longstanding of great concern to consumers and governments, thus reliable evaluation techniques for these residues are necessary to ensure food safety. Emerging ambient ionization mass spectrometry (AIMS), a transformative technology in the field of analytical chemistry, is becoming a promising and solid evaluation technology due to its advantages of direct, real-time and in-situ ionization on samples without complex pretreatments. To provide useful guidance on the evaluation techniques in the field of food safety, we offered a comprehensive review on the AIMS technology and introduced their novel applications for the analysis of residual pesticides in foodstuffs under different testing scenarios (i.e., quantitative, screening, imaging, high-throughput detection and rapid on-site analysis). Meanwhile, the creative combination of AIMS with high-resolution mass analyzer (e.g., orbitrap and time-of-flight) was fundamentally mentioned based on recent studies about the detection and evaluation of multi-residual pesticides between 2015 and 2021. Finally, the technical challenges and prospects associated with AIMS operation in food industry were discussed.

Pulsed Blue Laser Diode Thermal Desorption Microplasma Imaging Mass Spectrometry

An ambient air laser desorption, plasma ionization imaging method is developed and presented using a microsecond pulsed laser diode for desorption and a flexible microtube plasma for ionization of the neutral desorbate. Inherent parameters such as the laser repetition rate and pulse width are optimized to the imaging application. For the desorption substrate, copper spots on a copper-glass sandwich structure are used. This novel design enables imaging without ablating the metal into the mass spectrometer. On this substrate, fixed calibration markers are used to decrease the positioning error in the imaging process, featuring a 3D offset correction within the experiment. The image is both screened spot-by-spot and per line scanning at a constant speed, which allows direct comparison. In spot-by-spot scanning, a novel algorithm is presented to unfold and to reconstruct the imaging data. This approach significantly decreases the time required for the imaging process, which allows imaging even at decreased sampling rates and thus higher mass resolution. After the experiment, the raw data is automatically converted and interpreted by a second algorithm, which allows direct visualization of the image from the data, even on low-intensity signals. Mouse liver microtome cuts have been screened for dehydrated cholesterol, proving good agreement of the unfolded data with the morphology of the tissue. The method optically resolves 30 μm, with 30 μm diameter copper spots and a 10 μm gap. No conventional chemical matrices or vacuum conditions are required.

Plug-and-play laser ablation-mass spectrometry for molecular imaging by means of dielectric barrier discharge ionization

The plug-and-play hyphenation of UV-laser ablation (LA) and mass spectrometry is presented, using dielectric barrier discharge ionization (DBDI). The DBDI source employed here is characterized by its unique geometry, being directly mounted onto the inlet capillary of a mass spectrometer. In the literature, this particular kind of DBDI source is also referred to as active capillary plasma ionization. It has been commercialized as soft ionization by chemical reaction in transfer (SICRIT) and will be addressed as DBDI in this study. LA-DBDI-MS was used for the direct, molecule-specific and spatially resolved analysis of various solid samples, such as coffee beans and pain killer tablets without extensive sample preparation. The combination of fast washout UV-laser ablation and the principle of the DBDI source used here allowed for highly efficient soft ionization as well as high spatial resolution down to 10 μm for molecular imaging.