Novel Blotting Method for Mass Spectrometry Imaging of Metabolites in Strawberry Fruit by Desorption/Ionization Using Through Hole Alumina Membrane

Thin-section- and matrix-free mass spectrometry imaging: Reproducible sample transfer using novel platinum-coated porous plate formed of glass beads

RATIONALE

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) typically requires sample preparation such as sectioning and spraying of the matrix. Sample transfer using a Pt-coated porous plate formed of glass beads simplifies preparation and enables reproducible MSI measurements.

METHODS

The surface of a sintered-glass-bead porous plate was coated with Pt on one side of the plate. Polymer additives and the cross-section of a strawberry were chosen as the sample and transferred to the Pt-coated surface of the plate. This process was completely thin-section- and matrix-free. The prepared plates were analyzed using a commercial MALDI time-of-flight instrument.

RESULTS

Several constituents of the polymer additives (Irgafos 168 and Irganox 1010) and strawberry metabolites (hexose, citric acid, and sucrose) were detected without organic matrices. These ion images were obtained with a special distribution of the retained constituents. Duplicate prepared plates using the same strawberry cross-section reproduced the ion images.

CONCLUSIONS

A sample transfer process using a Pt-coated porous plate formed of glass beads was demonstrated as an alternative preparation method for MSI. This process has the potential to simplify MSI preparation and achieve easily reproducible MSI measurements.

Sugars and Organic Acids in 25 Strawberry Cultivars: Qualitative and Quantitative Evaluation

(1) The nutritional quality of strawberry (Fragaria × ananassa Duch) fruits, among others, is largely maintained by the presence of soluble sugars and organic acids. As the primary products of photosynthesis, they are energy depots in plants, necessary for the construction of cell constituents, but also serve as precursors of aromatic compounds and signaling molecules. (2) In this study, fruits of 25 strawberry cultivars were qualitatively and quantitatively characterized concerning individual sugars and organic acids by HPLC, FT-ICR-MS, and MS imaging analysis. In addition, the total quality index (TQI), as a novel mathematical model, was used to compare all individual parameters evaluated to obtain a quantitative single score, as an indicator of overall fruit quality. (3) Regardless of a large number of cultivars and monitored parameters that were studded, several cultivars stood out in terms of selected primary metabolites, such as 'Rumba', 'Jeny', and 'Sandra', while the latter had the best TQI score. (4) Intercultivar variations in sugars and organic acids profiles, along with other bioactive compounds, should be considered for selection of promising cultivars with improved naturally occurring nutraceutical traits. Besides the search for a pleasant taste, increased awareness of healthy nutrition resulted in heightening consumer demand for high-quality fruit.

Analysis of Immunogenic Galactose-α-1,3-galactose-Containing N-Glycans in Beef, Mutton, and Pork Tenderloin by Combining Matrix-Assisted Laser Desorption/Ionization-Mass Spectroscopy and Capillary Electrophoresis Hyphenated with Mass Spectrometry via Electrospray Ionization

Severe allergic reactions to certain types of meat following tick bites have been reported in geographic regions which are endemic with ticks. This immune response is directed to a carbohydrate antigen (galactose-α-1,3-galactose or α-Gal), which is present in glycoproteins of mammalian meats. At the moment, asparagine-linked complex carbohydrates (N-glycans) with α-Gal motifs in meat glycoproteins and in which cell types or tissue morphologies these α-Gal moieties are present in mammalian meats are still unclear. In this study, we analyzed α-Gal-containing N-glycans in beef, mutton, and pork tenderloin and provided for the first time the spatial distribution of these types of N-glycans in various meat samples. Terminal α-Gal-modified N-glycans were found to be highly abundant in all analyzed samples (55, 45, and 36% of N-glycome in beef, mutton, and pork, respectively). Visualizations of the N-glycans with α-Gal modification revealed that this motif was mainly present in the fibroconnective tissue. To conclude, this study contributes to a better understanding of the glycosylation biology of meat samples and provides guidance for processed meat products, in which only meat fibers are required as an ingredient (i.e., sausages or canned meat).

MALDI MS Imaging of Cucumbers

There are many different methodologies in the literature for the preparation of plant material for subsequent MALDI MSI analysis. This chapter overviews preparation of cucumbers (Cucumis sativus L.), with emphasis on sample freezing, cryosectioning, and matrix deposition. This should act as a representative example of sample preparation for plant tissue, and due to wide sample variation (e.g., leaves, seeds, and fruit) and analytes of interest, method optimization will be required for different samples.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

Unique distribution of ellagitannins in ripe strawberry fruit revealed by mass spectrometry imaging

Ellagitannins (ETs) are hydrolysable tannins composed of a polyol core, primarily glucose, which is esterified with hexahydroxydiphenic acid (HHDP), and in some cases, gallic acid. ETs are the major phenolic compounds found in strawberries and may contribute to the health-related properties of strawberries, because of their strong antioxidative activity. However, their distribution in the strawberry fruit remains unclear. In this study, matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI–MSI) was used to visualize ETs in ripe strawberry fruits. Five peaks, corresponding to the m/z values of ET [M−H]⁻ ions detected in the MALDI–MS spectrum of strawberry extracts, were identified as strictinin, pedunculagin, casuarictin, davuriicin M₁, and an unknown ET using MALDI–tandem MS (MS/MS). In addition, liquid chromatography–electrospray ionization–MS/MS of the extracts revealed the presence of pedunculagin isomers and the unknown ET. Ion images of these five ETs were reconstructed using MALDI–MSI. Strictinin was widely distributed in and around the achene seed coats, while the other ETs were dispersed in and around the seed coats, and at the bottom of the receptacle; pedunculagin was distributed in the epidermis and pith, whereas casuarictin, the unknown ET, and davuriicin M₁ were distributed in the pith. Moreover, MALDI–MSI of a casuarictin standard indicated that in-source fragmentation weakly affected the ion images. The results suggest that the distribution of ETs depends on the presence or absence of their constituents, namely galloyl units, HHDP, and bis-HHDP. To the best of my knowledge, this is the first report on the visualization of ETs in plant tissues using MSI, MALDI–MSI may be a useful tool for analyzing the distribution of ETs in the strawberry fruit.

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The ellagitannins (ETs) in strawberry fruits were identified.

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MALDI-MS/MS and LC-MS/MS were used.

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The ETs identified in the fruits were visualized using MALDI-MSI.

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The ETs showed unique distributions in the seeds and the lower receptacle.

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The distribution of ETs depends on their constituents: galloyl units and HHDP.

Matrix-Free High-Resolution Atmospheric-Pressure SALDI Mass Spectrometry Imaging of Biological Samples Using Nanostructured DIUTHAME Membranes

Applications of mass spectrometry imaging (MSI), especially matrix-assisted laser desorption/ionization (MALDI) in the life sciences are becoming increasingly focused on single cell analysis. With the latest instrumental developments, pixel sizes in the micrometer range can be obtained, leading to challenges in matrix application, where imperfections or inhomogeneities in the matrix layer can lead to misinterpretation of MS images. Thereby, the application of premanufactured, homogeneous ionization-assisting devices is a promising approach. Tissue sections were investigated using a matrix-free imaging technique (Desorption Ionization Using Through-Hole Alumina Membrane, DIUTHAME) based on premanufactured nanostructured membranes to be deposited on top of a tissue section, in comparison to the spray-coating of an organic matrix in a MALDI MSI approach. Atmospheric pressure MALDI MSI ion sources were coupled to orbital trapping mass spectrometers. MS signals obtained by the different ionization techniques were annotated using accurate-mass-based database research. Compared to MALDI MSI, DIUTHAME MS images captivated with higher signal homogeneities, higher contrast and reduced background signals, while signal intensities were reduced by about one order of magnitude, independent of analyte class. DIUTHAME membranes, being applicable only on tissue sections thicker than 50 µm, were successfully used for mammal, insect and plant tissue with a high lateral resolution down to 5 µm.

Desorption ionization using through-hole alumina membrane offers higher reproducibility than 2,5-dihydroxybenzoic acid, a widely used matrix in Fourier transform ion cyclotron resonance mass spectrometry imaging analysis

RATIONALE

DIUTHAME (desorption ionization using through-hole alumina membrane), a recently developed matrix-free ionization-assisting substrate, was examined for reproducibility in terms of mass accuracy and intensity using standard lipid and mouse brain sections. The impregnation property of DIUTHAME significantly improved the reproducibility of mass accuracy and intensity compared with 2,5-dihydroxybenzoic acid (DHB).

METHODS

Frozen tissue sections were mounted on indium tin oxide-coated glass slides. DIUTHAME and DHB were applied to individual sections. Subsequently, a solution of a phosphatidylcholine standard, PC(18:2/18:2), was poured onto the DIUTHAME and matrix. Finally, the samples were subjected to laser desorption ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry. The reproducibility was tested by calculating the mean ± standard deviation values of mass errors and intensities of individual ion species.

RESULTS

Analysis of the PC(18:2/18:2) standard showed significantly (p < 0.01) lower mass error for DIUTHAME-MS than for MALDI-MS. Endogenous PC(36:4) analysis in mouse brain section also showed significantly (p < 0.05) lower mass errors for DIUTHAME-MS. Furthermore, we investigated the mass error of some abundant lipid ions in brain sections and observed similar results. DIUTHAME-MS displayed lower signal intensity in standard PC analysis. Interestingly, it offered higher signal intensities for all the endogenous lipid ions. Lower fluctuations of both mass accuracies and signal intensities were observed in DIUTHAME-MS.

CONCLUSIONS

Our results demonstrated that DIUTHAME-MS offers higher reproducibility for mass accuracies and intensities than MALDI-MS in both standard lipid and mouse brain tissue analyses. It can potentially be used instead of conventional MALDI-MS and mass spectrometry imaging analyses to achieve highly reproducible data for mass accuracy and intensity.

Spatiotemporal Visualization of Insecticides and Fungicides within Fruits and Vegetables Using Gold Nanoparticle-Immersed Paper Imprinting Mass Spectrometry Imaging

Food safety issues caused by pesticide residue have exerted far-reaching impacts on human daily life, yet the available detection methods normally focus on surface residue rather than pesticide penetration to the internal area of foods. Herein, we demonstrated gold nanoparticle (AuNP)-immersed paper imprinting mass spectrometry imaging (MSI) for monitoring pesticide migration behaviors in various fruits and vegetables (i.e., apple, cucumber, pepper, plum, carrot, and strawberry). By manually stamping food tissues onto AuNP-immersed paper, this method affords the spatiotemporal visualization of insecticides and fungicides within fruits and vegetables, avoiding tedious and time-consuming sample preparation. Using the established MSI platform, we can track the migration of insecticides and fungicides into the inner region of foods. The results revealed that both the octanol-water partition coefficient of pesticides and water content of garden stuffs could influence the discrepancy in the migration speed of pesticides into food kernels. Taken together, this nanopaper imprinting MSI is poised to be a powerful tool because of its simplicity, rapidity, and easy operation, offering the potential to facilitate further applications in food analysis. Moreover, new perspectives are given to provide guidelines for the rational design of novel pesticide candidates, reducing the risk of food safety issues caused by pesticide residue.

Adhesive film applications help to prepare strawberry fruit sections for desorption electrospray ionization-mass spectrometry imaging

Desorption electrospray ionization-mass spectrometry imaging (DESI-MSI) is a powerful tool to analyze the distribution of metabolites in biological tissues. Cryosectioning of biological tissues is usually required prior to DESI-MSI, but it can be difficult for tissues that are fragile, hard, and have a high-water content. The Kawamoto method uses transparent adhesive films to prepare cryosections; however, its application for plant tissues, such as strawberry tissues, in DESI-MSI has not been verified. In this study, strawberry cryosections maintained original structures were prepared using adhesive film. Subsequently, numerous peaks were detected for the sections using the positive and negative ion modes of DESI-MSI. Several primary and specialized metabolites, such as amino acids, sugars, organic acids, and flavonoids, were identified and visualized. These results suggest the use of adhesive films when cryosectioning could improve DESI-MSI analysis of the metabolites in strawberry fruits and various tissues of other plant species.

Localization of Flavan-3-ol Species in Peanut Testa by Mass Spectrometry Imaging

Flavan-3-ols, procyanidins and their monomers are major flavonoids present in peanuts that show a wide range of biological properties and health benefits, based on their potent antioxidant activity. Procyanidin oligomers, especially A-type, are reportedly abundant in peanut skin; however, their localization in the raw peanut testa remains poorly understood. Therefore, we performed matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) to investigate the localization of flavan-3-ols in peanut testa. 1,5-Diaminonaphthalene was coated onto the peanut section by matrix vapor deposition/recrystallization, and MALDI-MSI measurements were performed in the negative-ion mode. Peaks matching the m/z values of flavan-3-ol [M - H]^- ions were observed in the mass spectrum extracted from the outer epidermis of the peanut testa, using the region of interest function. Catechin and/or epicatechin, five A-type, and one B-type procyanidins were assigned by the fragment ions generated by retro-Diels-Alder, heterocyclic ring fission, and quinone methide reactions detected in MALDI-tandem MS spectra. These flavan-3-ols were localized in the outer epidermis of the peanut testa. This information will contribute to improving the extraction and purification efficiencies of flavan-3-ols from peanut testa. As flavan-3-ols display anti-microbial activity, it is speculated that flavan-3-ols present in the outer epidermis of peanut testa act to prevent pathogen infection.