Application of Mass Spectrometry Imaging for Visualizing Food Components

Study on dynamic alterations of volatile organic compounds reveals aroma development over enzymatic-catalyzed process of Tieguanyin oolong tea production

To elucidate the formation of characteristic aroma over enzymatic-catalyzed processes (ECP), GC-MS-based volatile-metabolomic combined with desorption-electrospray-ionization coupled mass-spectrometry-imaging (DESI-MSI) were employed to analyze the changes of volatile organic compounds (VOCs) in Tieguanyin tea. A total of 579 VOCs were obtained, from which 24 components involved in five pathways were identified as biomarkers. Among these, four VOCs including 2-furancarboxylic acid, 4-methylbenzaldehyde, N-benzylformamide, cuminaldehyde, were detected in both DESI-MSI and GC-MS analysis, exhibiting dynamic changes along processing steps. RNA-sequencing analysis indicated the genes referring to stress response were activated during tea processing, facilitating the accumulation of flora-fruity aroma in tea leaf. Metabolic pathways analysis revealed that the increase in floral-fruity related components such as volatile terpenoids, phenylpropanoids/benzenoids, indole, alongside a decrease in green leaf volatiles including (E)-2-Hexenal, (Z)-3-Hexenol, played a crucial role in development of characteristic aroma, which could be a feasible index for evaluating processing techniques or quality of oolong tea.

Imaging Lipidomics and Metallomics of Brown Rice Cultivars Used for Sake Production

Many previous studies have reported various phospholipids and elements that affect sake production; however, it seems to be challenging to investigate individual types in each rice variety due to their high diversity, not to mention their distribution patterns. Since its introduction, mass spectrometry imaging (MSI) has gained attention in various fields as a simple compound visualization technique. The current study highlights the progress of powerful MSI in comprehensively analyzing phospholipids and minerals in brown rice for sake production. Multivariate analysis suggested phospholipids relating to each rice group based on regions of interest. Phospholipid classes connected with embryo and endosperm included fatty acylcarnitine, diacylglycerol, phosphatidylcholine, phosphatidylglycerol, and phosphatidylethanolamine. Meanwhile, the studied rice groups showed the same distribution of the investigated 12 minerals. This is the first study that reports a comprehensive imaging analysis of phospholipids and elements in brown rice for several cultivars for sake production.

New perspective on central nervous system disorders: focus on mass spectrometry imaging

An abnormally organized brain spatial network is linked to the development of various central nervous system (CNS) disorders, including neurodegenerative diseases and neuropsychiatric disorders. However, the complicated molecular mechanisms of these diseases remain unresolved, making the development of treatment strategies difficult. A novel molecular imaging technique, called mass spectrometry imaging (MSI), captures molecular information on the surface of samples in situ. With MSI, multiple compounds can be simultaneously visualized in a single experiment. The high spatial resolution enables the simultaneous visualization of the spatial distribution and relative content of various compounds. The wide application of MSI in biomedicine has facilitated extensive studies on CNS disorders in recent years. This review provides a concise overview of the processes, applications, advantages, and disadvantages, as well as mechanisms of the main types of MSI. Meanwhile, this review summarizes the main applications of MSI in studying CNS diseases, including Alzheimer's disease (AD), CNS tumors, stroke, depression, Huntington's disease (HD), and Parkinson's disease (PD). Finally, this review comprehensively discusses the synergistic application of MSI with other advanced imaging modalities, its utilization in organoid models, its integration with spatial omics techniques, and provides an outlook on its future potential in single-cell analysis.

Visualizing active fungicide formulation mobility in tomato leaves with desorption electrospray ionisation mass spectrometry imaging

Newer and safer agrochemicals are always in demand to meet the increasing needs of a growing population for affordable food. Spatial chemical monitoring of the active mobility of an agrochemical is essential to this agrochemical development process and mass spectrometry imaging (MSI) is proposed as a safer, easier alternative to the existing standard of autoradiography for the same. With desorption electrospray ionisation mass spectrometry imaging (DESI MSI) using leaf imprints, we were able to visualize the active agrochemical mobility of a commercial fungicide formulation with the active ingredient Azoxystrobin in whole tomato leaves. The leaf-imprinting method was optimized with precise control over the pressure conditions and time of imprinting to yield highly consistent samples for imaging. The reproducibility of this method was tested with the Azoxystrobin formulation applied to tomato leaves and was compared to the mobility of the unformulated Azoxystrobin standard in similar application conditions. The xylem mobility and the lateral-leaf lamina spreading of the fungicide were visualized with mass spectrometry imaging and validated using complementary LC-MS studies. The necessity and importance of the agrochemical application as a formulation were re-iterated by the limited mobility observed in Azoxystrobin standard studies compared to the Azoxystrobin formulation. This mass spectrometry imprint-imaging method could be translated for the visualization of any xenobiotic in further foliar systems particularly with soft leaves.

Mass spectrometry imaging as a promising analytical technique for herbal medicines: an updated review

Herbal medicines (HMs) have long played a pivotal role in preventing and treating various human diseases and have been studied widely. However, the complexities present in HM metabolites and their unclear mechanisms of action have posed significant challenges in the modernization of traditional Chinese medicine (TCM). Over the past two decades, mass spectrometry imaging (MSI) has garnered increasing attention as a robust analytical technique that enables the simultaneous execution of qualitative, quantitative, and localization analyses without complex sample pretreatment. With advances in technical solutions, MSI has been extensively applied in the field of HMs. MSI, a label-free ion imaging technique can comprehensively map the spatial distribution of HM metabolites in plant native tissues, thereby facilitating the effective quality control of HMs. Furthermore, the spatial dimension information of small molecule endogenous metabolites within animal tissues provided by MSI can also serve as a supplement to uncover pharmacological and toxicological mechanisms of HMs. In the review, we provide an overview of the three most common MSI techniques. In addition, representative applications in HM are highlighted. Finally, we discuss the current challenges and propose several potential solutions. We hope that the summary of recent findings will contribute to the application of MSI in exploring metabolites and mechanisms of action of HMs.

BitterMasS: Predicting Bitterness from Mass Spectra

Bitter compounds are common in nature and among drugs. Previously, machine learning tools were developed to predict bitterness from the chemical structure. However, known structures are estimated to represent only 5-10% of the metabolome, and the rest remain unassigned or "dark". We present BitterMasS, a Random Forest classifier that was trained on 5414 experimental mass spectra of bitter and nonbitter compounds, achieving precision = 0.83 and recall = 0.90 for an internal test set. Next, the model was tested against spectra newly extracted from the literature 106 bitter and nonbitter compounds and for additional spectra measured for 26 compounds. For these external test cases, BitterMasS exhibited 67% precision and 93% recall for the first and 58% accuracy and 99% recall for the second. The spectrum-bitterness prediction strategy was more effective than the spectrum-structure-bitterness prediction strategy and covered more compounds. These encouraging results suggest that BitterMasS can be used to predict bitter compounds in the metabolome without the need for structural assignment of individual molecules. This may enable identification of bitter compounds from metabolomics analyses, for comparing potential bitterness levels obtained by different treatments of samples and for monitoring bitterness changes overtime.

Mass spectrometry imaging reveals flavor distribution in edible mushrooms

The spatial distribution of molecules and compounds responsible for the flavor profile of edible button mushrooms (Agaricus bisporous) has never been determined. The food industry is interested in knowing the localization of these compounds. Such knowledge would enable extraction of flavor compounds from a particular regions of the mushroom, which is safer for consumption compared to alternatives such as synthetic flavoring agents. The present study utilizes matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI), to determine the spatial distribution of flavor compounds in a mushroom. As MALDI-MSI requires very thin sections, a sample preparation protocol was optimized and sectioning fresh frozen mushrooms at 35 µm thickness was considered the best method to evaluate the distribution of flavor compounds. Further, the effect of heat on the spatial distribution of flavor compounds was investigated by heating whole mushrooms to 140 ℃ prior to sectioning. Heating reduced the water content of the mushroom and thus enabled the generation of even-thinner 17 µm thick sections. MALDI-MSI measurements performed on underivatized and on-tissue derivatized fresh frozen and heat-treated mushroom sections elucidated the spatial distribution of several flavor-related compounds.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05883-0.

Sodium doping and trapped ion mobility spectrometry improve lipid detection for novel MALDI-MSI analysis of oats

Oat (Avena sativa L.) is an important cereal grain with a unique nutritional profile including a high proportion of lipids. Understanding lipid composition and distribution in oats is valuable for plant, food and nutritional research, and can be achieved using MALDI mass spectrometry imaging (MALDI-MSI). However, this approach presents several challenges for sample preparation (hardness of grains) and analysis (isobaric and isomeric properties of lipids). Here, oat sections were successfully mounted onto gelatin-coated indium tin oxide slides with minimal tearing. Poor detection of triacylglycerols was resolved by applying sodium chloride during mounting, increasing signal intensity. In combination with trapped ion mobility spectrometry (TIMS), lipid identification significantly improved, and we report the separation of several isobaric and isomeric lipids with visualisation of their "true" spatial distributions. This study describes a novel MALDI-TIMS-MSI analytical technique for oat lipids, which may be used to improve the discovery of biomarkers for grain quality.

Combined Widely Targeted Metabolomic, Transcriptomic, and Spatial Metabolomic Analysis Reveals the Potential Mechanism of Coloration and Fruit Quality Formation in Actinidia chinensis cv. Hongyang

Postharvest kiwifruit (Actinidia chinensis cv. Hongyang) pulp is mainly composed of outer yellow-flesh (LR) and inner red-flesh (HR). However, information about the differences in coloration and fruit quality between these two parts are limited. In this study, widely targeted metabolomic, transcriptomic, and spatial metabolomic analyses were used to reveal the potential mechanism of coloration and fruit quality formation. The results show that a total of 1001 metabolites were identified in Hongyang kiwifruit, and the accumulation of 211 metabolites were significantly higher in the HR than LR, including 69 flavonoids, 53 phenolic acids, and 38 terpenoids. There were no significant differences in the content of citric acid, quinic acid, glucose, fructose, or sucrose between the LR and HR. These results were consistent with the results from the RNA-seq profile and spatial metabolomic analysis. In addition, a total of 23 key candidate genes related to flesh color and fruit quality formation were identified and validated by qRT-PCR analysis. This study provides a theoretical basis for elucidating the underlying mechanism of the formation of kiwifruit flesh color and fruit quality.

Visualizing the Distribution of Jujube Metabolites at Different Maturity Stages Using Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

Chinese jujube (also called Chinese date, Ziziphus jujuba Mill.) is an economically important tree in China and provides a rich source of sugars, vitamins, and bioactive components, all of which are indispensable and essential for the composition and participation in life processes of the human body. However, the location of these metabolites in jujube fruits has not been determined. This study applied matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) to investigate the spatial distribution of sugars, organic acids, and other key components in jujube fruits at different developmental periods. Soluble sugars such as hexoses and sucrose/maltose significantly increase with fruit ripening, while organic acids show an overall trend of initially increasing and then decreasing. Procyanidins and rutin exhibit specific distributions in the fruit periphery and peel. These findings suggest that MALDI-MSI can be used to study the spatial distribution of nutritional components in jujube fruits, providing insights into the changes and spatial distribution of substances during jujube fruit development. This technique offers a scientific basis for jujube breeding, utilization, and production.

MALDI MS Imaging of Chickpea Seeds (Cicer arietinum) and Crab's Eye Vine (Abrus precatorius) after Tryptic Digestion Allows Spatially Resolved Identification of Plant Proteins

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) imaging following in situ enzymatic digestion is a versatile analytical method for the untargeted investigation of protein distributions, which has rarely been used for plants so far. The present study describes a workflow for in situ tryptic digestion of plant seed tissue for MALDI MS imaging. Substantial modifications to the sample preparation procedure for mammalian tissues were necessary to cater to the specific properties of plant materials. For the first time, distributions of tryptic peptides were successfully visualized in plant tissue using MS imaging with accurate mass detection. Sixteen proteins were visualized and identified in chickpea seeds showing different distribution patterns, e.g., in the cotyledons, radicle, or testa. All tryptic peptides were detected with a mass resolution higher than 60,000 as well as a mass accuracy better than 1.5 ppm root-mean-square error and were matched to results from complementary liquid chromatography-MS/MS (LC-MS/MS) data. The developed method was also applied to crab's eye vine seeds for targeted MS imaging of the toxic protein abrin, showing the presence of abrin-a in all compartments. Abrin (59 kDa), as well as the majority of proteins visualized in chickpeas, was larger than 50 kDa and would thus not be readily accessible by top-down MS imaging. Since antibodies for plant proteins are often not readily available, in situ digestion MS imaging provides unique information, as it makes the distribution and identification of larger proteins in plant tissues accessible in an untargeted manner. This opens up new possibilities in the field of plant science as well as to assess the nutritional quality and/or safety of crops.

Characterization of ginsenosides from Panax japonicus var. major (Zhu-Zi-Shen) based on ultra-high performance liquid chromatography/quadrupole time-of-flight mass spectrometry and desorption electrospray ionization-mass spectrometry imaging

BACKGROUND

Panax japonicus var. major (PJM) belongs to the well-known ginseng species used in west China for hundreds of years, which has the effects of lung tonifying and yin nourishing, and exerts the analgesic, antitussive, and hemostatic activities. Compared with the other Panax species, the chemical composition and the spatial tissue distribution of the bioactive ginsenosides in PJM have seldom been investigated.

METHODS

Ultra-high performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UHPLC/QTOF-MS) and desorption electrospray ionization-mass spectrometry imaging (DESI-MSI) were integrated for the systematic characterization and spatial tissue distribution studies of ginsenosides in the rhizome of PJM. Considering the great difficulty in exposing the minor saponins, apart from the conventional Auto MS/MS (M1), two different precursor ions list-including data-dependent acquisition (PIL-DDA) approaches, involving the direct input of an in-house library containing 579 known ginsenosides (M2) and the inclusion of the target precursors screened from the MS1 data by mass defect filtering (M3), were developed. The in situ spatial distribution of various ginsenosides in PJM was profiled based on DESI-MSI with a mass range of m/z 100-1500 in the negative ion mode, with the imaging data processed by the High Definition Imaging (HDI) software.

RESULTS

Under the optimized condition, 272 ginsenosides were identified or tentatively characterized, and 138 thereof were possibly not ever reported from the Panax genus. They were composed by 75 oleanolic acid type, 22 protopanaxadiol type, 52 protopanaxatriol type, 16 octillol type, 19 malonylated, 35 C-17 side-chain varied, and 53 others. In addition, the DESI-MSI experiment unveiled the differentiated distribution of saponins, but the main location in the cork layer and phloem of the rhizome. The abundance of the oleanolic acid ginsenosides was high in the rhizome slice of PJM, which was consistent with the results obtained by UHPLC/QTOF-MS.

CONCLUSION

Comprehensive characterization of the ginsenosides in the rhizome of PJM was achieved, with a large amount of unknown structures unveiled primarily. We, for the first time, reported the spatial tissue distribution of different subtypes of ginsenosides in the rhizome slice of PJM. These results can benefit the quality control and further development of PJM and the other ginseng species.

A Critical Overview of HPLC-MS-Based Lipidomics in Determining Triacylglycerol and Phospholipid in Foods

With the current advancement in mass spectrometry (MS)-based lipidomics, the knowledge of lipidomes and their diverse roles has greatly increased, enabling a deeper understanding of the action of bioactive lipid molecules in plant- and animal-based foods. This review provides in-depth information on the practical use of MS techniques in lipidomics, including lipid extraction, adduct formation, MS analysis, data processing, statistical analysis, and bioinformatics. Moreover, this contribution demonstrates the effectiveness of MS-based lipidomics for identifying and quantifying diverse lipid species, especially triacylglycerols and phospholipids, in foods. Further, it summarizes the wide applications of MS-based lipidomics in food science, such as for assessing food processing methods, detecting food adulteration, and measuring lipid oxidation in foods. Thus, MS-based lipidomics may be a useful method for identifying the action of individual lipid species in foods.

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.

Filling gaps in animal welfare assessment through metabolomics

Sustainability has become a central issue in Italian livestock systems driving food business operators to adopt high standards of production concerning animal husbandry conditions. Meat sector is largely involved in this ecological transition with the introduction of new label claims concerning the defense of animal welfare (AW). These new guarantees referred to AW provision require new tools for the purpose of authenticity and traceability to assure meat supply chain integrity. Over the years, European Union (EU) Regulations, national, and international initiatives proposed provisions and guidelines for assuring AW introducing requirements to be complied with and providing tools based on scoring systems for a proper animal status assessment. However, the comprehensive and objective assessment of the AW status remains challenging. In this regard, phenotypic insights at molecular level may be investigated by metabolomics, one of the most recent high-throughput omics techniques. Recent advances in analytical and bioinformatic technologies have led to the identification of relevant biomarkers involved in complex clinical phenotypes of diverse biological systems suggesting that metabolomics is a key tool for biomarker discovery. In the present review, the Five Domains model has been employed as a vademecum describing AW. Starting from the individual Domains-nutrition (I), environment (II), health (III), behavior (IV), and mental state (V)-applications and advances of metabolomics related to AW setting aimed at investigating phenotypic outcomes on molecular scale and elucidating the biological routes most perturbed from external solicitations, are reviewed. Strengths and weaknesses of the current state-of-art are highlighted, and new frontiers to be explored for AW assessment throughout the metabolomics approach are argued. Moreover, a detailed description of metabolomics workflow is provided to understand dos and don'ts at experimental level to pursue effective results. Combining the demand for new assessment tools and meat market trends, a new cross-strategy is proposed as the promising combo for the future of AW assessment.

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.

Visualizing the Distribution of Lipids in Peanut Seeds by MALDI Mass Spectrometric Imaging

Peanut (also called groundnut, Arachis hypogaea L.) seeds are used for producing edible oils and functional foods, and offer a rich source of lipids, proteins and carbohydrates. However, the location of these metabolites has not yet been firmly established. In the present study, the matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI-MSI) technique was applied to investigate spatial distribution of lipids and other key components in seeds of three peanut cultivars (ZH9, KQBH, HP). A total of 103 metabolites, including 34 lipid compounds, were putatively identified by MALDI-MSI. The abundance and spatial distribution of glycerolipids (GLs) and glycerophospholipids (GPs) were compared among the three peanut cultivars. All the identified lysophosphatidylcholine (LPC), phosphatidylethanolamine (PE) and phosphatidylcholines (PCs) were distributed mainly in the inner part of seeds. The visualization of phosphatidic acids (PAs) and triacylglycerols (TGs) revealed a dramatic metabolic heterogeneity between the different tissues making up the seed. The non-homogeneous spatial distribution of metabolites appeared to be related to the different functions of particular tissue regions. These results indicated that MALDI-MSI could be useful for investigating the lipids of foodstuffs from a spatial perspective. The present study may contribute to the development of oil crops with higher oil yields, and to improvement of food processing.

Toward Depth-Resolved Analysis of Plant Metabolites by Nanospray Desorption Electrospray Ionization Mass Spectrometry

Nanospray desorption electrospray ionization (nano-DESI) is one of the ambient desorption ionization methods for mass spectrometry (MS), and it utilizes a steady-state liquid junction formed between two microcapillaries to directly extract analytes from sample surfaces with minimal sample damage. In this study, we employed nano-DESI MS to perform a metabolite fingerprinting analysis directly from a Hypericum leaf surface. Moreover, we investigated whether changes in metabolite fingerprints with time can be related to metabolite distribution according to depth. From a raw Hypericum leaf, the mass spectral fingerprints of key metabolites, including flavonoids and prenylated phloroglucinols, were successfully obtained using ethanol as a nano-DESI solvent, and the changes in their intensities were observed with time via full mass scan experiments. In addition, the differential extraction patterns of the obtained mass spectral fingerprints were clearly visualized over time through selected ion monitoring and pseudo-selected reaction monitoring experiments. To examine the correlation between the time-dependent changes in the metabolite fingerprints and depth-wise metabolite distribution, we performed a nano-DESI MS analysis against leaves whose surface layers were removed multiple times by forming polymeric gum Arabic films on their surfaces, followed by detaching. The preliminary results showed that the changes in the metabolite fingerprints according to the number of peelings showed a similar pattern with those obtained from the raw leaves over time.

MALDI mass spectrometry imaging: From constituents in fresh food to ingredients, contaminants and additives in processed food

Mass Spectrometry imaging (MS imaging) provides spatial information for a wide range of compound classes in different sample matrices. We used MS imaging to investigate the distribution of components in fresh and processed food, including meat, dairy and bakery products. The MS imaging workflow was optimized to cater to the specific properties and challenges of the individual samples. We successfully detected highly nonpolar and polar constituents such as beta-carotene and anthocyanins, respectively. For the first time, the distributions of a contaminant and a food additive were visualized in processed food. We detected acrylamide in German gingerbread and investigated the penetration of the preservative natamycin into cheese. For this purpose, a new data analysis tool was developed to study the penetration of analytes from uneven surfaces. Our results show that MS imaging has great potential in food analysis to provide relevant information about components' distributions, particularly those underlying official regulations.

Mass Spectrometry Imaging Spatial Tissue Analysis toward Personalized Medicine

Novel profiling methodologies are redefining the diagnostic capabilities and therapeutic approaches towards more precise and personalized healthcare. Complementary information can be obtained from different omic approaches in combination with the traditional macro- and microscopic analysis of the tissue, providing a more complete assessment of the disease. Mass spectrometry imaging, as a tissue typing approach, provides information on the molecular level directly measured from the tissue. Lipids, metabolites, glycans, and proteins can be used for better understanding imbalances in the DNA to RNA to protein translation, which leads to aberrant cellular behavior. Several studies have explored the capabilities of this technology to be applied to tumor subtyping, patient prognosis, and tissue profiling for intraoperative tissue evaluation. In the future, intercenter studies may provide the needed confirmation on the reproducibility, robustness, and applicability of the developed classification models for tissue characterization to assist in disease management.

Mass Spectrometry Imaging for Spatial Chemical Profiling of Vegetative Parts of Plants

The detection of chemical species and understanding their respective localisations in tissues have important implications in plant science. The conventional methods for imaging spatial localisation of chemical species are often restricted by the number of species that can be identified and is mostly done in a targeted manner. Mass spectrometry imaging combines the ability of traditional mass spectrometry to detect numerous chemical species in a sample with their spatial localisation information by analysing the specimen in a 2D manner. This article details the popular mass spectrometry imaging methodologies which are widely pursued along with their respective sample preparation and the data analysis methods that are commonly used. We also review the advancements through the years in the usage of the technique for the spatial profiling of endogenous metabolites, detection of xenobiotic agrochemicals and disease detection in plants. As an actively pursued area of research, we also address the hurdles in the analysis of plant tissues, the future scopes and an integrated approach to analyse samples combining different mass spectrometry imaging methods to obtain the most information from a sample of interest.

The application of mass spectrometry imaging in traditional Chinese medicine: a review

Mass spectrometry imaging is a frontier technique which connects classical mass spectrometry with ion imaging. Various types of chemicals could be visualized in their native tissues using mass spectrometry imaging. Up to now, the most commonly applied mass spectrometry imaging techniques are matrix assisted laser desorption ionization mass spectrometry imaging, desorption electrospray ionization mass spectrometry imaging and secondary ion mass spectrometry imaging. This review gives an introduction to the principles, development and applications of commonly applied mass spectrometry imaging techniques, and then illustrates the application of mass spectrometry imaging in the investigation of traditional Chinese medicine. Recently, mass spectrometry imaging has been adopted to explore the spatial distribution of endogenous metabolites in traditional Chinese medicine. Data collected from mass spectrometry imaging can be further utilized to search for marker components of traditional Chinese medicine, discover new compounds from traditional herbs, and differentiate between medicinal plants that are similar in botanical features. Moreover, mass spectrometry imaging also plays a role in revealing the pharmacological and toxicological mechanisms of traditional Chinese medicine.

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.

Highest resolution chemical imaging based on secondary ion mass spectrometry performed on the helium ion microscope

This paper is a review on the combination between Helium Ion Microscopy (HIM) and Secondary Ion Mass Spectrometry (SIMS), which is a recently developed technique that is of particular relevance in the context of the quest for high-resolution high-sensitivity nano-analytical solutions. We start by giving an overview on the HIM-SIMS concept and the underlying fundamental principles of both HIM and SIMS. We then present and discuss instrumental aspects of the HIM and SIMS techniques, highlighting the advantage of the integrated HIM-SIMS instrument. We give an overview on the performance characteristics of the HIM-SIMS technique, which is capable of producing elemental SIMS maps with lateral resolution below 20 nm, approaching the physical resolution limits, while maintaining a sub-nanometric resolution in the secondary electron microscopy mode. In addition, we showcase different strategies and methods allowing to take profit of both capabilities of the HIM-SIMS instrument (high-resolution imaging using secondary electrons and mass filtered secondary sons) in a correlative approach. Since its development HIM-SIMS has been successfully applied to a large variety of scientific and technological topics. Here, we will present and summarise recent applications of nanoscale imaging in materials research, life sciences and geology.

Development of a Matrix Sublimation Device with Controllable Crystallization Temperature for MALDI Mass Spectrometry Imaging

The size and distribution of matrix crystals deposited on the surface of a tissue section play a key role in the performance of MALDI mass spectrometry imaging (MALDI-MSI). In this study, uniform distribution and a restricted size of matrix crystals were achieved via a homemade matrix sublimation device with controllable crystallization temperature. The crystallization temperature was stably controlled at a subzero temperature, and homogeneous matrix crystals with diameters <0.2 μm were generated on the sample surface. Typical MALDI-MSI experiments of endogenous and exogenous components in the tissues of strawberries, kidneys, and mussels were conducted to examine the performance of the sublimator. Good reproducibility of MALDI-MSI was achieved, and the quality of ion images was significantly improved. These results demonstrate that the developed sublimator should have potential in matrix deposition for further high resolution MALDI-MSI application.

Novel technical developments in mass spectrometry imaging in 2020: A mini review

The applicability of mass spectrometry imaging (MSI) has exponentially increased with the improvement of sample preparation, instrumentation (spatial resolution) and data analysis. The number of MSI publications listed in PubMed continues to grow with 378 published articles in 2020-2021. Initially, MSI was just sensitive enough to identify molecular features correlating with distinct tissue regions, similar to the resolution achieved by visual inspection after standard immunohistochemical staining. Although the spatial resolution was limited compared with other imaging modalities, the molecular intensity mapping added a new exciting capability. Over the past decade, significant improvements in every step of the workflow and most importantly in instrumentation were made, which now enables the molecular analysis at a cellular and even subcellular level. Here, we summarize the latest developments in MSI, with a focus on the latest approaches for tissue-based imaging described in 2020.

A trip of peptides to the brain

Abstract

Dietary di/tripeptides elicit preventive effects against lifestyle-related diseases such as hypertension, and hypercholesterolemia, etc. Although there have been evidential reports that the intake of protein hydrolysate improved impaired memory in human, limited studies on bioavailability, in particular, beyond the blood-brain barrier (BBB) of candidates in hydrolysate may prevent their extensive physiological studies. Thus, this review discusses the updated studies on BBB transport of peptides showing improved cognitive decline. Furthermore, their accumulation in the brain cerebral parenchyma is also introduced.

Graphical abstract

Mass Spectrometry Imaging of Flavonols and Ellagic Acid Glycosides in Ripe Strawberry Fruit

Flavonols and ellagic acid glycosides are major phenolic compounds in strawberry fruit. They have antioxidant activity, show protective functions against abiotic and biotic stress, and provide health benefits. However, their spatial distribution in ripe fruit has not been understood. Therefore, matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry imaging (MSI) was performed to investigate their distribution in fruit tissues. Using strawberry extract, five flavonols, namely, three kaempferols and two quercetins, and two ellagic acid glycosides, were tentatively identified by MALDI-tandem MS. To investigate the tentatively identified compounds, MALDI-MSI and tandem MS imaging (MS/MSI) analyses were performed. Kaempferol and quercetin glycosides showed similar distribution patterns. They were mainly found in the epidermis, while ellagic acid glycosides were mainly found in the achene and in the bottom area of the receptacle. These results suggested that the difference in distribution pattern between flavonols and ellagic acid glycosides depends on the difference between their aglycones. Seemingly, flavonols play a role in protective functions in the epidermis, while ellagic acid glycosides play a role in the achene and in the bottom side of the receptacle, respectively. These results demonstrated that MALDI-MSI is useful for distribution analysis of flavonols and ellagic acid glycosides in strawberry fruit.