Automated Ambient Desorption−Ionization Platform for Surface Imaging Integrated with a Commercial Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

Ambient ionization mass spectrometry provides screening of selective androgen receptor modulators

Ambient ionization mass spectrometry allows for analysis of samples in their natural state, i.e., with no sample pre-treatment. It can be viewed as a fast, simple, and economical analysis, but its main disadvantages include a lower analytical performance due to the presence of complex sample matrix and the lack of chromatographic separation prior to the introduction of the sample into the mass spectrometer. Here we present an application of two ambient ionization mass spectrometry techniques, i.e., Desorption Atmospheric Pressure Photoionization and Dielectric Barrier Discharge Ionization, for the analysis of known Selective Androgen Receptor Modulators, which represent common compounds of abuse in professional and semiprofessional sport. Eight real samples of illegal food supplements, seized by the local law enforcement, were used to test the performance of the ambient mass spectrometry and the results were validated against a newly developed targeted LC-UV-MS/MS method performed in multiple reaction monitoring mode with an external calibration for each analyte. In order to decide whether or not the compound can be declared as present, we proposed a system of rules for the interpretation of the obtained spectra. The criteria are based on mass spectrum matching (5-10 ppm accuracy from the theoretical exact mass and a correct isotopic pattern), duration of the mass signal (three or five consecutive scans, depending on the instrumentation used), and intensity above the background noise (threefold increase in intensity and absolute intensity above 5E4 or 1E5, depending on the instrumentation). When applying these criteria, good agreement was found between the tested methods. Ambient ionization techniques were effective at detecting SARMs at pharmacologically relevant doses, i.e., approximately above 1 mg per capsule, although they may fail to detect lower levels or isomeric species. It is demonstrated that when adhering to a set of clear and consistent rules, ambient mass spectrometry can be employed as a qualitative technique for the screening of illegal SARMs with sufficient confidence and without the necessity to perform a regular LC-MS analysis.

Mass Spectrometry Imaging Techniques: Non-Ambient and Ambient Ionization Approaches

Molecular information can be acquired from sample surfaces in real time using a revolutionary molecular imaging technique called mass spectrometry imaging (MSI). The technique can concurrently provide high spatial resolution information on the spatial distribution and relative proportion of many different compounds. Thus, many scientists have been drawn to the innovative capabilities of the MSI approach, leading to significant focus in various fields during the past few decades. This review describes the sampling protocol, working principle and applications of a few non-ambient and ambient ionization mass spectrometry imaging techniques. The non-ambient techniques include secondary ionization mass spectrometry and matrix-assisted laser desorption ionization, while the ambient techniques include desorption electrospray ionization, laser ablation electrospray ionization, probe electro-spray ionization, desorption atmospheric pressure photo-ionization and femtosecond laser desorption ionization. The review additionally addresses the advantages and disadvantages of ambient and non-ambient MSI techniques in relation to their suitability, particularly for biological samples used in tissue diagnostics. Last but not least, suggestions and conclusions are made regarding the challenges and future prospects of MSI.

Development of mass spectrometry imaging techniques and its latest applications

Mass spectrometry imaging (MSI) is a novel molecular imaging technology that collects molecular information from the surface of samples in situ. The spatial distribution and relative content of various compounds can be visualized simultaneously with high spatial resolution. The prominent advantages of MSI promote the active development of ionization technology and its broader applications in diverse fields. This article first gives a brief introduction to the vital parts of the processes during MSI. On this basis, provides a comprehensive overview of the most relevant MS-based imaging techniques from their mechanisms, pros and cons, and applications. In addition, a critical issue in MSI, matrix effects is also discussed. Then, the representative applications of MSI in biological, forensic, and environmental fields in the past 5 years have been summarized, with a focus on various types of analytes (e.g., proteins, lipids, polymers, etc.) Finally, the challenges and further perspectives of MSI are proposed and concluded.

Streamlined Multimodal DESI and MALDI Mass Spectrometry Imaging on a Singular Dual-Source FT-ICR Mass Spectrometer

The study of biological specimens by mass spectrometry imaging (MSI) has had a profound influence in the various forms of spatial-omics over the past two decades including applications for the identification of clinical biomarker analysis; the metabolic fingerprinting of disease states; treatment with therapeutics; and the profiling of lipids, peptides and proteins. No singular approach is able to globally map all biomolecular classes simultaneously. This led to the development of many complementary multimodal imaging approaches to solve analytical problems: fusing multiple ionization techniques, imaging microscopy or spectroscopy, or local extractions into robust multimodal imaging methods. However, each fusion typically requires the melding of analytical information from multiple commercial platforms, and the tandem utilization of multiple commercial or third-party software platforms—even in some cases requiring computer coding. Herein, we report the use of matrix-assisted laser desorption/ionization (MALDI) in tandem with desorption electrospray ionization (DESI) imaging in the positive ion mode on a singular commercial orthogonal dual-source Fourier transform ion cyclotron resonance (FT-ICR) instrument for the complementary detection of multiple analyte classes by MSI from tissue. The DESI source was 3D printed and the commercial Bruker Daltonics software suite was used to generate mass spectrometry images in tandem with the commercial MALDI source. This approach allows for the generation of multiple modes of mass spectrometry images without the need for third-party software and a customizable platform for ambient ionization imaging. Highlighted is the streamlined workflow needed to obtain phospholipid profiles, as well as increased depth of coverage of both annotated phospholipid, cardiolipin, and ganglioside species from rat brain with both high spatial and mass resolution.

Sphingolipids in Obesity and Correlated Co-Morbidities: The Contribution of Gender, Age and Environment

This paper reviews our present knowledge on the contribution of ceramide (Cer), sphingomyelin (SM), dihydroceramide (DhCer) and sphingosine-1-phosphate (S1P) in obesity and related co-morbidities. Specifically, in this paper, we address the role of acyl chain composition in bodily fluids for monitoring obesity in males and females, in aging persons and in situations of environmental hypoxia adaptation. After a brief introduction on sphingolipid synthesis and compartmentalization, the node of detection methods has been critically revised as the node of the use of animal models. The latter do not recapitulate the human condition, making it difficult to compare levels of sphingolipids found in animal tissues and human bodily fluids, and thus, to find definitive conclusions. In human subjects, the search for putative biomarkers has to be performed on easily accessible material, such as serum. The serum “sphingolipidome” profile indicates that attention should be focused on specific acyl chains associated with obesity, per se, since total Cer and SM levels coupled with dyslipidemia and vitamin D deficiency can be confounding factors. Furthermore, exposure to hypoxia indicates a relationship between dyslipidemia, obesity, oxygen level and aerobic/anaerobic metabolism, thus, opening new research avenues in the role of sphingolipids.

Increased throughput and ultra-high mass resolution in DESI FT-ICR MS imaging through new-generation external data acquisition system and advanced data processing approaches

Desorption electrospray ionisation-mass spectrometry imaging (DESI-MSI) is a powerful imaging technique for the analysis of complex surfaces. However, the often highly complex nature of biological samples is particularly challenging for MSI approaches, as options to appropriately address molecular complexity are limited. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) offers superior mass accuracy and mass resolving power, but its moderate throughput inhibits broader application. Here we demonstrate the dramatic gains in mass resolution and/or throughput of DESI-MSI on an FT-ICR MS by developing and implementing a sophisticated data acquisition and data processing pipeline. The presented pipeline integrates, for the first time, parallel ion accumulation and detection, post-processing absorption mode Fourier transform and pixel-by-pixel internal re-calibration. To achieve that, first, we developed and coupled an external high-performance data acquisition system to an FT-ICR MS instrument to record the time-domain signals (transients) in parallel with the instrument’s built-in electronics. The recorded transients were then processed by the in-house developed computationally-efficient data processing and data analysis software. Importantly, the described pipeline is shown to be applicable even to extremely large, up to 1 TB, imaging datasets. Overall, this approach provides improved analytical figures of merits such as: (i) enhanced mass resolution at no cost in experimental time; and (ii) up to 4-fold higher throughput while maintaining a constant mass resolution. Using this approach, we not only demonstrate the record 1 million mass resolution for lipid imaging from brain tissue, but explicitly show such mass resolution is required to resolve the complexity of the lipidome.

Desorption Electrospray Ionization Coupled with Ultraviolet Photodissociation for Characterization of Phospholipid Isomers in Tissue Sections

Desorption electrospray ionization (DESI) mass spectrometry imaging has become a powerful strategy for analysis of tissue sections, enabling differentiation of normal and diseased tissue based on changes in the lipid profiles. The most common DESI workflow involves collection of MS1 spectra as the DESI spray is rastered over a tissue section. Relying on MS1 spectra inherently limits the ability to differentiate isobaric and isomeric species or evaluate variations in the relative abundances of key isomeric lipids, such as double-bond positional isomers which may distinguish normal and diseased tissues. Here, 193 nm ultraviolet photodissociation (UVPD), a technique capable of differentiating double-bond positional isomers, is coupled with DESI to map differences in the double-bond isomer composition in tissue sections in a fast, high throughput manner compatible with imaging applications.

Recent developments in atmospheric pressure photoionization-mass spectrometry

Recent developments in atmospheric pressure photoionization (APPI), which is one of the three most important ionization techniques in liquid chromatography-mass spectrometry, are reviewed. The emphasis is on the practical aspects of APPI analysis, its combination with different separation techniques, novel instrumental developments - especially in gas chromatography and ambient mass spectrometry - and the applications that have appeared in 2009-2014. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:423-449, 2017.

Towards matrix-free femtosecond-laser desorption mass spectrometry for in situ space research

RATIONALE

There is an increasing interest in the quest for low molecular weight biomarkers that can be studied on extra-terrestrial objects by direct laser desorption mass spectrometry (LD-MS). Although molecular structure investigations have recently been carried out by direct LD-MS approaches, there is still a lack of suitable instruments for implementation on a spacecraft due to weight, size and power consumption demands. In this contribution we demonstrate the feasibility of LD-MS structural analysis of molecular species by a miniature laser desorption-ionization mass spectrometer (instrument name LMS) originally designed for in situ elemental and isotope analysis of solids in space research.

METHODS

Direct LD-MS studies with molecular resolution were carried out by means of a Laser Ablation/Ionization Mass Spectrometry (LIMS) technique. Two polymer samples served as model systems: neutral polyethylene glycol (PEG) and cationic polymerizates of imidazole and epichlorohydrin (IMEP). Optimal conditions for molecular fragmentation could be identified for both polymers by tuning the laser energy and the instrument-sample distance.

RESULTS

PEG and IMEP polymers show sufficient stability over a relatively wide laser energy range. Under mild LD conditions only moderate fragmentation of the polymers takes place so that valuable structural characterization based on fragment ions can be achieved. As the applied laser pulse energy rises, the abundance of fragment ions increases, reaches a plateau and subsequently drops down due to more severe fragmentation and atomization of the polymers. At this final stage, usually referred to as laser ablation, only elemental/isotope analysis can be achieved.

CONCLUSIONS

Our investigations demonstrate the versatility of the LMS instrument that can be tuned to favourable laser desorption conditions that successfully meet molecule-specific requirements and deliver abundant fragment ion signals with detailed structural information. Overall, the results show promise for use in similar studies on planetary surfaces beyond Earth where no or minimal sample preparation is essential.

Mass spectrometry imaging for visualizing organic analytes in food

The demand for rapid chemical imaging of food products steadily increases. Mass spectrometry (MS) is featured by excellent molecular specificity of analysis and is, therefore, a very attractive method for chemical profiling. MS for food imaging has increased significantly over the past decade, aided by the emergence of various ambient ionization techniques that allow direct and rapid analysis in ambient environment. In this article, the current status of food imaging with MSI is reviewed. The described approaches include matrix-assisted laser desorption/ionization (MALDI), but emphasize desorption atmospheric pressure photoionization (DAPPI), electrospray-assisted laser desorption/ionization (ELDI), probe electrospray ionization (PESI), surface desorption atmospheric pressure chemical ionization (SDAPCI), and laser ablation flowing atmospheric pressure afterglow (LA-FAPA). The methods are compared with regard to spatial resolution; analysis speed and time; limit of detection; and technical aspects. The performance of each method is illustrated with the description of a related application. Specific requirements in food imaging are discussed.

Ambient in situ analysis and imaging of both hydrophilic and hydrophobic thin layer chromatography plates by electrostatic spray ionization mass spectrometry

Direct coupling of ESTASI-MS with both hydrophilic and hydrophobic TLC for ambient in situ analysis and imaging with ultralow sample consumption.

Laser ablation atmospheric pressure photoionization mass spectrometry imaging of phytochemicals from sage leaves

RATIONALE

Despite fast advances in ambient mass spectrometry imaging (MSI), the study of neutral and nonpolar compounds directly from biological matrices remains challenging. In this contribution, we explore the feasibility of laser ablation atmospheric pressure photoionization (LAAPPI) for MSI of phytochemicals in sage (Salvia officinalis) leaves.

METHODS

Sage leaves were studied by LAAPPI-time-of-flight (TOF)-MSI without any sample preparation. Leaf mass spectra were also recorded with laser ablation electrospray ionization (LAESI) mass spectrometry and the spectra were compared with those obtained by LAAPPI.

RESULTS

Direct probing of the plant tissue by LAAPPI efficiently produced ions from plant metabolites, including neutral and nonpolar terpenes that do not have polar functional groups, as well as oxygenated terpene derivatives. Monoterpenes and monoterpenoids could also be studied from sage by LAESI, but only LAAPPI was able to detect larger nonpolar compounds, such as sesquiterpenes and triterpenoid derivatives, from the leaf matrix. Alternative MSI methods for nonpolar compounds, such as desorption atmospheric pressure photoionization (DAPPI), do not achieve as good spatial resolution as LAAPPI (<400 µm).

CONCLUSIONS

We show that MSI with LAAPPI is a useful tool for concurrently studying the distribution of polar and nonpolar compounds, such as phytochemicals, directly from complex biological samples, and it can provide information that is not available by other, established methods.

Mass spectrometry imaging of surface lipids on intact Drosophila melanogaster flies

The spatial distribution of neutral lipids and semiochemicals on the surface of six-day-old separately reared naive Drosophila melanogaster flies has been visualized and studied using matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry and laser-assisted desorption/ionization (LDI)-TOF imaging (MSI). Metal targets were designed for two-dimensional MSI of the surface of 3-D biological objects. Targets with either simple grooves or profiled holes designed to accurately accommodate the male and female bodies were fabricated. These grooves and especially holes ensured correct height fixation and spatial orientation of the flies on the targets after matrix application and sample drying. For LDI-TOF to be used, the flies were arranged into holes and fixed to a plane of the target using fast-setting glue. In MALDI-TOF mode, the flies were fixed as above and sprayed with a lithium 2,5-dihydroxybenzoate matrix using up to 100 airbrush spray cycles. The scanning electron microscopy images revealed that the deposits of matrix were homogenous and the matrix formed mostly into the clusters of crystals (40-80 µm) that were separated from each other by an uncovered cuticle surface (30-40 µm). The MSI using target with profiled holes provided superior results to the targets with simple grooves, eliminating the ion suppression/mass deviation due to the 3-D shape of the flies. Attention was paid to neutral lipids and other compounds including the male anti-attractant 11-cis-vaccenyl acetate for which the expected distribution with high concentration on the tip of the male abdomen was confirmed. The red and blue mass shift (PlusMinus1 colour scale) was observed associated with mass deviation predominantly between ±0.2 and 0.3 Da. We use in-house developed software for mass recalibration, to eliminate the mass deviation effects and help with the detection of low-intensity mass signals.

Electrostatic spray ionization mass spectrometry imaging

Imaging samples on a surface by mass spectrometry (MS) requires the combination of MS detection with a scanning mode that enables localized desorption and ionization and/or detection of sample analytes with good spatial resolution. We have developed a new mass spectrometry imaging (MSI) method based on electrostatic spray ionization. It works under ambient conditions and can be applied to a wide range of molecules providing quantitative MS analysis even in the presence of salts in excess. 2D MS images of protein and peptide spots, inkjet-printed black dye patterns, and cells were obtained. The presented novel ambient ionization mass spectrometry imaging method can find many applications in analytical and bioanalytical chemistry.

Mass spectrometry imaging of plant metabolites--principles and possibilities

Covering: up to the end of 2013 New mass spectrometry imaging (MSI) techniques are gaining importance in the analysis of plant metabolite distributions, and significant technological improvements have been introduced in the past decade. This review provides an introduction to the different MSI techniques and their applications in plant science. The most common methods for sample preparation are described, and the review also features a comprehensive table of published studies in MSI of plant material. A number of significant works are highlighted for their contributions to advance the understanding of plant biology through applications of plant metabolite imaging. Particular attention is given to the possibility for imaging of surface metabolites since this is highly dependent on the methods and techniques which are applied in imaging studies.

Mass spectrometry imaging, an emerging technology in neuropsychopharmacology

Mass spectrometry imaging is a powerful tool for directly determining the distribution of proteins, peptides, lipids, neurotransmitters, metabolites and drugs in neural tissue sections in situ. Molecule-specific imaging can be achieved using various ionization techniques that are suited to different applications but which all yield data with high mass accuracies and spatial resolutions. The ability to simultaneously obtain images showing the distributions of chemical species ranging from metal ions to macromolecules makes it possible to explore the chemical organization of a sample and to correlate the results obtained with specific anatomical features. The imaging of biomolecules has provided new insights into multiple neurological diseases, including Parkinson's and Alzheimer's disease. Mass spectrometry imaging can also be used in conjunction with other imaging techniques in order to identify correlations between changes in the distribution of important chemical species and other changes in the properties of the tissue. Here we review the applications of mass spectrometry imaging in neuroscience research and discuss its potential. The results presented demonstrate that mass spectrometry imaging is a useful experimental method with diverse applications in neuroscience.

Mass spectrometry imaging under ambient conditions

Mass spectrometry imaging (MSI) has emerged as an important tool in the last decade and it is beginning to show potential to provide new information in many fields owing to its unique ability to acquire molecularly specific images and to provide multiplexed information, without the need for labeling or staining. In MSI, the chemical identity of molecules present on a surface is investigated as a function of spatial distribution. In addition to now standard methods involving MSI in vacuum, recently developed ambient ionization techniques allow MSI to be performed under atmospheric pressure on untreated samples outside the mass spectrometer. Here we review recent developments and applications of MSI emphasizing the ambient ionization techniques of desorption electrospray ionization (DESI), laser ablation electrospray ionization (LAESI), probe electrospray ionization (PESI), desorption atmospheric pressure photoionization (DAPPI), femtosecond laser desorption ionization (fs-LDI), laser electrospray mass spectrometry (LEMS), infrared laser ablation metastable-induced chemical ionization (IR-LAMICI), liquid microjunction surface sampling probe mass spectrometry (LMJ-SSP MS), nanospray desorption electrospray ionization (nano-DESI), and plasma sources such as the low temperature plasma (LTP) probe and laser ablation coupled to flowing atmospheric-pressure afterglow (LA-FAPA). Included are discussions of some of the features of ambient MSI for example the ability to implement chemical reactions with the goal of providing high abundance ions characteristic of specific compounds of interest and the use of tandem mass spectrometry to either map the distribution of targeted molecules with high specificity or to provide additional MS information on the structural identification of compounds. We also describe the role of bioinformatics in acquiring and interpreting the chemical and spatial information obtained through MSI, especially in biological applications for tissue diagnostic purposes. Finally, we discuss the challenges in ambient MSI and include perspectives on the future of the field.

Surface analysis of lipids by mass spectrometry: more than just imaging

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age. The recent explosion of mass spectrometry-based surface analysis techniques is fuelling further expansion of the lipidomics field. This is evidenced by the numerous papers published on the subject of mass spectrometric imaging of lipids in recent years. While imaging mass spectrometry provides new and exciting possibilities, it is but one of the many opportunities direct surface analysis offers the lipid researcher. In this review we describe the current state-of-the-art in the direct surface analysis of lipids with a focus on tissue sections, intact cells and thin-layer chromatography substrates. The suitability of these different approaches towards analysis of the major lipid classes along with their current and potential applications in the field of lipid analysis are evaluated.

Infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) imaging source coupled to a FT-ICR mass spectrometer

Mass spectrometry imaging (MSI) allows for the direct monitoring of the abundance and spatial distribution of chemical compounds over the surface of a tissue sample. This technology has opened the field of mass spectrometry to numerous innovative applications over the past 15 years. First used with SIMS and MALDI MS that operate under vacuum, interest has grown for mass spectrometry ionization sources that allow for effective imaging but where the analysis can be performed at ambient pressure with minimal or no sample preparation. We introduce here a versatile source for MALDESI imaging analysis coupled to a hybrid LTQ-FT-ICR mass spectrometer. The imaging source offers single shot or multi-shot capability per pixel with full control over the laser repetition rate and mass spectrometer scanning cycle. Scanning rates can be as fast as 1 pixel/second and a spatial resolution of 45 μm was achieved with oversampling. Design and integration of a versatile IR-MALDESI imaging source offering multi-shot capability with a commercial FT-ICR mass spectrometer.

Advanced mass calibration and visualization for FT-ICR mass spectrometry imaging

Mass spectrometry imaging by Fourier transform ion cyclotron resonance (FT-ICR) yields hundreds of unique peaks, many of which cannot be resolved by lower performance mass spectrometers. The high mass accuracy and high mass resolving power allow confident identification of small molecules and lipids directly from biological tissue sections. Here, calibration strategies for FT-ICR MS imaging were investigated. Sub-parts-per-million mass accuracy is demonstrated over an entire tissue section. Ion abundance fluctuations are corrected by addition of total and relative ion abundances for a root-mean-square error of 0.158 ppm on 16,764 peaks. A new approach for visualization of FT-ICR MS imaging data at high resolution is presented. The "Mosaic Datacube" provides a flexible means to visualize the entire mass range at a mass spectral bin width of 0.001 Da. The high resolution Mosaic Datacube resolves spectral features not visible at lower bin widths, while retaining the high mass accuracy from the calibration methods discussed.

In-situ enrichment of phosphopeptides on MALDI plates modified by ambient ion landing

We report substantial in-situ enrichment of phosphopeptides in peptide mixtures using titanium and zirconium dioxide-coated matrix assisted laser desorption-ionization (MALDI) plates prepared by recently reported ambient ion landing deposition technique. The technique was able to modify four common materials currently used for MALDI targets (stainless steel, aluminum, indium-tin oxide glass and polymeric anchor chip). The structure of the deposited dioxide was investigated by electron microscopy, and different surfaces were compared and discussed in this study. Two standard proteins were used to test the enrichment capabilities of modified MALDI plates: casein and in-vitro phosphorylated trehalase. The enrichment of casein tryptic digest resulted in identification of 20 phosphopeptides (including miscleavages). Trehalase was used as a suitable model of larger protein that provided more complex peptide mixture after the trypsin digestion. All four possible phosphorylation sites in trehalase were identified and up to seven phosphopetides were found (including methionine oxidations and miscleavages). Two different mass spectrometers, MALDI-Fourier transform ion cyclotron resonance (FTICR) and MALDI-time of flight, were used to detect the phosphopeptides from modified MALDI plates after the enrichment procedure. It was observed that the desorption-ionization phenomena on the modified surfaces are not critically influenced by the parameters of the different MALDI ion sources (e.g. different pressure, different extraction voltages), and thus the presence of dioxide layer on the standard MALDI plate does not significantly interfere with the main MALDI processes. The detection of phosphopeptides after the enrichment could be done by both instruments. Desorption electrospray ionization coupled to the FTICR was also tested, but, unlike MALDI, it did not provide satisfactory results.

Analysis of lipids with desorption atmospheric pressure photoionization-mass spectrometry (DAPPI-MS) and desorption electrospray ionization-mass spectrometry (DESI-MS)

In this article, the effect of spray solvent on the analysis of selected lipids including fatty acids, fat-soluble vitamins, triacylglycerols, steroids, phospholipids, and sphingolipids has been studied by two different ambient mass spectrometry (MS) methods, desorption electrospray ionization-MS (DESI-MS) and desorption atmospheric pressure photoionization-MS (DAPPI-MS). The ionization of the lipids with DESI and DAPPI was strongly dependent on the spray solvent. In most cases, the lipids were detected as protonated or deprotonated molecules; however, other ions were also formed, such as adduct ions (in DESI), [M-H](+) ions (in DESI and DAPPI), radical ions (in DAPPI), and abundant oxidation products (in DESI and DAPPI). DAPPI provided efficient desorption and ionization for neutral and less polar as well as for ionic lipids but caused extensive fragmentation for larger and more labile compounds because of a thermal desorption process. DESI was more suitable for the analysis of the large and labile lipids, but the ionization efficiency for less polar lipids was poor. Both methods were successfully applied to the direct analysis of lipids from pharmaceutical and food products. Although DESI and DAPPI provide efficient analysis of lipids, the multiple and largely unpredictable ionization reactions may set challenges for routine lipid analysis with these methods.

Emerging technologies in mass spectrometry imaging

Mass spectrometry imaging (MSI) as an analytical tool for bio-molecular and bio-medical research targets accurate compound localization and identification. In terms of dedicated instrumentation, this translates into the demand for more detail in the image dimension (spatial resolution) and in the spectral dimension (mass resolution and accuracy), preferably combined in one instrument. At the same time, large area biological tissue samples require fast acquisition schemes, instrument automation and a robust data infrastructure. This review discusses the analytical capabilities of an "ideal" MSI instrument for bio-molecular and bio-medical molecular imaging. The analytical attributes of such an ideal system are contrasted with technological and methodological challenges in MSI. In particular, innovative instrumentation for high spatial resolution imaging in combination with high sample throughput is discussed. Detector technology that targets various shortcomings of conventional imaging detector systems is highlighted. The benefits of accurate mass analysis, high mass resolving power, additional separation strategies and multimodal three-dimensional data reconstruction algorithms are discussed to provide the reader with an insight in the current technological advances and the potential of MSI for bio-medical research.

Direct plant tissue analysis and imprint imaging by desorption electrospray ionization mass spectrometry

The ambient mass spectrometry technique, desorption electrospray ionization mass spectrometry (DESI-MS), is applied for the rapid identification and spatially resolved relative quantification of chlorophyll degradation products in complex senescent plant tissue matrixes. Polyfunctionalized nonfluorescent chlorophyll catabolites (NCCs), the "final" products of the chlorophyll degradation pathway, are detected directly from leaf tissues within seconds and structurally characterized by tandem mass spectrometry (MS/MS) and reactive-DESI experiments performed in situ. The sensitivity of DESI-MS analysis of these compounds from degreening leaves is enhanced by the introduction of an imprinting technique. Porous polytetrafluoroethylene (PTFE) is used as a substrate for imprinting the leaves, resulting in increased signal intensities compared with those obtained from direct leaf tissue analysis. This imprinting technique is used further to perform two-dimensional (2D) imaging mass spectrometry by DESI, producing well-resolved images of the spatial distribution of NCCs in senescent leaf tissues.

An external matrix-assisted laser desorption ionization source for flexible FT-ICR Mass spectrometry imaging with internal calibration on adjacent samples

We describe the construction and application of a new MALDI source for FT-ICR mass spectrometry imaging. The source includes a translational X-Y positioning stage with a 10×10 cm range of motion for analysis of large sample areas, a quadrupole for mass selection, and an external octopole ion trap with electrodes for the application of an axial potential gradient for controlled ion ejection. An off-line LC MALDI MS/MS run demonstrates the utility of the new source for data- and position-dependent experiments. A FT-ICR MS imaging experiment of a coronal rat brain section yields ∼200 unique peaks from m/z 400-1100 with corresponding mass-selected images. Mass spectra from every pixel are internally calibrated with respect to polymer calibrants collected from an adjacent slide.

Desorption electrospray ionization tissue imaging using heated nebulizing gas and high-resolution accurate mass spectra

A new method for tissue imaging using desorption electrospray ionization (DESI) mass spectrometry is described. The technique utilizes a DESI source with a heated nebulizing gas and high-resolution accurate mass data acquired with an LTQ-Orbitrap mass spectrometer. The two-dimensional (2D) automated DESI ion source creates images using the ions that are collected under high-resolution conditions. The use of high-resolution mass detection significantly improves the image quality due to exclusion of interfering ions. The use of a heated nebulizing gas increases the signal intensity observed at lower gas pressure. The technique developed is highly compatible with soft tissue imaging due to the minimal surface destruction.