Laserspray ionization imaging of multiply charged ions using a commercial vacuum MALDI ion source

New Processes for Ionizing Nonvolatile Compounds in Mass Spectrometry: The Road of Discovery to Current State-of-the-Art

This Perspective covers discovery and mechanistic aspects as well as initial applications of novel ionization processes for use in mass spectrometry that guided us in a series of subsequent discoveries, instrument developments, and commercialization. Vacuum matrix-assisted ionization on an intermediate pressure matrix-assisted laser desorption/ionization source without the use of a laser, high voltages, or any other added energy was simply unbelievable, at first. Individually and as a whole, the various discoveries and inventions started to paint, inter alia, an exciting new picture and outlook in mass spectrometry from which key developments grew that were at the time unimaginable, and continue to surprise us in its simplistic preeminence. We, and others, have demonstrated exceptional analytical utility. Our current research is focused on how best to understand, improve, and use these novel ionization processes through dedicated platforms and source developments. These ionization processes convert volatile and nonvolatile compounds from solid or liquid matrixes into gas-phase ions for analysis by mass spectrometry using, e.g., mass-selected fragmentation and ion mobility spectrometry to provide accurate, and sometimes improved, mass and drift time resolution. The combination of research and discoveries demonstrated multiple advantages of the new ionization processes and established the basis of the successes that lead to the Biemann Medal and this Perspective. How the new ionization processes relate to traditional ionization is also presented, as well as how these technologies can be utilized in tandem through instrument modification and implementation to increase coverage of complex materials through complementary strengths.

Direct sub-atmospheric pressure ionization mass spectrometry: Evaporation/sublimation-driven ionization is amazing, fundamentally, and practically

This paper covers direct sub-atmospheric pressure ionization mass spectrometry (MS). The discovery, applications, and mechanistic aspects of novel ionization processes for use in MS that are not based on the high-energy input from voltage, laser, and/or high temperature but on sublimation/evaporation within a region linking a higher to lower pressure and modulated by heat and collisions, are discussed, including how this new reality has guided a series of discoveries, instrument developments, and commercialization. A research focus, inter alia, is on how best to understand, improve, and use these novel ionization processes, which convert volatile and nonvolatile compounds from solids (sublimation) or liquids (evaporation) into gas-phase ions for analysis by MS providing reproducible, accurate, sensitive, and prompt results. Our perception on how these unprecedented versus traditional ionization processes/methods relate to each other, how they can be made to coexist on the same mass spectrometer, and an outlook on new and expanded applications (e.g., clinical, portable, fast, safe, and autonomous) is presented, and is based on ST's Opening lecture presentation at the Nordic Mass spectrometry Conference, Geilo, Norway, January 2023. Focus will be on matrix-assisted ionization (MAI) and solvent-assisted ionization (SAI) MS covering the period from 2010 to 2023; a potential paradigm shift in the making.

Instrumentation development, improvement, simplification, and miniaturization: The multifunctional plate source for use in mass spectrometry

In remembrance of Prof. Dr Przybylski, we are presenting a vision towards his beloved mass spectrometry (MS) and its far-reaching promises outside of the academic laboratory. Sub-atmospheric pressure (AP) ionization MS is well positioned to make a step-change in direct ionization, a concept that allows sublimation/evaporation ionization and mass analyses of volatile and nonvolatile molecules from clean or dirty samples, directly, accurately, sensitively, and in a straightforward manner that has the potential to expand the field of MS into unchartered application areas. Contrary to ambient ionization MS, ionization commences in the sub-AP region of the mass spectrometer, important for practical and safety reasons, and offers inter alia, simplicity, speed, sensitivity, and robustness directly from real-world samples without cleanup. The plate source concept, presented here, provides an easy to use, rapid, and direct sample introduction from AP into the sub-AP of a mass spectrometer. Utilizing sub-AP ionization MS based on the plate source concept, small to large molecules from various environments that would be deemed too dirty for some direct MS methods are demonstrated. The new source concept can be expanded to include multiple ionization methods using the same plate source "front end" without the need to vent the mass spectrometer between the different methods, thus allowing ionization of more compounds on the same mass spectrometer for which any one ionization method may be insufficient. Examples such as fentanyl, gamma-hydroxybutyric acid, clozapine, 1-propionyllysergic acid, hydrocodone angiotensin I and II, myoglobin, and carbonic anhydrase are included.

A tutorial: Laserspray ionization and related laser-based ionization methods for use in mass spectrometry

This Tutorial is to provide a summary of parameters useful for successful outcomes of laserspray ionization (LSI) and related methods that employ a laser to ablate a matrix:analyte sample to produce highly charged ions. In these methods the purpose of the laser is to transfer matrix-analyte clusters into the gas phase. Ions are hypothesized to be produced by a thermal process where emitted matrix:analyte gas-phase particles/clusters are charged and loss of matrix from the charged particles leads to release of the analyte ions into the gas phase. The thermal energy responsible for the charge-separation process is relatively low and not necessarily supplied by the laser; a heated inlet tube linking atmospheric pressure with the first vacuum stage of a mass spectrometer is sufficient. The inlet becomes the "ion source", and inter alia, pressure, temperature, and the matrix, which can be a solid, liquid, or combinations, become critical parameters. Injecting matrix:analyte into a heated inlet tube using laser ablation, a shockwave, or simply tapping, all produce the similar mass spectra. Applications are provided that showcase new opportunities in the field of mass spectrometry.

Recent advances in mass spectrometry analysis of neuropeptides

Due to their involvement in numerous biochemical pathways, neuropeptides have been the focus of many recent research studies. Unfortunately, classic analytical methods, such as western blots and enzyme-linked immunosorbent assays, are extremely limited in terms of global investigations, leading researchers to search for more advanced techniques capable of probing the entire neuropeptidome of an organism. With recent technological advances, mass spectrometry (MS) has provided methodology to gain global knowledge of a neuropeptidome on a spatial, temporal, and quantitative level. This review will cover key considerations for the analysis of neuropeptides by MS, including sample preparation strategies, instrumental advances for identification, structural characterization, and imaging; insightful functional studies; and newly developed absolute and relative quantitation strategies. While many discoveries have been made with MS, the methodology is still in its infancy. Many of the current challenges and areas that need development will also be highlighted in this review.

Advances in ionisation techniques for mass spectrometry-based omics research

Omics analysis by mass spectrometry (MS) is a vast field, with proteomics, metabolomics and lipidomics dominating recent research by exploiting biological MS ionisation techniques. Traditional MS ionisation techniques such as electrospray ionisation have limitations in analyte-specific sensitivity, modes of sampling and throughput, leading to many researchers investigating new ionisation methods for omics research. In this review, we examine the current landscape of these new ionisation techniques, divided into the three groups of (electro)spray-based, laser-based and other miscellaneous ionisation techniques. Due to the wide range of new developments, this review can only provide a starting point for further reading on each ionisation technique, as each have unique benefits, often for specialised applications, which promise beneficial results for different areas in the omics world.

Multiomics Imaging Using High-Energy Water Gas Cluster Ion Beam Secondary Ion Mass Spectrometry [(H2O)n-GCIB-SIMS] of Frozen-Hydrated Cells and Tissue

Integration of multiomics at the single-cell level allows the unambiguous dissecting of phenotypic heterogeneity at different states such as health, disease, and biomedical response. Imaging mass spectrometry holds the promise of being able to measure multiple types of biomolecules in parallel in the same cell. We have explored the possibility of using water gas cluster ion beam secondary ion mass spectrometry [(H2O)n-GCIB-SIMS] as an analytical tool for multiomics assay. (H2O)n-GCIB has been hailed as an ideal ionization source for biological sampling owing to the enhanced chemical sensitivity and reduced matrix effect. Taking advantage of 1 μm spatial resolution by using a high-energy beam system, we have clearly shown the enhancement of multiple intact biomolecules up to a few hundredfold in single cells. Coupled with the cryogenic sample preparation/measurement, the lipids and metabolites were imaged simultaneously within the cellular region, uncovering the pristine chemistry for integrated omics in the same sample. We have demonstrated that double-charged myelin protein fragments and single-charged multiple lipids and metabolites can be localized in the same cells/tissue with a single acquisition. Our exploration has also been extended to the capability of (H2O)n-GCIB in the generation of multiple charged peptides on protein standards. Frozen hydration combined with (H2O)n-GCIB provides the possibility of universal enhancement for the ionization of multiple bio-molecules, including peptides/proteins which has allowed "omics" to become feasible in the same sample using SIMS.

A Combination MAI and MALDI Vacuum Source Operational from Atmospheric Pressure for Fast, Robust, and Sensitive Analyses

Previously, vacuum matrix-assisted ionization (vMAI) was employed with matrix/analyte sample introduction into the vacuum of a mass spectrometer on a probe sample introduction device. Low attomole detection was achieved, while no carryover was observed even for concentrated samples. Here, we report a new vacuum ionization source designed to duplicate the sensitivity and robustness of probe device while providing fast multisample introduction to vacuum and rapid sequential ionization. Exposure of a sample to the vacuum of the mass spectrometer provides spontaneous ionization of volatile as well as nonvolatile analytes without the need for external energy input. However, the novel source design described herein, in addition to vMAI, can employ a laser to obtain vacuum matrix-assisted laser desorption/ionization (vMALDI). In particular, ionization by vMAI or vMALDI is achieved by using the appropriate matrix. Switching between ionization modes is accomplished in a few seconds. We present results demonstrating the utility of the two ionization methods in combination to improve the molecular analyses of sample composition. In both ionization modes, multiple samples can be sequentially and rapidly acquired to increase throughput in MS. With the prototype source, samples were acquired in as little as 1 s per sample. Exchanging multisample plates can be accomplished in as little as 2 s, suggesting low-cost high-throughput automation when properly developed.

An overview of biological applications and fundamentals of new inlet and vacuum ionization technologies

RATIONALE

The developments of new ionization technologies based on processes previously unknown to mass spectrometry (MS) have gained significant momentum. Herein we address the importance of understanding these unique ionization processes, demonstrate the new capabilities currently unmet by other methods, and outline their considerable analytical potential.

METHODS

The inlet and vacuum ionization methods of solvent-assisted ionization (SAI), matrix-assisted ionization (MAI), and laserspray ionization can be used with commercial and dedicated ion sources producing ions from atmospheric or vacuum conditions for analyses of a variety of materials including drugs, lipids, and proteins introduced from well plates, pipet tips and plate surfaces with and without a laser using solid or solvent matrices. Mass spectrometers from various vendors are employed.

RESULTS

Results are presented highlighting strengths relative to ionization methods of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization. We demonstrate the utility of multi-ionization platforms encompassing MAI, SAI, and ESI and enabling detection of what otherwise is missed, especially when directly analyzing mixtures. Unmatched robustness is achieved with dedicated vacuum MAI sources with mechanical introduction of the sample to the sub-atmospheric pressure (vacuum MAI). Simplicity and use of a wide array of matrices are attained using a conduit (inlet ionization), preferably heated, with sample introduction from atmospheric pressure. Tissue, whole blood, urine (including mouse, chicken, and human origin), bacteria strains and chemical on-probe reactions are analyzed directly and, especially in the case of vacuum ionization, without concern of carryover or instrument contamination.

CONCLUSIONS

Examples are provided highlighting the exceptional analytical capabilities associated with the novel ionization processes in MS that reduce operational complexity while increasing speed and robustness, achieving mass spectra with low background for improved sensitivity, suggesting the potential of this simple ionization technology to drive MS into areas currently underserved, such as clinical and medical applications.

A nanoparticle co-matrix for multiple charging in matrix-assisted laser desorption ionization imaging of tissue

RATIONALE

A two-component matrix of 2-nitrophloroglucinol (2-NPG) and silica nanoparticles was used for matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging of high-charge-state biomolecules in tissue. Potential advantages include increased effective mass range and efficiency of fragmentation.

METHODS

A mixture of 2-NPG matrix and silica nanoparticles was applied to cyrosectioned 10 μm thick mouse brain tissue. The mixture was pipetted onto the tissue for profiling and sprayed for tissue imaging. MALDI images were obtained under high vacuum in a commercial time-of-flight mass spectrometer.

RESULTS

The combined 2-NPG and nanoparticle matrix produced highly charged ions from tissue with high-vacuum MALDI. Nanoparticles of 20, 70, 400, and 1000 nm in diameter were tested, the 20 nm particles producing the highest charge states. Images of mouse brain tissue obtained from highly charged ions show similar spatial localization.

CONCLUSIONS

The combined 2-NPG and nanoparticle matrix produces highly charged ions from tissue through a mechanism that may rely on the high surface area of the particles which can dry the tissue, and their ability to bind analyte molecules thereby assisting in crystal formation and production of multiply charged ions on laser irradiation.

Toward understanding the ionization mechanism of matrix-assisted ionization using mass spectrometry experiment and theory

RATIONALE

Matrix-assisted ionization (MAI) mass spectrometry does not require voltages, a laser beam, or added heat to initiate ionization, but it is strongly dependent on the choice of matrix and the vacuum conditions. High charge state distributions of nonvolatile analyte ions produced by MAI suggest that the ionization mechanism may be similar to that of electrospray ionization (ESI), but different from matrix-assisted laser desorption/ionization (MALDI). While significant information is available for MAI using mass spectrometers operating at atmospheric and intermediate pressure, little is known about the mechanism at high vacuum.

METHODS

Eleven MAI matrices were studied on a high-vacuum time-of-flight (TOF) mass spectrometer using a 266 nm pulsed laser beam under otherwise typical MALDI conditions. Detailed comparisons with the commonly used MALDI matrices and theoretical prediction were made for 3-nitrobenzonitrile (3-NBN), which is the only MAI matrix that works well in high vacuum when irradiated with a laser.

RESULTS

Screening of MAI matrices with good absorption at 266 nm but with various degrees of volatility and laser energies suggests that volatility and absorption at the laser wavelength may be necessary, but not sufficient, criteria to explain the formation of multiply charged analyte ions. 3-NBN produces intact, highly charged ions of nonvolatile analytes in high-vacuum TOF with the use of a laser, demonstrating that ESI-like ions can be produced in high vacuum. Theoretical calculations and mass spectra suggest that thermally induced proton transfer, which is the major ionization mechanism in MALDI, is not important with the 3-NBN matrix at 266 nm laser wavelength. 3-NBN:analyte crystal morphology is, however, important in ion generation in high vacuum.

CONCLUSIONS

The 3-NBN MAI matrix produces intact, highly charged ions of nonvolatile compounds in high-vacuum TOF mass spectrometers with the aid of ablation and/or heating by laser irradiation, and shows a different ionization mechanism from that of typical MALDI matrices.

Novel ionization processes for use in mass spectrometry: 'Squeezing' nonvolatile analyte ions from crystals and droplets

Together with my group and collaborators, I have been fortunate to have had a key role in the discovery of new ionization processes that we developed into new flexible, sensitive, rapid, reliable, and robust ionization technologies and methods for use in mass spectrometry (MS). Our current research is focused on how best to understand, improve, and use these novel ionization processes which convert volatile and nonvolatile compounds from solids or liquids into gas-phase ions for analysis by MS using e.g. mass-selected fragmentation and ion mobility spectrometry to provide reproducible, accurate, and improved mass and drift time resolution. In my view, the apex was the discovery of vacuum matrix-assisted ionization (vMAI) in 2012 on an intermediate pressure matrix-assisted laser desorption/ionization (MALDI) source without the use of a laser, high voltages, or any other added energy. Only exposure of the matrix:analyte to the sub-atmospheric pressure of the mass spectrometer was necessary to initiate ionization. These findings were initially rejected by three different scientific journals, with comments related to 'how can this work?', 'where do the charges come from?', and 'it is not analytically useful'. Meanwhile, we and others have demonstrated analytical utility without a complete understanding of the mechanism. In reality, MALDI and electrospray ionization are widely used in science and their mechanisms are still controversially discussed despite use and optimization of now 30 years. This Perspective covers the applications and mechanistic aspects of the novel ionization processes for use in MS that guided us in instrument developments, and provides our perspective on how they relate to traditional ionization processes.

Fundamental Studies of New Ionization Technologies and Insights from IMS-MS

Exceptional ion mobility spectrometry mass spectrometry (IMS-MS) developments by von Helden, Jarrold, and Clemmer provided technology that gives a view of chemical/biological compositions previously not achievable. The ionization method of choice used with IMS-MS has been electrospray ionization (ESI). In this special issue contribution, we focus on fundamentals of heretofore unprecedented means for transferring volatile and nonvolatile compounds into gas-phase ions singly and multiply charged. These newer ionization processes frequently lead to different selectivity relative to ESI and, together with IMS-MS, may provide a more comprehensive view of chemical compositions directly from their original environment such as surfaces, e.g., tissue. Similarities of results using solvent- and matrix-assisted ionization are highlighted, as are differences between ESI and the inlet ionization methods, especially with mixtures such as bacterial extracts. Selectivity using different matrices is discussed, as are results which add to our fundamental knowledge of inlet ionization as well as pose additional avenues for inquiry. IMS-MS provides an opportunity for comparison studies relative to ESI and will prove valuable using the new ionization technologies for direct analyses. Our hypothesis is that some ESI-IMS-MS applications will be replaced by the new ionization processes and by understanding mechanistic aspects to aid enhanced source and method developments this will be hastened.

Direct Biological Sample Analyses by Laserspray Ionization Miniature Mass Spectrometry

With improved performances, miniature mass spectrometers are becoming suitable for more practical applications. At the same time, the coupling of an approximate ionization source is essential in terms of minimizing sample preparation and broadening the range of samples that could be analyzed. In this study, an atmospheric pressure laserspray ionization (AP-LSI) source was coupled with our home developed miniature ion trap mass spectrometer. The whole system is compact in size, and biological samples could be directly analyzed with minimum sample preparation. Direct detections of peptides, proteins, drugs in whole blood, and urine could be achieved with high sensitivity. The analyses of tissue sections were demonstrated, and different regions in a tissue section could be differentiated based on their lipid profiles. Results suggest that the coupling of AP-LSI with miniature mass spectrometer is a powerful technique, which could potentially benefit target molecule analysis in biological and medical applications.

A high resolution atmospheric pressure matrix-assisted laser desorption/ionization-quadrupole-orbitrap MS platform enables in situ analysis of biomolecules by multi-mode ionization and acquisition

Introduced in 2000, atmospheric pressure (AP)/matrix-assisted laser desorption/ionization (MALDI) has attracted substantial attention in the mass spectrometry community due to its ease of sample introduction and handling, interchangeability with ESI source and capability of analyzing volatile species. In this study, an AP/MALDI source with ultra-high spatial resolution was coupled to a Q Exactive HF orbitrap mass spectrometer for high resolution in situ analysis by MALDI, laserspray ionization (LSI) and matrix assisted ionization (MAI) without instrument modification. LSI and MAI generated multiply charged ions, which expanded the mass detection range and improved fragmentation efficiency. Full MS, targeted MS/MS, data dependent acquisition (DDA) and parallel reaction monitoring (PRM) acquisitions were performed on peptide and protein standards, tissue extracts and tissue sections for in depth characterization of various biomolecules. High resolution full MS and MS/MS images were obtained from crustacean and rat tissues with pixel size less than 30 μm. Overall, AP/MALDI-Q-Orbitrap is a fast scanning instrument that is capable of performing multiple types of ionization and multiple acquisition modes without instrument modification. This instrument platform provides an attractive alternative to other high resolution MALDI instruments.

"Magic" Ionization Mass Spectrometry

The systematic study of the temperature and pressure dependence of matrix-assisted ionization (MAI) led us to the discovery of the seemingly impossible, initially explained by some reviewers as either sleight of hand or the misinterpretation by an overzealous young scientist of results reported many years before and having little utility. The “magic” that we were attempting to report was that with matrix assistance, molecules, at least as large as bovine serum albumin (66 kDa), are lifted into the gas phase as multiply charged ions simply by exposure of the matrix:analyte sample to the vacuum of a mass spectrometer. Applied heat, a laser, or voltages are not necessary to achieve charge states and ion abundances only previously observed with electrospray ionization (ESI). The fundamentals of how solid phase volatile or nonvolatile compounds are converted to gas-phase ions without added energy currently involves speculation providing a great opportunity to rethink mechanistic understanding of ionization processes used in mass spectrometry. Improved understanding of the mechanism(s) of these processes and their connection to ESI and matrix-assisted laser desorption/ionization may provide opportunities to further develop new ionization strategies for traditional and yet unforeseen applications of mass spectrometry. This Critical Insights article covers developments leading to the discovery of a seemingly magic ionization process that is simple to use, fast, sensitive, robust, and can be directly applied to surface characterization using portable or high performance mass spectrometers.

Test purchase, synthesis, and characterization of 2-methoxydiphenidine (MXP) and differentiation from its meta- and para-substituted isomers

The structurally diverse nature of the 1,2-diphenylethylamine template provides access to a range of substances for drug discovery work but some have attracted attention as 'research chemicals'. The most recent examples include diphenidine, i.e. 1-(1,2-diphenylethyl)piperidine and 2-methoxydiphenidine, i.e. 1-[1-(2-methoxyphenyl)-2-phenylethyl]piperidine (MXP, methoxyphenidine, 2-MXP) that have been associated with uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist activity. Analytical challenges encountered during chemical analysis include the presence of positional isomers. Three powdered samples suspected to contain 2-MXP were obtained from three Internet retailers in the United Kingdom and subjected to analytical characterization by gas chromatography (GC) and high performance liquid chromatography (HPLC) coupled to various forms of mass spectrometry (MS). Nuclear magnetic resonance spectroscopy, infrared spectroscopy and thin layer chromatography were also employed. This was supported by the synthesis of all three isomers (2-, 3- and 4-MXP) by two different synthetic routes. The analytical data obtained for the three purchased samples were consistent with the synthesized 2-MXP standard and the differentiation between the isomers was possible. Distinct stability differences were observed for all three isomers during in-source collision-induced dissociation of the protonated molecule when employing detection under HPLC selected-ion monitoring detection, which added to the ability to differentiate between them. Furthermore, the analysis of a 2-MXP tablet by matrix assisted inlet ionization Orbitrap mass spectrometry confirmed that it was possible to detect the protonated molecule of 2-MXP directly from the tablet surface following addition of 3-nitrobenzonitrile as the matrix.

Inlet and Vacuum Ionization from Ambient Conditions

The discovery that laser ablation of a common MALDI matrix at atmospheric pressure without use of a voltage produced ions with nearly identical charge states to ESI led to a series of new ionization methods that we have given the general term inlet and vacuum ionization. The initial thought that the laser was necessary for matrix-assisted ionization gave way to ionization requiring a heated inlet with a pressure-drop region and then to a matrix that could be a solvent or no matrix. This in turn led to laser ablation in vacuum producing multiply charged ions without an inlet, and finally to the present where we have found matrices that lift molecules into the gas phase as ions without any external energy source. Our mechanistic view of this new ionization process developed into ionization methods for use in mass spectrometry will be discussed. These methods are simple to use, safe, robust, and sensitive. Several approaches for high-throughput analyses of compounds irrespective of their molecular weight will be presented using low- and high-performance mass spectrometers.

Transmission geometry laserspray ionization vacuum using an atmospheric pressure inlet

This represents the first report of laserspray ionization vacuum (LSIV) with operation directly from atmospheric pressure for use in mass spectrometry. Two different types of electrospray ionization source inlets were converted to LSIV sources by equipping the entrance of the atmospheric pressure inlet aperture with a customized cone that is sealed with a removable glass plate holding the matrix/analyte sample. A laser aligned in transmission geometry (at 180° relative to the inlet) ablates the matrix/analyte sample deposited on the vacuum side of the glass slide. Laser ablation from vacuum requires lower inlet temperature relative to laser ablation at atmospheric pressure. However, higher inlet temperature is required for high-mass analytes, for example, α-chymotrypsinogen (25.6 kDa). Labile compounds such as gangliosides and cardiolipins are detected in the negative ion mode directly from mouse brain tissue as intact doubly deprotonated ions. Multiple charging enhances the ion mobility spectrometry separation of ions derived from complex tissue samples.

Ion sources for mass spectrometric identification and imaging of molecular species

Covering: 2013 The ability to transfer molecular species to the gas phase and ionize them is central to the study of natural products and other molecular species by mass spectrometry (MS). MS-based strategies in natural products have focused on a few established ion sources, such as electron impact and electrospray ionization. However, a variety of other ion sources are either currently in use to evaluate natural products or show significant future promise. This review discusses these various ion sources in the context of other articles in this special issue, but is also applicable to other fields of analysis, including materials science. Ion sources are grouped based on the current understanding of their predominant ion formation mechanisms. This broad overview groups ion sources into the following categories: electron ionization and single photon ionization; chemical ionization-like and plasma-based; electrospray ionization; and, laser desorption-based. Laser desorption-based methods are emphasized with specific examples given for laser desorption postionization sources and their use in the analysis of intact microbial biofilms. Brief consideration is given to the choice of ion source for various sample types and analyses, including MS imaging.

Ambient femtosecond laser vaporization and nanosecond laser desorption electrospray ionization mass spectrometry

Recent investigations of ambient laser-based transfer of molecules into the gas phase for subsequent mass spectral analysis have undergone a renaissance resulting from the separation of vaporization and ionization events. Here, we seek to provide a snapshot of recent femtosecond (fs) duration laser vaporization and nanosecond (ns) duration laser desorption electrospray ionization mass spectrometry experiments. The former employs pulse durations of <100 fs to enable matrix-free laser vaporization with little or no fragmentation. When coupled to electrospray ionization, femtosecond laser vaporization provides a universal, rapid mass spectral analysis method requiring no sample workup. Remarkably, laser pulses with intensities exceeding 10(13) W cm(-2) desorb intact macromolecules, such as proteins, and even preserve the condensed phase of folded or unfolded protein structures according to the mass spectral charge state distribution, as demonstrated for cytochrome c and lysozyme. Because of the ability to vaporize and ionize multiple components from complex mixtures for subsequent analysis, near perfect classification of explosive formulations, plant tissue phenotypes, and even the identity of the manufacturer of smokeless powders can be determined by multivariate statistics. We also review the more mature field of nanosecond laser desorption for ambient mass spectrometry, covering the wide range of systems analyzed, the need for resonant absorption, and the spatial imaging of complex systems like tissue samples.

MSiMass list: a public database of identifications for protein MALDI MS imaging

The clinical application of mass spectrometry imaging has developed into a sizable subdiscipline of proteomics and metabolomics because its seamless integration with pathology enables biomarkers and biomarker profiles to be determined that can aid patient and disease stratification (diagnosis, prognosis, and response to therapy). Confident identification of the discriminating peaks remains a challenge owing to the presence of nontryptic protein fragments, large mass-to-charge ratio ions that are not efficiently fragmented via tandem mass spectrometry or a high density of isobaric species. A public database of identifications has been initiated to aid the clinical development and implementation of mass spectrometry imaging. The MSiMass list database ( www.maldi-msi.org/mass ) enables users to assign identities to the peaks observed in their experiments and provides the methods by which the identifications were obtained. In contrast with existing protein databases, this list is designed as a community effort without a formal review panel. In this concept, authors can freely enter data and can comment on existing entries. In such, the database itself is an experiment on sharing knowledge, and its ability to rapidly provide quality data will be evaluated in the future.

High-speed tandem mass spectrometric in situ imaging by nanospray desorption electrospray ionization mass spectrometry

Nanospray desorption electrospray ionization (nano-DESI) combined with tandem mass spectrometry (MS/MS), high-resolution mass analysis of the fragment ions (m/Δm = 17 500 at m/z 200), and rapid spectral acquisition enabled simultaneous imaging and identification of a large number of metabolites and lipids from 92 selected m/z windows (±1 Da) with a spatial resolution of better than 150 μm. Mouse uterine sections of implantation sites on day 6 of pregnancy were analyzed in the ambient environment without any sample pretreatment. MS/MS imaging was performed by scanning the sample under the nano-DESI probe at 10 μm/s, while higher-energy collision-induced dissociation (HCD) spectra were acquired for a targeted inclusion list of 92 m/z values at a rate of ∼6.3 spectra/s. Molecular ions and their corresponding fragments, separated by high-resolution mass analysis, were assigned on the basis of accurate mass measurement. Using this approach, we were able to identify and image both abundant and low-abundance isobaric and isomeric species within each m/z window. MS/MS analysis enabled efficient separation and identification of isomeric and isobaric phospholipids that are difficult to separate in full-scan mode. Furthermore, we identified several metabolites associated with early pregnancy and obtained the first 2D images of these molecules.