Mass spectrometry imaging (MSI) is a powerful tool that enables untargeted investigations into the spatial distribution of molecular species in a variety of samples. It has the capability to image thousands of molecules, such as metabolites, lipids, peptides, proteins, and glycans, in a single experiment without labeling. The combination of information gained from mass spectrometry (MS) and visualization of spatial distributions in thin sample sections makes this a valuable chemical analysis tool useful for biological specimen characterization. After minimal but careful sample preparation, the general setup of an MSI experiment involves defining an (x, y) grid over the surface of the sample, with the grid area chosen by the user. The mass spectrometer then ionizes the molecules on the surface of the sample and collects a mass spectrum at each pixel on the section, with the resulting spatial resolution defined by the pixel size. After collecting the spectra, computational software can be used to select an ...

Mass Spectrometry Imaging: A Review of Emerging Advancements and Future Insights.

Mass spectrometry imaging (MSI) has proven to be extremely useful for applications such as the spatial analysis of peptides and proteins in biological tissue, the performance assessment of drugs in vivo or the measurement of protein or metabolite expression as tissue classifiers or biomarkers from disease versus control tissue comparisons. The most popular MSI technique is MALDI mass spectrometry. First invented by Richard Caprioli in the mid-1990s, it is the highest performing MSI technique in terms of spatial resolution, sensitivity for intact biomolecules and application range today. The unique ability to identify and spatially resolve numerous compounds simultaneously, based on m/z values has inter alia been applied to untargeted and targeted chemical mapping of biological compartments, revealing changes of physiological states, disease pathologies and metabolic faith and distribution of xenobiotics. Many MSI applications focus on lipid species because of the lipids' diverse roles as structural components of cell membranes, their function in the surfactant cycle, and their involvement as second messengers in signalling cascades of tissues and cells. This article gives a comprehensive overview of lipid imaging techniques and applications using established MALDI and SIMS methods but also other promising MSI techniques such as DESI.

Lipid imaging by mass spectrometry – a review

In this Perspective we discuss photoelectron circular dichroism (PECD), a relatively novel technique that can detect chiral molecules with high sensitivity. PECD has an enantiomeric sensitivity of typically 1–10%, which is two to three orders of magnitude larger than that of the widely employed technique of circular dichroism (CD). In PECD a chiral molecule is photoionized with circular polarized light, and the photoelectron angular scattering distribution is detected using particle imaging techniques. We present the general physical principles of photoelectron circular dichroism and we address both single- and multiphoton excitation. PECD has been measured with synchrotron radiation in single-photon ionization as well as, very recently, with femtosecond laser radiation in multiphoton ionization. We discuss the experimental implementation of PECD, focusing on velocity map coincidence imaging where the momentum distribution of both the electron and the coincident ion is measured. The coincident detection of the mass and momentum of the ion adds very powerful mass-correlated information to the PECD measurement of the chiral molecule. We illustrate the capabilities and the potential of PECD with various experimental examples and introduce computational methods that are able to model quantitatively experimental PECD results. We conclude with an outlook on novel developments and (analytical) implementations of PECD that may further broaden the application of PECD for the sensitive detection of chirality in molecules.

Detecting chirality in molecules by imaging photoelectron circular dichroism

Imaging Mass Spectrometry (IMS) is strengthening its position as a valuable analytical tool. It has unique ability to identify structures and to unravel molecular changes that occur in the precisely defined part of the sample. These unique features open new possibilities in the field of various aspects of biological research. In this review we briefly discuss the main imaging mass spectrometry techniques, as well as the nature of biological samples and molecules, which might be analyzed by such methodology. Moreover, a novel approach, where different analytical techniques might be combined with the results of IMS study, is emphasized and discussed. With such a fast development of IMS and related methods, we can foresee the promising future of this technique. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 35: 147–169, 2016.

Imaging mass spectrometry: Instrumentation, applications, and combination with other visualization techniques

When protein crystals are submicrometre-sized, X-ray radiation damage precludes conventional diffraction data collection. For crystals that are of the order of 100 nm in size, at best only single-shot diffraction patterns can be collected and rotation data collection has not been possible, irrespective of the diffraction technique used. Here, it is shown that at a very low electron dose (at most 0.1 e− A−2), a Medipix2 quantum area detector is sufficiently sensitive to allow the collection of a 30-frame rotation series of 200 keV electron-diffraction data from a single ∼100 nm thick protein crystal. A highly parallel 200 keV electron beam (λ = 0.025 A) allowed observation of the curvature of the Ewald sphere at low resolution, indicating a combined mosaic spread/beam divergence of at most 0.4°. This result shows that volumes of crystal with low mosaicity can be pinpointed in electron diffraction. It is also shown that strategies and data-analysis software (MOSFLM and SCALA) from X-ray protein crystallography can be used in principle for analysing electron-diffraction data from three-dimensional nanocrystals of proteins.

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A Medipix quantum area detector allows rotation electron diffraction data collection from submicrometre three-dimensional protein crystals

https://link.springer.com/content/pdf/10.1016%2Fj.jasms.2010.08.014.pdf

Fast, High Resolution Mass Spectrometry Imaging Using a Medipix Pixelated Detector

A system to read-out images from four photon-counting Medipix chips at one Gigabit per second has been constructed in a bilateral collaboration between Nikhef and PANalytical. The system consists of two printed circuit boards. One board supports the four Medipix readout ASICs, flip-chipped to a single pixellized semiconducting sensor on top. This board is mounted perpendicular on the Relaxd read-out board. This Relaxd board supplies the necessary voltages to the Medipix readout ASICs, controls all signals to and from these chips via a low-power Field Programmable Gate Array (FPGA, Lattice SC series) and communicates with a PC through a standard one Gigabit per second Ethernet connection (GbE). The T-shaped mechanical topology allows multiple modules to be mounted adjacent to each other in all directions (2D tiling) and to minimise the insensitive area between separate Relaxd modules. An overview of the system layout and the functionality is presented as well as the first test results.

A Gigabit per second read-out system for Medipix Quads

The use of a new type in-vacuum pixel detector in velocity map imaging (VMI) is introduced The Medipix2 and Timepix semiconductor pixel detectors (256×256 square pixels, 55×55 μm2) are well suited for charged particle detection They offer high resolution, low noise, and high quantum efficiency The Medipix2 chip allows double energy discrimination by offering a low and a high energy threshold The Timepix detector allows to record the incidence time of a particle with a temporal resolution of 10 ns and a dynamic range of 160 μs Results of the first time application of the Medipix2 detector to VMI are presented, investigating the quantum efficiency as well as the possibility to operate at increased background pressure in the vacuum chamber

Velocity map imaging using an in-vacuum pixel detector

The implementation of the Timepix complementary metal oxide semiconductor pixel detector in velocity map slice imaging is presented. This new detector approach eliminates the need for gating the imaging detector. In time-of-flight mode, the detector returns the impact position and the time-of-flight of charged particles with 12.5 ns resolution and a dynamic range of about 100 μs. The implementation of the Timepix detector in combination with a microchannel plate additionally allows for high spatial resolution information via center-of-mass centroiding. Here, the detector was applied to study the photodissociation of NO2 at 452 nm. The energy resolution observed in the experiment was ΔE/E=0.05 and is limited by the experimental setup rather than by the detector assembly. All together, this new compact detector assembly is well-suited for slice imaging and is a promising tool for imaging studies in atomic and molecular physics research.

Jan Visschers

Papers

Fast, High Resolution Mass Spectrometry Imaging Using a Medipix Pixelated Detector

A Gigabit per second read-out system for Medipix Quads

Velocity map imaging using an in-vacuum pixel detector

A new imaging method for understanding chemical dynamics: Efficient slice imaging using an in-vacuum pixel detector