Mass spectrometry imaging

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Mass spectrometry imaging (MSI) is a technique used in mass spectrometry to visualize the spatial distribution of molecules, as biomarkers, metabolites, peptides or proteins by their molecular masses. After collecting a mass spectrum at one spot, the sample is moved to reach another region, and so on, until the entire sample is scanned. By choosing a peak in the resulting spectra that corresponds to the compound of interest, the MS data is used to map its distribution across the sample. This results in pictures of the spatially resolved distribution of a compound pixel by pixel. Each data set contains a veritable gallery of pictures because any peak in each spectrum can be spatially mapped. Despite the fact that MSI has been generally considered a qualitative method, the signal generated by this technique is proportional to the relative abundance of the analyte. Therefore, quantification is possible, when its challenges are overcome. Although widely used traditional methodologies like radiochemistry and immunohistochemistry achieve the same goal as MSI, they are limited in their abilities to analyze multiple samples at once, and can prove to be lacking if researchers do not have prior knowledge of the samples being studied. [1] Most common ionization technologies in the field of MSI are DESI imaging, MALDI imaging, secondary ion mass spectrometry imaging (SIMS imaging) and Nanoscale SIMS (NanoSIMS). [2] [3] [4]

Contents

History

More than 50 years ago, MSI was introduced using secondary ion mass spectrometry (SIMS) to study semiconductor surfaces by Castaing and Slodzian. [5] However, it was the pioneering work of Richard Caprioli and colleagues in the late 1990s, demonstrating how matrix-assisted laser desorption/ionization (MALDI) could be applied to visualize large biomolecules (as proteins and lipids) in cells and tissue to reveal the function of these molecules and how function is changed by diseases like cancer, which led to the widespread use of MSI. Nowadays, different ionization techniques have been used, including SIMS, MALDI and desorption electrospray ionization (DESI), as well as other technologies. Still, MALDI is the current dominant technology with regard to clinical and biological applications of MSI. [6]

Operation principle

The MSI is based on the spatial distribution of the sample. Therefore, the operation principle depends on the technique that is used to obtain the spatial information. The two techniques used in MSI are: microprobe and microscope. [7]

Microprobe

This technique is performed using a focused ionization beam to analyze a specific region of the sample by generating a mass spectrum. The mass spectrum is stored along with the spatial coordination where the measurement took place. Then, a new region is selected and analyzed by moving the sample or the ionization beam. These steps are repeated until the entire sample has been scanned. By coupling all individual mass spectra, a distribution map of intensities as a function of x and y locations can be plotted. As a result, reconstructed molecular images of the sample are obtained. [7]

Microscope

In this technique, a 2D position-sensitive detector is used to measure the spatial origin of the ions generated at the sample surface by the ion optics of the instruments. The resolution of the spatial information will depend on the magnification of the microscope, the quality of the ions optics and the sensitivity of the detector. A new region still needs to be scanned, but the number of positions drastically reduces. The limitation of this mode is the finite depth of vision present with all microscopes. [7]

Ion source dependence

The ionization techniques available for MSI are suited to different applications. Some of the criteria for choosing the ionization method are the sample preparation requirement and the parameters of the measurement, as resolution, mass range and sensitivity. Based on that, the most common used ionization method are MALDI, SIMS AND DESI which are described below. Still, other minor techniques used are laser ablation electrospray ionization (LAESI), laser-ablation-inductively coupled plasma (LA-ICP) and nanospray desorption electrospray ionization (nano-DESI).

SIMS and NanoSIMS imaging

Secondary ion mass spectrometry (SIMS) is used to analyze solid surfaces and thin films by sputtering the surface with a focused primary ion beam and collecting and analyzing ejected secondary ions. There are many different sources for a primary ion beam. However, the primary ion beam must contain ions that are at the higher end of the energy scale. Some common sources are: Cs+, O2+, O, Ar+ and Ga+. [8] SIMS imaging is performed in a manner similar to electron microscopy; the primary ion beam is emitted across the sample while secondary mass spectra are recorded. [9] SIMS proves to be advantageous in providing the highest image resolution but only over small area of samples. [10] More, this technique is widely regarded as one of the most sensitive forms of mass spectrometry as it can detect elements in concentrations as small as 1012-1016 atoms per cubic centimeter. [11] [note 1] [note 2]

Multiplexed ion beam imaging (MIBI) is a SIMS method that uses metal isotope labeled antibodies to label compounds in biological samples. [12]

Developments within SIMS: Some chemical modifications have been made within SIMS to increase the efficiency of the process. There are currently two separate techniques being used to help increase the overall efficiency by increasing the sensitivity of SIMS measurements: matrix-enhanced SIMS (ME-SIMS) - This has the same sample preparation as MALDI does as this simulates the chemical ionization properties of MALDI. ME-SIMS does not sample nearly as much material. However, if the analyte being tested has a low mass value then it can produce a similar looking spectra to that of a MALDI spectra. ME-SIMS has been so effective that it has been able to detect low mass chemicals at sub cellular levels that was not possible prior to the development of the ME-SIMS technique. [4] The second technique being used is called sample metallization (Meta-SIMS) - This is the process of gold or silver addition to the sample. This forms a layer of gold or silver around the sample and it is normally no more than 1-3 nm thick. Using this technique has resulted in an increase of sensitivity for larger mass samples. The addition of the metallic layer also allows for the conversion of insulating samples to conducting samples, thus charge compensation within SIMS experiments is no longer required. [13]

Subcellular (50 nm) resolution is enabled by NanoSIMS [2] allowing for absolute quantitative analysis at the organelle level.

MALDI imaging

Mouse kidney: (a) MALDI spectra from the tissue. (b) H&E stained tissue. N-glycans at m/z = 1996.7 (c) is located in the cortex and medulla while m/z = 2158.7 (d) is in the cortex, (e) An overlay image of these two masses, (f) untreated control tissue. MALDI imaging.png
Mouse kidney: (a) MALDI spectra from the tissue. (b) H&E stained tissue. N-glycans at m/z = 1996.7 (c) is located in the cortex and medulla while m/z = 2158.7 (d) is in the cortex, (e) An overlay image of these two masses, (f) untreated control tissue.

Matrix-assisted laser desorption ionization can be used as a mass spectrometry imaging technique for relatively large molecules. [4] It has recently been shown that the most effective type of matrix to use is an ionic matrix for MALDI imaging of tissue. In this version of the technique the sample, typically a thin tissue section, is moved in two dimensions while the mass spectrum is recorded. [15] Although MALDI has the benefit of being able to record the spatial distribution of larger molecules, it comes at the cost of lower resolution than the SIMS technique. The limit for the lateral resolution for most of the modern instruments using MALDI is 20 m. MALDI experiments commonly use either an Nd:YAG (355 nm) or N2 (337 nm) laser for ionization. [4]

Pharmacodynamics and toxicodynamics in tissue have been studied by MALDI imaging. [16]

DESI imaging

Desorption electrospray Ionization is a less destructive technique, which couples simplicity and rapid analysis of the sample. The sample is sprayed with an electrically charged solvent mist at an angle that causes the ionization and desorption of various molecular species. Then, two-dimensional maps of the abundance of the selected ions in the surface of the sample in relation with the spatial distribution are generated. [17] [10] This technique is applicable to solid, liquid, frozen and gaseous samples. Moreover, DESI allows analyzing a wide range of organic and biological compounds, as animal and plant tissues and cell culture samples, without complex sample preparation [6] [10] Although, this technique has the poorest resolution among other, it can create high-quality image from a large area scan, as a whole body section scanning. [10] Fn

Comparative between the ionization techniques

Comparison of typical parameters among MSI techniques [10]
Ionization SourceType of IonizationAnalytesSpatial ResolutionMass Range
SIMSIon gunHardElemental ions, small molecules, lipids<10 m0-1000 Da
MALDIUV laser beamSoftLipids, peptide, proteins20 m0-100 000 Da
DESISolvent SpraySoftSmall molecules, lipids, peptides50 m0-2000 Da

Combination of various MSI techniques and other imaging techniques

Combining various MSI techniques can be beneficial, since each particular technique has its own advantage. For example, when information regards both proteins and lipids are necessary in the same tissue section, performing DESI to analyze the lipid, followed by MALDI to obtain information about the peptide, and finalize applying a stain (haematoxylin and eosin) for medical diagnosis of the structural characteristic of the tissue. [10] On the other side of MSI with other imaging techniques, fluorescence staining with MSI and magnetic resonance imaging (MRI) with MRI can be highlighted. Fluorescence staining can give information of the appearance of some proteins present in any process inside a tissue, while MSI may give information about the molecular changes presented in that process. Combining both techniques, multimodal picture or even 3D images of the distribution of different molecules can be generated. [10] In contrast, MRI with MSI combines the continuous 3D representation of MRI image with detailed structural representation using molecular information from MSI. Even though, MSI itself can generate 3D images, the picture is just part of the reality due to the depth limitation in the analysis, while MRI provides, for example, detailed organ shape with additional anatomical information. This coupled technique can be beneficial for cancer precise diagnosis and neurosurgery. [10]

Data processing

Standard data format for mass spectrometry imaging datasets

The imzML was proposed to exchange data in a standardized XML file based on the mzML format. [18] Several imaging MS software tools support it. The advantage of this format is the flexibility to exchange data between different instruments and data analysis software. [19]

Software

There are many free software packages available for visualization and mining of imaging mass spectrometry data. Converters from Thermo Fisher format, Analyze format, GRD format and Bruker format to imzML format were developed by the Computis project. Some software modules are also available for viewing mass spectrometry images in imzML format: Biomap (Novartis, free), Datacube Explorer (AMOLF, free), [20] EasyMSI (CEA), Mirion (JLU), MSiReader (NCSU, free) [21] and SpectralAnalysis. [22]

For processing .imzML files with the free statistical and graphics language R, a collection of R scripts is available, which permits parallel-processing of large files on a local computer, a remote cluster or on the Amazon cloud. [23]

Another free statistical package for processing imzML and Analyze 7.5 data in R exists, Cardinal. [24]

SPUTNIK [25] is an R package containing various filters to remove peaks characterized by an uncorrelated spatial distribution with the sample location or spatial randomness.

Applications

A remarkable ability of MSI is to find out the localization of biomolecules in tissues, even though there are no previous information about them. This feature has made MSI a unique tool for clinical research and pharmacological research. It provides information about biomolecular changes related with diseases by tracking proteins, lipids, and cell metabolism. For example, identifying biomarkers by MSI can show detailed cancer diagnosis. In addition, low cost imaging for pharmaceuticals studies can be acquired, such as images of molecular signatures that would be indicative of treatment response for a specific drug or the effectiveness of a particular drug delivery method. [26] [27] [28]

Ion colocalization has been studied as a way to infer local interactions between biomolecules. Similarly to colocalization in microscopy imaging, correlation has been used to quantify the similarity between ion images and generate network models. [29]

Advantages, challenges and limitations

The main advantage of MSI for studying the molecules location and distribution within the tissue is that this analysis can provide either greater selectivity, more information or more accuracy than others. Moreover, this tool requires less investment of time and resources for similar results. [17] The table below shows a comparison of advantages and disadvantages of some available techniques, including MSI, correlated with drug distribution analysis. [5]

Comparison of advantages and disadvantages of techniques assessing drug distribution [5]
MethodologyQuestion answeredAdvantagesDisadvantages
AutoradiographyWhere and how much radioactivityVery high spatial resolution; reliable quantitationEx vivo; requires radio-labelled drug; does not distinguish drug from metabolites.
ImmunohistochemistryWhereShort processing time; easy interpretation; inexpensiveEx vivo; requires antibodies, which vary in sensitivity and specificity; difficulties assigning; detection threshold; lack of standard scoring system
FluorescenceWhereIn vivo possible; reasonable costNot quantitative; poor resolution; autofluorescent interference
Positron emission tomography (PET)Where, what and activityIn vivo possible; good resolution; can be coupled to CT X-ray, gamma cameraExpensive; short-lived isotopes; need cyclotron to produce isotopes
Coherent anti-Stokes

Raman scattering

microscopy (CARS)

Where and whatLabel-free; sub-cellular spatial resolutionNot quantitative; poor selectivity; high background noise
Electrochemical atomic

force microscopy (AFM)

Where and whatLabel-free imaging; high resolutionNot quantitative; poor reproducibility; high background
MSIWhere and whatMultiplex; label-free imaging; good spatial resolutionSemi-quantitative; ion-suppression effects; complex analysis

Notes

  1. by way of comparison, 1 cc of Carbon (diamond) contains about 1.8 x 1023 atoms. 1012 to 1016 corresponds to 6 parts per trillion (ppt) to 60 parts per billion (ppb).
  2. the sensitivity varies by element (or molecule) as well as by nature of the surface being analyzed and conditions of the analysis.

Further reading

"Imaging Trace Metals in Biological Systems" pp 81–134 in "Metals, Microbes and Minerals: The Biogeochemical Side of Life" (2021) pp xiv + 341. Authors Yu, Jyao; Harankhedkar, Shefali; Nabatilan, Arielle; Fahrni, Christopher; Walter de Gruyter, Berlin. Editors Kroneck, Peter M.H. and Sosa Torres, Martha. DOI 10.1515/9783110589771-004

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<span class="mw-page-title-main">Mass spectrometry</span> Analytical technique based on determining mass to charge ratio of ions

Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.

<span class="mw-page-title-main">Ion source</span> Device that creates charged atoms and molecules (ions)

An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.

<span class="mw-page-title-main">Lipidomics</span>

Lipidomics is the large-scale study of pathways and networks of cellular lipids in biological systems The word "lipidome" is used to describe the complete lipid profile within a cell, tissue, organism, or ecosystem and is a subset of the "metabolome" which also includes other major classes of biological molecules. Lipidomics is a relatively recent research field that has been driven by rapid advances in technologies such as mass spectrometry (MS), nuclear magnetic resonance (NMR) spectroscopy, fluorescence spectroscopy, dual polarisation interferometry and computational methods, coupled with the recognition of the role of lipids in many metabolic diseases such as obesity, atherosclerosis, stroke, hypertension and diabetes. This rapidly expanding field complements the huge progress made in genomics and proteomics, all of which constitute the family of systems biology.

<span class="mw-page-title-main">Matrix-assisted laser desorption/ionization</span> Ionization technique

In mass spectrometry, matrix-assisted laser desorption/ionization (MALDI) is an ionization technique that uses a laser energy-absorbing matrix to create ions from large molecules with minimal fragmentation. It has been applied to the analysis of biomolecules and various organic molecules, which tend to be fragile and fragment when ionized by more conventional ionization methods. It is similar in character to electrospray ionization (ESI) in that both techniques are relatively soft ways of obtaining ions of large molecules in the gas phase, though MALDI typically produces far fewer multi-charged ions.

Surface-enhanced laser desorption/ionization (SELDI) is a soft ionization method in mass spectrometry (MS) used for the analysis of protein mixtures. It is a variation of matrix-assisted laser desorption/ionization (MALDI). In MALDI, the sample is mixed with a matrix material and applied to a metal plate before irradiation by a laser, whereas in SELDI, proteins of interest in a sample become bound to a surface before MS analysis. The sample surface is a key component in the purification, desorption, and ionization of the sample. SELDI is typically used with time-of-flight (TOF) mass spectrometers and is used to detect proteins in tissue samples, blood, urine, or other clinical samples, however, SELDI technology can potentially be used in any application by simply modifying the sample surface.

Soft laser desorption (SLD) is laser desorption of large molecules that results in ionization without fragmentation. "Soft" in the context of ion formation means forming ions without breaking chemical bonds. "Hard" ionization is the formation of ions with the breaking of bonds and the formation of fragment ions.

<span class="mw-page-title-main">MALDI imaging</span> Imaging system

MALDI mass spectrometry imaging (MALDI-MSI) is the use of matrix-assisted laser desorption ionization as a mass spectrometry imaging technique in which the sample, often a thin tissue section, is moved in two dimensions while the mass spectrum is recorded. Advantages, like measuring the distribution of a large amount of analytes at one time without destroying the sample, make it a useful method in tissue-based study.

<span class="mw-page-title-main">Protein mass spectrometry</span> Application of mass spectrometry

Protein mass spectrometry refers to the application of mass spectrometry to the study of proteins. Mass spectrometry is an important method for the accurate mass determination and characterization of proteins, and a variety of methods and instrumentations have been developed for its many uses. Its applications include the identification of proteins and their post-translational modifications, the elucidation of protein complexes, their subunits and functional interactions, as well as the global measurement of proteins in proteomics. It can also be used to localize proteins to the various organelles, and determine the interactions between different proteins as well as with membrane lipids.

<span class="mw-page-title-main">Desorption electrospray ionization</span>

Desorption electrospray ionization (DESI) is an ambient ionization technique that can be coupled to mass spectrometry (MS) for chemical analysis of samples at atmospheric conditions. Coupled ionization sources-MS systems are popular in chemical analysis because the individual capabilities of various sources combined with different MS systems allow for chemical determinations of samples. DESI employs a fast-moving charged solvent stream, at an angle relative to the sample surface, to extract analytes from the surfaces and propel the secondary ions toward the mass analyzer. This tandem technique can be used to analyze forensics analyses, pharmaceuticals, plant tissues, fruits, intact biological tissues, enzyme-substrate complexes, metabolites and polymers. Therefore, DESI-MS may be applied in a wide variety of sectors including food and drug administration, pharmaceuticals, environmental monitoring, and biotechnology.

Sample preparation for mass spectrometry is used for the optimization of a sample for analysis in a mass spectrometer (MS). Each ionization method has certain factors that must be considered for that method to be successful, such as volume, concentration, sample phase, and composition of the analyte solution. Quite possibly the most important consideration in sample preparation is knowing what phase the sample must be in for analysis to be successful. In some cases the analyte itself must be purified before entering the ion source. In other situations, the matrix, or everything in the solution surrounding the analyte, is the most important factor to consider and adjust. Often, sample preparation itself for mass spectrometry can be avoided by coupling mass spectrometry to a chromatography method, or some other form of separation before entering the mass spectrometer. In some cases, the analyte itself must be adjusted so that analysis is possible, such as in protein mass spectrometry, where usually the protein of interest is cleaved into peptides before analysis, either by in-gel digestion or by proteolysis in solution.

<span class="mw-page-title-main">Matrix-assisted laser desorption electrospray ionization</span>

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<span class="mw-page-title-main">Desorption atmospheric pressure photoionization</span>

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<span class="mw-page-title-main">Laser ablation electrospray ionization</span>

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<span class="mw-page-title-main">Surface-assisted laser desorption/ionization</span>

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<span class="mw-page-title-main">Single-cell analysis</span> Testbg biochemical processes and reactions in an individual cell

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<span class="mw-page-title-main">Desorption/ionization on silicon</span> Soft laser desorption method

Desorption/ionization on silicon (DIOS) is a soft laser desorption method used to generate gas-phase ions for mass spectrometry analysis. DIOS is considered the first surface-based surface-assisted laser desorption/ionization (SALDI-MS) approach. Prior approaches were accomplished using nanoparticles in a matrix of glycerol, while DIOS is a matrix-free technique in which a sample is deposited on a nanostructured surface and the sample desorbed directly from the nanostructured surface through the adsorption of laser light energy. DIOS has been used to analyze organic molecules, metabolites, biomolecules and peptides, and, ultimately, to image tissues and cells.

<span class="mw-page-title-main">Matrix-assisted ionization</span>

In mass spectrometry, matrix-assisted ionization is a low fragmentation (soft) ionization technique which involves the transfer of particles of the analyte and matrix sample from atmospheric pressure (AP) to the heated inlet tube connecting the AP region to the vacuum of the mass analyzer.

<span class="mw-page-title-main">Ron Heeren</span> Dutch mass spectrometry researcher

Ron M.A. Heeren is a Dutch scientist in mass spectrometry imaging. He is currently a distinguished professor at Maastricht University and the scientific director of the Multimodal Molecular Imaging Institute (M4I), where he heads the division of Imaging Mass Spectrometry.

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