Mass spectrometry imaging
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.
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.
[2][3][4] 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.
[7] This technique is performed using a focused ionization beam to analyze a specific region of the sample by generating a mass spectrum.
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.
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.
[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.
[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.
Matrix-assisted laser desorption ionization can be used as a mass spectrometry imaging technique for relatively large molecules.
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.
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.
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.
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.
[29] 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.
[17] The table below shows a comparison of advantages and disadvantages of some available techniques, including MSI, correlated with drug distribution analysis.
Authors Yu, Jyao; Harankhedkar, Shefali; Nabatilan, Arielle; Fahrni, Christopher; Walter de Gruyter, Berlin.
