Question

How are photos of tissues made using MS imaging, so how do you get a photo of a sample? Because the mass spectrometer itself cannot take a picture of a tissue if I am correct.

Answer 1

Ans.

Materials are all types of tissue sections. These sections are covered with a matrix for extracting molecules from the tissue specimen into the matrix and aids desorption/ionization for the further analysis in the mass spectrometer. For the ionization process, the laser shoots only in the matrix layer, while the underlying tissue remains intact and thus allowing histological tissue examination after the measurement in the very same tissue section.

The matrix absorbs the laser energy and transfers the analytes to the gas phase, promoting ionization in the process. By selecting different matrices and depending on the technology used (MALDI-TOF or high resolution mass spectrometry as, eg, MALDI-FT-ICR), the analyte classes can be chosen.

In the mass spectrometer, the tissue specimen are then raster-scanned (with a spatial resolution ranging from approximately 200 μm down to 20 μm), generating a mass spectrum for each measuring spot. Recently, a method was developed for combining a spatial resolution in the low micrometer range and high mass accuracy for the analysis of biological samples.

Phospholipids, neuropeptides, and drug compounds were imaged with accurate mass at a pixel size between 5 and 10 μm. The molecular range that could be analyzed depends on the technique which is chosen for imaging mass spectrometry. With SIMS (secondary ion mass spectrometry), eg, detectable ions are typically limited to a narrow mass range of only a few hundred Daltons; however, cluster ion sources (eg, C60+ and Bi3+) have effectively extended this limit to ∼2 kDa.

High-resolution imaging mass spectrometry, as MALDI-FT-ICR, has its strength more in the field of low molecular weight compounds, while MALDI-TOF is used for analyzing peptides and small proteins up to 25 kDa.

Recently, several publications demonstrated that higher mass proteins could also be detected and imaged. A new detector enabled proteins up to 70 kDa to be imaged, and proteins up to 110 kDa to be detected, directly from tissue, indicating new directions by which the mass range amenable to MALDI Imaging MS and MALDI profiling MS may be extended.15 The use of the matrix ferulic acid remarkably increased signal acquisition in the mass range of 20k to 150k

Matrix spotted section Mass spectrum UV-Laser Acquisition y MALDI-TOF MS Acquisition x 5000 10000 15000 20000 m/z H&E staining Stained tissue section 5 mm m/z

Principle of MALDI Imaging mass spectrometry. A conventional tissue section is coated with matrix, which extracts molecules from the tissue. Afterwards, the sample is measured in a raster process in the mass spectrometer resulting in spatially resolved mass spectra, while the UV-laser only hits the matrix crystals by unaffecting the tissue section. Subsequent to the MALDI measurement, a histological staining is carried out, therefore allowing a histology-directed analysis of the mass spectra

Overall Average Spectrum Arbitrary units 10000 11000 12000 b Virtual microdissection Arbitrary units Arbitrary units 00 9000 12000 9000 10000 11000 12000 m/z 10000 11000 m/z 100% Stroma 100% Cancer t and ads, to provide social media features and to analyse our traffic. We also share infor analytics partners in accordance with our Privacy Policy. You can manage your prefereng Arbitrary units SA 9000 10000 11000 12000 mz Virtual microdisection. (a) A breast cancer tissue composed of cancer cells and tumor stroma is depicted (H&E) together with a MALDI Imaging average mass spectrum over the whole tumor section. (b) The green-colored mass represents a molecule, which is localized specifically in the tumor stroma, while the red colored mass is present in cancer cells. (c) To generate specific mass spectra for tumor stroma or cancer cells, virtual microdisection is performed. (d) An overlay of tumor stroma and cancer cell-specific mass spectra is depicted. Asterisk highlighting masses that are differentially expressed in tumor stroma or cancer cells demonstrating the advances of the MALDI Imaging approach in comparison to the liquid-based techniques that are not able to separate between different tissue compounds.