DESI Mass Spectrometry Imaging

For Mass Spectrometry Imaging (MSI), we employ the Xevo-G2XS instrument coupled with a 2D Desorption Electrospray Ionization (DESI) stage, equipped with an XS generation DESI sprayer. The DESI source facilitates direct material analysis under atmospheric conditions, requiring minimal sample preparation and eliminating the need for a matrix. It operates by spraying an ionized solvent at high velocity onto the sample surface, generating a charged plume of droplets. These ions are then directed into the mass spectrometry by the vacuum of the source. The Xevo-G2XS Quadrupole Time-of-Flight instrument is capable of both MS and MSe, offering over 30,000 FWHM resolution at 556.2771 m/z. MSI enables the visualization of the spatial distribution of various molecules, including lipids, metabolites, peptides, and proteins, within biological samples.

The DESI Imaging System was partly supported by the National Cancer Institute of the National Institutes of Health under Award Number NIH R01 CA204254 (PI: Baowei Fei).

Protocol

Cryosectioning using CMC embedding for mass spectrometry imaging

1. Make a 2.5% carboxymethylcellulose (CMC) solution in Milli-Q water (metal stir bar).
2. Store in a glass bottle.
3. To embed, pick the correct size of cryomold based on the sample size. For example, mouse brain embedding in the standard (25 mm × 20 mm × 5 mm) size works well.
4. Add a drop of CMC to the bottom of the mold, place it in a cryostat, and mount the sample directly (to avoid the sample floating) onto the drop in the correct orientation (coronal or sagittal) before it freezes.
5. Once the sample is secured to the bottom of the mold, pour CMC to completely cover the tissue section.
6. Wait till the CMC freezes; the color will go from clear to cloudy.
7. Remove the whole CMC block containing the tissue.
8. To mount the block to the sample mount (OCT-free), use a couple of drops of CMC to mount the block.
9. Clean cryostats and brush with ethanol to eliminate residue OCT.  
10. Sectioning can be done between -21°C to -23°C.
11. Slides can be 10 – 20 µm thickness.

Supplies:

• Carboxymethylcellulose sodium salt C4888
• Cryomold

By Cindy Nghe
Posted under Mass Spectrometry