A single colony is picked from an agar plate and transferred onto a MALDI target plate. The sample is then overlaid with 1-2 µL of matrix.
The matrix is a low-mass, organic compound that coats the targets, most commonly, α cyano-4 - hydroxycinnamic acid (CHCA). The compound is solubilized in a trifluoroacetic acid and acetonitrile (ACN) mixture. When combined with target molecules, cells are disrupted, and protein and other cellular components are extracted. When dry due to evaporation of solvents, components are crystallized; thus, the “matrix-assisted” component.
The target plate is now ready to be placed into the mass spectrometer’s ionization chamber where brief laser pulses are fired at the sample. The matrix “buffers” the sample, preserving the integrity of protein components. The transfer of laser light energy into heat allows the sample to be released or desorbed as singly-charged ions; thus, the “laser-desorption” component.
4. Time of flight
The desorbed and ionized molecules are mobilized and as they pass through the vacuum tube and hit the detector, their time of flight is measured. Molecules of smaller size travel faster than larger molecules and as they 'hit' the detector they generate a spectrum (or fingerprint) based on their mass to charge ratio. The fingerprint (or unique series of peaks) is then interpreted in a database to assess identification. For routine microbial identification, the range of mass to charge values for adequate identification is 2-20 KDa.