MALDI

MALDI, short for matrix-assisted laser desorption/ionization, is an ionization method for mass spectrometry. The sample is prepared with a MALDI matrix that absorbs the incident laser light and helps release sample molecules into the gas phase as ions. The MALDI matrix is a chemical preparation component; it is distinct from the mass analyzer that subsequently separates the ions. Combining MALDI with time-of-flight mass spectrometry gives MALDI-TOF. The measured mass-to-charge ratio characterizes the resulting ion, rather than directly reporting the mass of an uncharged molecule.

For an illustrative molecule of nominal mass $20000$ atomic mass units, adding one proton gives a singly charged ion with nominal mass-to-charge ratio $20001$, whereas adding two protons gives approximately $(20000+2)/2=10001$. These are simplified assignments that neglect exact mass defects. A second peak therefore need not mean a second molecular species: charge states, isotopes and attached ions also affect a mass spectrum.

In the 1997 tissue-imaging experiment, a MALDI matrix coated a tissue section or its blotted imprint, and an ordered array of mass spectra supplied a spatial map for a selected mass-to-charge ratio. The investigators mapped insulin in rat pancreas and peptides in rat pituitary. This connects the ion source to mass spectrometry imaging: sample position supplies the image coordinates, while the mass spectrum supplies chemical discrimination. A bright pixel alone does not establish an absolute molecular concentration.^[ref]