MALDI (source code)
= MALDI
{c}
\b[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 $20\,000$ atomic mass units, adding one <proton> gives a singly charged <ion> with nominal <mass-to-charge ratio> $20\,001$, whereas adding two <protons> gives approximately $(20\,000+2)/2=10\,001$. 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.<Molecular imaging of biological samples: localization of peptides and proteins using MALDI-TOF MS by R. M. Caprioli and others>[Abstract and pp. 4751–4760: matrix-coated sections, ordered spectral arrays and the pancreas/pituitary demonstrations.]{ref}
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