MALDI
/ MAWL-dee /
Imagine you want to lift a fragile snowflake off a table without melting it. Instead of touching it directly with a hot tool, you embed it in a block of sacrificial material, then flash a laser at the block: the material absorbs the blast, puffs into a gentle vapor, and carries the snowflake up unharmed. MALDI lifts delicate molecules into the gas phase in just this protective way.
MALDI stands for matrix-assisted laser desorption/ionization. The sample is mixed into a large excess of a small, light-absorbing matrix compound and dried into a crystal. A laser pulse hits the spot; the matrix absorbs the energy, vaporizes, and carries the embedded analyte molecules into the gas phase while gently ionizing them — usually as single-charge ions.
It matters because MALDI, like electrospray, opened mass spectrometry to large, fragile biomolecules — proteins, peptides, large polymers — and it is fast, tolerant of some contaminants, and well suited to imaging surfaces spot by spot. The caveat is that the choice of matrix is critical and somewhat empirical, the signal can vary spot to spot, and the low-mass region is cluttered by matrix ions.
In a hospital microbiology lab, MALDI-TOF identifies a bacterial species in minutes: a colony is smeared with matrix, zapped by a laser, and its pattern of protein masses is matched to a reference for that microbe.
A light-absorbing matrix lets a laser gently lift large molecules into the gas phase.
MALDI usually makes single-charge ions, unlike electrospray, which tends to make multiply charged ones; that is why MALDI is so often paired with a time-of-flight analyzer, whose wide mass range comfortably handles the resulting high m/z values.