MALDI (Matrix-Assisted Laser Desorption/Ionization)

MALDI (Matrix-Assisted Laser Desorption/Ionization) is a soft ionization technique developed in the late 1980s by Karas, Hillenkamp and, independently, Tanaka (2002 Nobel Prize). It transfers ions of fragile biomolecules (peptides, proteins, oligonucleotides, polysaccharides, polymers) into the gas phase without fragmenting them, paving the way for their analysis by mass spectrometry.

The principle relies on co-crystallization of the analyte with a large excess of an absorbing organic matrix. A UV laser pulse (typically 337 nm N₂ or 355 nm Nd:YAG) is focused on the crystalline deposit. The matrix absorbs laser energy, violently evaporates, and entrains and ionizes the analyte while minimizing fragmentation. The resulting ions, mostly singly charged for peptides, are then accelerated and analyzed.

Standard instrumentation couples MALDI with a TOF (Time-of-Flight) analyzer: ions accelerated by an electric field travel through a vacuum flight tube, and their arrival time at a detector is proportional to the square root of their m/z ratio. Resolution (ability to distinguish two close masses) reaches 10,000 to 50,000 in reflectron mode, and mass accuracy drops to 5-10 ppm with internal calibration, 50-100 ppm externally.

The most used matrices are α-cyano-4-hydroxycinnamic acid (CHCA) for peptides up to 10 kDa, 2,5-dihydroxybenzoic acid (DHB) for peptides and glycopeptides, sinapinic acid (SA) for whole proteins, and dithranol or HABA for polymers. Choice of matrix and deposition protocol (dried droplet, thin layer, sandwich) strongly influence spectrum quality.

Applications cover synthesis peptide quality control (verification of theoretical molecular mass), peptide mapping after proteolytic digestion (tryptic peptide fingerprints), top-down and bottom-up proteomics, MALDI tissue imaging (spatial distribution maps of biomolecules on thin sections), and characterization of post-translational modifications.

MALDI strengths: high tolerance to salts and contaminants (superior to ESI), fast sample preparation, simple single-charge spectra easing interpretation, possible miniaturization on MALDI-plate stencils. Limitations: lower temporal resolution than LC-MS coupling, difficult absolute quantification, matrix that can mask low-mass ions (< 500 Da), and ion suppression in complex mixtures.