Matrix-Assisted Laser Desorption/Ionization (MALDI)

Matrix-Assisted Laser Desorption/Ionization (MALDI)

Matrix-Assisted Laser Desorption/Ionization (MALDI) is a technique used for ionizing macromolecular analytes without fragmenting them, for subsequent analysis by Mass Spectrometry. MALDI utilizes the assistance of laser energizable matrix molecules, to perform Laser Desorption/Ionization of a prepared sample containing the analyte. The sample is prepared by co-crystallization of analyte and matrix molecules from a solution and deposition onto a target plate, for excitation by laser energy. The co-crystallized matrix is then bombarded by laser pulses, inducing localized desorption and gas-phase ionization of co-crystallized molecules. The analyte ions are then identified and quantified by Mass Spectrometry. MALDI usually employs UV lasers, though occasional use of Infrared lasers has been reported.

Suitable matrix compounds display low mass relative to the analyte and high laser energy absorbance, generally in the UV range. They typically have ‘conjugated pi-bond’ structures for UV absorbance, polar functional groups to form aqueous solutions with analyte and acidic groups to facilitate deprotonation. Proven combinations of analytes and matrix compounds are used for MALDI analysis. MALDI requires a very low concentration of analyte, which is mostly dispersed in the interstices of the crystallized matrix. For example, Cinnamic Acid derivatives, which display high absorbance of UV radiation, co-crystallized with polypeptide analytes can desorb/ionize one picomole of analyte for detection. Modern MALDI techniques and instrumentation, can detect molecular masses upto 350,000 Da.

Since MALDI causes minimal break-up of macromolecules, it is used for analysis of a range of biopolymers such as peptides, proteins, oligonucleotides, carbohydrates as well as synthetic macromolecules.

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Common Uses of Matrix-Assisted Laser Desorption/Ionization (MALDI)

• In Biochemistry, for Peptide mass fingerprinting
• Determining molar mass distribution or degree of polymerization of synthetic polymers

• Identification and molecular mass determination of biopolymers 
• In Microbiology, for the identification of microorganisms

 Advantages of Matrix-Assisted Laser Desorption/Ionization (MALDI)

• Sensitive, with high mass accuracy
• Small quantity of analyte is sufficient

 Limitations of Matrix-Assisted Laser Desorption/Ionization (MALDI)

• The Matrix assisted ionization produces limited analyte ions, leading to lower mass sensitivity
• Suitable  sample preparation is needed
• Soft ionization not effective for all molecules

Industrial Applications of Matrix-Assisted Laser Desorption/Ionization (MALDI)

• Biotechnology
• Polymer testing
• Parasitology
• Clinical microbiology