Matrix-Assisted Laser Desorption Ionization (MALDI)

The matrix is an organic compound, such as 2,5-dihydroxybenzoic acid, that absorbs laser energy and facilitates ionization. When mixed with the analyte and dried, the matrix forms small crystals incorporating the sample molecules. During laser irradiation, the matrix vaporizes and transfers protons to analyte molecules, generating the ions needed for mass analysis.

MALDI ionizes biomolecules through proton transfer between the matrix and analyte. When the pulsed laser heats the matrix, it vaporizes and donates protons to analyte molecules, creating protonated ions. These newly formed ions are then accelerated by an electric field for separation and analysis based on their mass-to-charge ratio.

MALDI can identify and characterize various biomolecules including proteins, peptides, nucleic acids, and carbohydrates. It is particularly effective for polar biopolymers with molecular masses ranging from a few to a hundred thousand Daltons, making it valuable in biological, medical, pharmacological, and biochemical research applications.

Proper sample preparation is critical for MALDI success. The analyte is mixed with matrix material and deposited onto a probe target, then dried to allow matrix crystals to form and incorporate analyte molecules within their structure. This crystalline incorporation ensures efficient laser energy absorption and subsequent ionization of the embedded sample molecules.

After protonated ions form through proton transfer, they are accelerated by an electric field and separated based on their mass-to-charge ratio in a time-of-flight mass analyzer. The resulting mass spectrum provides valuable information about the mass and relative abundance of analyzed biomolecules, enabling molecular identification and characterization.

Laser desorption is the process where matrix molecules absorb pulsed laser energy, heat up, and vaporize rapidly. This vaporization carries embedded analyte molecules into the gas phase where they can be ionized. Without efficient desorption, analyte molecules would remain in the solid phase and could not be analyzed by the mass spectrometer.

MALDI is one of several ionization methods used in mass spectrometry. Unlike electrospray ionization ESI mass spectrometry, which uses electrical spray to ionize samples in solution, MALDI uses laser energy and matrix-assisted proton transfer for solid or crystalline samples. Each technique has distinct advantages depending on the analyte type and research application.