Matrix-Assisted Laser Desorption/Ionization (Molecular Biology)

Matrix-assisted laser desorption/ionization (MALDI) is a method for producing ionic forms of molecules for analysis by mass spectrometry (MS). Gas phase ions are generated by the laser vaporization of a mixture of the molecule of interest, the analyte, in a solid matrix in which the matrix (usually a small crystalline organic compound) strongly absorbs the laser radiation and acts as a receptacle for energy deposition (Fig. 1) (1). This concentrated energy deposition results in the vaporization and ionization of both the matrix and the analyte ions, which contain relatively few charges. Low-molecular-weight analytes (<20 kDa) are typically ionized with only one or two charges, while larger analytes can accumulate as many as 3 to 5 charges, depending on the specific desorption conditions (ie the type of matrix material and the laser power). The relatively low number of charge states observed in MALDI makes the technique especially well suited for the analysis of multicomponent mixtures, as individual components can be easily identified by the mass spectral signal generated from their 1+ charge state.

Figure 1. The matrix-assisted laser desorption/ionization process.

The other commonly used ionization technique is electrospray ionization (ESI). Because ESI and MALDI are different ionization techniques, they offer different capabilities that in many cases are complementary (Table 1). For example, since MALDI is especially well suited for analyses like complex mixtures of peptides, a protein can often be identified by MS analysis of its constituent peptides produced by either chemical or enzymatic treatment of the sample (Fig. 2). This information can be especially useful when used in conjunction with protein databases or in identifying post-translational modifications (2).

Figure 2. Examples of data generated on a MALDI-MS. Proteins (left, inset) typically produce positive singly or doubly inset) also produce singly and doubly charged positive ions. Proteolytic digest of the proteins (left) offers information ab identify the protein when used in conjunction with a protein database searching program. Exonuclease digestion of DNA.

Table 1. Capabilities, Limitations, and Recent Improvements of ESI-MS and MALDI-MS