5.3.2 Matrix Assisted Laser Desorption/Ionization, MALDI

The MALDI technique has revolutionized the analysis of large molecular weight non-volatile compounds, especially synthetic polymers and biopolymers with molecular weights up to 300 000 Daltons. Unlike the Field Desorption technique that desorb and ionize pure analyte from a probe, MALDI volatilizes a mixture of a matrix and analyte in order to “transport” the non-volatile analyte into a vapor phase.

The MALDI technique is completed in two steps. First, a solution of solvent, analyte, and matrix compound are thoroughly mixed and placed on a disk to dry. As the solvent evaporates crystals of matrix containing evenly dispersed analyte molecules are formed. For the second step, the coated disk is placed in the vacuum chamber of the MS. Then the disk is repeatedly pulsed with a laser in the UV or visible spectrum depending on the matrix (Table 1.2). During each laser pulse, the matrix molecules are rapidly volatilized (sublimated/abulated) and carry the individual analyte molecules into a low pressure plasma. The wavelength of the laser is selected to heat and volatize the matrix and to avoid significant heat or degrade the analyte molecules. Analyte molecules are mostly ionized in the vapor phase by photoionization, excited-state proton transfer, ion-molecule reactions, desorption of preformed ions and most commonly by gas-phase proton transfer in the expanding plume by photoionized matrix molecules.

After the analyte molecules are ionized (to cations) they are drawn toward the negative accelerator plate and into the mass filter. A time-of-flight mass filter is always used because of its rapid scanning abilities and large mass range. The introduction of ions into the flight tube is controlled so that all ions reach the detector before the next group enters into the TOF tube. This requires carefully spacing the laser pulses and electric gates (discussed in Section 5.5.4). The spectrum of the analysis is considerably “clean” since only pure analyte is introduced into the MS and essentially no fragmentation occurs (matrix molecules/ions can be ignored by the mass filter due to their relatively low mass). Ionized analytes can acquire +1, +2, and +3 charges and multiple molecules can form dimer and trimer peaks (combined fragments of two or three molecular ions), so the confirmational molecular weights can easily be determined. A very simple illustration of a MALDI-Time-of-Flight MS (the most common combination) is shown in Animation 5.3.

Table 5.1. Frequently Used Matrix Compounds

Animation 5.3. Illustration of a MALDI-TOF MS System.

©Dunnivant & Ginsbach, 2008