Liquid secondary ion mass spectrometry (LSIMS)

LSIMS is similar to FAB, with only difference that an ion is used for bombarding the samples. Usually argon, xenon, or caesium ions are used for the LSIMS.

Laser desorption

Laser desorption or laser ablation is the ionization method wherein an intense laser beam is focused on a solid sample resulting in ablation of mass from the surface. The laser pulse causes both desorption and ionization of the molecules. The ions generated are short-lived and therefore detected simultaneously. Laser desorption, however, causes fragmentation for large molecules (Molecular mass >500 Da) therefore restricting its use to small molecules.
Owing to the difficulty in generating gas phase ions of biomolecules like proteins and nucleic acids, the application of mass spectrometry, till late 1980s, was largely restricted to the small organic molecules and biomolecules (amino acids, peptides, oligonucleotides, etc.). Therefore, mass spectrometry was not of much use for biochemists. Advent of two ionization methods around 1987-88 revolutionized the area of biomolecular mass spectrometry. Both these methods are routinely used for identifying and characterizing the biomolecules.

Matrix-assisted laser desorption ionization (MALDI)

MALDI is basically a laser desorption ionization method wherein ionization is assisted by small organic molecules, called matrix. The matrices used in positive ion mode MALDI mass spectrometry are organic acids that have strong absorption for the wavelength of the laser used. Some of the commonly used MALDI matrices are listed in Table 11.1.

The sample for MALDI is usually prepared in one of the following ways:

1. mixing the analyte solution with the matrix solution → deposition of the mixture on a metallic plate → complete drying of the sample
2. Deposition of the matrix on the metallic plate → drying of matrix → addition of analyte solution → drying of analyte solution

Desorption and ionization is achieved by applying the laser pulse on the dried sample. Although the exact mechanism behind MALDI is not completely understood, it is believed that the absorption of light by the matrix molecules causes sublimation of matrix crystals carrying along with them the analyte molecules into the gas phase (Figure 11.7).

 

Figure 11.7 Diagram showing ionization in a MALDI source