Ion source:

There are several types of ionization methods in mass spectrometry. The physical basis of ionization methods are very complex and outside the scope of the course. Most common methods are:

(a)  Matrix-assisted laser desorption/ionization ( MALDI)

This method of ionization is a soft ionization method and results in minimum fragmentation of sample. This method is used for non-volatile, and thermally labile compounds such as proteins, oligonucleotides, synthetic polymers. Sample is mixed with 1000 times molar excess of sample and spotted onto a metal plate and dried. Matrix plays a key role in this technique by absorbing the laser light energy and causing a small part of the target substrate to vaporize. Although, the process of forming analyte ions is unclear, it is believed that matrix which has labile protons, such as carboxylic acids, protonates neutral analyte molecules after absorbing laser light energy. Scheme of MALDI is shown in Fig. 3 and some common matrices are listed in Table 1

Figure 3: Scheme of Matrix-assisted laser desorption/ionization (MALDI)