Ion Sources for LDI and MALDI

Both LDI and MALDI make use of the absorption of laser light by a solid sample layer. The energy uptake upon laser irradiation then causes evaporation and ionization of the sample. Wavelengths ranging from ultraviolet (UV) to infrared (IR) have been employed, e.g., nitrogen lasers (337 nm), excimer lasers (193, 248, 308 and 351 nm), Q-switched, frequency-tripled and quadrupled Nd Yag lasers (355 and 266 nm, respectively), [24] Er Yag lasers (2.94 pm) [24,25] and TEA-CO2 lasers (10.6 pm). [16,26] 

The general setup of LDI/MALDI ion sources is comparatively simple (Fig. 10.2). [27] The pulse of laser light is focused onto a small spot which is typically 0.05-0.2 mm in diameter. [28] As laser irradiance is a critical parameter in MALDI, a variable beam attenuator in the laser optical path is employed to adjust 

Note The vast majority of MALDI instruments use UV nitrogen lasers (337 nm, 3 ns). IR-MALDI has been restricted to applications where its deeper penetration offers advantages, e.g., for the direct desorption of analytes from sodium dodecyl sulfate (SDS) gels or thin layer chromatographic (TLC) plates. 

The mechanisms of ion formation in MALDI are a subject of continuing research. [30-34] The major concerns are the relationship between ion yield and laser flu-ence, [28,35] the temporal evolution of the desorption process and its implications upon ion formation, [36] the initial velocity of the desorbing ions, [29,37,38] and the question whether preformed ions or ions generated in the gas phase provide the major source of the ionic species detected in MALDI. [39,40] 

Although lasers of both ultraviolet (UV) and infrared (IR) wavelengths are in use, UV lasers are by far the most important light sources in analytical LDI-MS and MALDI-MS. Among these, nitrogen lasers and frequency-tripled or quadrupled Nd Yag lasers serve for the majority of applications [27]. IR-MALDI is dominated by Er Yag lasers [27,28] while TEA-CO2 lasers (cf. IRMPD in Chap. 9.12) are rarely used (Table 11.1) [17,29]. 

Spechal range Wavelength Photon energy Laser type 

The mechanisms of ion formation in MALDI are a subject of continuing research [31-35]. The major concerns are the relationship between ion yield and laser flu-ence [26,36], the temporal evolution of the desorption process and its implications 

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