Laser desorption post-ionization methods

However, this high variability in ionization efficiencies implies that LDl is a very selective ionization method. In some cases, this selectivity is advantageous, for example, when one wishes to observe the presence or concentration of one, known, select molecule of interest.However, in mass spectrometry, one usually is interested in detecting all molecules that are present, including unknown species. Thus, many methods were explored to ionize the molecules after they were desorbed by the laser. These methods are collectively known as laser desorption post-ionization methods, and the post-ionization techniques include electron impact (El, diagrammed in Figure 6.2), chemical ionization (Cl), photoionization (PI), resonant-enhanced multiphoton ionization (REMPI), and many others. 

Ionization of large biomolecules, such as proteins and DNA, with LDI or any of the modified post-ionization methods was limited to very small fragments, typically under 1000 Da due to decomposition of the relatively fragile larger molecules upon ionization. Fragmentation during ionization isn t always a problem, but it s extremely useful to know the mass of the precursor species first, and these ions weren t usually observed. In 1985, two significant developments were made which led directly to the development of electrospray ionization and matrix-assisted laser desorption/ionization (MALDI), for which the Nobel prize was (jointly) awarded in 2002. Electrospray ionization is covered extensively elsewhere in this book, but the development of MALDI is to be addressed here. 

The development glycopeptide libraries obtained by the split-mix method is severely hampered by the lack of concurrent development of a general, facile separation and characterization technology. Some headway has been made with chemical coding of the libraries, but very few direct methods of analysis exist. One promising method that could be applied to the direct characterization of both types of libraries is mass spectrometry. More specifically, post-source-decay matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (PSD-MALDI-TOF-MS) and CID-FAB/MS/MS have been used to characterize glycopeptides.53-55... 

T. Yamagaki, Y. Ishizuka, S.-I. Kawahara, H. Nakanishi. Analysis of glycosidic linkages in saccharide compounds by post-source decay fragment methods in matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy. Rap. Comm. Mass. Spec. 1999, /1,527-531. 

Figure 5. Methods employed for identifiying and characterizing proteins separated by 2DE. Abbreviations used 2DE, two-dimensional electrophoresis ESI, electrospray ionization HPLC, high performance liquid chromatography IR, infrared M, relative molecular mass MALDI, matrix-assisted laser desorption ionization MS, mass spectrometry MS/MS tandem mass spectrometry p/, isoelectric point PSD, post-source decay.

As a first approach to post-ionization of desorbed neutral pol5uners we discuss laser-generated cationization. This approach differs from other desorption/ cationization techniques because the desorption step and the cationization step are completely separate. First polymers are gently laser desorbed at low fluence wifh one laser. Independenfly atomic metal ions are generated by a second pulsed laser that is tightly focused on a metal surface fo creafe a plume of mefal ions. Gas phase collisions above the sample surface subsequenfly produce fhe cafionized complexes. Separation of fhe desorption and cationization processes allows independent optimization of fhe fwo laser/maferial interactions. This approach is especially useful in sifuations where ofher ionization methods fail, such as fhe example of PFPEs discussed here. ... 

The experimental set-up used in this analysis method is shown schematically in Figure 30.13. Essentially, it consists of two independent high vacuum chambers the first chamber is used for both laser desorption and laser post-ionization of the sam-... 

Four of the most commonly used desorption/ionization methods for MSI are secondary ion mass spectrometry (SIMS), desorption electrospray ionization (DESI), matrix-assisted laser desorption/ionization (MALDI), and laser ablation (LA) with post-ionization. Other desorption/ionization approaches such as laser desorption/ionization (LDI) see Chapter 9, (12)), desorption/ionization on silicon (DIOS) (13), electrospray ionization (ESI) (14), and nanostructure-initiator mass spectrometry (NIMS) (15, 16) also have great potential in MSI. Importantly, many mass spectrometers equipped with a MALDI ion source can be used with related ionization processes such as LDI, DIOS, LA, and laser-NIMS. Erequently, a specific ion source arrangement is optimized for a specific mass analyzer for example, MALDI is often interfaced to a time-of-flight (TOF) mass analyzer (described below) although it can also be used with ICR-based instruments. 

The technique of electrospray-assisted laser desorption/ionization (ELDI) combines two well-matured techniques of ionization for the benefit of inproved analysis of samples under ambient conditions. The development of ELDI emanates from the fact that in (MA)LDI by far more neutrals an ions are released from the sample layer (Chap. 11) [45]. Consequently, post-ionization of laser-desorbed neutrals is promising and such methods have indeed been developed (cf. Refs, in [46]). The unique feature of ELDI is to laser-irradiate the sanple in the ambient close to the ESI plume, wherein the neutrals are then ionized by ion-molecule reactions [46]. 

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