Other Desorption Ionization Techniques

Several other desorption ionization methods have been described, including laser desorption ionization and SIMS [21, 34]. In Cf plasma desorption ionization 

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B. Matrix-Assisted Laser Desorption Ionization and Other Desorption Ionization Techniques for the Identification of Combinatorial Libraries... 

Which of these species is formed depends on the nature of the sample. Lipophilic or aprotic molecules tend to form M ions whereas [M + H]" ions form from polar molecules such as peptides and carbohydrates by intermolecular proton transfer. The intensity of [M + H] ions can often be enhanced in FD as well as in other desorption ionization techniques by treatment with acid. Because of the ambiguity as to whether or [M + H] is formed, precise molecular weight determination of an unknown sample by FD can be a problem. By mixing the sample with a metal salt, for example lithium chloride, [M +cation] ions may be produced, defining the molecular weight unambiguously 18). 

There are other MS ionization techniques applied to analyze pyrolysates such as secondary ion mass spectrometry (SIMS) [66], californium plasma desorption [67], fast atom bombardment (FAB) [68]. 

Other Recently Developed Desorption Ionization Techniques... 

The number of El/ED-MS publications rose rapidly in the early to mid-1970s, reaching a peak in the late 1970s. A noticeable drop in FI/ED use occurred after 1983 because of the advent of fast atom bombardment (EAB-MS), and later other desorption/ionization methods. A high percentage of FI/FD-MS articles have come from Germany, which is not surprising in view of the early development of the techniques in Borm. 

At any length, those who think of FI-MS and FD-MS as pioneering techniques diat are no longer useful should rethink their position. It is premature to put these methods on the endangered species list. FI-MS and FD-MS have some attractive features that make them both complementary to other desorption/ionization methods and unique in their applications. 

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. ... 

APCI is a relatively soft ionization technique. In fact, only few fragment ions are normally recorded. Nevertheless, analyte decomposition may occur due to heating. It can be used in the analysis of polar and low polar analytes with molecular weights up to 1500 Da [29] as long as the proton affinity of the analytes is higher than that of the solvents. On the other hand, ionization techniques such as ESI and matrix-assisted laser desorption/ionization (MALDI) are mainly used in the analyses of polar, less volatile, and thermally labile analytes, for example, large biomolecules. These two ionization techniques are discussed in the following (Sections 2.4 and 2.6). 

Of the two related techniques, FAB found far greater use in studies of enantioselective discrimination as compared to other desorption/ionization methods, such as MALDI and secondary ion mass spectrometry (SIMS). Chan and coworkers demonstrated enantiodiscrimina-tion of amino acids by a-, P-, and y-cyclodextrins using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) [28]. The observed levels of enantioselectivity were found to be dependent on the size of cyclodextrin cavities, as well as on the length and functionality of the amino acid side chain. Vairamani and coworkers demonstrated discrimination of amino acid methyl esters using various monosaccharide hosts by liquid secondary ion mass spectrometry (LSIMS) [29]. It is curious that more work has not been done using these sources. MALDI, in particular, is a simple and straightforward technique. Various researchers have demonstrated the observation of noncovalent complexes [30-32], for example, between peptides and proteins, but relatively little work has been performed that focuses on studying enantioselective noncovalent interactions by MALDI-MS. 

In addition to DESI and AP-MALDI, a large variety of other, sometimes closely related, atmospheric-pressure desorption ionization techniques have been introduced in the past decade, connected to a huge number of acronyms. Van Beikel et al. [76] tried to classify these emerging techniques into four categories, i.e., (1) thermal desorption ionization, (2) laser desorption/ablation ionization, (3) liquid-jet and gas-jet desorption ionization, and (4) hquid extraction surface sampling probe ionization. 

The 1970s and 1980s saw the advent of several new soft desorption/ionisation methods, many of which are now well-established in analytical MS. MALDI, electrospray ionization (ESI), and a few other desorption/ionisation techniques have important applications in polymer analysis. 

The outstanding success of FABMS has tended to obscure the earlier advances in other soft ionization techniques in obtaining molecular weight information on underivatized peptides. Field desorption (FD) and plasma desorption (PD) have been particularly successful in this respect. 

On the other hand, there are some ionization techniques that are very useful, particularly at very high mass, but produce ions only in pulses. For these sources, the ion extraction field can be left on continuously. Two prominent examples are Californium radionuclide and laser desorption ionization. In the former, nuclear disintegration occurs within a very short time frame to give a... 

The focus of this chapter is the development of a technique often called wholecell matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) or whole-cell MALDI-TOF MS. Some groups prefer to use terms such as intact or unprocessed rather than whole, but the intended meaning is the same regardless of which word is used. As noted in the first chapter of this book, there are many different methods for the analysis of bacteria. However, for the analysis of intact or unprocessed bacteria, whole-cell MALDI-TOF MS is the most commonly used approach. This method is very rapid. MALDI-TOF MS analysis of whole cells takes only minutes because the samples can be analyzed directly after collection from a bacterial culture suspension. Direct MALDI MS analysis of fungi or viruses is similar in approach1,2 but is not covered in this chapter. MALDI-TOF MS of whole cells was developed with very rapid identification or differentiation of bacteria in mind. The name (whole cell) should not be taken to imply that the cells are literally intact or whole. Rather, it should be taken to mean that the cells that have not been treated or processed in any way specifically for the removal or isolation of any cellular components from any others. In whole-cell analysis the cells have been manipulated only as necessary to... 

Other MS based analytical approaches have occasionally been applied to ancient resin samples, in particular for paint varnishes. Such techniques include FABS (fast atom bombardment mass spectrometry) [35], MALDI (matrix assisted laser desorption-ionization mass spectrometry) and GALDI (graphite assisted laser desorption-ionization mass spectrometry) [36 38]. 

The second advantage is the possibility of performing chemical imaging analyses. Even if other techniques, such as matrix-assisted laser desorption/ionization, also allow images to be acquired, ToF-SIMS is, for now, the mass spectrometric technique with the best spatial resolution performance. 

A recently developed technique that has found extensive use in the characterization of dendrimers, specifically in determining the purity and monodispersity of these novel materials, has been matrix-assisted laser desorption/ionization (MALDI) mass spectrometry [16,17]. For dendrimers grown by the divergent strategy, incomplete functionalization of the periphery can lead to subsequent failure sequences and loss of strict dendritic growth. Observation and quantification of these defects is extremely difficult by other techniques, however MALDI mass spectrometry has been successfully employed by a variety of authors to... 

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