Mass spectral techniques laser desorption

Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) is a direct mass spectral technique that has found application for the analysis of various phytochemicals. MALDI represents the ion source, whereas TOF is the mass analyzer. The two techniques may be coupled with other ion sources or mass analyzers, but are most commonly used in tandem. In MALDI, the analytes are mixed with a matrix, which is usually an aromatic organic acid, which aids in... 

Laser and thermal desorption mass spectral techniques provided complementary structural information, and when coupled with current analytical methods to characterise rubber compounds, can provide the necessary information to positively identify various organic species present on the surfaces of vulcanised rubber. 

HPPD-ozone reaction) and protection [202]. Mass spectra were obtained for intact molecular ions (M" ) of organic chemical rubber additives such as the aromatic processing oil, and the aromatic antiozo-nant and AOs incorporated to protect the rubber. Molecular weight information from the molecular ions and structural information from the fragmentation ions could be obtained without interference from the fragmentation peaks of the rubber backbone. Laser and thermal desorption mass spectral techniques provide complementary structural information and can allow positive identification of various organic species present on the surface of vulcanised rubber. 

Various mass spectral techniques have been used in characterizing various metal-containing macromolecules. The focus will be on two techniques. The first one is the most commonly employed mass spectrophotometer setup, called high-resolution electron impact mass spectrometry (HREl-MS). The second system is the matrix-assisted laser desorption ionization (MALDl) system. Illustrative examples of these two procedures are given for the products of Group IVB metallocene dichlorides and the antibacterial drug norfloxacin, scheme 8. 

Mass spectrometry is used to identify unknown compounds by means of their fragmentation pattern after electron impact. This pattern provides structural information. Mixtures of compounds must be separated by chromatography beforehand, e.g. gas chromatography/mass spectrometry (GC-MS) because fragments of different compounds may be superposed, thus making spectral interpretation complicated or impossible. To obtain complementary information about complex mixtures as a whole, it may be advantageous to have only one peak for each compound that corresponds to its molecular mass ([M]+). Even for thermally labile, nonvolatile compounds, this can be achieved by so-called soft desorption/ionisation techniques that evaporate and ionise the analytes without fragmentation, e.g. matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). 

Since its discovery in 1987, matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) has become a common technique in the mass spectral analysis of biopolymers (1, 2). Its ease of operation, theoretically unlimited mass range, and ability to acquire an entire mass spectrum without scanning make the technique an excellent method to analyze high mass biopolymers. Combining such advantages with the capability of analyzing sub-picomole quantities of biopolymers makes MALDI-TOF MS extremely useful for routine mass analysis. 

Lasers have been used as a source of energy for pyrolysis, and several experimental systems were described in Section 4.5. The main use of lasers in mass spectral analysis is associated with several desorption techniques where the pyrolysis is an undesired process. However, laser pyrolysis is also used in direct coupling with an MS system, and a schematic diagram of a laser Py-MS system is shown in Figure 5.4.5. 

MS, especially in combination with advanced separation techniques, is one of the most powerful and versatile techniques for the structural analysis of bacterial glycomes. Modern mass spectral ionization techniques such as electrospray (ESI) and matrix-assisted laser desorption/ionization (MALDI) provide detection limits in the high atto- to low femto-mole range for the identification of peptides and complex carbohydrates. Structural characterization of these trace level components can be achieved using tandem MS. This provides a number of specific scanning functions such as product, precursor ion, and constant neutral loss scanning to... 

Field desorption (FD) and fast atom bombardment (FAB) mass spectrometry provides mass spectral information about compotmds that are not very volatile but these two techniques are not used often in polymer science since they have several disadvantages. Electrospray ionization (ESI) mass spectrometry can also be used to obtain the above information about polymers, but ESI spectra are generally complicated due to differences in charge state distributions. Static secondary ion mass spectrometry (static SIMS) is a surface-sensitive MS technique, which is suitable for studying the interfaces of polymers with respect to chemical structure and molecular weight as well as end groups and surface contaminants. Laser desorption... 

To compensate for the energy spread, the first stage of the two-stage acceleration field can be activated shortly after ion production or ejection from the sample. This event typically occurs in the nanosecond to low microsecond range. The method is known as time-lag focusing or delayed extraction technique. In TOF-MS equipped with matrix-assisted laser desorption/ionization (MALDI) ion source, delayed extraction may improve the mass resolving power by a factor of two- to fivefold [9, 10]. Mass resolving power defines the sharpness of spectral features, which is explained in Chapter 5. 

This relationship between the final state of the sample and MS instramerrtation is mainly a consequence of the type of sample inlet and ionization technique used in a particular mass spectrometer. Referring to Fig. 2.1, the analysis of intact cells by matrix-assisted laser desorption/ionization-MS (MALDI-MS), (JaskoUa and Karas 2011) one of the simplest approaches for microorganism analysis by MS, (Holland et al. 1996) requires the isolation of a pure microbial colony, which is then deposited directly onto the MALDl plate. The subsequerrt mass spectral profiles. 

Successful imaging of samples directly from TLC plates with high spatial and mass resolution and sensitivity was demonstrated using FAB and a time-of-flight mass spectrometer (Busch et al., 1992). Sophisticated two-dimensional scanners have been based on LSIMS (Busch, 1996). The use of laser desorption (LD) in connection with TLC is of interest because very small areas of spots (a few micrometers) can be sampled, the majority of the sample is unchanged for further study, and a liquid matrix is not required (Somsen et al., 1995). The TLC/ FAB-MS-MS technique has been applied to the analysis of nucleotides and bases in order to gain more spectral information and eliminate background matrix interferences found with TLC/FAB-MS (Morden and Wilson, 1995). Instrumentation, procedures, and applications of TLC/MS were reviewed in detail by Busch (1992, 1996). 

---