Laser desorption techniques

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

Liquid (solvent) extraction is not the only way of sample preparation, but stands along with various forms of heat extraction (headspace, thermal desorption, pyrolysis, etc.) and with laser desorption techniques. 

Sterically hindered phenols and other additives containing thioesters, phosphites, phosphonites and hindered amine moieties were analysed by FAB-MS and LD-FTMS. The laser desorption technique was preferred for analysis of polymer additives because of undesirable fragmentation from FAB experiments [93]. 

Laser desorption methods (such as LD-ITMS) are indicated as cost-saving real-time techniques for the near future. In a single laser shot, the LDI technique coupled with Fourier-transform mass spectrometry (FTMS) can provide detailed chemical information on the polymeric molecular structure, and is a tool for direct determination of additives and contaminants in polymers. This offers new analytical capabilities to solve problems in research, development, engineering, production, technical support, competitor product analysis, and defect analysis. Laser desorption techniques are limited to surface analysis and do not allow quantitation, but exhibit superior analyte selectivity. 

Ideally, scientists would like to be able to perform laser desorption and analysis directly, but typical laser wavelengths cause fragmentation of bacteria and other particles. Due to the low energy produced by infrared lasers, however, bacterial fingerprints can indeed be obtained as shown by researchers at Lawrence Livermore National Laboratory. It is also possible to detect much larger species, an impossible task with earlier technology. Infrared laser desorption techniques are undergoing constant improvement. 

Imaging MS is and will become increasingly critical for many aspects of materials science. One example is in the semiconductor industry, where the ability to provide spatial and chemical information on the length scales of current integrated circuit fabrication (50 nm or better) with depth profiling to provide layer-by-layer maps of the fabricated layers is critical for the continued advancement of the computer industry. Maps of any heterogeneous surface are important in other areas of materials science. For example, using various laser desorption techniques, information about the molecules found in specific inclusions in meteorites or defects in reactive surfaces can be obtained. 

Luhrs et al. have observed the 0-0 band of the first itn transition of adenine and 9MA at 36105 cm ( 277 nm, 4.48 eV) and 36136 cm ( 276.7 mn, 4.48 eV), respectively, and these results are in accordance with the observation made by Kim et al. Similar results were also foimd fi om the REMPI study by Nir at al who used the laser desorption technique instead of heating the samples. 

Several modifications of MALDI have been developed to couple additional sampling and reaction capabilities to this technique. Surface-enhanced laser desorption ionization (SELDI) is one type of modified MALDI and describes an ionization process that involves reacting a sample with an enhanced surface. With SELDI, the sample interacts with a surface modified with some chemical functionality prior to laser desorption ionization and mass analysis. For example, an analyte could bind with receptors or affinity media on the surface, and be selectively captured and sampled by laser desorption. A SELDI surface can be modified for chemical (hydrophobic, ionic, immunoaffinity) or biochemical (antibody, DNA, enzyme, receptor) interactions with the sample. This technique can act as another dimension of separation or sample cleanup for analytes in complex matrices. As discussed before, one disadvantage of MALDI is that the matrix (usually a substituted cinnamic acid) that is mixed with the sample can directly interfere with the analysis of small molecules. There have been several areas of research to overcome this issue.Direct ionization on silicon (DIOS) is an example of a modification of MADLI that eliminates the matrix. In this case, analytes are captured on a silicon surface prior to laser desorption and ionization. Other examples of matrix-free laser desorption techniques include the use of siloxane or carbon-based polymers. 

Laser desorption technique has recently been shown to be useful for the ionization of non-volatile biomolecules . Quasimolecular ions are desorbed by submicrosecond laser pulses delivering about 10 Watt/cm to the sample. Ionization occurs mainly by alkali attachment and, for some classes of molecules, by proton transfer as well. The internal energy of these ions is rather low giving rise to a moderate fragmentation only. 

In 1988, a laser-based desorption method was developed (10) which produced ions of both high molecular mass synthetic polymers and biopolymers. In that same year, the laser desorption technique using an organic molecule matrix now commonly called MALDI was developed (11). The technique was originally described for biopol5miers. In 1992, it was shown that the MALDI technique could be applied to synthetic pol5miers (12-15). 

Gas phase spectroscopy of jet-cooled neutral molecules benefited from the development of laser-vaporisation and laser-desorption techniques as well as their coupling with a supersonic expansion. Improvements of spectroscopic procedures involving several lasers, e.g. the IRAJV double resonance spectroscopy, helped to collect information on weakly populated conformers. 

Additive analysis may be carried out by examination of extracts or dissolutions of the polymer, by non-destructive spectroscopic (in-polymer) testing of solid or melt, or by degradative testing using thermal methods mainly through the examination of volatiles released ( thermal extraction ). In this Chapter we consider thermo-analytical and pyrolysis methods applied to polymer/additive formulations as received Chp. 3 deals with laser desorption techniques. 

Laser desorption (ionisation) - mass spectrometry (LDMS or one-step LDI-MS) experiments may be manipulated by varying (i) sample presentation (ii) power density, wavelength and pulse duration of the laser desorption technique to ablate the sample and (Hi) mass analyser type combined with a suitable ionisation mode. At variance to MALDI experiments in LDI-MS no matrix is required and no special sample preparation of any kind is necessary. The sample can be subjected to the pulsed desorption laser in a variety of ways. Most commonly, the sample is presented as a solid layer, deposited from solution, on a substrate, which absorbs at the laser wavelength. There is a well-defined threshold, above which successful desorption of neutrals can... 

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