Laser-mode pyrolysis

Laser-Mode Pyrolysis. Directs very high energies to the sample, which usually result in ionization and the formation of plasma plumes. Thus, laser pyrolysis results in fewer and sometimes different products than thermal pyrolysis. 

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. 

In both cases, GC fingerprint libraries must be built before quantitative analysis can be routinely carried out. In analysis of QTLC by laser pyrolysis scanning (LPS), the TLC plates are placed in a chamber after development, and were irradiated with an IR laser to produce a high temperature at the location of the spot. The analyte is swept by a carrier gas to a GC, and detected with FID or ECD. The technique combines the separation power of TLC and the detection modes of GC [846]. 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

This fragmentation mode is not altered for silacyclobutanes bearing a vinyl group at the silicon17, as the same Arrhenius parameters are found for the decomposition of 1 and of 1 -methyl-1-vinylsilacyclobutane 3 (logA = 15.64 s 1, E = 62.6 kcalmol-1), in sharp contrast to the pyrolysis of cyclobutanes where a vinyl group accelerates the pyrolysis by a factor of nearly 60018. 2-Silabuta-l,3-diene 4 was produced in a laser-photosensitized (SFg) decomposition (LPD) of 1-methyl-1-vinylsilacyclobutane 3... 

In the past, this type of selective photolysis has been accomplished by exciting a rather complex precursive molecule with infrared laser radiation at a frequency which could select some initial vibrational mode of absorption that would ultimately lead to the desired product. Many of the impressive results that have been achieved with this approach have been summarized in a recent review (1). Unfortunately, two general difficulties have persisted in the conventional approaches. The first has been the tendency to obtain pyrolysis of the parent molecule rather than photolysis. The second is that if viewed as reactive species, laser photons are extremely expensive "substances," their cost being elevated further by the degree of spectral purity and stability that must be maintained. Both... 

The complex anion [HPT C WnC ci]4- has IR bands at 630 and 690 cm"1, assigned as vs, vas (respectively) of the Ti(02) unit.33 The Raman spectra of aqueous sulphuric acid solutions containing titanium indicate the presence of Ti(OI 1)2(804)2(1120)22, 34 The Raman spectrum of a well-characterised TiP207 catalyst has v, of TiOe at 620 cm-1, with v6 at 275/240 cm"1 (together with P03 and P-O-P modes.35 IR and Raman spectroscopy were used to characterise Ti02 nanosized powders formed by TiCl4 laser pyrolysis.36... 

Pyrolysis by laser has involved both high- and low-powered systems, with the former, in general, giving smaller, less-characteristic molecular fragments. The specialized nature of the technique has limited the applications of this pyrolysis mode. However, a relatively cheap system has now been designed that... 

Depending on the heating mechanism, pyrolysis systems have been classified into two groups continuous-mode pyrolyzers (e.g., furnace pyrolyzer) and pulsemode pyrolyzers (e.g., heated filament. Curie point, and laser pyrolysis). All of them are extensively used in polymer characterization and degradation smdies. 

The specialised nature of the technique has limited applications of this pyrolysis mode. High-energy LPyGC has been investigated as a tool in analysis of polymers [361]. CO2 laser PyGC has extensively been applied to polymers by Chinese authors [374-379]. According to this work CO2 LPy can be... 

X-ray fluorescence and photothermal spectrometry are also employed for in situ analysis. It is possible as well to determine elements reliably and quantitatively, after removal from a plate. Laser pyrolysis scanning (LPS) may also be used as a quantification method for TLC [102]. No spray reagent is required for TLC-LPS-FID/ECD. Low ng detection for LPS-FID and pg detection for LPS-ECD is possible. The technique combines the advantage of the separation power of TLC and GC detection modes. 

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