Laser pyrolysers

Laser pyrolysers are practically the only type of radiative heating pyrolyser with certain applicability. Attempts were made in the past to use a strong light/heat source and... 

The nature and distribution of pyrolysis products from a particular sample critically depend largely on the pyrolysis temperature and the specific set of pyrolysis conditions (i.e. temperature rise time, sample size, pyrolyser geometry). Laser pyrolysers are practically the only type of radiative heating pyrolyser with certain applicability. A laser pyrolyser consists of five components (i) laser (ii) fibre optics (Hi) probe for sample introduction (iv) pyrolyser body, containing the pyrolysis chamber and (v) heater of the pyrolysis chamber with dedicated control unit. The laser beam can be focused onto a small spot of a sample to deliver the radiative energy. This provides a special way to pyrolyse only a small portion of a sample. Only the sample itself is... 

Various laser pyrolyser designs have been reported [351-355]. A relatively cheap (non-commercial) low-power system has been designed that simplifies and improves access to laser pyrolysis (LPy). This system uses a Nd-Cr-GGG laser that delivers 600 mJ pulses of 500 /as with a slow repeat rate of 40 s [355]. The laser energy is delivered to the pyrolysis chamber via an optical fibre. 

The polymer is laser pyrolysed using the conditions described under Apparatus. 

A tunable diode laser system and optics (Laser Photonics, L5736) was used to monitor the species release rates during an experiment. It consists of a liquid-nitrogen cooled diode emitter. For CO absorption measurements the laser emits a beam tuned to a wavenumber of 2082 cm4. This line is chosen as it exhibits strong absorption and is not subject to interference from other species likely to be present in the pyrolysate. 

A problem with lasers is the difficulty of knowing precisely the equivalent temperature of pyrolysis. Also, due to some inherent characteristics of laser pyrolysis, its reproducibility is not always good. Several studies [e.g. 25] showed variability in the total mass of material pyrolysed and difficulties in the control of the pyrolysis temperature. The secondary reactions with the radicals from the plume (although catalytic reactions are probably absent) also make this technique less reproducible. 

Figure 4.7.1. The variation in chromatographic peak heights for n-alkanes generated from torbanite pyrolysed by several techniques [35] A - laser micropyrolysis, B - sealed vessel microscale furnace pyrolysis, C - resistiveiy heated pyrolysis (HP 18580 A Pyroprobe), and D - microfurnace pyrolysis (SGE Pyrojector).

Similarly to Curie point Py-MS. the expansion chamber may be needed for buffering the emission of the pyrolysate and for allowing a longer time for the MS to acquire scans (spectra). Different types of lasers, combinations of lasers, or other experimental setups were reported [52. 53] as utilized in Py-MS. 

The processes taking place in laser Py-MS are not well characterized because more than one effect may happen when the sample is irradiated with the laser beam—laser induced desorption (LID), melting, pyrolysis, ionization, etc. These processes depend on the laser intensity and energy (wavelength) and on the substrate and sample composition. Also, the vacuum in the MS system may play a role regarding the result of irradiation by diminishing any secondary reactions of the pyrolysate. 

In this reaction, the formation of two series of compounds is proposed because in the chromatographic separations of polypeptide pyrolysates, an additional peak is noticed for each 3-alkenyl-5-alkyl-pyrrolidin-2,4-dione. This second peak is assigned to the corresponding 2,4-dialkyl-3,5-diketopyrroline (position isomers are not possible when R2 and R3 are identical) [1]. The list of different compounds from these two classes that may be formed during laser irradiation of different mammalian tissues [13a] due to peptide (protein) pyrolysis and the amino acid pair that can generate them is given in Table 12.2.3. 

In recent years, such attention has been paid to laser Py-GC [56, 85—97]. Lasers were first applied in chemistry about 15 years ago. These were mostly photochemical studies, making use of the unique monochromaticity of laser radiation. Lasers can also be used in Py—GC, and substances can be subjected to laser pyrolysis for analytical purposes. Lasers are particularly suitable for controlled pyrolysis, bearing in mind that energy can be beamed at a definite wavelength on to a small portion of the sample to be pyrolysed. Laser pyrolysis conditions differ considerably from those of thermal pyrolysis, which is why we can speak of a separate Py—GC technique, laser pyrolysis—gas chromatography (LPy-GC). 

If the laser radiation is not absorbed by the sample (e.g., with transparent materials), a substance performing the function of absorption centres (such as powdered carbon or nickel) is introduced into the sample for the latter to be pyrolysed. For example, in ref. 93 it was proposed to decompose transparent polymers (e.g., polyethylene, polystyrene) exposed to a laser beam by placing the samples made in the form of a thin film on the flat surface of a blue cobalt glass rod. The light products are formed primarily in the plasma torch - the rapidly frozen plasma induced by the laser radiation. These products are essentially low-molecular-weight gases whose analysis permits the sample composition to be determined. Such an analysis is known as plasma-stoichiometric analysis [94]. 

The thermal degradation of poly(butylene terephthalate) was examined with the aid of a laser microprobe and mass spectrometry [506]. A complex multistage decomposition mechanism was observed that involves two reaction paths. The initial degradation takes place by an ionic mechanism. This results in an evolution of tetrahydrofuran. This is followed by concerted ester pyrolyses reactions that involve intermediate cyclic transition states and result in formation of 1,3-butadiene. Simultaneous decarboxylations occur in both decomposition paths. The latter stages of decomposition are... 

Further notable developments include renewed interest in the use of laser source pyrolysis, particularly in combination with time-of-flight mass spectrometry, which offers the possibility of targeting particular chemical bonds for degradation, and the coupling of analytical pyrolysis and tandem mass spectrometry, to induce the fragmentation of specific pyrolysate ions. The latter has culminated in the construction of a number of novel instruments, including a pyrolysis ion trap mass spectrometer capable of MS analysis. 

Several advanced PyMS configurations have been described. Boon et al. [712] have presented a multi-purpose external ion source FTICR mass spectrometer for rapid microscale analysis of complex mixtures. External source DT-FTlCR-MS allows obtaining nominal mass spectra, temperature windows, HRMS data and exact elemental composition and MS/MS data on selected ions. For more detailed structural analysis of the more volatile part of the pyrolysate PyGC-MS and PyGC-HRMS are frequently applied. Laser pyrolysis experiments benefit... 

Although FTIR can readily be utilised for the analysis of pyrolysates, and has some advantages over PyMS and TVA, a disadvantage of PyFTIR is the lower sensitivity relative to mass spectrometry. This explains the limited usage of this complementary technique. The sensitivity of pyrolysis-IR spectroscopy is surpassed by pyrolysis-laser photoacoustic spectroscopy, a combination of filament pyrolysis and CO2 laser photoacoustic detection [838]. 

---