Laser-induced pyrolysis

The reactions listed in Table II illustrate the utility that this technique may have in conventional pyrolysis reactions. The homogeneous nature of the laser-induced reaction minimizes undesirable secondary products and results in cleaner processes with higher yields. Laser-induced pyrolysis may prove to be a useful synthetic technique. 

Very fine boron carbide powders of spherical shape and 20-30 nm in size have been prepared by chemical vapor deposition according to (iii). In an Ar-H2-CH2-BCI3 atmosphere a radio frequency plasma produces stoichiometries between Bi5 gC and B3 9C [33, 166]. Also laser-induced pyrolysis of similar gas mixtures with or without acetylene has been employed for the preparation of nano-sized particles [167]. With similar success, composites of B4C and SiC have been produced by the pyrolysis of boron-containing polysilanes [168]. 

Pulsed lasers with relatively short pulse widths (<50 ns) are typically used for laser desorption/ablation techniques because of their high peak powers and because short pulses reduce sample consumption and minimize laser-induced pyrolysis of the sample. Only mass spectrometers that can measure ions of all mass-to-charge values near simultaneously (corresponding to a single short laser pulse) or mass spectrometers that can trap all the ions produced in a single laser pulse are compatible with this pulsed ionization teclmique. Time-of-flight (2,7-9) and Fourier transform mass spectrometers (9-74) are commercially proven for laser desorption/ablation mass spectrometry. The TOFMS detects all ions near simultaneously, and the FT/MS is a ion trapping mass spectrometer that detects all ions simultaneously. 

In another study several simple silenes RR Si=CH2 (R, R = Me, Vinyl etc.) were formed by laser-powered pyrolysis and were found to form linear polymers, in contrast to the usual behavior of silenes which yield cyclodimers when formed by conventional thermolysis techniques16. Reactions of the silenes in the presence of several monomers such as vinyl acetate, allyl methyl ether and methyl acrylate were also studied. Laser-induced decomposition of silacyclobutane and 1,3-disilacyclobutane gave rise to silenes and other oxygen-sensitive deposits17,18. 

Other techniques that have been used to determine polycyclic aromatic hydrocarbons in soil extracts include ELISA field screening [86], micellar elec-tr okinetic capillary chromatography [ 87], supersonic jet laser-induced fluorescence [88,89], fluorescence quenching [90], thermal desorption gas chromatography-mass spectrometry [81,90,100], microwave-assisted extraction [91], thermal desorption [92], immunochemical methods [93,94], electrophoresis [96], thin layer chromatography [95], and pyrolysis gas chromatography [35]. 

R. C. Sausa, V. Swayambunathan and G. Singh, Detection of Energetic Materials by Laser Photofragmentation/Fragment Detection and Pyrolysis/Laser-Induced Fluorescence, ARL-TR-2387, U.S. Army Research Laboratory (2001). 

The technique based on laser-induced breakdown coupled to mass detection, which should thus be designated LIB-MS, is better known as laser plasma ionization mass spectrometry (LI-MS). The earliest uses of the laser-mass spectrometry couple were reported in the late 1960s. Early work included the vaporization of graphite and coal for classifying coals, elemental analyses in metals, isotope ratio measurements and pyrolysis [192]. Later work extended these methods to biological samples, the development of the laser microprobe mass spectrometer, the formation of molecular ions from non-voIatile organic salts and the many multi-photon techniques designed for (mainly) molecular analysis [192]. 

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. 

Other techniques utilize lasers for sample evaporation/pyrolysis and excitation such as laser induced desorption (LID) or laser microprobe mass analysis (LAMMA) (see e g. [1]). Some of the sample introduction procedures in Py-MS enhance the information obtained from Py-MS by the use of time-resolved, temperature-resolved, or modulated molecular beams techniques [10]. In time-resolved procedures, the signal of the MS is recorded in time, and the continuous formation of fragments can be recorded. Temperature-resolved Py-MS allows a separation and ionization of the sample from a platinum/rhodium filament inside the ionization chamber of the mass spectrometer based on a gradual temperature increase [11]. The technique can be used either for polymer or for additives analysis. Attempts to improve selectivity in Py-MS also were done by using a membrane interface between the pyrolyzer and MS [12]. 

A number of anomeric D-hex-2-ulopyranosyl azides have been synthesized and their photochemistry examined. For both a- and P-azides, the major photoproducts arise from cleavage of the C-2C-3 bond and migration of the C-3 carbon to the nitrene centre. Decomposition reactions of a glycidyl azide polymer have been induced by pulsed laser infrared pyrolysis and UV photolysis of thin films at 17-77 K and monitored by IR spectroscopy. The initial step is elimination of N2 and formation of imines, which decompose on warming, possibly with secondary polymerization. 

Therefore we decided to gather new data about the laser-induced decomposition of polyimide and contrast them with results from pyrolysis using the same experimental technique, i.e., diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy. 

In the previous chapter polyimide was analyzed after UV laser irradiation using DRIFT spectroscopy. Various intermediates and products of the laser-induced decomposition could be identified. Experiments with the same material were performed to test whether it is possible to distinguish between UV laser-induced decomposition and thermally induced decomposition, i.e., pyrolysis. 

The aim of this work is to investigate the reaction scheme of the thermally induced decomposition of Kapton in air and compare it to the UV laser-induced decomposition. This should help to decide whether the laser-induced decomposition (ablation) of Kapton is comparable to pyrolysis. Using DRIFT spectroscopy the changes in the concentration of different functional groups of the polymer are monitored during the thermal decomposition process. This information is used to develop a kinetic reaction scheme and to calculate kinetic parameters. 

The experimental setup consists of a gas dosing system and the DRIFT spectroscopy apparatus. For the pyrolysis experiments KBr was selected as matrix, different to the laser-induced decomposition experiments [141], where SiC was used. KBr was chosen because the emissivity did not increase drastically, as in the case of SiC, where it interfered with the measurements. The Kapton-KBr mixtures are placed in the sample holder of the DRIFT cell and packed using a pressure of 1 MPa as described elsewhere [288, 306]. The sample is heated in an inert gas atmosphere to the desired temperature using a heating rate of 10 K min-1. The spectrum of the Kapton-KBr mixture at a given temperature is collected and used as background spectrum. The following experiments were carried out. 

On the other hand, it is unlikely that methylene is formed from methane or other hydrocarbons in the pyrolysis of coke. In contrast to methylene, its analog silylene is a product of the pyrolysis of silane and disilane (8-17 Purnell and Walsh, 1966 Bowrey and Purnell, 1970). Laser-induced fluorescence was used to study the formation of silylene and its reactivity (see, e.g., Baggott et al., 1988 Jasinski and Chu, 1988), but silylene is not within the scope of this book as there are no diazo compounds involved in its chemistry. Literature in which the reactivity of silylene is compared with that of methylene is reviewed briefly in a publication of Skancke (1993, p. 640). 

We have developed a new technique of laser pyrolysis/laser fluorescence, or LP/LF (J ), designed to furnish direct measurement of rate constants of reactions involving free radicals at elevated temperatures (800-1400K). A pulsed CO2 laser is used to heat a sample containing a precursor that pyrolyzes to form radicals. These radicals are then detected using laser-induced fluorescence (LIF). The measurement of the radical removal rates in the presence of added reactant then yields the rate constant for the selected conditions of temperature (T) and pressure (P). 

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. 

Spectral methods based on UV-visible spectrophotometry, laser-induced breakdown spectroscopy (LIBS), infrared (IR), Raman, nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), thermoanalytical and chromatographic methods, especially liquid chromatography (LC) or gas chromatography (GC) combined with pyrolysis are most common. 

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