Laser pyrolyzer

Most of the pyrolysis experiments in the field of natural polymer characterization involve the use of Curie-point instruments or resistance-heating apparatus. Both methods are described in the following sections. Laser pyrolyzers are not yet common in biopolymer analysis and will not be discussed here. 

Laser pyrolyzers are practically the only type of radiative heating pyrolyzer with certain applicability. Attempts were made in the past to use a strong light/heat source and focus the beam with lenses [20] to achieve the desired power output. However, the laser as a radiative energy source is much more convenient. The laser beam can be focused onto a small spot of a sample to deliver the radiative energy. This provides a special way to pyrolyze only a small portion of a sample. A variety of laser types were used for pyrolysis purposes normal pulsed, Q-switched, or continuous wave (cw) [21], at different energy levels. More common are the normal pulsed high-power lasers. 

The second apparatus uses a pulsed laser (pulse energy of about 1 joule) and an alignment laser. In order to direct the high energy laser beam to the right place on the sample, some commercially available pulsed laser pyrolyzers have a low energy laser, or alignment laser, which allows the selection of the desired spot on the sample to perform the pyrolysis. 

Another typical property of the laser pyrolysis is that it can achieve very short TRT times and also very short cooling times, in the range of 100 to 300 jis. This will contribute to the uniqueness of the degradation conditions for the laser pyrolysis, which are rather different from the other types. In addition to this, the capability to pyrolyze only a very small area of the sample is characteristic for most laser pyrolyzers. This directional nature can be of exceptional utility when combined with the microscopic inspection of a particular sample. Inclusions and inhomogeneities in the samples, etc. can be analyzed successfully using this technique. 

The synthesis and electrochemical properties of carbon films prepared from positive photoresist have been reported.The initial direction for this work was the fabrication of carbon interdigitated electrodes. In this work, positive photoresist was spin coated on a silicon substrate, patterned by photolithography, and pyrolyzed to form the carbon electrode. In more recent work, laser excitation has been used to both pyrolyze the film and to write the electrode pattern. ... 

Carbon films and graphite films have been prepared by vacuum evaporation [28,42,76,77], pyrolysis [29,36,78-83], screen printing [46,62,63,65,66,84], and laser photoactivation of sites on a graphite or glassy carbon substrate [85]. Various pyrolytic processes have been successful, most based on the deposition of volatile precursor compounds. For example, methane can be pyrolyzed while... 

CA 73, 100610 (1970) A pulsed ruby laser-mass spectrometry technique was developed and applied, wherein granular mixts of AP and lightabsorbing substrate materials were rapidly flash pyrolyzed (0.8msec) within the low-pressure ion-source chamber of a Bendix TOP mass... 

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 high. Several studies [27] showed variability in the total mass of material pyrolyzed 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. More recently, considerable effort was put into improving the reproducibility of the use of laser techniques for pyrolysis. 

The reproducibility of the results for heated filament pyrolyzers and Curie point pyrolyzers as well as the comparison between the two systems was reported for a number of materials [41], The reproducibility of the analysis was evaluated both qualitatively and quantitatively. It was found that for most samples the results are obtained with very good reproducibility for the same instrument. However, differences in the instrumentation may play an important role regarding the dissimilarity of the results, even when they are operated at comparable parameters. These differences are typically less pronounced between filament pyrolyzers and Curie point pyrolyzers. Also, microfurnace pyrolyzers are closer to filament pyrolyzers than large furnace ones. On the other hand, laser micropyrolyzers or sealed vessel furnace pyrolyzers may lead to quite different results. 

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]. 

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). 

To analyze for photoelectrolysis activity several groups use custom-built scanning laser screening systems where a laser is rastered over the printed and pyrolyzed substrates that are immersed in an aqueous electrol3Te. Scanning the... 

Mass spectrometry This is often used for analysis of plastics. Thermal degradation can be carried out using an ion source, or in a pyrolyzer connected directly to a mass spectrometer or via a chromatographic column. Two soft ionization techniques, electrospray ionization and matrix-assisted laser desorption ionization (MALDI), are the most common in the analyses of large molecules. MALDI-MS is usually used offline but online connection with a... 

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. 

Heating should be instantaneous to prevent drawn out transfer of the pyrolyzates through the injection port. Heated filament and Curie-point pyrolysis result in less secondary pyrolysis products compared to furnace pyrolysis. Curie-point pyrolyzers accurately reproduce pyrolysis conditions with a rapid temperature rise time, yet the choice of different pyrolysis temperatures is limited. Very little sample preparation or pretreatment is required for laser pyrolysis however, a specific laser wavelength may not be appropriate for aU types of samples. 

F. G., Ishigaki, T., Smith, D.R., Snznki, S., Willett, G.D. (1999) Laser ablation mass spectrometry of pyrolyzed Koppers coal-tar pitch a precursor for fullerenes and metallofullerenes. Journal of Physical Chemistry B, 103, 9450-9458. 

Mass spectrometry using alternative ionization and sample preparation methods are employed in ink and paint analysis. The oldest of these techniques is based on pjnrolysis of the sample (typically, a paint) prior to its introduction into the GC. Detectors for PyGC are and FID. Pyrolysis patterns can be examined in the same way accelerant patterns are (Chapter 10), but increasingly, GCMS is preferred over FID. Pyrolysis is, by definition, destructive, but the sample size is reasonably small, and recently a micropyrolysis GCMS has been developed and applied to photocopier toners and paint. A laser is focused on the sample through a microscope, and the pyrolysis vapor product is directed into the GCMS system. The pattern of the pyrolyzates and chemical composition... 

---