PYROLYSIS GAS CHROMATOGRAPHY
GC is always operated at elevated temperatures. As discussed previously in the section on stationary phases above, GC experiments can be run at temperatures of up to about 350 °C. At higher temperatures, the stationary phase will begin to bleed off the sides of the capillary and destroy the column. Any substance which is stable at these temperatures and which can easily be vaporized is a good candidate for GC. There are, however, many substances that could be otherwise analyzed by GC except that they do not vaporize to any appreciable degree at these temperatures and cannot be analyzed by normal GC methods. These substances include fibers, paints, plastics, hairs, and other polymers. A modification of GC, called pyrolysis gas chromatography (PGC), can make it possible for a gas chromatograph to analyze polymers. The term “pyrolysis” means essentially “to heat in the absence of air.” If a poly mer such as a fiber is heated to very high temperatures, up to 1000 °C, in the absence of oxygen, it will not burn but instead will decompose into stable fragments, called pyrolyzates. If this process is done repeatedly under the same conditions, the number, size, and relative amounts of the pyrolyzates will be the same for a particular polymer type. In PGC, an apparatus that can hold a small fragment of polymer is inserted directly into the injector of the gas chromatograph. There is no oxygen present in the injector because of the inert mobile phase gas, so only pyrolysis and not combustion takes place. The pyrolyzer is then heated to high temperatures, generally 700–1000 °C and the polymer decomposes. The fragments are then separated, the same as the components of any analyte. The resulting chromatogram is called a pyrogram. Figure 1: shows a typical apparatus used in pyrolysis. The wand has a coil made of platinum wire at the tip. A quartz glass tube is used to hold the sample. The tube is plugged at both ends with quartz glass wool. The tube is threaded into the platinum coil and then the tip of the wand is inserted directly into the inlet of the gas chromatograph. The coil is heated and the sample pyrolyzes, forming vapor phase pyrolyzates that are swept into the stationary phase by the mobile phase.
FIGURE 1: A pyrolysis unit. The quartz tubes hold the samples and can fit inside the platinum coil on the end of the probe. The probe is then inserted into the inlet of a gas chromatograph and then the platinum coil is heated under controlled conditions. The sample in the quartz tube pyrolyzes and the fragments are separated by the GC.
