Pyrolysis Gas Chromatography Mass Spectrometry (Py/GC/MS)

Pyrolysis gas chromatography mass spectrometry (Py/GC/MS) is an instrumental method that enables a reproducible characterisation of the intractable and involatile macromolecular complexes found in virtually all materials in the natural environment. It differs from GC/MS in the type of sample analysed and the method by which it is introduced to the GC/MS system. Instead of the direct injection of a highly refined organic solution, a few mg (or in the case of materials with a high organic carbon content, <mg) of the original natural material (e.g. soil, sediment, vegetation, insect cuticle, hair etc.) is analysed directly. The analysis is usually preceded by an extraction of the sample with an organic solvent to remove any free, unbound components with a low molecular mass that would otherwise obscure analytical data pertaining to the high molecular mass components of interest. Extracted samples are then inserted into a quartz chamber in a pyrolysis unit (Fig. 1) that is then heated resistively in an oxygen free enivironment at a pre-set temperature for a number of seconds (e.g. 610 ºC for 10 s). This results in a heat mediated cleavage of chemical bonds within the macromolecular structures of interest producing a suite of low molecular weight chemical moieties the composition of which is indicative of specific types of macromolecule (e.g. lignin, cellulose, chitin etc.). This mixture of compounds is then swept onto the analytical column of the GC and GC/MS proceeds as normal.

For some macromolecules it is desireable to introduce a chemical reagent that, upon heating, will react with the macromolecule in a more directed manner, e.g. the preferential cleavage of ester domains, to produce a mixture of derivatized low molecular weight moieties in a process know as thermally assisted chemolysis. Typical reagents that are used include tetramethylammonium hydroxide (TMAH) and trimethylsufonium hydroxide (TMSH).

A picture of the pyrolysis unit

Figure 1  A picture of the CDS 2500 pyrolysis autosampler used by NERC LSMSF (Bristol)