Figure 1. Pyrogram of a lyophilized sludge sample (700°C, split 1:50, MSD).
Figure 2. MSD TIC chromatograms of a lyophilized sludge sample; top: After thermal desorption at 275°C, bottom: Pyrogram resulting from pyrolysis of the same sample at 700°C immediately after the thermal desorption step.

Fractionated Pyrolysis

Using fractionated pyrolysis multiple thermal desorption and/or pyrolysis steps are performed on a single sample at increasing temperatures providing more in-depth information, e.g. for multicomponent samples.

Since it is the aim of pyrolysis GC to determine volatile degradation products of macromolecules it is important to distinguish between actual degradation products and previously adsorbed volatile organic compounds (VOCs). Performing a thermal desorption step before pyrolysis provides additional information on adsorbed compounds and allows drying wet samples. Aditionally a clean pyrolysis profile is obtained without interfering contaminants that are unrelated to the macromolecular structure

 

Features and Benefits

Fractionated pyrolysis
  • Multiple pyrolysis steps performed on a single sample at increasing temperatures
  • Provides more in-depth information, e.g. for multicomponent samples.
Thermal desorption with solvent venting and pyrolysis of the same sample
  • Thermal desorption analysis and pyrolysis is performed on a single sample resulting in separate GC/MS chromatograms
  • Sample is purged and cleaned using thermal desorption prior to the pyrolysis step
  • A clean pyrolysis profile is obtained without interfering contaminants that are unrelated to the macromolecular structure
  • Suitable e.g. for polymers in solution, enabling exact and reproducible sample amounts to be introduced
  • Automated drying of wet or humid samples, saves time by eliminating external drying step
  • Clearer and more accurate information is obtained from the sample

 

Application example

Fig. 1 shows a chromatogram of pyrolysis products from a lyophilized sludge pyrolyzed at 700°C. Besides other compounds, short chain fatty acids were identified. A first guess was that these compounds must originate from degradation processes because, given their volatility, they should have evaporated during the lyophilization step. In fig. 2, the top GC/MS chromatogram shows the compounds released from the lyophilized sludge during thermal desorption at 275 °C; the bottom chromatogram shows the pyrolysis products formed at 700 °C from the same sample immediately following the thermal desorption analysis.
The pyrogram doesn’t show the short chain fatty acids even though these were present in the thermal desorption chromatogram. This indicates that the acids were in fact adsorbed on the matrix and were not formed by degradation of the macromolecules.

Obviously, without the initial thermal desorption step, a pyrogram could have been interpreted incorrectly. Of course for a more detailed and more well-founded interpretation, a fractionation with higher resolution in terms of temperature steps (e.g. from 50° to 250°C in steps of 50°) should be considered.