A thermally labile compound is a compound that breaks down when exposed to high temperatures. Thermal breakdown is often attributed to inlet or column activity. However, this is not always the case. In hot inlets, thermally labile analytes are prone to decomposition in the presence of the high temperatures required for fast sample vaporization. Since gas chromatography (GC) separates compounds by heating them to make them volatile, how do you analyze these types of compounds without breakdown? The answer is to expose the analyte to the lowest temperatures possible that will elute the compound.
The inlet temperature is a critical factor in optimizing injections for thermally labile compounds. A balance should be reached between inlet discrimination due to low temperature and thermal breakdown at high temperature.
Oven Ramp Program
Optimizing the oven program is an additional critical factor in preventing breakdown of thermally labile compounds. You may be able to achieve faster run times with a higher final temperature or increased ramp rate, but this can come at the cost of compound breakdown or decreased resolution. There is a tradeoff between run time and analyte breakdown that should be balanced depending on your analytical needs.
An Example: Polybrominated Diphenyl Ethers (PBDEs)
Below is an example for PBDEs where the last congener, 209, is thermally labile. The conditions in both chromatograms are exactly the same – with the exception of the final hold temperature. Column used is a Zebron ZB-5MSi, 5 m x 0.18 mm x 0.18 µm. The top chromatogram elutes PBDE 209 at 250 °C, while the bottom chromatogram elutes PBDE 209 at 300°C. There are two things that are evident: there is a large difference in retention times, as well as a difference in peak shape and area.
PBDE 209 has a very long retention time of 40 min in the top chromatogram, but also has a very high peak area for easier quantitation. Even though the compound interacts with the column for a very long time, the column is not active and the compound does not break down. When the congener is exposed to higher temperature as in the bottom chromatogram, it spends less time interacting with the column and elutes much earlier, but has a much lower peak area. This is because the compound is thermally labile and is breaking down due to the higher oven temperature.
When dealing with thermally labile compounds, it is important to minimize the temperatures in the chromatographic system. A balance has to be reached between long retention times due to low temperatures vs. lower peak areas because of higher temperatures. Using on-column injection, columns with thinner stationary phases, shorter column lengths, and higher carrier gas flow rates can also elute compounds at lower temperatures and therefore minimize breakdown.
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