Using proper procedures for capillary GC column storage and conditioning can have a major impact on column lifetime and the quality of results obtained. This "Tips and Tricks" instalment covers everything you wanted to know, but were never told, about proper GC column maintenance.
Correct column storage is necessary to prevent two major occurrences — the ingress of atmospheric oxygen and moisture into the column and the oxidative degradation of the bonded stationary phase through UV catalyzed mechanisms. The following guidelines will help ensure longer column lifetime:
- Remember to seal the column ends when they are not in use to exclude atmospheric oxygen and moisture. The easiest way is to seal the column using silicone septa (cut them in half — it’s less expensive!!) or column sealing caps.
- Don’t leave the column out on the bench where it can be damaged. Store the column so it will not be scratched. If scratched, the stress to the column may cause it to crack during operation.
- Store the column boxed with the test chromatogram in a dark place. Exposure to high levels of ultraviolet light can initiate oxidization of the stationary phase.
If the column is to remain on the instrument a constant low flow of carrier gas should be maintained with the split flow on. If the split flow is switched off back diffusion of air into the column can occur; this air can then cause damage. In order to prevent a build-up of moisture and air in the oven it should be left on at a
temperature of 60 °C.
Oxygen rapidly degrades the stationary phase by cleaving bonds along the back-bone of the column. This is known as a “cyclic backbiting reaction” where the siloxane chain breaks into more thermodynamically stable, but also more volatile, cyclic siloxanes (Figure 1). It is the elution and detection of these cyclic siloxanes which constitutes column bleed. This damage is irreversible. The cyclic structures which are formed during this process have characteristic mass spectra at m/z 207, 281, and 355.
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