Why do fused silica GC-columns appear to have different colors?

Have you ever wondered why similar bonded phase-type fused silica columns from different suppliers, have different colors, while it’s a similar type phase?

Fused silica columns are coated on the outside with a 20-25 um layer of polyimide, which is a high temperature stable polymer, but when exposed above 300C, the color will change.. It turns from yellow into brown, dark brown, even black..

Today’s stationary phases can be used up to 330-340C isothermally, columns must be stable up to this temperature. If you have a bonded phase column which lists a Tmax above 300C, and it still looks “yellow”, be careful as that column may not have been subjected to its maximum temperature at all..

At Restek, before testing, all the bonded phases are conditioned at the max. temp. of the stationary phase. This to ensure that the column is stable up to Tmax, AND clean, so its stable within 30 minutes once put to work.

That’s why Restek bonded phase columns are always: brown..

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