Ripple

[Giant_Bones]

Noticing a ripple in my chromatograms and just wondering what might cause this? Sample solvent is heptane and using FID, DB5 column, split 20:1 and He carrier. The ripple is noticable when the temp starts to increase from 100 to 300 (10C/min)

TIA

[Don_Hilton]

A couple of things:
1) if you could post a picture, it may help
2) are all gas pressures from the cylinders at least 20 lb above the pressures set on in the GC? (Common cause of a ripple with a time constant of a few minutes is regulators attempting to control with too low a pressure drop across them.)

[cleh]

Yes, a picture would help. Do you have a high purity regulator with stainless steel diaphrams? What grade of He are you using and how old are your purifiers? Make sure your regulator cylinder valves are fully open. If they are partially open the flow can fluctuate and sometimes stop completely.