Chromatography Forum

We have a brand new GC with an FID and have a ghost peak around .8 minutes after the air peak. We are only injecting humidified air. We are trying to figure out why we are getting this ghost peak. The peak becomes bigger after the GC has been in standby for a while, compared to in a run that quickly follows the previous one. The peak almost disappears after 5 to 10 repetitive air injections that are made every 20 seconds or so. We also see this ghost peak if we use ONLY our calibration cylinder that is water free. Does anyone have any ideas?

The forum archives are a great resource for ghost peak and carryover troubleshooting. It sounds to me as if your peak is a contaminant rather than a ghost.

Peter

Maybe a contaminant in the carrier gas that's building up on the head of the column between runs? More time in standby = bigger peak.

The initial peak height and width is approximately the same after in standby (as little as 2 minutes) as compared to days of standby. With repetitive injections very close together (~20 seconds), the ghost peak disappears for the most point. Let it sit 2 minutes and we are back in full force. Let it sit a day, same initial ghost peak intensity as the 2 minute standby. I don't think it is the carrier gas since we are using two different carrier gases (automatic and manual injections). Has anyone had any problems with contaminants coming from the injection valves? If so, what could be done? Our problem is this ghost peak isn't going away even after tens of thousands of injections. You would think the intensity would decrease with usage of the injection valve. Very confused at this point.

It is not a carrier gas for sure. I guess it could be a contaminant in inlet. Try to clean liner (if you have one), septum and syringle.

There is an active site in the injector/column that collects an impurity which volatizes in the carrier gas. When the active site becomes saturated, the contamination reaches maximum concentration as a plug. Otherwise it doesn't get focused but only becomes a continuous column bleed artifact.

A good GC is like an operating room. clean clean clean. And it isn't always easy.

best wishes,

Rod

You have a "brand new" GC and yet you have done "tens of thousands of injections"

To have different carrier gasses for manual and automatic injections sounds like a rather odd setup - cetainly not the run of the mill split splitles inlet with liquid injections. What hardware do you have, and how are you using, and what are you analysing ?

Peter