Chromatography Forum

Then your interference may be a ghost peak. Is it a problem to simply extend the run time ?

Peter

I'm with Peter on this one, if there is junk at the end of the run, it could be eluting just after the end of data collection on the working instruments and just being trapped by the cooling oven on the non working instruments.

If that is not the case, the only thing I haven't seen mentioned is the gas supply tubing being changed. You mentioned changing gas traps, but if some contaminate got into the tubing between the trap and instrument, then it could be contaminating the inlet. If it only appears after a sample injection, then maybe something in the sample matrix is knocking it loose when just the solvent in the standards does not. That is a long shot, but can be considered if extending run time doesn't fix the problem.

My only hesitation towards extending the run time...

If we are going to go down the road of late eluters that were tapped by the cooling oven ... then shouldn't this be a universal thing? If this were the case, then no instruments should be able to run the method without problem.

I can literally take the same column, samples, and consumables from a "bad" instrument to a "good" instrument and everything will be fine.

The copper tubing has been mentioned. When I installed the gas traps I literally installed them at the back of the carrier inlet to the GC. My thinking was even if the copper gas lines needed to be replaced my placement of the traps should have at least temporairly eliminated the problem.

The difference between instruments could be due to how fast the oven flaps open to cool the instrument or how much heat the oven walls retain. It could be that one oven opens just a bit faster than another allowing the column to cool just fast enough to capture a late eluting analyte, while others let it pass through just before it cools too much. Add two minutes to the runs, if you still get the ghost peak then it is from something else.

As I'm quickly eliminating other possibilities, it almost has to be something unique to the instrument and operation, doesn't it?

My only hesitation towards extending the run time...

If we are going to go down the road of late eluters that were tapped by the cooling oven ... then shouldn't this be a universal thing? If this were the case, then no instruments should be able to run the method without problem.

I can literally take the same column, samples, and consumables from a "bad" instrument to a "good" instrument and everything will be fine.

The copper tubing has been mentioned. When I installed the gas traps I literally installed them at the back of the carrier inlet to the GC. My thinking was even if the copper gas lines needed to be replaced my placement of the traps should have at least temporairly eliminated the problem.

Exactly where in a subsequent run the ghost peaks elute depends on how fast the oven cools and how fast the gas flows return to initial conditions. These are very unlikely to be the same on different GCs. So on one instrument you get an overlap with your target, on another the ghost either does not appear, or is in a place where it causes no trouble.

Peter

The copper tubing has been mentioned. When I installed the gas traps I literally installed them at the back of the carrier inlet to the GC. My thinking was even if the copper gas lines needed to be replaced my placement of the traps should have at least temporarily eliminated the problem.

With you injecting 2ul and over loading the liner, stuff can condense in the copper tubing.
As a test, just replace it ( in my experience cleaning it is not optimal). All you need is some 1/8" Cu tubing, cut it about the same length of the one on the GC.

The most likely explanation is carry-over caused by injecting too much methanol, whose vapour pushes sample into places it should not be. The reason that some GCs have the problem and others not is probably differences in how fast their electronic flow and pressure controllers respond to the pressure pulse as the sample is injected. To test this you would need to swap the gas controls and inlets between GCs, but in the end you need to develop a method that is robust to inter-instrument differences.

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