Variation in retention time GC-FID

[Peter Griffin]

I'm experiencing variations in retention time after initially switching out columns. I reinserted the original one back in, but I'm still seeing around a 30-40 sec delay for when my peaks should appear. Flow rate, tempt, split flow, etc have not changed.

[dblux_]

... Flow rate, tempt, split flow, etc have not changed.

Flow rate probably has changed due to imperfect column installation (leaks) presuming it was the same column and not trimmed considerably.

[Peter Apps]

I'm experiencing variations in retention time after initially switching out columns. I reinserted the original one back in, but I'm still seeing around a 30-40 sec delay for when my peaks should appear. Flow rate, tempt, split flow, etc have not changed.

Welcome to the forum.

Like most newcomers (and some people who have been around long enough to know better !!) you expect us to troubleshoot your problem on the basis of the very minimum of information.

Unless you want to go the slow and painful route of troubleshooting by guesswork we need to know; what instrument are you using, what are the instrument settings for flow, temperature etc in all the zones (inlet, oven, detector), what are you analysing, dissolved in what and injected how, what column, length diameter and stationary phase.

What retention time did you used to get, what is it now ? When you say variation do you mean that it has shifted to a new value and is now stable, or that it has shifted and is not stable, or is just unstable ? Do you see any changes in peak shape ?

When you put the old column back in, did you put the original inlet end back into the inlet ?

Peter

[Peter Griffin]

I have a longer retention time than what I used to get. The retention time is consistent with every run. I used to see the peak around 3.9, but now it shows up around 4.1. No changes in peak shape. I'm using a PerkinElmer autosystem XL (has an auto injector). The flow rate that I'm using is the one that is suggested in the manual (flow rates are 450 mL/min O2, 45mL/min H2, and carrier gas is sitting at 8 psi.) I install the column with the exact same inlet piece.

[aidnai]

"carrier gas is sitting at 8 psi"
This is a clue -- have you checked what the actual flow is at the column end or detector, before and after to verify it is the same? How do you know the flow rate has not changed?

With constant inlet pressure control, your RT shift can be caused by any type of constriction or leak caused by installation. For example a bit of disintegrated graphite ferrule ends up in the column and now your column flow is lower.

The FID flows do not affect the retention time so they aren't relevant to your issue.

BTW, you said RT shift was 30-40 seconds, but 0.2 minutes is only about 12 seconds.

If your peak is smaller than you expect, suspect a leak in the inlet. If your peak is the same size, suspect a constriction/blockage.