retention time shift

[edmgc7788]

Hi,
i am using 5973B GCMS for F1 and MS. The RT sometimes shifted, but in my lab, other instruments were good at the same time. For example, i loaded 70samples, the RT shift happened from the 50 to 70 samples for FID and MS, which is the peaks come out late about 0.2m. If i change the RT and save as the new method, after 2 days, the RT shift back to before.
Is there anything wrong in the inlet or gas pressure?
thank you

[Don_Hilton]

Have you changed the GC septum in this long number of samples? Perhaps near a time when retention times changed?

[edmgc7788]

hi
thank you to answer my question. i loaded samples before i go home, then i come back to th lab next day and integrate. So the problem happened the last 20samples, but my closing calibration standard has no RT shift problem. That's i was confused. Same thing happened today(OCT 15), my daily opening standards are all good, after that all samples have RT time shift 0.2min late.
rebecca

[Don_Hilton]

There are a number of possibilities - some good some bad. And more information is needed to sort it out.

Having said that, with your second post, the next question that comes to mind is could this be a matter of peaks shifting as a result of column overload from an earler peak in the chromatogram? I would expect this to happen in samples but not a solution standard. And it woud not have to happen in all samples, just those whith a large matrix component that elutes just before the analytes of interst. Alternatively, the samples could leave garbage on the column that affects the chromatography, but that garbage goes away after the instrument stands for a while.

So, what does a total ion chromatogram for the samples look like? Is there a large peak that elutes just ahead of the shifted peaks? (This requires full scan acquisition to detect the extra compund.)

The standard at the end of the sequence give a clue. A shifted retention time would indicate a change to the instrument; a normal retention time suggests the problem is with individual samples -- like peak overloading mentioned above.

On a long sequence, several standards through the sequence are a good idea. When instrument operation becomes questionable, all results back to the last known good sample (a sample with a known result that ran correctly) are questionable. And if you the last sample you ran was just before all the samples in the run, you get to run them all again if you need to be sure of results. And the multiple control samples or standards would help to confim what you are seeing in the standard at the end of the sequence.

[lynchxiaoyi]

I just had the same problem. During an overnight run, my IS and target compound's retention time just changed, and not for all the samples, just for a few at the end.

I don't think the overloading caused the problem, since I ran the standard curve everyday, and my previous runs were all good.

Dose edmgc7788 find out the reason?