[Baileys]

I have a sample that was run through GC/MS on two seperate occasions; the same instrument and the same analytical conditions were used on both occasions.

Both spectra appear identical. The only thing that is different is the retention time of some of the peaks. Of the 9 peaks present, the first 4 peaks have the same retention time in both spectra, whereas the retention time of the remaining 5 peaks are very slightly different, e.g. 3.70 and 3.71, 4.52 and 4.55...

Does anyone know why the last few peaks would have different retention times, even though its the same sample with the same equipment? Could it be something to do with using a manual injection prehaps?

[Stepan]

Usually such shift is acceptable. It may occur in results of temperature variation in a GC oven.

[Toxophilus]

I think your problem is thinking that they are spectra when in fact they are chromatograms. Spectral data is indeed very precise but retention times in any chromatographic technique will vary slightly; in GC this would be variation in both the gas flow and oven temperatures.

[TheUnfocusedOne]

+1 to this being normal. The later eluting compounds are more succeptible to variation since they've been in the column longer.

What's your method? Are you running isothermal or are you ramping?