Different Rt of the same compound.

[chandra]

when i run a urine sample in a GC-MS,the peak of the some compounds occurs at different retention time for example peak of Acetic acid trimethylsilyl ester occurs at three different RT (8.707,10.441,13.981) in the same run.I don't understand which retention time should i consider for that compound.what is the reason for this?

[Don_Hilton]

If this is in a single chromatogram, I suspect that you have three different compounds. How are you identifying the peaks as acetic acid-TMS ester? If it is by library search, beware! A library hit is a suggestion, not proof! Many compunds look the same in the mass spectral library.

Give a bit more detail and we may be able to geive a better, and more specific, answer. A pictuyre of hte chromatogram could help. Ion traces for the major ions in acetic acid-TMS might be helpful as well. And it would be good to know the retention time for a standard sample of acetic acid-TMS as well.

[chandra]

Thanks Hilton... for your valuable suggestion.Till date i have been using Library search for the compound identification.

[Don_Hilton]

Always: an idenificaiton with a library search is tentative. It must be confirmed by some other means. That may be the analysis of a reference compund or some knowledge of the sample (my favorite example of the latter is: a large peak matching the spectrum for nicotine in a tobacco sample may usually be safely identified as nicotine.)