[martie]

Hi,

I am from an ecology background. Looking for steroids in sewage effluent samples- when i went to analyse my samples i found that some of my compounds of interest in a spiked sample had shifted in retention time (5 minutes) compared to the standard run on same day even though the conditions/method are the same.

I am a bit concerned that i have stored the samples incorrectly because after they were extracted i left them at -20C in a couple of mLs of solvent (DCM) for several months. It is possible that the compound of interest has undergone oxidation or a change in structure to cause this shift in RT?   

Thanks

[tmartin]

I don't have a lot of experience with long term storage of steroids (probably someone else on this forum will), but in general when storing complex organic compounds for long term I tend to store them neat, without solvent, at -20 C. Or if you are really concerned perhaps freezing them in benzene could help (benzene should actually freeze solid, whereas methylene chloride will remain a liquid).

Have you analyzed the compounds by NMR?  If you have a proton NMR of the material before storage, you could compare the NMR spectrum now and perhaps figure out what changes have occurred to cause a change in the retention time.  This may not be possible though if you are analyzing complex mixtures of compounds/samples, which I guess it seems you may be doing?

[orgopete]

Without a complete description, I cannot be certain, but if I understand you correctly, the answer is no. Since your samples have already been subject to environmental oxidation, I would rule out a rapid and complete oxidation to a new product. Therefore, the question is whether partial oxidation might have occurred. You did not describe that in your scenario. A partial oxidation should retain at least some of your original peaks and now should include new additional peaks. However, you describe a shift in retention time. That sounds like a change in column, solvent, flow rate, etc.