Chromatography Forum

Hello!

I am trying to run sterols extracted from marine sediments on a GCMS. For our lab method, which has previously and successfully been used, we add a-cholestane as an instrument standard to normalize peak areas too. Recently, I noticed the cholestane peak areas vary widely between the calibration curve and samples even though the same amount is added to each vial. I don't think there is any cholestane in the samples because even injecting the same exact sample twice, the cholestane peak areas vary by 10x.

Thanks,
Emily

We used cholestane as internal standard for assay of cholesterol, but used GC-FID.

Make sure your GC inlet temperature is high-enough. And is the solvent composition of your standards and samples EXACTLY the same?

Also consider how often you do inlet maintenance. If the liner has not been changed recently it could be causing problems and also could need to trim a few inches from the front end of the column in case it has become contaminated.