Cholestane Internal Standard Issues on GCMS

[egwatts]

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

[Consumer Products Guy]

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?

[James_Ball]

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.