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GC internal standards
Posted: Wed Apr 17, 2013 1:04 pm
by cody84
Most places I've worked have used an internal standard and ratios when testing alcohols on GC. (For eg, to test ethanol, use IPA standard. Then EtOH area/IPA area in standard vs. EtOH/IPA in sample used for calculation). Is there a reason to use the internal standard? I heard it's more reliable due to evaporation, but wouldn't each component evapourate at different rates? The new company I'm at uses direct standards, no internal standards/ratio.
Thanks!
Re: GC internal standards
Posted: Wed Apr 17, 2013 1:26 pm
by chromatographer1
An internal std can give you a warning about a bad injection, where without it the bad injection might be counted as a genuine number, just low in value.
Proof that an analysis is valid is always welcome in any regulated industry.
best wishes,
Rod
Re: GC internal standards
Posted: Wed Apr 17, 2013 3:15 pm
by Consumer Products Guy
Most published alcohol procedures (since the alcohol levels can be relatively high, and alcohol is volatile) employ either n-propyl alcohol or acetontrile (USP 611) as internal standard.
Although modern autosamplers are pretty darn good, I agree with the internal standard reasoning for products/assays like this. Alcohol assay is pretty much the only routine assay we do where internal standard is utilized.
IPA is more difficult to resolve from ethanol, and some products do contain IPA as an ingredient or active, so n-propyl alchol or acetonitrile can be a better internal standard.
Re: GC internal standards
Posted: Wed Apr 17, 2013 6:04 pm
by cody84
Awesome thanks guys!
We had great resolution between MeOH, EtOH and IPA on a DB-624 btw
(testing hand sanitizers and disinfectants.)
Re: GC internal standards
Posted: Thu Apr 18, 2013 12:13 am
by KM-USA
Awesome thanks guys!
We had great resolution between MeOH, EtOH and IPA on a DB-624 btw
(testing hand sanitizers and disinfectants.)
Actually, I've never tried ethanol-IPA separation on DB-624 column, even though we do use that column for USP 611 ethanol assay. Why? Because there hasn't been a business need for it. All the hand sanitizers we test contain ethanol.
but thanks for the tip !
Re: GC internal standards
Posted: Thu Apr 18, 2013 7:44 am
by Peter Apps
An internal standard can correct for problems with injection volume (a smaller volume makes all the peaks smaller) and for evaporation of the solvent that the sample is dissolved in (if the solvent evaporates all the peaks get bigger). Unless you can find an IS with the same chemistry and molecular weight as the analyte the IS will not correct completely for evaporation of the analyte, or for inlet discrimination.
Peter