Chromatography Forum

I'm running a free phenol % concentration on a 7890B GC. My ISTD value is showing up just fine and all peaks are appearing as normal, but my phenol % is being calculated at twice what it should be. No multiplier or dilution factor is being used.

We have run a 10% STD several times and it comes out fine, but our control and our samples are showing a phenol % higher than what we know is in them.

What could be the cause of this issue?

Thanks in advance!

Take a look at the actual chromatograms. Is there any difference in the peak shape? or the baseline?

My guess would be that you're seeing either a matrix effect distorting the peak shape and fouling up the integration (although it's hard to imagine doubling the area that way) or there are unresolved compounds in your matrix (controls & samples) coeluting with the phenol.

Are you talking about the AREA changing or the calculation and result not giving the result that you expect. Are you asking for a ISTD report or a ISTD% report.

Gasman

If you take the peak areas and do the calculation by hand, do you get the right answer ?

If you do the problem is in the data processing, if not it is in the chromatography, sample prep or integration.

Peter