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. The peak area calculation seems off.

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!

Hello

First of all you didn't specify quantitation type more specificaly - is it ISTD method or % normalization?

If you're using ISTD method and it is fine for your 10% phenol standard but for some reason your real sample and/or QC sample is higher it means that there is something wrong with quantitaion setup.

Try to check area for your phenol peak (sample) and check manually if it is in expected range - you can calculate result manually using ISTD area and sample peak area.

I'd recommend to check/setup processing method again.

Regards

Tomasz Kubowicz

1. Always make sure your calculations match your hand-done calculations.

2. Make sure your setting is for "ISTD%", not "ISTD.

3. Are you entering the amount of internal standard through a set amount, or on individual lines in the Sequence table.

Yes, automated calculations can be tricky to set up.

I'm using an ISTD method and it is set to ISTD %.

I'm entering the ISTD as a set amount in the quantitation settings when I edit entire method. I put in the sample amount (which should be around .5g) and the internal standard amount (cresol).

my retention times are 1.79 (phenol) and 2.03 (cresol). My areas in the STD are 1955 (phenol) and 2186 (cresol). When I run the control I notice the areas are 650 (phenol) and 2000 (cresol). The phenol should come out at 2.3% of the sample, but I keep getting around 4%. The cresol comes out ok at around 10% every time.

Where are you entering in the sample size? Should be only in the Sample Table.

The default cresol grams added to each sample goes into the ChemStation in the other window.