Chromatography Forum

My Methanol QC has been coming in high. Ex. 780 when the calculated value is 758 ppm. Started running a second QC from the same prep and the result averages at 760.

Simple suggestion I know but have you run a new calibration recently? Could a solution have been made not so accurately? Additionally, prepare a in house spike with different solutions and test that.

Preparing the methanol daily QC from the prepared STD and calculated 758 ppm. The QC working vial is weighed out at 25 grams +/- 0.02. ETOH INT STD added to working vial. Vortexed and added to both head space vials. So the question I am asking is why the first QC vial from the working vial is higher than the second. This is happening on both our GC's. Is contamination carrying over from samples that are being ran at the end of the queue that are at 4500 ppm. Trying to learn here.

Hi, can you suggest me what HS method you using? Are you using HS-GCMS or FID detector? I'm struggling to detect methanol and ethanol using HS_GCMSD, I,m using a HP-5 25m x 0.32 mm I.D x 0.52 um column?
I am heating the std at 80 degrees in the HS oven for 20 min and then GC oven starts with 37 degrees and goes gradually to 100oC. Any advice?
Thank you

Question 1
Yes, you are being affected by the high concentration runs prior to your calibration check. You need a blank in between to help your system clean out.

Question 2
HS-GCMS does not work nearly as well as HS-GCFID for methanol. Use a highly polar column for both methanol and ethanol (like a wax phase). You may not see a lot of difference for ethanol between MS and FID, but you most definitely will see a difference between the two with methanol. Your -5 phase is not polar enough for light alcohol work and that is also causing you issues.

I wasn't aware it was even possible to do methanol via GC/MS as its Mw is 32 same as oxygen. Also I agree I do ethanol with a .32mm 0.5um wax column though there are even better ones on the market (I hear the 624's work well for solvents).