help for GC sensitivity issue

[peijan]

Hi everyone,

I am trying to detect diethylene glycol(0.0125mg/ml) and ethylene glycol(0.0125mg/ml) by GC. However, I do not get any response for both. WHo can tell me what is going on?
I also tried to decrease split ratio but it dose not help.

Instrument setting:
GC: HP 6890A
Cloumn: DB-624, 30mX0.53mm, 3um
Column flow:4.5ml/min
Oven: 40C for 5min, ramp @ 10C/min to 260C, hold for 3mins
Inlet: 250C split 10:1
Detector:300C H2 flow:30ml/min, Air flow:300ml/min
injection volume:1ul

I can get small peak area (150pa*s for both of compounds) when I inject conc. is 1.25mg/ml solution in MeOH

Thank you for your info.

[Consumer Products Guy]

Sounds to me like you're trying to do the Diethylene Glycol and Ethylene Glycol Impurities test in the USP 31 May 15, 2008 Glycerin Monograph revision. OK, your GC conditions don't exactly match those listed in the USP 31 May 15, 2008 Glycerin Monograph revision, but they're not far off. We use a practically-identical 624 column from a different vendor, and we don't have any sensitivity issues. Our vendor lists 240C as the maximum temperature, and you're going to 260C, so maybe you've damaged your column by going to 260C, contact your supplier. Attached are the GC conditions we use (also on 6890A GC), the 60C/minute rate detailed by the USP is ridiculous.

Anyway, we use here as our standards the 0.025 mg/ml concentrations for EG and DEG in methanol as detailed in the USP update, see the attached chromatograms.

[peijan]

I agree "the 60C/minute rate detailed by the USP is ridiculous"

Actually, I had tried this method already but I still don't get any response.

However, The above method is made based on Agilent method. The DEG peak should elute at ~18min that means before Oven temp. reach 170C. So I think DEG should not degrade.

By the way, my column is from Agilent.

[Consumer Products Guy]

Go to basics. Take out the SS inlet liner, clean with solvent. Install a new split liner. New septum. Try a manual injection with a different syringe. Check inlet for leaks. One must make sure all the sample gets to the detector. When was the detector last cleaned? Is there a sensitivity issue with different assays on the same GC?

Agilent bought J&W (DB columns) a bunch of years ago. Their columns are good.

[AICMM]

peijan,

Some suggestions: I would check the position of the column in the liner since this is often a source of low response. I would also evaluate what liner I was using and I would try a higher column flow (easy to do with a 0.53.)

Best regards.

[peijan]

Today, I just get a whole new column (same type of J+W, DB-624) and make a test injection with it. Then I get the peaks. I guess DEG and EG are very sensitivity with column condition.

Thank you for your help and suggestions~

[Consumer Products Guy]

Good; just don't take the new column too high in temperature....by the way, there is also a typo in the USP update, in the second part. I'm not at work right now, but if you follow their directions as written you end up with a solution that you never use. And you'll need to keep purchasing a fair amount of their expensive USP EG, DEG, and glycerin reagents at $185 a pop, what a scam.