Chromatography Forum

Does anyone have any suggestions for method settings to obtain qantitative results for dipropylene glycol (DPG)? Currently I have a 5890 with an RTX-5 starting at 100C for 1 minute and ramping 25C/min to 300C. The sample is a 82% to 92% DPG mixture with other components which elute at a longer RT than the DPG. I take 1 gram of the sample, 9 grams of methanol and 0.2g of internal standard (dimethyl phthalate) and use traditional internal standard calculations to obtain the % w/w of the DPG. I use all three peaks of DPG summed together for the area of DPG.

The first time I developed this method and ran through with standards and production samples, everything worked well. However, it is not repeatable on different production runs. Do my parameters sound reasonable? Is there a better method for obtaining dipropylene glycol results?

What is varying?

Do you know the water content of the non-complying samples?

Your column choice would not be mine.

best wishes,

Rod

The standard sample that I was running is giving me a 2% decrease, then a 5% increase in DPG this go around. Kind of bouncing around... The water content is between 6 - 8%

Personally, I'd dilute the samples and standard with DMF, and derivatize with BSTFA, that's what we do here. Use the column that's in there; you'll still get multiple peaks for the DPG. This will be more reproducible than underivatized, especially on such a non-polar capillary.

I take 1 gram of the sample, 9 grams of methanol and 0.2g of internal standard (dimethyl phthalate)

When I read your post, that's REALLY concentrated. We don't use internal standards often.

Thanks... I will try derivatized. We do that with other products, I just never thought to apply it to DPG.

I agree with CPG.

That column with raw samples is not a good plan. CPG is correct about the multiple peaks but these will be close together. Whether other components may give you problems remain to be seen. The high water content may be a problem but hopefully you can dilute enough to overcome this problem.

best wishes,

Rod

The high water content may be a problem but hopefully you can dilute enough to overcome this problem.

Actually, the 6 -8% water level is pretty low; we typically dilute down products with 80% water in them and derivatize. Obviously one needs excess derivatizing chemical.

Hello all,

Derivitization worked very well and I will continue running samples to verify all is well. I'm in an industrial environment so the level of accuracy is not as criticial as an FDA lab. I appreciate the help in pointing to derivitization as a solution.

Take care.

Derivatization worked very well

CPG. Him smart sometimes. Him sometimes uses area summation to integrate the DPG as one peak, easier than using grouping function.

CPG learned pronoun use from Tonto and Tarzan, but good speller. Corrected spelling above.