Chromatography Forum

Hi all;
I recently started developing a method to do C1-C7 acids in waters that have %levels of ethylene glycol. I have a curious happening at the front end of the chromatogram bewteen acetic and propionic acids. I'm using a Dionex ICE AS1 ion exlcusion column with a mobile phase of 1 mM HFBA modified with 5% acetonitrile in order to elute up to the C7 aliphatic acid. Det is UV at 210 nm.

In the samples that have ethylene glycol we see 1 or two peaks between acetic acid and propionic acid. In fact the second peak is rather large and propionic acid is a peak riding on its tail. I see no such peaks in the calibration standards or the blanks. Does anyone have an idea as to what these peaks could be? We're thinking acrylic or adipic acids but are uncertain as to what they are at this point.

Glycolic acid? Glyoxylic acid? They both can be oxidation products coming from ethylene glycol. They can be found in biodegradation liquors from polyethylene glycol or some non-ionic surfactants biodegradation. So you don't even need to have oxidative compounds in your water samples as some bacteria are usually present in water.

Regards

Hi IC_Gal,

What is the composition of the blank/s? Water/solvent and that’s all? If so, try to add some ethylene glycol in a realistic concentration (corresponding to the concentration in your samples) to your blank and see whether or not the mysterious peaks are to be found in this run/injection.
Also, try to spike your standard solution with some ethylene glycol (only necessary if you don’t see the peaks in the spiked blank).

Good luck

Hi,

Good idea danko. IC_Gal will probably find these impurities in water spiked with pure ethylene glycol. Nevertheless, he needs some standards to confirm what kind of acids they are. I suggested glycolic and glyoxylic acids but I can be wrong. Who knows?

Good luck IC_Gal

Thanks to all for the information and for the suggestions.

Actually it is when the standards are made in a 5% soln wt/wt of ethylene glycol in water that we see these peaks. I don't see them in the standards made in pure water. The area between acetic and prop. acids is clean when we inject the stds made in water.

The process water samples we are developing the method for can have anywhere from low ppm to 70% ethylene glycol in them....amongst other things. Dilution of about 1:10 gets rid of alot of the problems but I'm still looking to identify these peaks since they are interfering with the target analyte acids.

I do know that glycolic acid is not amongst these peaks since that one elutes before acetic on this column.

They could be non-acid components in ethylene glycol, or ethlyene glycol itself (the 2nd peak?).

If you take a look of the data sheet of ICE AS-1 (available from Dionex website), you can see that this column retains and separates alchols.

So my guess is they could be ethylene glycol and impurity in ethylene glycol (maybe glycerol?)

a simple run of ethylene glycol, like danko suggested can give you such result.

Good luck.
DC

Technical grades of Ethylene glycol can have variable impurities according to the processes used. Are you using the purest grade grade available to make standards, I would start with such material to see if the impurities are present in pure products?.

There are also UV-absorbing ketones, such as 1,2-cyclopentanediones, and alkyl homologues of 2-hydroxycyclopent-2-en-1-one reported in the literature. A literature search should help identify
possible impurities in the material you use, if the supplier can't.

If your method only involves pure grades, or a known source of ethylene glycol, you should be OK. But if you intend to be analysing samples with product from unknown sources, you may have to run samples as blanks, and spike the samples.

Please keep having fun,

Bruce Hamilton

Thanks to all for your help here. Ultimately I decided to lower the conc of acid in the mobile phase and decrease the flow rate slightly to get propionic acid away from the impurities. We made two methods and used an organic modifier for the higher eluting acids (C5-C7). Luckily these samples don't come around too often. Once again, thanks for all of the input.