Chromatography Forum

Hello,

Im setting up GC Methods for our laboratory, I'm a bit new to this so forgive any foolish questions.

I am testing a variety of glycols, currently my peaks of interest are : monoethylene glycol, diethylene glycol and monopropylene glycol. I have the GC set up and seperated my peaks of interest. I now need to convert my data into an actual assay, for this I want a known standard to compare against.

First question: My colleague doesnt seem to think you can get GC standards for MEG, DEG or MPG at an accurate, certified assay. I simply don't believe this , can anyone recommend a chemical supplier who can provide suitable GC standards for these chemicals?

Second Question: Is this equation correct for working out the assay:

Purity (sample)=
Volume (std) x Purity (std) x peak area (sample)
Volume (sample) x peak area (std)

Third question: How does adding an Internal STD to both STD/sample affect the equation?

Thanks for your time,
lighterthief

We purchase our glycol standards from Aldrich, typically greater than 98% purity. DEG will give a mixture of peaks.

We typically dissolve in DMF and derivatize to assay glycols by GC.

But polar capillaries can do OK for non-derivatized. If using water as the solvent, keep the injection volume 0.5ul or less.

I am very familiar with the testing of all of these. The USP has a method for testing EG and DEG in PG so that may be helpful to you. Restek makes custom standards for this particular test method.(C/N 31892(EG,DEG,PG in MeOH and C/N 31894(internal std in MeOH). This method uses 2,2,2 trichloroethanol for an internal std and methanol as the diluent. Your calculation looks good, my only suggestion would be that you may want to weigh the sample and standard as opposed to going by volume. When using an internal std, instead of using the peak area of the peak of interest, you would divide the peak area of each peak of interest in the chromatogram by the peak area of the internal std peak in that chromatogram. Most chromatography software will calculate this for you if you specify what the internal std is. Also when developing these methods I would suggesting preparing stds at different concentrations to see if you are getting linear results. I would also spike some of your samples to see if you are getting adequate recovery.

If you are assaying glycols, the samples can be analyzed without derivatization using a carbowax column. Our current favorite is a TR-WAX from Thermo although other columns also work. The standards are prepared from reagent glycols checked for purity by area percent using the same conditions. I have never observed more than one peak for the DEG unless there is contamination. We usually do not dilute the samples. If the samples are glycol mixtures a total area normalization of weight percent works. If you are measuring trace amounts of glycols in other matrices an internal or external standard calibration will work.

Be wary of checking glycols by GC - since they are hygroscopic the main impurity can easily be water, which is invisible to an FID.

Peter