We are trying to identify and quantificate the presence of PEG in a resin.
we hae an HPLC with Refraction index detector. We use ACN:water as mobile phase and a C18 column. We can not see anything in the chromatogram.
Somebody knows another method?
Please have a look on this paper:
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.607.5582&rep=rep1&type=pdf
regards
The lack of signal usually means the material either did not elute from the column or was not present. My suspicion is that the solvent strength was too low. MeOH/water or THF/water are much better solvents for those oligomers, so you would expect easier elution of these materials.from a lipophilic column like a c18. With THF/water mixtures, I have separated the first 40 oligomers of polystyrene.(In that case I used a gradient and a uv detector). We did extensive non acqueous reversed phase chromatography of oligomers and polymers in the 1970's,,and reported them in a chapter in "Liquid Chromatography of Food and Beverages" ed George Charlambous, 1979, academic press.
We also used infrared detection in many cases.
One difficulty is that the relative differences in molecular characteristics decreases with molecular weight, so the higher oligomers tend to get harder to separate from one another. With samples having a wide range of molecular sizes (and therefore a wide range of retentivities), and no ability to use a gradient, an alternate approach is high resolution SEC/GPC using a column set with a single small pore size. I gave a talk at the Waters Symposium on that topic back in 2000. With that setup and a novel way to treat the data, we could get extreme precisions ( 1% !!) for Mn, Mw, Mz, over 30 day periods. I will post the slides here or on linked in, in the case the text of the talk is no longer in print or available.
Not really. An RID is only sensitive if the expected concentration is >0.1% or 1000 ppm.
Bruce, it goes without saying from my first sentence that samples of appropriate expected concentration were used, and detector sensitivity was adequate to detect eluting materials. Unlike with biological type samples, typical analysis of polymeric sample concentrations are not a problem because these materials are sold and processed neat (i.e. near 100% concentration). Samples usually are diluted 10 fold or more just to get them to be dilute enough for the chromatography.
The sensitivity of RID is not sufficient for analyzing the monomer polyethylene glycol (PEG) in resin.
In such cases GC-FID or MS with headspace injector is recommended.
. For HPLC analysis, if the sample is a residual analysis, an extraction technique for the unreacted material present in a resin might be needed for a low level residual analysis.
The HPLC detection sensitivity is entirely dependent of the solvent composition and the sample concentration as it elutes from the detector. There are a multitude ofl publications which report separation of peg's using refractive index detection. The issue here is not a limitation of the detection, but rather sample concentration. Either concentrate the sample or use a different technique. If one does not have the luxury of GCMS and headspace analysis, one can a GPC column that excludes the polymer and separates only the low mol wt materials. GPC is the highest capacity LC Type technique, so very large samples of high concentration can be used in this type of situation.
You won't be able to identify or quantify the presence of PEG in a resin with Refractive index detector. The RI detector is considered to be a universal detector but it is not very sensitive. For non-UV absorbing compounds, the evaporative light scattering
detector (ELSD) is the primary choice since the principle of detection does not rely on the optical properties of the solute.
For more details the attached document
Thank you, Scott, for your very clear and precise presentation. For all the naysayers, this is exactly how we used to do it back "in the day", so, yes, it works just fine.
One thing that Scott didn't mention is that the sensitivity is highly dependent upon having a difference in refractive index between the analyte (PEG400, in this case) and the solvent. I would certainly try another solvent system before I gave up.
One question for Laura. Do you know that the PEG400 is supposed to be present? Perhaps your data is correct and you don't have the PEG400 in your extract.
It would be straightforward to run a few spiked samples with peg400 and determine what the sensitivity of the aparatus and technique are.
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