Chromatography Forum

Does anyone know of a good method or literature reference for Ethoxylate Distribution analysis? I want to separate ethoxylates from a finished product and analyze them, preferably in one assay.

It depends on your type of ethoxylate. Please detail.

Well, I'm back to the ethoxylate distribution project. I found a gradient method for ELSD and ethoxylate distribution. I get nice distribution curves, but am having trouble detecting the alcohol (C10) and the alcohol+EO1 through EO3. Using this same method, I also have trouble detecting octyl phenol and 2-naphthol plus their ethoxylates (EO1 - EO3). The higher ethoxylates (EO4+) appear to have good response.

HPLC Parameters:
ELSD: Tried 30°C - 70°C drift tube. Tried 10°C - 40°C spray chamber. Start getting 2-naphthol response at 40°C spray chamber; but, can't go higher temp. Liquid in drain trap evaporates.
Column: Silica
Mobile Phase A: 93:7 n-hexanes : isopropanol
Mobile Phase B: 65:35 methanol : isopropanol
Gradient: Linear gradient over 20 minutes.
Retention: alcohol, 2-naphthol, and octyl phenol elute at about 3-4 minutes.

Would switching to a solvent other than n-hexanes give better response; for example, n-hexane (no isomers), ethyl acetate, THF, n-heptane, etc?

Anyone have another methodology that works?

I understand that I can switch to UV/Vis for the 2-naphthol and octyl phenol; but, we still want to use ELSD for the alcohol. Don't want to have to derivatize samples or use second methodology (GC, GC/MS, etc.).

I will be most appreciative for any advice on this problem.

Thank you for your time.

Sally - I think the smaller guys are evaporating, and that's why you don't see them, or much.

We extract from a product and typically use GCMS or GC after trimethylsilylation for this task. I think others have posted similar here in the past.

Thank you KM-USA for your advice. I've been trying to avoid using 2 different methodologies to characterize the ethoxylated products; but, I think that I may have to resort to this. Does your GC assay detect only the backbone molecule and lower ethoxylates, or does it detect the entire range from EO-0 through EO-26?

For the higher EO the molecules get big, so for the big guys we use a high-temperature capillary, such as Restek metal capillaries.

It's not that likely to have a range be over such wide EO, unless you feel you need to find the really tiny guys.

You'll learn to recognize the order of elution of the peaks on the GC, inject fatty alcohols first and find their retention indexes, then go to a "simpler" nonionic surfactant such as laureth-2. You'll see that a series pattern repeats, such as C16-0, then C14-1EO, then C12-2EO, C16-1EO, C14-2EO, C12-3EO, etc.

The fun starts with nonionics also containing C11, C13, C15.

For alcohol ethyoxysulfates, you can hydrolyze then extract out the nonionic part and use that, and do similar.