Anionic surfactants

[msimone]

I have a need to determine some anionic surfactants in aqueous solution. The specific analyte of interest is sodium stearoyl lactylate.

I have previously analyzed cationics - cetylpyridinium chloride using a cyano column with 70:30 MeOH:0.2mM KH2PO4 buffer at pH 2.5 with good results.

Using the same conditions I am having no luck with the anionic surfactant.

Since the surfactant is a sodium salt of a carboxylic acid, it makes sense to me to keep the pH low and try the same conditions, but no luck.

Where am I going wrong, can anyone suggest a better set of operating conditions?

Thanks,

Mike

[maris]

What do you mean "no lack"? No elution? No retention?Bad peak's shape?
Please, clarify.

[Einar Ponten]

Using a high ratio of organic solvent and a polar stationary phase is a HILIC mode of separation.

Without having any details about your analytes I recommend you to first try acetonitrile (80%) instead of methanol and change to a volatile buffer like formic acid or ammonium acetate. It is likely that you later on need a volatile buffer for detection by MS or ELSD. Dissolve your sample in the mobile phase (or inject minute volume if it has to be in water).

You can learn more about HILIC at http://www.sequant.com/

[msimone]

By no luck, I mean I am not finding any peak using a refractive index detector.

Using a UV PDA detector, I was seeing a very weak, poorly shaped peak at around 17 minutes into my run. I switched to RI detector feeling that the surfactant did not have a good UV absorbing chromophore I would have better results.

Probably what I will do next is switch back to the UV detector, and continue experimenting.

Does anyone have any specific experience with this type of analyte?

Thanks,

Mike

[XL]

Dear Mike,

Ususally traditional C18 columns work pretty well for anionic surfactants with decent peak shape and efficiencies. You can use either ion suppresion mode (like you did at lower pH, say pH2.5) or ion-paring mode (ammonium acetate buffer,say pH5). There are plenty of examples of separation of linear aliphatic carboxylic acids (C10 to C18 chain length) by C18 columns.

I worked with carboxylic acids separation before at pH2.0 using UV at 210 nm. You can detect them fairly easily provided that the concentration of the sample is above 100 ppm. If you use ammonium acetate buffer, UV is out of the question. But you can use RI or ELSD to do the trick.

Right now, I am working on surfactant analysis. I am wondering if it is possible for you to send the sample to me so that I can try various conditions, hoping to make it work for you. Let me know what you think, then we can go from there.

Regards,
X.L.

[msimone]

Thanks, X.L.

I will try a C18 at low pH. As for the sample, it is commonly available, we got ours frm Rita chemicals. I don't see any reason why I couldn't send you some to work with if you like, this is no secret material. Why don't you e-mail me to discuss.