Chromatography Forum

Hi,

I am a chemist at a company that specializes in cleaning products. We recently purchased an HPLC with UV detector and a now I'm developing techniques to analyze sufactants. No one at my company has any HPLC experience so I thought someone here might lead me in the right direction. I want to analyze surfactants that do not have chromophore groups, especially anionic and nonionic surfactant, for example, alcohol ethoxylates and sodium lauryl sulfate. Some papers listed derivatizing compound containing a chromophore groups. I want to use methylene blue for analysis of sodium lauryl sulfate, could someone help me.

Hope this is enough to give you a good idea of what we are looking for. Any advise would be great.

Refractive index detection has long been the "standard" for detecting non-chromophoric compounds; the alternative is one of the "evaporative" detectors (ELSD = Evaporative Light Scattering Detector; CAD = "Charged Aerosol Detector", etc.).

"UV tagging" to add a chromophore has been around for a long time, but has a number of drawbacks that combine to make it an unpopular technique.

And it depends upon what you want to do. Do you want to quantitate a single or a few known surfactants or explore the entire composition of a mixture? How big is your non-ionic?

Derivatization is a pain if even possible for your non-ionics. GC might be a better choice for non-ionics. If you are stuck with an LC and UV then you might also try indirect UV detection. Not ideal but it might work for now. Basically, add a UV absorber to your mobile phases. You will detect your analytes as a negative peak where they have displaced the absorbing mobile phase. You might try the thermo PGC column if you don't have a column already.

thank you very much for your help, I know that using another type of detector quantization is possible, but I do not have the resources to acquire other detector, is for this reason that I am exploring options for this problem.

I measure from one to two surfactant by product, usually a non-ionic with an anionic, alkyl chain contains between C12 to C16, I have a Varian Pursuit C18 column, I can indicate the conditions for an indirect UV detection, which absorber I can use UV, mobile phase, etc. Thank you kindly

This looks like a good reference. I don't have access to it but you should be able to purchase it for $30 or so. Or just search google on 'indirect UV HPLC" and you should be able to find a freebie if you look hard enough. Good luck!

Indirect UV detection of non-ionic substances in liquid chromatography by addition of a UV-absorbing uncharged compound to the mobile phase
Journal Chromatographia
Publisher Vieweg Verlag
ISSN 0009-5893 (Print) 1612-1112 (Online)
Issue Volume 19, Number 1 / January, 1984
DOI 10.1007/BF02687753
Pages 274-279
Subject Collection Chemistry and Materials Science
SpringerLink Date Monday, August 20, 2007

Thanks a lot for all suggestions, I hope to succeed, best regards

Since there is really no such thing as an organic compound without any chromophore it happens often that some low wavelength absorption tails into the region used in HPLC detectors. Especially if the mobile phase is mostly water, maybe with some acetonitrile, and if you can use high enough concentrations of your substances you may do UV absorption directly. Did you ever try this?

I'm sorry, but I could not understand your suggestion, can you explain a little more, please.

I think HW suggests low wavelength UV detection.

I can't help you more as most of our surfactant procedures are proprietary. I can say that we have RID, ELSD, and conductivity detectors.

Hey there -

This may be utterly dumb and if it is, there will be others here that will so advise, I'm sure.

If I recall correctly, the MBAS assay is based upon ion pairing of methylene blue and anionic surfactants extracted under acidic conditions into chloroform.

If the ionic bond is particularly stable AND you get a specific 1:1 binding between the methylene blue and each anionic surfactant species, then you might be able to separate the methylene blue-surfactant complex via reversed phase with detection in the visible range (~650nm?).

If not, add just a breath of methylene blue to your MP - not enough to saturate your detector at 650 nm, keep your MP acidic to favor pairing, then shoot your surfactants and pray. This will certainly take some fiddling.

If this is a critical situation, I'd consider trading your UV detector for an ELS.

While driving home last night in my old BMW listening to some outstanding funk (Galactic), I gave this a bit more thought and came up with the following quick and dirty experiment:

Because the methylene blue is visible, it would be a simple matter to see whether the methylene blue-surfactant complex would be retained via reversed phase using a reversed phase SPE cartridge. I'd prep 2 samples of methylene blue in dilute acid, add a small quantity of SLS to one, and load them onto individual cartridges that have been conditioned (rinse with water, then methanol, then your dilute acid minus MB). Once loaded, rinse with dilute acid. and see if anything is retained. With luck, your sample with SLS will have some material retained and your MB only sample will not. If this is the case, I'd then elute with progressively stronger methanol (or other organic) solutions until your complexed surfactants elute.

One thing that would concern me is the purity of the methylene blue. If you have more than one MB related compound present that will react with the surfactants, you'll have multiple peaks for each surfactant species, which could be a nightmare.

This wold also do absolutely nothing for your nonionics, obviously.

This is pretty much an analog to the experiment I did a while ago to assay for Fe2+ via reversed phase.

If I can assist any further, please give a holler anytime at
christopher-dot-judd-at-BASF-dot-com.

Best of luck!
Chris

Hi Cris:

Thanks a lot for your recommendations, I will adapt the proposed experiment.

Regards

Hey frasmx can you please tell me how did you do it in the end, or at least some tips if you succeeded? I am also interested in a solution to this problem. Thanks!

Hello,

I have not reached a solution to my problem, but I have some ideas, for example, nonionic surfactants, many papers mention the use of PHENYLISOCYANATE reagent for detection by UV / Vis. For anionic surfactants cetylpyridinium chloride can be used, but not yet designed the experiments, I hope this information is helpful.