Looking for a cyclic cationic ion pair reagent...

[David Blais]

...to bond to my short-chain organic acids (glycolic, lactic, etc) to help increase the signal strength/reduce noise. I'm hoping the pi-pi bonding of the reagent will allow me to change the wavelength from 210nm to something like 254nm or 280nm. Unfortunately, I can only find chain reagents, like tetrabutylammonium. A quick Google search only brought up more of the same. Can anyone help me out here? Thanks.

-Dave

[MG]

I found these two, but don't know about their suitability for HPLC:

Trimethylphenylammonium iodide, Sigma-Aldrich # S96440

TRIMETHYL(4-(TRIPHENYLSTANNYL)PHENYL)AMMONIUM METHOSULFATE, Sigma-Aldrich # S838764

If simply added to your mobile phase, or sample, I don't think you will get the results you expect (if your expectation is to simply shift the UV maxima to longer wavelength). Or perhaps I misunderstand what you are trying to do.

[Steve]

I don't know it you have tried a mixture of water/acetonitrile
gradient 20 - 80% B in 20min @ UV 265nm. Here is a procedure for that work.
https://www.macherey-nagel.ch/web/MN-WE ... =HPLC00620

[Kostas Petritis]

It is not clear to me if you are searching for an aromatic ion-pairing reagent in order to retain and detect your underviatized organic acid through indirect UV detection at higher UV lengths or a derivatization reagent in order to chemically bound it to your organic acids...

[Uwe Neue]

I don't know how long a chain you want to use. For something short, see if the bretylium ion is avalable without the tosylate counterion. Another good candidate would be the bephenium ion. If my memory serves me correctly, an option for a longer chain is the dodecyl-dimethyl-benzylammonium ion.

[Bill Tindall]

I can't help with the question you asked but I can offer information on what you should be able to achieve with what you got. We were able to easily see peaks for acetic acid at 1 ppm and modest sized injection with a fixed wavelength detector and 205 nm. The acids you listed will have larger extinction coefficients than acetic acid. Separations were done reversed phase, phosphoric acid, and a column that could operate with little to no organic modifier,for example aqueous compatible columns from ES Industries. Chris can comment but I think ion chromatography and conductivity detection is nearly as sensitive, though I was annoyed by bending calibration curves so we went with UV detection normally.

[Chris Pohl]

David,

As I understand, it you are proposing to use a UV absorbing counter-cation as a basis for boosting your sensitivity for organic acids. There are a number of classes of cationic surfactants which may be suitable for such an application including alkylpyridinium quaternary surfactants and benzalkonium quaternary surfactants. A couple of words of warning, though: this type of chromatography is "indirect to the second power" since you are visualizing your analyte by adding a visualizing reagent rather than directly increasing the signal for your analyte and to further complicate matters this visualizing reagent is actually an electrostatic "partner" to your analyte! The results can be quite perplexing and confusing to interpret, yielding both positive and negative peaks depending upon the details of the partition coefficient of your analytes. Furthermore, such a system can bring with it a large system peak at substantial retention times, compromising throughput and the whole system is typically fairly temperature dependent so it's important that you control your operating temperature if you are going to try this. In addition, your life will be even more complicated if you try using such a reagent with a UV absorbing counter-anion such as iodide. In this case, you can get all sorts of negative and positive analyte and system peaks. This I would definitely stay away from. To avoid this problem you'll probably need to prepare the corresponding quaternary hydroxide by passing your quat solution through a hydroxide form anion exchange resin.

As Bill suggests above, if you need better sensitivity, conductivity detection is probably a lot better choice as the entire operation is much more straightforward. When using a suppressor device in conjunction with anion exchange chromatography, one can typically achieve 20-100 times higher sensitivity than is possible with UV detection at low wavelengths.

[Kostas Petritis]

I agree with Chris that is quite challenging your approach.

In general indirect UV detection is (or should I say was) mostly used for completly UV transparent ions and cations. As Chris mentioned the use of conductivity detection in conjuction with a suppressor can be a good alternative. However, if you do not have a suppressor then I will refer in one of Chris old articles (C.A. Pohl, E.L. Johnson, J. Chromatogr. Sci. 1980, 18, 442-452) who point to the problem inherent in attempting to determine accurately the ofthen times small changes in eluent conductivity that acompany the replacement of eluent ions by sample ions, both of which are conducting.

If you still want to go towards this path (indirect UV detection), you may want to do some reading first, which will give you more information of what you should expect from this technique. So if I may suggest:

Denkert et al. Reversed-phase ion-pair chromatography with UV-absorbing ions in the mobile phase, J Chromatogr, 218, (1981), 31-43.

Small and Miller Indirect Photometric Chromatography Anal Chem 1982, 54, 462-469.

Bidlingmeyer and Warren Effect of ionic strength on retention and detector response in reverse-phase ion pair liquid chromatography with ultraviolet absorbing ion interaction reagents Anal Chem 1982 54 2351-2356.

Hackzell et al Construction of systems for detection and quantitation by UV-absorbing mobile phase ions in reversed phase chromatography J Chromatography 282 1983 179-191

Bidlingmeyer et al Ion-pair chromatographic determination of anions using an ultraviolet absorbing co-ion in the mobile phase Anal. Chem 1987 59 1843-1846

Crommen et al Indirect detection in liquid chromatography. I. Response models for reversed phase ion pairing systems Chromatographia 24, 1987, 252-260

I have hard copies of all the articles I suggested above. If you can not find some of the above I could make a photocopy and fax it to you (well... not all of them though... one or two max...).

[David Blais]

Hi everyone,

As usual, thank you for the detailed responses. Let me try to clarify my question. I am interested in developing an ion-pairing method for the analysis of glycolic acid (and in the future, other short chain organic acids) currently in a topical formulation. I had tried reverse phase in the past and it worked very well, until they formulation was changed, then co-elution with the new ingredients became an issue.

I would like to go back to ion-pair, but am interested in finding a different ion pair reagent to use. The chain reagents (like TBAHS, etc) do their job well, but I would like to try a cyclic IP reagent for two reasons: 1) to see what change the retention is 2) to move to a higher wavelength to reduce the background noise, thus making it easier to see the oligimers that tend to form in these types of molecules.

Hopefully this clears things up a bit and leads to some more suggestions. Thanks again to all.

Regards,
David

[MG]

I still don't understand how (2) is even possible, unless you are talking indirect detection (negative peaks), discussed at length above. Could you explain idea #2 in more detail?

[Kostas Petritis]

David,

From what you said before about bonding the ion-pairing reagent with your organic acids I think that you have some misconceptions of how the think works. I imagine you believe that each of your molecules will carry along one molecule of the UV absorbing ion (ion-pairing reagent) so by monitoring let's say at 254 (if this is the UV max of the ion-pairing reagent) you will get a nice high signal/noise chromatographic peak... am I right?

Well things are not quite like this as you have a continue background from the UV absrobing ion that you use in your mobile phase. Depending on the charge of your molecules (here they have the same charge), the UV absorbing concentration, the injected amount and the elution of analytes in relation of the system peak (that you will almost certainly have due to the relaxisation of the system) your peaks might be positive or negative... Furthermore, I hope you realise that you won't be able to use gradient elution with indirect UV detection..

Or do you mean something else...?