Has anyone had experience detecting low levels of SDS (sodium dodecyl sulfate or sodium lauryl sulfate) by HPLC? I presume one method would be low UV using a C18 column with an acetonitrile/water mix. Would conductivity detection with this separating system or with anion exchange chromatography be possible?

Please e-mail replies to herlt@rsc.anu.edu.au as well as replying to this forum.

TIA

Tony

Dear Tony,
I separated SDS from octyl and tetradecyl sulfates with RP HPLC using unsuppressed conductivity detection. The detector was pretty cheesy and the LOD was in the low ppm range. By avoiding the use of buffer in the mobile phase you can do without the suppressor module and get decent detection limits.

Marc Foster

While it is true that low ionic strength buffers can be used for the separation of alkyl sulfates in order to permit direct conductivity detection, generally this is not advisable because retention of these analytes in a reversed phase system is ionic strength dependent and thus will exhibit poor reproducibility with low ionic strength buffers. The book "Ion Chromatography, second edition" by Joachim Weiss (published by VCH) gives a lot of information about separation of surfactants under reversed phase conditions. On page 275 there is a separation of C8 through C16 alkyl sulfates using 20 mM ammonium hydroxide with the gradient from 20-80 percent acetonitrile. The separation column in this case is a macroporous polymeric reversed phase column (Dionex sells the MPIC NS1 for this purpose, but analogous materials are available from a variety of other sources). This is followed by suppressed conductivity detection. Such a system is far more robust for real samples than that which is achievable with a low ionic strength system (necessitated by utilization of "nonsuppressed" detection). If you are only interested in SDS and are certain that you do not have to concern yourself with homologs (many sources of SDS contain 10% of C10 alkyl sulfate and 10% of C14 alkyl sulfate) then it is possible to do the separation under isocratic conditions: 20 mM ammonium hydroxide, 30 percent acetonitrile, with a flow rate of 1 ml per minute using the above-mentioned MPIC NS1.

With regard to the rest of your question, both reversed phase and anion exchange separations are possible for this class of analytes. Generally, in either case is necessary to have solvent present in order to obtain good chromatographic properties. Usually, however, the best chromatographic performance for this class of compounds is achieved using a reversed phase separation rather than ion exchange. In both cases, conductivity detection is definitely possible and in both cases the best results are obtained when using a suppressor column prior to the conductivity detector.

Dear Tony
I've used ion pairing technique on RP column with evaporative light scattering detector. Fine resolution between Lauryl and Myristyl Sulfates was achieved.
The detector has non-linear responce in the wide concentration ranges.
Oleg