By Anonymous on Thursday, June 19, 2003 - 08:25 pm:
I'm trying to develop a method to separate peaks of salt (NaCl) from ionic surfactant by using reversed phase c18 column. But I still can't get a good separation. Anyone has any idea?
By Chris Pohl on Friday, June 20, 2003 - 08:55 am:
Are you trying to analyze the anions in the sample? If so you will need to add anion pair reagents such as tetrabutylammonium phosphate in order to obtain retention of the chloride. It won't be very sensitive but you can detect chloride at 210 nm. If you're after sodium, you'll need to use an ion pair reagent such as hexanesulfonate or heptafluorobutyric acid but you won't be able to detect the sodium with a UV detector unless you use the indirect mode and add a UV absorbing cation such as copper or histidine.
However, if you are simply after the ionic surfactant, this should be easily accomplished by use of a suitable eluent system. There are a number of threads already posted at this site on specific analytical conditions for various ionic surfactants. The book, "Ion Chromatography" by Joachim Weiss has an extensive discussion of separation conditions for ionic surfactants. In general, you need some ionic strength in your mobile phase in order to achieve reasonable chromatographic performance with ionic surfactants. You can't just use water and an organic solvent as you need some ionic content to help "define" the electrical double layer formed when the surfactant absorbs at the surface of the stationary phase.
By Anonymous on Thursday, June 26, 2003 - 04:19 pm:
Actually, I'm trying to analyze anionic surfactant in sample. But the sample contains 0.2M NaCl. Salt peak overlabs with surfactant peak when I use MeOH/DI water as mobile phase. So, I've tried to develop HPLC technique to separate peaks of salt and surfactant.
By Chris Pohl on Friday, June 27, 2003 - 12:06 pm:
Considering your application, the best method of increasing the separation between the surfactant and the ionic components in the sample is to either: increase the ionic strength of the mobile phase (this will increase the partition coefficient onto the stationary phase) or add an ion pair reagent. You don't need to add a potent reagent like tetrabutylammonium ion for this sort of application, although it can be used, since you'll need to use of fair amount of solvent to elute the surfactant in this case. You can probably get the separation you need by using tetraethyl ammonium ion or maybe even tetramethylammonium ion.