Separation of hydrophobic polymer additives by microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) has been applied to the separation of some phenolic antioxidants [Irganox 1024, Irganox 1035, Irga nox 1076, Irganox 1010, Irganox 1330, Irgafos 138, Irganox 168 and 2,6-di-t ert-butyl-4-methylphenol (BHT)I. Due to the extremely hydrophobic nature of these analytes, they could not be separated using standard MEEKC condition s and two alternative approaches were investigated. Using an acidic buffer (phosphate, pH 2.5) to effectively suppress the electroosmotic flow, the ad dition of 2-propanol to the aqueous phase of the microemulsion buffer to im prove partitioning of the analytes, and a negative separation voltage, sepa ration of five of the analytes in under 10 min was possible. The second app roach, using a basic buffer (borate, pH 9.2) and a positive separation volt age resulted in complete resolution of all eight analytes. A mixed surfacta nt system comprising the anionic sodium dodecyl sulfate (SDS) and neutral B rij 35 was used to reduce the overall charge and with it the mobility of th e droplets, and hence the separation time. Using an optimised MEEKC buffer consisting of 2.25% (w/w) SDS, 0.75% (w/w) Brij 35, 0.8% (w/w) n-octane, 6. 6% (w/w) l-butanol, 25% (w/w) 2-propanol and 64.6% (w/w) 10 mM berate buffe r (pH 9.2) the eight target analytes were baseline separated in under 25 mi n. For these analytes, MEEKC was found to be superior to micellar electroki netic chromatography in every respect. Specifically, the solubility of the analytes was better, the selectivity was more favourable, the analysis time was shorter and the separation efficiency was up to 72% higher when using the MEEKC method. Detection Limits from 5.4 to 26 mug/ml were obtained and the calibration plot was Linear over more than one order of magnitude. The optimised method could be applied to the determination of Irganox 1330 and Irganox 1010 in polypropylene. (C) 2001 Elsevier Science B.V. All rights re served.