Thesis (M.Sc.)--Chulalongkorn University, 2008

This study was aimed to enhance the separation efficiency of a monomeric C18 column on the separation of isomeric polycyclic aromatic hydrocarbons (PAHs) by adding a surfactant into the mobile phase with a concentration lower than its critical micellar concentration (CMC). This surfactant will be adsorbed onto the stationary phase. The adsorbed surfactants lead to an increase in stationary phase rigidity which increase the selectivity (α) and retention factor (k') of the compound separation. In this study, 5 types of surfactants (ethylammoniumchloride, diethylamine hydrochloride, butylamine, hexylamine and octylamine) were employed as rigidity inducers. A pH6 carbonate buffer/acetonitrile (30/70) was used as mobile phase. The amounts of surfactant adsorbed on the column were determined by frontal analysis. The temperature was varied from 0 to 40 ℃ with a 10 degree increment. Triphenylene, benz[a]anthracene, benzo[k]fluoranthrene and dibenz[a,c]anthracene were used as the solute probes. Our results showed that surfactants were adsorbed onto the surface of a monomeric C₁₈ stationary phase via electrostatic interactions. Selectivity and retention factors obtained from using ethylammoniumchloride as the rigidity inducer were higher than the other surfactants. In comparison with a system without addition of the surfactant, the selectivity and retention factors were most improved at 0 ℃. This technique gave the higher precision on the separation of PAHs (%RSD < 1.0%). This method was used for separation of 5- and 6-methoxyflavone. Selectivity and retention factors of 5- and 6-methoxyflavone obtained from using ethylammoniumchloride at 0 ℃ as the rigidity inducers were higher than the other surfactants.