High performance liquid chromatography of aromatic and polyaromatic hydrocarbons on a new chemically bonded liquid crystal phase

Summary

The chromatographic behaviour of 15 aromatic and 20 polyaromatic hydrocarbons (PAHs) on a new HPLC bonded liquid crystal stationary phase called BLC is presented. BLC is 4-(4-carboxy-benzyloxy)-3-methyl-2-dodecyloxy-4′-(2-(4′-dodecyloxy-benzyl)-ethylene-azobenzene bonded to LiChrospher Si 100 NH2. The method for PAH separation involved n-hexane in normal-phase HPLC. In reversed phase HPLC, which was applied to the aromatics, the mobile phase was acetonitrile/water (60/40, v/v). UV detection was carried out at 254 nm. The temperature of the chromatographic system was varied from 285 to 343 K. For the normal phase system, two transition temperatures were observed for all the PAHs studied. The first transition occurred at 317 K, and may be induced by the motion of the terminal O-dodecyl chain. When the lateral O-dodecyl moves, the second transition appears at 326 K. Compared to the normal phase separation of polyaromatic hydrocarbons presented in a previous study, the behaviour on the reversed phase seems to be more complex and only one large transition appears at 317 K. The chromatographic behaviour was investigated below, during and above the transition range between 315–330 K. Interesting analytical properties were observed. In the normal phase, the PAH retention factors were enhanced when the temperature was increased from below to above the temperature transitions. A better separation was obtained above the transition range. In the reversed phase, xylene and diethyl benzene isomers were clearly separated above the transitions. The xylene isomers were not resolved below the transitions, but good separation was found for the diethylbenzene isomers. During the transition, the xylenes and diethylbenzenes were not separated. The bonded material exhibited liquid crystal-like behaviour in both (normal and reversed) phases.