Characterization of cyanobiphenyl stationary phases using polychlorinated biphenyls as probe solutes

Polychlorinated biphenyls (PCBs) as a class are lipophilic, but the degree of lipophilicity varies among individual congeners. Nor are all congeners equally toxic. Thus there is a need to analyze for individual congeners, yet no single column has been found which is capable of separating such a complex mixture. A better understanding of chromatographic retention mechanisms is necessary if stationary phases capable of specific and complex separations are to be designed. To further the understanding of factors controlling the chromatographic retention of PCBs, the selectivity of six cyanobiphenyl (CBP) phases for PCB congeners was studied. Retention behavior was compared to the results obtained on a nonpolar 5% phenyl methylpolysiloxane phase (5% phenyl), and on a shape selective smectic liquid crystalline phase. Correlations between chromatographic retention and molecular structural descriptors were investigated. For all of the columns studied, the degree of ortho substitution of the congener was found to be a significant factor controlling gas chromatographic retention. The retention pattern on six different CBP phases followed the same general pattern as on the liquid crystalline column, though the trends were more exaggerated on the liquid crystalline phase. While general retention patterns were similar for the six CBP phases, subtle differences occurred, resulting in significant selectivity differences. For isomeric planar PCB congeners, elution order could be correlated to length-to-breadth ratios (L/B). This correlation did not exist for nonplanar congeners. Degree of ortho substitution was also a significant factor controlling retention in supercritical fluid chromatography (SFC). Dipole moment was more significant in SFC than in GC, while L/B correlations disappeared. The dominant retention mechanism in SFC changed with temperature and pressure, and van't Hoff plots indicated a phase transition at 60$\sp\circ{\rm C}$. The p,p-CBP phase was used for several quantitative environmental applications due to its ability to separate PCB 138 (monitored internationally) from PCBs 163 and 164. PCB 163 was found in significant concentrations in environmental samples. The differing selectivity of the phase relative to the 5% phenyl phase makes it an excellent choice in multidimensional GC systems or as a secondary confirmation column. It can be used for single column analysis of legislatively important congeners when used with mass selective detection.