Relationships between Retention Factors and Analyte Hydrophobicity on Cyanopropyl andn-Octadecyl Bonded Silicas, Cross-Linked Polymers and Porous Graphitic Carbon Stationary Phases. Consequences for the Trace Analysis of Highly Polar Organic Compounds

Abstract

LC retention data have been measured using various stationary phases with an emphasis on highly polar to moderately polar neutral organic compounds having octanol-water partition coefficients (K ow ) in log units between 0 and 3. The relationships between the retention factor measured in water and the octanol-water partition coefficient are linear but with different slopes for octadecyl (C 18 ) silicas, and two polystyrene divinylbenzene (PS-DVB) phases with low and high surface areas. These relationships confirm that highly cross-linked polymers can provide more than 1000-times higher retention values than C 18 silicas for moderately polar analytes but close values for highly polar ones. They also explain why C 18 silicas and polymers are equivalent for the separation of very polar analytes. In contrast, due to a different retention mechanism, no relation exists between the retention shown by porous graphitic carbons (PGC) and analyte hydrophobicity, but highly polar analytes are in general much more strongly retained than by any other sorbent. The potential of PGC for both the extraction and the separation of analytes is shown. Due to the difference in separation mechanism, PGC is the analytical phase that should be used for confirmation of the identity of analytes instead of a cyanopropylsilica column as recommended in some environmental procedures. Applications are presented for the trace-determination of triazines and polar degradation products in ground and surface water with detection limits below the 0.1 μg/L level.