SELECTIVITY OF POLYMER-COATED ALUMINA STATIONARY PHASES FOR REVERSED-PHASE LIQUID-CHROMATOGRAPHY .1. METHYLENE GROUP SELECTIVITY

The methylene selectivity of various polymer-coated alumina columns [polybutadiene (PBD), and polymeric CN and C-18 phases] are compared with those for three conventional silica bonded-phase C-18 columns. Retention and selectivity data were obtained using hydroorganic mixtures of acetonitrile and methanol for selected aromatic and aliphatic compounds, including n-2-ketones (C-4-C-9), n-alkylphenones (C-8-C-11), n-alkylbenzenes (C-7-C-15), and n-alkanes (C-5-C-7). For a given mobile phase, the polymer-coated alumina columns and silica based C-18 columns exhibited similar methylene selectivity, although the alumina columns were much less retentive. The latter characteristic suggests a potentially important advantage of polymer-coated aluminas, namely that for a given homologous series it should be possible to employ a weaker mobile phase on the polymer-coated columns, thereby achieving higher methylene selectivity with equivalent retention. The inherent selectivity advantage of the different polymer-coated aluminas may be especially important when separation selectivity is to be optimized by changing columns.