Shape-selective separation of polycyclic aromatic hydrocarbons on protoporphyrin-silica phases - Effect of surface porphyrin distribution on column efficiency

The chromatographic performance of various metalloprotoporphyrin-silica (MP roP-silica) packing materials prepared using different porphyrin immobiliza tion schemes is examined. Column efficiency and solute resolution for the s hape-selective separation of polycyclic aromatic hydrocarbons (PAHs) can be improved significantly by preparing phases with lower porphyrin coverages and with a more homogeneous distribution of the porphyrin species on the su rface. The latter is accomplished by spreading/diluting the number of amino propyl reactive sites on the silica surface via mixing an inert methyltrime thoxysilane with 3-aminopropyltriethoxysilane during this preliminary react ion step. Subsequent covalent attachment of the prop via amide bonds to the pendant amine sites results in a more even distribution of the porphyrins on the surface. Band shapes and retention times as a function of injected s olute concentration as well as HPLC separation of various test mixtures of PAHs (including standard reference material SRM 869) are used to confirm th e enhanced performance of these so-called "spread" phases. Changes in the n ature of the immobilized porphyrin distribution on the silica surface are f urther probed by a coupled redox/UV-Vis absorbance method, and results sugg est a decrease in the number of prop species immobilized as aggregates on t he surface. (C) 1999 Elsevier Science B.V. All rights reserved.