Shape-selective retention of polycyclic aromatic hydrocarbons on metalloprotoporphyrin-silica phases: effect of metal ion center and porphyrin coverage.

Various metalloprotoporphyrins (MProP) covalently linked to silica are examined as stationary phases for reversed-phase HPLC separation of polycyclic aromatic hydrocarbons (PAHs). The MProP-silica stationary phases are shown to exhibit extraordinary shape selectivity for planar over nonplanar PAHs, with the selectivity factors for the triphenylene/o-terphenyl solute pair approaching 30 on Cu(II)ProP-silica phases using 100% acetonitrile as the mobile phase. Shape selectivity and solute retention are highly dependent on the metal ion (M) within the center of the immobilized protoporphyrin (ProP) structure in accordance with the following sequence:  Cu(II) > Fe(III) > Ni(II) > H(2) > Zn(II) ≈ Cd(II). A face-to-face π-π interaction is believed to be the major retention mechanism of PAHs on ProP- and MProP-silica phases, with varying metal ion centers affecting the strength of this interaction. Beyond the influence of the central metal, varying surface coverages of metalloporphyrin also lead to significant changes in observed capacity factors and shape selectivities for PAH solutes. The extremely high shape selectivity for planar vs nonplanar PAHs suggests that MProP-silicas could be ideal materials for selectively preconcentrating the more toxic planar PAHs from environmental samples. Preliminary results relevant to this application are reported.