The distributions of the nickel and vanadyl geoporphyrins from the same sample are usually markedly different. Such differences may be attributed to one of two factors: preferential chelation of one of the two metals by different geoporphyrins species; the different chemistry of the nickel and vanadyl porphyrin complexes gave rise to the variation in the distributions. The results of a series of competitive metallation studies are presented, and their role in the preferential chelation of the geoporphyrins discussed. The reactivity of the nickel and vanadyl porphyrins at both the beta - and meso (bridge) positions are described, and their significance in the analysis of geoporphyrin mixtures outlined.
