Osmium-187/osmium-186 in manganese nodules and the cretaceous-tertiary boundary

As a result of the radioactive decay of rhenium-187 (4.6 x 10(10) years) the osmium-187/osmium-186 ratio changes in planetary systems as a function of time and the rhenium-187/osmium-186 ratio. For a value of the rhenium-187/osmium-186 ratio of about 3.2, typical of meteorites and the earth's mantle, the present-day osmium-187/osmium-186 ratio is about 1. The earth's continental crust has an estimated rhenium-187/osmium-186 ratio of about 400, so that for a mean age of the continent of 2 x 10(9) years, a present-day osmium-187/osmium-186 ratio of about 10 is expected. Marine manganese nodules show values (6 to 8.4) compatible with this expectation if allowance for a 25 percent mantle osmium supply to the oceans is allowed. The Cretaceous-Tertiary boundary iridium-rich layer in the marine section at Stevns Klint, Denmark, yields an osmium-187/osmium-186 ratio of 1.65, and the one in a continental section in the Raton Basin, Colorado, is 1.29. The simplest explanation is that these represent osmium imprints of predominantly meteoritic origin.     

Immobilized enzymes and cells as practical catalysts

Performance of enzymes and whole cells in commercial applications can often be dramatically improved by immobilization of the biocatalysts, for instance, by their covalent attachment to or adsorption on solid supports, entrapment in polymeric gels, encapsulation, and cross-linking. The effect of immobilization on enzymatic properties and stability of biocatalysts is considered. Applications of immobilized enzymes and cells in the chemical, pharmaceutical, and food industries, in clinical and chemical analyses, and in medicine, as well as probable future trends in enzyme technology are discussed.