DETERMINATION OF THE CONCENTRATION AND STABLE ISOTOPIC COMPOSITION OF NONEXCHANGEABLE HYDROGEN IN ORGANIC-MATTER

The hydrogen stable isotope ratio of organic matter containing hydrogen that is not solely conservative, nonexchangeable, carbon-bound hydrogen depends on sample preparation, because organic hydrogen bound to oxygen and nitrogen may exchange isotopically with ambient-water hydrogen. The method described here permits the determination of the concentration and stable isotopic composition of the nonexchangeable hydrogen in complex organic matter, such as geochemicals and biological and archaeological organic materials. Aliquots of organic substrates were independently equilibrated with water vapors of different hydrogen isotopic compositions, followed by determinations of the bulk D/H ratios. Mass-balance calculations permit eliminating or minimizing the interference of exchangeable hydrogen. The precision of the calculated stable isotope ratio of nonexchangeable hydrogen can be better than +/- 3 per mil, depending on the precision of the measured values for bulk hydrogen. The accuracy of D/H ratios of nonexchangeable hydrogen was improved over that of previously available methods, as shown for cellulose/cellulose nitrate.