The assay of deuterium in organic compounds has heretofore been based on combustion followed by reduction of the resultant D,O to deuterium, which is then measured in the mass spectrometer. Certain limitations in methods (1, 2) employing the combustion principle have led to consideration of another degradative method, developed by Rittenberg and Ponticorvo for the determination of IsO (3). Their procedure, previously used in this laboratory (4), is based on the quantitative conversion of organic oxygen to CO, by HgCl,; organic hydrogen at the same time is converted to hydrogen chloride, a by-product which is removed before admission of CO? to t,he mass spectrometer. The present investigation was designed to test the feasibility of using this reaction for mass spectrometric analysis of deuterated organic compounds after reduction of the analogous DC1 to D?. As in combustion methods, zinc was found to be the preferred metal for the reductive step. When applied to various organic compounds of known deuterium content, the met,hod was found to be accurate within the tested range of up to 5 atom prr cent excess deuterium. MATERIALS D,O, >99.50/0 enriched, was obtained from Stuart Oxygen Co., San Francisco. Succinic acid-a,a',DA, at various enrichments, was a gift of Dr. Dewitt Stetten, Jr. ; glycine-aD, and sodium benzoatc-D,, bot,ll >99% enriched, were purchased from Volk Radiochemical Co., Chicago. Analytical-grade HgCl,, dried over P,Oj, was used in the oxidation; for the reduction, zinc chips, about 3 mm square, were cut from 100% zinc ribbon and washed briefly with 0.1 N HCI, water, methanol. and methylene chloride, and finally oven-dried. 93 94 FRANK EISENBERG, JR.