Marine Chemistry, 41 (1993) 187-193 187 Elsevier Science Publishers B.V., Amsterdam Platinum-catalyzed combustion of DOC in sealed tubes for stable isotopic analysis Brian Fry, Susan Saupe, Meredith Hullar and Bruce J. Peterson The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543. USA (Received 8 July 1991; revision accepted 16 December 1991) ABSTRACT Fry, B., Saupe, S., Hullar, M. and Peterson, B.J., 1993. Platinum-catalyzed combustion of DOC in sealed tubes for stable isotopic analysis. Mar. Chem., 41: 187-193. Dissolved organic carbon (DOC) in water samples can be freeze dried and combusted to CO 2 at 580°C in sealed, evacuated tubes for carbon isotopic analysis. Dissolved platinum catalyst is added to promote combustion, and CuO is Used as an oxidant. Freshwater DOC samples from Massachusetts ponds and rivers have ~3C values of -26 to -31%o, and values for surface seawater from Woods Hole Harbor and the Gulf of Maine are -21 to -24%0. INTRODUCTION Dissolved organic carbon (DOC) is abundant in fresh and salt waters, and many studies have focused on determination of DOC concentrations and carbon isotopic compositions. Methods utilized in DOC determinations include per- sulfate oxidation, uv-assisted persulfate oxi- dation, combustion of dry residues after removing water via lyophilization, and direct injection of water into high temperature furnaces (Menzel and Vaccaro, 1964; Calder and Parker, 1968; Williams and Gordon, 1970; Sharp, 1973). Recent work has suggested that uv and uv- assisted methods give incomplete yields in marine waters owing to the refractory nature of an as yet uncharacterized portion of the DOC (Sugimura and Suzuki, 1988; Williams and Druffel, 1989). Previous studies of marine DOC with lyophilization techniques have had problems with large blanks, and variable yields owing to transfer problems involved with dried Correspondence to: B. Fry, The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, USA. salts and lack of a metal catalyst (Gordon and Sutcliffe, 1973; Mackinnon, 1978). New techniques with direct injection of water into a furnace with a platinized catalyst give much higher DOC concentrations, but can be used for only small 50-500 #1 samples (Sugimura and Suzuki, 1988). To obtain CO2 samples >0.5/zmol that are convenient for isotopic analyses, we adapted widely-used high tem- perature sealed tube combustion methods (Sofer, 1980; Boutton et al., 1983; Minagawa et al., 1984) for use with lyophilized 25 ml samples. EXPERIMENTAL SECTION Combustion tubes (22 mm outer diameter) were cut into 27 cm lengths, sealed at one end, and precombusted at 590°C overnight. DOC standards were made up in deionized water and added to combustion tubes; natural samples were prefiltered through a preleached 0.2/zm Millipore filter to remove particulate organic carbon (POC). To acidify samples and to add platinum catalyst in a dissolved form, 100/~1 of chloroplatinic acid (PtC14.2HC1.6H20) was 0304-4203/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved