The sealed tube zinc reduction method for converting CO 2 to graphite for AMS 14 C measurements was originally developed for rapid production of graphite in biomedical tracer experiments. The method was usually thought to have low precision and a high background. We have modified the zinc reduction method originally outlined in Vogel [J.S. Vogel, Radiocarbon 34 (3) (1992) 344] by carefully controlling the amounts of reagents (zinc, titanium hydride and Co or Fe catalyst) and now routinely obtain a precision of 2-3& and a relatively low background of $50,000 14 C years when analyzing for 14 C at the Keck Carbon Cycle AMS facility at UC Irvine. Fractionation of carbon isotopes does occur during graphitization and depends on the graphitization yield, which can be affected by the amounts of reagents used and other conditions. The d 13 C of our zinc-reduced graphite is usually lighter by 2-3& than the CO 2 from which it is made, but this is corrected for in our system by simultaneous measurement of 13 C/ 12

Much of our understanding about how carbon (C) is allocated in plants comes from radiocarbon ( 14 C) pulse-chase labeling experiments. However, the large amounts of 14

The ability to measure the radiocarbon content of compounds isolated from complex mixtures has begun to revolutionize our understanding of carbon transformations on earth. Because samples are often small, each new compound isolation method must be tested for background carbon contamination (C ex ). Here, we present a new method for compound-specific radiocarbon analysis (CSRA) of higher plant-derived lignin phenols. To test for C ex , we compared the ∆ 14C values of unprocessed lignin phenol containing standard materials (woods, leaves, natural vanillin, and synthetic vanillin) with those of lignin phenols liberated by CuO oxidation and purified by twodimensional high-pressure liquid chromatography (HPLC) coupled to mass spectrometry (MS) and UV detection. We assessed C ex associated with (1) microwave assisted CuO oxidation of bulk samples to lignin phenol monomers, (2) HPLC purification, and (3) accelerator mass spectrometry (AMS) sample preparation. The ∆ 14C of purified compounds (corrected for C ex ) agreed, within error, with those of bulk materials for samples that were >10 µg C. This method will allow routine analysis of the ∆ 14 C of lignin phenols isolated from terrestrial, aquatic, and marine settings, revealing the time scale for the processing of one of the single largest components of active organic carbon reservoirs on earth.Higher-plant derived organic carbon represents a significant fraction of the earth's annual primary productivity and is also an important reservoir of fixed carbon, both living and dead. Organic matter (OM) derived from higher plants in terrestrial ecosystems can be processed to soil debris or converted to carbon dioxide through respiration. Once in aquatic ecosystems, such as rivers, more biological processing occurs as plant-derived OM makes its way to the ocean. Once in the ocean, it can be further transformed in the marine dissolved and particulate organic carbon pools, buried in coastal marine sediments, or converted to carbon dioxide. Traditionally, the radiocarbon age of bulk carbon in terrestrial reservoirs is used to infer the average age of mobilized organic carbon. For example, measurements of the ∆ 14 C of bulk particulate organic carbon (POC) and dissolved organic carbon (DOC) suggest that local geology and in situ biological production determine the average age of organic matter in rivers.1-5 Still, terrestrial carbon is a complex mixture that integrates recent biological and ancient geological carbon sources into a complex mixture of carbon-based materials at various stages of decomposition. More in-depth knowledge about the age of various carbon sources can tell us how carbon from different sources is processed. The advent of compound-specific radiocarbon analysis (CSRA) has presented the possibility of measuring the age of individual components within complex mixtures of organic carbon.6,7 Some researchers have used the ∆ 14 C value of long chain fatty acids in river delta sediments to apportion sources of organic matter and, therefore, to understand the fate ...

[1] Long-chain, odd-carbon-numbered C 25 to C 35 n-alkanes are characteristic components of epicuticular waxes produced by terrestrial higher plants. They are delivered to aquatic systems via eolian and fluvial transport and are preserved in underlying sediments. The isotopic compositions of these products can serve as records of past vegetation. We have developed a rapid method for stable carbon isotopic analyses of total plant-wax n-alkanes using a novel, moving-wire system coupled to an isotope-ratio mass spectrometer (MW-irMS). The n-alkane fractions are prepared from sediment samples by (1) saponification and extraction with organic solvents, (2) chromatographic separation using silica gel, (3) isolation of straightchain carbon skeletons using a zeolite molecular sieve, and (4) oxidation and removal of unsaturated hydrocarbons with RuO 4 . Short-chain n-alkanes of nonvascular plant origin (