TM96-05 Collection, processing, and analysis of carbon isotope samples

The National Water Quality Laboratory (NWQL) of the U.S. Geological Survey (USGS) has contracted with the University of Waterloo for the analysis of carbon-13 and carbon-14 in solids and in dissolved inorganic carbon (DIC) in water samples.In addition to the conventional analysis of carbon-14 (by beta counting) and carbon-13 (by mass spectrometric analysis), the new contract includes the analysis of carbon-14 by accelerator mass spectrometry (AMS). The advantages in selecting AMS for carbon-14 determinations over 
conventional beta-counting procedures are in ease of on-site collection and capability of determining carbon-14 activity in small samples. Conventional beta-counting determinations of carbon-14 usually require at least 5 grams of carbon, with an average reported precision of approximately ±0.3 percent modern carbon (pmc), while AMS determinations of carbon-14 usually require at least 5 milligrams of carbon, with an average reported precision of about 1 to 3 pmc. On-site collection of samples for beta counting can require 2 to 4 hours--time needed to precipitate sufficient quantities of inorganic carbon from the water--while collection for AMS determinations can be as simple as filling an appropriate container (see below). With the new contract, both the AMS and betacounting Schedules for carbon-14 will automatically report carbon-13. For most aqueous samples, Schedule 1000 will probably be more cost effective than Schedule 994. However, if analytical precision is of highest priority, somewhat higher precision may be obtained by beta counting (Beukens, 1992), especially if the sample has a low carbon-14 activity (<5 pmc). Analytical precision, by either beta counting or AMS measurement, depends, in part, on duration of the measurement and sample size. As a general guide, results obtained from AMS determinations of carbon-14 activity of samples submitted to the University of Waterloo and analyzed by AMS under subcontract to IsoTrace Laboratory, Toronto, Ontario, during the past 2 years have had the following 1 sigma analytical precisions: carbon-14 activity>50 pmc, ±0.7 percent of the reported activity;carbon-14 activity of approximately 20 pmc, ±0.9 percent of the reported activity; carbon-14 activity of approximately 5 pmc, ±3 percent of the reported activity. Although AMS technology is capable of much higher precision (Gove, 1992; Beukens, 1992), these services are not provided under the USGS contract. For rocks and other carbonate solids, Schedule 990 will probably be more cost effective than Schedule 960 if the sample contains at least 5 grams of carbon. All carbon-14 determinations are reported in percent modern carbon (pmc) normalized to the 1950 National Bureau of Standards (NBS) oxalic acid standard (Stuiver and Polach, 1977; Wigley and Muller, 1981), with accompanying 1 sigma error in percent modern carbon. Carbon-13 samples are reported in per mil relative to the Vienna Peedee belemnite (VPDB) standard (Coplen, 1994).