Variations in Atmospheric 13C/14C Ratios within a Radiocarbon Preparatory Lab and Implications for AMS Measurements

At CAMS, we focus on measuring carbon isotopes (12/13/14C) for a variety of applications. High precision radiocarbon AMS measurements are used for measuring atmospheric radiocarbon. The problem with performing high precision radiocarbon AMS measurements is that there’s almost always background contamination introduced during preparation for the AMS. For graphite preparation, dry ice is used in an isopropyl-dry ice mixture to condense H2O in the CO2 extraction and reduction lines. Dry ice is composed of 12/13C and releases carbon into the atmosphere as it sublimes. The dry ice for preparation is held in containers inside and outside the lab. We believe this may be a possible source of background contamination during the graphite preparatory phase for AMS. This experiment will provide data for quality control for ultra precise measurements at CAMS, LLNL. We measured the 13C/14C ratios of air at multiple locations within the graphitization lab and compared them across the lab and to atmosphere to test the following hypotheses: H1: 13C/14C ratios of lab air vary from atmosphere. H2: 13C/14C ratios vary across sites within the lab. We found that air in the graphite lab varies from atmosphere and at different sites within the lab. The dry ice containers are releasing depleted 13C and 14C into the lab air and affect lab sites close to them. This could lead to contamination when weighing out samples when CO2 can remain adsorbed to the sample during preparation. It is recommended that the dry ice containers be removed from the lab to reduce background contamination levels