Physical and chemical pre-treatments vary depending on the type of material and the depositional environment from which it came. There are two steps:
Physical pre-treatment often involves removing rootlets with tweezers or scraping the outer layer of bones or wood, followed by the sampling step where the best material is picked out or the sample is crushed to a smaller size. For more details about sample preparation techniques please refer to Crann et al, 2017.
Chemical pre-treatment varies depending on the material being dated. For example, an acid-alkali-acid (AAA) wash uses HCl to remove carbonates, NaOH to remove humic acid, and HCl again to neutralize the sample and remove any CO2 introduced during the alkali wash. Between each stage, the sample is rinsed with highly purified (Milli-Q) water. For more details about sample preparation techniques please refer to Crann et al, 2017.
For solid samples, once the sample has undergone physical and chemical pre-treatment, it is freeze dried and ready for combustion, which produces CO2 for graphitization. Depending on the age, carbon content, and client requirements, combustion takes place on an elemental analyzer, which purifies the gas, and clean CO2 is trapped in 6mm pyrex tubes.
Elemental carbon is produced from CO2 at 550–570°C in the presence of a preconditioned (reduced) Fe powder catalyst following the Bosch hydrogen-reduction reaction:
CO2 + 2H2 → C + 2H20
The 14C preparation lab is equipped with newly designed, semi-automated 10-port graphitization lines (below). The vacuum line, cooling assembly, ovens, and touch screen controls are supported on a single frame. The stainless steel vacuum lines were orbital welded to ensure smoother interior joints and to minimize CO2 adherence and cross contamination. The cooling assembly was designed to provide equal, uniform cooling for each reaction module to optimize water extraction during graphitization.
All operations on the line are controlled with a touch screen monitor using Labview. Oven and water trap temperatures, and pressures are continuously monitored and the ovens and valves are all operated under program control, thus facilitating automation. Safety controls are programmed into the software to prevent human error, to avoid sample loss or vacuum accidents. These automation and safety features contribute to a streamlined design, making graphitization easy, safe, and operator-friendly. For more information about the semi-automated CO2 purification and graphitization equipment, please refer to St-Jean et al, 2017.
Waters & Gases
Dissolved Inorganic Carbon (DIC) and Dissolved Organic Carbon (DOC) in Freshwater:
The appropriate sample volume is selected to target 1mgC depending on the concentration of [DIC] or [DOC]. For DIC extraction, the sample is added to a baked and pre-cleaned round-bottom borosilicate glass reaction bottle with 85% phosphoric acid. The bottle is heated to 60°C for minimum 1 hour, the headspace is sparged with helium, and the gas is then transferred to a vacuum line for cryogenically purification, or discarded if only DOC is desired. The DOC is extracted using a wet oxidation technique modeled from Zhou et al (2015) and Lang et al. (2016). ~8ml of sodium persulfate oxidant solution (400g/L) is added to the DIC sparged water along with 1mL of 0.5N AgNO3 catalyst, and the vessel is heated to 95°C for minimum 60 minutes. Once the sample has cooled, the headspace is sparged with helium. CO2 from DIC or DOC is cryogenically purified on a vacuum extraction line, which is trapped in a pre-baked 6 mm OD Pyrex breakseal containing a few grains of silver cobaltous (previously baked at 500°C for two hours) to remove S-bearing species or halogens. The sealed breakseal is baked overnight at 200°C so any sulfur is removed by the silver cobaltous (Palstra & Meijer, 2014).
Methane combustion (CH4):
The sample is combusted to CO2 following the methods of Pack et al. 2015. In summary, CH4 in headspace is sampled from a septum-capped bottle via gas-tight syringe. The syringe and dead volume in the needle are flushed with He to eliminate atmospheric contamination. The sample is injected into a combustion line with a carrier of ultra-zero compressed air at a flow rate of 10 ml/min. CO2 is first cryogenically separated from the sample, followed by a CO trap (300°C oven packed with CuO to oxidize CO to CO2) followed by a second CO2 trap. The methane is combusted in a 975°C tube furnace packed with CuO (regenerated by the constant flow of UZ air). Any water produced from the combustion is separated from the CO2, and the is transferred to a vacuum line trapped in a pre-baked 6 mm OD Pyrex breakseal containing a few grains of silver cobaltous (previously baked at 500°C for two hours). The sealed breakseal is baked overnight at 200°C so any sulfur is removed by the silver cobaltous (Palstra & Meijer, 2014).
Gas samples requiring purification to CO2 are submitted in an airtight glass bottle with septa. CO2 is extracted from the gas sample by Helium carrier. The volume of the sample bottle is flushed minimum 10x at 200 ml/min and CO2 is trapped cryogenically on a U-trap packed with Silver wool. After the extraction period, the Helium flow is cut off and the sample is transferred to a vacuum line for cryogenic purification with a -80°C ethanol slurry to remove water and other non-condensable gases. The CO2 is then transferred to a pre-baked 6 mm OD Pyrex breakseal containing a few grains of silver cobaltous (previously baked at 500°C for two hours). The sealed breakseal is baked overnight at 200°C so any sulfur is removed by the silver cobaltous (Palstra & Meijer, 2014).
For more information about processing of water and gas samples at AEL-AMS, please refer to Murseli et al, YYYY.