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., 2019.
For the analysis of bone samples, collagen is extracted (media code “B”). Always send clean and dry bone samples as damp samples may contain mold which can cause deterioration of the collagen. The hard part of the bone (cortical bone) is ideal, we typically avoid the spongy part.
Please pre-screen bone samples prior to shipping to the lab. Bone samples can fail to produce collagen if the collagen has been degraded or the bone has been charred. Since collagen holds the inorganic part of the bone together, the easiest way to test for suitability is to try to scrape the bone with a scalpel or snap the bone by hand. If the bone is friable and easy to scrape or snap (for the exception of small ribs or bird bones), the collagen has likely been degraded. Where bones fail to produce collagen in the lab, we will try a second time. The first two failed attempts are never charged, but we reserve the right to charge for the collagen extraction at the discretion of laboratory personnel, especially if the bone has a negligible nitrogen content (as measured in the lab).
For teeth, the dentine is most reliable for dating as the enamel exchanges carbon with the environment. The tooth root (which is dentine) may have better collagen preservation if it was protected for some time in the bone. Dentine also has a higher initial collagen content.
Ultrafiltration (media code “BU”) may be requested. The theory is that is ultrafiltration will concentrate longer intact protein molecules and can be used to remove shorter chain proteins more likely to originate from contaminants.
Collagen yield, atomic C:N, δ13C and δ15N (by EA-IRMS) are reported with all bone collagen dates as a QA measure.
The radiocarbon age of wood corresponds to the growth year of the ring. Depending on the research question, the most appropriate date will often be from either the outer tree rings or small twigs. The majority of wood samples will undergo AAA pre-treatment (see Crann et al. 2017), but where year to year differences in F14C are desired, individual tree rings will undergo an α-cellulose extraction upon request. Try to ship only dry wood and please remove the bark if you can.
Charcoal samples should be picked out, floated, or sieved from any accompanying sediment. Prior to shipping, please dry charcoal samples for 12-24 hours at temperatures less than 70°C. Visually inspect all samples and remove any rootlets or other signs of physical contamination. Our chemical pre-treatment methods (media code “AAA”) will remove post-depositional carbonates as well as humic acids.
CONSULTATION REQUIRED: Where a sample preparation laboratory has been set up in an external laboratory, we can accept pre-treated material for direct combustion. Consultation is required to ensure the external laboratory poses no risk of tracer contamination, and the proper pre-treatment protocols are followed. We will not accept pretreated material without old and modern standards that have been treated alongside the unknown samples. We can provide aliquots of standards.
Samples should be sent to the lab in pre‐baked (500 °C for 3 hours) borosilicate glass bottles (i.e. Wheaton) with a septum seal (screw cap or crimp cap). Concentrations (ppmC) are required at the time of submission. Ideally, each sample should yield minimum 1mgC (2mgC is ideal); please sample accordingly. Minimum concentrations are required for CH4 analysis (1-2% v/v); please contact us first prior to sending samples.
Researchers sometimes request to date the humic acid (alkali-soluble) and humin (alkali insoluble) fractions of bulk sediment or peat. The general theory is that humic acids percolate downward and make radiocarbon dates appear younger. However, this is not always the case so it is important to test this method first before pursuing numerous analyses.
In many lacustrine environments, macrofossils are the preferred organic fraction used for dating. Macrofossils include charcoal, wood, plant, bone, shell, and seeds – not rootlets. We recommend isolating macrofossils with tweezers or by sieving in distilled, deionized water. Identification should be made prior to shipping. Macrofossils (except shell) will undergo the standard AAA pre-treatment. Bulk sediment can contain carbon from more than one reservoir and occasionally this can lead to a freshwater reservoir effect, such as in hard water regions or lakes where old carbon is washed in from the catchment during spring melt or floods. Having said that, bulk sediment is widely used for radiocarbon dating, especially in cases when macrofossils are unavailable. If using bulk sediment, be sure to remove rootlets. For bulk samples, some consultation may be needed to determine whether a simple acid wash (media code “A”), AAA method, or compound specific analysis is needed, but most of the time only an acid wash is performed to give a true “bulk” signature.
Peat can be analyzed as either selected macrofossils or as bulk material. If shipping macrofossils, they must be rinsed in deionised, distilled or ultra-pure (Milli-Q) water to remove residual sediment, and ideally dried prior to shipping to avoid mould. Bulk peat samples often contain a silty component and a vegetable component. Both can be dated separately or together. Please remove visible rootlets prior to shipping and do not ship in aluminum foil as it may degrade. Samples will undergo AAA treatment, unless specified otherwise.
Physical pre-treatment of shells involves removal of the outer layer of the shell with a hand drill or scalpel as well as chalky or recrystallized areas to isolate aragonite only. Samples may undergo a chemical pre-treatment in the lab involving the dissolution of the outermost shell in dilute HCl (media code “S”). Very small or powdered samples will not undergo this stage of pre-treatment (media code “SN”). It is not advised to store powdered carbonates for long periods as the large surface area exposes the sample to contamination by atmospheric CO2. If powdering the sample is necessary for sampling by drilling or powdered specific areas, we recommend that this be completed under an inert gas (i.e. N2, Ar, etc.). The powdered samples should be stored in small glass vials and shipped to the lab immediately. If the samples are of marine origin, the client should also provide a marine reservoir correction (∆R) on the submission form appropriate to their collection site to be incorporated in the calibration of results.
If the use of enriched 14C (i.e. tracer 14C) is suspected at a study site or in a user’s laboratory, a series of swipe tests must first be carried out to rule out gross levels of 14C contamination prior to sending any samples to AEL-AMS for radiocarbon analysis.
Swipes are conducted by liquid scintillation counter. To conduct a swipe, a pre-combusted quartz fiber filter wetted with methanol is rubbed over a surface suspected of being contaminated with 14C tracer (lab counters, fume hoods, fridge/freezers, door handles, common science store counters, etc.). The user should take care to wear gloves and change them between swipe locations. The filters are allowed to air dry, then wrapped in non-stick aluminum foil and placed in a Ziploc bag labelled with the swipe location and shipped to the lab for testing, along with a printed copy of the submission form. Blank filters (only wetted with methanol) should also be sent. A swipe kit can be sent out upon request. Please contact us first prior to conducting a swipe test.