Radiocarbon analytical services

Combustion of a solid sample for graphite analysis

(Boutton et al., Comparison of quartz and pyrex tubes for combustion of organic samples for stable carbon isotope analysis, Analytical Chemistry 1983, 1832–1833) The sample mass is selected to combust approximately 1 mg of carbon. The sample is sealed in a quartz tube under a vacuum with an excess of copper oxide. The sample is then heated to 900°C for 2 hours. The copper oxide reacts as an oxidizer, producing CO₂ from organic carbon in the sample. The sample CO₂ is then purified as part of the graphitization step.

Graphitization is performed on one of two systems: the Automated Graphitization Equipment (AGE 3, Ionplus) or a manual graphitization line. Both systems use the hydrogen reduction method (Vogel et al., Performance of catalytically condensed carbon for use in accelerator mass spectrometry, Nucl. Instrum. Meth. B 1984, 289–293). By this method, the sample CO₂ is mixed with H₂ in a 1:2.3 ratio in the presence of an iron catalyst. The reaction takes place in a gas reactor heated to 400°C. Over 3 hours of reaction time, the sample is converted to graphite coated on the iron catalyst. The graphite/iron mixture is then pressed into a cathode for AMS analysis.

Radiocarbon analysis of graphite is performed by Mini Carbon Dating System (MICADAS, Ionplus; Synal et al., MICADAS: A new compact radiocarbon AMS system,  Nucl. Instrum. Meth. B 2007, 7–13). Graphite analysis produces higher precision than direct gas analysis by analyzing more carbon and counting more ¹⁴C atoms. Ideally, 1 mg of carbon is analyzed, but we can graphitize much smaller samples. Analysis of smaller samples will achieve lower precision than large samples.

Often with solid samples, pretreatment is required to remove exogenous carbon (for example, acid-base-acid treatment).

Pretreatment and oxidation of a carbonate sample for graphite analysis

Carbonate samples are cleaned and pretreated with acid to remove about 30% by mass, the outer surface being most susceptible to exogenous carbon (Bard et al., 230Th-234U and 14C Ages Obtained by Mass Spectrometry on Corals, Radiocarbon 1993, 191–199). They are then sealed in an exetainer vial and vacuumed out, then 1 mL of phosphoric acid is added to convert the carbonate to CO₂ for graphitization.

Graphitization is performed on manual vacuum lines. We use the hydrogen reduction method (Vogel et al., Performance of catalytically condensed carbon for use in accelerator mass spectrometry, Nucl. Instrum. Meth. B 1984, 289–293). By this method, the sample CO₂ is mixed with H₂ in a 1:2.3 ratio in the presence of an iron catalyst. The reaction takes place in a gas reactor heated to 400°C. Over 3 hours of reaction time, the sample is converted to graphite coated on the iron catalyst. The graphite/iron mixture is then pressed into a cathode for AMS analysis.

Radiocarbon analysis of graphite is performed by the Mini Carbon Dating System (MICADAS, Ionplus; Synal HA et al., MICADAS: A new compact radiocarbon AMS system, Nucl. Instrum. Meth. B 2007, 7–13). Graphite analysis produces better precision than direct gas analysis by analyzing more carbon and counting more ¹⁴C atoms. Ideally, 1 mg of carbon is analyzed, but we can graphitize much smaller samples. Analysis of smaller samples will achieve lower precision than large samples.

Purification of CO₂ from a mixed gas sample for graphite analysis

A sample of the atmosphere is a typical example; we have also purified CO₂ from incubation headspace, soil gas, species respiration, and ecosystem respiration. The sample may be submitted in a large canister (typical for atmospheric sampling) or captured on a zeolite trap (typical for incubation and respiration samples). CO₂ in the sample is purified using cryogenic separation on a vacuum line.

Graphitization is performed on manual vacuum lines. We use the hydrogen reduction method (Vogel et al., Performance of catalytically condensed carbon for use in accelerator mass spectrometry, Nucl. Instrum. Meth. B 1984, 289–293). By this method, the sample CO₂ is mixed with H₂ in a 1:2.3 ratio in the presence of an iron catalyst. The reaction takes place in a gas reactor heated to 400°C. Over 3 hours of reaction time, the sample is converted to graphite coated on the iron catalyst. The graphite/iron mixture is then pressed into a cathode for AMS analysis.

Radiocarbon analysis of graphite is performed by Mini Carbon Dating System (MICADAS, Ionplus; Synal HA et al., MICADAS: A new compact radiocarbon AMS system, Nucl. Instrum. Meth. B 2007, 7–13). Graphite analysis produces better precision than direct gas analysis by analyzing more carbon and counting more ¹⁴C atoms. Ideally, 1 mg of carbon is analyzed, but we can graphitize much smaller samples. Analysis of smaller samples will achieve lower precision than large samples.

Combustion of a solid sample for direct gas analysis

Sample is combusted with an elemental analyzer (vario ISOTOPE select, Elementar). The CO₂ is transferred to a syringe (Gas Interface System, Ionplus; Fahrni, et al., Nucl. Instrum. Meth. B 2013, 320–327) and pumped directly into the Mini Carbon Dating System for analysis (MICADAS, Ionplus; Synal HA et al., MICADAS: A new compact radiocarbon AMS system, Nucl. Instrum. Meth. B 2007, 7–13).

We generally analyze 100 ug of carbon with this method, but we can analyze as little as 10 ug. Analysis of smaller samples will achieve lower precision than large samples.

Often with solid samples, a pretreatment is required to remove exogenous carbon (for example, acid-base-acid treatment for plant macrofossils or acidification for bulk sediment; see below for prices).

Pretreatment and oxidation of a carbonate sample for direct gas analysis

Carbonate samples are cleaned and pretreated with acid to remove about 30% by mass, the outer surface being most susceptible to exogenous carbon (Bard et al., 230Th-234U and 14C Ages Obtained by Mass Spectrometry on Corals, Radiocarbon 1993, 191–199). They are then sealed in an exetainer vial and vacuumed out, then 1 mL of phosphoric acid is added to convert the carbonate to CO₂ for graphitization.

The CO₂ is transferred to a syringe (Gas Interface System, Ionplus. Fahrni, et al., Nucl. Instrum. Meth. B 2013, 320–327) and pumped directly into the Mini Carbon Dating System for analysis (MICADAS, Ionplus; Synal HA et al., MICADAS: A new compact radiocarbon AMS system, Nucl. Instrum. Meth. B 2007, 7–13).

We generally analyze 100 ug of carbon with this method, but we can analyze as little as 10 ug. Analysis of smaller samples will achieve lower precision than large samples.

Purification of CO₂ from a mixed gas sample for graphite analysis

A sample of atmosphere is a typical example; we have also purified CO₂ from incubation headspace, soil gas, soil respiration, and ecosystem respiration. The sample may be submitted in a large canister (typical for atmospheric sampling) or captured on a zeolite trap (typical for incubation and respiration samples). CO₂ in the sample is purified using cryogenic separation on a vacuum line and sealed in a pyrex tube.

The CO₂ is transferred to a syringe (Gas Interface System, Ionplus. Fahrni, et al., Nucl. Instrum. Meth. B 2013, 320–327) and pumped directly into the Mini Carbon Dating System for analysis (MICADAS, Ionplus; Synal HA et al., MICADAS: A new compact radiocarbon AMS system, Nucl. Instrum. Meth. B 2007, 7–13).

We generally analyze 100 ug of carbon with this method, but we can analyze as little as 10 ug. Analysis of smaller samples will achieve lower precision than large samples.

Vegetation and other macrofossils are cleaned using one of two acid-base-acid (ABA) procedures depending on the mass and durability of the material. For large or resilient materials (wood, seeds) we use the protocol developed by University of California, Irvine (Santos & Ormsby, 2013, Radiocarbon 55, 534-544). Samples are treated with 1N HCl at 70 °C for 30 min, then 1N NaOH at 70 °C for 30 min. The base treatment is repeated until the solution is clear. Samples are then exposed to 1N HCl at 70 °C for 30 min. and then rinsed twice in DI H₂O at 70° C for 10 min. Small-bore plastic pipettes are used to remove fluids between each step. A pH of 5-6 is confirmed using test trips. (2) For small or delicate samples we use weaker acid (0.5 N) and base (0.1 N) with longer exposure times (3 hr for acid and 2 hr for base) (Zander et al., 2020, Geochronology 2, 63-79).

Other chemical pretreatments may be performed, but these must be arranged with ACE analytical laboratories before sample submission.

Bulk sediment is often pretreated with acid to remove carbonates from the sample (Mollenhauer et al., Nucl. Instrum. Meth. B 2021, 45–51). The sample is placed in a silver rolling capsule, and a few drops of hydrochloric acid are added to oxidize carbonates. The sample is then dried at 60°C and rolled in a second capsule before combustion.