MICADAS GIS
Sample-derived carbon dioxide is analyzed using a GIS (gas interface system) connected to a MICADAS (Mini Carbon Dating System), both from Ionplus. The GIS introduces CO2 directly into the attached ion source of the tandem accelerator mass spectrometer (AMS). The GIS is coupled with an elemental analyzer (EA) and carbonate handling system (CHS) to allow for different sample types to be introduced to the system. CO2 from the CHS or EA is transferred to the GIS via He flow and captured on a zeolite trap. The trap is then heated to 450°C to release the CO2 into a syringe, and the CO2 is diluted with helium to a ratio of 90% He and 10% CO2. The mixture is subsequently introduced directly into the source via a helium flow capillary onto specially designed aluminum/titanium cathodes.
Although the zeolite trap is heated and flushed between samples, a very small carry-over effect from the trap was determined at NOSAMS in 2024 to be 0.50 ± 0.25 µg C from the previous sample. To account for this small addition of carbon, a carry-over correction is made to all sample results in addition to normalization and blank corrections. Normalization and blank samples are analyzed in triplicate at the beginning and end of each set of samples, with additional measurements as needed to achieve a standard to unknown ratio of approximately 1:10. The first measurements of triplicate and contiguously measured normalizers and blanks are discarded to minimize the carry-over effect.
CHS-AMS
Carbonate GIS samples are analyzed at NOSAMS using the automated carbonate handling system (CHS). Samples are pre-weighed into and submitted in special Exetainer vials (4.5 ml) with septa-seals. Sample masses are provided in amounts expected to produce between 100-300 micrograms of carbon (1-3 mg carbonate). The vials are first flushed with high purity helium to eliminate air, then 400 µL of phosphoric acid (85%) is injected. Sample vials are then heated to 50°C for 1-5 h. CO2 is transferred by a two-way needle via a flow of He (60 mL/min) over a phosphorus pentoxide water trap to the GIS zeolite trap.
CHS-AMS results are normalized using aliquots from a tank of pure CO2 with a known fraction modern determined by calibration against NBS Oxalic Acid I (NIST-SRM-4990). A correction is made to normalize the sample result to a δ13CVPDB value of -25 ‰, assuming a quadratic mass fractionation dependency. This correction is made using simultaneously measured 13C/12C ratios on the MICADAS system. These 13C/12C ratios are not reported. A large blank correction for CHS samples is performed using IAEA-C1 Carrara marble samples that are included in the run and then a mass-balance process blank of 0.02 ± 0.02 µg of Fm 0.9 ± 0.1 is applied using a method similar to that as in Roberts et. al (2019).
EA-AMS
Organic carbon GIS samples are converted to pure CO2 using an Elementar vario ISOTOPE select elemental analyzer (EA). Samples are submitted pre-weighed into tin cups with expected yields of 100-300 µg C. Samples are combusted at 920°C in a reactor packed with copper oxide and silver wool. Oxides are removed following combustion in a copper reduction reactor at 650°C. After water removal by flow through a magnesium perchlorate trap, CO2 is retained on a zeolite adsorption trap and released by heating into the GIS.
EA-AMS samples are normalized using IAEA OxII (SRM 4990 C) Oxalic acid. A correction is made to normalize the sample result to a δ13CVPDB value of -25 ‰, assuming a quadratic mass fractionation dependency. This correction is made using simultaneously measured 13C/12C ratios on the AMS system. These 13C/12C ratios are not reported. A large blank correction for EA samples is performed using 14C-free acetanilide analyzed concurrently with the samples and then a mass-balance process blank of 1.6 ± 0.4 µg of Fm 0.30 ± 0.15 is applied using a method similar to that as in Roberts et. al (2019).
Samples analyzed as CO2 by EA-AMS can also be analyzed for δ13C by IRMS, as the EA is also interfaced to an Elementar isoprime precisION continuous-flow IRMS. EA-IRMS samples are calibrated by run with triplicate measurements of certified reference materials USGS61 (caffeine, -35.05 ± 0.04 ‰) and USGS66 (glycine, -0.67 ± 0.04 ‰). Internal laboratory standards of acetanilide and oxalic acid are analyzed at least in triplicate throughout each run for quality control and may be used to correct for drift. The long-term standard deviation of the system for 13C is 0.24 ‰. δ13C values are reported relative to the international standard of VPDB (Vienna Pee Dee Belemnite).
Headspace (DIC)-AMS
Dissolved inorganic carbon (DIC) samples are analyzed by online headspace-AMS using the CHS (carbonate handling system). Liquid DI14C samples are injected with a syringe prior to submission into specially prepared septum-sealed screw cap Exetainer vials (12 ml). The vials are pre-acidified with 200 µl 85% phosphoric acid and purged with helium. Samples volumes should target 100-300 µg C, but should not exceed 6 ml. The acidified water sample is sparged with helium using the CHS, and the resulting CO2 in the headspace is transferred via a flow of He (60 mL/min) over a phosphorus pentoxide water trap to the GIS zeolite trap.
Headspace-AMS samples are normalized using aliquots from a tank of pure CO2 with a known fraction modern determined by calibration against NBS Oxalic Acid I (NIST-SRM-4990). A correction is made to normalize the sample result to a δ13CVPDB value of -25 ‰, assuming a quadratic mass fractionation dependency. This correction is made using simultaneously measured 13C/12C ratios on the MICADAS system. These 13C/12C ratios are not reported. A large blank correction for headspace DIC samples is performed using an internal 14C-free groundwater laboratory standard analyzed concurrently with the samples and then a mass-balance process blank of 0.02 ± 0.02 µg of Fm 0.9 ± 0.1 is applied using a method similar to that as in Roberts et. al (2019).
Roberts, ML, Elder, KL, McNichol, AP, Jenkins, WJ, Gagnon, AR, Xu, L, Hlavenka, JD, Longworth, BE. 2019. 14C Blank Corrections for 25–100 μg Samples at the National Ocean Sciences AMS Laboratory. Radiocarbon, 61(5), 1403-1411 DOI: 10.1017/RDC.2019.74.