Exploring sample size limits of AMS gas Ion Source ¹⁴C analysis at Cologneams

Abstract

Increasing demands for small-scale radiocarbon (14C) analyses required the installation of a “SO-110 B” type ion source (HVE Europa B.V.) at our 6 MV Tandetron AMS (HVE) dedicated for the direct injection of CO2 using either the gas injection system (GIS) from Ionplus AG or a EuroVector EA 3000 elemental analyzer (EA). We tested both systems with multiple series of 14C-free and modern standards (2.5–50 µg C) combusted in quartz ampoules or EA containers and were able to quantify exogenous C introduced. In EA-GIS-AMS analysis exogenous C is mainly derived from the EA sample containers. Blank values for 50 µg C combusted in solvent-cleaned tin (Sn) vessels were 0.0127 ± 0.0012 F14C (boats) and 0.0090 ± 0.0010 F14C (capsules), while they were much higher for thermally cleaned silver (Ag) capsules. The processing of gas samples for GIS-AMS yields similar blank values corresponding to 0.30 ± 0.08 µg exogenous C with 0.93 ± 0.23 F14C consisting of 0.28 µg C modern and 0.02 µg C fossil C. The combustion of larger amounts of blank material (1 mg C) in a single quartz tube split into aliquots gives lower blanks (0.0064 ± 0.0008 F14C; 50 µg C). Thus, 14C analysis of small, gaseous samples is now possible at CologneAMS.

Preparation and Handling of Methane for Radiocarbon Analysis at Cologneams

“…The same CH 4 that was mixed with Ox-II derived CO 2 contained slightly more 14 C (0.006 ± 0.006 F 14 C; n = 13). The blank values obtained from these series are comparable to blank standards of a similar size prepared on our CO 2 vacuum line that is used for small samples (10-50 μg C) including compound-specific radiocarbon analysis (F 14 C 0.005 ± 0.0004; n = 4; Melchert et al 2019). However, the blank values shown here are up to one order of magnitude higher compared to other studies (Table 2), which we think is most likely related to the much smaller size of our samples.…”

“…14 C-enriched CO 2 standards (Ox-II; NIST SRM 4990C; nominal value 1.3407 F 14 C) were prepared with sealed tube combustion of oxalic acid crystals according to Melchert et al (2019). In summary, oxalic acid powder equal to 3 mg of C was weighed into DCM-cleaned Sn boats (4 × 4 × 11 mm, Elementar, Germany) and transferred into quartz tubes (MQ-washed and pre-combusted at 900°C) along with CuO as a combustion catalyst.…”

“…The amount of extraneous C introduced during sample processing was determined from the 14 C data of standard CH 4 , which was on average 0.20 ± 0.23 μg modern C. This amount of Pack et al (2015) and Sparrow and Kessler (2017). The 14 C data of the 14 C-enriched standards were expected to show a size dependency, i.e., an increase in F 14 C with decreasing sample size and vice versa, following the concept of the introduction of a constant amount of contamination (Ruff et al 2010;Rethemeyer et al 2013;Melchert et al 2019). Only weak to moderate correlation coefficients after Pearson could be determined for the distribution of F 14 C versus sample size for the injected series of samples ( 14 C-free CH 4 r = 0.5; 14 C-free CH 4 :Ox-II CO 2 r = 0.1; Ox-II CO 2 r = -0.64; biogas r = 0.46).…”

Current Sample Preparation and Analytical Capabilities of the Radiocarbon Laboratory at CologneAMS

“…Blank values and sample size limitations of GIS-AMS and EA-GIS-AMS analysis are discussed in detail elsewhere (Melchert et al submitted to this issue; Stolz et al 2017, 2019). In summary, both gas injection systems have very similar blank values corresponding to the addition of a constant contamination of about 0.27 ± 0.05 µg C for GIS-AMS and 0.27 ± 0.02 µg C for EA-GIS-AMS introduced during sample processing, combustion, and analysis.…”

DO RADIOCARBON AGES OF PLANT WAX BIOMARKERS AGREE WITH 14C-TOC/OSL-BASED AGE MODELS IN AN ARID HIGH-ALTITUDE LAKE SYSTEM?

“…In order to achieve quantities large enough for reliable 14 C dating of CO 2 using a gas ion source (Schiffer et al 2019;Melchert et al 2020), some of the individual compounds of the soil samples were combined as given in Table 2: For Soil #1 nC 29 -and nC 31 -alkanes were pooled, nC 29 -and nC 31 -alkanes from soil #2 and #3, as well as from soil #4 and #5 were combined to one sample respectively. For soil #6 and the sediment samples n-alkane homologues were dated individually.…”