Compound-Specific Radiocarbon Analysis by Elemental Analyzer–Accelerator Mass Spectrometry: Precision and Limitations

Abstract

We examine instrumental and methodological capabilities for microscale (10–50 μg of C) radiocarbon analysis of individual compounds in the context of paleoclimate and paleoceanography applications, for which relatively high-precision measurements are required. An extensive suite of data for 14C-free and modern reference materials processed using different methods and acquired using an elemental-analyzer–accelerator-mass-spectrometry (EA-AMS) instrumental setup at ETH Zurich was compiled to assess the reproducibility of specific isolation procedures. In order to determine the precision, accuracy, and reproducibility of measurements on processed compounds, we explore the results of both reference materials and three classes of compounds (fatty acids, alkenones, and amino acids) extracted from sediment samples. We utilize a MATLAB code developed to systematically evaluate constant-contamination-model parameters, which in turn can be applied to measurements of unknown process samples. This approach is computationally reliable and can be used for any blank assessment of small-size radiocarbon samples. Our results show that a conservative lower estimate of the sample sizes required to produce relatively high-precision 14C data (i.e., with acceptable errors of <5% on final 14C ages) and high reproducibility in old samples (i.e., F14C ≈ 0.1) using current isolation methods are 50 and 30 μg of C for alkenones and fatty acids, respectively. Moreover, when the F14C is >0.5, a precision of 2% can be achieved for alkenone and fatty acid samples containing ≥15 and 10 μg of C, respectively.

Towards Organic Carbon Isotope Records from Stalagmites: Coupled δ13C and 14C Analysis Using Wet Chemical Oxidation

“…The contribution of extraneous carbon to the WCO was evaluated by a suite of standards (WCO standards) prepared for each run by adding varying amounts of standard solution to vials containing 5 ml of ultrapure water, then taking them through the WCO procedure. To evaluate the mass and F 14 C of extraneous carbon for each run, we used the model of constant contamination described in Hanke et al (2017) and Haghipour et al (2018). The procedural blank of the decarbonation (chemical pre-treatment standard) was quantified by spiking vials containing acid with variable amounts of sucrose and phthalic acid, before taking them through the entire procedure (Fig.…”

“…Both methods assume constant contamination from each method step on all samples. Table 2), the average blank contamination over all runs was1.30 ± 0.52 μg C, F 14 C= 0.42 ± 0.17, when calculated with the method of constant contamination byHaghipour et al (2018; Suppl. Table 1).…”

Liquid Chromatographic Isolation of Individual Amino Acids Extracted From Sediments for Radiocarbon Analysis

“…For correcting the processed standards, a constant contamination of 2.2 ± 1.3 µgC with 0.25 ± 0.09 Fm (average of all five direct blank measurements) was applied which bins together the blanks isolated under elution time windows corresponding to those of the reported amino acids and purified with the PrimesepA column under isocratic 95-5-0.05 and 65-35-0.05 water-acetonitrile-TFA conditions. This simplified assignment of constant contamination has large uncertainty compared to more rigorously constrained contamination case studies (Haghipour et al, 2019), yet, using Equations 1 and 2 [following Wacker and Christl (2012); see Supplementary Material], corrects all standards (IIb) within error of their respective consensus radiocarbon isotopic compositions (Table 1). Nonetheless, this blank size is consistent with observations made by Ishikawa et al (2018) who quantify the total blank as approximately 2.4 µgC for samples without "post-purification" (filtration and rinsing with diethyl ether; see procedure 3b in Figure 1).…”

Analytical and Computational Advances, Opportunities, and Challenges in Marine Organic Biogeochemistry in an Era of “Omics”

“…Several recent developments suggest further improvements to natural abundance 14 C analysis. New CO 2 -accepting ion sources allow peripheral instruments, such as elemental analyzers, other oxidation or hydrolysis systems and potentially gas chromatography, to interface directly to AMS (Bronk Ramsey et al, 2004;Ruff et al, 2010;Haghipour et al, 2019). This eliminates the need for an offline graphitization step, which is labor-intensive and potentially introduces 14 C contamination.…”