• Crump, Matthew P.
• Knowles, Timothy D. J.
• Evershed, Richard
• Williams, Christopher
• Casanova, Emmanuelle

Abstract

Preparative capillary gas chromatography (PCGC) is the central technique used for the purification of volatile or semi-volatile organic compounds for radiocarbon analysis using accelerator mass spectrometry (AMS). While thicker film columns offer efficient separations, column bleed of cyclic poly(dimethyl siloxane) (PDMS) stationary phase has been highlighted as a potential source of contaminant carbon in ‘trapped’ compounds. The dimethylpolysiloxane CH3 groups are of ‘infinite’ radiocarbon age due to the fossil carbon origin of the feedstock used in production. Hence, column bleed, if present at sufficiently high concentrations, would shift the radiocarbon ages of trapped compounds to older ages. Quantification of the column bleed in trapped samples, however, is extremely challenging and up to now has only been achieved through indirect 14C determinations of chromatographic blanks, which are used for post 14C determination ‘corrections’. As part of wider investigations aimed at better understanding the chemical nature of contamination in compound-specific 14C-determinations, herein, we report a rigorous approach to column bleed identification and quantification. Using reference fatty acid methyl esters (FAMEs) 1H nuclear magnetic resonance spectroscopy (NMR), employing a 700 MHz instrument equipped with a 1.7 mm microcryoprobe optimised for 1H observation, was able to detect low sub-microgram amounts of low molecular weight compounds (<500 Da). Direct quantification of PCGC ‘trapped’ FAMEs was achieved based on the recorded 1H NMR spectra. Gravimetrically prepared calibration mixtures of cyclic PMDSs and FAMEs, showed column bleed abundance to be below 0.03% w/w of the ‘trapped’ FAMEs, which would lead to a maximum shift in radiocarbon age of <3 years toward older values. We therefore conclude that column bleed contamination has a negligible effect on the 14C determination of FAMEs prepared using the chromatographic method described. The 1H NMR analysis also revealed the absence of other protonated carbon-containing components that would affect radiocarbon determinations at the precisions achievable by AMS.INTRODUCTION A critical concern when preparing samples for radiocarbon dating is contamination of the sample through the introduction of exogenous carbon. Such contamination from sample treatment can lead to significant offsets (older or younger) of the actual sample age leading to erroneous dates1,2,3. Contamination becomes particularly problematic with the small sample sizes, i.e. less than 1 mg of C, that are increasingly commonly analysed due to advances in sample preparation methods and AMS technologies.Identification and quantification of exogenous carbon in samples for radiocarbon analysis has been attempted using various approaches, e.g. FTIR has been used to identify contamination in bone collagen4 and Raman to determine soil carbon contamination of charcoal5,6. However, these techniques are ineffective in the case of low-level contamination of small samples, due to a fundamental lack of sensitivity, precluding quantification of contamination at the part per thousand level, which would affect radiocarbon determinations.The question of exogenous contamination is especially critical in compound-specific 14C determinations in which preparative capillary gas chromatography (PCGC) is used to isolate compounds from extracts of various environmental matrices7,8,9,10, and archaeological pottery vessels11,12,13. The compound-specific approach routinely involves trapping sub-milligram amounts of analyte for 14C determinations, hence, the use of PCGC requires assessment of all potential sources of exogenous carbon likely to arise during the sample pre-treatment. As the analytes are purified exogenous carbon could potentially be introduced either from the PCGC used for compound purification, the handling of compounds between isolation, oxidation and graphitisation. A recognised source of exogenous carbon to compounds isolated by PCGC is column “bleed”14,15,16 (figure 1), derived from thermal degradation of the commonly used PDMS stationary phase coating the column through heating in the GC oven15,16. The cyclic degradation products of the polymer released typically are n = 3 and n = 4 cyclic oligomers of the monomer unit (-[Si(CH3)2-O]n-), with the possibility of higher homologues up to an n = 717,18.Several approaches have been considered to identify, limit, and correct for the effects of ‘column bleed’ from the GC column. Eglinton and co-workers7, who reported the first use of PCGC to isolate compounds for radiocarbon dating, suggested using columns coated with a thin film of ultra-low bleed stationary phase (≤0.5 µm). Stott et al.12 attempted to determine the column bleed concentration by the preparation of chromatographic blanks when isolating C16:0 and C18:0 FAMEs from archaeological pottery. This was achieved by trapping the column eluent for almost an entire run after injection of s...

Topics

• impedance spectroscopy
• compound
• polymer
• Carbon
• phase
• thin film
• laser emission spectroscopy
• molecular weight
• Nuclear Magnetic Resonance spectroscopy
• gas chromatography
• ester
• additive manufacturing
• spectrometry
• graphitisation
• Accelerator mass spectrometry