Use of a 700 MHz NMR Microcryoprobe for the Identification and Quantification of Exogenous Carbon in Compounds Purified by Preparative Capillary Gas Chromatography for Radiocarbon Determinations

Abstract

Preparative capillary gas chromatography (PCGC) is the central technique used for the purification of volatile or semivolatile organic compounds for radiocarbon analysis using accelerator mass spectrometry (AMS). While thicker film columns offer efficient separations, cyclic poly(dimethylsiloxanes) (PDMS) derived from the column's stationary phase have been highlighted as a potential source of contaminant carbon in "trapped" compounds. The PDMS CH 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 C determinations of chromatographic blanks, which are used for postC determination "corrections". As part of wider investigations aimed at better understanding the chemical nature of contamination in compound-specific C determinations, herein, we report a rigorous approach to column bleed identification and quantification. Using reference fatty acid methyl esters (FAMEs),H nuclear magnetic resonance spectroscopy (NMR), employing a 700 MHz instrument equipped with a 1.7 mm microcryoprobe optimized for H observation, was able to detect low submicrogram amounts of low molecular weight compounds (<500 Da). Direct quantification of PCGC "trapped" FAMEs was achieved based on the recorded H 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 C determination of FAMEs prepared using the chromatographic method described. TheH NMR analysis also revealed the absence of other protonated carbon-containing components that would affect radiocarbon determinations at the precisions achievable by AMS.

Practical Considerations in High-Precision Compound-Specific Radiocarbon Analyses: Eliminating the Effects of Solvent and Sample Cross-Contamination on Accuracy and Precision

“…The isolated FAME solutions were then blown to dryness under a gentle stream of N2. The samples were then re-dissolved in deuterated chloroform and transferred to 1.7 mm NMR (nuclear magnetic resonance) tubes for analysis by 700 MHz microcryoprobe 1 H NMR as described by Casanova et al 16 .…”

“…MHz spectrometer equipped with microcryoprobe for the quantification of poly(dimethyl siloxanes), deriving from 'column-bleed', and screening for residual solvent and other sources of exogenous C as described previously 16 .…”

“…In their original validation of the method, Eglinton et al 1 used capillary columns coated with thin films of stationary phase to limit the effects of column "bleed" arising through thermal degradation, concluding that "There is minimal C background contamination (<<5 µg of C) introduced by the pcGC system or by the GC column". Recently, we demonstrated for the first time that although present in analytes isolated by pcGC, cyclic poly(dimethyl siloxanes) resulting from the degradation of the stationary phase of the GC column, were not present in sufficient quantities to affect high precision 14 C determinations 16 .…”

Radiocarbon Sample Preparation Procedures and the First Status Report from the Bristol Radiocarbon AMS (BRAMS) Facility

“…Ongoing and planned work includes: i) investigations into exogenous carbon introduction and removal of post-depositional environmental contaminants during a range of different pretreatment methods using the high-field NMR approach developed during work by Casanova et al (2017) alongside organic mass spectrometric methods, ii) continue the acquisition and characterisation of inhouse standards, iii) continue to review sample scatter values for the full range of pretreatment methods as more standards are analysed, and iv) continue work to develop and adapt methodologies for the radiocarbon analyses of small samples. (Scott et al, 1997), b IAEA-C9 and FIRI A (Boaretto et al, 2002), c (Cook et al, 2012)…”

“…We have been developing various pretreatment and sample preparation techniques for the compoundspecific radiocarbon analysis of a range of sample types, including lipids absorbed within archaeological potsherds. These methods are described elsewhere (Casanova et al, 2017(Casanova et al, , 2018.…”