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

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 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 C-14 determinations of chromatographic blanks, which are used for post C-14 determination "corrections". As part of wider investigations aimed at better understanding the chemical nature of contamination in compound-specific C-14 determinations, herein, we report a rigorous approach to column bleed identification and quantification. Using reference fatty acid methyl esters (FAMEs), H-1 nuclear magnetic resonance spectroscopy (NMR), employing a 700 MHz instrument equipped with a 1.7 mm microcryoprobe optimized for 1H 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 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 C-14 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.