14C BLANK ASSESSMENT IN SMALL-SCALE COMPOUND-SPECIFIC RADIOCARBON ANALYSIS OF LIPID BIOMARKERS AND LIGNIN PHENOLS

Compound-specific radiocarbon (C-14) dating often requires working with small samples of < 100 mu g carbon (mu gC). This makes the radiocarbon dates of biomarker compounds very sensitive to biases caused by extraneous carbon of unknown composition, a procedural blank, which is introduced to the samples during the steps necessary to prepare a sample for radiocarbon analysis by accelerator mass spectrometry (i.e., isolating single compounds from a heterogeneous mixture, combustion, gas purification and graphitization). Reporting accurate radiocarbon dates thus requires a correction for the procedural blank. We present our approach to assess the fraction modern carbon ((FC)-C-14) and the mass of the procedural blanks introduced during the preparation procedures of lipid biomarkers (i.e. n-alkanoic acids) and lignin phenols. We isolated differently sized aliquots (6-151 mu gC) of n-alkanoic acids and lignin phenols obtained from standard materials with known (FC)-C-14 values. Each compound class was extracted from two standard materials (one fossil, one modern) and purified using the same procedures as for natural samples of unknown (FC)-C-14. There is an inverse linear relationship between the measured (FC)-C-14 values of the processed aliquots and their mass, which suggests constant contamination during processing of individual samples. We use Bayesian methods to fit linear regression lines between (FC)-C-14 and 1/mass for the fossil and modern standards. The intersection points of these lines are used to infer (FCblank)-C-14 and m(blank) and their associated uncertainties. We estimate 4.88 +/- 0.69 mu gC of procedural blank with (FC)-C-14 of 0.714 +/- 0.077 for n-alkanoic acids, and 0.90 +/- 0.23 mu gC of procedural blank with (FC)-C-14 of 0.813 +/- 0.155 for lignin phenols. These (FCblank)-C-14 and m(blank)( )can be used to correct AMS results of lipid and lignin samples by isotopic mass balance. This method may serve as a standardized procedure for blank assessment in small-scale radiocarbon analysis.