Mass-spectrometer analyses of radiocarbon standards.

Abstract

Twenty-four samples of CO (sub 2) from combustion of the NBS oxalic-acid radiocarbon standard, submitted from 13 radiocarbon laboratories, were analyzed for C (super 13) content (at the Dept. of Earth Sciences, University of California, La Jolla) to provide comparative data for normalizing the counting results. The samples were analyzed on Samson, at 60 degrees sector, 6-in. radius, McKinney-Nier type mass-spectrometer with magnetic switching between sample and standard gas feed to the instrument. The samples were analyzed in series against a single gas standard which had previously been well calibrated against the PDB Chicago C (super 13) standard. A table presents results in terms of C (super 13) enrichment relative to the Chicago PDB standard (delta (super ) C (super 13) ), and also as enrichments (delta (sub w) C (super 13) ) relative to the C (super 13) content of average wood which is taken as delta = -25% (sub o) relative to PDB. The table also shows the measured O (super 18) /O (super 16) enrichments in per mil (delta (super ) O (super 18) ) relative to PDB standard CO (sub 2) . Average values for both "direct" combustion (with O (sub z) ) and wet combustion were computed. Results are given. The total C (super 13) range is 5.5% (sub o) , indicating that the probable spread among various laboratories for careful work will be of the order of only 1% variation in reported activity when fractionation is neglected, in the case of a single sample. The range of C (super 13) variation in ancient and modern wood is of the order of 10%, and is thus a more important factor than most of the laboratory effects observed in the present study.