Yankuang ceshi (May 2019)
14C Sample Preparation Vacuum Line and Graphite Preparation Method for 14C-AMS Measurement
Abstract
BACKGROUND The technical keys of high-quality 14C-Accelerator Mass Spectrometry (AMS) analysis with low-background is the sample preparation method and the vacuum line rigs. However, the development of the graphite target preparation method is impeded by extraneous source carbon contamination, unsteady graphitization yield and isotope fractionation. OBJECTIVES To reduce the requirement of the traditional on-line method on the long-term statical vacuum performance of the sample preparation line, solve the problem of CH4 produced in the Zn-TiH2/Fe sealed tube method, while improving the stability of graphitization, and control carbon contamination. METHODS 14C sample preparation vacuum system and graphite preparation method based on Zn/Fe flame sealing method was established. The effects on the beam current and values of graphite (prepared from OXⅡ and blank samples) between the Zn/Fe on-line method and the Zn/Fe flame sealed tube method were compared. The precision of the Zn/Fe flame sealed tube method was checked with OXⅡ as the unknown sample. Two 'in-house' blank standards IHEG-Cal and IHEG-Coal were used to evaluate the chemical procedure background of both methods. The other 'known-value' reference materials of IAEA C2, C3, C5, C7, C8 and C9 were used to validate the accuracy of the Zn/Fe flame sealed tube method. RESULTS Zn/Fe on-line method can obviously overcome the air leakage, which yields a lower chemical process background (0.24-0.32pMC) and higher ultimate radiocarbon age (47000-48000ya) observed in Zn/Fe flame sealed tube method with long-term stable beam current output. It was demonstrated that Zn/Fe sealed tube method was more suitable for graphite target preparation than Zn/Fe on-line method. The results illustrated that the Zn/Fe flame sealed tube method had good reproducibility (RSD=0.35%, n=20, OXⅡ), and high accuracy for a variety of natural samples ranging from dead carbon samples to modern carbon samples (linear fitting formula y=0.9969x+0.0013, R2=1) with a low background (radiocarbon age of blank 46296±271ya for inorganic carbon and 48341±356ya for organic carbon). CONCLUSIONS The graphite sample preparation vacuum system and the Zn/Fe flame sealing method have the characteristics of excellent graphite quality, low chemical procedure background, high accuracy and high precision, and meet the sample preparation requirements of high-precision and low-background 14C-AMS determination for graphite.
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