| Professional Committee of Rock and Mineral Testing Technology of the Geological Society of China, National Geological Experiment and Testing Center | Host |
| Citation: |
Sheng-hua LIU, Yu-zhen YANG, Sheng XU, Hui ZHANG, Ya-xin JIANG, Hui-xia SHI. 14C Sample Preparation Vacuum Line and Graphite Preparation Method for 14C-AMS Measurement[J]. Rock and Mineral Analysis, 2019, 38(3): 270-279. doi: 10.15898/j.cnki.11-2131/td.201807120084
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14C Sample Preparation Vacuum Line and Graphite Preparation Method for 14C-AMS Measurement
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Abstract
BACKGROUNDThe 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. OBJECTIVESTo 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. METHODS14C 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. RESULTSZn/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). CONCLUSIONSThe 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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References
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Sheng-hua LIU, Yu-zhen YANG, Sheng XU, Hui ZHANG, Ya-xin JIANG, Hui-xia SHI. 14C Sample Preparation Vacuum Line and Graphite Preparation Method for 14C-AMS Measurement[J]. Rock and Mineral Analysis, 2019, 38(3): 270-279. doi: 10.15898/j.cnki.11-2131/td.201807120084
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- Figure 1. Sample preparation vacuum system for radiocarbon. (a) 14C sample preparation vacuum line rigs, consist of (A) vacuum maintain system, (B) CO2 gas handling vacuum line and (C) on-line graphitization vacuum units by Zn/Fe method. PT denoted for pressure transducer. (b) Schematic diagram of Zn/Fe flame-sealed tube method
- Figure 2. Zn/Fe on-line method: (a) the function of pressure at graphitization unit with reaction time, consist of (A) the process of gas rapid expansion with heating Zn reaction at 450℃, (B) the process of contant pressure with CO2 turn into CO, (C) the process of gas rapid expansion with heating Fe reaction tube at 550℃, (D) the process of pressure decrease with graphitization, and (E) the contant pressure with graphitization completement. However, a significant increase of pressure in graphitization unit after 10 hours. (b) Yield of graphite
- Figure 3. Effect of graphitization methods on the results. (a) OX Ⅱ sample, the stability of sealed tube method is better than on-line method, the reproducibility of STM is RSD=0.35%, n=20. The blue solid line is the certified value of OX Ⅱ (Fm=1.3407±0.0019). (b) Blank sample, the background of sealed tube method is as low as GIG PTA. Results show that with the modification of flame sealed tube technology, the radiocarbon ages (or blank of chemical procedure) of blank samples were improved, 46296±271ya for inorganic carbon and 48341±356ya for organic carbon. Ref.[10] refers to Zn-TiH2/Fe sealed tube method, the background 14C age is ~50000ya; Ref.[16] refers to Zn-TiH2/Fe sealed tube method, the background 14C age is 53382±7676ya; Ref.[17] refers to both Zn-TiH2/Fe and Zn/Fe sealed tube method, the background 14C age is with a rang of 48457-51227ya; Ref.[21] refers to Zn/Fe septa-sealed tube method, the background 14C age is 44400ya; Ref.[22] refers to Zn-TiH2/Fe sealed tube method, the background 14C age is 44354ya. GIG are the samples prepared with Ionplus AGE-3 H2/Fe method in Guangzhou Institute of Geochemsitry AMS lab, OLM are samples prepared with Zn/Fe on-line method, and STM prepared with Zn/Fe sealed tube method in this study
- Figure 4. Effect of graphitization methods on the beam current performce of graphite, 3mins per each run. GIG are the samples prepared with Ionplus AGE-3 H2/Fe method in Guangzhou Institute of Geochemsitry AMS laboratory, OLM are samples prepared with Zn/Fe on-line method, and STM prepared with Zn/Fe sealed tube method in this study
- Figure 5. Comparison of measured values with certified values of different reference materials prepared by Zn/Fe sealed tube method