14C Sample Preparation Vacuum Line and Graphite Preparation Method for 14C-AMS Measurement
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摘要: 14C制样真空系统和石墨制备方法是高精度低本底14C加速器质谱（AMS）测量的关键，而碳污染、石墨产率不稳定和同位素分馏等问题是限制该技术发展的主要难题。为了降低传统在线还原法对制样系统长时间静态真空的要求和解决Zn-TiH2/Fe火焰封管法中不可控的CH4等问题，提高石墨合成的稳定性和控制本底，本文建立了基于Zn/Fe火焰封管法的14C制样真空系统和石墨制备方法。通过比较Zn/Fe在线法和Zn/Fe火焰封管法对石墨束流性能以及标样的影响，发现Zn/Fe火焰封管法相较Zn/Fe在线法能明显克服大气泄漏问题，改善化学流程本底（0.24~0.32pMC），提高方法测年上限（47000~48000ya），同时石墨束流输出稳定。进一步利用标准样品和本底样品评估了Zn/Fe火焰封管法的技术特点，实验结果表明该法的精密度好（RSD=0.35%，n=20，标样OXⅡ），准确度高（IAEA系列标样的测定值与认定值线性拟合方程y=0.9969x+0.0013，R2=1），实验本底低（无机碳46296±271ya和有机碳48341±356ya）。因此，该石墨样品制备真空系统及Zn/Fe火焰封管法技术具有石墨品质优、化学流程本底低、准确度和精密度高等特点，满足高精度低本底14C-AMS测定石墨样品制备要求。
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关键词:
- 14C加速器质谱 /
- 石墨制样 /
- Zn/Fe在线法 /
- Zn/Fe火焰封管法
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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图 1 14C制样真空系统。(a)14C制样真空装置原理图，包括(A)真空维持系统、(B)CO2纯化真空系统以及(C)Zn/Fe在线法石墨化真空单元。PT为压力传感器。(b)Zn/Fe火焰封管法实验流程图
Figure 1.
图 2 Zn/Fe在线法：(a)石墨化真空单元反应曲线，反应曲线分为Zn反应管加热450℃快速膨胀过程(A段)、CO2转化成CO的恒压反应过程(B段)、铁粉反应管加热550℃快递膨胀过程(C段)、石墨化的减压反应过程(D段)以及反应完成过程(E段)。反应时间>10h后压力有明显增加的趋势。(b)石墨产率
Figure 2.
图 3 不同实验方法的影响。(a)OX Ⅱ样品，Zn/Fe火焰封管法稳定性优于Zn/Fe在线法，Zn/Fe火焰封管法的RSD=0.35%，n=20。蓝线为国际标样OX Ⅱ的认定值Fm=1.3407±0.0019。(b)空白样品，随火焰封管方法技术的逐步改进，Zn/Fe火焰封管法的本底年龄得到改善，分别为无机碳46296±271ya，有机碳48341±356ya。文献[10]为Zn-TiH2/Fe火焰封管法，本底年龄~50000ya；文献[16]为Zn-TiH2/Fe火焰封管法，本底年龄53382±7676ya；文献[17]为Zn-TiH2/Fe和Zn/Fe火焰封管法，本底年龄48457~51227ya；文献[21]为Zn/Fe膜封管法，本底年龄44400ya；文献[22]为Zn-TiH2/Fe火焰封管法，本底年龄44354ya。GIG为广州地球化学研究所Ionplus AGE-3 H2/Fe法；OLM为本实验室Zn/Fe在线法；STM为本实验室Zn/Fe火焰封管法
Figure 3.
图 4 不同实验方法对石墨性能的影响(3min/轮)。GIG为中国科学院广州地球化学研究所Ionplus AGE-3 H2/Fe法；OLM为本实验室Zn/Fe在线法；STM为本实验室Zn/Fe火焰封管法
Figure 4.
表 1 标准样品认定值与测试值
Table 1. Certified and measured values of reference materials
样品名 成分 样品数量(个) Fm RSD(%) 认定值 测试值 IHEG Cal 方解石 5 0 0.00365 18.4 IHEG Coal 无烟煤 5 0 0.00302 12.3 OXⅡ 草酸 20 1.3407±0.0019 1.3413 0.35 IAEA C2 大理石 3 0.4114±0.0003 0.4105 0.10 IAEA C3 纤维素 3 1.2941±0.0006 1.2891 0.43 IAEA C5 木头 3 0.2305±0.0002 0.2287 0.33 IAEA C7 草酸 3 0.4953±0.0012 0.4934 0.04 IAEA C8 草酸 3 0.1503±0.0017 0.1503 1.16 IAEA C9 木头 3 0.0012~0.0021 0.00307 11.3 
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