Research Progress on Graphite Target Preparation for Accelerator Mass Spectrometry 14C Analysis
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摘要: 加速器质谱（AMS）是进行14C同位素分析的主要技术手段，而高精度低本底加速器质谱14C分析主要受样品制备技术限制，因此探讨如何提高石墨制备的稳定性和控制碳污染降低本底将有助于产出高质量14C分析数据，突破14C测年上限（约5.0万年），进一步拓宽14C年代学和同位素示踪的应用范畴。本文详细阐述了催化还原法（H2/Fe法、Zn/Fe法和Zn-TiH2/Fe法）制备石墨样品的真空装置和主要工作原理，指出了微量样品石墨制备过程中同位素分馏、石墨产率、束流强度以及精密度与样品量之间存在严重的依赖关系及其抑制方法。着重探讨了石墨制备技术实验条件（还原剂、催化剂、温度等）的优化选择及其与石墨产率、同位素分馏、束流性能之间的内在联系，总结分析了碳污染来源并探寻合适的碳污染控制技术。目前的研究表明最佳实验条件为：H2/Fe法宜采用还原剂H2/CO2（体积比2~2.5），催化剂为源自氢还原单质铁粉（-325目球粒，Fe/C=2~5），温度500~550℃；Zn/Fe法宜采用还原剂Zn/C（质量比50~80），催化剂为源自氢还原单质铁粉（-325目球粒，Fe/C=2~5），Zn反应管温度400~450℃，Fe反应管温度500~550℃。碳污染来源于制备过程中的各个方面，除采用高温除碳的方式外还可采用适当的数学模型加以校正，但还需要更多详细的实验工作来加强现有认识，以期更好地消除碳污染对测试结果的影响。对测年目标组分不稳定的样品（如地下水中的溶解无机碳）应避免样品直接暴露于大气，以减少野外采样过程中现代大气CO2对测量结果的影响。要点
(1) 探讨了实验条件对石墨性能的影响，提出了石墨制备的最佳实验条件。
(2) 分析了碳污染来源，提出了低本底控制办法。
(3) 总结了微量样品制备技术的发展现状及其存在的问题。
HIGHLIGHTS(1) The effects of graphitization conditions on graphite performance were discussed and the optimum conditions were proposed.
(2) The carbon contamination source was analyzed, and the method of reducing background was advocated.
(3) The development of ultra-small sample preparation technology and its problems were reviewed.
Abstract:BACKGROUNDAccelerator mass spectrometry (AMS) is the most popular technique for radiocarbon analysis. However, yielding high precision and low background 14C data by AMS is hindered by sample preparation. Therefore, improving the stability of graphitization and reducing the carbon contamination in the background helps to produce high quality 14C data and break through the 14C dating upper limit (about 50000ya), further broadening the application range of 14C chronology and isotope tracer.OBJECTIVESTo provide basic reference for beginners or more experienced scientists who are going to set up a radiocarbon sample preparation vacuum line and methods.METHODSThe development of graphite preparation technology with respect to sample preparation vacuum line apparatus and the underlying principle of H2/Fe, Zn/Fe and Zn-TiH2/Fe methods were reviewed. The advantages and drawbacks of these methods were also discussed. Additionally, the optimization of experimental conditions from the perspective of reductant, catalyst and graphitization temperature, accompanied by the inner relationship with the graphite yield, isotope fractionation and beam performance were emphatically discussed. The sources of carbon contamination and suitable control technology were also argued.RESULTSThe optimized graphitization conditions by H2/Fe method was H2/CO2=2-2.5 (V/V), -325 meshes ion powder derived from hydrogen reduction with Fe/C=2-5 (m/m), and graphitization temperature of 500-550℃, while by Zn/Fe method it was Zn/C=50-80 (m/m), -325 meshes ion powder derived from hydrogen reduction with Fe/C=2-5 (m/m), and graphitization temperature of 400-450℃ for Zn reaction tube and 500-550℃ for Fe reaction tube. The carbon contamination originated from each step in the sample preparation procedure, which could be either reduced by high-temperature bake or calibrated by mathematical model, but it required more detailed study to strengthen our knowledge and eliminate the effects on results.CONCLUSIONSBoth of the sample preparation methods used and the optimum of graphitization conditions are critical for good performance of the graphite target and the final precision and accuracy of the measurement, especially for ultra-small size samples. However, these effects could be reduced or even eliminated by optimizing the experimental procedures and the graphitization conditions or subtracting by mathematical models.
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铌钽是稀有金属中的重要品种，在钢铁工业、超导材料、电子工业、医疗领域及铸造行业等领域有较广泛的应用，是国家战略资源中极为重要的部分。因此，铌钽矿的开发越来越受到重视[1]。我国是铌、钽矿藏较富足的国家[2-3]，但铌钽矿资源矿物分布粒度细，且矿石含量较低，要求选矿处理量大，所以铌钽选矿工艺普遍存在流程复杂、回收率低等特点，同时伴生可综合利用的锂、铍、长石等资源[4-6]。所以，化学分析数据对判定矿物在分选流程中的去向就十分重要，而标准物质是对分析数据准确性的考察指标之一。但在铌钽选矿过程中常因化学分析结果的时间比较长、没有高品位的精矿标准物质、结果准确度不高等原因，不能满足量值溯源、传递及高精度、高准确度的质量控制要求，严重影响到了选矿工艺的设计。所以有必要研制铌钽精矿的相关标准物质，来指导选冶试验工艺流程的合理性，提高矿物的综合利用率[7-8]。
我国常用的铌、钽元素矿石标准物质[9]GBW07185、GBW07152、GBW07153、GBW07154、GBW07155、GBW07184、GBW07185等，它们的铌钽元素(Nb2O5+Ta2O5)含量基本在200×10-6以下，只有GBW07155和GBW07185的铌钽元素(Nb2O5+Ta2O5)含量分别在1130×10-6和15400×10-6，而对于选厂和冶炼厂的铌钽中间产品及最终产品来说，Nb2O5+Ta2O5的含量大于10%，甚至达到60%以上，没有相对应的铌钽标准物质对分析过程进行监控。可见，已有的铌钽矿石标准物质只适用于边界品位、工业品位的铌钽矿石分析，无法满足选冶试验样品中铌钽精矿样品分析的要求。铌铁标准物质(DH2805-铌铁，组分含量Ta 0.35×10-2，Nb 65.40×10-2；ECRM576-1-铌铁，组分含量Ta 0.306×10-2，Nb 43.90×10-2；YSBC18606-08-铌铁，组分含量Ta 0.84×10-2，Nb 66.24×10-2)中的铌高钽低，铌钽含量差距太大，且合金类标准物质，其基体与组分和铌钽矿石均不匹配，不适合在铌钽精矿分析过程中使用。因此，铌钽精矿标准物质的研制，不仅可为铌钽矿资源的开发中得到的精矿品位数据提供可靠的质量保证，也将填补我国铌钽精矿标准物质的空白；同时与原有的铌钽矿标准物质形成一个完整的铌钽矿含量系列标准物质，能够满足铌钽矿勘查和选冶对标准物质的需求。
本文研制了4个铌钽矿化学成分标准物质，采用气流粉碎及高铝球磨细碎的两次破碎方法，保证满足标准物质粒度的要求，混合均匀后对全部定值元素进行均匀性和稳定性检验，选择8家具有资质的实验室，采取经典分析方法与现代仪器分析技术相结合的方式对该标准物质联合定值，依照JJF 1343—2012和一级标准物质技术规范给出了12项组分(包括主量、痕量元素)的标准值和不确定度。
1. 标准物质候选物的采集与制备
1.1 标准物质候选物的采集
系列样品的选采主要考虑：①采样区是该矿种的主要矿床成因类型和工业类型，矿石的组成具有代表性；②矿石主成分的含量能满足含量梯度的要求。
根据国内外铌钽矿资源的情况，结合铌钽矿产出的类型及性质，选取江西宜春铌钽矿区和尼日利亚宾盖地区铌钽矿为采集地点。江西宜春铌钽矿[10]是以铌钽锂铍为主要成分的的特大型稀有多金属矿床，也是目前我国产生最大的铌钽采选企业及铌钽原料生产基地；尼日利亚宾盖铌钽矿[11]为典型的沉积型砂矿。为了避免选矿的药剂污染及采集样品的稳定性，结合国内外的实际情况，确定了4个品位级别的候选物(编号为NTJK1、NTJK2、NTJK3和NTJK4)，宜春微晶岩型(3个)和尼日利亚砂矿类型(1个)两种类型经过重选加工后的不同含量段的4个铌钽精矿样品。对4个铌钽精矿标准物质候选物进行化学分析、光薄片鉴定和X射线衍射分析，其矿物组成和基本特征见表 1。
表 1 采集铌钽精矿候选物的基本特征Table 1. Basic characteristics of niobium-tantalum concentrate candidates样品编号 Ta2O5含量(×10-2) Nb2O5含量(×10-2) 采样地及采样量 主要矿物组成 NTJK-1 5.72 4.17 江西宜春，80 kg 长石30%，黄玉35%，钽铌锰矿15%，石英10%，锡石3%，萤石2% NTJK-2 12.07 8.48 江西宜春，80 kg 黄玉20%，钽铌锰矿30%~35%，锡石5%，细晶石6%~8%，磁铁矿1%，长石10%，石英2% NTJK-3 21.02 19.77 江西宜春，80 kg (钽铌+铌钽+锡钽)锰矿60%，黄玉10%，细晶石15%，锡石10%，磁铁矿1% NTJK-4 5.81 47.88 尼日利亚，80 kg 铌钽铁矿75%，钛铁矿+铁金红石15%，赤铁矿5%，锡石3%，角闪石3%，钍石2% 1.2 标准物质候选物的制备
在避免污染的前提下，将4个铌钽精矿候选物按照铌钽含量由低到高分别进行晾晒，混合后于105℃烘24 h，然后进行样品的细碎和混匀。加工后的样品存于聚乙烯塑料桶内密封保存，每桶的样品质量约25 kg。分装样品的最小单元，全部采用国际上推荐的中高密度的100 mL聚乙烯瓶，包装单位为100 g/瓶。样品加工流程见图 1。
铌钽矿物颗粒硬度大且具有脆性，不易粉碎，尤其对于铌钽矿物颗粒富集的精矿，因此，在本系列标准物质加工过程中采用两次粉碎的方法。首先采用气流粉碎，在对气流粉碎后的样品全部进行高铝球磨机细磨，同时时刻注意检查样品粒度，要求74 μm筛通过率大于98%。气流粉碎特别适用于硬度大、脆性大的样品，且气流粉碎技术在矿石加工和标准物质制备加工中已有应用[12-13]，而在铌钽精矿标准物质的制备中首次采用。
混合均匀后的4个铌钽精矿样品经激光粒度分析仪(BT-9300S型)进行分析，检测结果(图 2)表明：4个铌钽精矿标准物质的颗粒粒径主要集中在2~50.2 μm，占粒径分布的65.10%~84.85%，其中NTJK-3样品所占比例最小，为65.10%；＜50.2 μm的粒径分别占到97.33%、94.97%、99.44%和98.07%，NTJK-3样品所占比例最大；NTJK-4样品中＞74 μm的颗粒比例最大，为0.98%。4个铌钽精矿标准物质颗粒的粒径＜74 μm的含量均达到99%以上，符合国家一级标准物质技术规范的要求。
2. 候选物均匀性和稳定性检验
2.1 均匀性检验
样品的均匀性是研制标准物质的基础，是标准物质物质必须具备的特性，也是衡量标准物质加工质量的非常重要的因素。检验方法为：从分装的最小包装单元中随机抽取50个子样，每个样品进行双份测试。采用酸溶ICP-OES/MS法(取样量0.100 g)对定值元素Nb2O5、Ta2O5、Fe2O3、TiO2、WO3、MnO、P2O5、U、Th等9个元素进行了均匀性检验。采用碱熔ICP-OES/MS法(取样量0.100 g)对SiO2、Zr、Hf进行了均匀性检验。根据测试值的相对标准偏差(RSD)和瓶间与瓶内方差检验的F值结果，对标准物质候选物的均匀性作出评价[14-15]，均匀性检验结果见表 2。
表 2 候选物均匀性检验结果Table 2. Homogeneity tests of niobium-tantalum concentrate candidates元素 NTJK-1 NTJK-2 NTJK-3 NTJK-4 含量测定平均值 RSD(%) F 含量测定平均值 RSD(%) F 含量测定平均值 RSD(%) F 含量测定平均值 RSD(%) F Nb2O5 4.07 1.15 1.43 8.54 1.85 1.41 20.94 3.74 1.58 49.01 1.60 1.46 Ta2O5 5.39 1.52 1.41 10.80 1.51 0.79 19.58 2.92 1.46 5.45 1.37 1.51 SiO2 26.75 1.37 1.49 20.61 0.41 0.48 10.81 0.93 0.99 2.03 3.67 1.12 Fe2O3 3.47 1.73 1.32 4.47 1.39 1.00 5.81 1.18 1.23 23.89 1.60 1.36 TiO2 0.068 6.12 1.49 0.12 4.01 0.85 1.43 1.17 0.92 10.41 0.97 1.24 MnO 1.94 1.15 0.83 3.55 3.08 0.91 5.63 2.12 1.03 2.46 3.27 0.87 P2O5 0.37 1.51 1.50 0.26 5.05 0.26 0.26 4.61 1.27 0.090 1.56 0.98 Zr* 897.75 3.55 1.47 1549.44 1.52 1.14 1812.26 1.46 0.77 2733.70 1.75 0.32 Hf* 159.72 3.16 0.43 303.87 5.11 1.50 294.38 2.62 1.52 238.87 4.06 1.41 U* 957.05 3.42 1.52 2168.39 4.76 1.54 2955.06 2.95 1.51 340.01 4.19 1.59 Th* 98.94 0.69 1.55 192.83 2.91 0.94 377.83 3.50 1.18 1357.67 2.33 0.78 W* 532.68 4.20 1.59 1127.25 4.91 1.49 2283.44 2.89 1.39 2445.83 3.30 0.66 注：表中带“*”成分的测定平均值单位为10-6，其他成分的测定平均值单位为10-2。 从表 2中4个候选物的检验结果可以看出，大部分主量元素的相对标准偏差小于3%，微量元素的相对标准偏差小于5%，说明12个指标的分析方法精密度较高。经单因素方差检验，4个标准物质候选物中12项元素的F实测值均小于临界值F0.05(24，25)=1.76，说明组内和组间分析结果无明显差异，综合判断样品的均匀性良好。
2.2 稳定性检验
2.2.1 短期稳定性
标准物质在运输过程中不可避免地会发生颠震，铌钽精矿中部分矿物比重较大，运输过程中的颠震是否会对其造成影响而出现不均匀和不稳定的现象，值得关注。每个铌钽精矿标准物质候选物随机抽取2个最小包装单元的样品分别在50℃和-18℃温度条件下保存，常温下振荡器上振荡模拟运输过程中的颠簸情况，在颠振48 h后取样分析，试验后的样品每个取2份进行分析。对12个定值元素Nb2O5、Ta2O5、Fe2O3、TiO2、WO3、MnO、P2O5、U、Th、SiO2、Zr、Hf进行了测试，分析方法同均匀性检验。采用T检验法验证标准物质的稳定性，以Nb2O5和Ta2O5为例，分析结果见表 3，本系列标准物质在进行了48 h的颠振后，T检测值均小于T临界值，样品特性量值未发生显著变化，这说明本系列标准物质候选物的短期稳定性良好。
表 3 ICP-OES法测定Nb2O5和Ta2O5短期稳定性的结果Table 3. Short-term stability test results of Nb2O5 and Ta2O5 by ICP-OES样品编号 检验方式 取样部位 Nb2O5 Ta2O5 T临界值 平均测定值(×10-2) T检测值 平均测定值(×10-2) T检测值 NTJK-1 机器振荡 上部 4.11 0.9 5.47 0.7 2.3 下部 4.13 5.46 正常存放 上部 4.22 1.0 5.40 0.8 下部 4.21 5.36 NTJK-2 机器振荡 上部 8.52 2.0 11.28 1.1 2.3 下部 8.62 11.41 正常存放 上部 8.82 1.9 11.37 1.3 下部 8.83 11.36 NTJK-3 机器振荡 上部 21.04 1.2 19.26 1.2 2.3 下部 20.98 19.56 正常存放 上部 21.38 1.5 20.30 1.3 下部 21.33 20.31 NTJK-4 机器振荡 上部 48.92 1.3 5.45 1.0 2.3 下部 49.15 5.47 正常存放 上部 54.14 1.2 5.71 0.7 下部 54.12 5.72 2.2.2 长期稳定性
本次研制的系列铌钽精矿标准物质的长期稳定性按照“先密后疏”原则在第0、1、4、12、18、36、48个月时定期取样分析，每个铌钽精矿标准物质随机抽取2瓶样品进行分析，每瓶样品对12个定值元素Nb2O5、Ta2O5、Fe2O3、TiO2、WO3、MnO、P2O5、U、Th、SiO2、Zr、Hf进行了7次测试，7次不同时间分析结果的平均值均在正常的分析误差和标准值的不确定度范围内, 无明显偏向性变化, 表明本系列标准物质候选物的长期稳定性良好。
3. 定值分析和不确定度评定
3.1 样品定值方式及方法
标准物质的定值分析测试是标准物质研制的重要环节之一。铌钽精矿标准物质元素定值是按照国家一级标准物质技术规范，采用多个实验室、多种分析方法合作定值。邀请了经过计量认证、铌钽元素测试水平较高的检测机构参加样品测试，制定了分析测试细则，采用两套以上原理独立的方法进行检测，以提高定值的质量。每种方法对每一样品的每一元素至少报出4个数据，定值元素不少于8组数据。样品各定值元素的测定采用多种不同原理的分析方法分别进行分析，各元素的分析方法见表 4。
表 4 样品各定值元素的分析方法Table 4. Analytical methods of certified value elements in samples定值元素 分析方法 Nb2O5和Ta2O5 碱熔-纸上层析重量法；混合酸溶ICP-OES测定；混合碱熔ICP-OES测定 Fe2O3 磺基水杨酸比色法；混合酸溶ICP-OES测定 TiO2 二氨替比林甲烷比色法；混合酸溶ICP-OES测定 WO3 硫氰酸盐比色法；混合酸溶ICP-OES测定；混合碱熔ICP-OES测定 SiO2 动物胶凝聚重量法；硅钼蓝比色法；混合碱熔ICP-OES测定 U3O8 钒酸铵容量法；混合酸溶ICP-OES测定 3.2 测试数据统计处理
以各单位提供的各元素平均值数据为统计单元，用Grubbs准则剔除离群数据，共收集8家实验室466组平均值数据，剔除2组数据，占总数的0.43%。用夏皮罗-威尔克法(Shapiro-Wilk)进行正态检验。本次研制的4个铌钽精矿标准物质正态检验值W均大于置信概率95%的列表值，定值测试数据均呈正态分布或近似正态分布。
3.3 认定值的确定
按照《标准物质定值的通用原则及统计学原理》(JJF1343—2012)的要求，当数据为正态分布或近似正态分布时，以算术平均值为最佳估计值，当数据集属偏态分布时以中位值为最佳估计值。本次研制的4个铌钽精矿标准物质平均值全部为正态分布，以算术平均值为最佳估计值，计算得到认定值和不确定度。
3.4 不确定度评定
化学成分测量不确定度来源较多，其不确定度评定较为困难，对于地质标准物质不确定评定的表达也不尽统一[16-17]。本次铌钽精矿标准物质在研制过程中，不确定度的评定采用JJF1343—2012推荐的标准值的不确定度评定方法，各元素的不确定度主要由其稳定性不确定度(us)、均匀性不确定度(ubb)和定值不确定度(uchar)三部分构成[18-19]。合成标准不确定度(uCRM)为：
$ u{_{{\text{CRM}}}} = \sqrt {u_{\text{s}}^2 + u_{{\text{bb}}}^2 + u_{{\text{char}}}^2} $
使用扩展不确定度UCRM=k×uCRM表示最终不确定度的值，因子k取2，对应的置信水平大约为95%，不确定度的数字修约采用只进不舍的原则。地质标准物质定值组分多，受工作量和分析方法精密度的限制，通常只选择有代表性的组分进行均匀性和稳定性检验[20-21]。本次标准物质的认定值和扩展不确定度列于表 5。
表 5 铌钽精矿标准物质的认定值及不确定度Table 5. Certified values and expanded uncertainty of niobium-tantalum concentrates reference materials定值元素 认定值与扩展不确定度 NTJK-1 NTJK-2 NTJK-3 NTJK-4 MnO(×10-2) 1.84±0.065 3.59±0.094 5.82±0.158 2.47±0.124 P2O5(×10-6) 3785±414.33 2839±455.71 2189±183.94 1002±114.49 SiO2(×10-2) 27.88±0.542 21.60±0.586 10.99±0.7 2.12±0.282 Fe2O3(×10-2) 3.67±0.307 4.75±0.254 6.34±0.473 24.51±0.343 TiO2(×10-2) 0.075±0.013 0.13±0.016 1.45±0.041 11.28±0.485 Ta2O5(×10-2) 5.72±0.05 12.07±0.10 21.02±0.16 5.81±0.08 Nb2O5(×10-2) 4.17±0.225 8.48±0.267 19.77±0.550 47.88±0.968 W(×10-6) 742±19.62 1540±101.34 2899±107.37 2997±97.46 Th(×10-6) 103±16.10 170±12.11 383±26.91 1520±129.01 U(×10-6) 984±42.50 2084±118.444 3322±290.60 334±12.48 Zr(×10-6) 971±64.30 1624±88.53 1900±110.73 2898±189.44 Hf(×10-6) 171±17.28 283±14.26 295±25.38 166±19.03 3.5 标准物质的溯源
为了保证本次标准物质研制工作的溯源性，采取了如下具体措施：①所使用的仪器设备及计量器具按国家计量部门有关规定进行检定或校准，确保量值准确、可靠，可溯源到国家标准。②用作校正曲线的标准溶液由标准物质或基准物质配制，可溯源到测量国际单位制。③保证分析试剂和水的高纯度，每次分析进行空白试验，减空白和背景校正正确、合理。④所选用的定值分析方法是经实践经验证明为成熟的、准确的、可靠的方法。另外，本次定值是由多家通过国家级计量认证，并多次参加了标准物质定值工作的单位以及多种经过实践经验的准确、可靠的方法联合测定，而且各单位和各方法都使用了国家一级标准物质(GBW07155和GBW07185)进行质量监控。
4. 标准物质的应用
本批标准物质研制成功后，先后送江西宜春铌钽矿选厂和河南洛阳钼业公司进行应用分析，两个应用单位根据各自的条件，采用例行分析方法对本批标准物质进行了验证分析，分析数据见表 6，结果表明本批标准物质定值准确、可靠。同时，本批标准物质在河南三门峡市卢氏七里沟和卢氏火炎沟等地区的铌钽矿选矿过程样品分析中进行应用，分析结果表明，铌钽选矿过程样品的分析数据满足选矿金属量平衡的需要，证明本批标准物质能够对分析过程发挥很好的监控作用。
表 6 实际样品应用分析结果对照Table 6. Comparison of analytical results of actual samples样品编号 Nb2O5分析结果(×10-2) Ta2O5分析结果(×10-2) 宜春选厂 洛阳钼业 参考值 宜春选厂 洛阳钼业 参考值 NTJK-1 4.23 4.19 4.17 5.64 5.74 5.72 NTJK-2 8.49 8.36 8.48 12.25 12.04 12.07 NTJK-3 20.06 19.96 19.77 20.87 20.93 21.02 NTJK-4 48.09 47.97 47.88 5.72 5.87 5.81 注：参考值为8家实验室测定数据统计分析后的算术平均值。 5. 结论
研制的4个铌钽精矿标准物质，其主要成分Ta(Nb)2O5的含量为9.89%、20.55%、40.79%、53.69%，此系列标准物质多数成分含量呈梯度分布，定值成分12个，具有样品粒度均匀且分布范围窄、定值元素含量分布广泛的特点，形成了一个从粗精矿到精矿较为完整的含量体系，可以满足选冶和冶金试验各阶段流程样品对标准物质的需求。4个铌钽精矿标准物质在选冶试验流程样品和冶金过程样品中的应用良好，可以满足铌钽选矿与贸易、铌钽矿开发综合利用和冶金过程样品中对分析测试过程、仪器校正、方法验证等的要求，具有较好应用前景，解决了我国无铌钽精矿标准物质的问题。
在铌钽精矿标准物质研制过程中，采用气流粉碎和高铝球磨两次粉碎的技术对样品进行粉碎，解决了铌钽精矿中矿物颗粒硬度高、细碎难度大的问题，其粒度分布可满足日常质量监控的需要，并经实验证实；所应用的两次细碎的粉碎方法可以为今后类似矿石标准物质的研制提供借鉴。
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图 1 催化还原法石墨靶制备方法原理图
Figure 1. Schematic diagram of graphite target preparation by catalytic reduction method
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图 2 各制样方法真空系统图
(a)14C石墨制靶CO2纯化真空系统；(b)H2/Fe法石墨化单元；(c)在线Zn/Fe法石墨化单元；(d)Zn+TiH2/Fe或Zn/Fe火焰封管法石墨化单元；(e)Zn/Fe隔膜封管法石墨化单元。PT为压力传感器。
Figure 2. Schematic diagrams of vacuum line for different graphite preparation methods: (a) The CO2 purification vacuum system; (b) Graphitization unit by H2/Fe method; (c) Graphitization unit by online Zn/Fe method; (d) Graphitization unit by Zn+TiH2/Fe or Zn/Fe flame-sealed method; (e) Graphitization unit by Zn/Fe septa-sealed method. PT denotes pressure transducer
表 1 石墨化条件及流程本底
Table 1 Summary of different graphitization conditions applied in prior studies and their corresponding procedure background
还原剂 催化剂 反应温度 反应时间(h) 样品量(mg) 本底14C年龄(ka) 参考文献 Zn：9.8~11.4mg
TiH2：3.3~4.9mg铁粉4~5mg 马弗炉
500~550℃7 0.1 41.1~48.1 [11] Zn：2.5mg 375目铁粉，
0.4mg/mg或1mg/mg C720℃ 10 1~2 41.1~44.4 [12] Zn：100mg 铁粉5mg 500℃ 5 0.5~1.0 44.4 [13] Zn：2.5mg/25μgC 铁粉2.5mg 电热板450℃ 12 0.025~0.25 27.9±0.26 [14] Zn：15mg 铁粉，2.5mg/mg C 550℃ 10 1 51.2~49.9 [15] H2/C=2 铁粉，2mg/mg C 550℃ 2~3 1 45.7 [16] Zn：9.8~11.4mg -400目铁粉4~5mg 450℃ 7 0.015 34.8 [17] H2/C=2.2 325目铁粉，2mg/mg C 600℃ 4~5 1 40.0~47.5 [18] H2/C=2.5 10μm铁粉，3mg/mg C 600℃ 3~4 0.3 45.2~55.5 [19] Zn：35~40mg
TiH2：7~10mg铁粉2mg 马弗炉
550~560℃8 0.5~1.0 53.0±4.6 [20] Zn：30~35mg
TiH2：10~15mg350目铁粉，
3~5mg/mg C马弗炉
500~550℃7 1 50 [8] H2/C=2.2 160目铁粉，2~3mg/mg C 600℃ 2~3 1 52 [21] Zn：200mg 铁粉，2mg/mg C Zn：420℃
Fe：620~630℃18~20 0.03~1 46.1~51.2 [7] H2/C=2.2~5 325目铁粉 670℃ 2~4 0.015~1 50~60 [22] H2/C=2.5~3 325目铁粉，1mg/mg C 600℃ 4 0.2 46.7±1.5 [23] H2/C=2.4 200目铁粉，2mg/mg C 500℃ 5~6 / 47.3±0.7 [24] 注：部分文献中并没有直接给出14C年龄，而是给出了现代碳污染量和死碳污染量的数据。为了统一和便于读者比较，此表中的部分14C年龄是经过文献中的公式进行换算而来的。14C年龄=-8033×ln(Fm)，Fm=Mdc/(Ms+Mmc+Mdc)×Rmc，Ms代表典型样品量，Mmc代表现代碳污染量，Mdc代表死碳污染量，Rmc代表现代碳14C浓度。 
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表 2 不同石墨合成方法中可能发生的化学反应
Table 2 Potential chemical reactions during the graphitization process by H2/Fe and Zn/Fe or Zn-TiH2/Fe method
序号 化学反应方程 1 CO2(g)+H2(g)→CO(g)+H2O(g) 2 CO2(g)+Zn(s)→CO(g)+ZnO(s) 3 2CO(g)→Cgraphite(s)+CO2(g) 4 CO(g)+H2(g)→Cgraphite(s)+H2O(g) 5 CO2(g)+2H2(g)→Cgraphite(s)+2H2O(g) 6 2CO(g)+2H2(g)→CH4(g)+CO2(g) 7 CO(g)+3H2(g)→CH4(g)+H2O(g) 8 Cgraphite(s)+2H2(g)→CH4(g) 9 CO(g)+H2O(g)→CO2(g)+H2(g) 10 Zn(s)+H2O(g)→ZnO(s)+H2(g)
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表 3 碳污染来源
Table 3 Sources of carbon contamination
碳污染来源 污染量 参考文献 样品预处理阶段 7.05±4.02μg现代碳 [45] 样品预处理阶段 0.7±0.3μg现代碳 [43] 燃烧阶段 3.01±2.0μg现代碳 [45] 燃烧阶段 1.5±0.1μg现代碳 [43] 燃烧阶段 0.36±0.07μg现代碳(玻璃管路吸附) [47] 燃烧阶段 每500mg CuO引入0.44±0.13μg现代碳 [47] 燃烧阶段 每支玻璃管引入0.02~0.15μg现代碳 [26] 燃烧阶段 每100mg CuO引入0.1±0.01μg现代碳 [26] 石墨化阶段 0.36±0.19μg现代碳 [43] 石墨化阶段 每10mg铁粉引入1.8±0.7μg现代碳 [26] 储存阶段 < 0.2μg现代碳 [43] 转移压靶阶段 < 0.1μg现代碳 [43] 仪器本底 ≤0.5μg现代碳 [45]
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