Analytical Method for δ18O of Phosphate in Trace Apatite
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摘要: 生物磷灰石壳体的磷酸根氧同位素组成是重建古温度理想指标之一，在古环境研究中具有重要意义.针对牙形石等磷灰石量极少的情况，稳定可靠的前处理方法是分析其δ18OPO4的重要保障，目前仅有少数国外实验室已建立了相关提取分析方法.结合这些方法的优缺点对分析步骤进行改进优化，建立了微量磷灰石的磷酸根氧同位素分析方法，通过硝酸消解磷灰石并除去非磷酸根氧，利用KF溶液沉淀法分离Ca2+，采用氨缓冲溶液形式调节pH，并加入AgNO3溶液以氨挥发法将PO43-转化成Ag3PO4结晶分离，气体稳定同位素质谱仪在线测定Ag3PO4氧同位素组成.结果表明，方法全流程未产生明显的氧同位素分馏，样品最低仅需0.2 mg，标准偏差小于0.2‰（1σ），与目前国际报道的分析精度一致.Abstract: The oxygen isotope composition of phosphate (δ18OPO4) in bioapatite plays a significant role in paleo-environmental research as one of the ideal proxies for paleo-temperature reconstruction. However, for trace bioapatite(e.g., conodonts), a reliable pre-treatment technology is quite important and difficult for its δ18OPO4 analysis, resulting in analytical technique established only in several overseas laboratories. Here we combine the advantages of those methods and present a protocol on the analysis of δ18OPO4 as Ag3PO4 for bioapatite of total sample size as small as 0.2 mg using a thermal conversion elemental analyzer (TC/EA) coupled to a continuous flow isotope ratio mass spectrometer (CF-IRMS) via a helium stream. Ag3PO4 is precipitated by NH3 buffer method after apatite being dissolved with nitric acid, Ca2+ being removed with KF solution and the solution being neutralized with ammonia buffer solution. The results indicate that analysis of δ18OPO4 maintains an external precision of ±0.2‰(1σ), meeting the international analytical standards. This method for analyzing δ18OPO4 of phosphate extracted from bioapatite is robust to be used to reconstruct paleo-temperature.
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Key words:
- bioapatite /
- oxygen isotope of phosphate /
- analytical method /
- biogeochemistry
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图 3 不同样品量时NBS 120c和NBS 694的PO43-回收率
Fig. 3. Average PO43- yields of NBS 120c and NBS 694 under different sample amount
图 4 NBS 120c和NBS 694在不同PO43-回收率下的δ18OPO4值
Fig. 4. δ18OPO4 values of NBS 120c and NBS 694 under different PO43- yields
图 5 本方法均相沉淀得到的磷酸银晶体(偏光显微镜)
Fig. 5. Ag3PO4 crystals precipitated by homogeneous precipitation method
图 6 不同实验室对NBS 120c的δ18OPO4提取测试结果对比
1.Vennemann et al.(2002): 22.09‰±0.51‰; 2.LaPorte et al.(2009): 22.4‰±0.3‰; 3.Joachimski et al.(2009): 22.4‰±0.16‰; 4.Halas et al.(2011): 21.8‰±0.2‰; 5.Rosenau et al.(2014): 21.8‰±0.4‰; 6.Griffin et al.(2015): 22.5‰±0.3‰; 7.本文: 21.9‰±0.17‰(精度1σ)
Fig. 6. δ18OPO4values of NBS 120c from different laboratories and different methodsδ18OPO4values of NBS 120c from different laboratories and different methods
表 1 微量磷灰石中δ18OPO4分析的TC/EA法比较
Table 1. Comparison of TC/EA method for δ18OPO4 analysis of trace apatite
样品量(mg) 1σ(‰) 回收率(%) 流程简介 主要优缺点 参考文献 20
0.51
(氟化法0.09)- NaOCl除有机物，NaOH除腐殖酸，HF溶液溶解磷灰石并沉淀Ca2+，KOH中和溶液，银氨溶液回收PO43-. 流程简单；要求样品量大，精度低，HF对仪器有损害 Vennemann et al., 2002 0.10~0.45 0.15 85~97 0.5 mol/L HNO3溶解磷灰石，阳离子交换树脂除Ca2+，浓氨水中和，AgNO3回收PO43-. 精度高，可分析44Ca、稀土元素等；使用阳离子交换树脂费时费力 LaPorte et al., 2009 0.5~1.0 0.15 92~99 2.0 mol/L HNO3溶解磷灰石，KOH中和，HF沉淀Ca2+，银氨溶液回收PO43-. 精度高；中和工作量大 Joachimski et al., 2009 0.3~0.8 0.10~0.40 20~85 0.5 mol/L HNO3溶解磷灰石，KOH中和，KF沉淀Ca2+，银氨溶液回收PO43-. 方法完整系统；溶液量低难操作，回收率不稳定 Griffin et al., 2015 
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表 2 相关的仪器、材料和试剂
Table 2. Relative instrument, materials and reagents
类别 仪器、材料与试剂 测试仪器 Flash HT元素分析仪+Delta V气体稳定同位素比值质谱仪(Thermo Fisher公司) 前处理设备材料 高速离心机；隔膜真空泵抽滤设备一套；移液枪(1 mL)；PFA烧杯(5 mL)；离心管(2 mL)；玻璃纤维滤膜(0.45 μm)；阳离子交换树脂(AG50W-X12) 试剂 硝酸、硝酸银、氨水、氢氧化钾和氟化钾(分析纯) 标准样品 磷灰石标样NBS 120c(NIST)；磷灰石标样NBS 694(NIST)；磷酸银氧同位素标样(Elemental Microanalysis公司) 实际样品 大唇犀、三趾马和羚羊牙齿磷灰石(200目)
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表 3 不同消解温度对PO43-回收率的影响
Table 3. PO43- yields in different digestion temperature
样品名称 PO43-回收率(±1σ，n=3，%) 20 ℃ 70 ℃ 150 ℃ NBS 120c 93.9±2.1 94.0±4.1 93.3±2.2 NBS 694 92.8±0.8 93.2±1.1 92.9±1.6
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表 4 不同除Ca2+方法对NBS 120c PO43-回收率和δ18OPO4的影响
Table 4. Average PO43- yields and δ18OPO4 values of NBS 120c under different Ca2+ removing options
不除Ca2+(n=3) 使用树脂(n=6) 硝酸+KF(n=6) 后加KF(n=6) PO43-回收率(±1σ, %) 45.9±14.3 89.1±1.5 93.4±2.2 91.4±1.9 δ18OVSMOW(±1σ, ‰) 21.88±0.31 22.04±0.15 21.91±0.17 21.97±0.14
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表 5 不同pH调节方法对NBS 120c PO43-回收率和δ18OPO4的影响
Table 5. Average PO43- yields and δ18OPO4values of NBS 120c under different pH adjustment methods
2 mol/L KOH 2 mol/L氨水 回收率(±1σ, n=6, %) 91.0±1.7 93.4±2.2 δ18OVSMOW(±1σ, n=6, ‰) 21.70±0.21 21.91±0.17
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表 6 不同结晶温度和时间对NBS 120c PO43-回收率的影响
Table 6. Average PO43- yields of NBS 120c over different precipitation temperatures and times
温度 回收率(±1σ，n=3，%) 12 h 24 h 48 h 1周 20 ℃(室温) 27.6±4.9 79.9±2.4 91.8±0.9 (93.1±2.2)% 50 ℃ 55.0±3.3 93.6±2.1 92.7±1.9 -
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表 7 实际磷灰石样品的PO43-回收率和δ18OPO4值
Table 7. The average PO43- yields and δ18OPO4 values of practical samples
样品名称 样品来源 回收率(±1σ, n=5, %) δ18OVSMOW(±1σ, n=5, ‰) DCX1 大唇犀1 95.6±2.5 10.14±0.19 DCX2 大唇犀2 96.4±1.9 10.08±0.18 SZM 三趾马 89.3±2.7 9.18±0.13 LY 羚羊 82.5±3.0 9.62±0.18
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