黃土中宇宙成因核素10Be提取條件檢驗

趙國慶，張 麗，孔祥輝，付云翀，杜雅娟

1.中國科學(xué)院地球環(huán)境研究所 黃土與第四紀(jì)地質(zhì)國家重點實驗室，陜西省加速器質(zhì)譜技術(shù)及應(yīng)用重點實驗室，西安 710061

2.西安加速器質(zhì)譜中心，西安 710061

黃土中宇宙成因核素10Be提取條件檢驗

趙國慶1,2，張 麗1,2，孔祥輝1,2，付云翀1,2，杜雅娟1,2

1.中國科學(xué)院地球環(huán)境研究所 黃土與第四紀(jì)地質(zhì)國家重點實驗室，陜西省加速器質(zhì)譜技術(shù)及應(yīng)用重點實驗室，西安 710061

2.西安加速器質(zhì)譜中心，西安 710061

鑒于利用中國黃土中的宇宙成因核素10Be示蹤高分辨率的古地磁場變化研究工作需要高精度、低本底的化學(xué)實驗結(jié)果，為獲得現(xiàn)有樣品制備流程的詳細(xì)數(shù)據(jù)并優(yōu)化實驗流程，文章對酸提取效率進(jìn)行了檢驗。通過采用不同溫度、不同酸度在加熱板和微波消解儀下的提取效率對比檢驗，獲得了不同條件下的可靠實驗數(shù)據(jù)，檢驗了現(xiàn)有流程提取兩次可將樣品中90%以上的10Be提取出來，達(dá)到實驗要求。

黃土；10Be；BeO

10Be主要是由高能宇宙射線與大氣中的氮、氧發(fā)生散裂反應(yīng)生成，具有半衰期較長、地球化學(xué)行為較穩(wěn)定等特點，在深海和冰芯的環(huán)境示蹤研究中涵蓋了全球變化研究的多個領(lǐng)域。因10Be產(chǎn)率與地磁場強(qiáng)度變化呈反相關(guān)關(guān)系，可捕捉弱地磁場信號的變化，國際上已成功運(yùn)用深海和冰芯中的10Be重建了古地磁場變化曲線（Frank et al，1997；Wagner et al，2000）。

中國黃土10Be研究起步于20世紀(jì)80年代（Shen，1986），主要開展大氣成因10Be賦存狀態(tài)和地球化學(xué)行為研究（沈承德等，1989a；顧兆炎等，2000），后拓展至定年和古氣候等領(lǐng)域（沈承德等，1989b）。但鑒于黃土來源及其組分的復(fù)雜性，直到近年Zhou et al（2007a，2007b，2015）建立的數(shù)理方法解決了將黃土10Be中微弱的地磁場信號分離出來的關(guān)鍵性問題，中國黃土10Be示蹤高分辨率古地磁場變化的研究才取得進(jìn)展（趙國慶等，2010；周衛(wèi)健等，2010；孔祥輝等，2016），并成功重建了過去8萬年（Zhou et al，2007a）和13萬年（Zhou et al，2010）以來的古地磁場變化曲線。隨著研究工作的開展，為檢驗實驗工作的可靠性（武振坤等，2008）并探索提高實驗的精度和效率，筆者進(jìn)行了相關(guān)實驗探索，以期為黃土10Be樣品制備工作提供參考。

1 黃土中10Be提取效率對比實驗

1.1 儀器與裝置

實驗工作在中國科學(xué)院地球環(huán)境研究所10Be實驗室完成。

微控數(shù)顯電熱板為LabTech公司制造的FG20A型；離心機(jī)為上海安亭科學(xué)儀器廠制造的LXJ-ⅡB型；AR 2140電子天平（OHAUS公司）（量程210 g，可讀性0.0001 g）；微波萃取儀器為CEM MARS密閉微波消解儀；實驗用水為美國Millipore公司制造的Milli-Q超純水設(shè)備生產(chǎn)。

1.2 實驗內(nèi)容

實驗內(nèi)容主要包括四部分：

（1）現(xiàn)有實驗流程中樣品的提取效率；

（2）不同溫度條件下提取效率檢驗；

（3）不同酸濃度條件下提取效率檢驗；

（4） 微波萃取不同濃度酸條件下的對比檢驗。

1.3 實驗流程

將黃土、古土壤樣品烘干后研磨過60目篩，儲存于5 mL冷凍管內(nèi)。

（1）取黃土和古土壤樣品各1 g，分別放入100 mL 的Tefl on燒杯，加入3 mL去離子水和0.5 mg的9Be載體，加入3 mL雙氧水靜置至反應(yīng)完全，加入12 mol · L-1的鹽酸浸泡24 h，離心收集上清液后按圖1流程進(jìn)行制樣，即獲得一次提取結(jié)果。將離心沉淀攪勻加入10 mL的6 mol · L-1的鹽酸浸泡24 h，再次重復(fù)提取兩次后，將離心沉淀消解。共獲得4次提取結(jié)果，以檢驗現(xiàn)實驗流程對10Be的提取效率。

（2）取黃土和古土壤樣品各1 g，同上流程將樣品提取3次后，再次用10 mL的6 mol · L-1的鹽酸浸泡24 h后提取，共獲得4次提取結(jié)果，交叉檢驗實驗結(jié)果。

（3）取黃土和古土壤樣品各1 g，同上流程加入去離子水、9Be載體、雙氧水、鹽酸后置于加熱板上在不同的溫度（50℃、70℃）條件下每4 h提取一次，共提取2次，檢驗溫度對提取效率的影響。

（4）取黃土和古土壤樣品各1 g，同上流程加入去離子水、9Be載體、雙氧水后，再分別加入1 mol · L-1、2 mol · L-1、4 mol · L-1、6 mol · L-1、8 mol · L-1不同濃度的鹽酸，浸泡24 h后提取一次，共提取2次，獲得提取效率最高時的鹽酸濃度。

（5）取黃土和古土壤樣品各0.5 g，應(yīng)用微波消解儀將樣品在4 mol · L-1、6 mol · L-1、8 mol · L-1、10 mol · L-1、12 mol · L-1不同濃度的鹽酸條件下萃取一次，檢驗在微波條件下不同濃度酸的提取結(jié)果。

將提取實驗的上清液按照武振坤等（2008）建立的實驗流程進(jìn)行制樣，制備后在西安加速器質(zhì)譜中心進(jìn)行測量。

2 結(jié)果與討論

2.1 黃土中10Be提取效率對比實驗結(jié)果

對加速器測量后數(shù)據(jù)進(jìn)行計算，結(jié)果見表1。

2.2 黃土中10Be提取效率對比實驗結(jié)果分析

2.2.1 樣品消解提取結(jié)果

為檢驗現(xiàn)有實驗流程兩次酸提取的效率，使用6 mol · L-1鹽酸對樣品進(jìn)行提取三次后消解。如圖2、圖3所示，通過兩次酸提取，黃土和古土壤樣品中10Be的提取效率分別為93%和88%，即兩次酸提取10Be的提取率約90%。

2.2.2 樣品多次提取結(jié)果

使用6 mol · L-1鹽酸對樣品進(jìn)行四次提取（假設(shè)提取完全），結(jié)果顯示，經(jīng)過兩次酸提取可將黃土中91%、古土壤中87%的10Be提取出來（圖4，圖5），與樣品消解提取結(jié)果基本一致，驗證了實驗工作的可靠性。

通過上述兩次實驗的對比檢驗，驗證了兩次酸提取可將樣品中90%的10Be提取出來，現(xiàn)有實驗流程在酸提取后再次用去離子水對樣品進(jìn)行洗滌，可保證90%以上的提取率。實驗在獲得了提取步驟詳細(xì)數(shù)據(jù)的同時，也驗證了現(xiàn)有流程每一步驟的存在都是必不可少的。實驗結(jié)果中古土壤樣品的提取比率沒有黃土樣品的比率高，造成這種結(jié)果的原因推測是古土壤中粘土礦物等成分較復(fù)雜造成的。

圖 1 BeO樣品制備的實驗流程Fig.1 The experimental process of preparation BeO

表1 黃土中10Be提取效率對比實驗結(jié)果Tab.1 The experimental results of10Be extraction effi ciency in the loess /(atoms· g-1)

圖2 黃土樣品提取效率Fig.2 The extraction ratio of loess

圖3 古土壤樣品提取效率Fig.3 The extraction ratio of paleosol

圖4 黃土樣品提取效率Fig.4 The extraction ratio of loess

圖5 古土壤樣品提取效率Fig.5 The extraction ratio of paleosol

2.2.3 溫度對樣品提取的影響

由多次提取實驗結(jié)果得知，在常溫下提取兩次可將樣品中約90%的10Be提取出來。為檢驗溫度條件對提取結(jié)果的影響，采用在50℃和70℃條件下對黃土和古土壤樣品進(jìn)行兩次提?。? h提取一次）。由結(jié)果得知黃土樣品在50℃和70℃條件下提取結(jié)果相同（圖6），顯示溫度對酸提取結(jié)果基本無影響，驗證了現(xiàn)有流程中采用常溫條件下進(jìn)行酸提取步驟的可靠性。古土壤樣品在50℃和70℃條件下結(jié)果分別為4.08×108atoms · g-1和4.44× 108atoms · g-1（圖6），70℃條件下結(jié)果增高，應(yīng)是高溫影響到粘土礦物等成分的分解造成10Be提取效率的增高，這也解釋了常溫提取時古土壤提取效率低于黃土提取效率。

圖6 不同溫度條件下的提取結(jié)果（a：黃土；b：古土壤）Fig.6 The extraction results under different temperature conditions (a: loess; b: paleosol)

2.2.4 酸濃度對樣品提取的影響

對古土壤樣品分別用1 mol · L-1、2 mol · L-1、4 mol · L-1、6 mol · L-1、8 mol · L-1的鹽酸條件下浸泡，結(jié)果顯示6 mol · L-1鹽酸對古土壤中10Be的提取效率最高（圖7）。用黃土樣品進(jìn)行驗證，結(jié)果與古土壤樣品一致（圖7）。此條件實驗驗證了前人工作的可靠性，因此對不同黃土粒徑中10Be的賦存不再進(jìn)行驗證，認(rèn)為10Be主要吸附和結(jié)合在風(fēng)化過程中形成的細(xì)粒級粘土礦物中（Shen，1986；沈承德等，1989b）。

2.2.5 微波萃取樣品結(jié)果對比

將4 mol · L-1、6 mol · L-1、8 mol · L-1、10 mol · L-1、12 mol · L-1濃度的鹽酸分別加入裝有0.5 g黃土樣品的消解罐中，在140℃條件下萃取15 min，數(shù)據(jù)結(jié)果基本一致，顯示不同酸濃度在高溫高壓下對樣品提取結(jié)果基本沒有影響。

與四次提取實驗結(jié)果對比，微波萃取黃土樣品比四次提取實驗提取量平均高5.6%，古土壤樣品提取量平均高10.8%，即在高溫高壓下不僅將細(xì)顆粒表面吸附的10Be，還可將粘土礦物等各種成分間裹含的10Be提取出來。

圖7 不同濃度鹽酸提取黃土-古土壤中10Be的結(jié)果Fig.7 The extraction results of10Be in loess-paleosol sequence with different concentrations of hydrochloric acid

3 結(jié)語

應(yīng)用10Be示蹤高分辨率的古地磁場變化研究離不開高精度的實驗數(shù)據(jù)支撐，對現(xiàn)有實驗流程中樣品提取的比率、不同濃度酸條件下加熱板和微波萃取提取效率進(jìn)行了檢驗。通過各項條件實驗，驗證了現(xiàn)有流程酸提取兩次后水洗即可將樣品中90%以上的10Be提取出來，達(dá)到了實驗要求。

實驗中有些問題還有待進(jìn)一步討論：

（1）鑒于樣品量的原因五次條件實驗用了四組不同的樣品進(jìn)行檢驗，若采用同一樣品可獲得各項條件結(jié)果的縱向?qū)Ρ取?/p>

（2）古土壤樣品提取結(jié)果隨溫度的增高而增加，而微波萃取結(jié)果基本一致，可進(jìn)一步探討提取的臨界溫度。

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Optimizing experimental conditions of extraction cosmogenic nuclide10Be in loess

ZHAO Guoqing1,2, ZHANG Li1,2, KONG Xianghui1,2, FU Yunchong1,2, DU Yajuan1,2
1. State Key Laboratory of Loess and Quaternary Geology and Shaanxi Key Laboratory of Accelerator Mass Spectrometry Technology and Application, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
2. Xi’an Accelerator Mass Spectrometry Center, Xi’an 710061, China

Background, aim, and scope High precision and low background contimination are required in the research work of cosmogenic nuclide beryllium 10 to trace high precision geomagnetic change in Chinese loess. Wu et al (2008) have optimized and established a reliable experimental process for the preparation of BeO. In order to obtain the detail data of acid extraction steps in the experimental process used now, this paper discusses the effi ciency of the acid extraction. Materials and methods The experimental process used now extracts beryllium 10 from samples using 6 mol · L-1HCl soak overnight twice, then using deionized water to wash the sample again. This paper design 5 different conditions of the experiment to test the effi ciency of the acid extraction. (1) Using 6 mol · L-1HCl extract samples three times then digest it using HF/HNO3/HClO4to test the experimental extraction effi ciency of beryllium 10. (2) Using 6 mol · L-1HCl to extract samples four times to test the experimental extraction effi ciency, the results can crosscheck the results of experiment of using acid extract the sample three times and digest it. (3) Acid extraction steps in the experimental process used now under different temperature (50℃ and 70℃) to test the influence of temperature. (4) Acid extraction steps in the experiment process used now under different concentration of hydrochloric acid (1 mol · L-1, 2 mol · L-1, 4 mol · L-1, 6 mol · L-1and 8 mol · L-1) to test the infl uence of different concentration of hydrochloric acid. (5) Using microwave digestion instrument to test the results ofdifferent concentration of hydrochloric acid (4 mol · L-1, 6 mol · L-1, 8 mol · L-1, 10 mol · L-1and 12 mol · L-1) extraction. The experimental process in accordance with the Wu et al established in 2008, the measurement in Xi’an Accelerator Mass Spectrometry Center. Results (1) The results of using hydrochloric acid extract samples three times and then digest it show that using 6 mol · L-1HCl extract samples two times can extract about 90% of the beryllium 10. (2) Results of using hydrochloric acid extract samples four times show that using 6 mol · L-1HCl extract samples two times can extract about 90% of the beryllium 10. (3) Results of loess samples under 50℃ and 70℃ conditions are the same, but the results of paleosol samples under the condition of 70℃ is higher than the 50℃’s. (4) 6 mol · L-1HCl is the highest extraction effi ciency to extract beryllium 10. (5) Results of use microwave digestion instrument are basically consistent. Discussion (1) Through the experiment of using 6 mol · L-1HCl extract samples three times then digestion it and using 6 mol · L-1HCl extract samples four times, the experimental process used now which using hydrochloric acid extract twice, 90% of the beryllium 10 can be extracted. The experimental process using deionized water to wash the samples again after the acid extracting, guaranteeing the extraction ratio more than 90%. Experimental results show that the extraction ratio of paleosol samples are less than the loess samples, the reason should be the composition of clay minerals are much more complicated in paleosol samples. (2) Results of loess samples under 50℃ are same with 70℃ conditions, which shows that the temperature has no infl uence on the extraction effi ciency, verify the reliability of the experimental process under the condition of normal temperature of the acid extraction steps. Results of paleosol samples under the condition of 70℃ are higher than the 50℃’s, the results should be that high temperature affect decomposition of the clay mineral composition, increased the extraction effi ciency of beryllium 10, it also explains that under the normal temperature paleosol samples extraction effi ciency is lower than the loess samples extraction effi ciency. (3) 6 mol · L-1HCl is the highest extraction effi ciency to extract beryllium 10. Verify the reliability of the predecessors’ work, no longer test beryllium 10 in different particle size in the loess, considering beryllium 10 is adsorbed and combined with the clay mineral in the fi ne grained level in the process of weathering. (4) Results of use microwave digestion instrument are basically consistent, which show that different hydrochloric acid concentration under the high temperature and the high pressure had little infl uence upon results of the sample extraction. Compared with the results of using hydrochloric acid extract samples three times then digestion it and using hydrochloric acid extract samples four times, the results of using microwave digestion instrument are higher than the two contrast test. Under high temperature and high pressure will not only extract beryllium 10 adsorption on fi ne particle surface, it also can extract beryllium 10 containing between various ingredients such as the clay minerals. Conclusions The experiment process used now can extract more than 90% beryllium 10 in the samples, meets the experimental requirements. Recommendations and perspectives (1) Due to the fact that the amount of the sample is less, the experiment using four different sets of samples to compare the experimental extraction effi ciency respectively. If using the same sample we can obtain direct comparison results of various conditions. (2) Results of paleosol samples under the condition of 70℃ are higher than the 50℃’s, high temperature should be affect extraction efficiency of beryllium 10, but results of using microwave digestion instrument are basically consistent, the best temperature of extraction is still need to investigated.

loess;10Be; BeO

ZHAO Guoqing, E-mail: zhaogq@ieecas.cn

2016-11-25；錄用日期：2017-01-14

Received Date: 2016-11-25; Accepted Date: 2017-01-14

國家自然科學(xué)基金項目（41230525，11205161）

Foundation Item: National Natural Science Foundation of China (41230525, 11205161)

趙國慶，E-mail: zhaogq@ieecas.cn

趙國慶，張 麗，孔祥輝, 等. 2017. 黃土中宇宙成因核素10Be提取條件檢驗[J]. 地球環(huán)境學(xué)報, 8(2): 169 – 175.

: Zhao G Q, Zhang L, Kong X H, et al. 2017. Optimizing experimental conditions of extraction cosmogenic nuclide10Be in loess [J]. Journal of Earth Environment, 8(2): 169 – 175.