Study On ¹⁴C-AMS Sample Preparation Method

As one of the important elements in nature,carbon has extremely important scientific research and application value.Radiocarbon(¹⁴C)has extremely important applications in archaeology,environmental science,oceanography and biomedical science.The most suitable technique for measuring ¹⁴C is accelerator mass spectrometry（AMS）.In the process of using AMS to measure ¹⁴C requires first oxidizing the carbon in the sample to CO₂,and then reducing the above CO₂ to elemental carbon.The reduction method includes H₂-Fe method,or Zn-Fe method,or Zn-TiH₂-Fe method,and finally analyzed with AMS.The ¹⁴C-AMS measurement and analysis technology is a systematic science,and the each step such as sample pretreatment,graphitization,and AMS measurement and analysis directly affects the final analysis results.The main research content of this paper is based on the ¹⁴C graphite preparation system of Guangxi Normal University to explore and study the ¹⁴C graphite sample preparation method,to provide a basis for optimizing the graphite preparation process and improve the conditions for a high quality,high performance,high-efficiency graphite preparation method.This work was based on the first homemade single-stage electrostatic ¹⁴C dedicated accelerator mass spectrometry（GXNU-AMS）in China developed by Guangxi Normal University and China Institute of Atomic Energy,to develop ¹⁴C-AMS sample preparation methods and AMS measurements.The specific tasks include the following:1)Establish the experimental process for preparing graphite targets by Zn-Fe method,from four aspects:sample chemical pretreatment,sample oxidation,CO₂purification,and CO₂ graphitization;2)Explore the experimental conditions of the Zn-Fe method to synthesize graphite:design experiments from the reduction reaction temperature,reduction reaction time,reagent dosage,Fe powder activation and other factors to explore the best reaction conditions for the preparation of graphite targets.From the comprehensive analysis of graphite yield,¹²C beam current,measurement sensitivity and other factors,to obtain the best experimental conditions for graphite preparation;3)Prepare graphite targets with background samples,modern carbon standard samples and actual sample to evaluate the performance of preparation of graphite by Zn-Fe method.Through a series of experimental studies on the preparation of graphite by Zn-Fe method,it is concluded that:1)For the preparation of graphite targets by Zn-Fe method,the experimental conditions are:1 mg C,18-22 mg Zn,2.5-3.0 mg Fe,and Fe is activated by vacuum heating at 600℃ for 1 h.The tube is sealed by flame sealing,and the reduction process is heated at 650℃ for 8 hours in a condensed graphite reduction furnace.2)According to the experimental results of background samples:the background value of the ¹⁴C graphite preparation system is about（0.55±0.04）pMC,and the ratio of ¹⁴C/¹²C is（6.47±0.48）×10^-15,equivalent to ¹⁴C age is 44000 years;the background value of ¹⁴C measured by AMS is（0.27±0.02）pMC,the AMS detection sensitivity is about（3.14±0.27）×10^-15,which is equivalent to ¹⁴C age is 50,000 years.3)According to the experimental results of the processed standard samples:the mean value of the processed oxalic acid standard is（134.11±0.41）pMC,the mean value of the processed IAEA-C8 standard is（15.17±0.22）pMC,within the allowable range of error,the experimental results are consistent with the recognized standard value.It is consistent and verifies the stability and reliability of preparation of graphite by Zn-Fe method.Based on the above research and analysis,the ¹⁴C graphite preparation system independently built by Guangxi Normal University has reliable performance,and the background value of the preparation of graphite by Zn-Fe method is 0.55 pMC,which can meet the AMS measurement requirements within 40,000 years.It provides a basis for improving the ¹⁴C-AMS sample preparation method,experimental preparation process,and optimizing the ¹⁴C graphite preparation system,and lays a foundation for improving the background level of ¹⁴C graphite prepared by the Zn-Fe method.