INTRODUCTION
14C dating technology was first proposed by Libby in 1949 (Libby et al. Reference Libby, Anderson and Arnold1949). With the development and application of accelerator mass spectrometry (AMS) (Dong et al. Reference Dong, He and Jiang2006; Jiang et al. Reference Jiang, Dong and He2012; Kutschera et al. Reference Kutschera2013; Synal et al. Reference Synal2013; Chen et al. Reference Chen, Shen, Sasa, Lan, Matsunaka, Matsumura, Takahashi, Hosoya, He and He2019; Shen et al. Reference Shen, Sasa, Meng and Matsamura2019, Reference Shen, Shi and Tang2022), it has become unparalleled in terms of radiocarbon (14C) measurement and analytical technology (Paul et al. Reference Paul, Hollos, Kaufman, Kutschera and Magaritz1987). Compared with other 14C determination methods (Shen et al. Reference Shen and Hung1980; Suter et al. Reference Suter, Balzer, Bonani, Morenzoni, Nessi, Wolfli, Andree, Beer and Oeschger1984), AMS has the advantages of a short measurement time, high accuracy, and small sample size (milligram level or even less) (Nelson et al. Reference Nelson, Korteling and Stott1977; Zhou et al. Reference Zhou, Zhou and Yang1995; Jiang et al. Reference Jiang, Dong and He2012), which provides technical support for the development of geology, oceanography, archeology, nuclear technology, biomedical science, and other fields (Cuang et al. Reference Cuang and Kim2013; Wang et al. Reference Wang, Wang and Hu2013). The sample preparation system needs to meet the requirements of low contamination, high yield, easy operation, high safety (Choe et al. Reference Choe, Song, Lee, Song, Kang, Yun and Kim2013; Yang et al. Reference Yang, Pang and He2015) and must provide high-quality targets for AMS measurement (Zhao et al. Reference Zhao, Yang and Lin2016). As a conventional target for 14C-AMS, graphite has been prepared in many methods, such as the high-temperature and high-pressure method (Bonani et al. Reference Bonani, Balzer, Hofmann, Morenzoni, Nessi, Suter and Wölfli1984; Farwell et al. Reference Farwell, Grootes, Leach and Schmidt1984), pyrolysis (Wand et al. Reference Wand, Gillespie and Hedges1984), and catalytic reduction method (Vogel et al. Reference Vogel, Southon, Nelson and Brown1984). However, the catalytic reduction method is still considered the most convenient and economical method. The catalytic reduction method is mainly divided into the hydrogen reduction method (Vogel et al. Reference Vogel, Southon, Nelson and Brown1984; Marzaioli et al. Reference Marzaioli, Borriello, Passariello, Lubritto, Cesare, D’Onofrio and Terrasi2008), zinc reduction method (Slota et al. Reference Slota, Jull, Linick and Toolin1987; Ertunç et al. Reference Ertunç, Xu, Bryant, Maden, Murray, Currie and Freeman2005), and hydrogenated titanium method (Xu et al. Reference Xu, Trumbore, Zheng, Southon, McDuffee, Luttgen and Liu2007, Reference Xu, Hong and Pong2018; Macario et al. Reference Macario, Alves, Oliveira, Moreira, Chanca, Carvalho, Jou, Oliveira, Pereira and Hammerschlag2016). In the conventional methods of zinc and hydrogen reduction, graphitization is usually achieved through online reduction, which is done via heating in an electric furnace, and the pressure change is monitored using a vacuum gauge. These methods have achieved great success and obtained good results.
To better meet the needs of researchers and explore better experimental methods, a new vacuum line that can prepare 14C samples using different methods, such as zinc, hydrogen, and titanium hydride reduction methods, was developed for GXNU-AMS (He et al. Reference He, Bao and Pang2019; Shen et al. 2021). Herein, the experimental conditions and preliminary results of the 14C sample preparation system are reported.
MATERIALS AND METHODS
14C Sample Preparation Vacuum Line
The layout of the 14C sample vacuum line is shown in Figure 1. The main components include a vacuum pump, CO2 trap, vacuum tube, quartz tube, water trap, valve, and vacuum gauge. According to its functions, the device is divided into a vacuum maintenance unit, CO2 purification unit, and CO2 reduction unit. The entire device adopts quartz glass as the main structural material, which has good vacuum performance and allows the entire experimental process to be observed. In addition, it has the advantages of high-temperature resistance, high hardness, excellent chemical stability, and has a low expansion coefficient. The vacuum maintenance unit uses a double 4-cm diameter vacuum tube, a molecular pump system, and a series of vacuum gauges to ensure high vacuum conditions. The CO2 purification unit includes a CO2 release section (the combustion tube crusher is shown in Figure 2), a CO2 purification section (an alcohol liquid nitrogen cold trap at –90°C and a liquid nitrogen cold trap at –196°C), and a CO2 volumetric calibrated tube. CO2 reduction tubes can be disassembled and used for different reduction methods.
Figure 1 The layout of the 14C sample vacuum line.
Figure 2 The homemade combustion tube crusher.
The Experimental Method
Before sample preparation, the quartz glass tubes and sample preparation tools were placed in a high-temperature furnace at 500°C for 5 hr to remove carbon contamination. Then, a dry organic solid sample (e.g., wood, carbon powder) equivalent to approximately 1 mg of C (the reagents used below are the amount corresponding to 1 mg C) was added to a quartz glass tube (I.D. 8 mm, O.D. 10 mm, length 30 cm) with approximately 20 mg of CuO powder. The quartz tube with samples and reagents was connected to the vacuum system and evacuated to <5.0 × 10–4 mbar. The tube was then sealed with a torch and placed in a muffle furnace at 900°C for 2 hr to fully react the sample and generate CO2. Then, the burned quartz tube was connected to the crushing device of the vacuum line (Figure 2) and evacuated to <5.0 × 10-4 mbar. After the quartz tube was cracked, the CO2 gas first passed through the alcohol liquid nitrogen cold traps at –90°C to thoroughly remove the water vapor and then entered the liquid nitrogen cold trap at –196°C, where it was frozen. Any non-condensable gases, such as SO2, N2, and O2, were pumped away. The purified CO2 was heated and transferred to a known-volume reservoir (with a fixed volume of approximately 23 mL), quantified by measuring the CO2 pressure (1 mg carbon corresponds to approximately 80 mbar), and transferred to a reduction tube containing a catalyst and reducing agent using a liquid nitrogen cold trap, and finally sealed with a torch.
Zn-Fe Catalytic Reduction Method
The reduction process of graphite was carried out in the closed system of the reduction reaction tube, as shown in Figure 3. Before the preparation, 2.5 mg of iron powder was put into the small tube of the reduction tube (I.D. 4 mm, O.D. 6 mm, length: 2 cm), and 35 mg of zinc powder was placed into the large tube of the reduction tube (I.D. 8 mm, O.D. 10 mm, length: 30 cm) as the reducing agent, and then the small tube was slowly placed into the large tube. The iron and zinc powders were placed in a high-temperature oven at 400°C for 2 hr, at ambient air, to remove any carbon contamination. Then, the large tube was connected to the reduction unit to evacuated to <5 × 10–4 mbar. Approximately 1 mg C of CO2 (80 mbar CO2 in the calibrated volume) was frozen into the reaction tube using liquid nitrogen. After being sealed with a torch, the reaction tube (with a volume of approximately 25 mL) was placed inside a muffle furnace at 600°C for 8 hr, whereby graphite forms at the surface of the iron powder in the reduction tube. Finally the graphite and iron powder were pressed into the AMS cathodes for measurement.
Figure 3 Reduction device and process of the Zn/Fe method.
Zn-TiH2/Fe Catalytic Reduction Method
2.5 mg iron powder was placed into the small tube of the quartz device, 35 mg zinc powder, and 15 mg titanium hydride were placed into the large reduction tube as the reducing agent, and a reduction tube with the small tube was connected to the detachable reaction device as shown on the left side of Figure 4, and then the reaction device was connected to the vacuum system to evacuated to <5 × 10–4 mbar. The CO2 was frozen into the reaction device using liquid nitrogen, and the middle valve of the reaction device was closed to prevent outside air from entering the device. Since the reactant in the reduction reaction contains water, which inhibits the entire reduction process, a horizontal quartz tube was designed on the detachable reaction device for water vapor condensation. The reaction furnace includes a heating oven, semiconductor cooler, and control unit, as shown on the right side of Figure 4. The reaction device (a total volume of approximately 50 mL) with CO2 and reducing agents were removed from the vacuum system and then placed inside the reaction furnace at 500°C for 1 hr, and 550°C for 3 hr. The graphite then formed at the surface of the iron powder in the reduction device, while the water condensed in the horizontal tube on the semiconductor cooler.
Figure 4 Reduction device and oven for the H2-Fe and Zn-TiH2/Fe methods.
H2-Fe Catalytic Reduction Method
Similar to the previous section, the small tube with the 2.5 mg catalyst iron powder was slowly placed into the large tube and installed into the detachable reaction device, which was then connected to the vacuum system to evacuated to <5 × 10–4 mbar. The CO2 was frozen into the reaction device using liquid nitrogen, while the H2 gas, quantified by measuring the H2 pressure (two times the volume of CO2 i.e., 160 mbar), was introduced into the reaction tube. Then, the reaction device (approximately 50 mL) with CO2 and H2 gas was removed from the vacuum system and placed inside the reaction furnace at 550°C for 6 hr. Finally, graphite formed at the surface of the iron powder in the reduction device, while the water condensed in the horizontal tube on the semiconductor cooler.
RESULTS AND DISCUSSION
Graphite Recovery Rate
A batch of samples was prepared according to the experimental conditions and methods described previously. The sample descriptions are presented in Table 1. OXII standard sample was obtained from the National Institute of Standards and Technology (NIST). The cellulose sample C3 was obtained from the International Atomic Energy Agency (IAEA). The carbon powder samples were of commercial graphite (CAS#1333-86-4) obtained from Alfa Aesar Co., Ltd. The tree sample was collected from leaves of growing trees and dried after standard acid-alkali-acid treatments (Yang et al. Reference Yang, Pang and He2015; Dumoulin et al. Reference Dumoulin, Comby-Zerbino, Delqué-Količ, Moreau, Caffy, Hain, Perron, Thellier, Setti and Berthier2017).
Table 1 Data of sample recovery rate and AMS measurements.
The preliminary weighed carbon content was recorded as M1 (M1 = actual weighed value × 41% for wood samples). The measured result of the CO2 gas pressure in the calibrated volume was recorded as M2. The mass increment of iron powder before and after graphite reduction was recorded as M3. The graphite recovery rate was obtained according to the formula R = M3/M2 × 100% (Xu et al. Reference Xu, Trumbore, Zheng, Southon, McDuffee, Luttgen and Liu2007, Reference Xu, Hong and Pong2018). Table 1 lists the graphite recovery rates of a series of samples. The average graphite recovery rate of each sample was more than 80%, and the graphite recovery rate of some samples reached more than 90%, proving that the 14C vacuum line can successfully prepare graphite and meet the requirements for AMS measurement. The graphite recovery of the hydrogen method is comparably lower than that of the Zn method, which may be due to the formation of CH4 or H2O by hydrogen during the reaction process (McNichol et al. Reference McNichol, Gagnon, Jones and Osborne1992), or the different vacuum conditions in various types of reactor. Another possible reason for the few low-carbon recovery rates is that a small amount of iron powder was attached to the wall of the small tube during the weighing process.
System Calibration and Stability
The system calibration was performed by evaluating the relationship between the carbon content of commercial graphite (or wood) and the amount of CO2 obtained using the barometer on the calibration volume. As shown in Figure 5, the carbon content of commercial graphite (or wood) presents an excellent linear relationship with the CO2 pressure value, and the correlation coefficients, R2, were 0.99948 and 0.99926, respectively, indicating excellent air-tightness and high stability of the system. A series of standard samples (IAEA C8) were prepared using the Zn-Fe method and measured using GXNU-AMS. As shown in Figure 6, data from 18 independent standard samples showed relatively stable pMC (percentage modern carbon) values.
Figure 5 Relationship between graphite (or wood) content and CO2 pressure.
Figure 6 IAEA-C8 AMS results.
AMS Measurements of the Blank and Standard Samples
According to the established experimental conditions for preparing 14C graphite using the Zn-Fe method, the samples were oxidized, purified, graphitized, and finally measured with AMS. Commercial blank graphite was directly measured to check the background, and the results are shown in Figure 7. The background value of unprocessed commercial graphite was 0.27 ± 0.02 pMC, and the 14C/12C value was 3.14 ± 0.27 ×10–15, equivalent to a 14C age of approximately 47,000 years. The experimental results for the process blank of Zn-Fe method are shown in Figure 8. The background value ranges between 0.49 pMC and 0.62 pMC, and the mean value was 0.55 ± 0.04 pMC. The 14C/12C ratio was 6.47 ± 0.48 × 10–15, equivalent to a 14C age of approximately 40500 years. In addition, three international standard samples, OXII, Chinese sugar carbon (CSC), and IAEA-C8, were prepared and measured using AMS. The experimental results for the oxalic acid standard are shown in Figure 9. The average pMC of oxalic acid was 134.11 ± 0.41 pMC, which is consistent with the recognized standard value of 134.07 pMC within the allowable error. The pMC value of CSC was 136.32 ± 0.48 pMC, which was consistent with the recognized standard value of 136.2 pMC. And the pMC value of IAEA-C8 was 15.17 ± 0.22 pMC as shown in Figure 6, which was also consistent with the recognized standard value of 15.03 pMC.
Figure 7 AMS machine background.
Figure 8 Processed commercial graphite background.
Figure 9 OXII AMS results.
CONCLUSIONS
14C sample preparation occupies a very important position in AMS applications. In this study, a simple, economical, and versatile sample preparation system was designed and constructed, and a batch of samples prepared by the system was checked using GXNU-AMS. The results showed that the average graphite recovery rate of each sample was more than 80%, and the 12C– beam current extracted from the samples was more than 40 μA. The 14C/12C ratio of the process blank was 6.47 ± 0.48 × 10–15, equivalent to a 14C age of approximately 44,000 years. After deducting the machine background of 3.14 ± 0.27 ×10–15, the background induced by the sample preparation process with this system was 2.06 ± 0.55 × 10–15. The preliminary results show that this sample preparation system can meet the requirements of the hydrogen, zinc, and titanium hydride preparation methods. Considering the graphite recovery and the complexity of different methods, we adopted the Zn-Fe protocol as a routine sample preparation method. With this sealed offline reaction method, the daily sample throughput is not limited by the long-time online reaction, which saves time for the overall process of batch sample preparation. The successful development of this sample preparation system provides technical support for the development of 14C dating, tracing, and contributes to the fields of biology, and environmental science.
ACKNOWLEDGMENTS
This work was supported by the Guangxi Natural Science Foundation under Grant Nos. 2017GXNSFFA198016 and 2018JJA110037; the National Natural Science Foundation of China under Grant Nos. 11775057, 11765004, U1867212, 11965003, and 12164006; JSPS KAKENHI under Grant No. 21K18622; and Guilin Science and Technology Foundation Grant No. 20190209-2.
