SAMPLE PREPARATION SYSTEM FOR CARBONATE AND DIC IN WATER AT THE GXNU-AMS LABORATORY

Abstract

A new vacuum line to extract CO₂ from carbonate and dissolved inorganic carbon (DIC) in water was established at Guangxi Normal University. The vacuum line consisted of two main components: a CO₂ bubble circulation region and a CO₂ purification collection region, both of which were made of quartz glass and metal pipelines. To validate its reliability, a series of carbonate samples were prepared using this system. The total recovery rate of CO₂ extraction and graphitization exceeded 80%. Furthermore, the carbon content in calcium carbonate exhibited a linear relationship with the CO₂ pressure within the system, demonstrating its stability and reliability. The system was also employed to prepare and analyze various samples, including calcium carbonate blanks, foraminiferal, shell, groundwater, and subsurface oil-water samples. The accelerator mass spectrometry (AMS) results indicated that the average beam current for ¹²C- in the samples exceeded 40 μA. Additionally, the contamination introduced during the liquid sample preparation process was approximately (1.77 ± 0.57) × 10⁻¹⁴. Overall, the graphitized preparation system for carbonate and DIC in water exhibited high efficiency and recovery, meeting the requirements for samples dating back to approximately 30,000 years.

INTRODUCTION

Given its biological nature and long half-life (5730 yr), radiocarbon is widely used in geological, marine, climatic, environmental, groundwater, and archaeological research. With advances in technology, accelerator mass spectrometry (AMS) has become the most popular analytical tool for ¹⁴C measurement (Nelson et al. 1977; Chen et al. 2019; Shen et al. 2022a, 2023), with advantages such as short measurement times, high accuracy, and small sample sizes (Kutschera et al. 2013; Shen et al. 2012, 2013, 2015, 2019a, 2019b). However, the reliability of graphite preparation methods and sample purification systems, such as the graphite extraction method for carbonate and DIC in water, has been essential for basic research in many ¹⁴C laboratories (Xu et al. 2007; Walker et al. 2008; Bretzler et al. 2011; Bragança et al. 2021; Shen et al. 2022b, 2022c; Wood et al. 2023; Tang et al. 2023). To solve problems related to complicated and expensive water sample preparation procedures, a simple, economic, and versatile graphitization system for carbonate and DIC in water was designed and established. Herein, the experimental conditions and preliminary results for a series of calcium carbonate blank, foraminiferal, shell, groundwater, and underground oil-water samples are reported.

MATERIALS AND METHODS

The CO₂ Extraction Line

The CO₂ extraction line for carbonate and DIC in water is shown in Figure 1. The main components included a vacuum pump, a nitrogen bottle, a circulation pump, a bubbling bottle (acid blowing gas absorption device), two water traps, a CO₂ trap, metal pipes, quartz tubes, valves, and vacuum gauges. The device was divided mainly by function into a CO₂ bubble circulation area and a CO₂ purification collection area. The main vacuum tube in the CO₂ purification collection area was quartz glass, while the CO₂ bubble circulation area was metal piping. Water vapor was removed from the flow gas using an alcohol-liquid nitrogen cold trap, while the produced CO₂ was collected using a liquid nitrogen cold trap.

Figure 1 Layout of the carbonate sample preparation system.

The system’s principle is as follows. An appropriate amount of phosphoric acid (2 mL of phosphoric acid per 10 mg of foraminifera or shells sample and 2 mL of phosphoric acid per 50 mL of water sample) is added to the sample solution to release the carbonate or DIC from the sample in the form of CO₂, which is circulated, purified, and extracted in the preparation system under high purity nitrogen. CO₂ is then reduced into graphite using the Zn–Fe method to complete carbon reduction, which involves the following reactions:

(1) $$HCO_3^ - + {H^ + } = C{O_2} \uparrow + {H_2}O$$

(2) $$C{O_2} + Zn\mathop \to \limits^\Delta CO + ZnO$$

(3) $$CO\matrix{ {\mathop \to \limits^\Delta } \cr {Fe} \cr } C+ C{O_2}$$

Experimental Methods

Pretreatment of the Original Carbonate Samples

The pretreatment of the original carbonate samples, such as foraminiferal and shell samples, was as follows: (1) The samples were dried in an oven at 60°C, weighed, and placed in a 500 mL glass beaker with deionized water until the sediment was entirely dispersed. (2) The samples were sieved with a 240 mesh copper sieve, very fine particles and clay particles less than 0.063 mm in length were removed by washing with deionized water, and the samples on the sieve surface were poured into a funnel lined with filter paper, which was subsequently placed in a 60 °C oven until completely dry. (3) The weighed sample was sieved with 240 mesh and 120 mesh copper sieves, and the sample was divided into two components, i.e., 0.063 mm–0.125 mm particle size and >0.125 mm particle size. The samples of the two particle sizes were weighed and packaged in bags. The sample number, particle size, weight, and other information were subsequently recorded on the bags. (4) Selection of samples and different particle sizes was performed according to the size of the foraminifera based on microscopic examination. If foraminifer shells were abundant, a particle size > 0.125 mm was chosen for the final samples, or both particle sizes were selected. If there were no foraminifera in the sample, the shell fragments were selected. (6) Leaching was performed with 1 mol/L HCl to eliminate meteoric ¹⁴C and other potential contaminants, removing the outermost 10% of the foraminifer surfaces. The samples were washed in an ultrasonic bath with deionized water 3 times for 5 minutes each and dried at 60°C for 12 hours. The dried samples were ground into powder for further processing.

Pretreatment of the Oil-Water and Groundwater Samples

Based on the results of the preexperiment, to obtain a graphite sample of 1∼2 mg, the oil-water sample volume required was ∼300 mL, and the groundwater sample volume required was ∼50 mL. For each water sample, a certain amount of solution was subjected to impurity separation by decanting the solution and filtering it through a 0.45 µm PTFE membrane via vacuum filtration into a 500 mL round-bottom flask. The flask bottle was then attached to a vacuum line for CO₂ extraction and purification.

The CO ₂ Extraction Process

For each foraminiferal or shell sample, approximately 10 mg of sample powder was weighed, mixed with approximately 30 mL of deionized water, and decanted into a 50 mL round-bottom flask. For each water sample, approximately 300 mL of filtered oil-water sample was placed in a 500 mL round-bottom flask, while approximately 50 mL of filtered groundwater sample was placed in a 50 mL round-bottom flask. The flask bottle was then affixed to a bubbler on the vacuum line for CO₂ extraction and purification, as shown below.

1. Before CO₂ extraction, the bubbling circulation line was first flushed with high-purity nitrogen (9 in Figure 1) for a minimum of 5 min (typical pressure value of 1050 mbar) to remove any air CO₂ present in the metal pipeline (9→19→10→20→11→21→12→22 in Figure 1) and the glass line (9→19→17→6→5→14→13→22 in Figure 1). During this procedure, valves 15, 16 and 18 were closed, while adapter 22 was disconnected to vent the nitrogen.

2. After quickly connecting adapter No. 22 and closing nitrogen inlet valve 19, the circulation line was closed and evacuated to a pressure of approximately 800 mbar, after which 2 mL of 85% H₃PO₄ (in the funnel of the upper part of the bubbler) was introduced into the water solution. A portion of the phosphoric acid was left in the glass funnel to prevent outside air from entering the system. At this point, phosphoric acid entered the round-bottom flask and reacted with the carbonate in the water to form CO₂ (see Figure 2).

Figure 2 Photograph of the acidification-blowing device. (1) Phosphoric acid enters the flask and reacts with the sample to produce CO₂. (2) Nitrogen is introduced into the interior of the acidification-blowing device. (3) Nitrogen bubbles through the sample water and strips the dissolved gases to the sample headspace. (4) CO₂ flows outside the acidification-blowing device.

3. When the two cold traps (5 and 13 in Figure 1) at −90°C and one nitrogen trap (6 in Figure 1) at −196°C were in place and all the valves in the circulation loop were open, a recirculating diaphragm pump (10 in Figure 1) was turned on, forcing the carrier gas through the heated flask at 60°C (12 in Figure 1) and producing a stream of fine bubbles throughout the solution. The CO₂ gas first passed through the two cold traps at −90°C to thoroughly remove water vapor and then entered the cold trap at −196°C, where it was frozen. After 5 min of circulation, the pump was shut off, and the N₂ carrier and other impurity gases, such as SO₂ and O₂, were slowly removed.

4. The purified CO₂ was heated and transferred to a known-volume reservoir (7 in Figure 1, with a fixed volume of approximately 23 mL), quantified by measuring the CO₂ pressure (1 mg of carbon corresponds to approximately 80 mbar), transferred to a reduction tube containing 2.5 mg of catalyst Fe (Aldrich, 325 mesh, #209309) and 25 mg of reducing agent Zn (Aldrich, <150 μm, #324930) using a liquid nitrogen cold trap (8), and sealed with a torch.

Zn-Fe Catalytic Reduction Process

The reduction of graphite was carried out in a closed system consisting of reduction reaction tubes (Xu et al. 2007; Shen et al. 2022c), as shown in Figure 3. Iron powder (2.5 mg) was placed in a small glass tube (the reagents used below were generated from 1 mg of carbon), 25 mg of zinc powder was placed in the reduction tube as a reductant, and the small glass tube was slowly placed on the bottom of the reduction tube. The reduction tube with zinc powder and iron powder was placed in a high-temperature oven at 400°C in an air environment for 2 hours to remove carbon contamination from the reduction tube. The treated reduction tube was then connected to the reduction unit to evacuate to below 5 × 10⁻⁴ mbar, after which CO₂ was transferred to the reduction tube using liquid nitrogen and the tube was finally sealed with a torch. Then, the reaction tube was placed in a graphite furnace at 600°C for 8 hours, which resulted in the formation of graphite at the surface of the iron powder in the reduction tube. Finally, the graphite and iron powder were pressed into the AMS cathode for measurement.

Figure 3 Reduction device and Zn/Fe method procedure. (a) Schematic showing the setup after the reduction reaction. (b) A physical image after the reduction reaction. (c) A photograph of the reduction tube before the reduction reaction.

RESULTS AND DISCUSSION

Graphite Recovery Rate

According to the experimental conditions and methods described previously, a batch of blank and foraminiferal samples was prepared using the new preparation system. The sample descriptions are presented in Table 1. Commercial calcium carbonate (analytical grade, 99.99%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Standard carbonate C1 was obtained from the International Atomic Energy Agency (IAEA). Foraminifera and shell samples were provided by the South China Sea Monitoring Center of the State Oceanic Administration.

Table 1 Sample recovery rate data.

The preliminary weighed carbon content was recorded as M1 (M1 = actual weight value for calcium carbonate samples × 12%). The calculated carbon mass of the CO₂ gas obtained using the barometer (4 in Figure 1) on the known-volume reservoir (7 in Figure 1) was recorded as M2. The increase in mass of the iron powder before and after graphite reduction was recorded as M3. The graphite recovery rate was obtained using the formula R = M3/M1 × 100%. The results showed that the average graphite recovery rate of each sample was above 80%, with some samples showing more than 90% recovery (see Table 1). The ¹⁴C carbonate graphitization preparation system could successfully recover carbon from carbonate samples and DIC in water to meet the requirements for AMS measurements. The relatively low graphite recoveries of foraminifera and shells compared to those of commercial calcium carbonate were related to the purity of the actual carbonate samples. The recovery rate of some samples was less than 70%, which may have been due to the different vacuum conditions of the reactor.

System Calibration and Stability

System calibration was performed by evaluating the relationship between the carbon content of commercial carbonate (or foraminifera) and the amount of CO₂ obtained via the barometer on the known-volume reservoir. As shown in Figure 4, the carbon content of calcium carbonate (or foraminifera) showed a good linear fit with the barometric pressure values, with correlation coefficients of R² = 0.98958 and R² = 0.98508, respectively, indicating good gas tightness and high stability of the system. Standard samples (IAEA C1) were prepared using the system and measured with GXNU-AMS, and the data from the standard samples showed relatively stable pMC values, as shown in Table 2 and Figure 5.

Figure 4 Relationship between calcium carbonate (or foraminifera) content and CO₂ pressure.

Table 2 AMS measurement results for IAEA-C1.

Figure 5 AMS results for the machine background (a) and process blank of IAEA-C1 (b).

AMS Measurements of the Blank, Carbonate, and DIC in Water

According to the established experimental conditions, a series of samples were acidified, purified, graphitized, and finally measured via AMS. Commercial blank graphite was directly measured to check the machine background, and the results are shown in Figure 5 (a). 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 background conditions of the graphitization preparation system for carbonate and DIC in water were examined using IAEA-C1 standard samples, and the results are shown in Figure 5(b) and Table 2, with background values of 1.34∼2.16 pMC and a mean value of 1.77 ± 0.25 pMC. The mean ¹⁴C/¹²C value of the process blank was 2.09 ± 0.30 × 10⁻¹⁴, equivalent to a ¹⁴C age of approximately 30,000 years. After subtracting the machine background of (3.14 ± 0.27) × 10⁻¹⁵, the background induced by the sample preparation process was determined to be (1.77 ± 0.57) × 10⁻¹⁴, which was most likely introduced by the use of deionized water.

In addition, foraminifera, shell, and groundwater samples were prepared and measured using AMS. The AMS results in Table 3 show that the ¹⁴C ages of foraminifera and shells ranged from 525∼23200 yr BP, and the sample formation time and geological level of each batch exhibited a linear relationship. However, some ¹⁴C ages of foraminifera exceeded the sensitivity level of the sample preparation system: 30,000 years. The ¹⁴C ages of the groundwater samples ranged from 1175∼12080 yr BP, and the sample preparation system fully met the measurement requirements for groundwater in the Xinjiang region, China.

Table 3 AMS measurement results for foraminifera and shells.

To explore the geological conditions and improve oilfield recovery in the Sinopec Zhongyuan Oilfield in China, ¹⁴C crosswell tracer monitoring technology was applied to monitor the development status and characterize the heterogeneity of oil reservoirs (Shen et al. 2022b). After the injection of ¹⁴C-labeled urea capsules into the water well, the tracer response in the production well was tracked, and the water drive speed, porosity, permeability, and average pore-throat radius were obtained. The measurement results showed that the ¹⁴C concentration of the oil-water samples could accurately reflect the peak tracer concentration, as shown in Table 4. This indicates that the ¹⁴C-AMS technique is a promising analytical method for evaluating the underground characteristics of remaining reservoirs and providing crucial data support for the mid- to late-stage oilfield recovery process.

Table 4 AMS measurement results for oil-water samples.

^a Sample collection time after ¹⁴C tracer injection.

CONCLUSIONS

In this study, a simple, economic and versatile graphitization system for carbonate samples and DIC in water was designed and constructed, and a series of samples were prepared by the system and measured with GXNU-AMS. The results showed that the recoveries of the carbonate samples ranged from 70% to 95%, depending on sample type. The ¹²C^– beam current extracted from the samples was more than 40 μA. The sample preparation system and associated reagents introduced a process blank background of ¹⁴C/¹²C = (2.09 ± 0.30) × 10⁻¹⁴, equivalent to a ¹⁴C age of approximately 30,000 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 determined to be (1.77 ± 0.57) × 10⁻¹⁴. The preliminary results showed that the sample preparation system can meet the requirements for carbon content and 0.5–1 mg carbonate sample preparation, with a high graphite reduction rate, and can meet both carbonate sample and DIC requirements in water for sample ages less than 30,000 years. The successful development of this preparation system provides technical support for the use of GXNU-AMS in biology, geology, and environmental science.