INTRODUCTION
In 2016, the first accelerator mass spectrometry (AMS) facility in Türkiye (Turkey) was established at The Scientific and Technological Research Council of Türkiye (TÜBİTAK) Marmara Research Center (MRC) Earth and Marine Science Institute (EMSI). The laboratory is called (Türkiye) National 1 MV Accelerator Mass Spectrometry Laboratory (Doğan et al. Reference Doğan, İlkmen and Kulak2021). The establishment of the laboratory was financed by national resources from the Ministry of Development, Republic of Türkiye. Since 2016 the laboratory is providing international commercial radiocarbon (14C) analysis. In this respect, it is one of only three AMS laboratories in the Middle East and Balkans (Regev et al. Reference Regev, Steier, Shachar, Mintz, Wild, Kutschera and Boaretto2017; Sava et al. Reference Sava, Simion, Gâza, Stanciu, Păceșilă, Sava, Wacker, Ştefan, Moşu, Ghiţă and Vasiliu2019). Considering the demand from archeology, earth and environmental sciences, as well as for forensic investigations in Türkiye and the wider region, Türkiye’s first and only AMS laboratory is aiming to provide services to meet this demand as well as fulfilling international requests.
The AMS system at the TÜBİTAK-MRC-EMSI laboratory is based on a unique design of a 1 MV Pelletron tandem accelerator, commercially named as the Universal AMS system developed by National Electrostatic Corporation (NEC). This is a very similar AMS system to that used in New Zealand (Zondervan et al. Reference Zondervan, Hauser, Kaiser, Kitchen, Turnbull and West2015) with a higher energy accelerator; the 1 MV tandem pelletron.
In addition to the development in AMS technologies and its spread globally as the most commonly used system for 14C analysis, the number of studies towards developing and improving sample preparation protocols and methods has also increased (Dunbar et al. Reference Dunbar, Cook, Naysmith, Tripney and Xu2016; Kutschera Reference Kutschera2016). Detailed tests and developments of the methods prior to AMS measurement have been examined with the aim of producing more accurate and reliable results (Brock et al. Reference Brock, Higham and Bronk Ramsey2010a). Although AMS systems provide high precision during the measurement phase, all of the pre-AMS sample preparation protocols can have a direct effect on the results. Therefore, sample preparation methods and techniques can be considered to be as important as the actual measurement(s) by AMS within 14C analysis (Aerts-Bijma et al. Reference Aerts-Bijma, Paul, Dee, Palstra and Meijer2020; Salazar and Szidat Reference Salazar and Szidat2021).
The installation, commissioning, and acceptance of the TÜBİTAK 1MV AMS system was completed in December 2015. Trial experiments for sample preparation pretreatment methods and method validation started one year earlier, at the end of 2014. These test examinations with a range of standards and inter-comparison samples continued until the summer of 2016.
Commercial testing and analysis activities commenced in autumn 2016 as part of the industrial services (i.e., test and analysis services) of TÜBİTAK-MRC-EMSI. The 14C analysis service includes sample evaluation for 14C analysis, wet chemistry of sample preparation, graphite production, AMS measurement, data supervision, and calendar date calculations. For the users who are not familiar with interpreting 14C analysis reports support is provided to explain the report format to those researchers.
METHODS
Radiocarbon Analysis Process in the Laboratory
Acceptance of Samples
All samples with natural, or below, levels of 14C are accepted by the laboratory for analysis within the research scopes and agendas of the previously mentioned application fields (archaeology, forensic sciences, etc.). Researchers in the fields of forensic science, earth sciences, environmental science, and archeology are among the most common researchers/research groups submitting material for 14C analysis. Archeological samples and cultural and natural assets are only accepted through divisions of the Ministry of Culture and Tourism due to Law No. 2863 on the Protection of Cultural and Natural Heritage in Türkiye. The Marmara Research Center and the General Directorate of Cultural Heritage and Museums have a protocol that outlines the conditions of the 14C analysis process, from sampling in the field to the final reporting of the results. Samples from universities, institutes, businesses, or researchers are accepted for analysis if they fall under the purview of earth sciences, environmental science, or other content that is not governed by Law No. 2863. The laboratory also accepts submissions from other countries for 14C testing and analysis. National and international certified auditors have accredited and audited the laboratory for ISO 9001 and 14001 quality assurance systems.
Screening of Samples
Samples submitted to the laboratory are accepted for 14C analysis following completion of the proper and official application forms (downloadable at https://mam.tubitak.gov.tr/tr/icerik/basvuru-dilekcesi). The sample submission form is to be filled by those submitting the sample(s) with information such as expected age (chronological period), any processes previously applied to the material/sample, and the presence of any possible contaminants (such as glues, paints, varnishes, preserving organics, etc.). This information is very important for the planning of the analysis with regards to sample preparations as well as AMS measurements. After the samples are accepted for 14C analysis in the laboratory, screening of each sample is done by the laboratory manager. Brief primary evaluation information and options for analysis is given to the applicants, especially for special materials. This process will be outlined for each sample type in the following sections of this paper. Screening and inspection of the samples to evaluate conformity of the sample materials may also include tests such as elemental analysis, inspection using a microscope, gas chromatography, or mass spectrometry (MS) depending on the nature of the material. If a specific compound, such as a bio-based material are requested for 14C analysis, substances can be identified with MS. The method for the analysis is determined by the laboratory manager once a scientific evaluation of the samples’ conformance for the 14C analysis is performed. If different materials are present in the samples, the screening process also comprises selecting the material to actually be subjected to analysis.
Sample Preparation Methods
For the past 5 years, numerous types of samples have been subjected to 14C analysis in the TÜBİTAK-MRC-EMSI laboratory. Depending on the nature of the samples, routine sample preparation methods (for materials such as charcoal, bones, shells, etc.) or specific ones (mortar, beach rock, etc.) are carried out in the laboratory. All sample and pretreatment protocols are applied according to the procedures described and defined in laboratory quality assurance documents that are used in-house by the laboratory staff. In addition, all samples are photographed in several stages and the applied pretreatment process is recorded step by step in the analysis protocol sheets. These documents are archived in the laboratory’s records for the purpose of providing legacy data. If a sample contains multiple different types of materials, the dissociated materials are recorded on separate sheets and documented separately.
Each step of the procedures is controlled with blank 14C-free materials, or laboratory standards, to allow for background monitoring and the detection of any contamination in the laboratory. This enables us to determine whether there is contamination from any of the steps in the process, including wet chemistry and sample pretreatment, graphitization, cathode pressing, and AMS background. The primary standards used for quality control of 14C analysis include 14C free coal, bones, shells, archeological materials with known ages like seeds, phthalic anhydride (Sigma Aldrich Product number 1001697447), IAEA C1-C9 reference standard materials, and 14C free graphite powder with 100 mesh and a purity of 99.9995% (Alfa Aesar LOT: Q21A015). These reference materials are prepared along with 24 samples with unknown dates in accordance with the types of samples to be examined. Wet chemistry control standards include a number of IAEA reference standard materials, 14C free coal, bones, shells, and age known archaeological materials such as seeds. Phthalic anhydride (Sigma Aldrich Product number 1001697447) and other IAEA reference standard materials are used for graphitization quality control. AMS background and cathode pressing process background quality controls are measured with the graphite powder procured commercially (Alfa Aesar LOT: Q21A015), with and without iron powder (Alfa Aesar LOT: 10187554). In addition to these control parameters, reference standard materials of known age are graphitized and measured to verify the accuracy of the overall 14C analysis. All borosilicate and Pyrex glassware are heated to 500°C for a minimum of two hours. Most of the materials used during the analysis are for single-use only, and preferably the same brand and model are used, that have been tested for 14C analysis suitability. The temperature and dew point are monitored and controlled (≤ ±1°C) in the sample preparation room as well as in AMS system room. All of the laboratory equipment is dedicated for the use of 14C analysis with natural 14C levels.
Wet Chemistry Procedures According to Sample Types
Natural samples that come to the laboratory for 14C dating usually contain various contaminants. It is well known that these contaminants affect the final measurement result of the 14C analysis and, consequently, the date calculated. Depending on the sample’s composition, different preparation techniques are used to remove these contaminants. This section explains these steps in relation to the types of samples and the corresponding method employed in the lab. The most frequent protocols are described, and these protocols can be modified based on the unique characteristics of the samples. The majority of pretreatment is performed in single-use glass containers that have previously been baked at 500ºC for 2 hr to remove organic pollutants. Generally, sterile 50-mL polypropylene conical tubes (Falcon® a Corning Brand) are used for large volume samples such as soil, bone, etc. The majority of wet chemistry sample preparation steps are carried out in a fume hood or ventilator. Milli-Q® grade ultrapure, reverse osmosis water is used throughout all sample preparation procedures for rinsing, washing samples, preparation of solutions. The quality of this ultrapure water is monitored and tested to make sure that it is free of carbon. Analytical grade chemical reagents are used in the sample preparation procedures. Only a minimum amount of stock solutions is prepared to prevent any possible contamination from storage in the laboratory. Stock solution quality is regularly checked to observe for any suspended material in the solution. Alkali solutions are examined for any precipitation in and out of the bottle. If any of these contaminants are observed, these solutions are not used in the pretreatment chemical processes.
Charcoal, Plant Fossils, Seeds, Wood
For charcoal, charred fossils, seeds, wood, or similar organic material samples, an acid-base-acid (ABA) treatment is the main procedure applied to eliminate contaminant carbon (Mook and Streurman Reference Mook and Streurman1983). The application of the procedures slightly differs between 14C laboratories in terms of the molarity of acid and base, the temperature, and duration of the steps, etc. Nevertheless, the main principle behind this procedure is to remove carbonate in a first acid washing step, then the removal of humic acid using a base wash, and then a final acid wash to remove CO2 that may be captured from the air during the base wash step. Fulvic acid is also removed during both acid wash steps. Although this is a routine and basic procedure, each sample must be observed throughout the process and the behavior of the sample must be well understood with regards to the chemical reactions Involved at each step of the procedure. Before ABA treatment, other physical and chemical cleaning might be necessary; for example, if the sample material is covered in soil, or has been varnished, or is covered/coated by other detectable contaminants. Washing in ultra-pure water (with or without weak sonication), wet or dry sieving, and/or organic solvent washing in Soxhlet are some of the procedures that can be used before commencing with the ABA treatment.
The first acid wash involves applying the sample to 1M HCl at 70ºC for 30 min. If CO2 or bubbling is observed to continue then this step is repeated until all bubbling ceases. Base washing starts with subjecting the sample to a 1M NaOH wash at 70ºC for 30 min, and this is repeated until the solution becomes colorless. The third, and final, acid wash again involves subjecting the sample to 1M HCl at 70ºC for 30 min.
If there is only a small and/or limited amount of sample material then the base wash step must be planned and applied very carefully. In this case a weak base of 0.1M NaOH can be attempted, or the base wash can be omitted completely, and just an acid wash can be performed.
A routine ABA procedure begins with weighing out 10 mg of the sample. As the acid and base washes change the pH of the sample, it is rinsed thoroughly with ultrapure water after the completion of the ABA procedure until the supernatant solution reaches a neutral pH. The pH is checked with litmus paper from supernatant to make sure that neutral pH values are attained. Once the whole ABA procedure has been completed, the samples are dried overnight at 70ºC. In some cases, the treated and dried samples are inspected with a stereo microscope to examine if any visible contaminants remain.
Sediments
Sediment is one of the most challenging sample types for 14C analysis because of its complex geochemical content. Sediment may contain multiple types of materials such as non-carbonate macrofossils or macrofossils with carbonate content, pollens, bulk organic sediment, humic acid, and humin microfossils (Dalsgaard and Odgaard Reference Dalsgaard and Odgaard2001; Kristiansen et al. Reference Kristiansen, Dalsgaard, Holst, Aaby and Heinemeier2003; Molnár et al. Reference Molnár, Joó, Barczi, Szántó, Futó, Palcsu and Rinyu2004).
All sediment samples are screened in the laboratory for the 14C analysis options unless the researchers submitting the sample have requested a specific method during their submission. Sample submitters are informed about the findings after screening of the samples in terms of the 14C material types present in them. Macrofossils and bulk organic sediment, namely acid insoluble organic matter, in the sample are the widely preferred materials for 14C analysis by users. Pollen extraction is not done in the laboratory, but we collaborate with experienced university laboratories in Türkiye to perform this stage of the screening and analysis. Depending on the request, therefore, pollen extraction and 14C analysis can be provided by the laboratory. If macrofossils are recovered from the sediment to be analyzed, the procedure described above is chosen. For bulk organic sediment, the procedure involves taking a 1-g sediment sample and washing it in 0.5M HCl at 80ºC for 1-hr periods until all carbonates are removed. Acid insoluble organic parts of the sediment are neutralized by rinsing with ultrapure water. The sample is sieved and then dried overnight in an oven at 60ºC. Elemental analysis of the bulk sediment sample after the acid wash step is carried out to determine the necessary amount of sample for the final 1 mg carbon yield. If the carbon content is very low (less than 0.5%) customers are informed and asked if whether they wish to continue with the analysis, or not.
Bone, Tooth, Antler
The standard pretreatment procedure for bone begins with physical cleaning including washing with a brush, ultrapure water, and in an ultrasonic bath until no visible dust or dirt remains. After washing is completed, the sample is dried and further cleaned with a sandblaster or using a sanding attachment on a Dremel. Then 1 g is cut and evaluated further for the quality of the physical cleaning process. If acceptable, the 1 mg of cleaned bone is analyzed for its C:N elemental composition. A checklist parameter is regimented for collagen presence; see Table 1. For research purposes, even if the conditions given in Table 1 are not met then the bone collagen extraction procedure may be attempted if there is no other alternative material provided from the applicant. The aim of attempting this is to better understand, and test, the degree to which the criteria in Table 1 accurately reflect the presence of collagen in the bone sample. So far, our findings demonstrate that the parameters listed in Table 1 seem to be mostly consistent and accurate. For collagen extraction, bulk collagen extraction (including those using ultrafiltration) procedures can be performed in laboratory. Ultrafiltration is mostly commonly used in the TÜBİTAK-MRC-EMSI laboratory unless the customer requests otherwise, or in special cases where there is a small/limited amount of sample.
A checklist of quality control parameters for checking collagen presence in bone samples.
* From our experience %N is the most indicative parameter.
The protocol to extract collagen from bone begins with crushing and grinding 1 g of bone. The ground samples are treated with 0.5M HCl at 4ºC for 3–5 days to dissolve and remove the mineral part of the bone that contains carbonate. A base wash using 0.1M NaOH at room temperature for 30 min may be applied depending on the color of the sample at this stage. If this step is skipped and the quality of the obtained collagen is poor/unacceptable the collagen extraction procedure is repeated with this step included. Demineralized samples are rinsed with ultrapure water followed by heating in 0.01M HCl at 80ºC for 8 hr and left to cool for 7 hr, which is often an overnight process. In the morning Millipore ultrafiltration tubes are cleaned following the procedure outlined by Brock et al. (Reference Brock, Bronk Ramsey and Higham2007). After cleaning is completed, the gelatinized solution is filtered using the cleaned ultrafiltration tubes; centrifuged at 4000 rpm for >20 min to collect the long chain molecule fraction (>30 kD). The filtered gelatin product is transferred to glass test tubes and frozen immediately by immersion in liquid nitrogen, and then freeze dried for a day or longer (Hajdas et al. Reference Hajdas, Michczyński, Bonani, Wacker and Furrer2009). The quality of the obtained collagen is always tested using the established and accepted collagen quality criteria of yield, C%, N%, and C:N ratio (DeNiro Reference DeNiro1985; van Klinken Reference van Klinken1999; Brock et al. Reference Brock, Geoghegan, Thomas, Jurkschat and Higham2013). These quality control parameters are given in Table 2.
Quality control parameters for extracted collagen (after DeNiro Reference DeNiro1985; van Klinken Reference van Klinken1999).
Charred Bones
Charred bones usually look like a piece of charcoal. Carbon and nitrogen content of the charred bone sample is checked using elemental analysis before beginning treatment. Usually, collagen is not expected to be conserved in fully charred bone samples. Elemental analysis results are used to confirm the C:N composition of this kind of sample. An ABA procedure is applied to fully charred bones. Apatite or contaminated carbonate of bone is eliminated with the first acid wash. The base treatment must be carried out very carefully because of the possibility of easily losing charred material.
Bone Apatite
Although collagen is the preferred osteological component for 14C dating bones, researchers occasionally ask to know the dates of poorly preserved bones which often contain only bone carbonate and no other organic residue. Bone is known to contain carbonate in its mineral structure however this structure is prone to diagenesis as it easily absorbs other carbonates from environment. These secondary diagenetic carbonates cause deviations of the sample’s 14C age. Methods have been suggested to remove the secondary diagenetic carbonates including treatment with diluted acetic acid in a vacuum (Lanting et al. Reference Lanting, Aerts-Bijma and van der Plicht2001; Cherkinsky Reference Cherkinsky2009). This method is applied at the TÜBİTAK-MRC-EMSI laboratory if requested by researchers submitting the material(s), usually when there is no alternative material for 14C dating. After physical cleaning as described previously in the procedure for bone, the samples are treated with diluted acetic acid in vacuum as described by Cherkinsky (Reference Cherkinsky2009). Once the pretreatment procedure has been completed, the final pieces of bone are prepared as graphite, as explained later.
Tooth, Enamel
The dentine structure, which contains collagen proteins, is most commonly analyzed for 14C dating when the submitted material is a tooth sample. This method produces compatible results with those from the bone collagen extraction procedure (Brock et al. Reference Brock, Higham, Ditchfield and Bronk Ramsey2010b; Hajdas et al. Reference Hajdas, Michczyński, Bonani, Wacker and Furrer2009). Tooth enamel is not used for archeological 14C dating. However, tooth enamel can be used for forensic purposes in the laboratory, as described by Spalding et al. (Reference Spalding, Buchholz, Bergman, Druid and Frisen2005) and Ubelaker et al. (Reference Ubelaker, Buchholz and Stewart2006).
Wood, Paper
Cellulose extraction procedures (see Němec et al. Reference Němec, Wacker, Hajdas and Gäggeler2010), or the ABA procedure (as explained previously) are applied to wood and paper samples. To better understand the results of 14C dating, the ABA and cellulose extraction methods are both performed on some wood samples. Cellulose can yield more reliable 14C results, particularly if the organic carbon structure of the wood samples has been affected.
Shell, Other Carbonate Fossils
Depending on the conditions of the coral, shell, or other carbonate samples they are first physically cleaned. During physical cleaning, the surfaces of the samples are scraped with a Dremel and then roughly cut before cleaning in MilliQ water using an ultrasonic bath. Samples are then acid etched with 0.01 M HCl to remove carbonates that have possibly recrystallized on the surface of the samples. A 20–25% loss of samples is usually expected during the etching process. After samples have undergone acid etching, they are rinsed with MilliQ water, dried, and weighed. Next, CO2 is produced in a CHS inlet, and then the graphitization—the conversion of CO2 to elemental carbon—takes place in an AGE III.
Groundwater
Dissolved inorganic carbon is most commonly analyzed for the 14C analysis of the groundwater (Kalin Reference Kalin, Cook and Herczeg2000). Procedures for sample collection and transport to the laboratory are described to the researchers requesting this analysis. Pretreated 100–200-mL vials are provided and it is requested that the sampling is conducted without atmospheric contamination. it is suggested that the vials are rinsed with groundwater before collecting the actual sample. Silicon aluminum Septa which are provided with the vials are required to be locked as soon as the sample is collected. Once in the laboratory, the samples are transferred to argon or helium filled inert glove bags and prepared for the CO2 production process in these glove bags. Usually 100–150 mL of the groundwater sample is transferred to 200-mL vials, which is locked with silicon aluminum septa in this inert medium. Depending on the condition of the sample, if suspended solids or some other contaminations are observed, the groundwater samples are filtered using a glass particle filter with a pore size of approximately 60–80 micrometers. The dissolved inorganic carbon is prepared for graphite as explained below under “Graphite Production with Acid Reduction.”
Residues on Pottery Fragments
This type of sample is rarely requested, but the procedure in the laboratory involves collecting residues on pottery fragments using a small and sharp knife. As other studies have suggested, and from our experience, an acid treatment is more convenient for residues on pottery fragments. A base wash may cause significant loss of valuable material to be analyzed for 14C dating (Brock et al. Reference Brock, Higham and Bronk Ramsey2010a). Samples are kept in 1M HCl for 1–2 hr at room temperature followed by ultra-sonication for 10–20 min. This is followed by rinsing with ultrapure water in ultrasonic bath for 5–10 min and the sample continues to be rinsed until a neutral pH value of the supernatant solution is achieved. Samples are then freeze dried before the conversion of CO2 process (Brock et al. Reference Brock, Higham and Bronk Ramsey2010a).
Other Sample Types
Theoretically, since any organic or carbon containing material can be analyzed for 14C content, our laboratory is open to the application of any other sample types not previously described above. Methods explained in the literature and/or experimental methods can be attempted in the laboratory as a part of testing and analysis services or projects.
To prevent cross-contamination and keep 14C background at natural levels, samples containing 14C levels higher than natural abundance ratios are not accepted to be studied or analyzed in the laboratory.
Removal of Organic Preservation Contaminants
Some samples contain organic materials used for the purpose of preservation, especially artefacts from museums or storage facilities. Depending on the nature of the preservation materials used, pretreatment methods can be employed prior to the commencement of the standard pretreatment processes explained above. An acetone-methanol-chloroform pretreatment is usually employed, as described by Brock et al. (Reference Brock, Dee, Hughes, Snoeck, Staff and Bronk Ramsey2018). If there is no limitation on the size/amount of sample, other tests to determine the nature of the preservation materials can be advised and performed for researchers. In this case, a specifically determined and appropriate removal process is applied, after the chemical structure of the preservation materials has been identified.
CO2 Production and Graphitization
The basic chemistry of this method includes (1) CO2 production and purification from organic samples and (2) reduction of CO2 to graphite in the presence of iron powder.
For this purpose, two pieces of equipment are present in the laboratory. An AGE III (Ionplus AG, Switzerland) is used for routine CO2 and graphite production between 0.2 mg and 1 mg. There is also a manual graphitization equipment system composed of a muffle furnace and a high vacuum glass line.
Graphite Production with Combustion Oxidation
The Automated Graphitization Equipment (AGE) produces graphite by reducing the CO2 from sample combustion in the coupled EA with hydrogen on an iron catalyst (Wacker et al. Reference Wacker, Němec and Bourquin2010). This method also enables the measurement of the elemental composition of carbon and nitrogen during graphite production. For samples of unknown carbon content (for example, bulk sediment samples) elemental analysis can be performed prior to graphite production. The AGE III equipment makes it possible to produce 21 graphite pieces in a daily operation which is very big advantage for a high capacity 14C dating laboratory. The AGE III allows the TÜBİTAK-MRC-EMSI laboratory to produce, annually, approximately 2000 samples and necessary associated reference materials for AMS measurement.
The high vacuum glass line is also used to manually produce graphite in the laboratory; for example, when the capacity of the AGE is at its maximum. Typical vacuum value is 1 × 10–5 torr, however, below 3 × 10–6 torr can also be obtained. This high vacuum property provides very low background values for graphitization. Before the samples are burned, they are prepared in quartz tubes to ensure maximum combustion. These quartz tubes are evacuated in a separate vacuum line and sealed using an oxygen propane burner (Doğan et al. Reference Doğan, İlkmen and Kulak2021).
Prepared quartz reactors are burned in an ash furnace at 900ºC for 2 hr, oxidizing the organic sample to carbon dioxide. Then the reactor containing the carbon dioxide is connected to the vacuum line. Carbon dioxide is trapped using liquid nitrogen. The trapped carbon dioxide is transferred to reactors containing iron powder. The amount of collected gas is calculated using gas pressure sensors and a sufficient amount of hydrogen is added to this reactor. The prepared reactor is connected to special furnaces for graphite production. This system allows us to convert CO2 to elemental carbon in amounts ranging from tens of micrograms to 3–4 mg of graphite.
Graphite Production with Acid Reduction
Carbonate natural samples to be analyzed for 14C are reduced to carbon dioxide with phosphoric acid. Following the completion of the sample preparation procedures of shell or bone apatite, the samples are transferred into exetainer® vials. The AGE system also has a CHS inlet in the laboratory. This system is used to produce graphite from carbonate and groundwater samples. These exetainer® vials are sealed and the headspace exchanged with helium gas using an IonPlus CHS (Wacker et al. Reference Wacker, Fülöp, Hajdas, Molnár and Rethemeyer2013). Orthophosphoric acid (1–2 mL 85% v/v) is injected through the septa and the samples are heated at 70ºC until CO2 evolution has ceased. The CO2 generated during the acid hydrolysis is transferred to the AGE III graphitization system in a stream of helium. AGE III together with the CHS provides information about the amount of CO2 collected and the graphite produced. This is very useful for providing the benchmark for yield of CO2 production and makes it possible to replan graphite production to specifically requested amounts. This provides advantages, especially for carbonate containing samples such as sediments and groundwater.
AMS Measurement
It has been determined that the typical background of the TÜBİTAK 1MV AMS with Pelletron tandem accelerator is approximately 0.226 pMC, or roughly 49,000 years. Modern standard reference material OXII generally has a 0.113% uncertainty. For the analyzed samples, there is an uncertainty of 30 years for 14C ages from Modern to 6000 years, and an uncertainty of 60 years for 14C ages from 6000 to 15,000 years. The TÜBİTAK 1MV AMS system is equipped with a MC SNICS ion source which has capacity for 40 cathodes. We usually measure a batch of 24 unknown samples and 16 reference materials in a 24-hr operation. The graphite of the standard reference materials is graphite powder (100 mesh with a purity of 99.9995% procured from Alfa Aesar LOT: 10187554), a processed blank (phthalic anhydride, Sigma Aldrich Product number 1001697447), two IAEA 14C reference materials, and NIST OXII. These are all included in each batch in addition to the unknown samples. The low AMS system background is verified with the graphite powder cathode where 14C/12C is averaged to 6.85 × 10−16. Through the wheel-shaped cathode holder, cathodes are distributed evenly. Before beginning the AMS measurement, the system is tuned using the processed blank, and 13C/12C and 14C/12C ratios are confirmed for the cathodes of the processed blank, graphite powder, and OXII. Each cathode is measured for 8 cycles which lasts for a total of 24 min of collection time.
After measurement is completed in AMS, abc software that is a part of the AMS system is used to normalize measured 14C/12C ratios of unknown samples and quality control reference standards. OXII and the processed blank is used for the normalization process. The AMS delta 13C values are used for fractionation correction. IAEA C1-C9 are available and depending on the properties of the unknown samples different standards are chosen. C7 and C8 are the most commonly used ones for organic samples while C1 and C2 are used for carbonates. The other IAEA standards are also used if necessary. For quality control purposes, measured δ13C values and pMC are compared with consensus values.
Standards and SIRI Data
In every AMS measurement, the quality control of the AGE III and AMS background is checked using the processed blank and phthalic anhydride (Sigma Aldrich, Product number 1001697447) which is combusted and graphitized. The processed blanks are the basic control parameter to monitor 14C contamination and background levels in the laboratory. The average value for this blank material for the past five years is 0.255 pMC. The oldest dates that can be achieved are, on average, 48,000 years old. The samples that have a measured pMC value of less than 0.255, in other words, older than 48,000 years are not assigned a normalized date; instead, they are simply reported as being older than 48,000 years. These are valid samples that were processed for graphite using the elemental analyzer inlet of the AGE III. The background age for this purpose may vary and is thus recalculated with each AMS run. Therefore, the pMC value of the processed blank may vary in every AMS operation. The pMC value of the graphite generated for the blank samples by introducing it through the carbonate handling system’s inlet is lower. For the IAEA’s C1 standard reference material, it is 0.136 pMC or less. Carbonate samples can be dated back to 50,000 years or more.
Table 3 gives the IAEA standard reference materials used for obtaining the preliminary measured values during the acceptance tests in 2015. The IAEA C1 standard graphite processing has been enhanced in the last five years to be closer to the consensus value of 0.00–0.05 pMC by reducing contamination from the CHS interface. When the IAEA C9 standard is treated with acid base acid (ABA) before measurement, the result is 0.22 ± 0.03 pMC.
The IAEA standard reference materials used and their obtained values from the earliest measurements during acceptance tests in 2015. The results given in this table were significant parameters for the first performance of the AGE III and TÜBİTAK 1MV AMS systems and their integration with each other. All measured values are achieved with a single cathode. Uncertainties of the measured value (pMC) are calculated as standard deviations of the mean.
*By removing contamination from graphite production over the previous five years, the result was improved to be more in line with the consensus value of 0.00–0.05 pMC.
**When the pretreatment step with an acid-base acid (ABA) wash was used prior to measurement, it was measured at 0.22 ± 0.03 pMC.
In the early phase of the laboratory, some SIRI comparison samples were obtained. The AMS measurement results of SIRI inter-comparison samples are given in Table 4. The results are consistent with Scott et al. (Reference Scott, Naysmith and Cook2018). Since just one cathode was used for the measurement, the uncertainty is expected to be less than 1%, meaning that it is within a range of ±35 years every 5000 years. Uncertainties are calculated as standard deviation of the mean and reported as it is.
AMS measurement results of SIRI intercomparison samples.
Preparing Reports
For each application, a one-page report is prepared. After AMS measured 14C/12C ratios are processed in abc software, each measurement’s result is calculated as a Libby age with an uncertainty value. 14C dates reported are corrected for 13δ from AMS measurement. In routine reports, 14C dates and uncertainty values are reported. The bomb peak samples are reported as Fraction modern (Fm) value with uncertainty. Upon request, Oxcal software (Bronk Ramsey Reference Bronk Ramsey2009) and the Intcal database are used to calibrate the Libby age results as a calendar age (Reimer et al. Reference Reimer, Austin, Bard, Bayliss, Blackwell, Ramsey, Butzin, Cheng, Edwards, Friedrich, Grootes, Guilderson, Hajdas, Heaton, Hogg, Hughen, Kromer, Manning, Muscheler, Palmer, Pearson, van der Plicht, Reimer, Richards, Scott, Southon, Turney, Wacker, Adolphi, Büntgen, Capano, Fahrni, Fogtmann-Schulz, Friedrich, Köhler, Kudsk, Miyake, Olsen, Reinig, Sakamoto, Sookdeo and Talamo2020). If there is a special situation for the sample regarding sample preparation or AMS measurement, it is given in the explanation part. The type of sample preparation procedure used for a sample is also given in the report. All signed ISO 9001 accredited reports are sent to the submitters of the sampled material as a printed version and digital PDF copies of the report can also be sent upon request. Reports are prepared in either Turkish or English depending on what has been requested by the customer/collaborator. The science behind the reports can also be explained to researchers who are not familiar with 14C studies.
SUMMARY
In this paper, the general workflow of the implemented and applied procedures for 14C analysis at the TÜBİTAK-MRC-EMSI Radiocarbon Dating Laboratory are given. Since mid-2016, the 1MV National AMS Laboratory at TÜBİTAK, MRC, Türkiye, has offered and provided 14C testing and analysis services according to the information presented here. Reports are produced for each sample submitted for 14C testing and analysis. The process includes sample evaluation for 14C analysis, wet chemistry of sample preparation, graphite production, accelerator mass spectrometry (AMS) measurement, data supervision, and calendar date calculations, and explanations of the work conducted and the resulting data and information.
ACKNOWLEDGMENTS
We appreciate the reviewers’ thorough perusal of our paper and all of their insightful remarks and recommendations. We thank Benjamin Irvine for editing the English of the manuscript.
