A fully automated system for the extraction of in situ cosmogenic carbon-14 in the Tulane University cosmogenic nuclide laboratory

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A fully automated system for the extraction of in situ cosmogenic carbon-14 in the Tulane University cosmogenic nuclide laboratory Brent M. Goehringa, , Jim Wilsonb, Keir Nicholsa ⁎

a b

Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118, USA Aeon Laboratories, L.L.C., Tucson, AZ, USA

ARTICLE INFO

ABSTRACT

Keywords: In situ carbon-14 Automation Cosmogenic nuclide

During 2015 and culminating in early 2016, we acquired a new Carbon Extraction and Graphitization System (CEGS) from Aeon Laboratories, L.L.C. (hereafter, “Aeon”), and adapted it for in situ cosmogenic sample processing. The Tulane University CEGS (TU-CEGS) is fully automated starting from sample insertion into the tube furnace to generation of graphite material ready for accelerator mass spectrometry cathode preparation. The system implements an integrated sequence of sample processing functions: extraction/collection, purification, measurement, and graphite production, which are all integrated into one unified system. The extraction portion is derived from evolving designs of fusions of quartz via lithium metaborate (LiBO2) flux. A critical analysis of system design in concert with analysis of process parameters yield a nearly order of magnitude increase in sample throughput with total samples processed in our laboratory (320 since installation) with consistent process blank levels (0.98 ± 0.32 × 105 atoms 14C, n = 26) and secondary standard values (0.4953 ± 0.0012 Fm, n = 8). In this paper we detail system design, process algorithm, and line performance including system blanks and the results from the CRONUS-A (6.12 ± 0.32 × 105 atoms g−1 14C, n = 13) interlaboratory comparison material.

1. Introduction Recent years have seen somewhat of a proliferation of in situ carbon14 (14C) extraction systems. Less progress has been made in optimizing sample extraction to ease complexity and the time-consuming nature of the extraction, and to date the only semi-automated method of extraction remains that at Purdue University’s PRIME Lab [1] and more recently at ETH-Zurich [2]. The challenge that still remains is the quantitative extraction of carbon, including the cosmogenic 14C fraction, from quartz, while simultaneously isolating the sample gas yields from ubiquitous atmospheric and organic carbon sources. Three principle system designs exist to complete this process. The first, pioneered by Lifton et al. [3], relies on the fusion of quartz and other minerals in a LiBO2 flux and the evolved gas carried away in a O2 carrier gas for secondary oxidation to CO2 of all carbon species, followed by purification and graphitization of the sample CO2 [1,4–6]. Another system uses similar downstream steps, purification followed by either graphitization or collection of purified gas for gas source accelerator mass spectrometry (AMS), but differs in its liberation of intra-crystalline carbon species via electron bombardment of the sample within a conductive crucible [7–9]. Finally, a new approach relies on a sealed-tube combustion of quartz within fused silica tubes, and is subsequently purified and either ⁎

graphitized or prepared for gas source AMS [10,11]. A number of commonalities exist with these systems, notably the requirement of some human intervention between steps and a generally highly time-consuming nature with numerous processing steps where repeatability is paramount. During 2015, we installed at Tulane University a fully automated system for the extraction, purification, and graphitization of cosmogenic 14C from quartz and other mineral phases. The Tulane UniversityCarbon Extraction and Graphitization System (TU-CEGS) is one of only two completely or nearly completely automated in situ 14C system where no human intervention is needed between sample insertion into the furnace and completion of graphitization (Figs. 1 and 2). Below, we will outline basic system design, system operation, and performance, including process algorithms, graphitization performance, process blank levels, and results of measurements of the CRONUS-A intercomparison quartz material. 2. System design and process 2.1. System overview A major difference between TU-CEGS and many other designs is that extraction, purification, and graphitization are combined into a single

Corresponding author at: Department of Earth and Environmental Sciences, Tulane University, 6823 St Charles Ave, New Orleans, LA 70118, USA. E-mail address: [email protected] (B.M. Goehring).

https://doi.org/10.1016/j.nimb.2019.02.006 Received 2 December 2017; Received in revised form 24 July 2018; Accepted 3 February 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Goehring, B.M., Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2019.02.006

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Fig. 1. TU-CEGS system sections and components. Fig. 2. A. TU-CEGS as installed and fully operational. 1700 °C tube furnace and mullite tube are at the left. Graphite reactors are at the right. Total width of both tables is 10 feet. B. Typical plug valve and servo. Also shown is a FTC and Cu/Ag furnace. C. VTT with FTC removed. D. LN reservoir with phase separator at top, servo controlled valves and distribution tubing is at the bottom. E. Graphite reactor array with furnaces removed. FTC has individual reservoirs, but is combined into single unit for space considerations.

system. This eliminates the need to physically transfer samples between multiple systems. The hardware is simpler and occupies less space, only one vacuum system is required, and intermediate break seal ports are absent. Reduced sample handling means fewer opportunities for contamination and significantly lower risk from human error. Another significant difference from existing LiBO2 fusion style systems is the primarily stainless-steel construction. Glass is used only for cold fingers and high-temperature sections with furnaces, which require quartz tubing. Instead of diaphragm valves, the CEGS features Swagelok quarter-turn plug valves because they provide higher conductance and simpler operation. The entire system is evacuated by a small turbo pump backed by a scroll pump. From a construction viewpoint, the TU-CEGS is an Aeon CEGS (https://www.aeonlaboratories.com/?page=product/CEGS/CEGS), with the inlet manifold adapted to accept Tulane's in situ quartz carbon liberation system as a new sample source (Fig. 2a). Aeon wrote device drivers to interface the new hardware (mass flow controller and tube furnace), and we developed custom process control software to implement new extraction protocols. We retained the original general-

purpose functionality of the CEGS, including the standard inlet port that accepts typical organic, carbonate, and sealed gas samples that any radiocarbon laboratory might encounter. Because the TU-CEGS is the first outside commercial installation of a fully integrated CEGS, we will describe some of its basic hardware and functionality where these differ markedly from other systems that accomplish similar tasks. The fundamental elements of the added carbon liberation and CO2 trapping portions of the system are similar to those described in Pigati et al. [4]: a high temperature tube furnace capable of maintaining 1200 °C for hours at a time, a secondary oxidation furnace filled with granular quartz media, and a flow-through cryogenic trap for sample collection. However, several implementation details differ. A mullite tube in the sample furnace, with Pyrex ball joints on the ends for access, connects via corrugated stainless-steel tubing and hot quartz bed (850 °C) to the CEGS inlet manifold. The furnace tube can be evacuated via two routes, directly through a plug valve, or via an automated metering valve (see below) which limits the rate of pressure change to 0.4 kPa sec−1 to avoid disturbing the LiBO2 prior to fusion. 2

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The quartz media in our system is finer than previously used and the grains irregularly shaped. We use 0.64 to 0.86 mm graded, crushed chips instead of 2 mm × 2 mm cylindrical rods or beads. The reduced grain size and the variety in size and shape enable a much higher packing density. Additionally, the quartz media furnace operates at 850 °C compared to ca. 1025 °C [4]. The lower quartz bed temperature radiates less heat into the surroundings, permits a smaller furnace and likely will increase furnace service life, while still ensuring that any CO from the sample is fully oxidized to CO2 for collection. Hadman et al. [12] showed that slightly less than 700 °C initiates CO oxidation even in very unfavorable conditions; however, we run at 850 °C as a means to ensure complete oxidation. Sample collection is accomplished by the CEGS variable temperature trap (VTT; Fig. 2c), which immediately follows the inlet manifold (Fig. 1). The VTT is also used subsequently for cryogenic purification. Because the quality of results achievable by the system depends heavily on the performance capabilities of the VTT and its operation, we discuss in more detail below (section 2.3.3).

2.3. System automation As mentioned above, all aspects of system operation aside from sample loading and unloading are fully automated. Significant reductions in the hands-on labor required for sample processing have been achieved – to approximately 2 h total, inclusive of both sample processing days. The reduction in human fatigue alone has enabled us to run more samples more frequently than ever before. However, the improvements in processing capacity could not have been achieved without attendant improvements in system reliability. In particular, four general aspects of the CEGS system have been crucial in this regard: valve operation, handling of liquid N2, variable temperature trap operation, and carbon yield determination. 2.3.1. Valve actuators The CEGS uses Swagelok quarter-turn plug valves instead of Kontes type double O-ring glass stopcocks or Swagelok SS-4BK type bellowssealed diaphragm valves (Fig. 2b). Plug valves are affordable and easy to maintain, have a very large conductance, and fast but smooth opening and closing characteristics. The latter two features are especially attractive for vacuum applications. Additionally, they are readily adapted for automation, using “servos” based on those ubiquitous among the radio-control model and small-scale robotics communities. The CEGS software interfaces with an actuator controller and a 64channel multiplexer bank to provide one-at-a-time operation of up to 63 valves. Automatic operation of the plug valves requires minimal maintenance and have been virtually trouble-free. Plug valves do not work where rate of flow must be controlled. In this case, the CEGS employs Swagelok stainless-steel right-angle metering valves. Motor control for these valves is also achieved via pulse width commands, with the fully opened and closed positions being detected by microswitches calibrated to prevent the valve jamming at its limits. These actuators are similar to those installed on the PRIME Lab automated systems [1].

2.2. Process overview Like Pigati et al. [4] we follow a two-day extraction process, summarized briefly here. The first day is dedicated to quickly flame cleaning carbon contaminants from the surface of the quartz sleeve used to protect the mullite process tube (all other implements are cleaned with isopropanol soaked Kimwipe and compressed air), 2) fusing and degassing of 20 g LiBO2 (Claisse Pure LiBO2) in a 50 mL alumina boat at 1200 °C (Coorstek). A slight difference from the procedure outlined in Pigati et al. [4] is that instead of holding the furnace atmosphere at a static 6.66 kPa of O2, we fill with 66 kPa O2 and evacuate down to 6.66 kPa three times during the 60-minute period at 1200 °C, as the flushing of gases should more effectively remove any evolved contaminants. Total time to complete day one is 2.5 – 3 h. After cooling (typically overnight), the boat is removed from the furnace and the sample loaded by distributing evenly across the solid LiBO2 and returned to the furnace tube. The sample is then evacuated to 1.33 Pa while heating to 500 °C; once 500 °C is reached ∼6.66 kPa of O2 is added to the furnace tube and held for 1 h. The furnace is again evacuated to 1.33 Pa, filled with 6.66 kPa Ar and then evacuated immediately (i.e., no Ar dwell time) to 260 mPa. The sample is then combusted at 1100 °C for three hours in 6.66 kPa O2. After three hours, the furnace is allowed to cool and the liberated CO2 is collected in the VTT, while O2 and incondensable gases are evacuated away. Our static procedure is a major departure from the methods outlined in Pigati et al. [4] and Lifton et al. [1], where they collect evolved CO2 for one hour (after 2 h at 1100 °C) in a stream of 5 or 10 cc per minute at STP (sccm) O2 at 6.66 kPa. During collection, pressure inside the VTT is limited by flow control to about 53 Pa until the upstream pressure falls below that point, after which the remaining incondensable gas is evacuated as quickly as possible. Testing shows that this procedure ensures > 98% collection of entrained CO2. The remainder of the process is essentially the same as for any sample processed by the CEGS. Briefly, the system extracts the CO2 from the VTT, passes it through a Cu/Ag trap, measures the yield, and transfers it to one of six “graphite reactors” for hydrogen reduction to filamentous C on a Fe catalyst [13] (Fig. 2e). The required amount of H2 is automatically determined based on the sample size and admitted to each graphite reactor to provide an optimal 2.3:1 ratio of H2:CO2. A pressure sensor in each reactor monitors reaction progress; completion is detected when the lowest observed pressure during a reaction has not decreased for five minutes. Reaction completeness is later confirmed by comparing the residual pressure present in the graphite reactor against that expected for a complete stoichiometric reaction. Graphitization is restarted if excess residual pressure is observed. Total time to complete day two, neglecting graphitization time, is approximately 7 h. Typical graphitization time is 45 – 90 min, depending on the size of the sample.

2.3.2. Liquid N2 handling Crucial to system reliability is the handling and distribution of liquid N2 (LN). The primary components can be divided into two parts, 1) a central reservoir and distribution manifold and 2) a cold-finger containment and monitoring device, known as a freeze thaw cup (FTC; Fig. 3). Insofar as possible, all of the devices in the system that hold or transport LN are manufactured from extruded polystyrene (XPS) foam or fluorinated ethylene propylene (FEP) tubing. These low effusivity, cryogenic-tolerant materials minimize LN turbulence and loss, which reduces stabilization time and enables finer temperature control with smaller perturbations. Where possible, aluminum-backed denim insulation sleeves cover FEP tubing runs, to reduce coolant waste and minimize atmospheric moisture condensation. Silicone tubing is used primarily as a mechanical grip for FEP tubing, but also in a few places as a connector to join FEP parts where forming a single part is impractical. Bulk LN is stored in a standard, low pressure (22 psi), industrial vacuum-insulated cryogenic cylinder. An arrangement of brass fittings, including a pressure-relief valve for safety, connects the cylinder's liquid outlet to a solenoid valve. A short length of FEP tubing, formed to mate with a flare fitting on the solenoid valve, directs LN into the side of an XPS gas/liquid phase separator, which sits atop the LN manifold. Once it reaches the phase separator, the LN pressure falls to atmospheric pressure, and all further transport into the LN manifold reservoir takes place by gravity alone. The LN manifold reservoir is a small (∼400 mL) chamber made of XPS (Fig. 2d). Within the reservoir is a level sensor made from Type T thermocouple wire. Channels machined into the manifold to distribute LN to the appropriate FTC via built in valves. Freeze-thaw cups (FTCs) are machined from a solid block of XPS (Fig. 2b and 2c). A well, machined into the top of the body, accepts a 3

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Fig. 3. Schematic drawing of LN system.

cold finger and can contain a few milliliters of LN. Like the LN manifold, the well contains a Type T thermocouple to monitor temperature and detect the liquid level. LN is gravity-fed into the side of the well. A second passage directs warm air into the bottom of the well, whenever a thaw cycle is initiated, to quickly warm the FTC to room temperature. The initial puff of air blows any LN out of the well, and the continuing stream of warm air brings the cold finger to ambient temperature.

measurement, known as the measurement chamber (MC). The purpose of the blanket is to suppress unwanted gases that otherwise would begin to evolve as the VTT pressure drops due to the deposition of solid CO2 in the MC. The MC cold finger is then cooled to −196 °C by its FTC. With the MC cold finger frozen, the system calculates the expected equilibrium temperature for CO2 at a vapor pressure equal to that of the He blanket. The VTT temperature is then raised to precisely 3 °C above the calculated equilibrium temperature. A typical CO2 target pressure is −145 °C (Fig. 5). The CO2 evolves from the VTT, passes through a trap containing several alternating layers Cu and Ag wool heated to 600 °C, and deposits as a solid in the MC cold finger. The Cu/Ag trap is intended to remove potential sulfur, halogen, and nitrogen contaminants.

2.3.3. Variable temperature trap The variable temperature trap (VTT) contains a freeze-thaw cup (Fig. 2c; Fig. 4). However, in this device, the LN does not act directly on the cold finger, but indirectly, by means of an aluminum thermal conduit. The top half of the conduit is cup-shaped, to enclose the trap cold finger, while the bottom half is a simple rod, immersed in the LN. A thin neck at the top of the rod limits the rate of heat flow between the rod and cup. An insulated resistance-wire heating element surrounds the cup. Temperature is regulated by a PID temperature controller that monitors a thermocouple in the bottom of the cup. Additional thermocouples monitor the LN level, the cold finger temperatures at the top and bottom of the cup, and the temperature of the resistance wire leads. When the VTT device is in regulating mode, i.e., when it is working to reach or maintain a specific temperature, the LN feed is adjusted to provide a more-orless constant trickle that overflows a port on the back of the VTT's FTC. The constant level of LN thereby imposed on the thermal conduit minimizes variability in the heat flux, and greatly stabilizes the temperature. The VTT's temperature range extends from below −190 to +50 °C, with ± 2 °C absolute precision and repeatability to within 1 °C. Stability (typical drift at hold) is ± 0.3 °C. To extract the CO2 and exclude unwanted gases, a low-pressure “He blanket” (10–25 Pa 99.9999% He) is first introduced into the joined volume including the VTT, the Cu/Ag area, and the volume containing the capacitance manometer used for sample

2.3.4. Measurement chamber Completion of the transfer of sample gas from the VTT to the MC is detected when the pressure in both line sections are stable and equal to each other (at the He blanket pressure). At this point, the system isolates the MC from the VTT and Cu/Ag chambers, then (with the CO2 still frozen) evacuates the He blanket and any permeated incondensable gases from the MC. The FTC thaws the cold finger, and when the MC temperature is uniform, the host software records the chamber pressure, chamber temperature, and calculated mass of the carbon, assuming the pressure is entirely due to CO2. The pressure is measured by a 13.3 kPa heated MKS Instruments 627F Baratron (45 °C) capable of five decades of precision (0.001%). The temperature sensor is a calibrated MCP9701 silicon “Linear Active Thermistor”. The MC volume, along with all other system volumes, is calibrated based on a gravimetric volume determination of an ∼30 mL glass bulb using 18.2 MΩ water. Calibrated MC volume is 22.55 ± 0.09 (0.39%) cc. The calculated sample mass accuracy is on the order of 1 μg C. Once the mass has been determined, if the amount is below a set threshold (typically 90 μg), the sample is automatically diluted with 14C-free CO2 to an 4

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Fig. 4. Schematic drawing of VTT assembly.

approximate total mass of 110 μg. Note that for some AMS labs, dilution volumes are larger to optimize sample measurement conditions (e.g., beam current, run time). The change in dilution volume reflects the change from AMS measurement at Lawrence-Livermore National Laboratory Center for Accelerator Mass Spectrometry (LLNL-CAMS) to National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) at Woods Hole Oceanographic Institution.

3. System operation and performance 3.1. Sample processing times and throughput The net result of the automation and implemented extraction procedures mean that overall sample processing times are shorter (∼10 h total) and the required man-hours (∼2 h total) are as well. Sample 5

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3.2. Graphitization blanks and secondary standard performance Previous experience suggests an inverse dependence of the measured 14C/13C ratio (equivalently Fm) on the mass of the carbon in the sample [1,5,14]. We conducted experiments to characterize the mass dependence of the background 14C/13C ratio associated with the TUCEGS system. To do so, we used aliquots of effectively 14C-free CO2 (industrial welding gas). Aliquots ranged in carbon mass between approximately 50 μg and 500 μg (Table 1; Fig. 6). Results closely follow the expected inverse relationship with the exception of two clear outliers that are excluded from system characterization. The blank 14C/13C ratio for a typical 110 μg sample is equivalent to 2.56e-13 (Fig. 4). An additional measure of the performance of our graphitization reactors, and overall system performance is the repeat measurement of the IAEA C7 radiocarbon secondary standard [15]. C7 performance on our system corrected for the graphitization blank yields a mean ± onestandard deviation value of 0.4953 ± 0.0012 Fm (n = 8, 3 outliers removed). After approximately 1.5 years of use we observed a gradual rise in both C7 and dead CO2 levels and suspected memory within the Viton O-rings of the graphite reactors (Fig. 6; Table 2). Replacing Orings with new baked O-rings led to an immediate improvement in both C7 and dead CO2 Fm values (Fig. 5).

Fig. 5. Typical profile of pressure (top) and temperature (bottom) in the VTT. Table 1 Results of “graphite reactor” characterization. ID

Mass (μg)

14

C/13C (10−13)

DG50 DG100 DG150 DG200 DG250 DG111116 DG020217 DG111116 DG071117

60.7 ± 0.8 97.8 ± 1.3 145.2 ± 1.9 193.1 ± 2.5 242.9 ± 3.1 292.7 ± 3.7 475.7 ± 6.1 296.4 ± 3.8 479.2 ± 6.1

3.80 2.87 2.24 1.83 2.56 1.64 1.30 1.54 2.92

± ± ± ± ± ± ± ± ±

0.13 0.11 0.09 0.16 0.06 0.07 0.05 0.10 0.00

3.3. Process blanks A key aspect of in situ 14C measurement is process blank control both in terms of the absolute levels of the blank in terms of 14C atoms, but also in terms of the repeatability of the process blank. Over the time since the line began routine operation, we have run 24 full procedural blanks (Table 3). We initially ran a process blank every five samples, but have switched to running a process blank every eight samples to reduce costs and increase sample throughput. Fig. 6a shows the evolution of procedural blanks through time. Initially, the blanks were relatively high but have gradually decreased over time, similar to that observed in other mullite furnace tube/ LiBO2 based systems and presumed to be a result of the furnace tube releasing carbon or an initial “break-in” period (Fig. 7a) [4–6]. Thus, over the initial break-in period we use a time dependent blank for the purposes of background corrections. After April 2016, we notice little long-term trend and therefore use a continually updating mean value for background corrections. As of July 2018, the average blank is equivalent to (0.98 ± 0.32) × 105 atoms 14C (n = 26, COV = 32%) excluding two

throughput is therefore also increased. Standard protocol is to complete extraction of a single sample every two days. If need be, we have also found that after the day 2 extraction, we can start the next day 1 procedure, therefore increasing sample throughput to one complete extraction every day. For a five-day work week, we can extract up to four samples. Since early 2016, we have processed over 300 samples for in situ 14C analysis, which represents a monumental leap forward in analytical capacity.

Fig. 6. A. “Graphite reactor” characterization for background contamination with respect to carbon mass. An inverse relationship is observed consistent with past studies. Outliers are show in red and are omitted from the curve fit. B. IAEA C-7 measurements. The mean (solid line) and 1σ standard deviation (gray shading) are shown based on four of the seven measurements. The three outliers removed are all similarly high and are suspected to be a result of O-ring contamination. 6

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Table 2 IAEA-C7 results. ID

Mass (μg)

14

C/13C (10−11)

C7122915 C7112316 C7020717 TUC71 TUC72 TUC73 C7071117 C711022017 C701192018 C7030818 C7033018

433.1 390.7 385.5 176.1 182.5 182.8 437.0 208.2 497.4 475.2 318.3

5.52 5.45 5.44 5.62 5.66 5.63 5.39 5.40 5.41 5.43 5.42

± ± ± ± ± ± ± ± ± ± ±

δ13C (‰)

0.13 0.01 0.02 0.01 0.02 0.02 0.01 0.01 0.01 0.02 0.02

−14.40 −14.40 −14.40 −14.03 −14.54 −12.74 −14.40 −15.38 −14.40 −14.40 −14.40

± ± ± ± ± ± ± ± ± ± ±

14

1.59 1.77 1.77 1.49 1.96 1.59 1.77 0.83 1.77 1.77 1.77

C/C Total (10−13)

6.02 5.94 5.94 6.15 6.18 6.16 5.88 5.89 5.89 5.92 5.91

± ± ± ± ± ± ± ± ± ± ±

0.15 0.01 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.02 0.02

Fm-25‰ 0.5038 0.4970 0.4968 0.5139 0.5175 0.5140 0.4918 0.4940 0.4933 0.4953 0.4949

± ± ± ± ± ± ± ± ± ± ±

0.0122 0.0012 0.0017 0.0014 0.0014 0.0014 0.0011 0.0013 0.0012 0.0017 0.0016

Table 3 Results of all process blanks. Data reduction follows [20]. ID

Yield (μg C)

Diluted C (μg)

14

PB012016 PB011816 PB021516 PB022416 PB030116 PB032116 PB040116 PB051516 PB062016 PB070616 PB072816 PB082516 PB091116 PB101216 PB110116 PB112016 PB121416 PB012317 PB022017 PB031517 PB041917 PB060717 PB062217 PB072517 PB090717 PB100417 PB110917 PB120417 PB122817 PB012518 PB020818 PB021718 PB022818 PB032118 PB041218

10.0 ± 0.1 13.5 ± 0.2 8.9 ± 0.1 7.2 ± 0.1 7.2 ± 0.1 8.5 ± 0.1 7.5 ± 0.1 3.3 ± 0.0 4.6 ± 0.1 4.5 ± 0.1 4.5 ± 0.1 3.8 ± 0.0 3.8 ± 0.0 3.6 ± 0.0 4.6 ± 0.1 4.0 ± 0.1 4.7 ± 0.1 8.0 ± 0.1 4.1 ± 0.1 6.2 ± 0.1 7.3 ± 0.1 3.0 ± 0.0 3.2 ± 0.0 4.9 ± 0.1 2.8 ± 0.0 3.2 ± 0.0 4.7 ± 0.1 1.9 ± 0.0 2.1 ± 0.0 2.4 ± 0.0 3.0 ± 0.0 2.2 ± 0.0 3.3 ± 0.0 2.8 ± 0.0 2.9 ± 0.0

381.5 381.3 385.1 383.7 387.1 379.5 387.0 389.3 386.1 381.0 381.7 382.1 385.5 376.8 379.7 380.2 386.2 399.9 383.7 386.4 197.3 106.2 114.5 114.9 106.7 108.9 107.4 109.9 104.7 104.1 111.0 110.5 107.9 271.9 101.2

19.61 ± 0.60 29.49 ± 0.51 14.61 ± 0.29 13.37 ± 0.44 12.98 ± 0.57 15.44 ± 0.28 9.63 ± 0.07 3.09 ± 0.03 6.15 ± 0.03 8.19 ± 0.13 5.67 ± 0.13 5.64 ± 0.09 5.21 ± 0.08 5.61 ± 0.08 4.28 ± 0.07 4.09 ± 0.08 5.90 ± 0.11 13.45 ± 0.19 5.50 ± 0.10 6.43 ± 0.10 25.30 ± 0.27 15.66 ± 0.24 23.99 ± 1.05 17.70 ± 0.29 11.91 ± 0.23 14.24 ± 0.25 19.35 ± 0.48 9.19 ± 0.19 9.64 ± 0.17 9.09 ± 0.19 12.45 ± 0.21 9.78 ± 0.17 12.60 ± 0.20 5.44 ± 0.09 15.23 ± 0.31

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

4.9 4.9 4.9 4.9 5.0 4.9 5.0 5.0 4.9 4.9 4.9 4.9 4.9 4.8 4.9 4.9 4.9 5.1 4.9 4.9 2.5 1.4 1.5 1.5 1.4 1.4 1.4 1.4 1.3 1.3 1.4 1.4 1.4 3.5 1.3

C/13C × 10−13

14

C/C Total × 10−14

2.15 3.24 1.61 1.47 1.43 1.70 1.06 0.34 0.68 0.90 0.62 0.62 0.57 0.62 0.47 0.45 0.65 1.48 0.60 0.70 2.77 1.74 2.67 1.94 1.31 1.56 2.10 1.01 1.06 1.00 1.36 1.07 1.38 0.60 1.67

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.07 0.06 0.03 0.05 0.06 0.03 0.01 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.03 0.03 0.12 0.03 0.03 0.03 0.05 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.03

14

C (atoms)

41.18 ± 1.37 61.88 ± 1.33 31.00 ± 0.73 28.25 ± 1.00 27.67 ± 1.26 32.27 ± 0.71 20.52 ± 0.30 6.57 ± 0.10 13.08 ± 0.18 17.19 ± 0.36 11.92 ± 0.30 11.89 ± 0.24 11.08 ± 0.22 11.69 ± 0.23 8.99 ± 0.19 8.63 ± 0.20 12.63 ± 0.29 29.63 ± 0.57 11.63 ± 0.26 13.66 ± 0.28 27.42 ± 0.46 9.28 ± 0.19 15.33 ± 0.70 11.20 ± 0.23 7.02 ± 0.16 8.52 ± 0.18 11.30 ± 0.32 5.57 ± 0.14 5.55 ± 0.12 5.20 ± 0.13 7.60 ± 0.16 5.94 ± 0.13 7.48 ± 0.15 8.14 ± 0.17 8.49 ± 0.20

d13C (‰) −4.37 ± 0.50 −4.37 ± 0.50 −3.45 ± 0.50 −3.33 ± 0.50 −3.43 ± 0.50 −3.80 ± 0.50 −3.78 ± 0.50 −11.32 ± 0.50 −3.91 ± 0.50 −3.66 ± 0.50 −3.39 ± 0.50 −2.60 ± 0.50 −1.65 ± 0.50 −0.15 ± 0.50 1.10 ± 0.50 2.75 ± 0.50 3.23 ± 0.50 −3.39 ± 0.50 −3.76 ± 0.50 −4.80 ± 0.50 −6.31 ± 0.50 9.52 ± 2.24 9.96 ± 2.88 −2.98 ± 0.45 −0.32 ± 1.76 −5.29 ± 0.50 −16.66 ± 0.50 −2.06 ± 0.50 −4.42 ± 0.50 −4.47 ± 0.50 −5.08 ± 0.50 −4.30 ± 0.50 −4.99 ± 0.50 −3.45 ± 0.50 −3.45 ± 0.50

boat results in increases of both the carbon yield and 14C content, while the addition of LiBO2 results in no significant increase. Thus, even though we degas the alumina boat on day one, we are apparently not liberating all contaminant carbon and suspect that during the threehour combustion, not only are we fusing the quartz sample, but the alumina boat as well and further liberating intra-crystalline carbon and 14 C. Additional baking of the boat does not significantly improve carbon yields or 14C content. We are experimenting with alternate combustion boat materials, notably a Pt/Rh (90%/10%) alloy. Early experiments with a Pt/Rh combustion boat using identical system processes yield a process blank of (0.38 ± 0.01) × 105 atoms.

obvious outliers. The two outliers coincide with sample processing by visiting students lacking experience compared to Tulane students. Over this period of time, the minimum observed blank is (0.52 ± 0.01) × 105 atoms and the maximum is (2.96 ± 0.06) × 105 atoms, indicating that individual analytical precision is better than the scatter observed in all blanks and thus we can further potentially improve scatter in process blanks. Given that the magnitude of scatter exceeds that of an individual sample measurement, we investigated potential sources of contaminant 14 C. Fig. 7b and 7c show measured blank levels against blank carbon yield and diluted carbon mass. The observed dependence of the blank level against the carbon yield is striking (r2 = 0.96), while little relationship is observed for diluted carbon masses as would be expected when using 14C-dead CO2. Contaminant 14C is sourced from either the mullite tube, quartz tube sleeve, alumina boat, or the LiBO2. Systematic measurements of carbon yield and 14C indicate no significant difference between the mullite tube and tube plus sleeve. Insertion of the alumina

3.4. Cronus-A As part of characterizing the TU-CEGS, the measurement of a material with known 14C concentration is useful. While there is no 7

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Fig. 7. A. All measured process blanks for the TU-CEGS. Solid line and shading represent mean and 1 σ standard deviation for process blanks following April 2016 exclusive of the two outliers shown in red. B. All process blanks with respect to the carbon mass yield prior to dilution. A robust relationship is observed suggesting that most contaminant 14C is derived from sample processing procedures. C. As in A, except for the diluted carbon mass. No significant relationship is observed. Uncertainties in all panels are generally smaller than symbol.

Table 4 CRONUS-A results. Data reduction follows [20]. ID

Quartz (g)

Yield (μg C)

Unit Yield (μg C/g quartz)

Diluted C (μg)

14

CA1026151 CA122415 CA122215 CA041816 CA042016 CA070416 CA072716 NLCA112116a2 NLCA112116b2 CA032317 CA0404173 CA111417 CA050718 CA050718

9.9498 ± 0.00017 4.9991 ± 0.00017 5.0209 ± 0.00017 5.0531 ± 0.00017 5.0210 ± 0.00017 10.0766 ± 0.00017 10.0908 ± 0.00017 5.0303 ± 0.00017 5.0303 ± 0.00017 5.0519 ± 0.00017 5.0430 ± 0.00017 5.0144 ± 0.00017 4.9941 ± 0.00017 5.0458 ± 0.00017

61.2 ± 0.8 30.9 ± 0.4 31.1 ± 0.4 30.9 ± 0.4 44.0 ± 0.6 66.4 ± 0.9 59.7 ± 0.8 30.9 ± 0.4 30.9 ± 0.4 29.4 ± 0.4 217.9 ± 2.8 29.5 ± 0.4 28.9 ± 0.4 31.8 ± 0.4

6.15 6.18 6.19 6.12 8.76 6.59 5.92 6.14 6.14 5.82 43.21 5.88 5.79 6.30

187.7 ± 2.4 203.9 ± 2.6 205.0 ± 2.6 394.9 ± 5.1 396.4 ± 5.1 380.4 ± 4.9 386.7 ± 5.0 751.4 ± 9.6 751.4 ± 9.6 394.4 ± 5.1 217.9 ± 2.8 126.1 ± 1.6 113.1 ± 1.4 90.0 ± 1.2

0.00 2.83 2.75 1.32 1.36 3.02 2.94 0.75 0.72 1.42 2.85 4.18 5.27 5.91

1 2 3

C/13C × 10−11 ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.00 0.00 0.05 0.01 0.00 0.01 0.01 0.00 0.01 0.00 0.01 0.02 0.01 0.02

14

C/C total × 10−13

14

C (103 atoms g−1)

δ 13C (‰)

0.00 3.10 3.01 1.45 1.49 3.31 3.23 0.83 0.79 1.56 3.07 4.58 5.78 6.45

0.00 6.43 6.23 6.20 6.46 6.17 6.10 5.99 5.69 5.91 6.44 5.64 6.48 5.64

0.00 ± 0.00 −7.30 ± 0.50 −7.44 ± 0.50 −4.25 ± 0.50 −5.54 ± 0.50 −5.53 ± 0.50 −5.00 ± 0.50 −6.09 ± 0.50 −6.09 ± 0.50 −5.17 ± 0.50 −24.35 ± 0.50 −7.01 ± 0.50 −5.95 ± 0.50 −8.54 ± 0.50

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.00 0.01 0.05 0.01 0.00 0.01 0.01 0.00 0.01 0.01 0.01 0.02 0.02 0.02

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.00 0.35 0.34 0.31 0.33 0.08 0.08 0.08 0.08 0.08 0.09 0.08 0.08 0.08

Extraction done using bleed method outlined in [4]. Sample split into two aliquots and measured at LLNL CAMS and PRIME Lab. Sample diluted prior to extraction with Alfa Aesar Synthetic Graphite.

official 14C in quartz standard for this purpose, the CRONUS-A material is a ready substitute. CRONUS-A was developed as part of the CRONUS-Earth project to develop a large quantity of quartz that could be distributed to numerous laboratories to not only serve as an internal quality control material, but to also assess real internal laboratory uncertainty. The CRONUS-A material is saturated with respect to in situ 14C and thus is relatively easy to measure with high precision using a range of sample sizes. Ten measurements of CRONUS-A (Table 4) yield an average 14C concentration of (6.12 ± 0.32) × 105 atoms g−1 14C (n = 13, COV = 5.2%; Fig. 8). The average analytical uncertainty of all ten measurements is 0.18 × 105 atoms g−1 14C, and thus we still have slightly more

scatter in repeat measurements of CRONUS-A than would be expected. Curiously, the measured concentration from TU-CEGS is lower (ca. 5–10%) than that measured at other 14C laboratories [1,5,7–9,16,17], but with a similar coefficient of variation. We are investigating further the causes of the discrepancy, but note that other nuclides display similar unaccounted-for discrepancies (e.g., [18]). The COV for CRONUS-A also indicates that similar to process blanks we have scatter in excess of the precision reported for an AMS measurement. We have adopted reporting the analytical precision, but use effective precision based on the CRONUS-A COV and therefore consider our results conservative for geological samples following recommendations by the CRONUS-Earth project [19]. 8

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also like to thank Dr. Nat Lifton for guidance and advice over many years of collaboration and friendship. References [1] N. Lifton, B. Goehring, J. Wilson, T. Kubley, M. Caffee, Progress in automated extraction and purification of in situ C-14 from quartz: results from the Purdue in situ C-14 laboratory, Nucl. Instrum. Methods 361 (2015) 381–386. [2] K. Hippe, M. Lupker, L. Wacker, A second generation ETH in situ cosmogenic 14C extraction line, AMS-14 Conference, Canada, Ottawa, 2017. [3] N.A. Lifton, A.J.T. Jull, J. Quade, A new extraction technique and production rate estimate for in situ cosmogenic 14C in quartz, Geochim. Cosmochim. Acta 65 (2001) 1953–1969. [4] J.S. Pigati, N.A. Lifton, A.J.T. Jull, J. Quade, A Simplified In Situ Cosmogenic 14C Extraction System, Radiocarbon 52 (2010) 1236–1243. [5] B.M. Goehring, I. Schimmelpfennig, J.M. Schaefer, Capabilities of the LamontDoherty Earth Observatory in situ 14C extraction laboratory updated, Quat. Geochron. 19 (2014) 194–197. [6] R.H. Fülöp, P. Naysmith, G.T. Cook, D. Fabel, S. Xu, P. Bishop, Update on the Performance of the Suerc in Situ Cosmogenic C-14 Extraction Line, Radiocarbon 52 (2010) 1288–1294. [7] K. Hippe, F. Kober, H. Baur, M. Ruff, L. Wacker, R. Wieler, The current performance of the in situ 14C extraction line at ETH, Quat. Geochron. 4 (2009) 493–500. [8] K. Hippe, F. Kober, L. Wacker, S.M. Fahrni, S. Ivy-Ochs, N. Akcar, C. Schluchter, R. Wieler, An update on in situ cosmogenic 14C analysis at ETH Zürich, Nucl. Instrum. Methods 294 (2013) 81–86. [9] K. Hippe, L. Wacker, F. Kober, S. Fahrni, S. Ivy-Ochs, N. Akçar, C. Schlüchter, R. Wieler, Cosmogenic in-situ 14C analysis at ETH-Zürich, AMS-12 Conference, Wellington, New Zealand, 2011. [10] R.H. Fülöp, L. Wacker, T.J. Dunai, Progress report on a novel in situ 14C extraction scheme at the University of Cologne, Nucl. Instrum. Methods 361 (2015) 20–24. [11] R.-H. Fülöp, D. Fink, B. Yang, A.T. Codilean, A. Smith, L. Wacker, V. Levchenko, T.J. Dunai, The ANSTO – University of Wollongong in-situ14C extraction laboratory, Nucl. Instrum. Methods, Elsevier, 2018, pp. 1–0. [12] G. Hadman, H.W. Thompson, F.R.S.C.N. Hinshelwood, The oxidation of carbon monoxide, Proc. R. Soc. Lond. A 137 (1932) 87–101. [13] J. Southon, Graphite reactor memory – where is it from and how to minimize it? Nucl. Instrum. Methods 259 (2007) 288–292. [14] D. Donahue, A. Jull, L. Toolin, Radiocarbon measurements at the University of Arizona AMS Facility, Nucl. Instrum. Meth. B 52 (1990) 224–228. [15] M. Le Clercq, J. van der Plicht, M. Gröning, New 14C Reference Materials with Activities of 15 and 50 pMC, Radiocarbon 40 (1998) 295–297. [16] M. Lupker, K. Hippe, L. Wacker, F. Kober, C. Maden, R. Braucher, D. Bourlès, J.R.V. Romani, R. Wieler, Depth-dependence of the production rate of in situ14C in quartz from the Leymon High core, Spain, Quat. Geochron. 28 (2015) 80–87. [17] A.J.T. Jull, A. Jull, E.M. Scott, P. Bierman, The CRONUS-Earth inter-comparison for cosmogenic isotope analysis, Quat. Geochron. 26 (2015) 3–10. [18] P.-H. Blard, G. Balco, P.G. Burnard, K.A. Farley, C.R. Fenton, R. Friedrich, A.J.T. Jull, S. Niedermann, R. Pik, J.M. Schaefer, E.M. Scott, D.L. Shuster, F.M. Stuart, M. Stute, B. Tibari, G. Winckler, L. Zimmermann, An inter-laboratory comparison of cosmogenic 3He and radiogenic 4He in the CRONUS-P pyroxene standard, Quat. Geochron. 26 (2014) 11–19. [19] F.M. Phillips, D.C. Argento, G. Balco, M.W. Caffee, J. Clem, T.J. Dunai, R. Finkel, B. Goehring, J.C. Gosse, A.M. Hudson, A.J.T. Jull, M.A. Kelly, M. Kurz, D. Lal, N. Lifton, S.M. Marrero, K. Nishiizumi, R.C. Reedy, J. Schaefer, J.O.H. Stone, T. Swanson, M.G. Zreda, The CRONUS-Earth Project: a synthesis, Quat. Geochron. 31 (2016) 119–154. [20] K. Hippe, N.A. Lifton, Calculating Isotope Ratios and Nuclide Concentrations for In Situ Cosmogenic 14C Analyses, Radiocarbon 56 (2014) 1167–1174-1174.

Fig. 8. CRONUS-A concentrations measured since TU-CEGS installation shown as normal kernel density plot.

4. Conclusions The TU-CEGS system is now fully operational and performance to date is highly encouraging in changing in situ 14C into a more widely applied cosmogenic nuclide. Complete automation of all major processes is providing greater sample throughput. Further improvements can still be made, including data consistency and overall reduction in process blank levels. We look forward to continued operation and investigation of a variety of projects. Acknowledgements BMG acknowledges support from Tulane University for construction and acquisition of the TU-CEGS. We would also like to thank Susan Zimmerman and Tom Guilderson of Lawrence-Livermore Center for Accelerator Mass Spectrometry and Mark Roberts and Mark Kurz of the Woods Hole National Ocean Sciences Accelerator Mass Spectrometer for outstanding 14C measurements. We thank reviews by Jeff Pigati and Kristina Hippe that clarified and improved the manuscript. BMG would

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