CEGS 12X - Can process at least twelve samples per day
CEGS LL6 - A six-channel CEGS capable of handling extremely small samples
A typical CEGS graphical user interface (Cegs12X shown).
CEGS 12X
CEGS LL6
The CEGS is a laboratory instrument for radiocarbon dating. Various models are available, to provide general-purpose or specialized service and to balance system cost with the technical and logistical requirements of the laboratory receiving it.
Each CEGS features fully automated process control, with stepped combustion, CO2 collection, purification, measurement, and reduction to graphite integrated into a single, seamless process.
The operator loads pretreated samples into the configurable inlet ports and enters the sample data into the system computer, selecting the desired process for each sample. Once the “Start” button is clicked, the system automatically executes the selected process for each sample in sequence and notifies the operator by email when the run is complete.
Each system is capable of processing a full suite of organic samples in a 24-hour period. Throughput for digested carbonates and gas samples can be higher, potentially up to 24 per day, depending on sample sizes, process selection, and operator availability.
A brief synopsis of the radiocarbon dating process provides a useful context for clearer understanding of how the CEGS fits in. So, what is done to a sample in order to date it? It undergoes a series of functional phases:
First, a representative fraction of the submitted material is isolated for further processing. Any carbon-bearing matter that might be older or younger than the target fraction must be removed. Often, specific carbon compounds are selected by employing chemical reagents to separate them from the other substances present. Additionally, steps may be taken to remove or avoid the formation of non-carbonaceous chemicals that might interfere with subsequent steps. This phase is sometimes called “wet chemistry”.
After pretreatment, the sample’s carbon must be separated from the rest of the sample material. To obtain an accurate analysis, all non-sample carbon, such as CO2 from the air, must be excluded. A vacuum system is usually employed to remove air, and carbon-free gases such as research-grade oxygen, helium, argon, or nitrogen are introduced to participate in the extraction. The collected carbon must come exclusively from the sample material. Moreover, all of the the sample’s carbon must be collected. Due to fractionation effects, an incomplete or partial carbon extraction can compromise the result as well. The technique for liberating the carbon from pretreated material varies based on the type of sample, but most commonly it is either combusted (burned) or digested in acid.
In either case, oxygen combines with the carbon to form carbon dioxide (CO2). Other chemical reactions usually take place simultaneously, so steps are taken to remove unwanted reaction products, such as water, sulfur and halogen compounds, nitrogen oxides, and so on. To summarize in simple terms, extraction consists of carbon liberation, CO2 collection, and purification.
This phase uses hydrogen to remove the oxygen from the purified CO2, leaving behind solid carbon. The carbon is deposited onto powdered iron, which serves as a catalyst for the reduction and a substrate for the deposition. The solid carbon is commonly called “graphite”, although this term is not strictly correct, because it doesn’t have a layered graphene structure.
Finally, the “graphite” is compressed into a tiny, cylindrical aluminum “cathode” or “target” and inserted into the ion source of an accelerator mass spectrometer. Ionized cesium is sputtered onto the graphite’s surface, which ionizes the carbon atoms. Electric fields focus and accelerate the charged atoms through a strong magnet which deflects them toward a set of particle detectors. The lightest atoms (12C) are deflected more than the heaviest ones (14C), so they go into different detectors. The ratio of 14C atoms to 12C in the sample is then used to determine its age.
The CEGS is computer-controlled machine that automatically performs the middle part of the radiocarbon dating process–the bit between Wet Chemistry and Isotope Analysis in the synopsis above. These tasks traditionally were performed by a researcher using a separate specialized manual vacuum apparatus for each step. They fell collectively under a broader definition of Pretreatment that includes everything before Isotope Analysis.
Combining extraction, purification and graphitization into a single instrument eliminates sample transfers, which are a source of contamination. Automation significantly reduces sample loss due to human error, and ensures extremely consistent temperatures, pressures, and process step times. A important consequence of this is reduced scatter in the analytical data, which leads to better statistics, lower uncertainties, and more reliable dates.
Each CEGS contains graphite reactors and inlet ports, with a Process Section in between.
The reduction from CO2 to ‘graphite’ typically takes about two hours, but can require anywhere from 30 minutes to three hours, depending on the amount of carbon in the sample. For practical purposes, the number of reactors governs the throughput of the system (how many samples can be processed in a day), and this is the first figure in the CEGS model number. Throughput can also be influenced by managing the purification process and controlling sample size. Both of these may be accomplished by adjusting the instrument settings.
An important configuration option is the selection of valves in the sample path through the CEGS.
In a standard CEGS, plug valves are used throughout the system, owing to their high-conductance, low cost, and ease of operation. Plug valves contain elastomeric (O-ring) seals, which have a non-zero permeability to gases, including carbon dioxide. The total amount of CO2 that can diffuse through all of the seals in the CEGS while a sample is in process is very tiny, on the order of 0.01 µmol. The contamination potential from permeation is even lower than this figure suggests, though, because for most of the process time, the sample is in contact with very few seals, and gases permeating through the rest of the sample path are pumped away. This means that for most samples, the contamination potential from elastomeric seals is sufficiently far below the activity measurement uncertainties as to be negligible.
However, “low-level” samples have extremely small amounts of radiocarbon. These samples are very old (> 35,000 years), or very small (< 2.5 µmol, or 30 µg C), or they are both old and small. Especially in the last case, the contamination potential from elastomeric seal permeation may become significant. By replacing the valves in the sample path with ones that use only metal seals, this potential source of error can be practically eliminated. Unfortunately, metal-sealed valves have a lower conductance and much longer actuation times. Additionally, they cost significantly more and require more expensive actuators. Therefore, they are recommended only when the benefits outweigh the drawbacks.
The CEGS variants that feature all metal-sealed valves in the sample path have model numbers that begin with “LL”, to signify their ability to quantitatively handle low-level samples.
The number of inlet ports can be selected to meet the laboratory’s needs. Most often, the number of inlet ports either matches the number of reactors or there is only one (X1). For example, a CEGS 6X system has six inlet ports (matching the number of reactors), while a CEGS 6X1 has six reactors and one inlet port. The X1 variants have the advantage of lower cost while achieving a similar throughput, with a slightly different workflow.
When the number if inlet ports equals the reactors, the daily workflow is:
When there is only one inlet port:
If the lab has a modest throughput and someone is available to occasionally load a sample, an X1 can be a perfectly adequate solution. If lab staff is limited, or if full batches are produced regularly, the additional inlet ports are more convenient.
For special applications (such as in situ cosmogenic radiocarbon dating, or ramped pyrolysis/oxidation), additional dedicated inlet ports are frequently included, and other devices may be added, such as tube furnaces, tandem coil traps, and CO2 analyzers.
Where CO2 splits for independent stable isotope (δ13C) analysis are required, the CEGS is fitted with a supplementary outlet manifold of gas ports. The gas split ports may be of two types. One has a fitting to accept a 6 mm pyrex tube to collect the split in a flame-sealable ampule (“ampule port”). The other type (“vial port”) has a needle to pierce into a septum-sealed vial commonly used on IRMS gas bench autosamplers. With vial ports, the CEGS can automatically post-fill the vial to slightly over atmospheric pressure with inert gas (He or N2, not Ar) to facilitate processing at the IRMS lab.
Every CEGS includes the following features.
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