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== Configuration Options == Each CEGS contains graphite reactors and inlet ports, with a Process Section in between. === Graphite reactors === The reduction from CO<sub>2</sub> to 'graphite' typically takes about two hours, but can take anywhere between 30 minutes and 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. === The Low-Level Option === 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 CO<sub>2</sub> 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 positives outweigh the negatives. 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. === Inlet ports === 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 would have six reactors and just 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: # Start of day: Remove yesterday's samples and load new reagents. # Load a batch of samples into inlet ports and start the sequence. # Wait for tomorrow. When there is only one inlet port: # Start of day: Remove yesterday's samples and load new reagents. # Load the first sample into the inlet port and start it. # When the system notifies the operator, load the next sample and start it (''usually takes only a moment''). #* Repeat as needed as long as more samples and reactors are available. # Wait for tomorrow. When 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. === Other Configuration Options === 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 CO<sub>2</sub> analyzers. Where CO<sub>2</sub> splits for independent stable isotope (δ<sup>13</sup>C) 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.
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