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CEGS
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=== 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.
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