| Summary: | Progress Code: completed Statement: Data quality is good for all sediment records. Tin-wrapped samples are loaded into carousels with 49 samples in each tray and 3 trays making up a full run of 147 samples. The carousels are placed on an AS200 autosampler that drops a sample in response to a pulse of compressed air. Gases are swept through the columns and GC by an approximately 100 ml min-1 flow of high purity helium. The sample dropper uses a sliding port that maintains the approximately 1 atmosphere pressure of the helium required to force the gases through the columns and GC. The autosampler sits on top of a Carlo Erba CE1110 CHN-S analyser containing the two furnaces, the scrubber and the GC (Gas Chromatograph). After the GC in the gas stream sits a TCD (Thermal Conductivity Detector) which is used to measure peak area when the machine is used as an elemental analyser. The samples are dropped into the left-hand furnace which is the oxidising furnace and is maintained at 1050 degrees C. It consists of a silica glass tube filled with chromium oxide to the midpoint, with cobaltous silver oxide under it. A tube takes the carrier gas from the bottom of this reactor to the second, reducing furnace tube which the mixed gases flow up through. The second silica glass tube is filled with metallic copper pellets and is kept at 660 degrees C. The gases flow out of the top of this tube and through a scrubber filled with magnesium perchlorate to absorb all water vapour. The dried gases then pass through a packed GC consisting of a 1/4" stainless steel tube with Porapak QS 50-80 mesh. This is held at about 25 degrees C. Nitrogen gas exits from the GC, followed by carbon dioxide after about one minute. These flow to an open split where they are sucked into the mass spectrometer through a 75 micro silica capillary. The gas pulses, still mixed with the helium carrier, flow into the Fisons Isochrom CF-IRMS (Continuous-flow Isotope Ratio Mass Spectrometer) where their isotopic compositions are determined. Surface sediment ...
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