Our instruments determine organic carbon, hydrogen, and nitrogen by means of high-temperature (1000C) combustion (Dumas method) of microgram to milligram size samples in an oxygen-enriched helium atmosphere. The combustion products CO2, NOX (reduced to N2), and H2O are separated by means of sequential, selective chemical traps, and measured with three pairs of high-precision thermal conductivity detectors. Interfering gases of sulfur, halogens, and phosphorus are removed in the combustion reactor using the appropriate solid reagents. Samples, which have been precisely weighed into small tin capsules, placed in protective nickel sleeves, and loaded into the autosampler wheel, are introduced into the combustion furnace by means of a mechanically operated quartz ladle. When combustion is complete, the ladle is withdrawn, and the sample and capsule residue in the nickel sleeve is deposited in a waste chamber (also sealed under helium during the run), thus avoiding buildup of sample residue in the combustion tube. Analysis results are reported in weight percent of the element.
In addition to solid sample materials, particulate organic matter trapped on glass fiber filters can be analyzed. Filters are not weighed, but are placed directly into nickel sleeves for analysis. (A nickel sleeve can accommodate a filter of up to 25mm diameter.) Results are reported in absolute micrograms of the element on the filter.
The control software for the instruments is capable of having the autosampler deposit two sample capsules on the ladle for a single analysis. In this mode, called 'double-drop,' larger total sample weights (for low organic-content materials) and larger diameter glass fiber filters (up to 47mm) can be analyzed.