Theory | Preparation | Operation

For a schematic of the instrument, click here.

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The technique of elemental analysis is a very old and well understood, though it was not until the sixties that it was automated for laboratory use, and not until the nineties that such accurate results could be obtained as though generated by the Exeter Analytical 440 Elemental Analyzer.
Ÿ The principles of elemental analysis are very simple.  They rely on the tendency than all atoms prefer to be in their oxidation states.  In pure oxygen, at high temperatures, all available carbon will easily burn to become carbon dioxide, all hydrogen will burn to become water and all nitrogen will become various nitric oxides.  These fundamentals are enough for chemists to determine any compound's relative percents of carbon hydrogen and nitrogen. 
Ÿ In Colby's CHN analyzer, a sample is moved from a 'cool zone' to a 'combustion train' filled with pure diatomic oxygen.  The sample is heated to a temperature of 980 C, and the burning described above occurs.  After the burn, a flow of ultra high pure He moves the gases through the system.  Any other elements become oxidized, as well, but these compounds are removed by the reduction tube.  This component is a quartz tube filled with reduced copper held at 700 C.  It removes all unreacted oxygen and converts nitric oxides to free nitrogen species which quickly form N2.  Any species that is not CO2 H2O or N2 become adsorbed onto the copper.  The reduction tube consequently must periodically be replaced. 
Ÿ Several safe guards exist which ensure the entire sample is gaseous and moving through the system.  After the initial burn, there is an option to inject additional oxygen, in bursts, to attempt to achieve complete combustion.  Also a 'high-heat coil' vaporizes any condensates that may have settled on the joints in the chamber. 
Ÿ Following the reduction tube the entire sample is pushed by the He flow into the mixing volume chamber, where additional He is added to ensure all gaseous sample is in this chamber.  The flow continues until the chamber is under 1500 mmHg.  This chamber is designed to homogenize the gases in the sample, it takes about 20-50 seconds depending on the flow rate.  During this time all the detectors measure a zero reading. 
Ÿ The gas expands to atmospheric pressure when a pair of valves open to allow the sample gas to exit the mixing chamber through a tube of known volume.  This becomes the sample volume and all excess gas is allowed to exit the system.  The same flow of He then pushes the sample gas through a series of thermal conductivity detectors and traps.  The first detector is sensitive to the amount of water in the sample.  The gas is then forced through a trap (MgO4) which removes all the water, and into a detector which calculates the amount of water that was present in the sample by difference.  A third detector is sensitive to carbon dioxide, which is followed by a trap ('Ascarite') that is able to remove all the carbon dioxide.  Another detector determines the amount of carbon dioxide in the sample volume, again by difference.  These traps also must be periodically replaced.  Finally the gas, composed now of only N2 and He, is pushed through a final detector which is paired with a detector that is exposed to only He.  A difference calculation will reveal how much nitrogen gas was in the sample volume. 
Ÿ Further calculations will reveal the mass percent carbon, hydrogen, and nitrogen in the initial sample.  A final difference calculation can be made to determine the mass percent of the additional elements in the sample.  All these steps used to be apart of a manual process that was extraordinarily labor intensive, time consuming and error prone.