The sample weighed in milligrams housed in a tin capsule is dropped into a quartz tube at 1020°C with constant helium flow (carrier gas). A few seconds before the sample drops into the combustion tube, the stream is enriched with a measured amount of high purity oxygen to achieve a strong oxidizing environment which guarantees almost complete combustion/oxidation even of thermally resistant substances. The combustion gas mixture is driven through an oxidation catalyst (WO3) zone, then through a subsequent copper zone which reduces nitrogen oxides and sulphuric anhydride (SO3) eventually formed during combustion on catalyst reduction to elemental nitrogen and sulphurous anhydride (SO2) and retains the oxygen excess. The resulting four components of the combustion mixture are detected by a Thermal Conductivity detector in the sequence N2, CO2, H2O and SO2. In case of oxygen which is analyzed separately, the sample undergoes immediate pyrolysis in a Helium stream which ensures quantitative conversion of organic oxygen into carbon monoxide separated on a GC column packed with molecular sieves.