Dumas method

This automatic Dumas technique has replaced the Kjeldahl method as the standard method of analysis for nutritional labelling of protein content of foods (except in high fat content foods where the Kjeldahl method is still preferred due to fire risks). Combust a sample of known mass to 800–900 °C in the presence of an atmosphere that is oxygen-rich and nitrogen-free. Usually, this is a mixture of oxygen and helium (the carrier gas). This releases carbon dioxide, water and nitrogen. The gases are then passed over special columns that absorb the carbon dioxide and water. For example, the columns may contain an aqueous solution of potassium hydroxide. After these columns, the carrier gas contains mostly nitrogen, but also trace amounts of carbon dioxide and water. The final column uses gas chromatography to separate nitrogen from carbon dioxide and water.