Principle Difference of Kjeldahl & Dumas Nitrogen Analyzers

Nitrogen analyzer is a special instrument for detecting nitrogen content in seeds, dairy products, beverages, feed, soil and other agricultural and sideline products, which is also known as protein analyzer and crude protein analyzer. There are two commonly used nitrogen analyzers: Kjeldahl nitrogen analyzer and Dumas nitrogen analyzer, and the principles of the two are different.
I. Kjeldahl Nitrogen Analyzer
The principle is Kjeldahl method
Kjeldahl method was established by the Danish chemist Kedal in 1883. It has now been developed into the macro, trace, and flat trace Kjeldahl method and the automatic nitrogen analyzer method, which is a common method to analyze the nitrogen content of organic compounds. The theoretical basis of Kjeldahl method is that the nitrogen content in protein usually accounts for about 16% of its total mass (12% to 19%). Therefore, the total protein content in the substance can be estimated (assuming that all the nitrogen in the measured substance comes from protein) by measuring the nitrogen content in the substance, namely: protein content = nitrogen content/16%.
Kjeldahl method is a method to determine the total nitrogen in a compound or mixture. That is, under the condition of a catalyst, the sample is digested with concentrated sulfuric acid to convert all organic nitrogen into inorganic ammonium salt, and then the ammonium salt is converted into ammonia under alkaline conditions, which is distilled out with steam and absorbed by excess boric acid solution. With standard hydrochloric acid titration, the amount of nitrogen in the sample can be calculated. Since the nitrogen content of protein is relatively constant, the protein content can be calculated from its nitrogen content, so this method is a classic protein quantification method.
II. Dumas Nitrogen Analyzer
The principle is Dumas combustion method
Dumas combustion method is to encapsulate a certain amount of sample in an aluminum boat, and then burn it in a high-temperature furnace. In the environment of catalyst and oxygen, the gases generated after the sample is burned are: CO2, H2O, NOx. After these gases pass through the reduction furnace, NOx is reduced to N2, while CO2 and H2O are separated, and finally nitrogen is measured by a thermal conductivity detector (TCD). Dumas nitrogen analyzer detects the nitrogen/protein content in the sample through the steps of combustion, purification, reduction, and thermal conductivity detection. No sample pre-treatment is required, and no toxic and harmful substances are used or produced during the reaction process, which is safe and environmentally friendly.
Kjeldahl method is a mandatory standard for the measurement of nitrogen or protein content in food, feed, soil, environment, seeds and other samples. The measurement results are interoperable and comparable. Due to the different objects that Dumas combustion method and Kjeldahl method measure for their nitrogen content, the sample type and the composition are different, and the result deviation is also different.