Determination of nitrogen, Kjeldahl, Dumas
Determination of nitrogen, Kjeldahl, Dumas

Nitrogen determination instruments, Kjeldahl nitrogen determination instruments, and Dumas nitrogen determination instruments are special instruments for detecting nitrogen content in seeds, dairy products, beverages, feed, soil and other agricultural and sideline products. The nitrogen determination instrument is an instrument that calculates the protein content by measuring the nitrogen content in a sample based on the principle of a constant nitrogen content in the protein. Because of its protein content measurement method, it is called Kjeldahl method and Dumas combustion method, so it is called Kjeldahl and Dumass, it is also called protein analyzer, crude protein assay. instrument.

Nitrogen determination instrument - Kjeldahl method

Principle 1

The Kjeldahl method is a measure of the total nitrogen in a compound or mixture. That is, in the presence of a catalyst, the sample is nitrated with concentrated sulfuric acid to convert the organic nitrogen into an inorganic ammonium salt, and then the ammonium salt is converted into ammonia under alkaline conditions, distilled with steam and absorbed by excess acid, and then With standard acid titration, the amount of nitrogen in the sample can be calculated. Since the protein nitrogen content is relatively constant, the protein content can be calculated from its nitrogen content, so this method is a classical protein quantification method.

2 defects

It can be known from the Kjeldahl principle that the Kjeldahl method is to convert nitrogen-containing organic matter into inorganic nitrogen ammonium sulfate for detection, and obtain the protein content by multiplying the measured value of nitrogen content by a certain factor. The nitrogen-containing organic matter is not only protein, but also melamine and so on. The current national standard and internationally accepted method for determining the content of protein in foods is the classical Kjeldahl method, which provides an opportunity for counterfeiters. The nitrogen content in the protein does not exceed 30%. The most prominent feature of melamine is the high nitrogen content (66%). It is colorless and odorless after being dissolved in water. That is to say, melamine is added to a cup of water and Kjeldahl is used. The nitrogen test showed that it contained protein. Since the Kjeldahl method can only measure nitrogen content and cannot identify non-compliance of chemical substances in feeds, melamine-added milk powder can theoretically measure higher protein content.

3 applications

The universal applicability, accuracy and repeatability of the Kjeldahl method have been widely recognized internationally. It has been determined as the standard method for detecting protein content in foods. However, this method does not give a true protein content because the measured nitrogen may not only be converted by the protein. This can be reflected in food safety incidents such as the 2007 pet food contamination incident in the United States and the 2008 tainted milk powder incident in China: Melamine, a substance with higher nitrogen content, was added to food to counterfeit higher nitrogen content. . In addition, this method requires many different correction factors due to different amino acid sequences. Moreover, the Kjeldahl method also requires the use of concentrated sulfuric acid and longer heating (generally, greater than 1 hour). This also caused the Dumas method to be sometimes more convenient when measuring protein content roughly.

Nitrogen determination instrument - Dumas combustion and nitrogen determination

Principle 1

In 1831, French chemist Jean Baptiste Dumas pioneered a practical nitrogen determination method. In this method, lead carbonate is stored at the front end of the combustion tube. Before the sample is decomposed, the lead carbonate is heated so that the carbon dioxide released by the separation is completely excluded from the combustion of the air in the combustion tube and CuO is burnt. The generated gas is decomposed by PbCO. The CO gas flow arrives at the gas collection cylinder containing the KOH solution on the mercury tank. During combustion, occasionally some of the nitrogen is converted to nitrogen oxides, which are reduced after passing through the red hot copper powder, so that the nitrogen in the organic matter is all reduced to N. 1838, O. L_Erdman and R. F. Marchand introduces CO2 generators. Carbonate reacts with acid to produce CO. 1868, H. Schitt invented the nitrogen turbulence meter to replace Dumas's gas collection cylinder. Since then. The Dumas method has almost no major changes.

In fact, it was not until 1883 that the Danish scientist Kjeldahl proposed the Kjeldahl method, which was 52 years earlier than the Kjeldahl method. However, due to the early Dumas determination method can only determine a few milligrams of the sample, so that it has been greatly limited in the practical application of agricultural products and other fields.

In 1964, Heraeus (Heraeus, precursor of elementar) produced the world's first Rapidly Dumas method (Rapid N). In 1989, with the advent of the world's first Macross, a Dumas method, which can detect gram samples, the Heraeus company opened the Dumas method in food, feed, fertilizer, plants, and soil. And the prelude to the widespread application of clinical and other fields. Therefore, Dumas method has become a legal nitrogen/protein analysis method in some countries and regions in the world, especially in developed countries such as Europe and America. In some areas of the United States, Canada, and Germany, the Dumas method is even the sole standard for determining nitrogen. Dumas' method of nitrogen----An old technology rekindled youth!

2 Limitations

However, the Dumas method also has certain limitations, such as difficult analysis of complex samples, poor analysis of liquid samples. Samples need to be uniform powder, but also smaller. These limitations are due to certain limitations in the analysis principle. For the time being, it cannot be avoided. In addition, the price of 40-60,000 Dumass dignifiers limits the promotion and application of this method.

3 Application

Due to some differences in experimental principles with the Kjeldahl method, there will be a phenomenon in which the results of a sample detected by the two methods are different. A large number of experiments found that the results of the Dumas method are slightly higher than the Kjeldahl method. The measured value. This is because the Kjeldahl method has a recognized limitation: it is difficult to quantitatively convert organic matter into ammonia, and especially nitrate-containing samples affect the results of total nitrogen. However, the Dumas NOx method does not have this problem and can completely detect nitrogen in feedstuffs. Taking a fish meal sample as an example, the Kjeldahl method can only determine the nitrogen in the fish meal except for nitrate nitrogen, putrescine, and cadaverine, while the Dumas method can measure nitrate nitrogen, putrescine, and cadaverine. The nitrogen inside, that is to say, the detection of fishmeal by this method is 1-2 proteins higher than the Kjeldahl method. The contribution to feed formulation cannot be ignored.

In Western countries with high environmental and safety requirements, the Dumas method has become the legal method for the analysis of nitrogen/crude protein. The Canadian Grain Commission uses the Dumas method as a standard method to test the protein and total nitrogen in grains, seeds, and their products. The U.S. Department of Agriculture and the AOAC also used the Dumas method instead of the Kjeldahl method as a standard method for the determination of crude protein content in cereals and feedstuffs.
At present, the standard method for the determination of protein in the country is still limited to the Kjeldahl method. This method has been widely used for more than one hundred years. Even in a small laboratory, it can be realized with a simple Kjeldahl device. Determination of protein. However, in today’s developed China, more and more attention is paid to environmental protection and human health. Given the obvious advantages of the Dumas method, it is bound to be accepted by domestic developed laboratories and gradually occupy a place in the field of nitrogen/crude protein detection, especially In international trade arbitration of import and export, it is in line with international standards.

It is worth looking forward to that the Dumas CBD method for detecting crude protein in feeds will be included in China's national standards.
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