Determination of total protein content based on protein elemental composition

Summary

Determination of total protein content based on the elemental composition of proteins is a method of direct analysis based on the characteristics of the elemental composition of proteins. At present, there is one main method for determining the total protein content based on the elemental composition of proteins: e.g. Kjeldahl method.

Principle

Kjeldahl Nitrogen Determination The basic principle of determining the total protein contentis based on the fact that proteins usually contain about 16% nitrogen (14% - 19%), hence the name.

Operation method

Determination of total protein content by Kjeldahl method

Materials and Instruments

Equipment: Kjeldahl vaporization device
Reagents:
①
Sulfuric acid
②
Standard alkali solution

Move

The basic process of determining the precise molecular weight of proteins by mass spectrometry can be divided into the following steps, and this method introduces the whole process of the mass spectrometry technique using matrix-assisted laser desorption ionization time-of-flight mass spectrometry as an example.

A. The sample is heated and digested in sulfuric acid, which oxidizes proteins to carbon dioxide and water, and reduces nitrogen to ammonia (NH ₃ ), which combines with sulfuric acid to form ammonium sulfate.

B. A strong base is added to cause the sulfuric acid to release ammonia by decomposition, and the ammonia is collected in an inorganic acid solution by means of a special Kjeldahl apparatus for vaporizing the ammonia.

C. Titrate with a standard alkali solution to determine the amount of ammonia, from which the nitrogen content of the sample is calculated, and hence the protein content.

Caveat

1. The presence of non-protein nitrogen in the sample has a direct influence on the measured value.

2. In case of deviation of amino acids constituting protein, the nitrogen content will be significantly higher than the theoretical nitrogen content, resulting in measurement error.

3. Compared with other methods, the operation process is cumbersome and requires a high degree of technical skill from the operator.

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