Quantitative Analysis of Proteins in Food Based on Mass Spectrometry

Protein content in food is an important indicator of the nutritional value of food, and it is also related to food safety and many other issues, therefore it is significant to accurately determine the protein content for the valuation of food nutrition and safety. The main elements in food include carbon, nitrogen, hydrogen and sulphur, of which nitrogen mainly comes from protein. The average nitrogen content in protein is 16%, so the conversion factor for “nitrogen-to-protein” is 6.25. Therefore, the protein content in food can be obtained by measuring the nitrogen content of food combined with “nitrogen-to-protein” conversion factor. The traditional methods of protein detection are Kjeldahl method, spectrophotometry, and Dumas combustion. The Kjeldahl method is time-consuming and consumes a large amount of reagents, which can cause environmental pollution. Spectrophotometry is susceptible to interference and the quantitative curve is not easy to draw. Dumas combustion is expensive in terms of consumables. In this paper, a platform of tube furnace-quadrupole mass spectrometer was set up to analyze the protein content in food. Food samples were heated at high temperatures in a tube furnace and combusted in an oxygen-rich environment. After the gas products were ionized, they were directly introduced into a quadrupole mass analyzer for detection, which eliminated the need for processes such as oxidation-reduction, adsorption-desorption by using the selection ion scan function of the mass spectrometer. Five kinds of food samples with different protein contents were selected as research object. Each food sample was weighed at 1 g. The food samples were fully combusted in tube furnace at high temperature (1 300 ℃). The gas products were infused through a drying and filtering tube to remove water and smoke, and then entered the EI ion source through a capillary for ionization. By electronic ionization, nitrogen element characteristic ions, N⁺, NO⁺, and NO₂⁺ are generated. The peak intensity of the NO₂⁺ ions were detected by the selection ion scan function of the quadrupole mass analyzer. A standard curve for nitrogen quantification is plotted to be y=115.64x+1 896.9 with the correlation coefficient (R²) of 0.99992 and the relative standard deviation (RSD) of 2.1%-6.1%. The nitrogen content can be calculated by using the peak intensity of NO₂⁺ ions according to the standard curve. The experimental platform of tube furnace-mass spectrometer provides a green, rapid, accurate, and low-cost detection method for quantitative analysis of protein content in food.