Brief description of physical and chemical methods for determining...

10.2.6. Brief description of physical and chemical methods for determining the quality of goods

Chromatographic methods - is a combination of methods for separation and analysis of multicomponent mixtures based on the use of the sorption phenomenon under dynamic conditions.

The chromatographic process takes place in a system of two immiscible phases, one of which is mobile, the other is stationary. The mobile phase containing the sample of the test substance can be gas (gas chromatography) or liquid (liquid chromatography), while a stationary phase is a porous or granular solid (sorbent) or a thin liquid film adsorbed on a solid (thin layer or paper chromatography). The preferability of a chromatographic method is determined by the nature of the analytes (for example, volatile or non-volatile compounds), as well as the efficiency of their separation and detection. Chromatographic methods are used to determine amino acids - threonine, valine, methionine, isoleucine, lysine, phenylalanine, tryptophan, histidine, cystine and others in food products. For example, gas-liquid chromatography is used to determine fatty acids in vegetable oils, organochlorine and organophosphorus pesticides, volatile nitrosamides; gas chromatography - when analyzing the smell of food; liquid - in the determination of antibiotics, hormonal drugs.

The gas-liquid chromatography method (GLC) is widely used for the analysis of volatile components (alcohols, ethers, volatile fatty acids, aldehydes, etc.) in the identification of alcoholic and non-alcoholic beverages, vegetable and animal fats and other food products.

Electrochemical methods are used to determine the content of heavy metals and other elements, many organic substances - alcohols, phenols, to study the ionic composition of water and measure some of the total characteristics, reduction potential (redox potential, Eh). These methods have a number of advantages: high efficiency, lack or low consumption of reagents, high sensitivity, low operating costs, lack of exceptional requirements for the qualification of personnel and, as a result, low cost of a single analysis. Electrochemical devices can be made in a portable or field configuration.

Potentiometric methods (ionometry) are designed to directly determine the concentration of ions in solution using an ion-selective electrode. The method is based on the direct measurement of electrode potentials and the determination of the concentration by a calibration graph or by calculations.

The conductometric method is designed to determine the concentration of a known electrolyte in its pure solution or melt in electrical conductivity. The measurements are carried out at a fixed temperature in solutions containing only one electrolyte.

The voltammetric method is intended to determine the content of toxic elements in food and water. The essence of the method consists in the accumulation on the electrode (from a carbon material or noble metal) of elements present in the aqueous solution, followed by their dissolution with a strictly controlled voltage change at the electrode, which leads to the appearance of current peaks whose height is related to the concentration of elements in the solution .

These methods are an alternative to atomic absorption spectrophotometry.

Capillary electrophoresis is based on the separation of complex mixtures of components in a quartz capillary whose internal diameter is 50-100 μm when applied to a voltage.

The separation occurs due to the difference in the rate of movement of charged particles in the solution under the action of the electric field. The velocity of the particles depends on the magnitude of the charge and mass, determining the degree of their acceleration in the electric field, and also on their size and shape, which determine the frictional resistance that prevents their movement. Diluted solutions of organic and inorganic compounds (salts, acids, alkalis) are used as buffers. Detection of sample components can be performed by spectrophotometric (SF), conductometric, fluorescent or mass spectrometric (MS) detectors. Qualitative and quantitative determination of sample components is carried out by calibration of standard solutions or using a library of spectra (in the case of using SF or MS detectors).

This method is designed to analyze the various classes of organic compounds contained in aqueous samples, as well as to study the ion composition and separation of isomer mixtures.

The method is used to determine the quality indicators of alcoholic and nonalcoholic beverages, meat, fish, dairy, egg products.

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