1. Introduction.

2. The emergence and development of chromatography.

3. Classification of chromatographic methods.

4. Chromatography on a solid stationary phase:

a) gas (gas adsorption) chromatography;

b) liquid (liquid-adsorption) chromatography.

5. Chromatography on liquid stationary phase:

a) gas-liquid chromatography;

b) gel chromatography.

6. Conclusion.

As the rays of the spectrum, in the column of calcium carbonate, various components of the mixture of pigments are regularly distributed, making it possible to determine their qualitative and quantitative determination. I call the preparation obtained in this way a chromatogram, and the proposed method - chromatographic.

M.S.Tsvet, 1906

Introduction

The need to separate and analyze a mixture of substances is faced not only by a chemist, but also by many other specialists.

In the powerful arsenal of chemical and physicochemical methods of separation, analysis, study of the structure and properties of individual chemical compounds and their complex mixtures, chromatography occupies one of the leading places.

Chromatography is a physicochemical method for separating and analyzing mixtures of gases, vapors, liquids or solutes and determining the physicochemical properties of individual substances, based on the distribution of the separated components of mixtures between two phases: mobile and stationary. The substances that make up the stationary phase are called sorbents. The stationary phase can be solid or liquid. A mobile phase is a liquid or gas flow that is filtered through a sorbent bed. The mobile phase acts as a solvent and carrier for the analyzed mixture of substances, converted into a gaseous or liquid state.

There are two types of sorption: adsorption - the absorption of substances by a solid surface and absorption - the dissolution of gases and liquids in liquid solvents.

2. The emergence and development of chromatography

The emergence of chromatography as a scientific method is associated with the name of the outstanding Russian scientist Mikhail Semenovich Tsvet (1872 - 1919), who in 1903 discovered chromatography in the course of researching the mechanism of solar energy conversion in plant pigments. This is the year and should be considered the date of creation of the chromatographic method.

M.S. The color passed a solution of the analytes and the mobile phase through a column of adsorbent in a glass tube. In this regard, his method was called column chromatography. In 1938 N.A. Izmailov and M.S. Schreiber suggested modifying the Tsvet method and separating a mixture of substances on a plate covered with a thin layer of adsorbent. This is how thin-layer chromatography arose, which makes it possible to carry out analysis with a trace amount of a substance.

In 1947 T.B. Gapon, E.N. Gapon and F.M. Shemyakin was the first to carry out the chromatographic separation of a mixture of ions in a solution, explaining it by the presence of an exchange reaction between sorbent ions and ions contained in the solution. Thus, another direction of chromatography was discovered - ion exchange chromatography. At present, ion exchange chromatography is one of the most important areas of the chromatographic method.

E.N. and G.B. Gapon in 1948 implemented what M.S. The color idea of \u200b\u200bthe possibility of chromatographic separation of a mixture of substances based on the difference in the solubility of hardly soluble precipitates. Sediment chromatography appeared.

In 1957 M. Golay proposed to apply a sorbent to the inner walls of a capillary tube - capillary chromatography. This option allows you to analyze trace amounts of multicomponent mixtures.

In the 60s, it became possible to synthesize both ionic and uncharged gels with strictly defined pore sizes. This made it possible to develop a variant of chromatography, the essence of which is to separate a mixture of substances on the basis of the difference in their ability to penetrate into gel - gel chromatography. This method allows the separation of mixtures of substances with different molecular weights.

Currently, chromatography has undergone significant development. Today, a variety of chromatographic methods, especially in combination with other physical and physicochemical methods, help scientists and engineers to solve a variety of, often very complex, problems in scientific research and technology.

3. Classification of chromatographic methods

The variety of modifications and variants of the chromatographic method requires their systematization or classification.

The classification can be based on various features, namely:

1. state of aggregation of phases;

2. separation mechanism;

3. method of carrying out the process;

4. the purpose of the process.

Classification by the state of aggregation of phases:

gas (mobile phase - gas), gas-liquid (mobile phase - gas, stationary phase - liquid), liquid (mobile phase - liquid) chromatography.

Classification by the separation mechanism.

Adsorption chromatography is based on the selective adsorption (absorption) of individual components of the analyzed mixture by the corresponding adsorbents. Adsorption chromatography is subdivided into liquid (liquid-adsorption chromatography) and gas (gas-adsorption chromatography).

Ion exchange chromatography is based on the use of ion exchange processes occurring between mobile adsorbent ions and electrolyte ions when passing a solution of the analyte through a column filled with an ion exchanger (ion exchanger). Ion exchangers are insoluble inorganic and organic high molecular weight compounds. As ion exchangers used are alumina, permutite, sulfocarbon and various synthetic organic ion-exchange substances - ion-exchange resins.

Sediment chromatography is based on the different solubility of precipitates formed by the components of the analyzed mixture with special reagents. For example, when a solution of a mixture of Hg (II) and Pb salts is passed through a column with a carrier previously impregnated with a KI solution, 2 colored layers are formed: the upper, colored orange-red (HgI 2), and the lower, colored yellow (PbI 2).

Classification by the way the process is carried out.

Column chromatography is a type of chromatography in which a column is used as a carrier for a stationary solvent.

Paper chromatography is a type of chromatography in which strips or sheets of filter paper that do not contain mineral impurities are used instead of a column as a carrier for a stationary solvent. In this case, a drop of the test solution, for example, a mixture of solutions of Fe (III) and Co (II) salts, is applied to the edge of the paper strip. The paper is suspended in a closed chamber (Fig. 1) by dropping its edge with a drop of the test solution applied on it into a vessel with a mobile solvent, for example, n-butyl alcohol. A mobile solvent, moving along the paper, wets it. In this case, each substance contained in the analyzed mixture moves with its inherent speed in the same direction as the solvent. At the end of the ion separation, the paper is dried and then sprayed with a reagent, in this case a solution of K 4, which forms colored compounds with the substances to be separated (blue - with iron ions, green - with cobalt ions). The resulting areas in the form of colored spots make it possible to establish the presence of individual components.

Paper chromatography in combination with the use of organic reagents allows qualitative analysis of complex mixtures of cations and anions. A number of substances can be detected on one chromatogram with the help of one reagent, since each substance is characterized not only by the corresponding color, but also by a certain localization location on the chromatogram.

Thin layer chromatography is a type of chromatography that is similar to paper chromatography in its separation mechanism. The difference between them lies in the fact that, instead of paper sheets, separation is carried out on plates coated with a thin layer of sorbent made of powdered alumina, cellulose, zeolites, silica gel, diatomaceous earth, etc. and retaining an immobile solvent. The main advantage of thin-layer chromatography is the simplicity of the apparatus, the simplicity and high speed of the experiment, the sufficient clarity of the separation of the mixture of substances, and the possibility of analyzing ultramicro-quantities of the substance.

Classification according to the purpose of the chromatographic process.

Chromatography is of greatest importance as a method for the qualitative and quantitative analysis of mixtures of substances (analytical chromatography).

Preparative chromatography is a type of chromatography in which a mixture of substances is separated for preparative purposes, i.e. to obtain more or less significant amounts of substances in a pure, free from impurities form. The task of preparative chromatography can also be the concentration and subsequent separation from the mixture of substances contained in the form of trace impurities to the basic substance.

Non-analytical chromatography is a type of chromatography used as a method of scientific research. It is used to study the properties of systems, such as solutions, the kinetics of chemical processes, the properties of catalysts and adsorbents.

So, chromatography is a universal method for analyzing mixtures of substances, obtaining substances in pure form, as well as a method for studying the properties of systems.

4. Chromatography on a solid stationary phase

a) Gas (gas adsorption) chromatography

Gas chromatography is a chromatographic method in which the mobile phase is gas. Gas chromatography has found the greatest application for the separation, analysis and study of substances and their mixtures, passing without decomposition into a vapor state.

One of the options for gas chromatography is gas adsorption chromatography, a method in which the stationary phase is a solid adsorbent.

In gas chromatography, an inert gas is used as a mobile phase (carrier gas): helium, nitrogen, argon, much less often hydrogen and carbon dioxide. Sometimes the carrier gas is a pair of highly volatile liquids.

The gas chromatographic process is usually carried out in special devices called gas chromatographs (Figure 3). Each of them has a system for supplying a carrier gas flow, a system for preparing and introducing the mixture under study, a chromatographic column with a system for regulating its temperature, an analyzing system (detector), and a system for recording separation and analysis results (recorder).

Temperature is of great importance in gas adsorption chromatography. Its role primarily consists in changing the sorption equilibrium in the gas - solid system. Correct selection of the column temperature determines the degree of separation of the mixture components, the column efficiency, and the overall analysis speed. There is a certain column temperature range in which chromatographic analysis is optimal. Typically, this temperature range is in the region close to the boiling point of the determined chemical compound. When the boiling points of the mixture components differ greatly from each other, column temperature programming is used.

Separation in a chromatographic column is the most important, but preliminary, operation of the entire process of gas chromatographic analysis. Binary mixtures (carrier gas - component) leaving the column, as a rule, enter the detecting device. Here, changes in the concentrations of components over time are converted into an electrical signal, which is recorded using a special system in the form of a curve called a chromatogram. The results of the entire experiment largely depend on the correct choice of the type of detector and its design. There are several classifications of detectors. Distinguish between differential and integral detectors. Differential detectors record the instantaneous value of one of the characteristics (concentration or flow) over time. Integral detectors add up the amount of substance over a certain period of time. They also use detectors of various types, sensitivity and purpose: thermoconductometric, ionization, spectroscopic, mass spectrometric, coulometric and many others.

Application of gas adsorption chromatography

Gas adsorption chromatography is used in the chemical and petrochemical industries to analyze the products of chemical and petrochemical synthesis, the composition of oil fractions, to determine the purity of reagents and the content of key products at different stages of technological processes, etc.

Gas chromatography analysis of permanent gases and light hydrocarbons, including isomers, takes 5-6 minutes. Previously, on traditional gas analyzers, this analysis lasted 5 - 6 hours. All this led to the fact that gas chromatography began to be widely used not only in research institutes and control and measuring laboratories, but also entered the systems of complex automation of industrial enterprises.

Today, gas chromatography is also used in the search for oil and gas fields, making it possible to determine the content of organic matter in samples taken from soils, indicating the proximity of oil and gas fields.

Gas chromatography is successfully used in forensic science, where it is used to establish the identity of samples of blood stains, gasoline, oils, counterfeit expensive food products, etc. Gas chromatography is very often used to determine the blood alcohol content of car drivers. A few drops of blood from a finger are enough to know how much, when and what kind of alcoholic drink he drank.

Gas chromatography allows us to obtain valuable and unique information about the composition of odors of food products, such as cheese, coffee, caviar, brandy, etc. Sometimes the information obtained by gas chromatographic analysis does not please us. For example, often excessive amounts of pesticides are found in food or fruit juice contains trichlorethylene, which, contrary to prohibitions, was used to increase the degree of extraction of carotene from fruits, etc. But it is this information that protects human health.

However, it is not uncommon for people to simply ignore the information they receive. This primarily applies to smoking. Detailed gas chromatographic analysis has long established that the smoke of cigarettes and cigarettes contains up to 250 different hydrocarbons and their derivatives, of which about 50 have a carcinogenic effect. That is why lung cancer occurs in smokers 10 times more often, but still millions of people continue to poison themselves, their colleagues and relatives.

Gas chromatography is widely used in medicine to determine the content of numerous drugs, to determine the level of fatty acids, cholesterol, steroids, etc. in the patient's body. Such analyzes provide extremely important information about the state of human health, the course of his illness, the effectiveness of the use of certain drugs.

Scientific research in metallurgy, microbiology, biochemistry, in the development of plant protection products and new drugs, in the creation of new polymers, building materials and in many other very different areas of human practice cannot be imagined without such a powerful analytical method as gas chromatography.

Gas chromatography is successfully used to determine the content of polycyclic aromatic compounds hazardous to human health in water and in the air, the level of gasoline in the air of filling stations, the composition of automobile exhaust gases in the air, etc.

This method is widely used as one of the main methods of environmental cleanliness control.

Gas chromatography plays an important role in our lives, providing us with an enormous amount of information. In the national economy and in research organizations, more than 20 thousand of a wide variety of gas chromatographs are used, which are indispensable helpers in solving many complex problems that arise every day before researchers and engineers.

b) Liquid (liquid-adsorption) chromatography

Liquid chromatography is a group of chromatography variants in which the mobile phase is liquid.

One of the options for liquid chromatography is liquid adsorption chromatography - a method in which the stationary phase is a solid adsorbent.

Although liquid chromatography was discovered earlier than gas chromatography, it only entered a period of extremely intensive development in the second half of the 20th century. At present, in terms of the degree of development of the theory of the chromatographic process and the technique of instrumental design, in terms of the efficiency and speed of separation, it is hardly inferior to the method of gas chromatographic separation. However, each of these two main types of chromatography has its own preferred field of application. If gas chromatography is suitable mainly for the analysis, separation and study of chemicals with molecular weights of 500 - 600, then liquid chromatography can be used for substances with molecular weights from several hundred to several million, including extremely complex macromolecules of polymers, proteins and nucleic acids. At the same time, the opposition of various chromatographic methods is inherently devoid of common sense, since chromatographic methods successfully complement each other, and the very task of a specific study must be approached in a different way, namely, which chromatographic method allows solving it with greater speed, information content and at a lower cost.

As in gas chromatography, modern liquid chromatography uses detectors to continuously record the concentration of the analyte in the liquid flow from the column.

There is no single universal detector for liquid chromatography. Therefore, in each case, the most suitable detector should be selected. The most widely used are ultraviolet, refractometric, microadsorption and transport flame ionization detectors.

Spectrometric detectors. Detectors of this type are highly sensitive selective devices that make it possible to determine very small concentrations of substances in the flow of the liquid phase. Their readings depend little on temperature fluctuations and other random changes in the environment. One of the important features of spectrometric detectors is the transparency of most solvents used in liquid-adsorption chromatography in the working wavelength range.

Most often used absorption in the UV, less often in the IR region. In the UV region, devices are used that operate in a wide range - from 200 nm to the visible part of the spectrum, or at certain wavelengths, most often at 280 and 254 nm. Low pressure (254 nm) and medium pressure (280 nm) mercury lamps and corresponding filters are used as radiation sources.

Microadsorption detectors. The action of microadsorption detectors is based on the release of heat during the adsorption of a substance on the adsorbent, which is filling the detector cell. However, it is not the heat that is measured, but the temperature of the adsorbent to which it is heated as a result of adsorption.

A microadsorption detector is a fairly highly sensitive instrument. Its sensitivity depends primarily on the heat of adsorption.

Microadsorption detectors are versatile, suitable for detecting both organic and inorganic substances. However, it is difficult to obtain sufficiently clear chromatograms on them, especially with incomplete separation of the mixture components.

5. Liquid stationary phase chromatography

a) Gas-liquid chromatography

Gas-liquid chromatography is a gas chromatographic method in which the stationary phase is a low-volatile liquid deposited on a solid carrier.

This type of chromatography is used to separate gases and vapors of liquids.

The main difference between gas-liquid chromatography and gas-adsorption chromatography is that in the first case the method is based on the use of the process of dissolution and subsequent evaporation of gas or vapor from a liquid film held by a solid inert carrier; in the second case, the separation process is based on the adsorption and subsequent desorption of gas or vapor on the surface of a solid - an adsorbent.

The chromatography process can be schematically represented as follows. A mixture of gases or vapors of volatile liquids is introduced by a flow of a carrier gas into a column filled with a stationary inert carrier, on which a non-volatile liquid (stationary phase) is distributed. The gases and vapors under investigation are absorbed by this liquid. Then the components of the mixture to be separated are selectively displaced in a certain order from the column.

In gas-liquid chromatography, a number of detectors are used that specifically react to any organic substances or to organic substances with a certain functional group. These include ionization detectors, electron capture detectors, thermionic, spectrophotometric and some other detectors.

Flame ionization detector (FID). The operation of the FID is based on the fact that organic substances, entering the flame of a hydrogen burner, undergo ionization, as a result of which an ionization current arises in the detector chamber, which is also an ionization chamber, the strength of which is proportional to the number of charged particles.

The PID is sensitive only to organic compounds and is insensitive or very weakly sensitive to gases such as air, sulfur and carbon oxides, hydrogen sulfide, ammonia, carbon disulfide, water vapor and a number of other inorganic compounds. The insensitivity of the FID to air allows it to be used to determine air pollution with various organic substances.

FID uses 3 gases: carrier gas (helium or nitrogen), hydrogen and air. All 3 gases must be of high purity.

Argon detector. In an argon detector, ionization is caused by the collision of molecules of the analyte with metastable argon atoms formed as a result of exposure to radioactive B-radiation.

Thermoionic detector. The principle of operation of a thermionic detector is that salts of alkali metals, evaporating in the flame of a burner, selectively react with compounds containing halogens or phosphorus. In the absence of such compounds, an equilibrium of alkali metal atoms is established in the ionization chamber of the detector. The presence of phosphorus atoms due to their reaction with alkali metal atoms violates this equilibrium and causes the appearance of an ion current in the chamber.

Since the thermionic detector has the highest sensitivity to phosphorus-containing compounds, it is called phosphoric. This detector is used mainly for the analysis of organophosphate pesticides, insecticides and a number of biologically active compounds.

b) Gel chromatography

Gel chromatography (gel filtration) is a method of separating mixtures of substances with different molecular weights by filtering the analyzed solution through cross-linked cellular gels.

The separation of a mixture of substances occurs if the sizes of the molecules of these substances are different, and the pore diameter of the gel grains is constant and can only let through those molecules whose sizes are less than the diameter of the pore holes of the gel. When filtering a solution of the analyzed mixture, smaller molecules, penetrating into the pores of the gel, are retained in the solvent contained in these pores, and move along the gel layer more slowly than large molecules that are unable to penetrate into the pores. Thus, gel chromatography makes it possible to separate a mixture of substances depending on the size and molecular weight of the particles of these substances. This separation method is simple, fast and, most importantly, it allows the separation of mixtures of substances under milder conditions than other chromatographic methods.

If you fill a column with gel granules and then pour a solution of various substances with different molecular weights into it, then when the solution moves along the gel layer in the column, this mixture will separate.

The initial period of the experiment: application of a solution of the analyzed mixture to a gel layer in a column. The second stage - the gel does not interfere with the diffusion of small molecules into the pores, while large molecules remain in the solution surrounding the gel granules. When the gel layer is washed with a pure solvent, large molecules begin to move at a rate close to that of the solvent, while small molecules must first diffuse from the internal pores of the gel into the volume between the grains and, as a result, are retained and washed out by the solvent later. A mixture of substances is separated according to their molecular weight. The substances are washed out from the column in order of decreasing molecular weight.

Application of gel chromatography.

The main purpose of gel chromatography is the separation of mixtures of high molecular weight compounds and the determination of the molecular weight distribution of polymers.

However, gel chromatography is equally used to separate mixtures of substances of average molecular weight and even low molecular weight compounds. In this case, it is of great importance that gel chromatography allows separation at room temperatures, which favorably distinguishes it from gas-liquid chromatography, which requires heating to transfer the analytes to the vapor phase.

The separation of a mixture of substances by gel chromatography is also possible when the molecular weights of the analyzed substances are very close or even equal. In this case, the interaction of solutes with the gel is used. This interaction can be so significant that it cancels out the differences in molecular sizes. If the nature of the interaction with the gel is not the same for different substances, this difference can be used to separate the mixture of interest.

An example is the use of gel chromatography for the diagnosis of thyroid diseases. The diagnosis is established by the amount of iodine determined during the analysis.

The given examples of gel chromatography application show its wide possibilities for solving a wide variety of analytical problems.

Conclusion

As a scientific method of cognition of the world around us, chromatography is constantly developing and improving. Today it is used so often and so widely in scientific research, medicine, molecular biology, biochemistry, technology, and the national economy that it is very difficult to find a field of knowledge in which chromatography is not used.

Chromatography as a research method with its exceptional capabilities is a powerful factor in cognition and transformation of an increasingly complex world in the interests of creating acceptable living conditions for humans on our planet.

LIST OF REFERENCES

1. Aivazov B.V. Introduction to chromatography. - M .: Higher school, 1983 - p. 8-18, 48-68, 88-233.

2. Kreshkov A.P. Fundamentals of Analytical Chemistry. Theoretical basis. Qualitative Analysis, Book One, 4th Edition, rev. M., "Chemistry", 1976 - p. 119-125.

3. Sakodynsky K.I., Orekhov B.I. Chromatography in Science and Technology. - M .: Knowledge, 1982 - p. 3-20, 28-38, 58-59.

2. The emergence and development of chromatography

The emergence of chromatography as a scientific method is associated with the name of the outstanding Russian scientist Mikhail Semenovich Tsvet (1872 - 1919), who in 1903 discovered chromatography in the course of researching the mechanism of solar energy conversion in plant pigments. This is the year and should be considered the date of creation of the chromatographic method.

M.S. The color passed a solution of the analytes and the mobile phase through a column of adsorbent in a glass tube. In this regard, his method was called column chromatography. In 1938 N.A. Izmailov and M.S. Schreiber suggested modifying the Tsvet method and separating a mixture of substances on a plate covered with a thin layer of adsorbent. This is how thin-layer chromatography arose, which makes it possible to carry out analysis with a trace amount of a substance.

In 1947 T.B. Gapon, E.N. Gapon and F.M. Shemyakin was the first to carry out the chromatographic separation of a mixture of ions in a solution, explaining it by the presence of an exchange reaction between sorbent ions and ions contained in the solution. Thus, another direction of chromatography was discovered - ion exchange chromatography. At present, ion exchange chromatography is one of the most important areas of the chromatographic method.

E.N. and G.B. Gapon in 1948 implemented what M.S. The color idea of \u200b\u200bthe possibility of chromatographic separation of a mixture of substances based on the difference in the solubility of hardly soluble precipitates. Sediment chromatography appeared.

In 1957 M. Golay proposed to apply a sorbent to the inner walls of a capillary tube - capillary chromatography. This option allows you to analyze trace amounts of multicomponent mixtures.

In the 60s, it became possible to synthesize both ionic and uncharged gels with strictly defined pore sizes. This made it possible to develop a variant of chromatography, the essence of which is to separate a mixture of substances on the basis of the difference in their ability to penetrate into gel - gel chromatography. This method allows the separation of mixtures of substances with different molecular weights.

Currently, chromatography has undergone significant development. Today, a variety of chromatographic methods, especially in combination with other physical and physicochemical methods, help scientists and engineers to solve a variety of, often very complex, problems in scientific research and technology.

Dmitry Ivanovich Mendeleev: contribution to the development of chemistry

Dmitry Mendeleev was born on January 27 (February 8) 1834 in Tobolsk in the family of the director of the gymnasium and the trustee of public schools in the Tobolsk province Ivan Pavlovich Mendeleev and Maria Dmitrievna Mendeleeva, nee Kornilieva ...

Fat-soluble vitamins

Hypovitaminosis is a disease associated with a lack of vitamins in the body. Lack of certain vitamins - vitamin deficiency. With an excess intake of vitamins with the diet, hypervitaminosis, diseases associated with an excess of vitamins ...

History of the Russian Chemical Society

Alexander Abramovich Voskresensky (1809-1880) - Russian organic chemist, founder (together with Nikolai Nikolaevich Zinin) of a large school of Russian chemists, corresponding member of the Petersburg Academy of Sciences (1864) ...

Historical overview of the main stages in the development of chemistry

Colloidal systems in the body and their functions

Development of ideas about colloidal systems and their properties. Colloidal processes such as dyeing and gluing have been used since ancient Egypt. The word "colloid" (from the Greek word meaning "glue") was introduced by T. Graham in 1862 ...

Polyhalogenated derivatives of alkanes

The history of fluorine chemistry does not begin in ancient Egypt or Phenicia, or even in medieval Arabia. The beginning of the emergence of fluorine chemistry was the discovery of hydrogen fluoride (Scheele, 1771) and then elemental fluorine (Moissant, 1886) ...

Traditionally, an experiment in a laboratory practice forms empirical thinking. Students explore the phenomenon, identify structural elements in it, classify them, describe connections, but all this is divided in consciousness ...

Formation of chemistry

one). Pre-alchemical period: up to III century. AD Chemistry, the science of the composition of substances and their transformations, begins with the discovery by man of the ability of fire to change natural materials. Apparently, people knew how to smelt copper and bronze, burn clay products ...

The basis of one or another classification of chromatographic methods can be based on various characteristic features of the process ...

Physical and chemical foundations of the chromatographic process

The task of the theory of chromatography is to establish the laws of motion and diffusion of chromatographic zones. The main factors underlying the classification of chromatography theories ...

Chemistry of oil and gas

The ingenious guess of M.V. ...

Chromatography as a method of separation and analysis

chromatography mixture sorption desorption Chromatography is a physicochemical process based on repeated repetition of acts of sorption and desorption of a substance when it moves in a flow of a mobile phase along a stationary sorbent ...

Evolution of chemistry - near-term prospects

What are chemical compounds made of? How are the smallest particles of matter arranged? How are they located in space? What unites these particles? Why do some substances react with each other ...

Very little is known about conducting analyzes in ancient Russia. Naturally, it was always necessary to check the composition of various materials, and in Russia this was done by herbalists, dyers, blacksmiths; there were even special mining specialists ...

Stages of formation of analytical chemistry in Russia

1. Introduction.

2. The emergence and development of chromatography.

3. Classification of chromatographic methods.

4. Chromatography on a solid stationary phase:

a) gas (gas adsorption) chromatography;

b) liquid (liquid-adsorption) chromatography.

5. Chromatography on liquid stationary phase:

a) gas-liquid chromatography;

b) gel chromatography.

6. Conclusion.

As the rays of the spectrum, in the column of calcium carbonate, various components of the mixture of pigments are regularly distributed, making it possible to determine their qualitative and quantitative determination. I call the preparation obtained in this way a chromatogram, and the proposed method - chromatographic.

M.S.Tsvet, 1906

Introduction

The need to separate and analyze a mixture of substances is faced not only by a chemist, but also by many other specialists.

In the powerful arsenal of chemical and physicochemical methods of separation, analysis, study of the structure and properties of individual chemical compounds and their complex mixtures, chromatography occupies one of the leading places.

Chromatography is a physicochemical method for separating and analyzing mixtures of gases, vapors, liquids or solutes and determining the physicochemical properties of individual substances, based on the distribution of the separated components of mixtures between two phases: mobile and stationary. The substances that make up the stationary phase are called sorbents. The stationary phase can be solid or liquid. A mobile phase is a liquid or gas flow that is filtered through a sorbent bed. The mobile phase acts as a solvent and carrier for the analyzed mixture of substances, converted into a gaseous or liquid state.

There are two types of sorption: adsorption - the absorption of substances by a solid surface and absorption - the dissolution of gases and liquids in liquid solvents.

2. The emergence and development of chromatography

The emergence of chromatography as a scientific method is associated with the name of the outstanding Russian scientist Mikhail Semenovich Tsvet (1872 - 1919), who in 1903 discovered chromatography in the course of researching the mechanism of solar energy conversion in plant pigments. This is the year and should be considered the date of creation of the chromatographic method.

M.S. The color passed a solution of the analytes and the mobile phase through a column of adsorbent in a glass tube. In this regard, his method was called column chromatography. In 1938 N.A. Izmailov and M.S. Schreiber suggested modifying the Tsvet method and separating a mixture of substances on a plate covered with a thin layer of adsorbent. This is how thin-layer chromatography arose, which makes it possible to carry out analysis with a trace amount of a substance.

In 1947 T.B. Gapon, E.N. Gapon and F.M. Shemyakin was the first to carry out the chromatographic separation of a mixture of ions in a solution, explaining it by the presence of an exchange reaction between sorbent ions and ions contained in the solution. Thus, another direction of chromatography was discovered - ion exchange chromatography. At present, ion exchange chromatography is one of the most important areas of the chromatographic method.

E.N. and G.B. Gapon in 1948 implemented what M.S. The color idea of \u200b\u200bthe possibility of chromatographic separation of a mixture of substances based on the difference in the solubility of hardly soluble precipitates. Sediment chromatography appeared.

In 1957 M. Golay proposed to apply a sorbent to the inner walls of a capillary tube - capillary chromatography. This option allows you to analyze trace amounts of multicomponent mixtures.

In the 60s, it became possible to synthesize both ionic and uncharged gels with strictly defined pore sizes. This made it possible to develop a variant of chromatography, the essence of which is to separate a mixture of substances on the basis of the difference in their ability to penetrate into gel - gel chromatography. This method allows the separation of mixtures of substances with different molecular weights.

Currently, chromatography has undergone significant development. Today, a variety of chromatographic methods, especially in combination with other physical and physicochemical methods, help scientists and engineers to solve a variety of, often very complex, problems in scientific research and technology.

3. Classification of chromatographic methods

The variety of modifications and variants of the chromatographic method requires their systematization or classification.

The classification can be based on various features, namely:

1. state of aggregation of phases;

2. separation mechanism;

3. method of carrying out the process;

4. the purpose of the process.

Classification by the state of aggregation of phases:

gas (mobile phase - gas), gas-liquid (mobile phase - gas, stationary phase - liquid), liquid (mobile phase - liquid) chromatography.

Classification by the separation mechanism.

Adsorption chromatography is based on the selective adsorption (absorption) of individual components of the analyzed mixture by the corresponding adsorbents. Adsorption chromatography is subdivided into liquid (liquid-adsorption chromatography) and gas (gas-adsorption chromatography).

Ion exchange chromatography is based on the use of ion exchange processes occurring between mobile adsorbent ions and electrolyte ions when passing a solution of the analyte through a column filled with an ion exchanger (ion exchanger). Ion exchangers are insoluble inorganic and organic high molecular weight compounds. As ion exchangers used are alumina, permutite, sulfocarbon and various synthetic organic ion-exchange substances - ion-exchange resins.

Sediment chromatography is based on the different solubility of precipitates formed by the components of the analyzed mixture with special reagents. For example, when a solution of a mixture of Hg (II) and Pb salts is passed through a column with a carrier previously impregnated with a KI solution, 2 colored layers are formed: the upper, colored orange-red (HgI 2), and the lower, colored yellow (PbI 2).

Classification by the way the process is carried out.

Column chromatography is a type of chromatography in which a column is used as a carrier for a stationary solvent.

Paper chromatography is a type of chromatography in which strips or sheets of filter paper that do not contain mineral impurities are used instead of a column as a carrier for a stationary solvent. In this case, a drop of the test solution, for example, a mixture of solutions of Fe (III) and Co (II) salts, is applied to the edge of the paper strip. The paper is suspended in a closed chamber (Fig. 1) by dropping its edge with a drop of the test solution applied on it into a vessel with a mobile solvent, for example, n-butyl alcohol. A mobile solvent, moving along the paper, wets it. In this case, each substance contained in the analyzed mixture moves with its inherent speed in the same direction as the solvent. At the end of the ion separation, the paper is dried and then sprayed with a reagent, in this case a solution of K 4, which forms colored compounds with the substances to be separated (blue - with iron ions, green - with cobalt ions). The resulting areas in the form of colored spots make it possible to establish the presence of individual components.

Paper chromatography in combination with the use of organic reagents allows qualitative analysis of complex mixtures of cations and anions. A number of substances can be detected on one chromatogram with the help of one reagent, since each substance is characterized not only by the corresponding color, but also by a certain localization location on the chromatogram.

Thin layer chromatography is a type of chromatography that is similar to paper chromatography in its separation mechanism. The difference between them lies in the fact that, instead of paper sheets, separation is carried out on plates coated with a thin layer of sorbent made of powdered alumina, cellulose, zeolites, silica gel, diatomaceous earth, etc. and retaining an immobile solvent. The main advantage of thin-layer chromatography is the simplicity of the apparatus, the simplicity and high speed of the experiment, the sufficient clarity of the separation of the mixture of substances, and the possibility of analyzing ultramicro-quantities of the substance.

Classification according to the purpose of the chromatographic process.

Chromatography is of greatest importance as a method for the qualitative and quantitative analysis of mixtures of substances (analytical chromatography).

Preparative chromatography is a type of chromatography in which a mixture of substances is separated for preparative purposes, i.e. to obtain more or less significant amounts of substances in a pure, free from impurities form. The task of preparative chromatography can also be the concentration and subsequent separation from the mixture of substances contained in the form of trace impurities to the basic substance.

Non-analytical chromatography is a type of chromatography used as a method of scientific research. It is used to study the properties of systems, such as solutions, the kinetics of chemical processes, the properties of catalysts and adsorbents.

So, chromatography is a universal method for analyzing mixtures of substances, obtaining substances in pure form, as well as a method for studying the properties of systems.

4. Chromatography on a solid stationary phase

a) Gas (gas adsorption) chromatography

Gas chromatography is a chromatographic method in which the mobile phase is gas. Gas chromatography has found the greatest application for the separation, analysis and study of substances and their mixtures, passing without decomposition into a vapor state.

1. INTRODUCTION.

2. The emergence and development of chromatography.

3. Classification of chromatographic methods.

4. Chromatography on a solid stationary phase:

a) gas (gas adsorption) chromatography;

b) liquid (liquid-adsorption) chromatography.

5. Chromatography on liquid stationary phase:

a) gas-liquid chromatography;

b) gel chromatography.

6. Conclusion.

As the rays of the spectrum, various components of the mixture of pigments are regularly distributed in the column of calcium carbonate, giving the possibility of their qualitative and quantitative determination. I call the preparation obtained in this way a chromatogram, and the proposed method - chromatographic.

M.S.Tsvet, 1906

INTRODUCTION

The need to separate and analyze a mixture of substances is faced not only by a chemist, but also by many other specialists.

In the powerful arsenal of chemical and physicochemical methods of separation, analysis, study of the structure and properties of individual chemical compounds and their complex mixtures, chromatography occupies one of the leading places.

Chromatography is a physicochemical method for separating and analyzing mixtures of gases, vapors, liquids or solutes and determining the physicochemical properties of individual substances, based on the distribution of the separated components of mixtures between two phases: mobile and stationary. The substances that make up the stationary phase are called sorbents. The stationary phase can be solid or liquid. A mobile phase is a liquid or gas flow that is filtered through a sorbent bed. The mobile phase acts as a solvent and carrier for the analyzed mixture of substances, converted into a gaseous or liquid state.

There are two types of sorption: adsorption - the absorption of substances by a solid surface and absorption - the dissolution of gases and liquids in liquid solvents.

2. Arosedevelopment and development of chromatography

The emergence of chromatography as a scientific method is associated with the name of the outstanding Russian scientist Mikhail Semenovich Tsvet (1872 - 1919), who in 1903 discovered chromatography in the course of researching the mechanism of solar energy conversion in plant pigments. This is the year and should be considered the date of creation of the chromatographic method.

M.S. The color passed a solution of the analytes and the mobile phase through a column of adsorbent in a glass tube. In this regard, his method was called column chromatography. In 1938 N.A. Izmailov and M.S. Schreiber suggested modifying the Tsvet method and separating a mixture of substances on a plate covered with a thin layer of adsorbent. This is how thin-layer chromatography arose, which makes it possible to carry out analysis with a trace amount of a substance.

In 1947 T.B. Gapon, E.N. Gapon and F.M. Shemyakin was the first to carry out the chromatographic separation of a mixture of ions in a solution, explaining it by the presence of an exchange reaction between sorbent ions and ions contained in the solution. Thus, another direction of chromatography was discovered - ion exchange chromatography. At present, ion exchange chromatography is one of the most important areas of the chromatographic method.

E.N. and G.B. Gapon in 1948 implemented what M.S. The color idea of \u200b\u200bthe possibility of chromatographic separation of a mixture of substances based on the difference in the solubility of hardly soluble precipitates. Sediment chromatography appeared.

In 1957 M. Golay proposed to apply a sorbent to the inner walls of a capillary tube - capillary chromatography. This option allows you to analyze trace amounts of multicomponent mixtures.

In the 60s, it became possible to synthesize both ionic and uncharged gels with strictly defined pore sizes. This made it possible to develop a variant of chromatography, the essence of which is to separate a mixture of substances on the basis of the difference in their ability to penetrate into gel - gel chromatography. This method allows the separation of mixtures of substances with different molecular weights.

Currently, chromatography has undergone significant development. Today, a variety of chromatographic methods, especially in combination with other physical and physicochemical methods, help scientists and engineers to solve a variety of, often very complex, problems in scientific research and technology.

3. Classication of chromatographic methods

The variety of modifications and variants of the chromatographic method requires their systematization or classification.

The classification can be based on various features, namely:

1. state of aggregation of phases;

2. separation mechanism;

3. method of carrying out the process;

4. the purpose of the process.

Classification by the state of aggregation of phases:

gas (mobile phase - gas), gas-liquid (mobile phase - gas, stationary phase - liquid), liquid (mobile phase - liquid) chromatography.

Classification by the separation mechanism.

Adsorption chromatography is based on the selective adsorption (absorption) of individual components of the analyzed mixture by the corresponding adsorbents. Adsorption chromatography is subdivided into liquid (liquid-adsorption chromatography) and gas (gas-adsorption chromatography).

Ion exchange chromatography is based on the use of ion exchange processes occurring between mobile adsorbent ions and electrolyte ions when passing a solution of the analyte through a column filled with an ion exchanger (ion exchanger). Ion exchangers are insoluble inorganic and organic high molecular weight compounds. As ion exchangers used are alumina, permutite, sulfocarbon and various synthetic organic ion-exchange substances - ion-exchange resins.

Sediment chromatography is based on the different solubility of precipitates formed by the components of the analyzed mixture with special reagents. For example, when a solution of a mixture of Hg (II) and Pb salts is passed through a column with a carrier previously impregnated with a KI solution, 2 colored layers are formed: the upper, colored orange-red (HgI 2), and the lower, colored yellow (PbI 2).

Classification by the way the process is carried out.

Column chromatography is a type of chromatography in which a column is used as a carrier for a stationary solvent.

Paper chromatography is a type of chromatography in which strips or sheets of filter paper that do not contain mineral impurities are used instead of a column as a carrier for a stationary solvent. In this case, a drop of the test solution, for example, a mixture of solutions of Fe (III) and Co (II) salts, is applied to the edge of the paper strip. The paper is suspended in a closed chamber (Fig. 1) by dropping its edge with a drop of the test solution applied on it into a vessel with a mobile solvent, for example, n-butyl alcohol. A mobile solvent, moving along the paper, wets it. In this case, each substance contained in the analyzed mixture moves with its inherent speed in the same direction as the solvent. At the end of the ion separation, the paper is dried and then sprayed with a reagent, in this case a solution of K 4, which forms colored compounds with the substances to be separated (blue - with iron ions, green - with cobalt ions). The resulting areas in the form of colored spots make it possible to establish the presence of individual components.

Paper chromatography in combination with the use of organic reagents allows qualitative analysis of complex mixtures of cations and anions. A number of substances can be detected on one chromatogram with the help of one reagent, since each substance is characterized not only by the corresponding color, but also by a certain localization location on the chromatogram.

Thin layer chromatography is a type of chromatography that is similar in its separation mechanism to paper chromatography. The difference between them is that, instead of sheets of paper, separation is carried out on plates covered with a thin layer of sorbent made of powdered alumina, cellulose, zeolites, silica gel, diatomaceous earth, etc. and retaining an immobile solvent. The main advantage of thin-layer chromatography is the simplicity of the apparatus, the simplicity and high speed of the experiment, the sufficient clarity of the separation of a mixture of substances, and the possibility of analyzing ultramicro-quantities of a substance.

Classification according to the purpose of the chromatographic process.

Chromatography is of greatest importance as a method for the qualitative and quantitative analysis of mixtures of substances (analytical chromatography).

Preparative chromatography is a type of chromatography in which a mixture of substances is separated for preparative purposes, i.e. to obtain more or less significant amounts of substances in a pure, free from impurities form. The task of preparative chromatography can also be the concentration and subsequent separation from the mixture of substances contained in the form of trace impurities to the basic substance.

Non-analytical chromatography is a type of chromatography used as a method of scientific research. It is used to study the properties of systems, such as solutions, the kinetics of chemical processes, the properties of catalysts and adsorbents.

So, chromatography is a universal method for analyzing mixtures of substances, obtaining substances in pure form, as well as a method for studying the properties of systems.

4. Chromatografia on a solid stationary phase

and)Gas (gazo-adsorbtional) chromatography

Gas chromatography is a chromatographic method in which the mobile phase is gas. Gas chromatography has found the greatest application for the separation, analysis and study of substances and their mixtures, passing without decomposition into a vapor state.

One of the options for gas chromatography is gas adsorption chromatography - a method in which the stationary phase is a solid adsorbent.

In gas chromatography, an inert gas is used as a mobile phase (carrier gas): helium, nitrogen, argon, much less often hydrogen and carbon dioxide. Sometimes the carrier gas is a pair of highly volatile liquids.

The gas chromatographic process is usually carried out in special devices called gas chromatographs (Figure 3). Each of them has a system for supplying a carrier gas flow, a system for preparing and introducing the mixture under study, a chromatographic column with a system for regulating its temperature, an analyzing system (detector), and a system for recording separation and analysis results (recorder).

Temperature is of great importance in gas adsorption chromatography. Its role, first of all, is to change the sorption equilibrium in the gas - solid system. Correct selection of the column temperature determines both the degree of separation of the mixture components, the column efficiency, and the overall analysis speed. There is a certain temperature range of the column in which the chromatographic analysis is optimal. Typically, this temperature range is in the region close to the boiling point of the determined chemical compound. When the boiling points of the mixture components differ greatly from each other, column temperature programming is used.

Separation in a chromatographic column is the most important, but preliminary, operation of the entire process of gas chromatographic analysis. Binary mixtures (carrier gas - component) leaving the column, as a rule, enter the detecting device. Here, changes in the concentrations of components over time are converted into an electrical signal, which is recorded using a special system in the form of a curve, called a chromatogram. The results of the entire experiment largely depend on the correct choice of the type of detector and its design. There are several classifications of detectors. Distinguish between differential and integral detectors. Differential detectors record the instantaneous value of one of the characteristics (concentration or flow) over time. Integral detectors add up the amount of substance over a certain period of time. They also use detectors of various types, sensitivity and purpose: thermoconductometric, ionization, spectroscopic, mass spectrometric, coulometric and many others.

Application of gas adsorption chromatography

Gas adsorption chromatography is used in the chemical and petrochemical industries to analyze the products of chemical and petrochemical synthesis, the composition of oil fractions, to determine the purity of reagents and the content of key products at different stages of technological processes, etc.

Analysis of permanent gases and light hydrocarbons, including isomers, by gas chromatography takes 5 - 6 minutes. Previously, on traditional gas analyzers, this analysis lasted 5 - 6 hours. All this has led to the fact that gas chromatography has become widely used not only in research institutes and control and measuring laboratories, but also entered the systems of complex automation of industrial enterprises.

Today, gas chromatography is also used in the search for oil and gas fields, making it possible to determine the content of organic matter in samples taken from soils, indicating the proximity of oil and gas fields.

Gas chromatography is successfully used in forensic science, where it is used to establish the identity of samples of blood stains, gasoline, oils, counterfeit expensive food products, etc. Gas chromatography is very often used to determine the blood alcohol content of car drivers. A few drops of blood from a finger are enough to know how much, when and what kind of alcoholic drink he drank.

Gas chromatography allows us to obtain valuable and unique information about the composition of odors of food products, such as cheese, coffee, caviar, brandy, etc. Sometimes the information obtained by gas chromatographic analysis does not please us. For example, often excessive amounts of pesticides are found in food or fruit juice contains trichlorethylene, which, contrary to prohibitions, was used to increase the degree of extraction of carotene from fruits, etc. But it is this information that protects human health.

However, it is not uncommon for people to simply ignore the information they receive. This primarily applies to smoking. Detailed gas chromatographic analysis has long established that the smoke of cigarettes and cigarettes contains up to 250 different hydrocarbons and their derivatives, of which about 50 have a carcinogenic effect. That is why lung cancer occurs in smokers 10 times more often, but still millions of people continue to poison themselves, their colleagues and relatives.

Gas chromatography is widely used in medicine to determine the content of numerous drugs, to determine the level of fatty acids, cholesterol, steroids, etc. in the patient's body. Such analyzes provide extremely important information about the state of human health, the course of his illness, the effectiveness of the use of certain drugs.

Scientific research in metallurgy, microbiology, biochemistry, in the development of plant protection products and new drugs, in the creation of new polymers, building materials and in many other very different areas of human practice cannot be imagined without such a powerful analytical method as gas chromatography.

Gas chromatography is successfully used to determine the content of polycyclic aromatic compounds hazardous to human health in water and in the air, the level of gasoline in the air of filling stations, the composition of automobile exhaust gases in the air, etc.

This method is widely used as one of the main methods of environmental cleanliness control.

Gas chromatography plays an important role in our lives, providing us with an enormous amount of information. In the national economy and in research organizations, more than 20 thousand of a wide variety of gas chromatographs are used, which are indispensable helpers in solving many complex problems that arise every day before researchers and engineers.

b)Liquid (liquid-adsorption)chromatography

Liquid chromatography is a group of chromatography variants in which the mobile phase is liquid.

One of the options for liquid chromatography is liquid adsorption chromatography - a method in which the stationary phase is a solid adsorbent.

Although liquid chromatography was discovered before gas chromatography, it only entered a period of extremely intensive development in the second half of the 20th century. At present, in terms of the degree of development of the theory of the chromatographic process and the technique of instrumental design, in terms of the efficiency and speed of separation, it is hardly inferior to the method of gas chromatographic separation. Moreover, each of these two main types of chromatography has its own primary field of application. If gas chromatography is suitable mainly for the analysis, separation and study of chemicals with molecular weights of 500 - 600, then liquid chromatography can be used for substances with molecular weights from several hundred to several million, including extremely complex macromolecules of polymers, proteins and nucleic acids. At the same time, the opposition of various chromatographic methods is inherently devoid of common sense, since chromatographic methods successfully complement each other, and the very task of a specific study must be approached in a different way, namely, which chromatographic method allows solving it with greater speed, information content and at a lower cost.

As in gas chromatography, modern liquid chromatography uses detectors to continuously record the concentration of the analyte in the liquid flow from the column.

There is no single universal detector for liquid chromatography. Therefore, in each case, the most suitable detector should be selected. The most widely used are ultraviolet, refractometric, microadsorption and transport flame ionization detectors.

Spectrometric detectors. Detectors of this type are highly sensitive selective devices that make it possible to determine very small concentrations of substances in the flow of the liquid phase. Their readings depend little on temperature fluctuations and other random changes in the environment. One of the important features of spectrometric detectors is the transparency of most solvents used in liquid-adsorption chromatography in the working wavelength range.

Most often, UV absorption is used, less often in the IR region. In the UV region, devices are used that operate in a wide range - from 200 nm to the visible part of the spectrum, or at certain wavelengths, most often at 280 and 254 nm. Low pressure (254 nm) and medium pressure (280 nm) mercury lamps and corresponding filters are used as radiation sources.

Microadsorption detectors. The action of microadsorption detectors is based on the release of heat during the adsorption of a substance on the adsorbent, which is filling the detector cell. However, it is not the heat that is measured, but the temperature of the adsorbent to which it is heated as a result of adsorption.

A microadsorption detector is a fairly highly sensitive instrument. Its sensitivity depends primarily on the heat of adsorption.

Microadsorption detectors are versatile, suitable for detecting both organic and inorganic substances. Moreover, it is difficult to obtain sufficiently clear chromatograms on them, especially with incomplete separation of the mixture components.

5. Chromatografiya on a liquid stationary phase

a) Gas-liquid chromatography

Gas-liquid chromatography is a gas chromatographic method in which the stationary phase is a low-volatile liquid deposited on a solid carrier.

This type of chromatography is used to separate gases and vapors of liquids.

The main difference between gas-liquid chromatography and gas-adsorption chromatography is that in the first case the method is based on the use of the process of dissolution and subsequent evaporation of gas or vapor from a liquid film held by a solid inert carrier; in the second case, the separation process is based on the adsorption and subsequent desorption of gas or vapor on the surface of a solid substance - an adsorbent.

The chromatography process can be schematically represented as follows. A mixture of gases or vapors of volatile liquids is introduced by a flow of a carrier gas into a column filled with a stationary inert carrier, on which a non-volatile liquid (stationary phase) is distributed. The gases and vapors under investigation are absorbed by this liquid. Then the components of the mixture to be separated are selectively displaced in a certain order from the column.

In gas-liquid chromatography, a number of detectors are used that specifically react to any organic substances or to organic substances with a certain functional group. These include ionization detectors, electron capture detectors, thermionic, spectrophotometric and some other detectors.

Flame ionization detector (FID). The operation of the FID is based on the fact that organic substances, entering the flame of a hydrogen burner, undergo ionization, as a result of which an ionization current arises in the detector chamber, which is also an ionization chamber, the strength of which is proportional to the number of charged particles.

The PID is sensitive only to organic compounds and is insensitive or very weakly sensitive to gases such as air, sulfur and carbon oxides, hydrogen sulfide, ammonia, carbon disulfide, water vapor and a number of other inorganic compounds. The insensitivity of the FID to air allows it to be used to determine air pollution with various organic substances.

FID uses 3 gases: carrier gas (helium or nitrogen), hydrogen and air. All 3 gases must be of high purity.

Argon detector. In an argon detector, ionization is caused by the collision of molecules of the analyte with metastable argon atoms formed as a result of exposure to radioactive B-radiation.

Thermoionic detector. The principle of operation of a thermionic detector is that salts of alkali metals, evaporating in the flame of a burner, selectively react with compounds containing halogens or phosphorus. In the absence of such compounds, an equilibrium of alkali metal atoms is established in the ionization chamber of the detector. The presence of phosphorus atoms due to their reaction with alkali metal atoms violates this equilibrium and causes the appearance of an ion current in the chamber.

Since the thermionic detector has the highest sensitivity to phosphorus-containing compounds, it is called phosphoric. This detector is used mainly for the analysis of organophosphate pesticides, insecticides and a number of biologically active compounds.

b)Gel chromatographfia

Gel chromatography (gel filtration) is a method of separating mixtures of substances with different molecular weights by filtering the analyzed solution through cross-linked cellular gels.

The separation of a mixture of substances occurs if the sizes of the molecules of these substances are different, and the pore diameter of the gel grains is constant and can only let through those molecules whose sizes are less than the diameter of the pore holes of the gel. When filtering a solution of the analyzed mixture, smaller molecules, penetrating into the pores of the gel, are retained in the solvent contained in these pores, and move along the gel layer more slowly than large molecules that are unable to penetrate into the pores. Thus, gel chromatography makes it possible to separate a mixture of substances depending on the size and molecular weight of the particles of these substances. This separation method is simple, fast and, most importantly, it allows the separation of mixtures of substances under milder conditions than other chromatographic methods.

If you fill a column with gel granules and then pour a solution of various substances with different molecular weights into it, then when the solution moves along the gel layer in the column, this mixture will separate.

The initial period of the experiment: application of a solution of the analyzed mixture to a gel layer in a column. The second stage - the gel does not interfere with the diffusion of small molecules into the pores, while large molecules remain in the solution surrounding the gel granules. When the gel layer is washed with a pure solvent, large molecules begin to move at a rate close to that of the solvent, while small molecules must first diffuse from the internal pores of the gel into the volume between the grains and, as a result, are retained and washed out by the solvent later. A mixture of substances is separated according to their molecular weight. The substances are washed out from the column in order of decreasing molecular weight.

Application of gel chromatography.

The main purpose of gel chromatography is the separation of mixtures of high molecular weight compounds and the determination of the molecular weight distribution of polymers.

At the same time, gel chromatography is equally used to separate a mixture of substances of average molecular weight and even low molecular weight compounds. In this case, it is of great importance that gel chromatography allows separation at room temperatures, which compares favorably with gas-liquid chromatography, which requires heating to convert the analytes into the vapor phase.

The separation of a mixture of substances by gel chromatography is also possible when the molecular weights of the analyzed substances are very close or even equal. In this case, the interaction of solutes with the gel is used. This interaction can be so significant that it cancels out the differences in molecular sizes. If the nature of the interaction with the gel is not the same for different substances, this difference can be used to separate the mixture of interest.

An example is the use of gel chromatography for the diagnosis of thyroid diseases. The diagnosis is established by the amount of iodine determined during the analysis.

The given examples of gel chromatography application show its wide possibilities for solving a wide variety of analytical problems.

Conclusion

As a scientific method of cognition of the world around us, chromatography is constantly developing and improving. Today it is used so often and so widely in scientific research, medicine, molecular biology, biochemistry, technology, and the national economy that it is very difficult to find a field of knowledge in which chromatography is not used.

Chromatography as a research method with its exceptional capabilities is a powerful factor in cognition and transformation of an increasingly complex world in the interests of creating acceptable living conditions for humans on our planet.

S P I S O KL I T E R A T U R S

1. Aivazov B.V. INTRODUCTION to chromatography. - M .: Higher school, 1983 - p. 8-18, 48-68, 88-233.

2. Kreshkov A.P. Fundamentals of Analytical Chemistry. Theoretical basis. Qualitative Analysis, Book One, 4th Edition, rev. M., "Chemistry", 1976 - p. 119-125.

3. Sakodynsky K.I., Orekhov B.I. Chromatography in Science and Technology. - M .: Knowledge, 1982 - p. 3-20, 28-38, 58-59.