Electrochemical methods of analysis- this is a set of methods of qualitative and quantitative analysis based on electrochemical phenomena occurring in the medium under study or at the phase boundary and associated with a change in structure, chemical composition or analyte concentration.
Varieties of the method are electrogravimetric analysis (electroanalysis), internal electrolysis, contact metal exchange (cementation), polarographic analysis, coulometry, etc. In particular, electrogravimetric analysis is based on weighing a substance released on one of the electrodes. The method allows not only to carry out quantitative determinations of copper, nickel, lead, etc., but also to separate mixtures of substances.
In addition, electrochemical methods of analysis include methods based on measuring electrical conductivity (conductometry) or electrode potential (potentiometry). Some electrochemical methods are used to find the end point of a titration (amperometric titration, conductometric titration, potentiometric titration, coulometric titration).
There are direct and indirect electrochemical methods. In direct methods, the dependence of the current strength (potential, etc.) on the concentration of the analyte is used. In indirect methods, the current strength (potential, etc.) is measured in order to find the end point of the titration of the analyte component with a suitable titrant, i.e. use the dependence of the measured parameter on the volume of the titrant.
Any kind of electrochemical measurement requires an electrochemical circuit or an electrochemical cell, integral part which is the analyzed solution.
Electrochemical methods are classified depending on the type of phenomena measured during the analysis. There are two groups of electrochemical methods:
1. Methods without superimposing an extraneous potential, based on measuring the potential difference that occurs in an electrochemical cell consisting of an electrode and a vessel with a test solution. This group of methods is called potentiometric. Potentiometric methods use the dependence of the equilibrium potential of the electrodes on the concentration of ions involved in the electrochemical reaction on the electrodes.
2. Methods with the imposition of an extraneous potential, based on the measurement of: a) the electrical conductivity of solutions - conductometry; b) the amount of electricity passed through the solution - coulometry; c) the dependence of the current on the applied potential - voltammetry; d) the time required for the passage of an electrochemical reaction - chronoelectrochemical methods(chronovoltammetry, chronoconductometry). In the methods of this group, an extraneous potential is applied to the electrodes of the electrochemical cell.
The main element of devices for electrochemical analysis is an electrochemical cell. In methods without the imposition of an extraneous potential, it is galvanic cell, in which, due to the occurrence of chemical redox reactions, electricity. In a cell of the galvanic cell type, two electrodes are in contact with the analyzed solution - an indicator electrode, the potential of which depends on the concentration of the substance, and an electrode with a constant potential - a reference electrode, relative to which the potential of the indicator electrode is measured. Measurement of the potential difference is carried out with special devices - potentiometers.
In methods with superimposed extraneous potential, electrochemical cell, so named because electrolysis occurs on the electrodes of the cell under the action of an applied potential - the oxidation or reduction of a substance. Conductometric analysis uses a conductometric cell in which the electrical conductivity of a solution is measured. According to the method of application, electrochemical methods can be classified into direct methods, in which the concentration of substances is measured according to the indication of the instrument, and electrochemical titration, where the indication of the equivalence point is fixed using electrochemical measurements. In accordance with this classification, there are potentiometry and potentiometric titration, conductometry and conductometric titration, etc.
Instruments for electrochemical determinations, in addition to the electrochemical cell, stirrer, and load resistance, include devices for measuring the potential difference, current, solution resistance, and the amount of electricity. These measurements can be carried out by pointer instruments (voltmeter or microammeter), oscilloscopes, automatic recording potentiometers. If the electrical signal from the cell is very weak, then it is amplified with the help of radio amplifiers. In devices of methods with superimposed extraneous potential, an important part is the devices for supplying the cell with the appropriate potential of a stabilized direct or alternating current (depending on the type of method). The power supply unit for electrochemical analysis instruments usually includes a rectifier and a voltage stabilizer, which ensures the stability of the instrument.
Potentiometry combines methods based on measuring the emf of reversible electrochemical circuits when the potential of the working electrode is close to the equilibrium value.
Voltammetry is based on the study of the dependence of the polarization current on the voltage applied to the electrochemical cell, when the potential of the working electrode differs significantly from the equilibrium value. It is widely used to determine substances in solutions and melts (for example, polarography, amperometry).
Coulometry combines methods of analysis based on measuring the amount of a substance released at an electrode during an electrochemical reaction in accordance with Faraday's laws. In coulometry, the potential of the working electrode differs from the equilibrium value.
Conductometric analysis is based on a change in the concentration of a substance or the chemical composition of the medium in the interelectrode space; it is not related to the potential of the electrode, which is usually close to the equilibrium value.
Dielectrometry combines methods of analysis based on measuring the dielectric constant of a substance, due to the orientation of particles (molecules, ions) with a dipole moment in an electric field. Dielectrometric titration is used to analyze solutions.
Electrochemical analysis methods are a set of methods of qualitative and quantitative analysis based on electrochemical phenomena occurring in the medium under study or at the phase boundary and associated with a change in the structure, chemical composition or concentration of the analyte.
Electrochemical methods of analysis (ECMA) are based on processes occurring on electrodes or in the interelectrode space. Their advantage is high accuracy and comparative simplicity of both equipment and analysis methods. High accuracy is determined by very precise laws used in ECMA. A great convenience is that this method uses electrical influences, and the fact that the result of this influence (response) is also obtained in the form of an electrical signal. This provides high speed and accuracy of counting, opens up wide possibilities for automation. ECMA are distinguished by good sensitivity and selectivity, in some cases they can be attributed to microanalysis, since sometimes less than 1 ml of solution is sufficient for analysis.
According to the types of analytical signal, they are divided into:
1) conductometry - measurement of the electrical conductivity of the test solution;
2) potentiometry - measurement of the currentless equilibrium potential of the indicator electrode, for which the test substance is potentiodetermining;
3) coulometry - measurement of the amount of electricity required for the complete transformation (oxidation or reduction) of the substance under study;
4) voltammetry - measurement of stationary or non-stationary polarization characteristics of electrodes in reactions involving the test substance;
5) electrogravimetry - measurement of the mass of a substance released from a solution during electrolysis.
27. Potentiometric method.
potentiometry - measurement of the currentless equilibrium potential of the indicator electrode, for which the test substance is potentiodetermining.
A) standard (reference electrode) - has a constant potential, independent of external. Terms
B) individual electrode - its potential depends on the concentration of the substance.
The potential depends on the concentration: E = f(c)
Nerist's equation E= E° + lna kat
E° - standard. Electron. Potential (const)
R- Univer. Gas constantconst)
T is the absolute rate (t)- +273 °
.n is the number of electrons involved. In oxidation/recovery Reactions
. a - active concentration
Potentiometry method
Ionometry potentiometry
Equivalence point
EСх Vх = l t *Vt
28. Conductometric method.
conductometry - measurement of the electrical conductivity of the test solution.
Conductometric titration
Conductometer (instrument)
Conductometric analysis (conductometry) is based on the use of the relationship between the electrical conductivity (electrical conductivity) of electrolyte solutions and their concentration.
The electrical conductivity of electrolyte solutions - conductors of the second kind - is judged on the basis of measuring their electrical resistance in an electrochemical cell, which is a glass vessel (glass) with two electrodes soldered into it, between which the test electrolyte solution is located. An alternating current is passed through the cell. The electrodes are most often made of metallic platinum, which, to increase the surface of the electrodes, is coated with a layer of spongy platinum by electrochemical deposition from solutions of platinum compounds (platinum platinum electrodes).
29. Polarography.
Polarography is a method of qualitative and quantitative chemical analysis based on obtaining curves of the dependence of the magnitude of the current on the voltage in a circuit consisting of the test solution and electrodes immersed in it, one of which is strongly polarizable, and the other is practically non-polarizable. Such curves - polarograms - are obtained using polarographs.
The polarographic method is characterized by high sensitivity. To perform the analysis, 3-5 ml of the test solution is usually sufficient. Analysis with an auto-registering polarograph lasts only about 10 minutes. Polarography is used to determine the content of toxic substances in objects of biological origin (for example, compounds of mercury, lead, thallium, etc.), to determine the degree of blood oxygen saturation, to study the composition of exhaled air, and harmful substances in the air of industrial enterprises. The polarographic method of analysis is highly sensitive and makes it possible to determine substances at very low (up to 0.0001%) concentrations in solution.
30. Classification of spectral methods of analysis. Spectrum concept.
Spectral analysis is a set of methods for determining the quality and quantity. Composition, as well as the structure of matter (based on the interaction of the research object with various types of radiation.)
All spectroscopic methods are based on the interaction of atoms, molecules or ions that make up the analyzed substance with electromagnetic radiation. This interaction is manifested in the absorption or emission of photons (quanta). Depending on the nature of the interaction of the sample with electromagnetic radiation, two groups of methods are distinguished -
Emission and absorption. Depending on which particles form the analytical signal, there are methods of atomic spectroscopy and methods of molecular spectroscopy.
Issue
In emission methods, the analyzed sample emits photons as a result of its excitation.
absorption
In absorption methods, radiation from an external source is passed through the sample, while some of the quanta are selectively absorbed by atoms or molecules.
Spectrum- distribution of values of a physical quantity (usually energy, frequency or mass). A graphical representation of such a distribution is called a spectral diagram. Usually, the spectrum means the electromagnetic spectrum - the frequency spectrum (or the same as the quantum energies) of electromagnetic radiation.
1.light reflection
2.turning the light beam(defraction)
3. light scattering: nephelometry, turbidimetry
4.Light absorption
5reradiation
A) phosphorescence (lasts for a long time)
B) fluorescence (very short)
By the nature of the distribution of the values of a physical quantity, the spectra can be discrete (linear), continuous (continuous), and also represent a combination (superposition) of discrete and continuous spectra.
Examples of line spectra are mass spectra and spectra of bound-bound electronic transitions of an atom; examples of continuous spectra are the spectrum of electromagnetic radiation of a heated solid and the spectrum of free-free electronic transitions of an atom; examples of combined spectra are the emission spectra of stars, where chromospheric absorption lines or most of the sound spectra are superimposed on the continuous spectrum of the photosphere.
31. Photometry: the principle of the method, application in forensic research.
Photometry - the spectral method is based on the absorption of electromagnetic radiation in the visible and near ultraviolet range (the method is based on the absorption of light)
Molecular Atomic
Spectroscopy spectroscopy (In electron.Analysis)
Cuvette - light passes through it
l
I (output light intensity)
I° is the intensity of the incident light.
Photometry is a branch of physical optics and measuring technology devoted to methods for studying the energy characteristics of optical radiation in the process of its emission, propagation in various environments and interactions with bodies. Photometry is carried out in the ranges of infrared (wavelengths - 10 -3 ... 7 10 -7 m), visible (7 10 -7 ... 4 10 -7 m) and ultraviolet (4 10 -7 ... 10 -8 m) optical radiation. When electromagnetic radiation of the optical range propagates in a biological medium, a number of main effects are observed: absorption and scattering of radiation by atoms and molecules of the medium, scattering of medium inhomogeneities on particles, depolarization of radiation. By recording data on the interaction of optical radiation with the medium, it is possible to determine the quantitative parameters associated with the medical and biological characteristics of the object under study. Photometers are used to measure photometric quantities. In terms of photometry, light is radiation capable of producing a sensation of brightness when exposed to the human eye. Photometry as a science is based on the theory of the light field developed by A. Gershun.
There are two general methods of photometry: 1) visual photometry, in which the ability of the human eye to perceive differences in brightness is used to equalize the brightness of two comparison fields by mechanical or optical means; 2) physical photometry, in which various light receivers of a different kind are used to compare two light sources - vacuum photocells, semiconductor photodiodes, etc.
32. Law of Bouguer-Lambert-Beer, its use in quantitative analysis.
A physical law that determines the attenuation of a parallel monochromatic beam of light as it propagates in an absorbing medium.
The law is expressed by the following formula:
,
where is the intensity of the incoming beam, is the thickness of the substance layer through which the light passes, is the absorption index (not to be confused with the dimensionless absorption index, which is related to the formula, where is the wavelength).
The absorption index characterizes the properties of a substance and depends on the wavelength λ of the absorbed light. This dependence is called the absorption spectrum of the substance.
For solutions of absorbing substances in solvents that do not absorb light, the absorption index can be written as
where is the coefficient characterizing the interaction of an absorbing solute molecule with light with a wavelength λ, is the concentration of the solute, mol/l.
The statement that does not depend on is called Beer's law (not to be confused with Beer's law). This law assumes that the ability of a molecule to absorb light is not affected by other surrounding molecules of the same substance in solution. However, numerous deviations from this law are observed, especially at large .
If a layer of a solution or gas with a thickness (a light flux of intensity I passes through, then, according to the Lambert-Beer law, the amount of absorbed light will be proportional to the intensity /, the concentration c of the substance that absorbs light, and the thickness of the LAYER), the BMB law, which relates the intensity of light incident on substance and passed it, with the concentration of the substance and the thickness of the absorbing layer Well, this is the same as refraction, only attenuation in the substance. Which light absorbs under a certain percentage. That is, the remainder of the output of light is
33. IR spectroscopy.
This method of analysis is based on the recording of infrared absorption spectra of a substance. Absorption by a substance in the infrared region occurs due to the vibrations of atoms in molecules. Vibrations are subdivided into valence (when the distances between atoms change during the vibration) and vibrational (when the angles between bonds change during the vibration). Transitions between different vibrational states in molecules are quantized, due to which absorption in the IR region has the form of a spectrum, where each vibration has its own wavelength. It is clear that the wavelength for each vibration depends on which atoms participate in it, and besides, it depends little on their environment.
IR spectroscopy is not a separating method, that is, when studying a substance, it may turn out that a mixture of several substances was actually studied, which, of course, will greatly distort the results of spectrum interpretation. Well, it’s still not entirely correct to talk about the unambiguous identification of a substance using the IR spectroscopy method, since the method rather allows you to identify certain functional groups, and not their number in the connection and their way of communication with each other.
IR spectroscopy method is used in the study of polymeric materials, fibers, paint coatings, drugs(when identifying a filler, which is often carbohydrates, including polysaccharides). The method is especially indispensable in the study of lubricants, in that it makes it possible to simultaneously determine the nature of both the base of the lubricant and possible additives (additives) to this base.
34. X-ray fluorescence analysis.
(XRF) is one of the modern spectroscopic methods for studying a substance in order to obtain its elemental composition, that is, its elemental analysis. It can analyze various elements from beryllium (Be) to uranium (U). The XRF method is based on the collection and subsequent analysis of the spectrum obtained by exposing the material under study to X-ray radiation. When irradiated, the atom goes into an excited state, which consists in the transition of electrons to higher energy levels. An atom stays in an excited state for an extremely short time, on the order of one microsecond, after which it returns to a quiet position (ground state). In this case, electrons from the outer shells either fill the formed vacancies, and the excess energy is emitted in the form of a photon, or the energy is transferred to another electron from the outer shells (Auger electron)
Ecology and protection environment: determination of heavy metals in soils, sediments, water, aerosols, etc.
Geology and mineralogy: qualitative and quantitative analysis of soils, minerals, rocks and etc.
Metallurgy and chemical industry: quality control of raw materials, production process and finished products
Paint industry: analysis of lead paints
35. Atomic emission spectroscopy.
Atomic emission spectral analysis is a set of elemental analysis methods based on the study of the emission spectra of free atoms and ions in the gas phase. Usually, emission spectra are recorded in the most convenient optical wavelength range from 200 to 1000 nm.
AES (atomic emission spectrometry) is a method for determining the elemental composition of a substance from the optical emission spectra of atoms and ions of the analyzed sample, excited in light sources. As light sources for atomic emission analysis, a burner flame or various types of plasma are used, including electric spark or arc plasma, laser spark plasma, inductively coupled plasma, glow discharge, etc. AES is the most common express highly sensitive method for identifying and quantifying elements. impurities in gaseous, liquid and solid substances, including high-purity ones.
Areas of use:
Metallurgy: analysis of the composition of metals and alloys,
Mining industry: exploration of geological samples and minerals,
Ecology: water and soil analysis,
Technique: analysis of motor oils and other technical fluids for metal impurities,
Biological and medical research.
Operating principle.
The principle of operation of an atomic emission spectrometer is quite simple. It is based on the fact that the atoms of each element can emit light of certain wavelengths - spectral lines, and these wavelengths are different for different elements. In order for atoms to emit light, they must be excited - by heating, by an electric discharge, by a laser, or in some other way. The more atoms of a given element present in the analyzed sample, the brighter the radiation of the corresponding wavelength will be.
The intensity of the spectral line of the analyzed element, in addition to the concentration of the analyzed element, depends on a large number various factors. For this reason, it is impossible to theoretically calculate the relationship between the line intensity and the concentration of the corresponding element. That is why analysis requires standard samples that are close in composition to the analyzed sample. Previously, these standard samples are exposed (burned) on the device. Based on the results of these burns, a calibration graph is constructed for each analyzed element, i.e. dependence of the intensity of the spectral line of an element on its concentration. Subsequently, during the analysis of samples, these calibration curves are used to recalculate the measured intensities into concentrations.
Preparation of samples for analysis.
It should be borne in mind that a few milligrams of a sample from its surface is actually analyzed. Therefore, to obtain correct results, the sample must be homogeneous in composition and structure, and the composition of the sample must be identical to the composition of the analyzed metal. When analyzing metal in a foundry or smelter, it is recommended to use special molds for casting samples. In this case, the shape of the sample can be arbitrary. It is only necessary that the analyzed sample has a sufficient surface and can be clamped in a tripod. For the analysis of small samples, such as bars or wires, special adapters can be used.
Advantages of the method:
non-contact,
Possibility of simultaneous quantitative determination of a large number of elements,
High accuracy,
Low limits of detection,
Ease of sample preparation
Low cost.
36. Atomic absorption spectroscopy.
method of quantities. determination of the elemental composition of the test substance by atomic absorption spectra, based on the ability of atoms to selectively absorb electromagnetic radiation in decomp. sections of the spectrum. A.-a.a. carried out on a special devices - absorption. spectrophotometers. A sample of the analyzed material is dissolved (usually with the formation of salts); the solution in the form of an aerosol is fed into the flame of the burner. Under the action of a flame (3000°C), salt molecules dissociate into atoms, which can absorb light. Then a beam of light is passed through the flame of the burner, in the spectrum of which there are spectral lines corresponding to one or another element. From the total radiation, the investigated spectral lines are isolated by a monochromator, and their intensity is fixed by a recording unit. Mat. processing is carried out according to the formula: J = J0 * e-kvI,
where J and J0, are the intensities of the transmitted and incident light; kv - coefficient. absorption, depending on its frequency; I - absorbing layer thickness
more sensitive than nuclear power plant
37. Nephelometry and turbidimetry.
S = lg (I°/I) incident intensity. In solution (I °) we divide by the intensity coming out of solution (I) \u003d
k-const turbidity
b is the path length of the light beam
N is the number of particles in units. r-ra
Nephelometric and turbidimetric analysis uses the phenomenon of light scattering by solid particles suspended in solution.
Nephelometry is a method for determining the dispersion and concentration of colloidal systems by the intensity of the light scattered by them. Nephelometry, measurements are made in a special nephelometer device, the operation of which is based on comparing the intensity of light scattered by the medium under study with the intensity of light scattered by another medium that serves as a standard. The theory of light scattering by colloidal systems, in which the particle sizes do not exceed the half-wavelength of the incident light, was developed by the English physicist J. Rayleigh in 1871. According to Rayleigh's law, the intensity of light I scattered in a direction perpendicular to the incident beam is expressed by the formula I \u003d QNvlk - where q is the intensity of the incident light, N is total number particles per unit volume, or partial concentration, v is the volume of one particle, \ is the wavelength of the incident light, k is a constant depending on the refractive indices of colloidal particles and their surrounding dispersion medium, the distance from the light source, and also on the accepted units of measurement
Turbidimetry is a method for analyzing turbid media based on measuring the intensity of light absorbed by them. Turbidimetric measurements are made in transmitted light using visual turbidimeters or photoelectric colorimeters. The measurement technique is similar to the colorimetric one and is based on the applicability of Bouguer-Lambert to turbid media - Beer's law, which in the case of suspensions is valid only for very thin layers or at significant dilutions. In turbidimetry, careful observance of the conditions for the formation of a dispersed phase, similar to the conditions observed in nephelometry, is required. A significant improvement in turbidimetry is the use of turbidimetric turbidity peak titration using photoelectric colorimeters. Turbidimetry is successfully used for the analytical determination of sulfates, phosphates, chlorides, cyanides, lead, zinc, etc.
The main advantage of nephelometric and turbidimetric methods is their high sensitivity, which is especially valuable in relation to elements or ions for which there are no color reactions. In practice, for example, the nephelometric determination of chloride and sulfate in natural waters and similar objects is widely used. In terms of accuracy, turbidimetry and nephelometry are inferior to photometric methods, which is mainly due to the difficulties in obtaining suspensions with the same particle sizes, stability over time, etc. suspension properties.
Nephelometry and turbidimetry are used, for example, to determine SO4 in the form of a suspension of BaSO4, Cl- in the form of a suspension of AgCl, S2- in the form of a suspension of CuS with a lower. the limits of the determined contents ~ 0.1 µg/ml. To standardize the conditions of analysis in experiments, it is necessary to strictly control the temperature, the volume of suspension, the concentration of reagents, the stirring speed, and the time of measurements. Precipitation must be fast and the particles to be deposited must be small and of low p-value. To prevent coagulation of large particles, a stabilizer is often added to the solution, for example. gelatin, glycerin.
38. Chromatography: history of occurrence, principle of the method, application to the court. Research.
Chromatography is a dynamic sorption method for separating and analyzing mixtures of substances, as well as studying the physicochemical properties of substances. It is based on the distribution of substances between two phases - stationary (solid phase or liquid bound on an inert carrier) and mobile (gas or liquid phase, eluent). The name of the method is associated with the first experiments on chromatography, during which the developer of the method, Mikhail Tsvet, separated brightly colored plant pigments.
The chromatography method was first used by the Russian botanist Mikhail Semenovich Tsvet in 1900. He used a column filled with calcium carbonate to separate plant pigments. The first report on the development of the chromatography method was made by Tsvet on December 30, 1901 at XI Congress of Naturalists and Physicians in St. Petersburg. The first printed work on chromatography was published in 1903 in the journal Proceedings of the Warsaw Society of Naturalists. First time term chromatography appeared in two printed works of Color in 1906 published in a German magazine Berichte der Deutschen Botanischen Gesellschaft. In 1907 Color demonstrates his method German Botanical Society.
In 1910-1930, the method was undeservedly forgotten and practically did not develop.
In 1931, R. Kuhn, A. Winterstein and E. Lederer isolated α and β fractions in crystalline form from crude carotene using chromatography, which demonstrated the preparative value of the method.
In 1941, A. J. P. Martin and R. L. M. Sing developed a new form of chromatography based on the difference in the distribution coefficients of substances to be separated between two immiscible liquids. The method is called " partition chromatography».
In 1947, T. B. Gapon, E. N. Gapon and F. M. Shemyakin developed the method of "ion-exchange chromatography".
In 1952, J. Martin and R. Singh were awarded the Nobel Prize in Chemistry for the creation of a partition chromatography method.
From the middle of the 20th century to the present day, chromatography has developed rapidly and has become one of the most widely used analytical methods.
Classification: Gas, Liquid
Fundamentals of chromatography. process. For carrying out chromatographic separation in-in or determination of their physical.-chemical. characteristics usually use special. devices - chromatographs. Main nodes of the chromatograph - chromatographic. column, detector, and sample injection device. The column containing the sorbent performs the function of separating the analyzed mixture into its constituent components, and the detector performs the function of their quantities. definitions. The detector, located at the outlet of the column, automatically continuously determines the concentration of the separated compounds. in the flow of the mobile After entering the analyzed mixture with the flow of the mobile phase into the column, the zones of all in-in are located at the beginning of the chromatographic. columns (Fig. 1). Under the action of the flow of the mobile phase, the components of the mixture begin to move along the column with decomp. speeds, the values of which are inversely proportional to the distribution coefficients K of the chromatographed components. Well-sorbed substances, the values of distribution constants for which are large, move along the sorbent layer along the column more slowly than poorly sorbed ones. Therefore, component A leaves the column fastest, then component B, and component C is the last to leave the column (K A<К Б <К В). Сигнал детектора, величина к-рого пропорциональна концентрации определяемого в-ва в потоке элюента, автоматически непрерывно записывается и регистрируется (напр., на диаграммной ленте). Полученная хроматограмма отражает расположение хроматографич. зон на слое сорбента или в потоке подвижной фазы во времени.
Rice. one. Separation of a mixture of three components (A, B and C) on a chromatographic column K with a detector D: a - the position of the chromatographic zones of the components to be separated in the column at certain time intervals; b - chromatogram (C - signal, t - time) .
With flat layer chromatography. separation, a sheet of paper or a plate with a layer of sorbent coated with samples of the investigated in-va is placed in a chromatographic. camera. After separation, the components are determined by any suitable method.
39. Classification of chromatographic methods.
Chromatography is a method of separation and analysis of substances based on the distribution of the analyser. V-va between 2 phases: movable and stationary
A solution of a mixture of substances to be separated is passed through a glass tube (adsorption column) filled with an adsorbent. As a result, the components of the mixture are held at different heights of the adsorbent column in the form of separate zones (layers). Things are better adsorber. Nah in the top of the column, and worse adsorbed in the lower part of the column. In-va not able to be adsorbed - pass through the column without stopping and are collected in the filter.
Classifications:
1. According to the state of aggregation of the phases.
1) Movable
A) gas (inert gases: helium, argon, ozone)
B) liquid
2. according to the method of conducting
1) on a plane (planar); paper thin layer
2) column
A) packed (packed column filled with sorbent)
B) capillary (thin glass / quartz capillary on the inner surface of which the stationary phase is applied)
Can def. Items in small quantities.
Volatile matter is separated.
40. Chromatogram. Basic parameters of the chromatographic peak.
The chromatogram is the result of recording the dependence of the concentration of components at the outlet of the column on time.
H S
Each peak in the chromatogram is characterized by two basic parameters
1. Retention time ( t R) is the time from the moment of injection of the analyzed sample to the moment of registration of the maximum of the chromatographic peak. It depends on the nature of the substance and is a qualitative characteristic.
2. Height ( h) or area ( S) peak
S = ½ ω × h. (4)
The peak height and area depend on the amount of substance and are quantitative characteristics.
The retention time consists of two components - the residence time of substances in the mobile phase ( t m) and residence time in the stationary phase ( t s):
Identification of peaks of unknown components of the analyzed mixture is carried out by comparison (comparison) refers. values determined directly from the chromatogram, with corresponding tabular data for known compounds. When identifying in chromatography, only negative is reliable. answer; for example, peak i is not in-tion A if the retention times of peak i and in-va A do not match. The coincidence of the retention times of peak i and in-va A is a necessary but not sufficient condition for the conclusion that peak i is in-in A.
In practical work, the choice of one or another parameter for the quantitative interpretation of chromatograms is determined by the combined influence of several factors, the speed and convenience of calculation, the shape (wide, narrow) and the degree of asymmetry of the chromatographic peak, the efficiency of the column used, the completeness of separation of the mixture components, the availability of the necessary automated devices (integrators, computer systems for data processing of chromatographic analysis).
The determined parameter of the chromatographic peak is measured by the operator on the chromatogram manually at the end of the cycle of separation of the components of the analyzed mixture
The determined parameter of the chromatographic peak is measured automatically using digital voltmeters, integrators or specialized computers simultaneously with the separation of the components of the analyzed mixture in the column and recording the chromatogram
Since the technique of deciphering chromatograms is reduced to measuring the parameters of the chromatographic peaks of the compound of interest and the standard, the chromatographic conditions should ensure their complete separation, if possible, all other components of the original sample under the accepted analysis conditions may not be separated from each other or even not appear on the chromatogram at all (this is advantage of the internal standard method over the internal normalization method)
41. Qualitative chromatographic analysis.
With sufficient column length, complete separation of the components of any mixture can be achieved. And after elution of the separated components into separate fractions (eluates), determine the amount of mixture components (it corresponds to the number of eluates), establish their qualitative composition, determine the amount of each of them using appropriate methods of quantitative analysis.
Qualitative chromatographic analysis, i.e. identification of a substance by its chromatogram can be performed by comparing the chromatographic characteristics, most often the retained volume (i.e., the volume of the mobile phase passed through the column from the beginning of the mixture input to the appearance of this component at the column outlet), found under certain conditions for the components of the analyzed mixtures and for the standard.
42. Quantitative chromatographic analysis.
Quantitative chromatographic analysis is usually carried out on a chromatograph. The method is based on the measurement of various parameters of the chromatographic peak, depending on the concentration of the chromatographed substances - height, width, area and retained volume or the product of the retained volume and the height of the peak.
In quantitative gas chromatography, the methods of absolute calibration and internal normalization, or normalization, are used. An internal standard method is also used. With absolute calibration, the dependence of the height or area of the peak on the concentration of the substance is experimentally determined and calibration graphs are built or the corresponding coefficients are calculated. Next, the same characteristics of the peaks in the analyzed mixture are determined, and the concentration of the analyte is found from the calibration curve. This simple and accurate method is the main one in the determination of microimpurities.
When using the internal normalization method, the sum of any peak parameters, for example, the sum of the heights of all peaks or the sum of their areas, is taken as 100%. Then the ratio of the height of an individual peak to the sum of the heights or the ratio of the area of one peak to the sum of the areas, when multiplied by 100, will characterize the mass fraction (%) of the component in the mixture. With this approach, it is necessary that the dependence of the value of the measured parameter on the concentration be the same for all components of the mixture.
43. Planar chromatography. Using thin layer chromatography for ink analysis.
The first form of use of cellulose in thin layer chromatography was paper chromatography. Available plates for TLC and high-throughput TLC allow the separation of mixtures of polar substances, while at least ternary mixtures of water, an organic solvent immiscible with it, and a water-soluble solvent that promotes the formation of one phase) are used as eluent)
