What is spectral analysis? Signal spectral analysis

Spectral analysis is one of the most important physical methods for studying substances. Designed to determine the qualitative and quantitative composition of a substance based on its spectrum.

Chemists have long known that compounds of certain chemical elements, if added to a flame, give it characteristic colors. Thus, sodium salts make the flame yellow, and boron compounds make it green. The color of a substance occurs when it either emits waves of a certain length or absorbs them from the full spectrum of white light incident on it. In the second case, the color visible to the eye turns out to correspond not to these absorbed waves, but to others - additional ones, which, when added to them, give white light.

These patterns, established at the beginning of the last century, were generalized in 1859-1861. German scientists G. Kirchhoff and R. Bunsen, who proved that each chemical element has its own characteristic spectrum. This made it possible to create a type of elemental analysis - atomic spectral analysis, with the help of which it is possible to quantitatively determine the content of various elements in a sample of a substance decomposed into atoms or ions in a flame or in an electric arc. Even before the creation of a quantitative version of this method, it was successfully used for “elemental analysis” of celestial bodies. Spectral analysis already in the last century helped to study the composition of the Sun and other stars, as well as to discover some elements, in particular helium.

With the help of spectral analysis, it became possible to distinguish not only different chemical elements, but also isotopes of the same element, which usually give different spectra. The method is used to analyze the isotopic composition of substances and is based on different shifts in the energy levels of molecules with different isotopes.

X-rays, named after the German physicist W. Roentgen who discovered them in 1895, are one of the shortest wavelength parts of the full spectrum of electromagnetic waves, located in it between ultraviolet light and gamma radiation. When X-rays are absorbed by atoms, deep electrons located near the nucleus and bound to it especially tightly are excited. The emission of X-rays by atoms, on the contrary, is associated with transitions of deep electrons from excited energy levels to ordinary, stationary ones.

Both levels can have only strictly defined energies, depending on the charge of the atomic nucleus. This means that the difference between these energies, equal to the energy of the absorbed (or emitted) quantum, also depends on the charge of the nucleus, and the radiation of each chemical element in the X-ray region of the spectrum is a set of waves characteristic of this element with strictly defined vibration frequencies.

X-ray spectral analysis, a type of elemental analysis, is based on the use of this phenomenon. It is widely used for the analysis of ores, minerals, as well as complex inorganic and organoelement compounds.

There are other types of spectroscopy based not on radiation, but on the absorption of light waves by matter. So-called molecular spectra are observed, as a rule, when solutions of substances absorb visible, ultraviolet or infrared light; In this case, no decomposition of molecules occurs. If visible or ultraviolet light usually acts on electrons, causing them to rise to new, excited energy levels (see Atom), then infrared (thermal) rays, which carry less energy, excite only vibrations of interconnected atoms. Therefore, the information that these types of spectroscopy provide chemists is different. If from the infrared (vibrational) spectrum one learns about the presence of certain groups of atoms in a substance, then spectra in the ultraviolet (and for colored substances - in the visible) region carry information about the structure of the light-absorbing group as a whole.

Among organic compounds, the basis of such groups, as a rule, is a system of unsaturated bonds (see Unsaturated hydrocarbons). The more double or triple bonds in a molecule, alternating with simple ones (in other words, the longer the conjugation chain), the easier the electrons are excited.

Molecular spectroscopy methods are used not only to determine the structure of molecules, but also to accurately measure the amount of a known substance in a solution. Spectra in the ultraviolet or visible region are especially convenient for this. Absorption bands in this region are usually observed at a solute concentration of the order of hundredths and even thousandths of a percent. A special case of such an application of spectroscopy is the colorimetry method, which is widely used to measure the concentration of colored compounds.

Atoms of some substances are also capable of absorbing radio waves. This ability manifests itself when a substance is placed in the field of a powerful permanent magnet. Many atomic nuclei have their own magnetic moment - spin, and in a magnetic field nuclei with unequal spin orientation turn out to be energetically “unequal”. Those whose spin direction coincides with the direction of the applied magnetic field find themselves in a more favorable position, and other orientations begin to play the role of “excited states” in relation to them. This does not mean that a nucleus in a favorable spin state cannot go into an “excited” state; the difference in the energies of the spin states is very small, but still the percentage of nuclei in an unfavorable energy state is relatively small. And the more powerful the applied field, the smaller it is. The nuclei seem to oscillate between two energy states. And since the frequency of such oscillations corresponds to the frequency of radio waves, resonance is also possible - the absorption of energy from an alternating electromagnetic field with the corresponding frequency, leading to a sharp increase in the number of nuclei in an excited state.

This is the basis for the work of nuclear magnetic resonance (NMR) spectrometers, capable of detecting the presence in a substance of those atomic nuclei whose spin is equal to 1/2: hydrogen 1H, lithium 7Li, fluorine 19F, phosphorus 31P, as well as isotopes of carbon 13C, nitrogen 15N, oxygen 17O, etc.

The more powerful the permanent magnet, the higher the sensitivity of such devices. The resonant frequency needed to excite nuclei also increases in proportion to the magnetic field strength. It serves as a measure of the class of the device. Middle class spectrometers operate at a frequency of 60-90 MHz (when recording proton spectra); cooler ones - at a frequency of 180, 360 and even 600 MHz.

High-class spectrometers - very accurate and complex instruments - allow not only to detect and quantitatively measure the content of a particular element, but also to distinguish the signals of atoms occupying chemically “unequal” positions in the molecule. And by studying the so-called spin-spin interaction, which leads to the splitting of signals into groups of narrow lines under the influence of the magnetic field of neighboring nuclei, one can learn a lot of interesting things about the atoms surrounding the nucleus under study. NMR spectroscopy allows you to obtain from 70 to 100% of the information needed, for example, to establish the structure of a complex organic compound.

Another type of radio spectroscopy - electron paramagnetic resonance (EPR) - is based on the fact that not only nuclei, but also electrons have a spin of 1/2. EPR spectroscopy is the best way to study particles with unpaired electrons - free radicals. Like NMR spectra, EPR spectra make it possible to learn a lot not only about the “signaling” particle itself, but also about the nature of the atoms surrounding it. EPR spectroscopy instruments are very sensitive: to record the spectrum, a solution containing several hundred millionths of a mole of free radicals per liter is usually quite sufficient. And a device with record sensitivity, recently created by a group of Soviet scientists, is capable of detecting the presence of only 100 radicals in a sample, which corresponds to their concentration of approximately 10 -18 mol/l.

Chemical composition of the substance– the most important characteristic of materials used by mankind. Without its exact knowledge, it is impossible to plan technological processes in industrial production with any satisfactory accuracy. Recently, the requirements for determining the chemical composition of a substance have become even more stringent: many areas of industrial and scientific activity require materials of a certain “purity” - these are requirements for an accurate, fixed composition, as well as strict restrictions on the presence of impurities of foreign substances. In connection with these trends, increasingly progressive methods for determining the chemical composition of substances are being developed. These include the method of spectral analysis, which provides an accurate and rapid study of the chemistry of materials.

Fantasy of light

Nature of Spectral Analysis

(spectroscopy) studies the chemical composition of substances based on their abilities to emit and absorb light. It is known that each chemical element emits and absorbs a light spectrum characteristic only of it, provided that it can be reduced to a gaseous state.

In accordance with this, it is possible to determine the presence of these substances in a particular material based on their unique spectrum. Modern methods of spectral analysis make it possible to determine the presence of a substance weighing up to billionths of a gram in a sample - the radiation intensity indicator is responsible for this. The uniqueness of the spectrum emitted by an atom characterizes its deep relationship with the physical structure.

Visible light is radiation from 3,8 *10 -7 to 7,6*10 -7 m, responsible for various colors. Substances can emit light only in an excited state (this state is characterized by an increased level of internal energy) in the presence of a constant source of energy.

Receiving excess energy, the atoms of the substance emit it in the form of light and return to their normal energy state. It is this light emitted by atoms that is used for spectral analysis. The most common types of radiation include: thermal radiation, electroluminescence, cathodoluminescence, chemiluminescence.

Spectral analysis. Flame coloring with metal ions

Types of spectral analysis

There are emission and absorption spectroscopy. The emission spectroscopy method is based on the properties of elements to emit light. To excite the atoms of a substance, high-temperature heating equal to several hundred or even thousands of degrees is used - for this, a sample of the substance is placed in a flame or in the field of powerful electrical discharges. Under the influence of high temperatures, the molecules of a substance are divided into atoms.

Atoms, receiving excess energy, emit it in the form of light quanta of different wavelengths, which are recorded by spectral devices - devices that visually depict the resulting light spectrum. Spectral devices also serve as a separating element of the spectroscopy system, because the light flux is summed up from all substances present in the sample, and its tasks include dividing the total array of light into the spectra of individual elements and determining their intensity, which will allow in the future to draw conclusions about the amount of the element present in the total mass of substances.

Depending on the methods of observing and recording spectra, spectral instruments are distinguished: spectrographs and spectroscopes. The former record the spectrum on photographic film, and the latter make it possible to view the spectrum for direct observation by a person through special spotting scopes. To determine dimensions, specialized microscopes are used that allow the wavelength to be determined with high accuracy.
Once the light spectrum is recorded, it is subjected to careful analysis. Waves of a certain length and their position in the spectrum are identified. Next, a correlation is made between their position and their belonging to the desired substances. This is done by comparing wave position data with information located in methodological tables indicating typical wavelengths and spectra of chemical elements.
Absorption spectroscopy is carried out similarly to emission spectroscopy. In this case, the substance is placed between the light source and the spectral apparatus. Passing through the analyzed material, the emitted light reaches the spectral apparatus with “dips” (absorption lines) along certain wavelengths - they constitute the absorbed spectrum of the material under study. The further sequence of the study is similar for the above process of emission spectroscopy.

Opening Spectral Analysis

The importance of spectroscopy for science

Spectral analysis allowed mankind to discover several elements that could not be determined by traditional methods of recording chemical substances. These are elements such as rubidium, cesium, helium (it was discovered using spectroscopy of the Sun - long before its discovery on Earth), indium, gallium and others. The lines of these elements were detected in the emission spectra of gases, and at the time of their study were unidentifiable.

It became clear that these were new, hitherto unknown elements. Spectroscopy had a serious influence on the formation of the current type of metallurgical and mechanical engineering industries, the nuclear industry, and agriculture, where it became one of the main tools for systematic analysis.

Spectroscopy has acquired enormous importance in astrophysics.

Provoking a colossal leap in the understanding of the structure of the Universe and the affirmation of the fact that everything that exists consists of the same elements, which, among other things, abound on the Earth. Today, the spectral analysis method allows scientists to determine the chemical composition of stars, nebulae, planets and galaxies located billions of kilometers from Earth - these objects, naturally, are not accessible to direct analysis methods due to their great distance.

Using the absorption spectroscopy method, it is possible to study distant space objects that do not have their own radiation. This knowledge allows us to establish the most important characteristics of space objects: pressure, temperature, structural features and much more.

Have you ever thought about how we know about the properties of distant celestial bodies?

Surely you know that we owe such knowledge to spectral analysis. However, we often underestimate the contribution of this method to understanding itself. The advent of spectral analysis overturned many established paradigms about the structure and properties of our world.

Thanks to spectral analysis, we have an idea of the scale and grandeur of space. Thanks to him, we no longer limit the Universe to the Milky Way. Spectral analysis revealed to us a great diversity of stars, telling us about their birth, evolution and death. This method underlies almost all modern and even future astronomical discoveries.

Learn about the unattainable

Two centuries ago, it was generally accepted that the chemical composition of planets and stars would forever remain a mystery to us. Indeed, in the minds of those years, space objects will always remain inaccessible to us. Consequently, we will never get a sample of any star or planet and will never know its composition. The discovery of spectral analysis completely refuted this misconception.

Spectral analysis allows you to remotely learn about many properties of distant objects. Naturally, without such a method, modern practical astronomy is simply meaningless.

Lines on a rainbow

Dark lines on the spectrum of the Sun were noticed back in 1802 by the inventor Wollaston. However, the discoverer himself was not particularly fixated on these lines. Their extensive research and classification was carried out in 1814 by Fraunhofer. During his experiments, he noticed that the Sun, Sirius, Venus and artificial light sources have their own set of lines. This meant that these lines depended solely on the light source. They are not affected by the earth's atmosphere or the properties of the optical instrument.

The nature of these lines was discovered in 1859 by the German physicist Kirchhoff together with the chemist Robert Bunsen. They established a connection between the lines in the spectrum of the Sun and the emission lines of vapors of various substances. So they made the revolutionary discovery that each chemical element has its own set of spectral lines. Consequently, by the radiation of any object one can learn about its composition. This is how spectral analysis was born.

Over the next decades, many chemical elements were discovered through spectral analysis. These include helium, which was first discovered in the Sun, which is how it got its name. Therefore, it was initially thought to be exclusively a solar gas until it was discovered on Earth three decades later.

Three types of spectrum

What explains this behavior of the spectrum? The answer lies in the quantum nature of radiation. As is known, when an atom absorbs electromagnetic energy, its outer electron moves to a higher energy level. Similarly with radiation - to a lower level. Each atom has its own difference in energy levels. Hence the unique frequency of absorption and emission for each chemical element.

It is at these frequencies that the gas emits and emits. At the same time, solid and liquid bodies, when heated, emit a full spectrum, independent of their chemical composition. Therefore, the resulting spectrum is divided into three types: continuous, line spectrum and absorption spectrum. Accordingly, a continuous spectrum is emitted by solids and liquids, and a line spectrum is emitted by gases. The absorption spectrum is observed when continuous radiation is absorbed by a gas. In other words, multi-colored lines on a dark background of a line spectrum will correspond to dark lines on a multi-colored background of an absorption spectrum.

It is the absorption spectrum that is observed in the Sun, while heated gases emit radiation with a line spectrum. This is explained by the fact that the photosphere of the Sun, although it is a gas, is not transparent to the optical spectrum. A similar picture is observed in other stars. Interestingly, during a total solar eclipse, the spectrum of the Sun becomes lined. Indeed, in this case it comes from the transparent outer layers of it.

Principles of spectroscopy

Optical spectral analysis is relatively simple in technical implementation. Its work is based on the decomposition of the radiation of the object under study and further analysis of the resulting spectrum. Using a glass prism, in 1671 Isaac Newton carried out the first "official" decomposition of light. He also introduced the word “spectrum” into scientific use. Actually, while arranging the light in the same way, Wollaston noticed black lines on the spectrum. Spectrographs also operate on this principle.

Light decomposition can also occur using diffraction gratings. Further analysis of light can be done using a variety of methods. Initially, an observation tube was used for this, then a camera. Nowadays, the resulting spectrum is analyzed by high-precision electronic instruments.

So far we have been talking about optical spectroscopy. However, modern spectral analysis is not limited to this range. In many fields of science and technology, spectral analysis of almost all types of electromagnetic waves is used - from radio to X-rays. Naturally, such studies are carried out using a variety of methods. Without various methods of spectral analysis, we would not know modern physics, chemistry, medicine and, of course, astronomy.

Spectral analysis in astronomy

As noted earlier, it was from the Sun that the study of spectral lines began. Therefore, it is not surprising that the study of spectra immediately found its application in astronomy.

Of course, the first thing that astronomers began to do was use this method to study the composition of stars and other cosmic objects. Thus, each star acquired its own spectral class, reflecting the temperature and composition of their atmosphere. The parameters of the atmosphere of the planets of the solar system have also become known. Astronomers have come closer to understanding the nature of gas nebulae, as well as many other celestial objects and phenomena.

However, using spectral analysis, you can learn not only about the qualitative composition of objects.

Measure speed

Doppler effect in astronomyDoppler effect in astronomy

The Doppler effect was theoretically developed by an Austrian physicist in 1840, after whom it was named. This effect can be observed by listening to the whistle of a passing train. The pitch of the whistle of an approaching train will be noticeably different from that of a moving train. This is roughly how the Doppler Effect was proven theoretically. The effect is that, to the observer, the wavelength of the moving source is distorted. It increases as the source moves away and decreases as it approaches. Electromagnetic waves have a similar property.

As the source moves away, all the dark bands in its emission spectrum shift to the red side. Those. all wavelengths increase. In the same way, when the source approaches, they shift to the violet side. Thus it has become an excellent addition to spectral analysis. Now, from the lines in the spectrum, it was possible to recognize what had previously seemed impossible. Measure the speed of space objects, calculate the orbital parameters of double stars, the rotation speed of planets and much more. The “red shift” effect played a special role in cosmology.

The discovery of the American scientist Edwin Hubble is comparable to the development of the heliocentric system of the world by Copernicus. By studying the brightness of Cepheids in various nebulae, he proved that many of them are located much further than the Milky Way. By comparing the obtained distances with the spectra of galaxies, Hubble discovered his famous law. According to it, the distance to galaxies is proportional to the speed of their removal from us. Although his law differs somewhat from modern ideas, Hubble's discovery expanded the scale of the Universe.

Spectral analysis and modern astronomy

Today, almost no astronomical observation occurs without spectral analysis. With its help, new exoplanets are discovered and the boundaries of the Universe are expanded. Spectrometers are carried on Mars rovers and interplanetary probes, space telescopes and research satellites. In fact, without spectral analysis there would be no modern astronomy. We would continue to gaze at the empty, faceless light of the stars, about which we would know nothing.

Spectral analysis

Spectral analysis- a set of methods for qualitative and quantitative determination of the composition of an object, based on the study of the spectra of interaction of matter with radiation, including the spectra of electromagnetic radiation, acoustic waves, mass and energy distributions of elementary particles, etc.

Depending on the purposes of analysis and the types of spectra, several methods of spectral analysis are distinguished. Atomic And molecular spectral analyzes make it possible to determine the elemental and molecular composition of a substance, respectively. In the emission and absorption methods, the composition is determined from the emission and absorption spectra.

Mass spectrometric analysis is carried out using the mass spectra of atomic or molecular ions and allows one to determine the isotopic composition of an object.

Story

Dark lines in spectral stripes have been noticed for a long time, but the first serious study of these lines was undertaken only in 1814 by Joseph Fraunhofer. In his honor, the effect was called “Fraunhofer lines”. Fraunhofer established the stability of the positions of the lines, compiled a table of them (he counted 574 lines in total), and assigned an alphanumeric code to each. No less important was his conclusion that the lines are not associated with either the optical material or the earth's atmosphere, but are a natural characteristic of sunlight. He discovered similar lines in artificial light sources, as well as in the spectra of Venus and Sirius.

It soon became clear that one of the clearest lines always appeared in the presence of sodium. In 1859, G. Kirchhoff and R. Bunsen, after a series of experiments, concluded: each chemical element has its own unique line spectrum, and from the spectrum of celestial bodies one can draw conclusions about the composition of their substance. From this moment on, spectral analysis appeared in science, a powerful method for remote determination of chemical composition.

To test the method, in 1868 the Paris Academy of Sciences organized an expedition to India, where a total solar eclipse was coming. There, scientists discovered: all the dark lines at the moment of the eclipse, when the emission spectrum replaced the absorption spectrum of the solar corona, became, as predicted, bright against a dark background.

The nature of each of the lines and their connection with chemical elements were gradually clarified. In 1860, Kirchhoff and Bunsen discovered cesium using spectral analysis, and in 1861, rubidium. And helium was discovered on the Sun 27 years earlier than on Earth (1868 and 1895, respectively).

Operating principle

The atoms of each chemical element have strictly defined resonant frequencies, as a result of which it is at these frequencies that they emit or absorb light. This leads to the fact that in a spectroscope, lines (dark or light) are visible on the spectra in certain places characteristic of each substance. The intensity of the lines depends on the amount of substance and its state. In quantitative spectral analysis, the content of the substance under study is determined by the relative or absolute intensities of lines or bands in the spectra.

Optical spectral analysis is characterized by relative ease of implementation, the absence of complex sample preparation for analysis, and a small amount of substance (10-30 mg) required for analysis of a large number of elements.

Atomic spectra (absorption or emission) are obtained by transferring the substance into a vapor state by heating the sample to 1000-10000 °C. A spark or an alternating current arc are used as sources of excitation of atoms in the emission analysis of conductive materials; in this case, the sample is placed in the crater of one of the carbon electrodes. Flames or plasmas of various gases are widely used to analyze solutions.

Application

Recently, emission and mass spectrometric methods of spectral analysis, based on the excitation of atoms and their ionization in argon plasma of induction discharges, as well as in a laser spark, have become most widespread.

Spectral analysis is a sensitive method and is widely used in analytical chemistry, astrophysics, metallurgy, mechanical engineering, geological exploration and other branches of science.

In signal processing theory, spectral analysis also means the analysis of the energy distribution of a signal (for example, audio) over frequencies, wave numbers, etc.

Applications

Spectral expertise, common in:

Metallurgy;
Geology;
Chemistry;
Mineralogy;
Astrophysics;
Biology;
medicine, etc.

Allows you to find the smallest amounts of an established substance in the objects being studied (up to 10 - MS). Spectral analysis is divided into qualitative and quantitative.

Methods

The method of establishing the chemical composition of a substance based on the spectrum is the basis of spectral analysis. Line spectra have a unique personality, just like human fingerprints or the pattern of snowflakes. The uniqueness of patterns on the skin of a finger is a great advantage for searching for a criminal. Therefore, thanks to the peculiarities of each spectrum, it is possible to establish the chemical content of the body by analyzing the chemical composition of the substance. Even if its mass of an element does not exceed 10 - 10 g, using spectral analysis it can be detected in the composition of a complex substance. This is a fairly sensitive method.

Emission spectral analysis

Emission spectral analysis is a series of methods for determining the chemical composition of a substance from its emission spectrum. The basis for the method of establishing the chemical composition of a substance - spectral examination - is based on the patterns in the emission spectra and absorption spectra. This method allows you to identify millionths of a milligram of a substance.

There are methods of qualitative and quantitative examination, in accordance with the establishment of analytical chemistry as a subject, the purpose of which is to formulate methods for establishing the chemical composition of a substance. Methods for identifying a substance become extremely important within qualitative organic analysis.

Based on the line spectrum of vapors of any substance, it is possible to determine which chemical elements are contained in its composition, because any chemical element has its own specific emission spectrum. This method of determining the chemical composition of a substance is called qualitative spectral analysis.

X-ray spectral analysis

There is another method for identifying a chemical called X-ray spectral analysis. X-ray spectral analysis is based on the activation of the atoms of a substance when it is irradiated with X-rays, a process called secondary or fluorescent. Activation is also possible when irradiated with high-energy electrons; in this case, the process is called direct excitation. As a result of the movement of electrons in the deeper inner electron layers, X-ray lines appear.

The Wulff-Bragg formula allows you to set the wavelengths of X-ray radiation when using a crystal of a popular structure with a known distance d. This is the basis of the determination method. The substance being studied is bombarded with high-speed electrons. It is placed, for example, on the anode of a dismountable X-ray tube, after which it emits characteristic X-rays that fall on a crystal of a known structure. The angles are measured and the corresponding wavelengths are calculated using the formula, after photographing the resulting diffraction pattern.

Techniques

Currently, all methods of chemical analysis are based on two techniques. Either at the physical test, or at the chemical test, comparing the established concentration with its unit of measurement:

Physical

The physical technique is based on the method of correlating a unit of quantity of a component with a standard by measuring its physical property, which depends on its content in a sample of the substance. The functional relationship “Property saturation – component content in the sample” is determined by trial by calibrating the means for measuring a given physical property according to the component being installed. From the calibration graph, quantitative relationships are obtained, constructed in the coordinates: “saturation of a physical property - concentration of the installed component.”

Chemical

A chemical technique is used in the method of correlating a unit of quantity of a component with a standard. The laws of conservation of quantity or mass of a component during chemical interactions are used here. Chemical interactions are based on the chemical properties of chemical compounds. In a sample of a substance, a chemical reaction is carried out that meets the specified requirements to determine the desired component, and the volume or mass involved in the specific chemical reaction of the components is measured. Quantitative relationships are obtained, then the number of equivalents of a component for a given chemical reaction or the law of conservation of mass is written down.

Devices

Instruments for analyzing the physical and chemical composition of a substance are:

Gas analyzers;
Alarms for maximum permissible and explosive concentrations of vapors and gases;
Concentrators for liquid solutions;
Density meters;
Salt meters;
Moisture meters and other devices similar in purpose and completeness.

Over time, the range of analyzed objects increases and the speed and accuracy of the analysis increases. One of the most important instrumental methods for establishing the atomic chemical composition of a substance is spectral analysis.

Every year more and more complexes of instruments appear for quantitative spectral analysis. They also produce the most advanced types of equipment and methods for spectrum recording. Spectral laboratories are organized initially in mechanical engineering, metallurgy, and then in other areas of industry. Over time, the speed and accuracy of the analysis increases. In addition, the area of analyzed objects is expanding. One of the main instrumental methods for determining the atomic chemical composition of a substance is spectral analysis.