Ionization by electronic. Ionization by electron impact
"Introduction to Organic Chemistry" - Did organic chemistry develop? C6H12O6. Al2S3. C2H5OH. C10H22. NH3. Thesis: What is organic chemistry? HNO3. Are there more organic substances than inorganic ones? How to distinguish genuine leather from artificial? Arguments: Chemistry is one of the rapidly developing sciences. CH3COOH. CaCO3. L-aspartylaminomalonic acid methylphenyl ester is 33,000 times sweeter than sugar.
"Theory of organic chemistry" - Organic chemistry. Main classes of organic compounds. Aldehydes. Alcohols. Hypotheses of chemistry. The times of the Middle Ages. Products. Development of the theory of valency. Students. Functions. A bit of history. Ethers. Ions. Human. Halogens. Definition of organic chemistry. The structure of organic molecules.
"Composition of the body" - The structure of the carbohydrate molecule. Oxygen. Fats make up an essential part of our food. During digestion, fat is broken down into its component parts - glycerol and fatty acids. Nitrogen; We get carbohydrates from grains, legumes, potatoes, fruits and vegetables. Nutrients are proteins, fats and carbohydrates.
"Subject of organic chemistry" - Classification of organic matter. Synthetic - created by a person in the laboratory, there are no similar substances in nature. Earthy (mineral). 2) The composition necessarily includes (C) and (H) - hydrocarbons (HC). Plastic. Inorganic. Organ. Molecular CR. Petrol. Organic. 1) Numerous (about 27 million).
"Butlerov's theory of structure" - The quantitative composition of the molecule. Friedrich Wöhler. The doctrine of the mutual influence of atoms. Properties of organic compounds. Hydrogen atom. Comparative characteristics of ethane and ethylene. Alexander Mikhailovich Butlerov. Structural levels and systemic organization of matter. Stereochemistry. The possibility of establishing the “order of bonds” in molecules.
"Theory of the structure of chemical compounds" - Prerequisites for the emergence of the theory. Weller Friedrich. Berzelius Jens Jacob. Properties of organic compounds. Organic chemistry. Ethanol. Creation of the theory of the structure of substances. Spatial isomerism. Kekule Friedrich August. Structural isomerism. Frankland Edward. The main provisions of the theory of the structure of chemical compounds.
Electron ionization (EI)
Rice. 5.
Electron ionization is one of the most important ionization methods for routine analysis of small, hydrophobic, thermally stable molecules and is still widely used today. Since EI usually produces a large number of fragment ions, this is a "hard" way of ionization.
However, fragmented information can also be very useful. For example, using databases containing over 200,000 electron ionization mass spectra, it is possible to determine an unknown compound within a few seconds (of course, if it is in the database). These databases, as well as the amount of memory and search algorithms of modern computers, allow quick browsing of such databases (such as the NIST database), thus greatly facilitating the identification of small molecules.
The electron ionization device is straight forward (Fig. 5). The sample must be supplied in gaseous form, which is accomplished by "boiling" the sample through thermal desorption or by introducing gas through a capillary. The capillary is often the outlet of the capillary column of a gas chromatography instrument. In this case, a capillary column provides separation (this is also known as gas chromate mass spectrometry - GC/MS). Desorption of solid or liquid samples is carried out by heating the mass spectrometer in vacuum. After entering the gas phase, the compounds are transferred to an electron ionization device, where the electrons excite the molecule, thereby causing electron ionization and fragmentation.
The applicability of electron ionization is greatly reduced for compounds with molecular weights above 400 daltons because the necessary thermal desorption of the sample leads to thermal decomposition before evaporation occurs. The principal problems associated with thermal desorption during electron ionization are 1) non-volatility of large molecules, 2) thermal decomposition, 3) excessive fragmentation.
The mechanism of electron detachment during the formation of a positive ion is carried out as follows:
- · The sample is thermally evaporated.
- · Electrons are emitted from a heated cathode and accelerated by an electric field with a potential difference of 70 V to form a continuous electron beam.
- · The sample molecules pass through the electron beam.
- · Electrons with a kinetic energy of 70 eV transfer part of their energy to molecules. This transfer causes ionization (detachment of an electron) so that the ion usually retains no more than 6 eV of excess energy.
- · An excess of internal energy (6 eV) in the molecule leads to some fragmentation.
Electron capture is usually much less efficient than electron abstraction, although it is sometimes used in the same way, working with high sensitivity for compounds with high electron affinity: M + e - > M - .
Advantages of the method:
- · The method of electron impact ionization gives mass spectra rich in fragments, which unambiguously characterize the structure of the molecule, which is convenient for identifying substances;
- · Electron impact mass spectrometry - a highly sensitive method of analysis, allows you to analyze pikomolar amounts of a substance;
- · There are "libraries" of mass spectra containing spectra of more than 200,000 organic compounds, which can be used for their identification using a computer.
Disadvantages of the method:
- · Molecular ions are formed only in 20% of organic compounds;
- the method is applicable only to the determination of highly volatile thermally stable compounds;
- · ions with large m/z values, which give information about the molecular weight and the presence of functional groups, provide a small contribution to the values of the total ionic current;
- · Negatively charged ions, which are of great importance in structural analysis, are formed in a very small amount and a limited number of organic compounds.
Electron ionization
Electron ionization(EI, electron impact ionization, EI - Electron Ionization or Electron Impact) is the most common method of ionization of substances in the gas phase in mass spectrometry.
During electron ionization, the molecules of the analyte enter the flow of electrons moving from the emitting cathode to the anode. The energy of moving electrons is usually 70 eV, which, according to the de Broglie formula, corresponds to the length of a standard chemical bond in organic molecules (about 0.14 nm). Electrons cause ionization of the analyzed molecules with the formation of radical cations:
M + e − = M .+ + 2e −
Electron ionization takes place in a vacuum (compared to chemical ionization) to prevent the mass production of atmospheric gas ions that can recombine with and destroy analyte ions.
Since the energy of the electrons is much greater than the energy of the chemical bond, fragmentation of the ions occurs. The chemistry of ion fragmentation during electronic fragmentation is well studied, therefore, knowing the masses of fragments and their intensities, one can predict the initial structure of the substance. Mass spectra obtained using the electron ionization method are well reproducible, therefore, today there are libraries containing hundreds of thousands of spectra of various substances, which greatly facilitate qualitative analysis.
Some substances undergo very intense fragmentation, generating only low molecular weight fragments that make identification difficult. For the analysis of such substances, there is an alternative method of chemical ionization.
| Mass spectrometry Mass Mass spectrum MS software |
|
|---|---|
| EI CI IA ( English) BBA DVEP MALDI APCI ESI DESI GD ICP MIP TS DART | |
| Mass analyzer | Magnetic sector WP Quadrupole Quadrupole ion trap Orbitrap Tandem ICR |
| Detector | Electron multiplier Microchannel plate detector Daly detector |
| MS combination | MS/MS GC/MS LC/MS IMS/MS |
Wikimedia Foundation. 2010 .
See what "electronic ionization" is in other dictionaries:
E. theory is a very bold attempt to consider an atom of any substance as an aggregate of the same number of atoms of positive and negative electricity, the so-called positive and negative electrons, forming ... ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron
A device for obtaining flows (beams) of electrons in a volume from which air is removed (in vacuum). The electrons in the electric field fly out of the cathode and are accelerated by the electric field (Fig. 1). The emission of electrons from the cathode occurs mainly ... ...
Term Auger electron spectroscopy English term Auger electron spectroscopy Synonyms Auger spectroscopy Abbreviations EOS, AES Related terms ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy… …
Ionization in the upper atmosphere due to the intrusion of meteoric matter into it. Active M. and. occurs mainly in the collisions of evaporated and pulverized meteoric atoms with air molecules. Average… … Great Soviet Encyclopedia
See Art. (see MULTI-PHOTON PROCESSES). Physical Encyclopedic Dictionary. Moscow: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1983. MULTI-PHOTON IONIZATION ... Physical Encyclopedia
The steadily increasing process of electron multiplication as a result of ionization of atoms and molecules, as a rule, by electron impact; is the main element of electricity. breakdown of gases. In most cases, L. e. develops into electrical or email magn… Physical Encyclopedia
- (see IONIZATION). Physical Encyclopedic Dictionary. Moscow: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1983. STEP IONIZATION ... Physical Encyclopedia
The term molecular electron spectroscopy The term in English molecular electron spectroscopy Synonyms UV spectroscopy, UV spectroscopy Abbreviations Related terms electron vibrational spectroscopy Definition method of determination ... ... Encyclopedic Dictionary of Nanotechnology
Electron ionization (EI, electron impact ionization, EI Electron Ionization or Electron Impact) is the most common method of ionization of substances in the gas phase in mass spectrometry. In the electron ionization of the analyzed molecule ... ... Wikipedia
- (mass spectroscopy, mass spectrography, mass spectral analysis, mass spectrometric analysis) method of studying a substance by determining the ratio ... Wikipedia
Electric current in gases.
Non-self-sustained electrical discharge. Experience shows that two oppositely charged plates separated by a layer of air do not discharge.
Usually, a substance in the gaseous state is an insulator, since the atoms or molecules of which it is composed contain the same number of negative and positive electric charges and are generally neutral.
We introduce the flame of a match or spirit lamp into the space between the plates (Fig. 164).
In this case, the electrometer will begin to discharge quickly. Consequently, the air under the action of the flame became a conductor. When the flame is removed from the space between the plates, the discharge of the electrometer stops. The same result can be obtained by irradiating the plates with the light of an electric arc. These experiments prove that gas can become a conductor of electric current.
The phenomenon of the passage of an electric current through a gas, observed only under the condition of some external influence, is called a non-self-sustained electric discharge.
Thermal ionization. Heating a gas makes it a conductor of electric current, because some of the atoms or molecules of the gas turn into charged ions.
To detach an electron from an atom, it is necessary to do work against the forces of Coulomb attraction between a positively charged nucleus and a negative electron. The process of detachment of an electron from an atom is called ionization of the atom. The minimum energy that must be expended to detach an electron from an atom or molecule is called the binding energy.
An electron can be detached from an atom when two atoms collide if their kinetic energy exceeds the binding energy of the electron. The kinetic energy of the thermal motion of atoms or molecules is directly proportional to the absolute temperature, therefore, with increasing gas temperature, the number of collisions of atoms or molecules, accompanied by ionization, increases.
The process of the emergence of free electrons and positive ions as a result of collisions of atoms and molecules of a gas at high temperature is called thermal ionization.
Plasma. A gas in which a significant part of the atoms or molecules are ionized is called plasma. The degree of thermal ionization of plasma depends on temperature. For example, at a temperature of 10,000 K, less than 10% of the total number of hydrogen atoms is ionized; at temperatures above 20,000 K, hydrogen is almost completely ionized.
Plasma electrons and ions can move under the action of an electric field. Thus, at low temperatures, the gas is an insulator, at high temperatures it turns into a plasma and becomes a conductor of electric current.
Photoionization. The energy required to detach an electron from an atom or molecule can be transferred by light. The ionization of atoms or molecules by the action of light is called photoionization.
Independent electrical discharge. With an increase in the electric field strength to a certain value, depending on the nature of the gas and its pressure, an electric current appears in the gas even without the influence of external ionizers. The phenomenon of electric current passing through the gas, which does not depend on the action of external ionizers, is called an independent electric discharge.
In air at atmospheric pressure, an independent electric discharge occurs at an electric field strength of approximately
The main mechanism of gas ionization in a self-sustained electric discharge is the ionization of atoms and molecules due to electron impacts.
Ionization by electron impact. Ionization by electron impact becomes possible when the free path of the electron acquires a kinetic energy that exceeds the binding energy W of the electron with the atom.
The kinetic energy Wk of an electron, acquired under the action of an electric field strength, is equal to the work of the forces of the electric field:
where l is the free path length.
Hence, the approximate condition for the onset of ionization by electron impact has the form
The binding energy of electrons in atoms and molecules is usually expressed in electron volts (eV). 1 eV is equal to the work that an electric field does when an electron (or other particle with an elementary charge) moves between field points, the voltage between which is 1 V:
The ionization energy of a hydrogen atom, for example, is 13.6 eV.
self-discharge mechanism. The development of an independent electric discharge in a gas proceeds as follows. A free electron under the action of an electric field acquires acceleration. If the electric field strength is high enough, the free path of the electron increases the kinetic energy so much that when it collides with the molecule, it ionizes it.
The first electron, which caused the ionization of the molecule, and the second electron, released as a result of ionization, acquire acceleration in the direction from the cathode to the anode under the action of an electric field. Each of them, during the next collisions, releases one more electron, and the total number of free electrons becomes equal to four. Then, in the same way, it increases to 8, 16, 32, 64, etc. The number of free electrons moving from the cathode to the anode increases like an avalanche until they reach the anode (Fig. 165).
Positive ions formed in the gas move under the action of an electric field from the anode to the cathode. When positive ions hit the cathode and under the action of light emitted during the discharge process, new electrons can be released from the cathode. These electrons, in turn, are accelerated by the electric field and create new electron-ion avalanches, so the process can continue continuously.
The concentration of ions in the plasma increases as the self-sustained discharge develops, while the electrical resistance of the discharge gap decreases. The current strength in the self-discharge circuit is usually determined only by the internal resistance of the current source and the electrical resistance of other circuit elements.
Spark discharge. Lightning. If the current source is not able to maintain an independent electrical discharge for a long time, then the ongoing independent discharge is called a spark discharge. The spark discharge stops a short time after the start of the discharge as a result of a significant decrease in voltage. Examples of a spark discharge are sparks that occur when combing hair, separating sheets of paper, discharging a capacitor.
An independent electrical discharge is also represented by lightning observed during a thunderstorm. The current strength in the lightning channel reaches 10,000-20,000 A, the duration of the current pulse is several tens of microseconds. An independent electric discharge between a thundercloud and the Earth after several lightning strikes stops by itself, since most of the excess electric charges in a thundercloud are neutralized by an electric current flowing through the lightning plasma channel (Fig. 166).
With an increase in the current strength in the lightning channel, the plasma is heated to a temperature of over 10,000 K. Pressure changes in the lightning plasma channel with an increase in current strength and the termination of the discharge cause sound phenomena called thunder.
glow discharge. As the gas pressure in the discharge gap decreases, the discharge channel becomes wider, and then the entire discharge tube is uniformly filled with luminous plasma. This type of independent electrical discharge in gases is called a glow discharge (Fig. 167).
Electric arc. If the current strength in an independent gas discharge is very high, then the impacts of positive ions and electrons can cause the cathode and anode to heat up. Electrons are emitted from the cathode surface at a high temperature, which ensures the maintenance of a self-sustained discharge in the gas. A long-term independent electrical discharge in gases, maintained by thermionic emission from the cathode, is called an arc discharge (Fig. 168).
Corona discharge. In highly inhomogeneous electric fields, which are formed, for example, between a tip and a plane or between a wire and a plane (power line), an independent discharge of a special type occurs, called a corona discharge. In a corona discharge, ionization by electron impact occurs only near one of the electrodes, in a region with a high electric field strength.
The use of electrical discharges. The impacts of electrons accelerated by an electric field lead not only to the ionization of atoms and molecules of the gas, but also to the excitation of atoms and molecules, accompanied by the emission of light. The light radiation of a plasma of an independent electric discharge is widely used in the national economy and in everyday life. These are fluorescent lamps and gas-discharge lamps for street lighting, an electric arc in a film projector, and mercury-quartz lamps used in hospitals and clinics.
The high temperature of the arc discharge plasma allows it to be used for cutting and welding metal structures, for melting metals. With the help of a spark discharge, parts from the hardest materials are processed.
An electrical discharge in gases is also an undesirable phenomenon, which must be combated in technology. So, for example, a corona electrical discharge from the wires of high-voltage power lines leads to useless losses of electricity. The increase in these losses with increasing voltage puts a limit on the way to further increase the voltage in the power line, while in order to reduce energy losses for heating the wires, such an increase is highly desirable.
Recombination.
Recombination is a process that is the reverse of ionization. It consists in the capture of a free electron by an ion. Recombination leads to a decrease in the charge of the ion or to the transformation of the ion into a neutral atom or molecule. The recombination of an electron and a neutral atom (molecule) is also possible, leading to the formation of a negative ion, and in more rare cases, the recombination of a negative ion with the formation of a two- or three-fold charged negative ion. In some cases, instead of an electron, other elementary particles, such as mesons, can act, creating mesoatoms or mesomolecules. In the early stages of the development of the universe, the reaction of hydrogen recombination took place.
Recombination is the reverse process of breaking a chemical bond. Recombination is associated with the formation of an ordinary covalent bond due to the socialization of unpaired electrons belonging to different particles (atoms, free radicals)
Recombination examples:
H + H → H2 + Q ;
Cl + Cl → Cl2 + Q;
CH3 + CH3 → C2H6 + Q, etc.
In order to knock out one electron from a molecule (atom), it is necessary to expend a certain amount of energy. The minimum value of such energy is called the ionization energy of a molecule (atom), its value for atoms of various substances lies within 425 eV.
Simultaneously with the process of gas ionization, the reverse process always goes on - the process of recombination: positive and negative ions and molecules. The more ions appear under the action of the ionizer, the more intense is the recombination process. As a result of recombination, the conductivity of the gas disappears or returns to its original value.
As mentioned above, to detach an electron from an atom (ionization of an atom), a certain amount of energy must be expended. In the case of recombination of a positive ion and an electron, this energy, on the contrary, is released. Most often, it is emitted in the form of light, and therefore the recombination of ions is accompanied by luminescence (luminescence of recombination). If the concentration of positive and negative ions is high, then the number of recombination events occurring every second will also be large, and the recombination glow can be large, and the recombination glow can be very strong.
Ionization under the action of an external ionizer is taken into account only in the case of relatively weak electric fields, when the kinetic energy eEL accumulated by an electron (or ion) over the mean free path L is less than the ionization energy Ei
and, consequently, when colliding with neutral particles, electrons only change the direction of motion (elastic scattering).
In addition to this ionization, ionization by electron impacts is also possible.
3.2 Ionization by electron impacts.
This process consists in the fact that a free moving electron with sufficient kinetic energy knocks out one (or several) of atomic electrons when it collides with a neutral atom. As a result, the neutral atom turns into a positive ion (which can also ionize the gas) and, in addition to the primary one, new electrons appear that ionize more atoms. Thus, the number of electrons will increase like an avalanche, this process is called an electron avalanche. This type of ionization is observed in strong fields, when
To quantify the ionizing power of electrons and ions, Townsend (1868 - 1957) introduced two "volume ionization coefficients" and . is defined as the average number of ions of the same sign produced by an electron per unit length of its path. The coefficient characterizing the ionizing ability of positive ions has the same meaning. The electron ionization coefficient significantly exceeds the positive ion ionization coefficient.
The following classical experiment by Townsend proves this assertion.
An experience: An ionization chamber is taken in the form of a cylindrical capacitor, the inner electrode of which is a thin metal thread (Fig. 1). Between the filament and the outer cylinder of the capacitor, a potential difference V is pretended to be sufficient for impact ionization gas. The latter will practically occur only near the filament, where the electric field is very strong. Let us assume that a positive potential is applied to the filament. Then electrons will rush to the filament and will ionize the gas near it. Positive ions, rushing to the outer cylinder, will pass through the region of a weak field and cause practically no ionization. Let us now change the polarity of the voltage V without changing its value. Then the roles of positive and negative ions will change places. Positive ions will rush to the filament, and ionization in the chamber will be excited almost exclusively by them. Experience shows that in the first case, the ionization current increases more and faster with voltage V than in the second (Fig. 2, curve I refers to the case when the internal electrode is positive, and curve II to the case when it is negative).
Thus, the main role is played by ionization by electron impacts, in comparison with which the ionization by positive ions can be neglected in many cases.
3.3 Independent and non-independent category.
Before proceeding to the consideration of Townsend's theory, we will give the concept of an independent and non-self-sustaining discharge.
A discharge that exists only under the action of an external ionizer is called non-self-sustained discharge.
If the ions necessary to maintain the electrical conductivity of the gas are created by the discharge itself (as a result of the processes occurring in the discharge), such a gas discharge is called independent.
Townsend's theory of the passage of electric current through a gas.
It takes into account the impact ionization of atoms and molecules of the gas by electrons and positive ions. For simplicity, the electrodes of the discharge tube will be considered flat. We will neglect the recombination of ions and electrons, assuming that during the time of passage between the cathode and the anode, these particles do not have time to recombine. In addition, we restrict ourselves to the stationary regime, when all quantities characterizing the discharge do not depend on time. Let us place the origin of coordinates on the surface of the cathode K, directing the X axis towards the anode A. Let ne(x) and np(x) be the concentrations of electrons and positive ions, and ve and vp be their average drift velocities. Take an infinitely thin flat layer in a gas. Every second, ne(x) vp(x) electrons enter the layer through this area from the left, and ne(x+dx) ve(x+dx) exit from the right. In the volume of the dx layer, due to ionization by electrons, ne vedx electrons and the same number of positive ions arise every second. Similarly, due to ionization by positive ions, npvpdx electrons and the same number of positive ions are formed. Finally, there may be an external source of ionization that creates q pairs of ions every second per unit volume of gas. And since in the case of stationarity of the process, the number of electrons in the layer does not change, the relation must be satisfied
ne(x)ve(x)-ne(x+dx)ve(x+dx) + (neve + npvp)dx +qdx=0
Similarly, for positive ions moving from the anode to the cathode,
np(x+dx)vp(x+dx) – np(x)vp(x) + (neve + npvp)dx +qdx=0
Replacing the differences with the corresponding differentials and canceling by dx, we get
