Detectors in the masses of spectrometry. Chromatographic methods and their use in identifying pollutants of natural environments

Mass spectrometry is a method of studying substances, mass calculation and the number of ions when the substance ionization.

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Equipment to which mass spectrometry is produced is a mass spectrometer. It analyzes the sample and provides data in the form of graphs (mass spectra).

In this way, any material that is amenable to ionization can be explored.

Wide use of mass spectrometry acquired in such areas as:

• medicine and pharmaceuticals;
• genetic engineering and biochemistry;
• chemical industry;
• fOOD INDUSTRY;
• cosmetic and perfumery;
• laboratory diagnostics for determining substances in criminalistics, doping control, ecology;
• manufacture of polymeric and plastic materials;
• semiconductor industry;
• nuclear energy;
• metallurgical production;
• refineries and petrochemical industries;
• biology, geology, hydrology, mineralogy and other industries.

The study paths of mass spectrometry in different areas vary depending on which data must be obtained in the end.

The mass spectrometry can be obtained by the following data:

• establish the structure of the compound;
• research substance on components;
• establish the age of geological rock to examine the composition of isotopes;
• chromato mass spectral analysis for the ecological sphere;
• explore ionization processes, ionic reactions;
• measure the potential and energy of molecules.

The advantage of the method of mass spectrometry is that a very small amount of substance is enough for research.

The disadvantage consists of the destruction of the material, which is studied, i.e. Conversion products are analyzed.

Note. The mass spectrometric method in essence does not relate to the spectrometric method, since there is no interaction of the sample with electromagnetic radiation. But due to the graphical type of dependence of the power of the ion flow from the mass ratio to the charge, which is similar to the spectrum, this method and got its name.

Very available and in detail mass spectrometry is illuminated in tutorials, like Lebedev A.T. "Mass spectrometry in organic chemistry."

Mass spectrometry method

The mass spectrometry method is the sequential execution of the following operations:

1. Ionization of the substance, namely the deprivation of molecules at least one ion. The mass of it below the mass of the molecule is many times, so it will not affect the result of the study.
2. The acceleration of charged particles in the vacuum medium in the electric field followed by moving them into a magnetic field.
3. Analysis of the movement of particles in a magnetic field, namely their speed, curvature of the trajectory of movement. More charged particles are rapidly accelerated and react better to a magnet. Particles with a large mass are not such controlled due to inertia of movement.

Note. The vacuum is necessary for the free movement of charged particles and preventing them in turning them into uncharged.

Sample ionization can be performed in several ways and depends on the required target.

There are such ionization methods in mass spectrometry:

1. Electronic blow - adapted for isotopic and molecular analysis of inorganic materials.
2. Chemical ionization - to study organic materials.
3. Electrosprony.
4. Laser radiation.
5. Bombardment by the bunch of ions.

The last three methods are used to study substances with large molecules.

In addition, the ionization method is divided into several types of substances before the study, namely gas, liquid or solid.

The gas state (phase) of the sample is carried out by such ionization methods:

• electronic (isotope mass spectrometry);
• chemical;
• electronic grip;
• ionization in the electric field.

The liquid state (phase) of the sample is carried out by such ionization methods in mass spectrometry:

• thermospair;
• on open air;
• electrospray;
• chemical outdoors;
• photoInization.

The solid state (phase) of the sample is carried out in such ways of ionization:

• direct laser desorption;
• matric-activated laser desorption / ionization (Moldi mass spectrometry);
• mass spectrometry of secondary ions (ionic mass spectrometry);
• bombing by rapid atoms;
• desorption in the electric field;
• plasma desorption;
• ionization in inductive-bound plasma (mass spectrometry with inductive-bound plasma);
• thermoionization (superficial ionization);
• ionization in glowing discharge (spark ionization);
• ionization in the laser ablation process.

The last four options are sufficiently rigid, but without them it is impossible to get ions in samples with very durable connections.

Mass spectrometric helium leak detector

The method of mass spectrometry in helium leak detectors, for example, PH-10, T1-50 and others practiced very widely.

The studied systems or tanks are filled with helium and then with the help of the mass spectrometric method, places are wanted, where helium seeps through the cracks.

The sensitivity of the mass spectrometric method allows to find even very small leaks of inert gas in very small quantities, therefore the helium mass spectrometric leak detector is one of the most accurate and used devices in the industry.

Method of chromato mass spectrometry

The method of chromato-mass spectrometry is a tandem mass spectrometry of chromatography and mass spectrometry, i.e. The combination of these two methods.

Chromatography is engaged in the splitting of molecules on charged particles, and the mass spectrometry analyzes them.

There are two types of chromato mass spectrometry:

• gas;
• liquid.

The determination by the method of chromato mass spectrometry of the composition of organic substances, which are most often multicomponent, is perhaps the only available method. The best is the combination of gas chromatography and an ion detector of the mass spectrometer.

That is why the chromato mass spectrometry received great consumption in medical practice to diagnose and analyze the diseases and their pathogens, including the definition of microbiocenosis of different organs of any concentration by the method of chromato mass spectrometry or mass spectrometry of microbial markers of biological materials (blood, urine and Other). Microbiocenosis by the method of chromato mass spectrometry provides the ability to identify many microbes that cannot be identified by other methods, even those that are in sleeping state in protective capsules. And, therefore, people get the opportunity to take advantage of the right and timely treatment, which is impossible to overestimate.

In addition, the chromato mass spectrometry is extensively used in pharmaceuticals to create new drugs, chemical industries, the ecological sphere for sample assessment ambient, genetic engineering, technical control different regions Industry, laboratory surveys for the presence in the blood of prohibited drugs and so on.

Gas chromatography

Gas chromatography mass spectrometry provides for the addition of an inert gas carrier (often this helium), which is a moving element. The studied substance is a fixed element.

The gas mass spectrometry allows you to analyze gases, liquids and solids that have a molecular weight below 400. The studied substances must have the required volatile, inert and thermal stable properties.

The gas chromatograph scheme is proposed in the diagram below.

Spectrometric analysis

Spectrometric analysis proceeds in mass analyzers and mass spectrometer detectors.

Mass analyzers are continuous and impulse. They differ in the fact that the receipt of ions in them is constantly (continuously) or portions, respectively.

Continuous analyzers belong to the magnetic and quadrupole, to the pulse - ion trap, time-of-flight mass analyzer and an analyzer of ion-cyclotron resonance with Fourier transform.

The main task of the analyzer is the redistribution of ions with different parameters of the movement.

After that, the ions fall into the detector, which registers different spectra of ions.

Most often, a diode secondary-electronic multiplier or photomultiplier is used as detectors. The first registers the quantitative indicators of various ions with electron beams, the second registers flickering from the bombardment by the ions of the phosphor.

There are also other types of detectors, these are microchannel multipliers, systems such as diode matrices and collectors.

What is a mass spectrometer

The mass spectrometer is called vacuum equipment, which is capable of analyzing the substance according to the laws of displacement of charged particles in a magnetic and electric field.

In a simplified form, the description of the mass spectrometer can be represented as: the main components of the device are an ion source, mass analyzer and detector.

The ion source turns conventional sample molecules to charged particles and places them into an electric and magnetic field to accelerate.

The mass analyzer divides the ions on the speed of the speed of movement, namely the time of moving to some distance.

The detector registers data on the relative amount of each group.

In addition to the main components, the mass spectrometer is equipped with even vacuum installations with a pump and a fan to produce a vacuum, a pressure gauge, a system for setting a trial sample, an electronic circuit, indicators, stabilizer and other.

Depending on the ionization of the substance, mass spectrometers are static and dynamic.

There are also mass spectrometers with two mass analyzers, i.e. Tandem spectrometers. They are used mainly at the soft ways of ionization.

How to distinguish molecules of different connections? It turns out the easiest way - weighing them on special scales, which are called mass spectrometer

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The principle of operation of mass spectrometers Electrostatic and magnetic sector in a double focusing device. The ions flying out from the source are focused in the slits not only in different directions, but also with different energies

Quadrupole analyzer. Ions with a selected mass ratio (M) to charge (z) pass along the axis of the analyzer and fall into the detector, and the ions with other M / Z relations face rods or fly beyond the working space

Perhaps, soon familiar bandwidth users for the passenger check will be much more smarter. Imagine a man near the detector passes, a lung breeze touches his clothes - and soon the security service already has information about whether this passenger has something with any hazardous substances. Trial samples of such detectors are so sensitive that they are able to detect traces of a chemical compound, even if only a few molecules remained. And they are made on the basis of a mass spectrometer - a device that can distinguish between molecules by mass and determine the percentage of each variety of molecules in the sample of the substance.

In essence, the mass spectrometer is precision electromagnetic scales, which can be "weighing" atoms with an accuracy of 10-31 grams. It is due to this invention in the twenties of the last century, isotopes of all known chemical elementsand when the curiosity of scientists was sufficiently satisfied, the turn came applied tasks. In the forties, in the laboratories of the Occroja, the mass spectrometer was used in the separation of uranium isotopes for the first atomic bombsAnd at the same time, the first civil consumers of these devices appeared - oil concerns. They used mass spectrometers for quantitative analysis of the mixture of organic gases.

Operating principle

Modern mass spectrometers are undoubtedly more accurate and more advanced of their predecessors of centenary limitations, but the basic principle of their work remains unchanged, and the design, as a hundred years ago, includes three main elements: ionizer, analyzer and detector.

At first, the molecules should be ionized, that is, to deprive them at least one electron. Since the electron in thousands (and sometimes in tens of thousands) times lighter than the molecule, ionization practically does not affect its mass. After the ionizer, the particles fall into the analyzer, which is a vacuum chamber with electrical and magnetic fields. The ions are accelerated by an electric field, and then sent to a magnetic field, where the trajectory of the charged particle is twisted. All particles move in the same field, and among themselves they differ in electrical charge and mass. The more charge, the stronger you can disperse the ion and the easier it is to turn it with a magnet, but the more his mass, the harder it is because of the inertia. What kind of energy will acquire a particle, what will its speed and the degree of curvature of the trajectory, depends on the size of the field and the ratio of the mass of the particle to its charge.

If we assume that when ionization from each molecule was managed to disrupt only one electron (as most often and occurs), all ions will be discharged and the nature of their movement will depend only on the mass. The heavier ion, the harder it is "turning" and the less the curvature of his trajectory. It turns out that particles with different masses will literally fly out in different directions.

On the last stage You need to register these ions by some detector of charged particles, such as a photoflastic or a secondary-electronic multimeter. Putting B. suitable place A number of detectors, we will see that particles with a different mass (but with the same charge) will fall into different detectors. Now we will build a schedule: the horizontal will postpone the coordinate of the detector, which registered ion, and vertically - the number of these ions. We will get a mass spectrum - the picture similar to the radiation spectrum: the greater the difference in the masses, the further points of the hitting from each other, and the more particles arrive at this place, the greater the signal and the above corresponding peak.

Actually in modern systems Only one detector is used. With a specific field value, the ions of a certain mass are focused. Gradually changing the size of the field, you can direct the detector in turns of different ions and register them. The computer calculates the corresponding masses by the field values, compares with the database and builds the mass spectrum.

First experiments

In a series of founders, mass spectrometry is the first name of the electron opener Sir Joseph John Thomson. At that time, at the end of the beginning of the century, many physicists actively studied electrical discharges in gases. First of all, they were interested in the charged particles that occurred. After putting a number of witty experiments, Thomson was able to determine the parameters of negatively charged particles (which we now know as electrons), and then engaged positively charged - ions. To explore the ions, he had to assemble a separate installation. Thomson posted a cathode with holes in the middle of a glass tube, a magnet behind him, and even further behind the tube - a photoplastic. Positively charged ions of different chemical compoundswho were in the tube, flew to the cathode, hit the magnetic field through the holes and eventually left the traces in different places of the photoflastic. According to the coordinates of particles on the photoflastic and the known values \u200b\u200bof the Thomson fields, the ratio of the mass ions to their charges and noted in photographs of the trajectory of hydrogen ions, atomic and molecular oxygen, carbon dioxide and carbon monoxide, mercury and neon. So the era of mass spectrometry began.

Excess weight

After World War II, Thomson's research continued his assistant Francis William Aston. An improved device that Aston called the mass spectrograph allowed not only to see the lines corresponding to particles with different masses, but also had sufficient accuracy to determine the quantitative relations between them. Most Aston and his staff struck the fact that the atomic weights of all light elements expressed in relative units, with amazing accuracy corresponded to the whole numbers. Taking a mass of an oxygen atom for 16 units, a value of 12 were obtained for carbon, for nitrogen - 14, etc. For heavy elements, with atomic weight more than 30, it "the rule of an integer" began to be slightly broken, but the most strange was the value atomic mass hydrogen - not 1, and 1,008. Moreover, the accuracy of the mass spectrograph was such that this, at first glance, a minor, the difference could not be written to the error of measurements. The first who understood the importance, and most importantly, the meaning of this anomaly was the Aston himself. In his opinion, this experimental fact confirmed nothing more than a mutual transition of mass and energy predicted by the theory of relativity: when several protons (hydrogen nuclei) are connected, forming another element, part of their mass goes into energy, and as a result, mass, for example, Helium is somewhat less than the sum of the masses of its particles.

"The results obtained using a mass spectrograph eliminated any doubts about this issue ..." said Aston in his Nobel lecture in 1922. - We can be quite sure that when the hydrogen turning into helium, a certain part of the mass should disappear ... Perhaps future researchers will open some way to exemplate this energy that will allow it to be used. Then humanity will receive at its disposal such opportunities that are superior to any fantasy. " Aston supported his words with numbers. According to its calculations, made on the basis of mass spectrometric measurements and theory of relativity, if all the hydrogen contained in only 9 grams of water, turn into helium, the energy of 200,000 kW / hour will be distinguished, which is enough for modern standards to illuminate the usual urban apartment For several years. Now we know exactly what exactly nuclear reactions - Source of solar energy, but people can manage it only in the mode of thermonuclear bomb, in other words, they still do not know how to at all.

So experiments with gas-discharge lamps allowed physicists to make far-reaching conclusions about the fundamental properties of matter, and at the same time creating a wonderful device - mass spectrometer.

Quadrupole mass spectrometers

With the advent of new methods of detection, together with photoflasts, he gradually went into the past and invented aston name - mass spectrograph. Modern mass spectrometers, which mostly retained the magnetic field as the main element, came to replace. Mass spectrometers with a magnet remain unsurpassed by sensitivity, and, despite the huge sizes and high energy consumption, they have no alternative where high accuracy is needed. The search for a more compact and economical solution led in the mid-50s of Professor Wolfgang Paul and his staff from the University of Bonn to the creation of a mass spectrometer without a magnetic field - a quadrupole analyzer with an alternating electric field. This analyzer consists of four rods, a radio frequency alternating voltage is applied to pairs of opposite rods and additionally - a constant voltage between pairs. Depending on the values \u200b\u200bof the voltage and frequency, only ions with a certain ratio of the mass are moving between the rods between the rods, and the rest flies out. The design was really compact and very practical.

The miniature quadrupole mass spectrometer was made specifically to ensure the safety of astronauts of international space station, Including when working in open space. This device with a box size for shoes and weighing 2.3 kg can continuously control ammonia leaks, nitrogen, rocket fuel, oxygen, water and different substances.

Who quickly

Even before the quadrupole, in 1946, an employee of Pennsylvania University William Stephens came up with another way of sorting molecules by weight without a magnet - a time-of-flight mass spectrometer. It remains only a small section of the electric field to overclock the ions,

and the main part was involved in the useless space. The principle of operation of this device was remarkably simple: heavy ions are harder to disperse due to their inertia, and therefore they, having a smaller speed after overclocking and moving more slowly in the drift space without a field, arrive to the detector later lungs. If we assume that all ions are charged the same, the time on the way will be directly proportional to square root From the mass. First, light ions will arrive in the detector, then those that are heavier, and the last thing is the hardest. Such a device was easier (although there was a smaller accuracy than magnetic) and cheaper, and also possessed huge speed, since the entire spectrum of ions in a wide range of masses was recorded in one pass and it was not necessary to spend time on a gradual change in the field.

With the help of the gun mass spectrometer in 1985 was opened whole class New substances are fullerenes. By that time it was already known that clusters consisting of molecules consisting of carbon of different numbers Carbon atoms (up to 24). Thanks to mass spectrometers, these clusters managed to distinguish between these clusters and determine their masses. When the vapor moved to the study of carbon plasma directed into the stream of helium, molecules from a larger number of atoms were visible on the mass spectra, including C60 and C70. And with certain modes of creating a plasma, the peak corresponding to C60 became several times higher than all the others, which indicated the stability of this compound. Thus, unusual molecules in the form of a soccer ball were found, consisting of 60 carbon atoms, for which fullerene plates in 1996 were awarded Nobel Prize in chemistry.

Delicate approach

Truly the limitless scope of the use of mass spectrometry is an analysis of complex organic substances, without which modern medicine and biology is unthinkable. However, this was only possible after the emergence of new ionization methods. After all, for mass spectrometric analysis, you need to get free ions, and therefore evaporate the substance. Most biological molecules do not make such violence over themselves and disintegrate under the action of high temperatures accompanying the evaporation process. Therefore, more delicate methods of transformation into free ions were invented for them. One of them is an ionization of electrical exploitation. The solution of the substance under pressure enters the metal capillary, to which the high voltage (3-4 kV) is submitted. From the narrow nozzle of the capillary, drops are squeezed out, which, being highly charged, disintegrate, losing the solvent molecule, and the voltage is selected in such a way that the mass spectrometer includes biomolecules. The second method called "matrix-activated laser desorption / ionization" is even more cunning. The sample studied is applied to the matrix from a specially selected substance capable of effectively absorb laser radiation. With the rapid heating of this sandwich, the laser pulse of the sample molecule is ionized, not good to get enough on the part.

Thanks to new methods of ionization, the mass spectrometry of biomolecules using relatively simple and cheap quadrupole and time mass spectrometers began to be widely used in practice - in the development of new drugs, the determination of traces of psychotropic and narcotic substances, DNA studies, proteins and other substances. There are whole banks of data, with which it is possible to identify the organic matter according to its components detected in the mass spectrometer.

A combination of mass spectrometry and other physicochemical method intended for separation and analysis of mixtures - chromatography was very fruitful. First, using a chromatograph, the components of the mixture are isolated and then separately they are directed to the input of the mass spectrometer. Such devices are equipped with doping control laboratories. With the help of chromato mass spectrometers, the content of anabolic steroids, analgesics, diuretics, stimulants and corticosteroids is determined. No matter how hard the athlete tried, for whom a medal is more expensive than his own health, to hide the use of anabolics, it will not be able to do it - a modern mass spectrometer is able to find in the blood or urine even a billion share of these prohibited drugs. True, there is a kind of struggle: someone synthesizes new doping funds, and someone tries to discover them, and without such a tool, as a mass spectrometer, the latter, most likely, this race would be lost.

For all occasions

Nowadays, various use of mass spectrometry came far beyond the framework of unique projects, and to describe the numerous designs of mass analyzers and ionization methods, would not have the whole number of the magazine. Portable chromato mass spectrometers are in service american army In Iraq. They allow you to detect minor traces of chemical weapons reagents and are used for preliminary analysis of the surrounding environment. High-precision mass spectral analysis devices purchase customs services - this is a way to carefully monitor the composition of petroleum products and determine the origin of oil literally up to the well, since the isotope composition is unique for each field.

A modern mass spectrometer can occupy an experimental hall or placed in a small box on the table, contain a superconducting magnet or to do without a magnetic field. The sensitivity of these devices amazing imagination. There is enough a milligram of an organic pollutant on a ton of water so that the mass spectrometer may be doubteding in its quality, and inorganic impurities - and less. Paradoxically, the high sensitivity can itself become a source of problems: for example, when checking passengers, insignificant traces of drugs, accidentally falling on cash bills, can be found on the hands of a perfectly respectable citizen! However, this is the task of another variety, and, having such a wonderful tool at its disposal, as a mass spectrometer, a person will certainly be able to solve it.

Chromato mass spectrometry is an analytical method based on combination of chromatograph and mass spectrometer, used for quantitative and qualitative definition of individual components in complex mixtures. This article will consider the main issues relating to the essence of chromato mass spectrometry and its features:

The device by which the study is carried out, the name of the chromato mass spectrometer or HCMS is called. Passing through a chromatograph, the sample is divided into components, and the mass spectrometer is responsible for their identification and analysis. Depending on the characteristics of the studied composition and requirements for the accuracy of the result, one of two techniques are used: or high-precision liquid chromatography, or gas chromatography with mass spectrometric detection of GC-MS.

The studied composition is introduced into the evaporator of chromatograph and is instantly translated into a gaseous form, mixed with an inert carrier gas and under pressure is supplied to the column. Passing through a chromatographic column, the sample is divided into components that are supplied to MS and are passed through the spectrometric component of the device.

To obtain the spectrum, the sample component molecules are ionized, the special sensor reads the change in the ion current, on the basis of which the chromatogram is written. Chromatogram processing software allows you to verify the obtained peaks with registered previously, and thereby, by carrying out their exact qualitative and quantitative definition. At the same time, a multiple of the mass spectrum, which gives an idea of \u200b\u200bthe structure of components, including those not identified earlier.

The chromato mass spectrometry was developed in the 50s of the last century, and the first device was assembled and tested in the 60s.

The effectiveness and effectiveness of chromato mass spectrometry is defined by the sensitivity of the CHS, which are constantly being improved, which allows to expand the use of the GC-MS system.

High accuracy shows selective detection. Its essence comes down to records of testimony not throughout the volume of the incoming ion current, but at the maximum for the alleged molecules of ions. This reduces the mixture method and allows you to detect the minimum content of a given substance in any compositions. Therefore, chromato mass spectrometry is actively used in medicine and pharmacology to search for specific markers: for example, hormones or drugs in biological fluids.

High sensitivity has a chromato mass spectrometer with ISS MSD. The features of the detector used in it are to:

• use of special materials providing high ion output in any operating modes;
• automatic signal processing system by means of software capabilities;
• automatic setting system MS;
• automatic diagnostic system MS;
• combining high-quality electrodes with a digital detection system that allows you to increase the scanning speed;
• special noise reduction system from residual helium.

High sensitivity and wide sphere of applying a chromatomass spectrometer, quite justifies its price.

The quality of the result also affects the speed of the mass spectrum, which should be significantly higher than the construction of a chromatographic peak. If the speed decreases, peak impositions and distortion of the analysis result appear.

This parameter depends on the installed mass analyzer. Optimal currently is a quadrupole system operating by next principle. The stream passes through four magnets creating a high-frequency field. Finding into it, particles with a certain ratio of mass and charge fall into the trap, all the others are "sifted."

MS at an equal period of time scans the spectra of analyzed substances. Then each statistical snapshot is processed, and the total value gives an idea of \u200b\u200bthe set of spectra at each time. Most of the modern MS (for example, on units with ISS MSA, which was described above), this type of analyzers were installed.

Equipment for mass chromatography is characterized by its parameters and capabilities. To find a technique that meets the needs of a modern user, you must consider the following parameters:

• used source of ionization (electronic blow, chemical ionization);
• the sensitivity of the most common MS allows you to reach 10-9 ... 10-12 g at different scanning modes;
• the ability to scan: It is desirable that the chromato mass spectrometer supports selective search by specified particle groups (SIM mode), and also performed the full scan in the specified range (Full Scan mode).

Large importance for chromato-mass spectrometry acquires software that is supplied in the kit. It determines the possibility of constructing a chromatogram in real time, control over the stability of the specified parameters, automatic receiving reporting in a convenient form. It depends on how far the chromato mass spectrometer is convenient. Additionally, developers offer a set of libraries that contain spectra for various industrial and scientific spheres: medicine and pharmacology (hormones, drugs, drugs), oil producing industry (hydrocarbons), ecology (pesticides and other organic pollutants), etc.

Selecting the chromato mass spectrometer, it is necessary to take into account all specifications. Then the acquired device will fully respond to the needs of the user.

Ministry of Health of the Russian Federation

General Pharmacopoeia article

Mass spectrometryOFS.1.2.1.1.0008.15

Entered for the first time

Mass spectrometry method - a method of high-quality and quantitative analysis of drugs based on direct measurement of mass ratios to the number of elementary positive or negative charges of ions ( m./ z.) In the gas phase derived from the test substance. The charge may be due to the addition or loss of an electron, proton, cation or anion depending on the conditions of ionization and sample composition. This ratio is expressed in atomic units of mass (A.E.m.) or in Dalton (yes). Ions formed in the ion source of the device are accelerated and before entering detector separated using the mass analyzer. These processes occur in the chamber in which the pump system supports vacuum from 10 -3 to 10 -6 pa. The signal corresponding to the ion is represented by several peaks corresponding to the statistical distribution of various isotopes of this ion. This signal is called isotopic profile (for small molecules), and a separate peak representing the most common isotope for an atom, - monoisotope peak. The resulting mass spectrum is a graph of the dependence of the number of different ions from the relationship m / Z.. When analyzing complex molecules, there is a need for two and more consecutive mass analyzers to decipher the molecular structure. In the MS / MS device (MS N) ( tandem mass spectrometer) Mass analyzers are built consistently after each other. From the ions divided in the first mass analyzer, the particles are unidentified by their structure ( parental ions) and split them into smaller fragments by a collision with inert gas atoms ( dissociation activated by collision - CID) or laser radiation. This process is implemented before the second mass analyzer, with which the decay products analyze ( daughter ions).

Mass spectrometric analysis gives an important qualitative and quantitative (using external or internal standards) information (determination of molecular masses, fragments structure of defined molecules) with a detection limit from pekomole [PMOL (10 -12)] to fempomol [FMOL (10 -15) ].

The methods of the method are characterized by the method of entering the sample into the device, the mechanism of the formation of ions (type ion source) and the method of separating ions in terms of mass to charge (type mass analyzer).

Technical characteristics of mass spectrometers

The most important technical characteristics of mass spectrometers are the speed of scanning, sensitivity, dynamic range, resolution.

Scan speed

The mass analyzer skips ions with a certain mass and charge ratio ( m./ z.) At a certain time (except for multicollector devices, ion-cyclotron resonance, orbital ion traps). In order to analyze all ions in relation m./ z.The mass analyzer must scan all the values \u200b\u200bneeded to pass to the detector of all ions of interest. The field deployment rate is called the scanning speed, which must be maximal (respectively, the scan time must be as small as possible), since the mass spectrometer must register a signal during the yield of chromatographic peak, which can be several seconds. At the same time, the larger the mass spectra will be measured during the yield of chromatographic peak, the more precisely the chromatographic peak will be described, and the less the likelihood will skip its maximum value.

The slowest mass analyzer is a magnet, the minimum scanning time of which, without a special loss of sensitivity, is a fraction of a second. The quadrupole mass analyzer can turn the spectrum over the tenths of the second, the ion trap and the linear ion trap - faster, and the mass spectrometer of the ion-cyclotron resonance is slower.

Any scanning in all listed types of mass analyzers is compromise - with an increase in the scanning speed, the sensitivity decreases, because Less time is spent on recording a signal for each mass number. For typical methods for analyzing the speed of scanning a quadrupole analyzer or ion trap, it is enough to obtain satisfactory results. At the same time, for high-performance analysis of complex molecular systems, it is advisable to use a time-over mass spectrometer, which is capable of recording mass spectra at a speed of 40,000 spectra per second.

Resolution

The resolution or resolution of the mass spectrometer is defined as the possibility of mass analyzer to divide ions with close masses. It is very important to determine the masses of ions as accurately as possible, this allows you to calculate the atomic composition of the ion or identify the molecule by comparing with the database, reducing the number of possible candidates from thousands and hundreds to units or one. For magnetic mass analyzers, in which the distance between the mass spectrum peaks does not depend on the mass ions, the resolution is an amount equal to M / ΔM. This value is usually determined by 10% peak height. Thus, resolution 1000 means that peaks with the masses of 100.0 AE.m. and 100.1 A.Y.m. They are separated from each other, that is, they are not superimposed up to 10% of height.

For analyzers in which the distance between the peaks changes in the operating range of the masses (than more MassaThe smaller the distance), such as quadrupole analyzers, ion traps, time-of-flight analyzers, resolution (M / ΔM) has a different meaning: it characterizes a particular mass. Therefore, these mass analyzers are characterized by the width of peaks - the value remaining constant in the entire mass range. The width of the peaks is measured at 50% of their height. For such devices, the peak width on a half-life equal to 1 is a good indicator and means that such a mass analyzer is able to distinguish between the nominal masses, differing in the atomic unit of mass in almost all of its operating range.

A nominal mass or mass number is called an integer in the scale of atomic versions of the mass. For example, the mass of hydrogen ion H + is equal to 1.00787 AE.M., and its mass number is 1. Mass analyzers that measure the nominal masses are called low-resolution analyzers. Mass spectrometers with double focusing (magnetic and electrostatic), ion-cyclotron resonance refer to the instruments of the average or high resolution. A typical permit for a magnetic spectrometer is a value exceeding 60000, and the operation at the resolution level is 10,000 - 20000 is a routine. On the mass spectrometer of the ion-cyclotron resonance when analyzing a sample with a mass of about 500 A.M. It can be easily achieved permission of 500,000, which allows measurements of mass ions with an accuracy of the fourth - the fifth sign after the comma. Resolutions of several thousand can be achieved when using time-taking mass analyzers; However, exploring samples with a large molecular weight for which this type of instrument has an advantage over other analyzers, this permission is enough only to measure the mass of ion with an accuracy of ± tens A.E.M.

The mass analyzer resolution is closely connected with another important characteristic - the accuracy of measurement of the ion mass. For example, the masses of molecular nitrogen ions (N 2 +) and carbon monoxide (CO +) are 28,00615 and 27,99491 A.M. Accordingly, both ions are characterized by a mass number 28. These ions will be recorded by the mass spectrometer of the browse at a resolution of 2500, and the measured accurate weight value will show which of these gases is recorded. Measurement of the exact mass is available on double focusing devices, at a time of time mass spectrometers (in the low molecular weight range) and on mass spectrometers of ion-cyclotron resonance.

Dynamic range

Dynamic range - the ratio of maximum and minimum detectable signals. When analyzing a mixture containing 99.99% of one compound or any element and 0.01% of any impurity, the linearity range must be fourth order. Mass spectrometers for analyzing organic compounds are characterized by a dynamic range of 5-6 orders, and mass spectrometers for elemental analysis - 9 - 12 orders.

Sensitivity

Sensitivity is one of the most important characteristics of analytical instruments. Typically, the parameter associated with sensitivity is the minimum determined amount of the substance or the detection threshold. The typical magnitude of the detection threshold of a good chromatomass spectrometer used to analyze organic compounds is 1 ∙ 10 -12 g with a form of 1 microlometer solution.

Detection limits inorganic substances The ICP / MS method (AC / MS - mass spectrometry with inductive-bound plasma) is 1 ∙ 10 -15 (one share on quadrillion).

Scope of the method

Destiny authentication

Fragmented mass spectrum is a "fingerprint" chemical structure. Therefore, the identity of the mass spectra unequivocally indicates the identity of molecules, especially in combination with the use of libraries of mass spectra and chromatographic data. The high resolution mass spectrum allows you to determine the atomic composition of the molecule (gross formula) by exact mass.

Quantitative determination of pharmaceutical substances and impurities in dosage forms

Quantitative analysis is carried out using standard samples in combination with traditional chromatographic techniques, and the exact reproduction of chromatography conditions does not need, since the peak on the chromatogram is identified by the mass spectrum, and the integration of selected iones or peaks of elected reactions of the formation of a particular ion is usually Allows you to quantify the component in case of incomplete separation of peaks on the chromatogram.

Identification of impurities and establishing an unknown structure

The mass spectrum allows you to determine the molecular weight of the compound on the molecular ion, and in many cases it is possible to find out from which fragments the molecule is that, in combination with the use of spectra libraries and NMR spectroscopy data makes it possible to unambiguously establish a chemical structure.

Quantification of trace amounts of substances in pharmacokinetics and metabolic

Selectivity in SIM modes (monitoring of elected ions) and SRM (monitoring of selected reactions) Along with very high sensitivity allows the use of a combination of HPLC and mass spectrometry to determine the analyzed substances against the background of such complex multicomponent mixtures such as biological fluids or vegetable extracts.

Quantitative determination of more than 70 elements with a measurement limits from 10 to 0.1 PRT ( parts. per. trillion. ) Mass spectrometry with inductively coupled plasma.

Equipment

The mass spectrometer consists of the following blocks that have several varieties: sample input systems, ion source, mass analyzer, detector, and data processing systems.

Sample input system

The first stage of analysis is to enter a sample of the test substance into the device without a significant violation of the vacuum.

The input system is most applicable to analyze the components of the mixture separated by the appropriate instrument connected to the mass spectrometer.

Gas chromatography / mass spectrometry (GC / MS) ( GC. / MS. ).

When using suitable capillary columns, it is possible to directly administer the end of the column to the ion source of the device without the use of the separator.

It is used to analyze chemical compounds having a boiling point of about 400 ºС.

Liquid chromatography / mass spectrometry (LC / MS).

Such a combination of devices is particularly effective when analyzing non-volatile polar compounds or thermolabile substances. Due to the difficulty of obtaining ions in the gas phase, with this method requires the use of special interfaces: electrospray (ESI), thermosprint (TSI), chemical ionization at atmospheric pressure (APCI), photoInization at atmospheric pressure (APPI), etc., which are Independent methods of ionization and will be discussed below.

Supercritical Fluid Chromatography / Mass Spectrometry

This method of input sample lies in the fact that the mobile phase, usually consisting of a dioxide carbon in the supercritical state, passes into a gaseous state after passing through the heated valve between the column and the ion source.

Capillary electrophoresis / mass spectrometry ( CE / MS. )

The eluent is introduced into the ion source, in some cases, after adding an additional solvent, while the flow rate can reach several milliliters per minute. The limitations of this method are small amounts of the input sample and the need to use volatile buffer solutions.

Sample direct input devices

The sample is entered into the device through a vacuum gateway using a valve, rods, conveyor or autosampler, evaporates thermally or during desorption from the surface directly in the ion source. With this method of input, it is necessary to use pure samples or meant that the mass spectrum obtained may be a spectrum of a mixture of several connections.

Ion source

Electronic ionization ( EI )

A sample of the test substance in a gaseous state is ionized by the electron flow, the energy of which (usually 70 eV) is greater than the sample ionization energy. In addition, besides the molecular ion of M +, fragmentation ions of smaller mass are formed, characteristic of this molecular structure. The main limitation of this method is the need to evaporate the sample, which makes it impossible to study polar, thermolabile or high molecular weight connections. Electronic ionization can be used in gas chromatography in combination with mass spectrometry and only in some cases - in liquid chromatography.

Chemical ionization ( CI )

In this case, the method of ionization uses gas reagent (methane, isobutane, ammonia, nitrogen monooxide, nitrogen dioxide or oxygen). The spectrum contains type ions (M + H) +, (Mn) -, as well as ionic complexes formed by an analyte with a gas-used reagent. Fragmentation in chemical ionization manifests itself to a lesser extent than when the ionization is an electron impact.

For thermolabile substances, a type of this method of ionization is used, in which the sample applied to the wire is evaporated very quickly due to the Joule - Thomson effect (desorption chemical ionization).

Bombing by rapid atoms ( Fab. ) or ionization by bombardment by fast ions (secondary-ion mass spectrometry - Sims).

Sample dissolved in a viscous matrix (glycerin or m.-Nitrobenzyl alcohol) are applied to the metal surface, ionized by the stream of neutral atoms (argon or xenon) or the cesium ions with a large kinetic energy. Ions are observed (M + H) + and (Mn) - types or ionic complexes formed by the medium (matrix) and sample. This type of ionization is well suited for polar, thermolabile compounds, allowing you to obtain the spectra of molecules with a mass of up to 10,000. It is important that the sample is evenly distributed in the matrix, otherwise the quality of the spectrum deteriorates greatly, and attempts to quantify mixtures lead to unpredictable results. Known Fab FUB, which can be used for liquid chromatography, but the flow rate of the mobile phase should be very low (less than 10 μl / min).

Field desorption and field ionization

Sample evaporate near tungsten wire emitter covered with mini (field ionization) or placed on this wire (Field desorption).

The electrical field (voltage of about 10 kV), formed by the emitter, ionizes the sample. The energy that is transferred under the data of the methods of ionization is only a fraction of eV, i.e. The excess energy of the molecular ion is significantly lower than with other ionization methods. In addition, other electrons of the ionizing molecule are not excited, and M + turns out to be mainly (unexcited) electronic conditionAnd the spectrum is often the only peak belonging to the molecular ion.

Matrix Laser Desorption Ionization (Maldi)

A sample mixed with the corresponding medium (matrix) and placed on a metal substrate is ionized by short laser pulses with a wavelength from UV to an IR range (the duration of the pulses can be from picosecond to several nanoseconds). UV absorbing organic compounds are commonly used as a matrix (2,5-dihydroxybenzoic, synapic acid, 2,6-dihydroxyacetophenone, etc.). This method of ionization is used mainly when analyzing compounds with a very large molecular weight (more than 100,000 yes).

Inductively connected plasma ( ICP. )

A sample dissolved in strong mineral acid (nitric acid, chloride hydrochloric acid, plastic acid, tsarist vodka, etc.), is supplied to the zone of combustion of argon plasma, where at a temperature of several thousand degrees there is a decay of the sample to atoms with ionization. The method is used to determine more than 70 items. Due to the presence of molecular interferences, the optimal use of high-resolution devices or combined mass analyzers with chamber of collisions. Isotopic interferences, as a rule, can be resolved by mathematical methods.

Electrosprony (electrospray) ( ESI. )

The sample in the solution is introduced into the source through the capillary, at the end of which there is a potential of about 5 square meters. At the exit of the capillary, an aerosol of charged droplets with a high surface charge is formed. The evaporation of the solvent molecules from the generated microcapel leads to the formation in the gas phase of one-charged (M + H) +, (Mn) - or multiply charged ions (M + NN) + N, (M-NN) - n. The flow rate of the mobile phase at this form of ionization can vary from several NL / min to 1 - 2 ml / min. This ionization method is used for polar compounds. The use of electrosprey is particularly effective to establish the structure of polypeptides, proteins and nucleic acids with molecular masses up to 10,000,000 and higher. Very good electrospray is combined with liquid chromatography and capillary electrophoresis.

Chemical ionization at atmospheric pressure ( APCI )

The sample ionization is carried out at atmospheric pressure in the zone of the corona discharge, placed on the path of the moving phase, which is sprayed both due to thermal effects and through the use of nitrogen flow. Discharged ions are formed (M + H) + or (Mn) -. The method has proven itself to analyze relatively small polar and non-polar molecules with a weighing less than 1200. The possibility of using high flow rates of the mobile phase (up to 2 ml / min) makes this method of ionization ideal for a combination with liquid chromatography.

PhotoInization at atmospheric pressure ( Appi. )

In the ion source, Appi use a krypton lamp, which emits photons with an energy of 10.0 and 10.6 eV. These photon energies are sufficient to ionize most of the analyzed compounds, while for the ionization of typical solvents (water, methanol, acetonitrile, etc.) for facing phase liquid chromatography with mass spectrometric detection, radiation is needed with greater energy. The use of low-energy photons as a source of ionization leads to a mass spectra free from "chemical noise", and also guarantees minimal fragmentation of ions, allowing you to identify protonated ions or radical cations.

In addition to the above varieties of ion sources, there are a number of less common ionization methods, such as thermospair, plasma desorption, laser ablation, etc.

Mass spectrometry Dart

Dart mass spectrometry (Direct Analysis in Real Time) - fast method obtaining the spectra of low molecular weight connections in ON-LINE mode directly during analysis, practically not requiring sample preparation. The method allows for the ultra-cut identification of components of any solid or liquid objects. The analysis procedure is reduced to the fact that the object is introduced by tweezers (in the case of solid samples) or a stick (in the case of liquid objects) to the ion source of DART, where there is evaporation of the substance and its ionization with the subsequent registration of ions by the mass spectrometer. At the same time, very simple spectra are formed, usually containing protonated molecular ions of low molecular weight components of the sample. The DART mass spectrometry method is applicable to track the completeness of the flow of the organic synthesis of new drug substances, direct analysis of mixtures components separated on the TLC plate, from its surface, detecting falsifications when analyzing pharmaceutical substances and drugs.

Mass analyzer

Double focus

The principle of operation of all mass analyzers is based on physical law The movements of the charged particles, according to which the trajectory of charged particles in the magnetic field is twisted, and the radius of curvature depends on the mass of the particles. It is in the registering device ions are distributed through the masses. An additional electrostatic analyzer is set to increase permissions on the ions paths. Magnetic mass spectrometers have a high resolution, which makes it possible to use them in the study of organic compounds with high resolution, when analyzing isotopic ratios, elemental analysis on marginal sensitivity.

Quadrupole analyzer

The instrument of the analyzer of the specified type is based on the principle of a quadrupole, which is 4 rods for which a certain combination of a constant and radio frequency variable electrical voltage is supplied in opposite polarity. The ions moving parallel to the axes of these rods fall into the hyperbolic field. The possibility of transmitting ions depends on the relationship m / Z. and voltage of the radio frequency field. Changing field voltage scan all values m / Z. In the operating range of the device (usually from 1 to 2000). Some devices scan up to 4000 AE.m.

Quadrupole mass spectrometers do not require the use of high voltages about thousands of volts, in contrast to magnetic mass spectrometers. This allows you to simplify the design, because to create a vacuum in the device requires smaller dimensions of the vacuum chamber.

Time Analyzer (Time o. f FLIGHT, TOF)

In such analyzers, the ions are distributed by mass in the useless space, and not at the expense of the patterns of motion of charged particles in the field (magnetic or electrostatic). The ions from the source accelerate the electric field, purchasing quite large kinetic energy, and fall into the useless space. At the entrance to this space, all ions have the same kinetic energy and, in accordance with the formula E. = mV 2/2, will move with different speeds. Depending on the mass of the ions, the detector has been reached at different times. Registration of ions and measurement of time when you get into the detector allows you to calculate their mass.

Based on the time of the mass analyzer, very rapid (and sensitive) mass spectrometers are designed.

The time-of-flight mass analyzer, in contrast to the quadrupole analyzer, allows you to register a wide range of masses and measure the masses of very large molecules, and the most suitable ionization method was described above the MALDI method (ionization of laser desorption with the assistance of the matrix).

Tree mass analyzers are used mainly due to their simplicity, speed and relatively low cost.

Quadrupole ion trap

The development of quadrupole analyzers led to the creation of a "ion trap".

In a quadrupole ion trap, the ions are fixed inside the quadrupole due to locking potentials at the input and output ends of the trap. Then, when applying a variable resonant radio frequency, ions are output from the trap according to the magnitude m / Z. and registered by an electronic multiplier. Such a mechanism can significantly increase the population of trapped ion trap, which leads to the expansion of the dynamic range and to improve sensitivity.

The ion trap allows you to hold the ions that are necessary to establish the structure, without focusing on the remaining fragments of the molecule, while the fragmentation process can be repeated repeated, up to 10 to 10 times (the generally accepted designation MS N).

Ion-cyclotron resonance

Ions exposed to a strong magnetic field move along circular trajectories with frequencies that can be directly related to values m / Z. For these ions by means of Fourier transform. Analyzers of this type have a very high resolution (up to 1,000,000 and higher), and also allow to obtain MS N spectra.

The disadvantage of mass analyzers based on the ion-cyclotron resonance is the need to use very low pressure (about 10 -7 PA) and the use of superconducting magnets operating at a temperature of liquid helium 4.2 K.

Orbital trap ions

In the orbital trap ions do not use magnetic fields (Mass spectrometer with double focusing or ion-cyclotron resonance) or radio frequency (quadrupole ion traps). The principle of operation of mass analyzers of this type is based on an electrostatic axial-harmonic orbital ion trap, which uses a symmetric static electric field between the external and internal electrodes of a special form.

By analogy with mass analyzers based on the ion-cyclotron resonance in the spectrometer with an orbital ion trap, the ion is detected by the induced value of the current on the external electrodes; Frequencies corresponding to different m / Z.Mixed using the Fourier transformation algorithm, and then convert to the mass spectrum.

The orbital trap is also characterized by a greater tank of ions. The large capacity of the spatial charge compared to the ion-cyclotron and quadrupole traps allows to achieve greater accuracy of the mass measurement (resolution of about 100,000 on the half-life of the peak), a wider dynamic range and range of ratios of quantities m / Z..

Signal detection and data processing

The ions separated by the analyzer are converted to electrical signals by detecting systems, in particular, an electronic multiplier, photomultiplier or a pharade cylinder. Control of various physical parameters required for the coordinated operation of all instrument systems, data processing, including calibration, spectra visualization, automatic quantitative calculations, data archiving, creating and using mass spectra libraries are carried out by a computer with the corresponding software.

Registration of spectra

There are three basic ways to register spectra: on complete ion current (TIC); monitoring of the elected ion (SIM) or multiple ions (MIM); Selective registration of elected decay reactions ion (SRM) or multiple ions (MRM).

Registration on the full ion current and dissociation, initiated by collision, makes it possible to obtain mass spectra, uniquely associated with the structure of a particular molecule.

Based on the spectra thus obtained, libraries (databases) were created to determine the structure of the molecule on the reference spectra.

Selective registration of ions allows you to determine the small concentrations of the analyte against the background of a complex matrix, and also leads to a huge win in sensitivity: the time that is spent on the recording of the full mass spectrum, when selective registration is used to record only one or more ions.

Registration of selected reactions is an even more selective method for determining the desired compound in a complex mixture.

This method is fundamentally different from the above spectroscopic methods. Structural mass spectrometry is based on the destruction of the organic molecule as a result of ionization in one way or another.

The resulting ions are sorted by their ratio of mass / charge (m / z), then the number of ions is recorded for each value of this ratio in the form of the spectrum. In fig. 5.1. A general scheme of a typical mass spectrometer is presented.

Fig. 5.1. Block diagram of a typical mass spectrometer

To maintain a sample to the mass spectrometer, a kind of chromatography is usually used, although in many devices there is an opportunity to directly enter the sample in the ionization chamber. All mass spectrometers have devices for the ionization of the sample and the separation of ions in the magnitude of M / z. After separation you need to detect ions and measure their number. The typical header of ions consists of collimating gaps that are sent to the collector at the moment only the ions of one species, where they are detected, and the detection signal is enhanced by an electronic multiplier. Modern mass spectrometers are equipped with specialized software: computers control accumulation, storage and visualization of data.

Currently, it was the usual practice of combining the mass spectrometer with gas (GC-MS) or liquid (LC-MS) chromatograph.

All mass spectrometers are divided into two classes: low (single) and high resolution devices (R). Low resolution spectrometers are devices on which entire masses can be divided to m / z 3000 (R \u003d 3000 / (3000-2990) \u003d 3000). On this device of compounds C 16 H 26 O 2 and C 15 H 24 NO 2 is indistinguishable, since the device will be fixed in the first and in the second case a mass 250.

High resolution devices (R \u003d 20000) will be able to distinguish between C 16 H 26 O 2 (250.1933) and C 15 H 24 NO 2 (250.1807), in this case R \u003d 250.1933 / (250.1933 - 250.1807) \u003d 19857.

Thus, on low-resolution devices, it is possible to establish a structural formula of substance, however, it is often necessary for this purpose. Additionally, it is necessary to attract data from other methods of analysis (IR-, NMR spectroscopy).

High resolution devices can measure the mass of ion with an accuracy sufficient to determine the atomic composition, i.e. Determine the molecular formula of the studied substance.

In the last decade, the rapid development and improvement of mass spectrometers took place. Without discussing their device, we note that they are divided into types depending on 1) of the ionization method, 2) of the ion separation method. In general, the method of ionization does not depend on the method of separation of ions and vice versa, although there are exceptions. Further information on these issues is set out in the literature [SINSB. Lebedev].

In this manual, the mass spectra obtained by ionization by electron impact will be considered.

5.2. Mass spectra with ionization by electronic blow

Electronic strike (EU, Electron Impact, EI) is the most common method of ionization in mass spectrometry. The advantage of this method is the ability to use search engines and databases (EU method was historically the first method of ionization, the main bases of experimental data were obtained on devices with EU).

The sample substance molecule in the gas phase is subjected to a bombardment of high-energy electrons (usually 70 eV) and ejects an electron, forming a radical cation-called molecular ion:

M + E → M + (molecular ion) + 2e

The smallest energy of bombarding (ionized) electrons, in which the formation of a given ion molecule is called energy (or, less well, the "potential") of the ionization of the substance (U E).

The energy of ionization is a measure of strength, with which molecule he keeps the least strongly associated electron.

As a rule, for organic molecules, the energy of ionization is 9-12 eV, therefore, the bombardment by electrons with an energy of 50 eV and above reports the excess internal energy of the emerging molecular ion. This energy is partially dissipated due to the breaking of covalent bonds.

As a result of such a gap, the molecular ion decay on particles of a smaller mass (fragments) occurs. Such a process is called fragmentation.

Fragmentation occurs selectively, is high-performance and featured for this connection.. Moreover, fragmentation processes are predictable, and it is they determining the wide possibilities of mass spectrometry for structural analysis. In essence, a structural analysis by mass spectrometry is to identify fragmentation ions and retrospective restoration of the structure of the original molecule, based on the directions of fragmentation of the molecular ion. For example, methanol forms a molecular ion according to the scheme:

ABOUT
bottom point is the remaining odd electron; When the charge is localized on a separate atom, the charge sign is indicated on this atom.

Many of these molecular ions are disintegrated during 10 -10 - 10 -3 C and give a number of fragmentation ions (primary fragmentation):

If some of the molecular ions have a sufficiently long lifetime, they reach the detector and are recorded in the form of a peak of a molecular ion. Since the charge of the original ion is equal to one, attitudem./ z. For this peak gives a molecular weight of the substance under study.

In this way, mass spectrum is the representation of the relative concentrations of positively charged fragments (including molecular ion), depending on their masses.

The special literature presents the tables of the most common fragment ions, where the structural formula of the ion and its value M / z [Prosch, Gordon, Silverstain] are indicated.

The height of the most intense in the peak spectrum is taken for 100%, and the intensities of other peaks, including the peak of the molecular ion, are expressed as a percentage of the maximum peak.

In certain cases, the intense molecular ion peak can be. In general: the intensity of the peak depends on the stability of the formed ion.

In mass spectra there is often a series of peaks of fragment ions, differing in homologous difference (CH 2), i.e. 14 A.E.M. Homological series of ions are characteristic of each class of organic substances, and therefore they carry important information about the structure of the studied substance.