Mass spectrometer
mass spectrometer

Mass spectrometer - a device for determining the masses of atoms (molecules) by the nature of the movement of their ions in electric and magnetic fields.
A neutral atom is not affected by electric and magnetic fields. However, if one or more electrons are taken away from it or one or more electrons are added to it, then it will turn into an ion, the nature of the movement of which in these fields will be determined by its mass and charge. Strictly speaking, in mass spectrometers, it is not mass that is determined, but the ratio of mass to charge. If the charge is known, then the mass of the ion is uniquely determined, and hence the mass of the neutral atom and its nucleus. Structurally, mass spectrometers can differ greatly from each other. They can use both static fields and time-varying magnetic and/or electric fields.

Consider one of the simplest options.
The mass spectrometer consists of the following main parts:
a) of the ion source, where neutral atoms turn into ions (for example, under the influence of heating or a microwave field) and are accelerated by an electric field, b) areas of constant electric and magnetic fields, and in) an ion receiver that determines the coordinates of the points where the ions that cross these fields fall.
From the ion source 1 accelerated ions through the slot 2 fall into the region 3 of constant and uniform electric E and magnetic B 1 fields. Direction electric field is set by the position of the capacitor plates and is shown by arrows. The magnetic field is directed perpendicular to the plane of the figure. In region 3, the electric E and magnetic B 1 fields deflect the ions into opposite sides and the magnitude of the electric field strength E and induction magnetic field B 1 are chosen so that the forces of their action on the ions (qE and qvB 1, respectively, where q is the charge and v is the ion velocity) compensate each other, i.e. was qЕ = qvB 1 . At the speed of the ion v = E/B 1 it moves without deviating in region 3 and passes through the second slot 4, falling into region 5 of a uniform and constant magnetic field with induction B 2 . In this field, the ion moves along the circle 6, the radius R of which is determined from the relation
Mv 2 /R = qvB 2, where M is the mass of the ion. Since v \u003d E / B 1, the mass of the ion is determined from the relation

M = qB 2 R/v = qB 1 B 2 R/E.

Thus, with a known ion charge q, its mass M is determined by the radius R circular orbit in region 5. For calculations, it is convenient to use the ratio in the system of units given in square brackets:

M[T] = 10 6 ZB 1 [T]B 2 [T]R[m]/E[V/m].

If a photographic plate is used as an ion detector 7, then this radius will show with high accuracy black dot in the place of the developed photographic plate where the ion beam hit. Modern mass spectrometers usually use electron multipliers or microchannel plates as detectors. The mass spectrometer makes it possible to determine the masses with a very high relative accuracy ΔM/M = 10 -8 - 10 -7 .
Analysis of a mixture of atoms of different masses by a mass spectrometer also makes it possible to determine their relative content in this mixture. In particular, the content of various isotopes of any chemical element can be established.

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

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

Rice. 5.1. Block diagram of a typical mass spectrometer

Some form of chromatography is usually used to guide the sample into the mass spectrometer, although many instruments have the ability to directly introduce the sample into the ionization chamber. All mass spectrometers have devices for sample ionization and separation of ions by m/z value. After separation, it is necessary to detect ions and measure their number. A typical ion collector consists of collimating slots that are guided into the collector at this moment only ions of one kind, where they are detected, and the detection signal is amplified by an electron multiplier. Modern mass spectrometers are equipped with specialized software: computers control the accumulation, storage and visualization of data.

It has now become common practice to combine a mass spectrometer with a gas (GC-MS) or liquid (LC-MS) chromatograph.

All mass spectrometers are divided into two classes: devices of low (single) and high resolution(R). Low resolution spectrometers are devices that can separate whole masses up to m/z 3000 (R = 3000/(3000-2990) = 3000). On such a device, the compounds C 16 H 26 O 2 and C 15 H 24 NO 2 are indistinguishable, since the device will fix the mass 250 in both the first and second cases.

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

Thus, it is possible to establish the structural formula of a substance on low-resolution instruments, but often for this purpose it is additionally necessary to involve data from other methods of analysis (IR, NMR spectroscopy).

High-resolution instruments can measure the mass of an ion with an accuracy sufficient to determine the atomic composition, i.e. determine the molecular formula of the test substance.

In the last decade, there has been a rapid development and improvement of mass spectrometers. Without discussing their structure, we note that they are divided into types depending on 1) the ionization method, 2) the method of ion separation. In general, the ionization method is independent of the ion separation method and vice versa, although there are exceptions. More complete information on these issues is presented in the literature [Sainsb. Lebedev].

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

5.2. Mass spectra with electron impact ionization

Electron impact (EI, electron impact, EI) is the most common ionization method in mass spectrometry. The advantage of this method is the possibility of using search engines and databases (the EI method was historically the first ionization method, the main experimental data bases were obtained on EI devices).

A sample substance molecule in the gas phase is bombarded with high-energy electrons (typically 70 eV) and ejects an electron, forming a radical cation called molecular ion:

M + e → M + (molecular ion) + 2e

The lowest energy of bombarding (ionizing) electrons, at which the formation of an ion from a given molecule, is called the energy (or, less successfully, "potential") of the ionization of a substance (U e).

The ionization energy is a measure of the strength with which a molecule retains the electron least strongly bound to it.

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

As a result of such a break, the molecular ion decays into particles of smaller mass (fragments). Such a process is called fragmentation.

Fragmentation occurs selectively, is highly reproducible, and is characteristic of a given compound.. Moreover, fragmentation processes are predictable, and it is they that determine the wide possibilities of mass spectrometry for structural analysis. In fact, structural analysis by mass spectrometry consists in the identification of fragment ions and the retrospective reconstruction of the structure of the original molecule, based on the directions of fragmentation of the molecular ion. So, for example, methanol forms a molecular ion according to the scheme:

O
bottom point - the remaining odd electron; when the charge is localized on a single atom, the sign of the charge is indicated on that atom.

Many of these molecular ions decay within 10 -10 - 10 -3 s and give rise to a number of fragment ions (primary fragmentation):

If some of the molecular ions have enough big time lifetime, they reach the detector and are recorded as a molecular ion peak. Since the charge of the initial ion is equal to unity, the ratiom/ zfor that peak gives the molecular weight of the analyte.

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

Special literature contains tables of the most common fragment ions, where the structural formula of the ion and its m/z value are indicated [Prech, Gordon, Silverstein].

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

In certain cases, the peak of the molecular ion may also be the most intense. In general: the intensity of the peak depends on the stability of the resulting ion.

Mass spectra often contain a series of fragment ion peaks that differ by a homologous difference (CH2), i.e. 14 amu Homologous series of ions are characteristic of each class of organic substances, and therefore they carry important information about the structure of the substance under study.

Capabilities of mass spectrometry

The mass spectrum can be used to determine the molecular weight of a substance. This is necessary to establish molecular formula substances (general formula). The mass of an atom, measured with high accuracy, differs from the mass number. So, for CO 2 and C 3 H 8 the mass number is 44, but their exact relative molecular masses are 43.989828 and 44.062600, respectively, i.e. the difference is 0.072772 amu. The mass spectrometer makes it possible to separate the CO 2 + and C 3 H 8 + ion beams when they are obtained simultaneously.

Determination of the atomic composition by exact value mass is carried out using tables of exact masses for various ratios of the number of atoms C, H, O and N as the most common elements. Accurate mass measurement does not replace elemental analysis. Both methods complement each other.

When studying the mass spectrum, in addition to determining the type of molecular ion (M + ) measure peaks and for isotopic ions, including lighter or heavier isotopes (with mass numbers M ± 1, M ± 2, M ± 3, etc.). The simultaneous presence of several isotopes in a molecule is unlikely, because the natural abundance of the heavier isotopes C, H, O, and N is negligible. For example, 13 C: 12 C = 1×10 -2 ; 2 H: 1 H = 1.6×10 -4 ; 15 N: 14 N = 4×10 -3 etc. However, for chlorine 35 Cl: 37 Cl = 3:1; for bromine 79 Br: 81 Br = 1:1. Consequently, in the mass spectrum, along with the M ion + an ion will be present (M+1) + with an intensity proportional to the abundance of isotopes. In widely used reference tables, the ratios of the peak intensities of molecular ions with mass numbers M + 1 and M + 2 are usually given.

Maximum value m/z in the mass spectrum of a substance can have a molecular ion (M + ), the mass of which is equal to the molecular mass of the test compound. The intensity of the peak of a molecular ion (M +) is the higher, the more stable this ion is.

In practice, it is rarely possible to establish the complete structure of a compound only on the basis of the mass spectrum. The most efficient way to use multiple physical and chemical methods. Mass spectrometry, especially in combination with chromatography, is one of the most informative methods for studying the structure of a substance (chromato-mass spectrometry).

Thus, the possibilities of the method are: determination of the molecular weight and gross formulas of substances; establishing the structure of a substance by the nature of the resulting fragments; quantitative analysis of mixtures, including the determination of trace impurities; determination of the purity of a substance; determination of the isotopic composition of a substance.

Consider, as an example, the mass spectrum of ethanol (Fig. 2). Typically, the spectrum is presented in the form of histograms.

Rice. 2. Mass spectrum of ethanol

AT modern appliances the processing of the intensity of electrical impulses corresponding to peaks with different m/z values ​​is performed using a computer.

Mass spectra are given in the following notation: m/z values ​​are indicated, and relative intensity (%) in parentheses. For example, for ethanol:

C 2 H 5 OH mass spectrum (m/z): 15(9), 28(40), 31(100), 45(25), 46(14).

Interview Questions

1. Theoretical basis method.

2. Energy of ionization. Fragmentation types.

3. Schematic diagram of the mass spectrometer.

4. Ionization methods: electron impact, chemical ionization, etc.

5. Patterns of molecular ion fragmentation.

6. Possibilities of mass spectrometry.

Test tasks

1. Types of molecular ion fragmentation:

a). Dissociation - the disintegration of a molecular ion with the preservation of the sequence of bonds. As a result of the process, a cation and a radical are formed, and fragments with even values ​​of the m/z ratio are formed.

Rearrangement - a change in the sequence of bonds, a new radical cation of smaller mass and a neutral stable molecule are formed, the fragments are characterized by an odd value of the m / z ratio.

b) Rearrangement - the disintegration of a molecular ion while maintaining the sequence of bonds. As a result of the process, a cation and a radical are formed, and fragments with odd values ​​of the m/z ratio are formed.

Dissociation is a change in the sequence of bonds, a new radical cation of smaller mass and a neutral stable molecule are formed, the fragments are characterized by an even value of the m/z ratio.

c) Dissociation - the disintegration of a molecular ion with the preservation of the sequence of bonds. As a result of the process, a cation and a radical are formed, and fragments with odd values ​​of the m/z ratio are formed.

Rearrangement - a change in the sequence of bonds, a new radical cation of smaller mass and a neutral stable molecule are formed, the fragments are characterized by an even value of the m / z ratio.

2. Capabilities of the mass spectrometry method:

a) determination of the molecular weight and gross formulas of substances, quantitative analysis of mixtures;

b) establishing the structure of the substance by the nature of the formed fragments, determining the isotopic composition of the substance;

c) determination of the molecular weight and gross formulas of substances; establishing the structure of a substance by the nature of the resulting fragments; quantitative analysis of mixtures, including the determination of trace impurities; determination of the purity of a substance; determination of the isotopic composition of a substance.

3. Choose the correct answer:

a) Probability of rupture S-N connections decreases with increasing hydrocarbon chain; bond breaking energy S-S less; in aromatic derivatives, the rupture of the β-bond with the formation of a rearrangement tropylium ion is most likely;

a) The probability of breaking the C-H bond decreases with the increase in the hydrocarbon chain; bond breaking energy S-S more; in aromatic derivatives, the rupture of the β-bond with the formation of a rearrangement tropylium ion is most likely;

c) The probability of breaking the C-H bond decreases with the increase in the hydrocarbon chain; breaking energy C-C connections less; in aromatic derivatives, the breaking of the a-bond with the formation of a rearrangement tropylium ion is most likely;

1. Kazin V.N., Urvantseva G.A. Physical and chemical research methods in ecology and biology: tutorial(neck UMO) / V.N. Kazin, G.A. Urvantsev; Yaroslavl state un-t im. P.G. Demidov. - Yaroslavl, 2002. - 173 p.

2. Under. ed. A.A. Ishchenko. Analytical chemistry and physical and chemical methods of analysis / N.V. Alov and others - M .: Publishing Center "Academy", 2012. (in 2 volumes, 1 volume - 352 p., 2 volume - 416 p.) - (Ser. Baccalaureate)

3. Vasiliev V.P. Analytical chemistry. - book. 2. Physical and chemical methods of analysis. Moscow: Ministry of Education of the Russian Federation. 2007. 383 p.

4. Kharitonov Yu.Ya. Analytical chemistry, book. 1, book. 2, graduate School, 2008.

5. Otto M. Modern methods analytical chemistry (in 2 volumes). Moscow: Technosphere, 2008.

6. Ed. Yu.A. Zolotova. Fundamentals of Analytical Chemistry, Higher School, 2004.

7. Vasiliev V.P. Analytical chemistry. - book. 2. Physical and chemical methods of analysis. M.: Bustard, 2009.

8. Kazin V.N. Physical and chemical methods of analysis: laboratory workshop/ V.N. Kazin, T.N. Orlova, I.V. Tikhonov; Yaroslavl state un-t im. P.G. Demidova. - Yaroslavl: YarSU, 2011. - 72 p.

(mass spectroscopy, mass spectrography, mass spectral analysis, mass spectrometric analysis) - a method of studying a substance by determining the ratio of mass to charge (quality) and the number of charged particles formed during a particular process of exposure to a substance. The history of mass spectrometry begins with the fundamental experiments of John Thomson at the beginning of the 20th century. The ending "-metria" was given to the term after the ubiquitous transition from the detection of charged particles using photographic plates to electrical measurements ionic currents.

The essential difference between mass spectrometry and other analytical physicochemical methods is that optical, x-ray and some other methods detect the emission or absorption of energy by molecules or atoms, and mass spectrometry directly detects the particles of matter themselves (Fig. 6.12).

Rice. 6.12.

Mass spectrometry in broad sense is the science of obtaining and interpreting mass spectra, which, in turn, are obtained using mass spectrometers.

A mass spectrometer is a vacuum instrument that uses physical laws movement of charged particles in magnetic and electric fields, necessary to obtain a mass spectrum.

The mass spectrum, like any other spectrum, narrow sense is the dependence of the intensity of the ion current (quantity) on the ratio of mass to charge (quality). Due to mass and charge quantization, a typical mass spectrum is discrete. Usually (in routine analyses) this is true, but not always. The nature of the analyte, the characteristics of the ionization method, and secondary processes in the mass spectrometer can leave their mark on the mass spectrum. Thus, ions with the same mass-to-charge ratios can end up in different parts spectrum and even make part of it continuous. Therefore, the mass spectrum in a broad sense is something more that carries specific information and makes the process of its interpretation more complex and exciting. Ions are singly charged and multiply charged, both organic and inorganic. Majority small molecules during ionization acquires only one positive or negative charge. Atoms can acquire more than one positive charge and only one is negative. Squirrels, nucleic acids and other polymers are capable of acquiring multiple positive and negative charges. atoms chemical elements have a specific weight. In this way, precise definition the mass of the analyzed molecule allows you to determine its elemental composition. Mass spectrometry also makes it possible to obtain important information about the isotopic composition of the analyzed molecules. In organic substances, molecules are specific structures formed by atoms. Nature and man have created a truly incalculable variety organic compounds. Modern mass spectrometers are capable of fragmenting detected ions and determining the mass of the resulting fragments. In this way, data on the structure of a substance can be obtained.

The principle of operation of the mass spectrometer

Instruments that are used in mass spectrometry are called mass spectrometers or mass spectrometric detectors. These devices work with material substance, which consists of smallest particles- Molecules and atoms. Mass spectrometers determine what kind of molecules they are (i.e. what atoms make them up, what is their molecular mass, what is the structure of their arrangement) and what kind of atoms they are (i.e. their isotopic composition). The essential difference between mass spectrometry and other analytical physicochemical methods is that optical, x-ray and some other methods detect the emission or absorption of energy by molecules or atoms, while mass spectrometry deals with the particles of matter themselves. Mass spectrometry measures their masses, or rather, the ratio of mass to charge. For this, the laws of motion of charged particles of matter in a magnetic or electric field are used. A mass spectrum is a sorting of charged particles according to their masses (mass-to-charge ratios).

First, in order to obtain a mass spectrum, it is necessary to turn neutral molecules and atoms that make up any organic or inorganic substance into charged particles - ions. This process is called ionization and is carried out differently for organic and inorganic substances. In organic substances, molecules are specific structures formed by atoms.

Secondly, it is necessary to convert the ions into the gas phase in the vacuum part of the mass spectrometer. High vacuum ensures the unhindered movement of ions inside the mass spectrometer, and in its absence, the ions will scatter and recombine (turn back into uncharged particles).

Conventionally, the methods of ionization of organic substances can be classified according to the phases in which the substances are located before ionization.

Gas phase:

• electron ionization (EI, El - Electron ionization);
• chemical ionization (CI, Cl - Chemical Ionization);
• electronic capture (EZ, EU - Electron capture);
• ionization in an electric field (PI, FI - Field ionization).

Liquid phase:

• thermospray;
• ionization at atmospheric pressure(ADI, AR - Atmospheric Pressure Ionization);
• electrospray (ES, ESI - Electrospray ionization);
• chemical ionization at atmospheric pressure (APCI - Atmospheric pressure chemical ionization);
• – photoionization at atmospheric pressure (FIAD, APPI – Atmospheric pressure fotoionization).

Solid phase:

• direct laser desorption - mass spectrometry (PLDMS, LDMS - Direct Laser Desorption - Mass Spectrometry);
• matrix-assisted laser desorption (ionization) (MALDI, MALDI - Matrix Assisted Laser Desorbtion (Ionization));
• mass spectrometry of secondary ions (MSVI, SIMS - Secondary-Ion Mass Spectrometry);
• bombardment by fast atoms (FAB, FAB - Fast Atom Bombardment);
• desorption in an electric field (FD, FD - Field Desorption);
• plasma desorption (PD, PD - Plasma desorption).

In not organic chemistry for analysis of elemental composition

apply hard methods ionization, since the binding energy of atoms in a solid is much greater, which means that much more stringent methods must be used in order to break these bonds and obtain ions:

• ionization in inductively coupled plasma (ICP, IC - Pinductively coupled plasma);
• thermal ionization or surface ionization;
• glow discharge ionization and spark ionization;
• ionization during laser ablation.

Historically, the first ionization methods were developed for the gas phase. Unfortunately, very many organic substances cannot be evaporated; transfer to the gas phase without decomposition. This means that they cannot be ionized by electron impact. But among such substances, almost everything that makes up living tissue (proteins, DNA, etc.) is physiologically active substances, polymers, i.e. everything that is of particular interest today. Mass spectrometry did not stand still and in last years have been developed special methods ionization of such organic compounds. Today, two of them are mainly used - atmospheric pressure ionization and its subspecies - electrospray (ES), atmospheric pressure chemical ionization and atmospheric pressure photoionization, as well as matrix-assisted laser desorption ionization (MALDI).

The ions obtained during ionization are transferred to the mass analyzer with the help of an electric field. There begins the second stage of the mass-spring-stretch analysis - sorting of ions by mass (more precisely, by the ratio of mass to charge).

There are the following types of mass analyzers.

• 1. Continuous mass analyzers:
  • magnetic and electrostatic sector mass analyzer;
  • quadrupole mass analyzer.
• 2. Pulse mass analyzers:
  • time-of-flight mass analyzer;
  • ion trap;
  • quadrupole linear trap;
  • mass analyzer of ion-cyclotron resonance with Fourier transform;
  • orbittrap.

Difference between continuous and pulse mass analyzers lies in the fact that the first ions enter in a continuous stream, and the second - in portions, at certain time intervals.

The mass spectrometer can have two mass analyzers. Such a mass spectrometer is called tandem. Tandem mass spectrometers are used, as a rule, together with "soft" ionization methods, in which there is no fragmentation of ions of the analyzed molecules (molecular ions). Thus, the first mass analyzer analyzes molecular ions. Leaving the first mass analyzer, molecular ions are fragmented under the action of collisions with inert gas molecules or laser radiation, after which their fragments are analyzed in the second mass analyzer. The most common configurations of tandem mass spectrometers are quadrupole-quadrupole and quadrupole-time-of-flight.

The last element of the simplified mass spectrometer we are describing is the detector of charged particles. The first mass spectrometers used a photographic plate as a detector. Now dynode secondary electron multipliers are used, in which an ion, hitting the first dynode, knocks out a beam of electrons from it, which, in turn, hitting the next dynode, knock out more large quantity electrons, etc. Another option is photomultipliers that detect the glow that occurs when bombarded with phosphor ions.

In addition, microchannel multipliers, systems such as diode arrays, and collectors are used that collect all ions that have fallen into given point space (Faraday collectors).

Mass spectrometers are used to analyze organic and inorganic compounds. Organic substances in most cases are multicomponent mixtures individual components. For example, it is shown that the smell of fried chicken is 400 components (ie 400 individual organic compounds). The task of analytics is to determine how many components make up organic matter, find out which components they are (identify them) and how much of each compound is contained in the mixture. For this, the combination of chromatography with mass spectrometry is ideal. Gas chromatography is best suited to be combined with the ion source of a mass spectrometer with electron impact ionization or chemical ionization, since the compounds are already in the gas phase in the chromatograph column. Instruments in which a mass spectrometric detector is combined with a gas chromatograph are called chromato-mass spectrometers ("Chromass").

Many organic compounds cannot be separated into components using gas chromatography, but can be separated using liquid chromatography. To combine liquid chromatography with mass spectrometry, electrospray ionization sources and atmospheric pressure chemical ionization sources are used today, and the combination of liquid chromatography with mass spectrometers is called LC/MS. The most powerful systems for organic analysis, demanded by modern proteomics, are built on the basis of a superconducting magnet and operate on the principle of ion-cyclotron resonance.

The most widespread in recent times mass analyzer, which allows the most accurate measurement of the mass of the ion, and has a very high resolution. The high resolution makes it possible to work with polyprotonated ions formed during the ionization of proteins and peptides in an electrospray, and the high accuracy of mass determination makes it possible to obtain the gross formula of ions, making it possible to determine the structure of amino acid sequences in peptides and proteins, as well as to detect post-translational modifications of proteins. This made it possible to sequence proteins without their prior hydrolysis into peptides. This method is called "Top-down" proteomics. Obtaining unique information became possible due to the use of an ion-cyclotron resonance mass analyzer with a Fourier transform. In this analyzer, ions fly into a strong magnetic field and rotate there in cyclic orbits (as in a cyclotron, accelerator elementary particles). Such a mass analyzer has certain advantages: it has a very high resolution, the range of measured masses is very wide, and it can analyze ions obtained by all methods. However, for its operation, it requires a strong magnetic field, which means that the use strong magnet with a superconducting solenoid maintained at a very low temperature (liquid helium, approximately -270°C).

The most important technical specifications mass spectrometers are sensitivity, dynamic range, resolution, scanning speed.

The most important characteristic in the analysis of organic compounds is sensitivity. In order to reach as far as possible greater sensitivity when the signal-to-noise ratio improves, detection is resorted to for individual selected ions. The gain in sensitivity and selectivity in this case is colossal, but when using low-resolution devices, one has to sacrifice another important parameter- credibility. The use of high resolution on devices with dual focusing allows you to achieve high level reliability without sacrificing sensitivity.

To achieve high sensitivity, tandem mass spectrometry can also be used, when each peak corresponding to a single ion can be confirmed by the mass spectrum of the daughter ions. The absolute champion in sensitivity is a high-resolution organic chromatography-mass spectrometer with double focusing.

According to the characteristics of the combination of sensitivity with the reliability of the determination of components, ion traps follow high-resolution devices. Classic next-generation quadrupole instruments are enhanced by a number of innovations, such as the use of a curved quadrupole prefilter to reduce noise, preventing neutral particles from reaching the detector.

Applications of mass spectrometry

• · Nuclear energy;
• · Archeology;
• · Petrochemistry;
• · Geochemistry (isotope geochronology);
• · Agrochemistry;
• · Chemical industry;
• · Analysis of semiconductor materials, ultra-pure metals, thin films and powders (for example, oxides of U and REE);
• · Pharmaceuticals - to control the quality of manufactured drugs and detect counterfeits;
• · Medical diagnostics;
• · Biochemistry - identification of proteins, study of drug metabolism.

Chromato-mass spectrometry

Chromato-mass spectrometry is a method for analyzing mixtures of mainly organic substances and determining trace amounts of substances in a liquid volume. The method is based on a combination of two independent methods - chromatography and mass spectrometry. With the help of the first, the mixture is separated into components, with the help of the second - identification and determination of the structure of the substance, quantitative analysis. There are 2 variants of chromato-mass spectrometry, which are a combination of mass spectrometry with either gas-liquid chromatography (GLC) or high-performance liquid chromatography.

Rice. ten.

First studies analytical capabilities chromato-mass spectrometry were carried out in the 1950s, the first industrial instruments that combined a gas-liquid chromatograph and

mass spectrometer, appeared in the 60s. The fundamental compatibility of these two instruments is due to the fact that in both cases the analyzed substance is in the gas phase, the operating temperature intervals are the same, and the limits of detection (sensitivity) are close. The difference is that a high vacuum (10 -5 - 10 -6 Pa) is maintained in the ion source of the mass spectrometer, while the pressure in the chromatographic column is 10 5 Pa. To reduce the pressure, a separator is used, which is connected at one end to the outlet of the chromatographic column, and at the other end to the ion source of the mass spectrometer. The separator removes the main part of the carrier gas from the gas stream leaving the column, and the organic matter passes into the mass spectrometer. In this case, the pressure at the outlet of the column is reduced to the operating pressure in the mass spectrometer.

The principle of operation of separators is based either on the difference in the mobility of the molecules of the carrier gas and the analyte, or on their different permeability through a semipermeable membrane. In industry, injector separators are most often used, which work according to the first principle. Single-stage separators of this type contain two nozzles with small diameter holes, which are installed exactly opposite each other. A pressure of 1.33 Pa is created in the volume between the nozzles. The gas flow from the chromatographic column through the first nozzle at supersonic speed enters the vacuum region, where the molecules propagate at velocities inversely proportional to their mass. As a result, the lighter and faster molecules of the carrier gas are pumped out, and the slower molecules of organic matter enter the second nozzle hole and then into the ion source of the mass spectrometer. Some instruments are equipped with a two-stage separator equipped with another similar nozzle block. A high vacuum is created in the volume between them. The lighter the carrier gas molecules, the more efficiently they are removed from the gas stream and the higher the enrichment organic matter.

The most convenient carrier gas for chromato-mass spectrometry is helium. Separator efficiency, i.e. the ratio of the amount of organic matter in the gas stream leaving the column to its amount entering the mass spectrometer depends to a large extent on the flow rate of the carrier gas entering the separator. At an optimal flow rate of 20-30 ml/min, up to 93% of the carrier gas is removed, and more than 60% of the analyte enters the mass spectrometer. This carrier gas flow rate is typical for packed columns. In the case of using a capillary chromatographic column, the carrier gas flow rate does not exceed 2–3 ml/min; therefore, an additional amount of carrier gas is added to the gas stream at its outlet so that the flow rate entering the separator reaches 20–30 ml/min. This ensures the best efficiency of the separator. Flexible quartz capillary columns can be injected directly into the ion source. In this case, the ion source must be provided with a powerful pumping system that maintains a high vacuum.

Mass spectrometers connected to gas chromatographs use electron impact ionization, chemical or field ionization. Chromatographic columns must contain non-volatile and thermostable stationary liquid phases so that the mass spectrum of their vapors does not overlap with the spectrum of the analyte.

The analyte (usually in solution) is introduced into the evaporator of the chromatograph, where it instantly evaporates, and the vapors mixed with the carrier gas under pressure enter the column. Here, the mixture is separated, and each component in the carrier gas flow, as it elutes from the column, enters the separator. In the separator, the carrier gas is mainly removed and the gas stream enriched with organic matter enters the ion source of the mass spectrometer, where the molecules are ionized. The number of ions formed in this case is proportional to the amount of incoming substance. Using a sensor installed in the mass spectrometer, which responds to changes in the total ion current, chromatograms are recorded. Thus, the mass spectrometer can be considered as a universal detector for a chromatograph. Simultaneously with the recording of the chromatogram at any point, usually at the top of the chromatographic peak, a mass spectrum can be recorded, which makes it possible to establish the structure of the substance.

An important condition for the operation of the device is the rapid recording of the mass spectrum, which must be recorded in a time much shorter than the time of the chromatographic peak. Slow recording of the mass spectrum can distort the ratio of peak intensities in it. The registration rate of the mass spectrum (scanning speed) is determined by the mass analyzer. The shortest scanning time of the full mass spectrum (several milliseconds) is provided by a quadrupole analyzer. In modern mass spectrometers equipped with a computer, the construction of chromatograms and processing of mass spectra is performed automatically. Through equal intervals time as the components of the mixture are eluted, mass spectra are recorded, quantitative characteristics which are stored in computer memory. For each scan, the intensities of all registered ions are added. Since this total value (total ion current) is proportional to the concentration of the substance in the ion source, it is used to build a chromatogram (this value is plotted along the ordinate axis, along the abscissa axis - the retention time and scan number). By setting the scan number, you can recall the mass spectrum from memory at any point in the chromatogram.

As described above, mixtures of substances can be analyzed that are sufficiently well separated on suitable columns of gas chromatography-mass spectrometry. Sometimes unresolved chromatographic peaks can also be investigated. The substances under study should be thermally stable, chromatographically mobile within the range of the operating temperature of the column, and be easily transferred into the vapor phase at the temperature of the evaporator. If substances do not meet these requirements, they can be chemically modified, for example, by silylation, alkylation or acylation of hydroxy, carboxy, mercapto, amino groups.

The sensitivity of the gas chromatography-mass spectrometry (usually 10 -6 -10 -9 g) is determined by the sensitivity of the mass spectrometer detector. A more sensitive (10 -12 -10 -15 g) variety of chromato-mass spectrometry is mass fragmentography, also called selective ion or multi-ion detection. Its essence lies in the fact that the recording of chromatograms is carried out not according to the total ion current, but according to the most characteristic for given substance ions. This type of chromato-mass spectrometry is used to search, identify and quantitative analysis substances with a known mass spectrum in a complex mixture, for example, when quantification traces of substances in large volumes biological fluids(medicine, pharmacology, toxicology, doping control, biochemistry). Carry out mass fragmentography on chromato-mass spectrometers using a special device - a multi-ion detector or using a computer that can build chromatograms for one or more ions. Such a chromatogram, unlike the usual one, contains peaks of only those components whose mass spectra contain such ions. The analysis is carried out using an internal standard, which is often used as an analogue of the desired substance, labeled stable isotopes(2 H, 13 C, 15 N, 18 O).

Another option for chromato-mass spectrometry is the combination of high performance liquid chromatography and mass spectrometry. The method is intended for the analysis of mixtures of hardly volatile, polar substances that cannot be analyzed by the GJ chromato-mass spectrometry method. To maintain vacuum in the ion source of the mass spectrometer, it is necessary to remove the solvent coming from the chromatograph at a rate of 0.5–5 ml/min. To do this, part of the liquid flow is passed through a hole of several microns, as a result of which drops are formed, which then fall into the heated zone, where most of the solvent evaporates, and the remaining part, together with the substance, enters the ion source and is chemically ionized.

A number of industrial devices implement the principle of a belt conveyor. The eluate from the column enters a moving belt that passes through an IR-heated chamber where the solvent evaporates. Then the tape with the substance passes through the area heated by another heater, where the analyte evaporates, after which it enters the ion source and is ionized. More effective method The combination of a high-performance gas-liquid chromatograph and a mass spectrometer is based on electro- and thermal spraying. In this case, the eluate is passed through a capillary heated to 150°C and sprayed into a vacuum chamber. The buffer ions present in the solution participate in ion formation. The resulting droplets carry a positive or negative charge. Due to its small diameter, a high electric field gradient is created along the drop, and this gradient increases as the drop breaks up. In this case, desorption from droplets of protonated ions or clusters (substance molecule + buffer cation) occurs.

The method of chromato-mass spectrometry is used in structural and analytical studies in organic chemistry, petrochemistry, biochemistry, medicine, pharmacology, for the protection environment and etc.