Mass spectrometry. Coursework: Mass spectrometric method of analysis

Applications of mass spectrometry

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

Chromatography-mass spectrometry

Chromatography-mass spectrometry is a method for analyzing mixtures of mainly organic substances and determining trace amounts of substances in a volume of liquid. 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, and quantitative analysis are carried out. There are 2 known variants of chromatography-mass spectrometry, which are a combination of mass spectrometry with either gas-liquid chromatography (GLC) or high-performance liquid chromatography.

Rice. 10.

The first studies of the analytical capabilities of gas chromatography-mass spectrometry were carried out in the 1950s, the first industrial instruments combining a gas-liquid chromatograph and

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

The principle of operation of separators is based either on the difference in mobility of the molecules of the carrier gas and the analyte, or on their different permeability through a semi-permeable membrane. In industry, injection separators operating on the first principle are most often used. 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 molecules propagate at speeds inversely proportional to their mass. As a result, lighter and faster molecules of the carrier gas are pumped out by the pump, and slower molecules of organic matter enter the hole of the second nozzle and then into the ion source of the mass spectrometer. Some devices are equipped with a two-stage separator equipped with another similar nozzle unit. 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 in organic matter.

The most convenient carrier gas for gas chromatography-mass spectrometry is helium. The efficiency of the separator, i.e. the ratio of the amount of organic matter in the gas stream leaving the column to its amount entering the mass spectrometer largely depends 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 analyzed substance 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, at its outlet, an additional amount of carrier gas is added to the gas flow so that the flow rate entering the separator reaches 20-30 ml/min. This ensures best efficiency separator. Flexible quartz capillary columns can be introduced 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 coupled to gas chromatographs use electron impact, chemical, or field ionization. Chromatographic columns must contain low-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 chromatograph evaporator, where it instantly evaporates, and the vapor, mixed with a carrier gas, enters the column under pressure. Here the mixture is separated, and each component in a stream of carrier gas, as it elutes from the column, enters the separator. In the separator, the carrier gas is largely removed and the organic-enriched gas stream 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 that responds to changes in the total ion current, chromatograms are recorded. Thus, a mass spectrometer can be considered as a universal detector for a chromatograph. Simultaneously with recording 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 it takes for the chromatographic peak to appear. Slow recording of a mass spectrum can distort the ratio of peak intensities in it. The mass spectrum registration speed (scanning speed) is determined by the mass analyzer. The shortest scanning time for 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 the processing of mass spectra are carried out automatically. At regular intervals, as the components of the mixture elute, mass spectra are recorded, the quantitative characteristics of which are accumulated in the computer memory. For each scan, the intensities of all detected ions are added together. Since this total value (total ion current) is proportional to the concentration of the substance in the ion source, it is used to construct a chromatogram (this value is plotted along the ordinate axis, and the retention time and scan number are plotted along the abscissa axis). By specifying the scan number, you can recall the mass spectrum at any point in the chromatogram from memory.

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

The sensitivity of 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) type of chromatography-mass spectrometry is mass fragmentography, also called selective ion or multi-ion detection. Its essence is that chromatograms are recorded not by the total ion current, but by the ions most characteristic of a given substance. This type of chromatography-mass spectrometry is used to search, identify and quantitatively analyze a substance with a known mass spectrum as part of a complex mixture, for example, in the quantitative determination of traces of substances in large volumes of biological fluids (medicine, pharmacology, toxicology, doping control, biochemistry) . Mass fragmentography is carried out on chromatography-mass spectrometers using a special device - a multi-ion detector or using a computer that can build chromatograms for one or several ions. Such a chromatogram, unlike a conventional one, contains peaks only of those components whose mass spectra contain such ions. The analysis is carried out using an internal standard, which is often an analog of the desired substance, labeled with stable isotopes (2 H, 13 C, 15 N, 18 O).

Another option for gas chromatography-mass spectrometry is to combine high-performance liquid chromatography and mass spectrometry. The method is intended for the analysis of mixtures of highly volatile, polar substances that cannot be analyzed by gas chromatography-mass spectrometry. To maintain a 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 a heated zone, where most of the solvent evaporates, and the remaining part, along with the substance, enters the ion source and is chemically ionized.

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

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

The essence of mass spectrometry

Mass spectrometry is a method of measuring the ratio of the mass of charged particles to their charge (m/z).

To perform mass spectrometric analysis, the sample is converted into ionized form. After this, in one way or another, the ions are separated according to the ratio of their mass to charges and the registration of these ions, which can be either positive or negative.

Mass spectrometric analysis provides important information for determining the molecular weight, molecular formula or elemental composition and structure of molecules.

Mass spectrometry is used to determine relative molecular weight M g compound, which is expressed in atomic mass units (amu) or daltons, Da, (1 Da = 1 amu = 1.660541 - 10 -27 kg, which is equal to 1/12 of the mass of the carbon isotope with mass number 12). The mass of the main carbon isotope 12 C is expressed as an integer and is equal to 12.000000 Da. The masses of all isotopes of any other elements will be expressed in non-integer numbers.

In the mass spectrum, peaks or lines with a certain ratio m/z, correspond to molecular fragments and are also designated by an integer obtained by rounding the exact value m/z.

There are three different concepts of mass in mass spectrometry. Average molecular weight calculated based on the elemental composition and average atomic masses. Average molecular weight is important when studying large molecules. Nominal molecular weight is calculated taking into account the elemental composition and nominal atomic masses of the most common isotopes in nature. Exact molecular weight calculated from the exact masses of the most common isotopes.

With the help of mass spectrometry, the following are possible: analysis of organic compounds, inorganic analysis, studies to elucidate reaction mechanisms in organic chemistry and surface analysis.

Using mass spectrometry as an analytical method, a huge number of qualitative and quantitative problems are solved. Qualitative research involves determining the structure of an unknown compound, in particular natural substances, metabolites of drugs and other xenobiotics, and synthetic compounds. For quantitative analysis, mass spectrometry is used in the development of arbitration and comparison methods. Mass spectrometry today is developing very quickly, covering ever wider areas of application. Combining mass spectrometry with chromatography has significantly increased the capabilities of the method and expanded the range of objects studied.

5.12. Electrogravimetry

In electrogravimetric analysis, the analyte is quantitatively isolated from a solution by electrolysis, and the content of the analyte in the sample is calculated from the mass of the released metal or its oxide on the electrode.

Electrolysis is the chemical decomposition of a substance under the influence of electric current. Reduction occurs at the cathode:

Cu 2+ + 2e → Cu 0

and at the anode – oxidation:

2Cl - - 2e → Cl 2 (g) and 2OH - - 2e → 1\2O 2 + H 2 O

Under the influence of applied voltage, charged particles (ions) move towards the electrodes. However, their discharge, i.e. electrolysis, begins when a certain voltage value is reached, called the decomposition voltage

where E a, E k – EMF of the galvanic cell;

iR – ohmic voltage drop;

η – overvoltage of the anode and cathode during the release of electrolysis products.

The installation diagram for electrolysis is shown in Fig. 5.14.

Electrolysis is most often carried out at constant current. For getting direct current usually an AC rectifier or battery 1 is used. Sliding contact 2 allows you to regulate the applied voltage, which is measured by a voltmeter V. The current is controlled by an ammeter A. When separating metals, cathode 5 is usually used in the form of a platinum grid, anode 4 in the form of a platinum spiral or plate . When oxides are released, the signs of the electrodes change: the platinum grid becomes the anode, and the spiral becomes the cathode. The solution is mixed with a mechanical or magnetic stirrer 3.

Rice. 5.14. Installation diagram for electrolysis: 1 – direct current source; 2 – variable resistance (rheostat); 3 – magnetic stirrer;

4 – anode; 5 – cathode

In electrogravimetric methods of analysis, in addition to potential and current strength, it is important to control a number of experimental conditions.

5.13. Coulometry

Coulometric methods determine the amount of electricity that is consumed during an electrochemical reaction. A distinction is made between direct coulometry and coulometric titration.

In direct coulometry methods, the analyte is directly subjected to electrochemical transformation in a coulometric cell (the process is carried out at a constant controlled potential) (Fig. 5.15.).

Rice. 5.15. Installation diagram for direct coulometry at constant E:

1 - electrolyzer; 2 - DC source with adjustable voltage: 3 - device for determining the amount of electricity: 4 - working electrode; 5 - auxiliary electrode; 6 - reference electrode, against which the potential of the working electrode is controlled: 7 - device that measures the potential difference.

In the coulometric titration method, the analyte is reacted with a titrant, which is produced in a coulometric cell through electrolysis of a specially selected solution.

Coulometric titration is carried out at constant current.

Coulometric methods are based on Faraday's laws. A necessary condition for quantitative determination is 100% current efficiency. The current output is determined by the ratio of the amount of substance released during the electrolysis process to the theoretical amount calculated based on Faraday's law. Not 100% current efficiency may be due to current consumption for side processes:

1) decomposition of water into hydrogen and oxygen;

2) reduction or oxidation of impurities, for example, oxygen dissolved in water;

3) reaction involving electrolysis products;

4) reaction involving the electrode material (oxidation of mercury, etc.).

When carrying out coulometric determinations, it is necessary to provide all conditions that ensure 100% current efficiency, pH control, selection of electrodes, separation of the cathode and anode spaces.

5.14. Conductometry

The conductometric method of analysis is based on measuring the specific electrical conductivity of the analyzed solution.



Electrical conductivity called the reciprocal of electrical resistance R. The unit of electrical conductivity is Siemens (Cm) or Ohm -1. Electrolyte solutions, being conductors of the second kind, obey Ohm's law. By analogy with the resistance of type I conductors, the resistance of the solution is directly proportional to the distance between the electrodes d and inversely proportional to their surface area A:

Where R - resistivity, Ohm cm.

At d =1 cm And A =1 cm 2 we have R = p, therefore, the resistivity is equal to the resistance of 1 cm 3 of solution.

The reciprocal of resistivity is called electrical conductivity:

Specific electrical conductivity (S cm ∙ cm -1) is numerically equal to the current (A) passing through a layer of solution with a cross-section equal to unity under the influence of a potential gradient of 1 V per unit length.

The electrical conductivity of dilute electrolyte solutions depends on the number of ions in the solution (i.e., on the concentration), the number of elementary charges carried by each ion (i.e., on the charge of the ion), and on the speed of movement of equally charged ions to the cathode or anode under the influence of electric current. fields (Fig. 5.16.). Taking into account all these factors, the electrical conductive properties of ions characterize equivalent ionic conductivity (mobility).

Rice. 5.16. Conductivity meter OK 102/1: 1 – device body; 2 – measuring

scale; 3 – toggle switch “Network”; 4 - switch of measurement limits “Range”; 5 – knob for calibrating the potentiometer “Calibration”; 6 – calibration button “Calibration”.

Distinguish direct and indirect conductometry, or conductometric titration.

Direct conductometry little used in analytical chemistry. The reason for this is that electrical conductivity is an additive quantity and is determined by the presence of all ions in the solution. Direct conductometric measurements are used to control the quality of water used in a chemical laboratory, and modern installations for the distillation or demineralization of water are equipped with conductometric sensors - conductometers for measuring the specific electrical conductivity of solutions. Electrical conductivity detectors are used in ion chromatography.

The advantages of the conductometric titration method include the possibility of highly accurate measurements even in very dilute solutions.

For conductometric titration acid-base or precipitation reactions are suitable, accompanied by a noticeable change in electrical conductivity due to the formation of poorly dissociating or poorly soluble compounds.

5.15. Titrimetry

Titrimetric analysis (titration) is a method of quantitative/mass analysis, which is often used in analytical chemistry, based on measuring the volume of a reagent solution of precisely known concentration consumed for the reaction with the substance being determined (Fig. 5.17.).

Rice. 5.17. Benchtop electrochemical device

OHAUS Starter 2100

Titration is the process of determining the titer of the test substance. Titration is carried out using a burette filled with titrant to the zero mark. It is not recommended to titrate starting from other marks, since the burette scale may be uneven. The burettes are filled with the working solution through a funnel or using special devices, if the burette is semi-automatic. The end point of titration (not to be confused with the equivalence point) is determined by indicators or physicochemical methods (electrical conductivity, light transmission, potential of the indicator electrode, etc.). The analysis results are calculated based on the amount of working solution used for titration.

Titration methods

The titration process is accompanied by a change in the equilibrium concentrations of the reagent, analyte and reaction products. It is convenient to depict this graphically in the form of the so-called. titration curve in the coordinates the concentration of the substance being determined (or a value proportional to it) - the volume (mass) of the titrant.

(1) Indirect titration or substituent titration is a titration that is used when there is no suitable reaction or indicator for direct titration. In this case, a reaction is used in which the analyte is replaced by an equivalent amount of another substance and then titrated with a working solution.

(2) Volumetric (titrometric) analysis method is a quantitative determination method based on measuring the volume of reagent required to react with the analyte.

(3) Back titration is a titration used when direct titration is not possible or when the analyte is unstable. In this case, two working solutions are taken, one of which is added in excess, and the excess of the first is titrated with the second.

(4) Direct titration is the most common and convenient technique, when a working solution of known concentration is directly added to the analyzed solution of a substance.

(5) Titration is the process of gradually adding a solution of precisely known concentration to the solution being studied.

(6) Equivalence point - establishing the end point of the titration.

Volumetric methods of analysis. Titration as a method for the quantitative determination of a substance: direct, indirect and reverse

Volumetric (titrometric) analysis method (2) It is a quantitative determination method based on measuring the volume of reagent required to react with the analyte.

Volumetric methods of analysis are based on the occurrence of reactions of neutralization, precipitation, ion exchange, complexation, oxidation-reduction, etc. They must satisfy the following conditions:

Strict adherence to stoichiometric ratios between reaction substances;

Fast and quantitative reactions;

Accurate and strict fixation of the equivalence point;

Foreign substances in the test sample must not react with the added reagent to interfere with the titration.

Titration (5) is the process of gradually adding a solution of precisely known concentration to the test solution.

One of the main stages of this process, which largely determines the accuracy of the volumetric method, is the establishment of the titration end point, called equivalence point (6). The equivalence point is determined visually by a change in the color of the solution, the indicator, the appearance of turbidity, or by instrumental methods - conductometric, potentiometric titration.

For titration, 1-3 drops of an indicator solution with a mass fraction of 0.1-0.5% per 10-100 cm 3 of the analyzed solution is sufficient.

Titrometric determination is carried out by direct, indirect and reverse titration.

Direct titration (4) the most common and convenient technique is when a working solution of known concentration is directly added to the analyzed solution of the substance.

Indirect titration or titration of a substituent(1) are used when there is no suitable reaction or indicator for direct titration. In this case, a reaction is used in which the analyte is replaced by an equivalent amount of another substance and then titrated with a working solution.

Back titration (3) used in cases where direct titration is not possible or when the analyte is unstable. In this case, two working solutions are taken, one of which is added in excess, and the excess of the first is titrated with the second.

Calculation mass fraction analyte X(in%) through the mass concentration of the working solution is carried out according to the formula

Х=100 VСМ /(1000t), (5.5)

Where V- volume of working solution used for titration, cm 3 ;

WITH-molar concentration of the working solution, mol/dm 3 ;

M - molecular equivalent mass of the analyte, g/mol;

m- weight of a sample of the analyzed substance, g.

6. TYPES OF METAL DEFECTS

6.1. Classification of defects

A defect is called each individual non-compliance of a product with the requirements established by regulatory documentation (GOST, OST, TU, etc.). Inconsistencies include a violation of the continuity of materials and parts, heterogeneity in the composition of the material: the presence of inclusions, changes in the chemical composition, the presence of other phases of the material other than the main phase, etc.

Defects are also any deviations of the parameters of materials, parts and products from the specified ones, such as dimensions, quality of surface treatment, moisture and heat resistance and a number of other physical quantities.

Defects are divided into obvious (those that are detected by the eye) and hidden (internal, subsurface, indistinguishable by the eye).

Depending on the possible influence of the defect on the service properties of the part, defects can be:

Critical (defects in the presence of which the use of the product for its intended purpose is impossible or is excluded for reasons of safety and reliability);

Significant (defects that significantly affect the use of the product and/or its durability, but are not critical);

Insignificant (do not affect the performance of the product).

Based on their origin, product defects are divided into production and technological (metallurgical, arising during casting and rolling, technological, arising during manufacturing, welding, cutting, soldering, riveting, gluing, mechanical, thermal or chemical treatment, etc.); operational (arising after some operating time of the product as a result of material fatigue, metal corrosion, wear of rubbing parts, as well as improper use and maintenance) and design defects resulting from design imperfections due to designer errors.

For the purpose of selection optimal methods and control parameters, defects are classified according to various criteria: by the size of the defects, by their number and shape, by the location of the defects in the controlled object, etc.

The size of defects a can vary from fractions of millimeters to arbitrarily large values. In practice, the size of the defects lies within the range of 0.01 mm ≤ a ≤ 1 cm.

In ultrasonic flaw detection, for example, the value of a influences the choice of operating frequency.

When quantitatively classifying defects, three cases are distinguished (Fig. 6.1): a – single defects, b – group (multiple) defects, c – continuous defects (usually in the form of gas bubbles and slag inclusions in metals).

Rice. 6.1. Quantitative classification of defects: a – single;

b – group; c – solid

When classifying defects by shape, three main cases are distinguished (Fig. 6.2): ​​a – defects of regular shape, oval, close to cylindrical or spherical, without sharp edges; b – lenticular-shaped defects, with sharp edges; c – defects of arbitrary, indefinite shape, with sharp edges – cracks, breaks, foreign inclusions.

The shape of the defect determines its danger from the point of view of structural destruction. Defects of regular shape, without sharp edges, are the least dangerous, because there is no stress concentration around them. Defects with sharp edges, as in Fig. 6.2, b and c, are stress concentrators. These defects increase during the operation of the product along lines of mechanical stress concentration, which, in turn, leads to destruction of the product.

Rice. 6.2. Classification of defects by shape: a – correct form;

b – lenticular shape with sharp edges; in – arbitrary,

indeterminate shape with sharp edges

When classifying defects by position, four cases are distinguished (Fig. 6.3): a - surface defects located on the surface of a material, semi-finished product or product - these are cracks, dents, foreign inclusions; b – subsurface defects – these are defects located under the surface of the tested product, but near the surface itself; c – volumetric defects are defects located inside the product.

The presence of phosphorus and nitride inclusions and interlayers can lead to the formation of defects of the fourth type – through ones.

According to the cross-sectional shape, through defects are round (pores, fistulas, slag inclusions) and slot-shaped (cracks, lack of penetration, structural defects, discontinuities in the locations of oxide and other inclusions and interlayers).

Based on the effective diameter (for defects of a round cross-section) or the width of the opening (for gaps, cracks), through defects are divided into ordinary (> 0.5 mm), macrocapillary (0.5...2·10 -4 mm) and microcapillary (< 2·10-4 мм).

Rice. 6.3. Classification of defects by position in the controlled

object: a – superficial; b – subsurface; c – volumetric

Based on the nature of the internal surface, through defects are divided into smooth and rough. The inner surface of the slag channels is relatively smooth. Inner surface cracks, lack of penetration and secondary pore channels are usually rough.

The position of the defect affects both the choice of testing method and its parameters. For example, in ultrasonic testing, the position of a defect affects the choice of wave type: surface defects are best determined by Rayleigh waves, subsurface defects by head waves, and volume defects by body (longitudinal) waves.

The danger of defects affecting performance depends on their type, type and quantity. Classification of possible defects in a product allows you to correctly select the method and means of control.

6.2. Manufacturing and technical defects

Defects in metals are formed mainly during melting, during metal forming (forging, stamping and rolling) and during grinding.

According to GOST 19200-80, defects in castings made of cast iron and steel are divided into five main groups. It should be noted that the adopted terminology is also widely used for castings from alloys based on aluminum, magnesium, titanium and others and therefore can be considered universal.

6.2.1. Casting defects

Geometric mismatch.

This group unites 14 types of defects caused by irregularities in shape, inaccuracy of dimensions and weight of the casting.

1. Underfilled- a defect in the form of incomplete formation of the casting due to failure to fill the mold cavity with metal (Fig. 6.4.a). One of the main reasons for underfilling is an insufficient amount of liquid metal.

2. Nezaliv- discrepancy between the casting configuration and the drawing due to wear of the pattern equipment or mold defects (Fig. 6.4. b). The reason for non-filling may also be a violation of the technological conditions of filling.

3. Neslitina- a through gap or hole in the wall of the casting, formed as a result of non-merging of counter flows of metal (Fig. 6.4. c). Neslitin is characteristic of alloys with a wide crystallization range and is usually observed in the thin walls of castings. These defects are easily detected by visual inspection of castings.

4. Crimping- this is a local violation of the casting configuration due to deformation of the mold during its assembly or pouring (Fig. 6.4. d). The crimp usually forms near the parting plane in the form of a bulge or thickening of arbitrary shape.

5. Puffiness is a local thickening of the casting, resulting from the expansion of an insufficiently compacted mold by the metal being poured (Fig. 6.4. e).

6-8. Skew and rod misalignment - defects in the form of displacement of one part of the casting relative to the axes or surfaces of another part along the mold connector, model due to their inaccurate installation (Fig. 6.4. e) or in the form of displacement of a hole, cavity or part of the casting made using a rod, due to its distortion (Fig. 6.4. g). These defects are caused by inaccurate fixation of the flasks or misalignment of the rod during its installation. In the latter case, there is also a difference in thickness - an increase or decrease in the thickness of the casting walls (Fig. 6.4. h). The difference in thickness is detected visually or using measuring instruments.

9. Rod Bay- a defect in the form of a hole or cavity filled with metal in a casting, arising due to a core not being inserted into the mold or its collapse (Fig. 6.4. i).

10. Warping- distortion of the casting configuration under the influence of stresses arising during cooling of the casting or due to deformation of the pattern equipment. Warping can manifest itself in various forms, the most typical being the appearance of concavity or convexity on the flat surfaces of castings (Fig. 6.4. j). The defect is detected using measuring instruments. The deflection arrow 6 can serve as a measure of warpage.

11. Break and cut- defects in the form of violations of the casting configuration when knocking out rods, cutting off gates (Fig. 6.4. l), cleaning castings or transporting them.

12. Breakthrough and metal leakage - defects caused by metal leakage* from the mold due to its insufficient strength or weak fastening of its parts. In this case, either incomplete filling of the mold cavity occurs with the simultaneous formation of tides of arbitrary shape, or a defect appears in the form of a void in the body of the casting, limited by a thin crust of hardened metal (Fig. 6.4. m).

Rice. 6.4. Casting defects - discrepancy in geometry (arrows indicate the location of the defect)

This method is fundamentally different from the spectroscopic methods discussed 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 as a spectrum. In Fig. 5.1. The general diagram 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 introduce a sample into a mass spectrometer, although many instruments have the ability to directly introduce the sample into an ionization chamber. All mass spectrometers have devices for ionizing the sample and separating ions by m/z value. After separation, the ions must be detected and their quantity measured. A typical ion collector consists of collimating slits that direct only ions of one type into the collector at a time, 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: low (single) and high resolution (R) devices. Low resolution spectrometers are instruments 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 record a mass of 250 in both the first and second cases.

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

Thus, using low-resolution instruments, it is possible to determine the structural formula of a substance, 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 substance under study.

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

This tutorial will discuss mass spectra obtained by electron impact ionization.

5.2. Electron impact ionization mass spectra

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

A sample molecule in the gas phase is bombarded with high energy electrons (usually 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 possible is called the ionization energy (or, less successfully, “potential”) of a substance (U e).

Ionization energy is a measure of the strength with which a molecule holds the least strongly bound electron.

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

As a result of such a rupture, the molecular ion disintegrates into particles of smaller mass (fragments). This process is called fragmentation.

Fragmentation occurs selectively, is highly reproducible and characteristic of a given compound. Moreover, fragmentation processes are predictable, and it is they that provide the broad capabilities of mass spectrometry for structural analysis. In essence, structural analysis by mass spectrometry consists of identifying fragment ions and retrospectively reconstructing 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 following scheme:

ABOUT
the bottom point is the remaining odd electron; when a charge is localized on an individual 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 a series of fragment ions (primary fragmentation):

If some of the molecular ions have a sufficiently long lifetime, they reach the detector and are recorded as a molecular ion peak. Since the charge of the parent ion is equal to unity, the ratiom/ zfor this peak gives the molecular weight of the test substance.

Thus, mass spectrum is a representation of the relative concentrations of positively charged fragments (including a molecular ion) depending on their mass.

Special literature provides tables of the most frequently occurring fragment ions, which indicate the structural formula of the ion and its m/z value [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 homologous difference (CH 2), 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.

Mass spectrometers

devices for separating ionized particles of matter (molecules, atoms) by their masses, based on the effect of magnetic and electric fields on beams of ions flying in a vacuum. In M.-s. Registration of ions is carried out by electrical methods, in mass spectrographs - by darkening of the sensitive layer of a photographic plate placed in the device.

M.-s. ( rice. 1 ) usually contains a device for preparing the test substance 1; ion source 2, where this substance is partially ionized and an ion beam is formed; mass analyzer 3, in which ions are separated by mass, more precisely, usually by mass ratio m ion to its charge e; ion receiver 4, where the ion current is converted into electrical signal, which is then amplified and recorded. In addition to information about the number of ions (ion current), recording device 6 also receives information about the mass of ions from the analyzer. M.-s. also contains systems electrical supply and devices that create and maintain a high Vacuum in the ion source and analyzer. Sometimes M.-s. connected to a computer.

For any method of recording ions, the mass spectrum ultimately represents a dependence of the magnitude of the ion current I from m. For example, in the mass spectrum of lead ( rice. 2 ) each of the ion current peaks corresponds to singly charged ions of lead isotopes. The height of each peak is proportional to the content of a given isotope in lead. The ratio of the ion mass to the peak width δ m (in mass units) R at different levels is also different. So, for example, in the spectrum rice. 2 in the region of the 208 Pb isotope peak at a level of 10% relative to the peak top R= 250, and at 50% (half height) R= 380. To fully characterize the resolution of the device, it is necessary to know the shape of the ion peak, which depends on the pln. factors. Sometimes called resolution. the value of the highest mass at which two peaks differing in mass by 1 are resolved to a specified level. Because for plural types M.-s. R does not depend on the m/e ratio, then both given definitions R match up. It is customary to say that M.-s. With R up to 10 2 has low resolving power, with R Mass spectrometers 10 2 - 10 3 - average, s R Mass spectrometers 10 3 - 10 4 - high, s R> 10 4 - 10 5 - very high.

The generally accepted definition of sensitivity M.-s. does not exist. If the substance under study is introduced into the ion source in the form of a gas, then the sensitivity of the M.-S. often referred to as the ratio of the current created by ions of a given mass of a given substance to the partial pressure of this substance in the ion source. This value in devices different types and with different resolutions lies in the range from 10 -6 to 10 -3 a/mmHg Art. Relative sensitivity is the minimum content of a substance that can still be detected using MS. in a mixture of substances. For different devices, mixtures and substances it lies in the range from 10 -3 to 10 -7%. Absolute sensitivity is sometimes taken to mean a minimum amount of a substance in r, which must be entered into M.-s. to detect this substance.

Mass analyzers. The classification of M.-s. is based on. lies the principle of the mass analyzer. There are static and dynamic M.-s. In static mass analyzers, electric and magnetic fields are used to separate ions, constant or practically unchanged during the flight of the ion through the device. The separation of ions is in this case spatial: ions with different meanings m/e move in the analyzer along different trajectories. In mass spectrographs, beams of ions with different magnitudes m/e are focused in different places on the photographic plate, forming traces in the form of stripes after development (the outlet of the ion source is usually made in the form of a rectangular slit). In static M.-s. beam of ions with a given m/e focuses on the ion receiver slit. The mass spectrum is formed (unfolded) when the magnetic or electric field changes, as a result of which beams of ions with different values ​​successively enter the receiving slit m/e. By continuously recording the ion current, a graph with ion peaks is obtained ( rice. 2 ). To obtain a mass spectrum in this form, recorded by a mass spectrograph on a photographic plate, a microphotometer is used.

On rice. 3 A diagram of a common static mass analyzer with a uniform magnetic field is shown. Ions formed in the ion source emerge from a slit of width S 1 in the form of a diverging beam, which in a magnetic field is divided into beams of ions with different

and a beam of ions with mass m b focuses on the slit S 1 of the ion receiver. Magnitude m b/e is determined by the expression:

Where m b- ion mass (in atomic mass units (See Atomic mass units)) , e- charge of the ion (in units of elementary electric charge (See Elementary electric charge)) , r- radius of the central trajectory of ions (in cm), N- tension magnetic field(in e), V- applied potential difference (in V), with the help of which ions are accelerated in the ion source (accelerating potential).

The mass spectrum is scanned by changing N or V. The first is preferable, because in this case, during the sweep, the conditions for “pulling” ions from the ion source do not change. Resolution of such M.-s.:

where σ 1 is the width of the beam at the place where it enters the receiver slit S 2.

If the focusing of ions were ideal, then in the case of a mass analyzer in which X 1 = X 2 (rice. 3 ), σ 1 would be exactly equal to the source slit width S 1. In reality σ 1 > S 1, which reduces the resolution of M.-s. One of the reasons for beam broadening is the spread in the kinetic energy of ions emitted from the ion source. This is more or less inevitable for any ion source (see below). Other reasons are: the presence of a significant divergence in a given beam, scattering of ions in the analyzer due to collisions with molecules of residual gas, “pushing apart” of ions in the beam due to the similarity of their charges. To weaken the influence of these factors, an “oblique entry” of the beam into the analyzer and curvilinear boundaries of the magnetic field are used. In some M.-s. non-uniform magnetic fields are used, as well as the so-called. prism optics (see Electronic and ion optics). To reduce ion scattering, they strive to create a high vacuum in the analyzer (≤10 -8 mmHg cm. in devices with medium and high R values). To weaken the influence of energy dispersion, M.-s. is used. with dual focusing that focus on the slit S 2 ions with the same m/e, flying out not only in different directions, but also with different energies. To do this, the ion beam is passed not only through a magnetic field, but also through a deflecting electric field using special shapes ( fig. 4 ).

Do S 1 And S 2 a few less µm technically difficult. In addition, this would lead to very small ionic currents. Therefore, in devices to obtain high and very high resolution it is necessary to use large values r and accordingly long ion trajectories (up to several m).

In dynamic mass analyzers for separating ions with different m/e are usually used different times ions travel a certain distance. There are dynamic analyzers, which use a combination of electric and magnetic fields, and purely electrical analyzers. What is common for dynamic mass analyzers is the impact on ion beams of pulsed or radio frequency electric fields with a period less than or equal to the time of flight of ions through the analyzer. More than 10 types of dynamic mass analyzers have been proposed, including time-of-flight (1), radio frequency (2), quadrupole (3), farvitron (4), omegatron (5), magnetic resonance (6), cyclotron resonance ( 7). The first four analyzers are purely electrical, the last three use a combination of constant magnetic and radio frequency electric fields.

During the flight time M.-s. ( rice. 5 ) ions are formed in the ion source by a very short electrical pulse and are “injected” in the form of an “ion packet” through grid 1 into analyzer 2, which represents an equipotential space. “Drifting” along the analyzer towards ion collector 3, the initial package “stratifies” into a number of packages, each of which consists of ions with the same m/e. The separation is due to the fact that in the initial packet the energy of all ions is the same, and their velocities and, therefore, flight times t analyzer are inversely proportional

In radiofrequency M.-s. ( rice. 6 ) ions acquire the same energy in the ion source eV and pass through a system of sequential grid cascades. Each cascade consists of three plane-parallel grids 1, 2, 3, located at an equal distance from each other. A high-frequency electric field ω is applied to the middle grid relative to the two outer ones U hf At a fixed frequency of this field and ion energy eV only ions with a certain m/e have such a speed υ that, moving between grids 1 and 2 in a half-cycle, when the field between them is accelerating for ions, they cross grid 2 at the moment the field sign changes and pass between grids 2 and 3 also in the accelerating field. So, they get max. energy gain and end up on the collector. Ions of other masses, passing through these cascades, are either inhibited by the field, i.e., they lose energy, or receive an insufficient increase in energy and are rejected at the end of the path from the collector by a high braking potential U 3. As a result, only ions with a certain m/e. The mass of such ions is determined by the relation:

Where A- numerical coefficient, S- distance between grids. The analyzer can be adjusted to register ions of other masses by changing either the initial energy of the ions or the frequency of the high-frequency field.

In quadrupole M.-s. ( rice. 7 ) ion separation is carried out transversely electric field with hyperbolic potential distribution. The field is created by a quadrupole capacitor (quadrupole), consisting of four rods of round or square cross-section, located symmetrically relative to the center, axis and parallel to it. The opposite rods are connected in pairs, and constant and variable high-frequency potential differences are applied between the pairs. An ion beam is introduced into the analyzer along the quadrupole axis through hole 1. At fixed values ​​of the frequency ω and the amplitude of the alternating voltage U 0 only for ions with a certain value m/e the amplitude of vibrations in the direction transverse to the analyzer axis does not exceed the distance between the rods. Such ions, due to the initial speed, pass through the analyzer and, leaving it through outlet 2, are recorded and enter the ion collector. Ions whose mass satisfies the condition pass through the quadrupole:

Where A- constant of the device. The amplitude of vibrations of ions of other masses increases as they move in the analyzer so that these ions reach the rods and are neutralized. Adjustment to registration of ions of other masses is carried out by changing the amplitude Uo or frequency ω of the alternating voltage component.

In the farvitron ( rice. 8 ) ions are formed directly in the analyzer itself during the ionization of molecules by electrons flying from the cathode, and oscillate along the axis of the device between electrodes 1 and 2. When the frequency of these oscillations ω coincides with the frequency of the alternating voltage U hf, supplied to the grid, the ions acquire additional. energy, overcome the potential barrier and arrive at the collector. The resonance condition has the form:

Where A- constant of the device.

In dynamic M.-s. with a transverse magnetic field, the separation of ions by mass is based on the coincidence of the cyclotron frequency (See Cyclotron frequency) of rotation of the ion along circular trajectories in a transverse magnetic field with the frequency of the alternating voltage applied to the electrodes of the analyzer. So, in the omegatron ( rice. 9 ) under the influence of an applied high-frequency electric field E and constant magnetic field N ions move along circular arcs. Ions whose cyclotron frequency coincides with the frequency ω of the field E, move in a spiral and reach the collector. The mass of these ions satisfies the relation:

Where A- constant of the device.

In magnetic resonance M.-s. ( rice. 10 ) the constancy of the time of flight of ions of a given mass along a circular trajectory is used. From ion source 1 ions similar in mass (the region of their trajectories I shaded), moving in a uniform magnetic field N , enter modulator 3, where a thin packet of ions is formed, which, due to the acceleration obtained in the modulator, begin to move in orbit II . Further mass separation is carried out by accelerating “resonant” ions, the cyclotron frequency of which is a multiple of the modulator field frequency. After several revolutions, such ions are again accelerated by the modulator and enter the ion collector 2.

In cyclotron resonance M.-s. ( rice. eleven ) resonant absorption of electromagnetic energy by ions occurs when the cyclotron frequency of the ions coincides with the frequency of the alternating electric field in the analyzer; ions move along cycloids in a uniform magnetic field N with cyclotron frequency of orbital motion:

(With- speed of light).

The resolution for each type of dynamic mass analyzer is determined by a complex set of factors, some of which, for example, the influence of space charge and ion scattering in the analyzer, are common to all types of mass analyzers, both dynamic and static. For devices (1), an important role is played by the ratio of the time during which the ions fly a distance equal to the width of the ion packet to the total time of flight of the ions in the drift space; for devices (3) - the number of ion vibrations in the analyzer and the ratio of the constant and variable components of the electric fields; for instruments (5) - the number of revolutions that an ion makes in the analyzer before it hits the ion collector, etc. For some types of dynamic microscopy. high resolution has been achieved: for (1) and (3) R Mass spectrometers 10 3 , for (6) R Mass spectrometers 2.5․10 4 , for (7) R Mass spectrometers 2․10 3 .

For M.-s. with very high resolution, as well as for laboratory instruments for general purposes, which simultaneously require high resolution, high sensitivity, a wide range of measured masses and reproducibility of measurement results, best results are achieved with the help of static M.-s. On the other hand, in some cases dynamic M.-s. are most convenient. For example, time-of-flight measurements are convenient for recording processes lasting from 10 -2 to 10 -5 sec; radiofrequency M.-s. due to their small weight, dimensions and power consumption, they are promising in space research; quadrupole M.-s. due to the small size of the analyzer, a wide range of measured masses and high sensitivity, they are used when working with molecular beams (see Molecular and atomic beams) . Magnetic resonance M.-s. due to high R values ​​at low levels intensities are used in helium isotope geochemistry to measure very large isotope ratios.

Ion sources. M.-s. They are also classified according to ionization methods, which are: 1) electron impact ionization; 2) photoionization; 3) ionization in a strong electric field (field ion emission) ; 4) ionization by ion impact (ion-ion emission); 5) Surface ionization ; electric spark in a vacuum (vacuum spark); 6) ionization under the influence laser beam(see Laser radiation).

In analytical mass spectroscopy (See Mass spectroscopy), the following methods are most often used due to the relative technical simplicity and sufficiently large ion currents created: 1 - in the analysis of evaporated substances; 6 - when working with difficult-to-evaporate substances and 5 - when isotope analysis of substances with low ionization potentials. Method 6, due to the large energy spread of the ions, usually requires analyzers with double focusing even to achieve a resolving power of several hundred units. Values ​​of average ion currents created by an ion source with electron impact ionization at an ion energy of 40 - 100 ev and the width of the source slit Mass spectrometers are several tens µm(typical for laboratory M.-s.), are 10 -10 - 10 -9 A. For other ionization methods these currents are usually lower. “Soft” ionization, i.e. ionization of molecules, accompanied by slight dissociation of ions, is carried out with the help of electrons, the energy of which is only 1 - 3 ev exceeds the ionization energy of the molecule, as well as using methods 2, 3, 4. The currents obtained with “soft” ionization are usually Mass spectrometers 10 -12 - 10 -14 A.

Registration of ion currents. The magnitude of the ion currents created in microscopy determines the requirements for their amplification and registration. The sensitivity of those used in M.-s. amplifiers Mass spectrometers10 -15 - 10 -16 A at a time constant from 0.1 to 10 sec. Further increase in sensitivity or speed of action of M.-s. is achieved by using electronic multipliers, which increase the sensitivity of measuring currents in M.-S. until 10 -18 - 10 -19 a.

Approximately the same sensitivity values ​​are achieved when using photographic registration of ions due to long exposure. However, due to the low accuracy of measuring ion currents and the cumbersomeness of the devices for introducing photographic plates into the vacuum chamber of the analyzer, photorecording of mass spectra retained a certain value only at very high temperatures. precise measurements mass, as well as in cases where it is necessary to simultaneously record all lines of the mass spectrum due to the instability of the ion source, for example, in elemental analysis in the case of ionization by a vacuum spark.

In the USSR, many different mass spectral equipment are being developed and produced. The accepted index system for M.-s. classifies devices mainly not by device type, but by purpose. The index consists of two letters (MI - MS isotopic, MX - for chemical analysis, MS - for physicochemical, including structural, research, MV - a device with high resolution) and four numbers, of which the first indicates the method used for separating ions by mass (1 - in a uniform magnetic field, 2 - in a non-uniform magnetic field, 4 - magnetodynamic, 5 - time-of-flight, 6 - radio frequency), the second - on the conditions of use (1 - indicators, 2 - for production, control, 3 - for laboratory research, 4 - for special conditions), and the last two are the model number. On rice. 12 two M.-s. made in the USSR are shown. Abroad M.-s. are produced by several dozen companies (USA, Japan, Germany, Great Britain, France and Sweden).

Lit.: Aston F., Mass spectra and isotopes, trans. from English, M., 1948; Rafalson A. E., Shereshevsky A. M., Mass spectrometric instruments, M. - L., 1968; Beynon J., Mass spectrometry and its applications in organic chemistry, trans. from English, M., 1964; Materials of the 1st All-Union Conference on Mass Spectrometry, L., 1972; Jairam R., Mass Spectrometry. Theory and applications, trans. from English, M., 1969; Polyakova A. A., Khmelnitsky R. A., Mass spectrometry in organic chemistry, Leningrad, 1972.

V. L. Talrose.

Rice. 12. On the table of a large double-focusing mass spectrometer for structural-chemical analysis MS-3301 with a resolving power of RMass spectrometers 5 · 10 4 lies a miniature mass spectrometer MX-6407M (circled in a square), which was used for studying the ionosphere on artificial Earth satellites.

Rice. 10. Diagram of a magnetic resonance mass analyzer; a magnetic field N perpendicular to the plane of the drawing.

Rice. 6. Diagram of a radio frequency mass analyzer: 1, 2, 3 - grids forming a three-grid cascade, high-frequency voltage U HF is applied to the middle grid 2. Ions with a certain speed and, therefore, a certain mass, accelerated inside the cascade by a high-frequency field, receive a greater increase in kinetic energy, sufficient to overcome the retarding field and hit the collector.

Rice. 5. Diagram of a time-of-flight mass analyzer. A package of ions with masses m 1 and m 2 (black and white circles), “thrown” into the analyzer through grid 1, moves in drift space 2 so that heavy ions (m 1) lag behind light ions (m 2); 3 - ion collector.

Rice. 4. Example of a mass analyzer with dual focusing. A beam of accelerated ions emerging from the slit S 1 of the ion source sequentially passes through the electric field of a cylindrical capacitor, which deflects the ions by 90°, then through a magnetic field, which deflects the ions by another 60°, and is focused into the slit S 2 of the ion collector receiver.

Rice. 3. Scheme of a static magnetic analyzer with a uniform magnetic field; S 1 and S 2 - slits of the ion source and receiver; OAV - region of uniform magnetic field N , perpendicular to the plane of the figure, thin solid lines are the boundaries of ion beams with different m/e; r is the radius of the central trajectory of ions.

Rice. 2. Mass spectrum of thorium lead (δm 50% - peak width at half maximum; δm 10% - peak width at 1/10 of the maximum intensity).

Rice. 1. Skeletal diagram of a mass spectrometer: 1 - system for preparing and introducing the substance under study; 2 - ion source; 3 - mass analyzer; 4 - ion receiver; 5 - amplifier; 6 - recording device; 7 - computer; 8 - electrical power system; 9 - pumping devices. The dotted line outlines the evacuated part of the device.


Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

See what “Mass spectrometers” are in other dictionaries:

    mass spectrometers- Devices for separation of ionizers. particles of a thing (molecules, atoms) by their masses, basic. under the influence of magnetic and electric fields on beams of ions flying in vacuum. In m.s. ions registered electric methods, in mass spectrographs - by darkening... ... Technical Translator's Guide

    Mass spectrometers- devices for separating ionized particles of matter (molecules, atoms) by their masses, based on the effect of magnetic and electric fields on beams of ions flying in a vacuum. In mass spectrometers, ions are recorded... ... encyclopedic Dictionary in metallurgy

(mass spectroscopy, mass spectrography, mass spectral analysis, mass spectrometric analysis) - a method for studying a substance by determining the ratio of mass to charge (quality) and the quantity of charged particles formed during a particular process of exposure to a substance. The history of mass spectrometry dates back to the seminal experiments of John Thomson at the beginning of the 20th century. The term “-metry” received the ending “-metry” after the widespread transition from the detection of charged particles using photographic plates to electrical measurements of ion currents.

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

Rice. 6.12.

Mass spectrometry in a 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 device that uses the physical laws of motion of charged particles in magnetic and electric fields, necessary to obtain a mass spectrum.

The mass spectrum, like any spectrum, in a narrow sense, is the dependence of the intensity of the ion current (quantity) on the ratio of mass to charge (quality). Due to the quantization of mass and charge, a typical mass spectrum is discrete. Usually (in routine tests) this is true, but not always. The nature of the analyte, features 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 may end up in different parts spectrum and even make part of it continuous. Therefore, the mass spectrum in the broad sense is something more, carrying specific information and making the process of its interpretation more complex and fascinating. Ions can be single-charged or multi-charged, both organic and inorganic. Most small molecules acquire only one positive or negative charge when ionized. Atoms are capable of acquiring more than one positive charge and only one negative charge. Proteins, nucleic acids, and other polymers are capable of acquiring multiple positive and negative charges. Atoms chemical elements have a specific mass. Thus, accurate determination of the mass of the analyzed molecule makes it possible to determine its elemental composition. Mass spectrometry also provides important information about the isotopic composition of the molecules being analyzed. In organic substances, molecules are specific structures formed by atoms. Nature and man have created a truly innumerable variety of organic compounds. Modern mass spectrometers are capable of fragmenting detected ions and determining the mass of the resulting fragments. In this way, it is possible to obtain data on the structure of a substance.

Operating principle of the mass spectrometer

The instruments used in mass spectrometry are called mass spectrometers or mass spectrometric detectors. These devices work with material matter, which consists of tiny particles - molecules and atoms. Mass spectrometers determine what kind of molecules they are (i.e., what atoms make them up, what their molecular weight is, what their arrangement is) and what kind of atoms they are (i.e., their isotopic composition). A significant difference between mass spectrometry and other analytical physicochemical methods is that optical, x-ray and some other methods detect radiation 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. To do this, the laws of motion of charged particles of matter in a magnetic or electric field are used. The mass spectrum is the sorting of charged particles by their masses (mass-to-charge ratios).

First, in order to obtain a mass spectrum, it is necessary to transform the 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 transfer the ions into the gas phase in the vacuum part of the mass spectrometer. A deep vacuum allows ions to move freely within the mass spectrometer, and in its absence, the ions will scatter and recombine (turn back into uncharged particles).

Conventionally, 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 (EC, EC – Electron capture);
  • ionization in an electric field (PI, FI – Field ionization).

Liquid phase:

  • thermal spray;
  • 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 (APPI – Atmospheric pressure fotoionization).

Solid phase:

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

In inorganic chemistry for the analysis of elemental composition

hard ionization methods are used, since the binding energy of atoms in a solid is much greater, which means that much more hard 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;
  • ionization in glow discharge and spark ionization;
  • ionization during laser ablation.

Historically, the first ionization methods were developed for the gas phase. Unfortunately, many organic substances cannot be evaporated, i.e. 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.), physiologically active substances, polymers, i.e. everything that is of particular interest today. Mass spectrometry did not stand still and last years They were designed special methods ionization of such organic compounds. Today, mainly two of them are used - atmospheric pressure ionization and its subtypes - 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 using an electric field. There the second stage of mass-specific analysis begins - sorting ions by mass (more precisely, by the ratio of mass to charge).

The following types of mass analyzers exist.

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

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

A mass spectrometer can have two mass analyzers. This 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 the ions of the analyzed molecules (molecular ions). Thus, the first mass analyzer analyzes molecular ions. Leaving the first mass analyzer, molecular ions are fragmented due to collisions with molecules inert gas or laser radiation, after which their fragments are analyzed in a second mass analyzer. The most common tandem mass spectrometer configurations are quadrupole-quad and quadrupole-to-flight.

The last element of the simplified mass spectrometer we are describing is a charged particle detector. The first mass spectrometers used a photographic plate as a detector. Nowadays, 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, knocks out even more electrons from it, etc. Another option is photomultiplier tubes, which record 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 the ions that enter the this point space (Faraday collectors).

Mass spectrometers are used to analyze organic and inorganic compounds. Organic substances in most cases are multicomponent mixtures of individual components. For example, it has been shown that smell fried chicken make up 400 components (i.e. 400 individual organic compounds). The goal of analytics is to determine how many components make up an organic substance, find out what those components are (identify them), and how much of each compound is present in the mixture. For this purpose, the combination of chromatography with mass spectrometry is ideal. Gas chromatography is ideally suited for combination with the ion source of an electron impact ionization or chemical ionization mass spectrometer because 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 chromatography-mass spectrometers ("Chromass").

Many organic compounds cannot be separated into their components using gas chromatography, but can be separated using liquid chromatography. To combine liquid chromatography with mass spectrometry, electropress ionization sources and atmospheric pressure chemical ionization sources are now used, and the combination of liquid chromatographs with mass spectrometers is called LC/MS. The most powerful systems for organic analysis, in demand in modern proteomics, are based on a superconducting magnet and operate on the principle of ion cyclotron resonance.

Most widely used in Lately a mass analyzer that allows you to most accurately measure the mass of an ion and has a very high resolution. High resolution allows you to work with polyprotonated ions formed during the ionization of proteins and peptides in electrospray, and high accuracy mass determination allows one to obtain the gross formula of ions, making it possible to determine the structure of the sequences of amino acid residues in peptides and proteins, as well as to detect post-translational modifications of proteins. This made it possible to sequence proteins without first hydrolyzing them into peptides. This method is called “Top-down” proteomics. Obtaining unique information became possible thanks to the use of an ion cyclotron resonance mass analyzer with Fourier transform. In this analyzer, ions fly into a strong magnetic field and rotate there in cyclic orbits (as in a cyclotron, an accelerator of 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, it requires a strong magnetic field to operate, which means the use of a strong magnet with a superconducting solenoid maintained at a very low temperature (liquid helium, approximately -270°C).

The most important technical characteristics Mass spectrometers are sensitivity, dynamic range, resolution, scanning speed.

The most important characteristic when analyzing organic compounds is sensitivity. In order to achieve the highest possible sensitivity while improving the signal-to-noise ratio, detection by individual selected ions is used. The gain in sensitivity and selectivity is enormous, but when using low-resolution devices one has to sacrifice another important parameter– reliability. The use of high resolution on dual focus instruments allows for a high level of confidence without sacrificing sensitivity.

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

In terms of the combination of sensitivity with the reliability of determining components, ion traps come next after high-resolution devices. New generation classic quadrupole instruments have improved performance due to a number of innovations applied to them, such as the use of a curved quadrupole prefilter to reduce noise, preventing neutral particles from reaching the detector.

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