Capabilities of mass spectrometry

The mass spectrum can be used to determine the molecular weight of a substance. This is necessary to establish the molecular formula of a substance (the gross formula). The mass of an atom measured with high precision, is different 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.

The determination of the atomic composition by the exact value of the 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 the 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.

The maximum m/z value 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 effective is the joint use of several physicochemical 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 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

V 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: indicate the values ​​of m/z, and in brackets relative intensity(%). 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. circuit diagram 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; breaking energy C-C connections 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; the energy of breaking the C-C bond is greater; 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; bond breaking energy S-S 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 physico-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, Higher 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 sectroscopy, 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 the physical laws of motion of charged particles in magnetic and electric fields 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 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 of the 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. Most small molecules acquire only one positive or negative charge when ionized. Atoms can acquire more than one positive charge and only one negative one. Proteins, nucleic acids, and other polymers are capable of acquiring multiple positive and negative charges. Atoms of chemical elements have a specific mass. In this way, precise definition the mass of the analyzed molecule makes it possible 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 of 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 matter, which consists of the 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 weight, what is the structure of their arrangement) and what kind of atoms are they (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).

V inorganic chemistry for analysis of elemental composition

harsh ionization methods are used, since the binding energies of atoms in a solid are much higher, 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.), physiologically active substances, polymers, i.e. everything that is of particular interest today. Mass spectrometry did not stand still and in last years special methods have been developed for the 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 that collect all ions that have fallen into a given point in space (Faraday collectors) are used.

Mass spectrometers are used to analyze organic and inorganic compounds. Organic substances in most cases are multicomponent mixtures of 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, ionization sources in electropress and chemical ionization at atmospheric pressure are now used, 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 widely used recently mass analyzer, which allows you to most accurately measure 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, it requires a strong magnetic field to operate, and thus 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 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 achieve the greatest possible sensitivity while improving the signal-to-noise ratio, detection is resorted to for individual selected ions. In this case, the gain in sensitivity and selectivity is colossal, but when using low-resolution devices, another important parameter has to be sacrificed - reliability. 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 pre-filter to reduce noise, preventing neutral particles from reaching the detector.

What happens to the blood samples you donate for clinical analysis? How much does your hemoglobin weigh? How do scientists even weigh molecules? tiny particles substances that cannot be seen or touched? Ekaterina Zhdanova, a 5th year student of the Department of Chemical Physics of the FMHF, an employee of the Laboratory of Ionic and Molecular Physics of the Moscow Institute of Physics and Technology, spoke about all this within the framework of the “Just about the complex” T&P rubric.

Very often, research methods are of interest only to specialists in specific fields and remain in the shadow of more fundamental problems, such as the origin of life or the principles of the human mind. However, in order to find an answer to main question of life, the universe and everything else”, you first need to learn how to answer simpler questions. For example, how to weigh a molecule? Ordinary scales are unlikely to help here: the mass of a methane molecule is about 10 ^ (-23) grams. Hemoglobin molecule, large and complex protein, weighs several times more - 10 ^ (-20) grams. It is clear that some other approach to the problem is needed, because the usual measuring instruments are not applicable to it. We must also understand that when weighing apples in the store or standing on the scales after training, we actually measure the force acting on the device - the scales. Then there is a conversion into the usual units - grams and kilograms.

But how do you weigh a molecule? Here nature has left us a loophole. It turns out that charged particles "feel" the presence of electric and magnetic field and change the trajectory and nature of their movement. Forces also act on charged particles, the magnitude of which can be recalculated in the ratio of mass to charge. This method is quite popular today and is called mass spectrometry. The pioneer of mass spectrometry is considered to be Sir J. J. Thomson, Nobel laureate in physics. He drew attention to the fact that charged particles move in a magnetic field along parabolic trajectories proportional to the ratio of their mass to charge.

The scheme of operation of the mass spectrometer consists of several stages. First of all, the analyte must undergo ionization. Then it enters the ion transport system, which must deliver the charged particles to the mass analyzer. In the mass analyzer, the separation of ions occurs depending on the ratio of mass to charge. Finally, the ions reach the detector, the data from which are analyzed using special software. The picture thus obtained is a spectrum, that is, the distribution of particles. One of the axes of this graph is the ratio of mass to charge, the second is the intensity. Each of the peaks on such a graph will be characteristic of the ions of a particular substance, so the ingress of foreign substances into the device, such as air, can lead to distortion of the results. To avoid this, a vacuum system is used.

The relatively simple physical concept of this method requires a number of non-trivial engineering solutions. How to ionize molecules? How to create an electromagnetic field? Atoms and molecules are electrically neutral, therefore, in order to carry out mass spectrometric measurements, it is necessary to ionize them, that is, to remove electrons from outer atomic orbitals or to add a proton. An important role is played by the type of sample with which to work. For the study of inorganic substances - metals, alloys, rocks - it is necessary to use one method, for organic substances others are suitable. Very many organic substances (such as DNA or polymers) are difficult to evaporate, i.e. turn into a gas, without decomposition, which means that the study of living tissue or biological samples requires the use of special methods. In addition, during ionization, molecules can break up into separate fragments. So we again face the question: what exactly are we going to measure? The mass of the whole molecule or the mass of fragments? Both are important. Moreover, after measuring the mass of a whole molecule, researchers often deliberately break it into pieces. Thus, by determining the mass structural elements protein, we also determine their quantity, which allows us to draw conclusions about its chemical composition and structure.

All this indicates a variety of existing mass spectrometers, each of which is used to solve problems in a particular area. This method is practically indispensable in cases where scientists need to determine the chemical composition of a substance. Pharmacists use mass spectrometric experiments in drug development, pharmacokinetics (that is, the biochemical processes that occur in the body when a drug is taken), and metabolism. Biological scientists use mass spectrometry to analyze proteins, peptides and nucleic acids. In addition, if we want to check the quality of water or food, then again we cannot do without this method.

A separate innovative field of application of mass spectrometry is medical diagnostics. Structural changes in the proteins of our body lead to the development of many diseases: they are usually classified according to the formation of a characteristic piece, a marker peptide. If such a mutation is detected in time, then it becomes possible to treat the disease at an early stage. In addition, thanks to modern mass spectrometers, it becomes possible to conduct studies of this kind in real time - for example, during a neurosurgical operation. This allows you to accurately determine the boundaries between healthy tissue and tumor, which is critical for surgeons.

Seemingly dry and narrow-profile at first glance, mass spectrometry, upon closer examination, turns out to be a surprisingly rich field, combining a wide class of applications with unusual engineering solutions. Science shows that the answers to less fundamental questions are sometimes just as interesting.

• Introduction
• A Brief History of Mass Spectrometry
• Ionization
• Mass Analyzers
• Detector
• Natural and artificial isotopy
• Mass spectrometers for isotopic analysis
• Scan speed
• Permission
• Dynamic Range
• Sensitivity
• What are mass spectrometers

So, mass spectrometers are used to analyze organic compounds and inorganic ones.

Organic substances in most cases are multicomponent mixtures of 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 find out 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 gas chromatography-mass spectrometers.

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, today electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) sources are used, and the combination of liquid chromatography with mass spectrometers is called LC/MS or LC/MS in English. 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. They are also called FT/MS because they use the Fourier transform of the signal.

A new class of mass spectrometers are hybrid instruments. They are called hybrid because they actually include two mass spectrometers, at least one of which can operate as an independent instrument. Examples of such devices are the FINNIGAN LTQ FT ion cyclotron resonance mass spectrometer, in which the FINNIGAN LTQ linear quadrupole ion trap can operate as a stand-alone device that detects ions after MS or MSn using two secondary electron multipliers, and can also prepare and send ions to cyclotron cell, pushing them out in a direction parallel to the quadrupole axis. Also hybrid is LTQ QRBITRAP, which works in exactly the same way. The advantages of such schemes are obvious, the linear trap has the highest sensitivity, operates in the tandem mass spectrometry mode from n to 10, performs a variety of intelligent scan functions, and the ion cyclotron resonance mass spectrometer and orbital ion trap have high resolution and can with the highest accuracy measure the ratio of mass to charge of ions. Inductively coupled plasma mass spectrometers are the most attractive for elemental composition analysis. With the help of this device, it is determined from which atoms a substance is composed. The same method of analysis can also show the isotopic composition. But it is best to measure the isotopic composition using specialized isotope instruments that register ions not on one detector at different times of their arrival at it, but each ion on its own personal collector and simultaneously (the so-called parallel detection).

However, before moving on to instruments for measuring isotopic composition, let us briefly dwell on what isotopes are.

Natural and artificial isotopy Atoms consist of a nucleus and electron shells. The properties of atoms are determined by how many protons (positively charged elementary particles) the nucleus contains. The nucleus contains neutrons in addition to protons. Nature decreed that with an equal number of protons, the nucleus can contain a different number of neutrons. Atoms with the same number of protons in the nucleus but different numbers of neutrons differ in mass by one or more atomic mass units (a.m.u.) and are called isotopes. Most elements have a certain set of stable isotopes. Radioactive isotopes are not stable and decay to form stable isotopes. The natural abundance of isotopes for each element is known. Some elements in nature are monoisotopic, that is, 100% of the natural abundance falls on one isotope (for example, Al, Sc, Y, Rh, Nb, etc.), while others have many stable isotopes (S, Ca, Ge, Ru , Pd, Cd, Sn, Xe, Nd, Sa, etc.). In technological activities, people have learned to change the isotopic composition of elements in order to obtain any specific properties of materials (for example, U235 has the ability to spontaneous chain reaction and can be used as fuel for nuclear power plants or an atomic bomb) or to use isotope labels (for example, in medicine ).

Since the masses of isotopes differ, and mass spectrometry measures mass, naturally, this method becomes the most convenient for determining the isotopic composition. At the same time, information on isotopic composition helps identify organic compounds and provides answers to many questions, from determining the age of rocks for geology, to determining the falsification of many products and determining the origin of goods and raw materials.

Mass spectrometers for isotope analysis. Mass spectrometers to determine the isotopic composition must be very accurate. Electron impact ionization is used to analyze the isotopic composition of light elements (carbon, hydrogen, oxygen, sulfur, nitrogen, etc.). In this case, all gas phase injection methods are suitable, as in organic mass spectrometers (DELTA Plus ADVANTAGE, FINNIGAN DELTA Plus XL and FINNIGAN MAT253).
For the analysis of isotopes of heavier elements, thermal ionization (FINNIGAN TRITON TI) or inductively coupled plasma ionization with parallel detection (FINNIGAN NEPTUNE, and FINNIGAN ELEMENT2 single-collector detection) is used.
Practically all types of isotope mass spectrometers use magnetic mass analyzers.

Characteristics of mass spectrometers and mass spectrometric detectors

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

Scanning speed. The mass analyzer, as we showed above, passes ions with a certain ratio of mass and charge at a certain time (except for multicollector devices and ion-cyclotron resonance, an orbital ion trap). In order to analyze all the ions in relation to their mass to charge, it must scan, that is, the parameters of its field must pass through all the values ​​​​needed to pass all the ions of interest to the detector in a given period of time. This field unfolding speed is called the scan rate and should be as fast as possible (respectively, the scan time should be as short as possible), since the mass spectrometer must be able to measure the signal in a short time, for example, the time of the chromatographic peak, which can be several seconds. At the same time, the more mass spectra are measured during the release of the chromatographic peak, the more accurately the chromatographic peak will be described, the less likely it will be to slip past its maximum value, and using mathematical processing to determine whether it is individual and “finish” it using mass spectrometry.
The slowest mass analyzer is a magnet, the minimum scanning time without much loss of sensitivity is a fraction of a second (MAT 95XP). A quadrupole mass analyzer can sweep the spectrum in tenths of a second (TSQ QUANTUM), and an ion trap even faster (POLARISQ, FINNIGAN LCQ ADVANTAGE MAX, FINNIGAN LCQ DECA XP MAX), a linear ion trap even faster (LTQ) and a little slower mass FINNIGAN LTQ FT ion-cyclotron resonance spectrometer.
The innovative FINNIGAN TRACE DSQ quadrupole chromato-mass spectrometer and its economical counterpart FINNIGAN FOCUS DSQ are capable of scanning at about 11,000 amu. per second. This opens up new opportunities, for example, it is possible to almost simultaneously obtain a complete mass spectrum of a compound for its unambiguous identification and conduct selective ion monitoring (SIM), which lowers the detection limit by several orders of magnitude.
Any scanning of all the mass analyzers listed above is a compromise - the higher the scanning speed, the less time spent on recording the signal for each mass number, the worse the sensitivity. However, for routine speed analysis, a quadrupole analyzer or ion trap is sufficient. Another issue is when it comes to high-performance analysis of complex matrices. In this case, it would be good to use ultrafast chromatography (on thin, short, rapidly heated columns). For such a task, a time-of-flight mass spectrometer (TEMPUS) is best suited. It is capable of recording mass spectra at a rate of 40,000 per second!

Permission. Visually, resolution (resolution) can be defined as the ability of an analyzer to separate ions from neighboring masses. It is very important to be able to accurately determine the mass of ions, this allows you to calculate the atomic composition of an ion or identify a peptide by comparison with a database, reducing the number of candidates from thousands and hundreds to units or a single one. For magnetic mass analyzers, for which the distance between the peaks of the mass spectrum does not depend on the ion masses, the resolution is a value equal to M/DM. This value is usually determined by 10% of the peak height. For example, a resolution of 1000 means that peaks with masses of 100.0 a.m.u. and 100.1 a.m.u. are separated from each other, that is, they do not overlap up to 10% of the height.
For analyzers in which the distance between peaks varies in the operating mass range (the larger the mass, the smaller the distance), such as quadrupole analyzers, ion traps, time-of-flight analyzers, strictly speaking, the resolution has a different meaning. Resolution defined as M/DM in this case characterizes a specific mass. It makes sense to characterize these mass analyzers by the width of the peaks, a value that remains constant over the entire mass range. This peak width is usually measured at 50% of their height. For such devices, a peak width at half maximum equal to 1 is a good indicator and means that such a mass analyzer is able to distinguish nominal masses that differ per atomic mass unit in almost its entire operating range. The nominal mass or mass number is the nearest integer to the exact mass of the ion in the scale of atomic mass units. For example, the mass of the hydrogen ion H+ is 1.00787 amu, and its mass number is 1. And such mass analyzers, which mainly measure nominal masses, are called low-resolution analyzers. We wrote “mostly” because today there are mass analyzers that formally belong to low-resolution ones, but in reality they are no longer such. High technology, primarily from the most advanced developer Thermo Electron, has already offered high-resolution quadrupole instruments to the market for analytical equipment. For example, the newest FINNIGAN TSQQuantum easily works with a mass spectrum peak width at half maximum of 0.1 amu. Knowledgeable people may object: “But this peak width can be obtained on every quadrupole mass spectrometer!” And they will be right, indeed, every quadrupole can be tuned to this level of resolution. But what happens to the signal? When moving from a peak width at half maximum of 1 a.m.u. to 0.1 amu the signal strength at all quadrupoles will drop by almost two orders of magnitude. But not on TSQ Quantum, on it it will decrease by only two and a half times. Ion traps in a narrow mass range can operate as high-resolution mass spectrometers, providing at least separation of peaks separated by 1/4 a.m.u. from each other. Mass spectrometers with double focusing (magnetic and electrostatic), ion-cyclotron resonance - instruments of medium or high resolution. A typical resolution for a magnetic instrument is >60,000, and operation at a resolution level of 10,000 - 20,000 is routine. On an ion-cyclotron resonance mass spectrometer at a mass of about 500 a.m.u. a resolution of 500,000 can easily be achieved, which makes it possible to measure the mass of ions with an accuracy of 4-5 decimal places. A resolution of several thousand can also be achieved using time-of-flight mass analyzers, however, at high masses, in the region of which, in fact, this device has an advantage over others, and this resolution is only enough to measure the ion mass with an accuracy of +/- tens a.u.m. As can be seen from the above, resolution is closely related to another important characteristic - mass measurement accuracy. The meaning of this characteristic can be illustrated with a simple example. The masses of molecular ions of nitrogen (N2+) and carbon monoxide (CO+) are 28.00615 amu. and 27.99491 amu, respectively (both are characterized by the same mass number 28). These ions will be registered by the mass spectrometer separately at a resolution of 2500, and exact value mass will give an answer - which of the gases is recorded. Accurate mass measurement is available on dual focus instruments, on the TSQ Quantum tandem quadrupole mass spectrometer, and on ion cyclotron resonance mass spectrometers.

dynamic range. If we analyze a mixture containing 99.99% of one compound or element and 0.01% of some impurity, we must be sure that we are correctly determining both. In order to be sure of the definition of the components in this example, you need to have a linearity range of 4 orders of magnitude. Modern mass spectrometers for organic analysis are characterized by a dynamic range of 5-6 orders of magnitude, and mass spectrometers for elemental analysis of 9-12 orders of magnitude. A dynamic range of 10 orders of magnitude means that an impurity in the sample will be visible even when it is 10 milligrams per 10 tons.

Sensitivity. This is one of the most important characteristics of mass spectrometers. Sensitivity is a value indicating how much of a substance must be introduced into the mass spectrometer so that it can be detected. For simplicity, we will consider a sensitivity-related parameter - the minimum detectable amount of a substance, or the detection threshold. A typical detection threshold for a good gas chromatography-mass spectrometer used for the analysis of organic compounds is 1 picogram when 1 microliter of liquid is injected. Let's imagine what it is. If we draw 1 microliter of liquid (one millionth of a liter) with a special syringe and release it onto a sheet of clean white paper, then when we examine it through a magnifying glass, we will see a speck equal in size to the trace of a prick with a thin needle. Now imagine that we threw 1 gram of a substance (for example, one aspirin tablet) into 1000 tons of water (for example, a pool 50 meters long, 10 meters wide and 2 meters deep). Thoroughly mix the water in the pool, draw 1 microliter of this water with a syringe and inject it into the gas chromatography-mass spectrometer. As a result of the analysis, we will get a mass spectrum that we can compare with the library spectrum and use the fingerprint method to make sure that this is really acetylsalicylic acid, otherwise called aspirin.

The detection limits for inorganic substances, for example by ICP/MS (FINNIGAN ELEMENT2) are even more impressive. Here, the pool will already be too small to prepare a solution with a concentration corresponding to the detection limit. The limit of detection for FINNIGAN ELEMENT2 for a number of metals is 1 ppq (one part per quadrillion). This means that the sensitivity of the device is sufficient to detect 1 kilogram of metal (for example, mercury, lead, etc.) dissolved in Lake Baikal (provided it is mixed and completely dissolved)!

In isotope mass spectrometry, for example, 800 - 1000 molecules of carbon dioxide (CO2, carbon dioxide) are sufficient to obtain a carbon signal. In order to demonstrate the accuracies and isotopic sensitivities that isotope mass spectrometry deals with, let us resort to the following allegory. Suppose for one thousand exactly the same apples, each of which weighs 100 grams, there are 11 apples that weigh 8% more, that is, 108 grams. All these apples are collected in one bag. This example corresponds to the ratio of carbon isotopes in nature - there are 11 13C atoms per 1000 12C atoms. Isotope mass spectrometry measures ratios, that is, it is able to distinguish not just these 11 apples, but to find among many bags those in which out of 1000 one hundred gram apples, not 11 one hundred and eight grams, but 10 or 12. This example is very easy for isotope mass spectrometry , in fact, instruments such as the FINNIGAN DELTAPlus ADVANTAGE, DELTA Plus XP and FINNIGAN MAT253 are able to determine the difference of one isotope (one hundred and eight gram apple) among ten million atoms (ten million apples).

The most important characteristic in the analysis of organic compounds is sensitivity. In order to achieve the highest possible sensitivity while improving the signal-to-noise ratio, detection is resorted to for individual selected ions. In this case, the gain in sensitivity and selectivity is colossal, but when using low-resolution devices, another important parameter has to be sacrificed - reliability. After all, if you recorded only one peak from the entire characteristic mass spectrum, you will need a lot of work to prove that this peak corresponds to exactly the component that you are interested in. How to solve this problem? Use high resolution on dual focus instruments where a high level of fidelity can be achieved without sacrificing sensitivity. Or use tandem mass spectrometry, where each peak corresponding to a single ion can be confirmed by the mass spectrum of the daughter ions. So, the absolute champion in sensitivity is a high-resolution organic chromatography-mass spectrometer with double focusing. So, for example, the passport characteristic of DFS states that 2,3,7,8-tetrachloro-p-dibenzodioxin, introduced through a chromatographic column in the amount of 10 femtograms, will give a peak characterized by a signal-to-noise ratio = 80: 1. Not achievable on any another instrument result!
According to the characteristics of the combination of sensitivity with the reliability of the determination of components, ion traps follow high-resolution devices. The classic new generation quadrupole instruments (TRACE DSQ II) have improved performance due to a number of innovations, such as the use of a curved quadrupole pre-filter, which prevents neutral particles from reaching the detector and, therefore, reduces noise.

Why mass spectrometry is needed

Deep physical laws, advanced scientific and engineering developments, high-tech vacuum systems, high electrical voltages, the most best materials, the highest quality of their processing, the most modern high-speed digital and analog electronics and computer equipment, sophisticated software - this is what a modern mass spectrometer is made of. And what is all this for? To answer one of the most important questions of the universe - what matter is composed of. But this is not a question of high science, but of everyday human life.

For example, the development of new drugs to save people from previously incurable diseases and the control of drug production, genetic engineering and biochemistry, proteomics. Mass spectrometry has given researchers a tool to identify proteins, to determine what changes have occurred in their structure due to various interactions, during their reproduction, to determine the metabolic pathways of various drugs and other compounds and to identify metabolites, to develop new targeted drugs. Mass spectrometry is the only method that solves all these and many other problems of analytical biochemistry.
Without mass spectrometry, control over the illegal distribution of narcotic and psychotropic drugs, forensic and clinical analysis of toxic drugs, and analysis of explosives is inconceivable.

Finding out the source of origin is very important for solving a number of issues: for example, determining the origin of explosives helps to find terrorists, drugs - to fight their distribution and block their traffic routes. The economic security of the country is more reliable if the customs services can not only confirm by analysis in doubtful cases the country of origin of the goods, but also its compliance with the declared type and quality. And the analysis of oil and oil products is needed not only to optimize oil refining processes or geologists to search for new oil fields, but also to identify those responsible for oil spills in the ocean or on land.

In the era of "chemization Agriculture» The presence of trace amounts of applied chemicals (eg pesticides) in food products has become very important. In trace amounts, these substances can cause irreparable harm to human health.

A number of technogenic (that is, not existing in nature, but resulting from human industrial activity) substances are supertoxicants (having a toxic, carcinogenic or harmful effect on human health in extremely low concentrations). An example is the well-known dioxin.

The existence of nuclear energy is unthinkable without mass spectrometry. With its help, the degree of enrichment of fissile materials and their purity are determined.

Of course, medicine is not complete without mass spectrometry. Isotope mass spectrometry of carbon atoms is used for direct medical diagnosis of human infection with Helicobacter Pylori and is the most reliable of all diagnostic methods.
HPLC/MS systems are the main analytical tool in the development of new drugs. Without this method, the quality control of manufactured drugs and the detection of such a common phenomenon as their falsification cannot be dispensed with.
Proteomics has given medicine the possibility of ultra-early diagnosis of the most terrible diseases of mankind - cancerous tumors and cardiac dysfunctions. Determination of specific proteins, called biomarkers, allows for early diagnosis in oncology and cardiology.

It is difficult to imagine an area of ​​human activity where there would be no place for mass spectrometry. We restrict ourselves to simply listing: biochemistry, clinical chemistry, general chemistry and organic chemistry, pharmaceuticals, cosmetics, perfumery, food industry, chemical synthesis, petrochemistry and oil refining, environmental control, production of polymers and plastics, medicine and toxicology, forensic science, doping control, drug control, control of alcoholic beverages, geochemistry, geology, hydrology, petrography, mineralogy, geochronology, archeology, nuclear industry and energy, semiconductor industry, metallurgy.

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) an ion source, where neutral atoms are converted into ions (for example, under the influence of heating or a microwave field) and accelerated by an electric field, b) areas of constant electric and magnetic fields, and v) 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. The direction of the 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 in opposite directions, and the magnitudes of the electric field strength E and magnetic field induction B 1 are chosen so that the forces of their action on the ions (respectively qE and qvB 1 , where q is the charge, and v is the ion velocity) compensated 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 be shown with high accuracy by a 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.