Today, more and more patients are referred not for radiography or ultrasound, but for nuclear magnetic resonance imaging. This research method is based on core magnetism. Let's consider what NMR tomography is, what are its advantages and in what cases it is performed.

What is this study?

This diagnostic method is based on nuclear magnetic resonance. In an external magnetic field, the nucleus of a hydrogen atom, or proton, is in two mutually opposite states. You can change the direction of the magnetic moment of the nucleus by acting on it with electromagnetic rays with a certain frequency.

Placing a proton in an external magnetic field causes a change in its magnetic moment with a return to its original position. This releases a certain amount of energy. Magnetic resonance tomography captures the change in the amount of such energy.

The tomograph uses very strong magnetic fields. Electromagnets are usually capable of developing a magnetic field with a strength of 3, sometimes up to 9 T. It is completely harmless to humans. The tomography system allows you to localize the direction of the magnetic field in order to obtain the highest quality images.

Nuclear magnetic tomograph

The diagnostic method is based on fixing the electromagnetic response of the nucleus of an atom (proton), which occurs due to its excitation by electromagnetic waves in a high-voltage magnetic field. Magnetic resonance imaging was first discussed in 1973. Then the American scientist P. Laterbur proposed to study the object in a changing magnetic field. The works of this scientist served as the beginning of a new era in medicine.

With the help of a magnetic resonance tomograph, it became possible to study the tissues and cavities of the human body due to the degree of tissue saturation with hydrogen. Magnetic resonance imaging contrast agents are often used. Most often, these are preparations of gadolinium, which are able to change the response of protons.
The term "nuclear MRI" existed until 1986.

In connection with radiophobia among the population in connection with the disaster at the Chernobyl nuclear power plant, it was decided to remove the word “nuclear” from the name of the new diagnostic method. However, this allowed magnetic resonance imaging to quickly enter the practice of diagnosing many diseases. Today, this method is the key to identifying many more recently difficult-to-diagnose diseases.

How is the diagnosis carried out?

An MRI uses a very strong magnetic field. And although it is not dangerous for humans, nevertheless, the doctor and the patient need to adhere to certain rules.

First of all, before the diagnostic procedure, the patient fills out a special questionnaire. In it, he indicates the state of health, as well as statements about himself. The examination is done in a specially prepared room with a cabin for changing clothes and personal belongings.

In order not to harm himself, and also to ensure the correctness of the results, the patient should take off all things that contain metal, leave mobile phones, credit cards, watches, etc. in the locker for personal belongings. It is desirable for women to wash off decorative cosmetics from the skin.
Next, the patient is placed inside the tomograph tube. At the direction of the doctor, the examination area is determined. Each zone is examined for ten to twenty minutes. During this time, the patient must remain still. The quality of the pictures will depend on this. The doctor can fix the position of the patient, if necessary.

During the operation of the device, uniform sounds are heard. This is normal and indicates that the study is proceeding correctly. To obtain more accurate results, a contrast agent may be administered intravenously to the patient. In some cases, with the introduction of such a substance, a surge of heat is felt. This is completely normal.

Approximately half an hour after the study, the doctor can receive the study protocol (conclusion). A disk with the results is also issued.

Benefits of Nuclear MRI

The benefits of such a survey include the following.

1. The ability to obtain high-quality images of body tissues in three projections. This greatly enhances the visualization of tissues and organs. In this case, MRI is much better than computed tomography, radiography and ultrasound diagnostics.
2. High-quality 3D images provide an accurate diagnosis, which improves treatment and increases the likelihood of recovery.
3. Since it is possible to obtain a high-quality image on an MRI, such a study is the best for detecting tumors, disorders of the central nervous system, and pathological conditions of the musculoskeletal system. Thus, it becomes possible to diagnose those diseases that until recently were difficult or impossible to detect.
4. Modern devices for tomography allow you to get high-quality images without changing the position of the patient. And for encoding information, the same methods are used as in computed tomography. This facilitates diagnosis, as the doctor sees three-dimensional images of entire organs. Also, the doctor can get images of a particular organ in layers.
5. Such an examination well determines the earliest pathological changes in the organs. Thus, it is possible to detect the disease at a stage when the patient does not yet feel symptoms.
6. During such a study, the patient is not exposed to ionizing radiation. This significantly expands the scope of MRI.
7. The MRI procedure is completely painless and does not cause any discomfort to the patient.

Indications for MRI

There are many indications for magnetic resonance imaging.

• Cerebral circulation disorders.
• Suspicions of a neoplasm of the brain, damage to its membranes.
• Assessment of the state of organs after surgery.
• Diagnosis of inflammatory phenomena.
• Convulsions, epilepsy.
• Traumatic brain injury.
• Assessment of the condition of the vessels.
• Assessment of the condition of bones and joints.
• Diagnosis of the soft tissues of the body.
• Diseases of the spine (including osteochondrosis, spondyloarthrosis).
• Spinal injury.
• Assessment of the state of the spinal cord, including suspicion of malignant processes.
• Osteoporosis.
• Assessment of the state of the peritoneal organs, as well as the retroperitoneal space. MRI is indicated for jaundice, chronic hepatitis, cholecystitis, cholelithiasis, tumor-like liver damage, pancreatitis, diseases of the stomach, intestines, spleen, kidneys.
• Diagnosis of cysts.
• Diagnosis of the state of the adrenal glands.
• Diseases of the pelvic organs.
• Urological pathologies.
• Gynecological diseases.
• Diseases of the organs of the chest cavity.

In addition, magnetic resonance imaging of the whole body is indicated if a neoplasm is suspected. MRI can be used to search for metastases if a primary tumor is diagnosed.

This is not a complete list of indications for magnetic resonance imaging. It is safe to say that there is no such organism and disease that could not be detected using this diagnostic method. Since the possibilities of medicine are growing, doctors have almost unlimited possibilities for diagnosing and treating many dangerous diseases.

When is magnetic resonance imaging contraindicated?

There are a number of absolute and relative contraindications for MRI. Absolute contraindications include:

1. Presence of a pacemaker. This is due to the fact that fluctuations in the magnetic field are able to adapt to the rhythm of the heart and thus can be fatal.
2. The presence of installed ferromagnetic or electronic implants in the middle ear.
3. Large metal implants.
4. The presence of ferromagnetic fragments in the body.
5. Availability of Ilizarov apparatus.

Relative contraindications (when research is possible under certain conditions) include:

When performing MRI with contrast, contraindications are anemia, chronic decompensated renal failure, pregnancy, individual intolerance.

Conclusion

The importance of magnetic resonance imaging for diagnosis cannot be overestimated. It is a perfect, non-invasive, painless and harmless way of detecting many diseases. With the introduction of magnetic resonance imaging, the treatment of patients has also improved, as the doctor knows accurate diagnosis and features of all processes occurring in the patient's body.

No need to be afraid of an MRI. The patient does not feel any pain during the procedure. It has nothing to do with nuclear or x-ray radiation. It is also impossible to refuse such a procedure.

Nuclear magnetic resonance

VK. Ravens

Irkutsk State Technical University

INTRODUCTION

Until recently, our ideas about the structure of atoms and molecules were based on studies using optical spectroscopy methods. In connection with the improvement of spectral methods, which have advanced the field of spectroscopic measurements into the range of ultrahigh (approximately 10^ 3 - 10^ 6 MHz; microradio waves) and high frequencies (approximately 10^ (-2) - 10^ 2 MHz; radio waves), new sources of information about the structure of matter. During the absorption and emission of radiation in this frequency range, the same basic process occurs as in other ranges of the electromagnetic spectrum, namely, when moving from one energy level to another, the system absorbs or emits a quantum of energy.

The energy difference between the levels and the energy of the quanta participating in these processes are about 10^(-7) eV for the radio frequency region and about 10^(-4) eV for microwave frequencies. In two types of radio spectroscopy, namely, nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spectroscopy, the difference in the energy levels is associated with different orientations, respectively, of the magnetic dipole moments of nuclei in an applied magnetic field and electric quadrupole moments of nuclei in molecular electric fields, if the latter are not spherically symmetrical.

The existence of nuclear moments was first discovered when studying the hyperfine structure of the electronic spectra of some atoms using high-resolution optical spectrometers.

Under the influence of an external magnetic field, the magnetic moments of the nuclei are oriented in a certain way, and it becomes possible to observe transitions between nuclear energy levels associated with these different orientations: transitions that occur under the action of radiation of a certain frequency. The quantization of the energy levels of the nucleus is a direct consequence of the quantum nature of the angular momentum of the nucleus receiving 2 I+ 1 values. The spin quantum number (spin) I can take on any value that is a multiple of 1/2; the highest known value I(> 7) possesses Lu. The largest measurable value of the angular momentum (the largest value of the projection of the moment on the selected direction) is equal to i ћ , where ћ = h /2 π , a h is Planck's constant.

Values I it is impossible to predict for specific nuclei, but it has been observed that isotopes in which both mass number and atomic number are even have I= 0, and isotopes with odd mass numbers have half-integer spins. Such a situation, when the numbers of protons and neutrons in the nucleus are even and equal ( I= 0) can be considered as a state with “complete pairing”, similar to the complete pairing of electrons in a diamagnetic molecule.

At the end of 1945, two groups of American physicists led by F. Bloch (Stanford University) and E.M. Purcell (Harvard University) were the first to receive nuclear magnetic resonance signals. Bloch observed resonant absorption by protons in water, and Purcell was successful in discovering nuclear resonance by protons in paraffin. For this discovery, they were awarded the Nobel Prize in 1952.

The essence of the NMR phenomenon and its distinctive features are outlined below.

HIGH RESOLUTION NMR SPECTROSCOPY

The essence of the NMR phenomenon

The essence of the NMR phenomenon can be illustrated as follows. If a nucleus with a magnetic moment is placed in a uniform field H 0 , directed along the z axis, then its energy (with respect to the energy in the absence of a field) is equal to μ z H 0, where μ z, is the projection of the nuclear magnetic moment on the direction of the field.

As already noted, the nucleus can be located in 2 I+ 1 states. In the absence of an external field H 0 all these states have the same energy. If we denote the largest measurable value of the magnetic moment component through μ , then all measurable values ​​of the magnetic moment component (in this case μ z,) are expressed as m, where m is the quantum number, which, as is well known, can take the values

m= I, I- 1,I- 2...-(I- 1),-I.

Since the distance between the energy levels corresponding to each of the 2 I+ 1 states, equals m H 0 /I, then the nucleus with spin I has discrete energy levels

- μ H0,-(I-1)μ z H 0 /I,..., (I-1)μ z H 0 /I, μ H0.

The splitting of energy levels in a magnetic field can be called nuclear Zeeman splitting, since it is similar to the splitting of electronic levels in a magnetic field (the Zeeman effect). Zeeman splitting is illustrated in fig. 1 for system with I= 1 (with three energy levels).

Rice. 1. Zeeman splitting of nuclear energy levels in a magnetic field.

The NMR phenomenon consists in the resonant absorption of electromagnetic energy due to the magnetism of the nuclei. This implies the obvious name of the phenomenon: nuclear - we are talking about a system of nuclei, magnetic - we mean only their magnetic properties, resonance - the phenomenon itself is resonant in nature. Indeed, it follows from Bohr's frequency rules that the frequency ν of the electromagnetic field causing transitions between adjacent levels is determined by the formula

, (1)

Since the vectors of momentum (angular momentum) and magnetic momentum are parallel, it is often convenient to characterize the magnetic properties of nuclei by the value γ defined by the relation

, (2)

where γ is the gyromagnetic ratio having the dimension radian * oersted^(- 1) * second^(- 1) (rad * E^(- 1) * s*(- 1) ) or radian/(oersted * second) (rad/ (E * s)). With this in mind, we find

, (3)

Thus, the frequency is proportional to the applied field.

If, as a typical example, we take the value of γ for a proton, equal to 2.6753 * 10: 4 rad / (E * s), and H 0 \u003d 10,000 Oe, then the resonant frequency

Such a frequency can be generated by conventional radio techniques.

NMR spectroscopy is characterized by a number of features that distinguish it from other analytical methods. About half (~150) of the nuclei of known isotopes have magnetic moments, but only a minority of them are used systematically.

Before the advent of spectrometers operating in a pulsed mode, most studies were carried out using the NMR phenomenon on hydrogen nuclei (protons) 1 H (proton magnetic resonance - PMR) and fluorine 19 F. These nuclei have properties ideal for NMR spectroscopy:

The high natural abundance of the “magnetic” isotope ( 1H 99.98%, 19 F 100%); for comparison, it can be mentioned that the natural abundance of the “magnetic” isotope of carbon 13 C is 1.1%;

Large magnetic moment;

Spin I = 1/2.

This is primarily responsible for the high sensitivity of the method in detecting signals from the nuclei mentioned above. In addition, there is a strictly theoretically justified rule according to which only nuclei with a spin equal to or greater than unity have an electric quadrupole moment. Hence, NMR experiments 1H and 19 F are not complicated by the interaction of the nuclear quadrupole moment of the nucleus with the electric environment. A large number of works have been devoted to resonance at other (besides 1H and 19 F) kernels such as 13 C, 31 P, 11 B, 17 O in the liquid phase (same as on nuclei 1 1H and 19F).

The introduction of pulsed NMR spectrometers into everyday practice has significantly expanded the experimental possibilities of this type of spectroscopy. In particular, the recording of NMR spectra 13 C solutions - the most important isotope for chemistry - is now actually a familiar procedure. The detection of signals from nuclei, the intensity of NMR signals of which is many times less than the intensity of signals from 1 H, including in the solid phase.

High-resolution NMR spectra usually consist of narrow, well-resolved lines (signals) corresponding to magnetic nuclei in various chemical environments. The intensities (areas) of the signals during the recording of the spectra are proportional to the number of magnetic nuclei in each group, which makes it possible to carry out a quantitative analysis using NMR spectra without preliminary calibration.

Another feature of NMR is the influence of exchange processes, in which resonating nuclei participate, on the position and width of resonant signals. Thus, NMR spectra can be used to study the nature of such processes. NMR lines in liquid spectra typically have a width of 0.1 - 1 Hz (high-resolution NMR), while the same nuclei examined in the solid phase will cause the appearance of lines with a width of the order of 1 * 10^ 4 Hz (hence the concept of NMR broad lines).

In high-resolution NMR spectroscopy, there are two main sources of information about the structure and dynamics of molecules:

Chemical shift;

Spin-spin interaction constants.

chemical shift

Under real conditions, resonating nuclei whose NMR signals are detected are a constituent of atoms or molecules. When the test substances are placed in a magnetic field ( H 0 ) there is a diamagnetic moment of atoms (molecules), due to the orbital motion of electrons. This movement of electrons forms effective currents and, therefore, creates a secondary magnetic field proportional, in accordance with Lenz's law, to the field H 0 and opposite direction. This secondary field acts on the nucleus. Thus, the local field in the place where the resonating nucleus is located,

, (4)

where σ is a dimensionless constant, called the screening constant and independent of H 0 , but strongly dependent on the chemical (electronic) environment; it characterizes the decrease Hlok compared with H 0 .

Value σ varies from a value of the order of 10^(- 5) for a proton to values ​​of the order of 10^(- 2) for heavy nuclei. Taking into account the expression for Hlok we have

, (5)

Screening effect is to reduce the distance between the levels of nuclear magnetic energy or, in other words, leads to the convergence of the Zeeman levels (Fig. 2). In this case, the energy quanta that cause transitions between levels become smaller and, consequently, resonance occurs at lower frequencies (see expression (5)). If we conduct an experiment by changing the field H 0 until resonance occurs, the applied field strength must be large compared to the case when the core is not shielded.

Rice. Fig. 2. Effect of electron screening on the Zeeman levels of the nucleus: (a) unscreened, (b) screened.

In the vast majority of NMR spectrometers, spectra are recorded when the field changes from left to right, so the signals (peaks) of the most shielded nuclei should be in the right part of the spectrum.

The shift of the signal depending on the chemical environment, due to the difference in screening constants, is called the chemical shift.

For the first time, messages about the discovery of a chemical shift appeared in several publications in 1950-1951. Among them, it is necessary to single out the work of Arnold et al. (1951), who obtained the first spectrum with separate lines corresponding to chemically different positions of identical nuclei. 1 H in one molecule. We are talking about ethyl alcohol CH 3 CH 2 OH, typical NMR spectrum 1 H of which at low resolution is shown in fig. 3.

Rice. 3. Low-resolution proton resonance spectrum of liquid ethyl alcohol.

There are three types of protons in this molecule: three protons of the methyl group CH 3 –, two protons of the methylene group –CH 2 – and one proton of the hydroxyl group –OH. It can be seen that three separate signals correspond to three types of protons. Since the intensity of the signals is in the ratio 3: 2: 1, the decoding of the spectrum (assignment of signals) is not difficult.

Since chemical shifts cannot be measured on an absolute scale, that is, relative to a nucleus devoid of all its electrons, the signal of a reference compound is used as a conditional zero. Usually, the chemical shift values ​​for any nuclei are given as a dimensionless parameter 8 defined as follows:

, (6)

where H- Hat is the difference in chemical shifts for the test sample and the standard, Hat is the absolute position of the reference signal with the applied field H 0 .

Under real experimental conditions, it is possible to measure the frequency more accurately than the field, so δ is usually found from the expression

, (7)

where ν - ν floor is the difference between the chemical shifts for the sample and the standard, expressed in units of frequency (Hz); NMR spectra are usually calibrated in these units.

Strictly speaking, one should use ν 0 is the operating frequency of the spectrometer (it is usually fixed), and the frequency ν floor, that is, the absolute frequency at which the resonant signal of the reference is observed. However, the error introduced by such a replacement is very small, since ν 0 and ν floor almost equal (the difference is 10 ^ (-5), that is, by the amount σ for a proton). Since different NMR spectrometers operate at different frequencies ν 0 (and, consequently, for different fields H 0 ), it is obvious that the expression δ in dimensionless units.

The unit of chemical shift is one millionth of the field strength or resonant frequency (ppm). In foreign literature, this reduction corresponds to ppm (parts per million). For most of the nuclei that make up diamagnetic compounds, the range of chemical shifts of their signals is hundreds and thousands of ppm, reaching 20,000 ppm. in case of NMR 59 Co (cobalt). In spectra 1 H proton signals of the vast majority of compounds lie in the range 0 – 10 ppm.

Spin-spin interaction

In 1951-1953, when recording the NMR spectra of a number of liquids, it was found that there are more lines in the spectra of some substances than follows from a simple estimate of the number of nonequivalent nuclei. One of the first examples is the resonance on fluorine in the POCl molecule 2 F. Spectrum 19 F consists of two lines of equal intensity, although there is only one fluorine atom in the molecule (Fig. 4). Molecules of other compounds gave symmetrical multiplet signals (triplets, quartets, etc.).

Another important factor found in such spectra was that the distance between the lines, measured in the frequency scale, does not depend on the applied field. H 0 , instead of being proportional to it, as it should be if the multiplicity arises from a difference in screening constants.

Rice. 4. Doublet in the resonance spectrum at fluorine nuclei in the POCl molecule 2F

Ramsey and Purcell in 1952 were the first to explain this interaction by showing that it is due to an indirect coupling mechanism through the electronic environment. The nuclear spin tends to orient the spins of the electrons surrounding the given nucleus. Those, in turn, orient the spins of other electrons and through them - the spins of other nuclei. The spin-spin interaction energy is usually expressed in hertz (that is, the Planck constant is taken as a unit of energy, based on the fact that E=h ν ). It is clear that there is no need (unlike the chemical shift) to express it in relative units, since the discussed interaction, as noted above, does not depend on the strength of the external field. The magnitude of the interaction can be determined by measuring the distance between the components of the corresponding multiplet.

The simplest example of splitting due to spin-spin coupling that can be encountered is the resonance spectrum of a molecule containing two kinds of magnetic nuclei A and X. The nuclei A and X can be either different nuclei or nuclei of the same isotope (for example, 1 H) when the chemical shifts between their resonant signals are large.

Rice. 5. View of the NMR spectrum of a system consisting of magnetic nuclei A and X with spin I = 1/2 when the condition is met δ AX > J AX .

On fig. 5 shows what the NMR spectrum looks like if both nuclei, i.e. A and X, have spin 1/2. The distance between the components in each doublet is called the spin-spin coupling constant and is usually denoted as J (Hz); in this case it is the constant J AH.

The occurrence of doublets is due to the fact that each nucleus splits the resonance lines of the neighboring nucleus into 2I+1 component. The energy differences between different spin states are so small that at thermal equilibrium the probabilities of these states, in accordance with the Boltzmann distribution, turn out to be almost equal. Consequently, the intensities of all lines of the multiplet resulting from interaction with one nucleus will be equal. In the case where there is n equivalent nuclei (that is, equally shielded, so their signals have the same chemical shift), the resonant signal of the neighboring nucleus is split into 2nI + 1 lines.

CONCLUSION

Soon after the discovery of the phenomenon of NMR in condensed matter, it became clear that NMR would be the basis of a powerful method for studying the structure of matter and its properties. Indeed, when studying NMR spectra, we use as a resonant system of nuclei that are extremely sensitive to the magnetic environment. Local magnetic fields near the resonating nucleus depend on intra- and intermolecular effects, which determines the value of this type of spectroscopy for studying the structure and behavior of many-electron (molecular) systems.

At present, it is difficult to point to a field of natural sciences where NMR is not used to some extent. NMR spectroscopy methods are widely used in chemistry, molecular physics, biology, agronomy, medicine, in the study of natural formations (mica, amber, semi-precious stones, combustible minerals and other mineral raw materials), that is, in such scientific areas in which the structure of matter is studied, its molecular structure, the nature of chemical bonds, intermolecular interactions and various forms of internal movement.

NMR methods are increasingly being used to study technological processes in factory laboratories, as well as to control and regulate the course of these processes in various technological communications directly in production. Research over the past fifty years has shown that magnetic resonance methods can detect disturbances in the course of biological processes at the earliest stage. Installations for the study of the entire human body by magnetic resonance methods (NMR tomography methods) have been developed and are being produced.

As for the CIS countries, and above all Russia, magnetic resonance methods (especially NMR) have by now taken a firm place in the research laboratories of these states. In various cities (Moscow, Novosibirsk, Kazan, Tallinn, St. Petersburg, Irkutsk, Rostov-on-Don, etc.), scientific schools arose on the use of these methods with their own original problems and approaches to their solution.

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2. Kerrington A., McLechlan E. Magnetic resonance and its application in chemistry. M.: Mir, 1970. 447 p.

3. Bovi F.A. High resolution NMR of macromolecules. Moscow: Chemistry, 1977. 455 p.

4. Heberlen W., Mehring M. High resolution NMR in solids. M.: Mir, 1980. 504 p.

5. Slikter Ch. Fundamentals of the theory of magnetic resonance. M.: Mir, 1981. 448 p.

6. Ionin B.I., Ershov B.A., Koltsov A.I. NMR spectroscopy in organic chemistry. L.: Chemistry, 1983. 269 p.

7. Voronov V.K. Methods of paramagnetic additives in NMR spectroscopy. Novosibirsk: Nauka, 1989. 168 p.

8. Ernst R., Bodenhausen J., Vokaun A. NMR in one and two dimensions. M.: Mir, 1990. 709 p.

9. Deroum E. Modern NMR methods for chemical research. M.: Mir, 1992. 401 p.

10. Voronov V.K., Sagdeev R.Z. Fundamentals of magnetic resonance. Irkutsk: Vost.-Sib. book. publishing house, 1995.352 p.

The same nuclei of atoms in different environments in a molecule show different NMR signals. The difference between such an NMR signal and the signal of a standard substance makes it possible to determine the so-called chemical shift, which is due to the chemical structure of the substance under study. In NMR techniques, there are many opportunities to determine the chemical structure of substances, the conformations of molecules, the effects of mutual influence, and intramolecular transformations.

Physics NMR

The splitting of the energy levels of the nucleus with I = 1/2 in a magnetic field

The phenomenon of nuclear magnetic resonance is based on the magnetic properties of atomic nuclei consisting of nucleons with half-integer spin 1/2, 3/2, 5/2 .... Nuclei with even mass and charge numbers (even-even nuclei) do not have a magnetic moment , while for all other nuclei the magnetic moment is nonzero.

Thus, the nuclei have an angular momentum related to the magnetic moment by the relation

,

where is Planck's constant, is the spin quantum number, is the gyromagnetic ratio.

The angular momentum and magnetic moment of the nucleus are quantized and the eigenvalues ​​of the projection and the angular and magnetic moments on the z-axis of an arbitrarily chosen coordinate system are determined by the relation

and ,

where is the magnetic quantum number of the eigenstate of the nucleus, its values ​​are determined by the spin quantum number of the nucleus

that is, the kernel can be in states.

So, for a proton (or another nucleus with I = 1/2- 13 C, 19 F, 31 P, etc.) can only be in two states

,

such a core can be represented as a magnetic dipole, the z-component of which can be oriented parallel or antiparallel to the positive direction of the z-axis of an arbitrary coordinate system.

It should be noted that in the absence of an external magnetic field, all states with different states have the same energy, that is, they are degenerate. The degeneracy is removed in an external magnetic field, while the splitting with respect to the degenerate state is proportional to the external magnetic field and the magnetic moment of the state and for a nucleus with a spin quantum number I in an external magnetic field, a system of 2I+1 energy levels, that is, nuclear magnetic resonance has the same nature as the Zeeman effect of the splitting of electronic levels in a magnetic field.

In the simplest case, for a nucleus with spin c I = 1/2- for example, for a proton, splitting

and energy difference of spin states

Larmor frequencies of some atomic nuclei

The frequency for proton resonance is in the short wave range (wavelength about 7 m).

Application of NMR

Spectroscopy

Main article: NMR spectroscopy

Devices

The heart of the NMR spectrometer is a powerful magnet. In an experiment first put into practice by Purcell, a sample placed in a glass ampoule about 5 mm in diameter is placed between the poles of a strong electromagnet. Then the ampoule begins to rotate, and the magnetic field acting on it is gradually increased. A high-quality RF generator is used as a radiation source. Under the action of an increasing magnetic field, the nuclei to which the spectrometer is tuned begin to resonate. In this case, the shielded cores resonate at a frequency slightly lower than the nominal resonance frequency (and the device).

The energy absorption is recorded by an RF bridge and then recorded by a chart recorder. The frequency is increased until it reaches a certain limit, above which resonance is impossible.

Since the currents coming from the bridge are very small, they are not limited to taking one spectrum, but make several dozen passes. All received signals are summarized on the final graph, the quality of which depends on the signal-to-noise ratio of the device.

In this method, the sample is exposed to radio frequency radiation at a constant frequency while the strength of the magnetic field changes, hence it is also called the constant field (CW) method.

The traditional method of NMR spectroscopy has many disadvantages. First, it takes a lot of time to build each spectrum. Secondly, it is very picky about the absence of external interference, and as a rule, the resulting spectra have significant noise. Thirdly, it is unsuitable for creating high-frequency spectrometers (300, 400, 500 and more MHz). Therefore, in modern NMR instruments, the method of the so-called pulsed spectroscopy (PW) is used, based on the Fourier transform of the received signal. At present, all NMR spectrometers are built on the basis of powerful superconducting magnets with a constant magnetic field.

In contrast to the CW method, in the pulsed version, the excitation of nuclei is carried out not with a “constant wave”, but with the help of a short pulse, several microseconds long. The amplitudes of the frequency components of the pulse decrease with increasing distance from ν 0 . But since it is desirable that all nuclei be irradiated equally, it is necessary to use "hard pulses", that is, short pulses of high power. The pulse duration is chosen so that the frequency bandwidth is greater than the spectrum width by one or two orders of magnitude. Power reaches several watts.

As a result of pulsed spectroscopy, not an ordinary spectrum with visible resonance peaks is obtained, but an image of damped resonant oscillations, in which all signals from all resonating nuclei are mixed - the so-called "free induction decay" (FID, free induction decay). To transform this spectrum, mathematical methods are used, the so-called Fourier transform, according to which any function can be represented as the sum of a set of harmonic oscillations.

NMR spectra

Spectrum of 1 H 4-ethoxybenzaldehyde. In the weak field (singlet ~9.25 ppm) the signal of the proton of the aldehyde group, in the strong field (triplet ~1.85-2 ppm) - the proton of the methyl ethoxy group.

For qualitative analysis using NMR, spectral analysis is used, based on such remarkable properties of this method:

• the signals of the nuclei of atoms included in certain functional groups lie in strictly defined regions of the spectrum;
• the integral area limited by the peak is strictly proportional to the number of resonant atoms;
• nuclei lying through 1-4 bonds are capable of producing multiplet signals as a result of the so-called. splits on each other.

The position of the signal in the NMR spectra is characterized by their chemical shift relative to the reference signal. As the latter in 1 H and 13 C NMR, tetramethylsilane Si(CH 3) 4 is used. The unit of chemical shift is the parts per million (ppm) of the instrument frequency. If we take the TMS signal as 0, and consider the signal shift to a weak field as a positive chemical shift, then we will obtain the so-called δ scale. If the resonance of tetramethylsilane is equated to 10 ppm and reverse the signs, then the resulting scale will be the τ scale, which is practically not used at present. If the spectrum of a substance is too complicated to interpret, one can use quantum chemical methods for calculating screening constants and correlate the signals based on them.

NMR introscopy

The phenomenon of nuclear magnetic resonance can be used not only in physics and chemistry, but also in medicine: the human body is a combination of all the same organic and inorganic molecules.

To observe this phenomenon, an object is placed in a constant magnetic field and exposed to radio frequency and gradient magnetic fields. In the inductor surrounding the object under study, an alternating electromotive force (EMF) arises, the amplitude-frequency spectrum of which and the time-transition characteristics carry information about the spatial density of resonating atomic nuclei, as well as about other parameters specific only for nuclear magnetic resonance. Computer processing of this information generates a three-dimensional image that characterizes the density of chemically equivalent nuclei, the relaxation times of nuclear magnetic resonance, the distribution of fluid flow rates, the diffusion of molecules, and the biochemical processes of metabolism in living tissues.

The essence of NMR introscopy (or magnetic resonance imaging) consists, in fact, in the implementation of a special kind of quantitative analysis of the amplitude of the nuclear magnetic resonance signal. In conventional NMR spectroscopy, the aim is to realize the best possible resolution of the spectral lines. To do this, the magnetic systems are adjusted in such a way as to create the best possible field uniformity within the sample. In the methods of NMR introscopy, on the contrary, the magnetic field is created obviously inhomogeneous. Then there is reason to expect that the frequency of nuclear magnetic resonance at each point of the sample has its own value, different from the values ​​in other parts. By setting some code for NMR signal amplitude gradations (brightness or color on the monitor screen), you can get a conditional image (

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GENERAL PHARMACOPEIAN AUTHORIZATION

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Nuclear magnetic resonance spectroscopy (NMR) is a method based on the absorption of radio frequency electromagnetic radiation by the nuclei of a sample with a nonzero magnetic moment placed in a constant magnetic field ( B 0). Non-zero magnetic moments have isotopes of nuclei of elements with an odd atomic mass (1 H, 13 C, 15 N, 19 F, 31 P, etc.).

General principles

A nucleus rotating around its axis has its own moment of momentum (angular momentum, or spin) P. The magnetic moment of the nucleus μ is directly proportional to the spin: μ = γ ∙ P(γ is the proportionality factor or gyromagnetic ratio). The angular and magnetic moments are quantized, i.e. can be in one of 2 I+ 1 spin states ( I – spin quantum number). Different states of the magnetic moments of nuclei have the same energy if they are not affected by an external magnetic field. When nuclei are placed in an external magnetic field B 0, the energy degeneracy of the nuclei is removed and the possibility of an energy transition from one level to another arises. The process of distribution of nuclei between different energy levels proceeds in accordance with the Boltzmann distribution law and leads to the appearance of a macroscopic equilibrium longitudinal magnetization M z . The time it takes to create M z after turning on the external magnetic field AT 0 , is called time longitudinal or spin—lattice relaxation (T one). Violation of the equilibrium distribution of nuclei occurs under the action of a radio frequency magnetic field ( B 1), perpendicular B 0 , which causes additional transitions between energy levels, accompanied by energy absorption (the phenomenon nuclear magnetic resonance). Frequency ν 0 , at which the absorption of energy by nuclei occurs ( Larmorova or resonant absorption frequency), varies depending on the value of the constant field B 0: ν 0 = γ B 0 /2π. At the moment of resonance, there is an interaction between the individual nuclear magnetic moments and the field AT 1 , which outputs a vector M z from its equilibrium position along the axis z. As a result, there appears transverse magnetization M xy. Its change associated with the exchange within the spin system is characterized by the time transverse or spin-spin relaxation (T 2).

Dependence of the intensity of energy absorption by nuclei of the same type on the frequency of the radio-frequency magnetic field at a fixed value AT 0 is called one-dimensional spectrumnuclear magnetic resonance kernels of this type. The NMR spectrum can be obtained in two ways: by continuously irradiating the sample with an RF field of varying frequency, as a result of which the NMR spectrum is recorded directly (continuous exposure spectroscopy), or by exposing the sample to a short RF pulse ( pulsed spectroscopy). In pulsed NMR spectroscopy, time-decayed coherent radiation emitted by nuclei upon returning to the initial spin state ( free induction decay signal) followed by the transformation of the time scale into frequency ( Fourier transform).

In molecules, the electrons of atoms reduce the magnitude of the acting external magnetic field B 0 at the location of the kernel, i.e. appears diamagnetic shielding:

B loc = B 0 ∙ (1 – σ),

B lok is the intensity of the resulting field;

σ is the screening constant.

The difference in the resonant frequencies of the signals of the nuclei, equal to the difference in their screening constants, is called chemical shift signals, indicated by the symbol δ , measured in parts per million (ppm). Interaction of magnetic moments of nuclei through chemical bond electrons ( spin-spin interaction) causes splitting of the NMR signal ( multiplicity, m). The number of components in multiplets is determined by the nuclear spin and the number of interacting nuclei. The measure of the spin-spin interaction is spin-spin coupling constant (J, measured in hertz, Hz). Values ​​δ, m and J do not depend on the magnitude of the constant magnetic field.

The intensity of the nuclear NMR signal in the spectrum is determined by the population of its energy levels. Of the nuclei with a natural abundance of isotopes, the most intense signals are produced by hydrogen nuclei. The intensity of NMR signals is also affected by the time of longitudinal-transverse relaxation (large T 1 lead to a decrease in signal intensity).

The width of NMR signals (difference between frequencies at half maximum of the signal) depends on T 1 and T 2. small times T 1 and T 2 cause wide and poorly interpreted spectrum signals.

The sensitivity of the NMR method (maximum detectable concentration of a substance) depends on the intensity of the nuclear signal. For 1 H nuclei, the sensitivity is 10 -9 ÷ 10 -11 mol.

Correlations of various spectral parameters (for example, chemical shifts of different nuclei within the same molecular system) can be obtained by homo- and heteronuclear methods in 2D or 3D format.

device

High resolution NMR pulse spectrometer (NMR spectrometer) consists of:

• magnet to create a constant magnetic field B 0 ;
• a temperature-controlled sensor with a sample holder for applying an RF pulse and detecting the radiation emitted by the sample;
• an electronic device for creating a radio frequency pulse, recording, amplifying and converting the free induction decay signal into digital form;
• devices for tuning and adjusting electronic circuits;
• data collection and processing devices (computer);

and may also include:

a flow cell for NMR liquid chromatography or flow-injection analysis;

• system for creating a pulsed magnetic field gradient.

A strong magnetic field is generated by a superconductivity coil in a Dewar vessel filled with liquid helium.

The proper functioning of the NMR spectrometer should be checked. For verification, appropriate tests are carried out, including, as a rule, the measurement of the spectral linewidth at half-height of certain peaks under certain conditions ( permission), signal position reproducibility and signal-to-noise ratio (the ratio between the intensity of a specific signal in the NMR spectrum and random fluctuations in the region of the spectrum that does not contain signals from the analyte, S/N) for standard mixtures. The spectrometer software contains algorithms for determining S/N. All instrument manufacturers provide specifications and measurement protocols for these parameters.

NMR Spectroscopy of Samples in Solutions

Methodology

The test sample is dissolved in a solvent to which an appropriate chemical shift calibration standard may be added as specified in the regulatory document. The value of the relative chemical shift of the nucleus of a substance (δ in-in) is determined by the following expression:

δ in-in \u003d (ν in-in - ν standard) / ν of the device,

ν in-in - the resonance frequency of the core of the substance, Hz;

ν etalon is the resonance frequency of the etalon core, Hz;

ν of the device is the operating frequency of the NMR spectrometer (the frequency at which the resonance conditions for hydrogen nuclei are satisfied for a given B 0, MHz).

For solutions in organic solvents, the chemical shift in the 1H and 13C spectra is measured relative to the tetramethylsilane signal, the position of which is taken as 0 ppm. The chemical shifts are counted in the direction of a weak field (to the left) from the tetramethylsilane signal (delta is the scale of chemical shifts). For aqueous solutions, sodium 2,2-dimethyl-2-silanepentane-5-sulfonate is used as a reference in the 1 H NMR spectra, the chemical shift of the protons of the methyl group of which is 0.015 ppm. For the spectra of 13 C aqueous solutions, dioxane is used as a reference, the chemical shift of which is 67.4 ppm.

When calibrating the 19 F spectra, trifluoroacetic acid or trichlorofluoromethane is used as the primary standard with zero chemical shift; spectra 31 P - 85% solution of phosphoric acid or trimethyl phosphate; spectra 15 N - nitromethane or saturated ammonia solution. In 1 H and 13 C NMR, as a rule, an internal standard is used, which is directly added to the test sample. 15 N, 19 F, and 31 P NMR often use an external standard, which is held separately in a coaxial cylindrical tube or capillary.

When describing NMR spectra, it is necessary to indicate the solvent in which the substance is dissolved and its concentration. Easily mobile liquids are used as solvents, in which hydrogen atoms are replaced by deuterium atoms to reduce the intensity of solvent signals. The deuterated solvent is selected based on the following criteria:

• 1) the solubility of the test compound in it;
• 2) no overlap between the signals of residual protons of the deuterated solvent and the signals of the test compound;
• 3) no interaction between the solvent and the test compound, unless otherwise indicated.

Solvent atoms give signals that are easily identified by their chemical shift and can be used to calibrate the chemical shift axis (secondary standard). The chemical shifts of the residual proton signals of deuterated solvents have the following values ​​(ppm): chloroform, 7.26; benzene, 7.16; water - 4.7; methanol -3.35 and 4.78; dimethyl sulfoxide - 2.50; acetone - 2.05; the position of the signal of water and the protons of the hydroxyl groups of alcohols depends on the pH of the medium and temperature.

For quantitative analysis, solutions must be free of undissolved particles. For some assays, it may be necessary to add an internal standard to compare test and reference intensities. Appropriate standard samples and their concentrations should be specified in the normative documentation. After placing the sample in a test tube and capping, the sample is introduced into the magnet of the NMR spectrometer, the test parameters are set (settings, registration, digitization of the free induction decay signal). The main test parameters given in the regulatory documentation are recorded or stored in a computer.

To prevent spectrum drift over time, a stabilization procedure (deuterium lock) is performed using the deuterium signal induced by deuterated solvents, unless otherwise indicated. The instrument is adjusted to obtain the most optimal resonance conditions and the maximum ratio S/N(shimming).

During the test, it is possible to perform multiple sequences of cycles "impulse - data acquisition - pause" with subsequent summation of individual signals of the decay of free induction and averaging the noise level. The delay time between pulse sequences during which the system of nuclear spins restores its magnetization ( D 1), for quantitative measurements must exceed the longitudinal relaxation time T 1: D 1 ≥ 5 T one . The spectrometer software contains algorithms for determining T one . If the value T 1 is unknown, it is recommended to use the value D 1 = 25 sec.

After carrying out the Fourier transform, the signals in the frequency representation are calibrated to the selected standard and their relative intensity is measured by integration - measuring the ratio of the areas of the resonant signals. In the 13 C spectra, only signals of the same type are integrated. The signal integration accuracy depends on the ratio signal – noise (S/N):

where u(I) is the standard uncertainty of integration.

The number of free induction decay accumulations required to achieve a satisfactory ratio S/ N, should be given in the regulatory documentation.

Along with one-dimensional for analytical purposes, homo- and heteronuclear two-dimensional correlation spectra are used, based on a certain sequence of pulses (COSY, NOESY, ROESY, HSQC, HMBC, HETCOR, CIGAR, INADEQUATE, etc.). In two-dimensional spectra, the interaction between nuclei manifests itself in the form of signals called cross peaks. The position of the cross peaks is determined by the values ​​of the chemical shifts of the two interacting nuclei. Two-dimensional spectra are preferably used to determine the composition of complex mixtures and extracts, because the probability of signal superposition (cross peaks) in two-dimensional spectra is significantly lower than the probability of signal superposition in one-dimensional spectra.

To quickly obtain the spectra of heteronuclei (13 C, 15 N, etc.), methods (HSQC, HMBC) are used that allow one to obtain spectra of other nuclei on 1 H nuclei using the mechanisms of heteronuclear interaction.

The DOSY technique, based on recording the loss of phase coherence of nuclear spins due to translational displacements of molecules under the action of a magnetic field gradient, makes it possible to obtain spectra of individual compounds (spectral separation) in a mixture without their physical separation and to determine the sizes, degrees of aggregation, and molecular weights of molecular objects (molecules , macromolecules, molecular complexes, supramolecular systems).

Areas of use

The variety of structural and analytical information contained in nuclear magnetic resonance spectra makes it possible to use the nuclear magnetic resonance method for qualitative and quantitative analysis. The use of nuclear magnetic resonance spectroscopy in quantitative analysis is based on the direct proportionality of the molar concentration of magnetically active nuclei to the integral intensity of the corresponding absorption signal in the spectrum.

1. Identification of the active substance. Identification of the active substance is carried out by comparing the spectrum of the test sample with the spectrum of a standard sample or with a published reference spectrum. The spectra of standard and test samples should be obtained using the same methods and conditions. The peaks in the compared spectra should coincide in position (deviations of the values δ test and standard samples within ± 0.1 ppm. for nuclear magnetic resonance 1 N and ± 0.5 ppm. for nuclear magnetic resonance 13 C), integrated intensity and multiplicity, the values ​​of which should be given when describing the spectra. In the absence of a standard sample, a pharmacopoeial standard sample can be used, the identity of which is confirmed by independent structural interpretation of the spectral data and alternative methods.

When confirming the authenticity of samples of non-stoichiometric composition (for example, natural polymers of variable composition), the peaks of the test and standard samples are allowed to differ in position and integral intensity of the signals. The spectra to be compared must be similar, i.e. contain the same characteristic regions of the signals, confirming the coincidence of the fragment composition of the test and standard samples.

To establish the authenticity of a mixture of substances (extracts), one-dimensional NMR spectra can be used as a whole, as “fingerprints” of an object, without detailing the values ​​of δ and the multiplicity of individual signals. In the case of using two-dimensional NMR spectroscopy in the description of spectra (spectrum fragments) claimed for authenticity, the values ​​of cross peaks should be given.

1. Identification of foreign matter/residual organic solvents. Identification of impurities/residual organic solvents is carried out similarly to the identification of the active substance, tightening the requirements for sensitivity and digital resolution.
2. Determination of the content of foreign impurities / residual organic solvents in relation to the active substance. The NMR method is a direct absolute method for determining the molar ratio of the active substance and the impurity compound ( n/n impurity):

where S‘ and S‘ impurity - normalized values ​​of the integral intensities of the signals of the active substance and impurity.

Normalization is carried out according to the number of nuclei in the structural fragment, which determine the measured signal.

Mass fraction of impurity / residual organic solvent relative to the active substance ( X pr) is determined by the formula:

M pr is the molecular weight of the impurity;

M is the molecular weight of the active substance;

S‘ pr is the normalized value of the integral intensity of the impurity signal;

S'– normalized value of the integral intensity of the signal of the active substance.

1. Quantitative determination of the content of the substance (active substance, impurity / residual solvent) in the pharmaceutical substance. Absolute content of matter in a pharmaceutical substance, it is determined by the internal standard method, which is chosen as a substance whose signals are close to the signals of the analyte, without overlapping with them. The signal intensities of the analyte and the standard should not differ significantly.

The percentage of the analyte in the test sample in terms of dry matter ( x,% mass) is calculated by the formula:

x,% mass = 100 ∙ ( S‘ /S‘ 0) ∙ (M ∙ a 0 /M 0 ∙ a) ∙ ,

S' is the normalized value of the integral intensity of the signal of the analyte;

S‘ 0 is the normalized value of the integrated signal intensity of the standard;

M is the molecular weight of the analyte;

M 0 – molecular weight;

a- weighing of the test sample;

a 0– weight of the standard substance;

W- moisture contents, %.

The following compounds can be used as standards: maleic acid (2H; 6.60 ppm, M= 116.07), benzyl benzoate (2H; 5.30 ppm, M= 212.25), malonic acid (2H; 3.30 ppm, M= 104.03), succinimide (4H; 2.77 ppm, M= 99.09), acetanilide (3H; 2.12 ppm, M = 135,16), tert-butanol (9H; 1.30 ppm, M = 74,12).

Relative substance content as the proportion of a component in a mixture of components of a pharmaceutical substance is determined by the method of internal normalization. molar ( X mol) and mass ( X mass) component fraction i in a mixture n substances is determined by the formulas:

1. Determination of the molecular weight of proteins and polymers. The molecular weights of proteins and polymers are determined by comparing their mobility with that of reference compounds of known molecular weight using DOSY techniques. Self-diffusion coefficients are measured ( D) of the test and standard samples, build a graph of the dependence of the logarithms of the molecular weights of the standard compounds on the logarithms D. From the graph thus obtained, the unknown molecular weights of the test samples are determined by linear regression. A full description of the DOSY experiment should be given in the regulatory documentation.

NMR spectroscopy of solids

Samples in the solid state are analyzed using specially equipped NMR spectrometers. Certain technical operations (rotation of a powdered sample in a rotor inclined at a magic angle (54.7°) to the magnetic field axis AT 0 , force depairing, polarization transfer from highly excitable nuclei to less polarizable nuclei - cross-polarization) make it possible to obtain high-resolution spectra of organic and inorganic compounds. A full description of the procedure should be given in the regulatory documentation. The main area of ​​application of this type of NMR spectroscopy is the study of polymorphism of solid drugs.

Nuclear magnetic resonance
nuclear magnetic resonance

Nuclear magnetic resonance (NMR) - resonant absorption of electromagnetic waves by atomic nuclei, which occurs when the orientation of the vectors of their own moments of momentum (spins) changes. NMR occurs in samples placed in a strong constant magnetic field, while simultaneously exposing them to a weak alternating electromagnetic field of the radio frequency range (the lines of force of the alternating field must be perpendicular to the lines of force of the constant field). For hydrogen nuclei (protons) in a constant magnetic field with a strength of 10 4 oersted, resonance occurs at a radio wave frequency of 42.58 MHz. For other nuclei in magnetic fields of 103–104 oersted NMR is observed in the frequency range of 1–10 MHz. NMR is widely used in physics, chemistry and biochemistry to study the structure of solids and complex molecules. In medicine, using NMR with a resolution of 0.5–1 mm, a spatial image of the internal organs of a person is obtained.

Let's consider the phenomenon of NMR on the example of the simplest nucleus - hydrogen. The hydrogen nucleus is a proton, which has a certain value of its own mechanical moment of momentum (spin). In accordance with quantum mechanics, the proton spin vector can have only two mutually opposite directions in space, conventionally denoted by the words “up” and “down”. The proton also has a magnetic moment, the direction of the vector of which is rigidly tied to the direction of the spin vector. Therefore, the vector of the magnetic moment of the proton can be directed either “up” or “down”. Thus, the proton can be represented as a microscopic magnet with two possible orientations in space. If you place a proton in an external constant magnetic field, then the energy of the proton in this field will depend on where its magnetic moment is directed. The energy of a proton will be greater if its magnetic moment (and spin) is directed in the direction opposite to the field. Let's denote this energy as E ↓ . If the magnetic moment (spin) of the proton is directed in the same direction as the field, then the energy of the proton, denoted E, will be less (E< E ↓). Пусть протон оказался именно в этом последнем состоянии. Если теперь протону добавить энергию Δ Е = E ↓ − E , то он сможет скачком перейти в состояние с большей энергией, в котором его спин будет направлен против поля. Добавить энергию протону можно, “облучая” его квантами электромагнитных волн с частотой ω, определяемой соотношением ΔЕ = ћω.
Let's move from a single proton to a macroscopic sample of hydrogen containing a large number of protons. The situation will look like this. In the sample, due to the averaging of random orientations of spins, approximately equal numbers of protons, when a constant external magnetic field is applied, will appear relative to this field with spins directed “up” and “down”. Irradiation of a sample with electromagnetic waves with a frequency ω = (E ↓ − E )/ћ will cause a “massive” spin flip (magnetic moments) of protons, as a result of which all protons of the sample will be in a state with spins directed against the field. Such a massive change in the orientation of protons will be accompanied by a sharp (resonant) absorption of quanta (and energy) of the irradiating electromagnetic field. This is NMR. NMR can only be observed in samples with a large number of nuclei (10 16) using special techniques and highly sensitive instruments.