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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5647_Библиотеки_им_академика_М_И_Перельмана.pdf
10.11 Types of NMR Spectroscopy 481
• Motion artifacts: Movem ents from breathing, heartbeat or even slight body
movements can introduce artifacts into the NMR signal, making data analysis
challenging.
• Complexity of analysis: The heterogeneous nature of tissues and the overlapping
resonance signals from various metabolites can complicate the interpretation of
spectra, requiring advanced processing and deconvolution techniques.
• High equipment costs: In vivo NMR systems are expensive to maintain and
require strong magnetic fields and advanced computational tools for data acquisition and analysis.
10.11.9 MRI
NMR imaging, more commonly known as MRI, is a noninvasive imaging technique
based on the principles of NMR. It uses strong magnetic fields, radiofrequency
pulses, and gradients to generate detailed images of soft tissues, organs, and other
internal structures within the body. MRI is widely used in clinical settings for
diagnosing and monitoring various medical conditions, particularly in the brain,
spine, muscles, and joints.
10.11.9.1 Principle of MRI
MRI is based on the behavior of hydrogen nuclei (protons) in a strong magnetic field.
Protons, primarily found in H₂O, possess a property called spin, which gives them a
magnetic moment. When placed in a strong external magnetic field, these protons
align either parallel or antiparallel to the field, creating a net magnetization in the
direction of the magnetic field. To generate an image, the following steps occur:
• Magnetization alignment: When the body is placed inside the MRI machine, the
protons align along the direction of the external magnetic field.
• RF pulse application: A short burst of RF energy is applied at a specific
frequency, known as the Larmor frequency, which tips the aligned protons
away from their equilibrium position. This RF pulse excites the protons, causing
them to move out of alignment with the main magnetic field.
• Relaxation: After the RF pulse is turned off, the excited protons begin to return to
their original alignment with the magnetic field. During this process, they release
energy in the form of electromagnetic signals, which are detected by the MRI
machine. This process is described by two relaxation times:
1. T1 relaxation: This describes the recovery of longitudinal magnetization along
the direction of the magnetic field.
2. T2 relaxation:
protons interact with each other and lose coherence.
• Signal detection: The MRI machine detects the emitted signals, which vary
depending on the tissue type and the local environment of the protons. The
signals are then transformed into detailed images of the body’ s internal structures
using a mathematical technique known as Fourier transformation.
This
describes the dephasing of the transverse magnetization as
482 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
• Gradient coils for spatial encoding: To create a three-dimensional image, gradi-
ent magnetic fields are applied along different axes (x, y, z). These gradients alter
the magnetic field strength slightly in specific directions, allowing the MRI
system to localize the origin of the signal and generate a detailed image of the
tissue’s spatial structure.
10.11.9.2 Types of MRI Scans
• T1-weigh ted MRI: Images generated by focusing on T1 relaxation times, which
provide excellent anatomical detail and are useful for visualizing structures like
the brain and spinal cord.
• T2-weigh ted MRI: Images emphasizing T2 relaxation times, which are particu-
larly useful for detecting pathological conditions such as tumors, inflammation,
and fluid accumulation (e.g., in edema or cysts).
• Diffusion-weighted imaging (DWI): A specialized form of MRI that measures the
diffusion of water molecules in tissues. DWI is particularly useful in detecting
early signs of stroke or cancer.
• Functional MRI (fMRI): Measures brain activity by detecting changes in
It is widely used in neuroscience to map the brain function and understand
flow.
blood
how different regions of the brain are activated during specific tasks.
• Magnetic resonance angiography (MRA): A variation of MRI used to image
blood vessels and diagnose vascular diseases such as aneurysms, stenosis, and
blockages.
• MRS: Provides chemical information by measuring the concentration of
metabolites in tissues, often used in cancer research and metabolic studies.
10.11.9.3 Applications of MRI
MRI is used across various medical disciplines for diagnostic purposes and is
particularly valued for its ability to generate high-resolution images without ionizing
radiation. Key applications include:
• Neurology: MRI is crucial for imaging the brain and spinal cord. It is used to
diagnose conditions such as brain tumors, multiple sclerosis, stroke, Alzheimer’s
disease, and epilepsy. fMRI also helps map brain activity and is valuable in
presurgical planning.
• Orthopedics : MRI is
widely used
to image joints, muscles, and bones. It provides
detailed images of soft tissues such as cartilage, tendons, and ligaments, maki ng it
essential for diagnosing sports injuries, arthritis, and musculoskeletal disorders.
• Cardiology: MRI provides detailed images of the heart and blood vessels. It is
used to evaluate heart structure, function, and blood flow, making it important for
diagnosing conditions such as heart disease, congenital heart defects, and
aneurysms.
• Oncology: MRI plays
a vital role in cancer diagnosis and treatment monitoring. It
can differentiate between different types of tissues and tumors, helping in the
early detection of cancers in organs such as the brain, breast, prostate, and liver.
10.11 Types of NMR Spectroscopy 483
• Abdominal imaging: MRI is useful for imaging organs in the abdomen, such as
the liver, kidneys, pancreas, and intestines. It helps detect abnormalities such as
tumors, cysts, and liver diseases.
• Pelvic imaging: MRI is commonly used to image the pelvic region, especially in
women, for diagno
prostate cancer in men.
sing condition
s such as uterine fibroids, ovarian cysts, and
10.11.9.4 Advantages of MRI
• Non-ionizing radiation: Unlike CT scans and X-rays, MRI does not use ionizing
radiation, making it safer for repeated use and for vulnerable populations such as
children and pregnant women.
• High-re solution soft tissue imaging: MRI provides excellent contrast and resolu-
tion for soft tissues, making it superior to other imaging techniques for studying
the brain, muscles, and internal organs.
• Multiplanar imaging: MRI can capture images in multiple planes (axia
coronal),
• Functional imaging: MRI techniques such as fMRI and diffusion-weight
imaging provi
brain activity, blood flow, and tissue metabolism.
providing a comprehensive view of complex anatomical structures.
de functional insights beyond just anatomical details, revealing
l, sagittal,
ed
10.11.9.5 Limitations of MRI
• Cost and availability: MRI is more expensive and less widely available compa red
to other imaging techniques such as CT or ultrasound.
• Longer scan times: MRI scans can take longer to perform, often ranging from
20 min to over an hour, which may be uncomfortable for patients, especially those
who are claustrophobic.
• Motion artifacts: Movement during the scan (such as breathing or involuntary
movements) can cause artifacts, reducing image quality.
• Magnetic field contraindications: Patients with metal implants (e.g., pacemakers,
cochlear implants, or aneurysm clips) may not be eligible for MRI due to the
strong magnetic fields.
10.11.10 Diffusion NMR
Diffusion NMR spectroscopy is a specialized NMR technique used to study the
diffusion of molecules in various media. It provides information about molecular
size, shape, interactions, and dynamics based on how molecules move in a solution
or other environments. The technique exploits the random motion of particles ,
known as Brownian motion, which causes molecules to diffuse through a medium.
This diffusion behavior is influenced by factors such as the medium’s viscosity,
temperature, and molecular size.
484 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
10.11.10.1 Principle of Diffusion NMR
The key principle behind diffusion NMR lies in the fact that molecules in a fluid
exhibit random motion, and this motion can be modulated by their environment. In
diffusion NMR, special magnetic field gradients are applied during the experiment,
allowing for the measurement of how far and how fast the molecules have moved
within the sample over time. This information is translat ed into diffusion
coefficients, which reveal insights into molecular size, shape, and interactions.
10.11.10.2 Steps in Diffusion NMR
• Pulsed field gradients (PFG): The technique applies magnetic field gradients in
short pulses, which cause the spins of the nuclei in different regions of the sample
to experience slightly different magnetic fields.
• Diffusion and dephasing: Molecules that diffuse within the sample move between
regions of varying magnetic field strength during the time between gradient
pulses. As a result, their nuclear spins become dephased, which leads to a
decrease in the intensity of the NMR signal.
• Rephasing and signa l attenuation: After diffusion has occurred, a second gradient
pulse is applied to reverse the dephasing. If a molecule has moved significantly
during the diffusion period, the rephasing will be incomplete, leading to signal
attenuation.
• Measuring diffusion coefficients: The degree of signal
attenuation
is directly
related to how far the molecules have diffused. By measuring this attenuation,
the self-diffusion coefficient (D) of the molecules can be calculated.
10.11.10.3 Applications of Diffusion NMR
Diffusion NMR spectroscopy has a wide range of applications in various fields,
including chemistry, biology, and material science:
• Characterizing molecular size and shape: The diffusion coefficient is inversely
related to the size of the molecule. Larger molecules diffuse more slowly, so
diffusion NMR can be used to estimate molecular sizes and shapes. For example,
it is used to study polymers, proteins, and complex organic molecules in
solutions.
• Molecular interactions: Diffusion NMR
is sensiti
ve to interactions between
molecules. If two or more molecules are interacting (e.g., forming complexes or
aggregates), their combined size will result in slower diffusion. This allows for
the study of molecular binding, aggregation, and self-assembly processes.
• Studying mixtures:
mixtures of molecules with different diffusion behaviors,
In
diffusion NMR can separate signals based on their diffusion coefficients, effectively allowing the analysis of individual components without physically
separating them. This is useful for studying heterogeneous systems such as
emulsions or micelles.
• Membrane permeability: Diffusion NMR can measure how molecules pass
through biological membranes, making it useful for studying drug delivery
systems and understanding the permeability of cell membranes.
10.12 Components of NMR Spectroscopy 485
• Porous materials: In materials science, diffusion NMR is used to p robe the pore
structure of materials such as zeolites or polymers by measuring how molecules
move through their internal structure.
10.11.10.4 Diffusion Ordered Spectroscopy
A commonly used diffusion NMR technique is diffusion ordered spectroscopy
(DOSY). DOSY separates the NMR signals of species in a mixture based on their
diffusion rates. In a DOSY experiment, the NMR spectrum is plotted in two
dimensions: one axis represents chemical shifts, while the other represents diffusion
coefficients. This allows for the identification of different molecular species within
the sample, even if their NMR spectra overlap.
10.11.10.5 Advantages of Diffusion NMR
• Noninvas ive: The technique is nondestructive and can be applied to molecules in
their natural environment, such as in solution or inside biological systems.
• No need for labels: Diffusion NMR does not require any external labeling or
modification of the molecules being studied, unlike other techniques such as
fluorescence or radiolabeling.
• Versatile: It can be used to study a wide range of molecular systems, including
small organic molecules, polymers, prote ins, and even complex biological
mixtures.
10.11.10.6 Limitations of Diffusion NMR
• Sensitivity: Diffusion NMR requires high signal-to-noise ratios, which can be
challenging for samples at low concentrations or with weak signals.
• Longer experiment times: The acquisition of diffusion data can be time-
consuming, especially for complex systems, as multiple gradient pulse sequences
are often needed.
• Complex data interpretation: Analyzing and interpreting diffusion NMR data,
particularly in DOS Y experiments, can be complex, requiring advanced
processing techniques.
10.12 Components of NMR Spectroscopy
NMR spectroscopy is a powerful analytical technique that relies on several essential
components (Fig.
atomic nuclei. Figure
10.3) and instruments to study the nuclear magnetic properties of
10.3 shows the key components of NM R spectroscopy.
10.12.1 The Magnet
The magnet is a critical component of an NMR spectrometer, providing a strong,
stable, and homogeneous magnetic field, typically measured in tesla (T). The
strength of the magnetic field influences the resolution and sensitivity of NMR
486 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
Fig. 10.3 Schematic representation of the components of an NMR spectrophotometer. The figure
depicts the basic components of an NMR spectrophotometer setup. The system includes the
following components: “Magnet”: a strong superconducting magnet (labeled as N and S poles)
that creates a powerful and uniform magnetic field, essential for aligning nuclear spins within the
sample; “Sample in tube”: the sample containing the nuclei of interest is placed in a glass tube inside
the magnet. The sample is exposed to the magnetic field and subjected to radiofrequency pulses for
resonance; “Radiofrequency generator”: this component generates the radiofrequency energy
necessary to perturb the aligned nuclear spins, causing them to resonate; “Detector and amplifier”:
the resonance signals from the nuclei are detected and amplified for better signal processing; “LCD
for display”: the processed data is displayed on a screen, typically showing the NMR spectrum,
where chemical shifts and peaks representing different nuclei, environments can be analyzed. This
setup demonstrates the fundamental components that work together to acquire NMR spectra,
providing critical information on molecular structures
spectra. Superconducting magnets, often used in modern NMR instruments, help
achieve this. In the absence of a magnetic field, all nuclear spin states are well
populated, resulting in no net polarization. An external magnetic field is applied to
achieve preferential population of nuclear energy spin states. Higher magnetic field
strengths lead to greater separation of energy levels and enhanced polarization at
or a c
equilibrium. The magnet may be a powerful permanent magnet
cooled
superc
onduct
ing
electromagnet, both of which align the nuclear spins in the
ryogenically
sample.
10.12.2 RF Oscillator
The RF oscillator generates precisely tuned radiofrequency pulses that are applied to
the sample to excite nuclear spins and induce NMR signals. The frequency of the RF
pulses is determined by the Larmor frequency, which depends on the strength of the
magnetic field and the gyromagnetic ratio of the nuclei under investigation. It
consists of radiofrequency synthesizers and amplifiers, which generate a pulse
sequence containing radiofrequency pulse s with specific frequency, phase,
10.12 Components of NMR Spectroscopy 487
amplitude, shape, and time duration. In some cases, multiple radiofrequency
oscillators are required as certain NMR experiments necessitate the simultaneous
application of radiofrequency pulses at different frequencies.
10.12.3 Sample Holder
The samp le holder, also known as an NMR tube, is a specialized container that holds
the NMR sample. The sample is typically dissolved in a deu terated solvent and
placed in the NMR tube. The NMR tube must be made of nonmagnetic material to
avoid interfering with the magnetic field.
10.12.4 Radiofrequency Receiver
The receiver detects the NMR signals emitted by the sample after the RF pulses. It
amplifies the weak NMR signals and sends them for further processing and data
acquisition. The receiver is equipped with various tuning and matching circuits to
optimize signal reception.
10.12.5 Pulse Programmer
The pulse programmer controls the timing and sequence of RF pulses applied to the
sample. It ensures that the correct pulse sequence is executed to acquire NMR data,
including 1D or 2D spectra.
10.12.6 Gradient Coils (Optional)
In some NMR experiments, gradient coils are used to introduce spatial encoding.
This allows for the acquisition of images in MRI or for determining the spatial
distribution of nuclei within the sample.
10.12.7 Computer and Data Processing Software
A computer is used to control the spectrometer, acquire and process NMR data, and
display the resulting spectra. Specialized NMR software programs are employed for
data analysis, spectral interpretation, and structure elucidation.
488 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
10.12.8 Shimming System
A shimming system is used to fine-tune the homogeneity of the magnetic field.
Magnetic field inhomogeneities can result in line broadening and distorted spectra,
so shimming is crucial for obtaining high-quality data.
10.12.9 Sample Changer (Optional)
For high- throughput NMR applications, sample changers are used to automate the
loading and unloading of samples. This is particularly useful for labs that process
numerous samples.
10.12.10 NMR Probes
NMR probes are specialized components that interact with the sample and generate
RF pulses and receive NMR signals. Different probes are designed for specific
applications, including liquid-state NMR, solid-state NMR, and multinuclear
experiments.
10.13 Working of NMR
NMR spectroscopy is a powerful analytical technique used to study the nuclear
magnetic properties of atomic nuclei in a sample. The NMR experiment begins with
the placement of the sample inside the NMR spectrometer, which has a strong and
homogeneous magnetic field, typically produced by superconducting magnets. In
the absence of an external magnetic field, nuclear spins are randomly oriented. To
initiate the NMR experiment, an RF pulse is applied perpendicular to the magnetic
field, with a specific frequency matching the Larmor frequency of the nuclei being
observed. This resona nce causes nuclei to temporarily transition from lower to
higher energy states. After the RF pulse is turned off, the excited nuclei relax back
to their lower energy state, emitting RF signals known as FIDs. These FIDs are
detected by the spectrometer’s receiver, digitized, and processed using Fourier
transformation to produce an NMR spectrum. The resulting spectrum provides
information about the sample’s composition, structure, and environment, making
NMR a versatile and widely used analytical tool in various scientific disciplines.
10.14 Sample Preparation for NMR Analysis 489
10.14 Sample Preparation for NMR Analysis
Proper sample preparation is critical for obtaining high-quality data in NMR spectroscopy. The proces s involves preparing a solution or solid sample in a way that
maximizes signal intensity, minimizes impurities, and provides a clear and interpretable spectrum.
10.14.1 Choosing a Solvent
• Deuterated solvents: In NMR, the solvent should ideally be deuterated
(containing deuterium,
2
H) to prevent solvent signals from overlapping with the
sample’s signals. Common deuterated solvents include:
– CDCl₃ (deuterated chloroform) for organic samples
– D₂O (deuterium oxide) for water-soluble compounds
– CD₃OD (deuterated methanol) for polar organic compounds
– C₆D₆ (deuterated benzene) for nonpolar organic compoun
• The choice of
solvent depends on the solubility and chemical compatibility with
ds
the sample. The solvent should fully dissolve the sample and not react with it.
10.14.2 Sample Concentration
• Optimal concentration: A typical NMR sample concentration for a standard 1 H
NMR experiment is around 5–50 mg/mL. However, this varies with the type of
NMR experiment and the sensitivity of the nucleus being analyzed.
1
– For
– For
H NMR, lower concentrations (1–5 mg in 0.5 mL solvent) are often
sufficient.
13
C NMR, a higher concentration (5–20 mg in 0.5 mL solvent) is needed
due to the lower natural abundance of
13
C and weaker signal strength.
• Avoiding overconcentration: Overly concentrated samples can lead to signal
broadening, excessive line-width, and loss of resolution due to viscosity effects.
10.14.3 Sample Volume
• Standard NMR tube: Typically, a 5 mm NMR tube is used. The ideal volume of
the sample solution is 0.5–0.7 mL, which is enough to fully cover the NMR
detection coil. Fill ing the tube too much or too little can lead to suboptimal signal
detection.
490 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
10.14.4 Sample Purity
• High purity required: Impurities can lead to unwanted signals that obscure the
analysis of the sample. Ensure the sample is as pure as possible by removing any
solvents, by-products, or contaminants before preparation.
• Filtering: Filtering the sample through a fine filter (e.g., 0.45 μm) helps remove
dust or particulates that can interfere with the homogeneity of the solution and
lead to baseline distortions in the spectrum.
10.14.5 Degassing (Optional)
Some NMR experiments, especially those sensitive to relaxation properties or when
working at low concentrations, benefit from removing dissolved gases (such as
oxygen) that can interfere with the signal. This is done by sparging the sample
with an inert gas (e.g., nitrogen or argon) or by freeze–pump–thaw cycles.
10.14.6 NMR Tubes
• Clean and dry tubes: Ensure the NMR tubes are thoroughly cleaned and dried
before use to prevent contamination.
• Tube quality: High-quality NMR tubes with minimal imperfections should be
used to avoid artifacts or distortions in the spectrum. Tubes with outer diameters
of 5 mm are the standard, but some experiments may use different diameters (e.g.,
3 mm tubes for limited sample quantities).
10.14.7 Internal Standards (Optional)
• Chemical shift reference: Internal standards such as TMS are often added for
calibration of chemical shifts, especially in
gives a sharp signal at 0 ppm, which can be used to reference all other shifts in the
spectrum.
• Locking solvent: For locking purposes, deuterated solvents also provide a refer-
ence for stabilizing the magnetic field.
1
H and
13
C NMR experiments. TMS
10.14.8 Solubility and Homogeneity
Ensure the sample is fully dissolved and homogeneous. Partially dissolved samples
can lead to broad or split signals. If the sample is not soluble in deuterated solvents, it
may be dissolved in a non-deuterated solvent with a small amount of deuterated
solvent added for field locking.