Name: nmr peak shape and magnetic field shimming
Uploaded: 2024-04-04T10:51:41.000Z
Duration: 1 min 14 s
Channel: JoVE

NMRスペクトルのローレンツピークは、その位置、振幅、および半値での全幅によって定義されます。
完全に均質な磁場で適切に調製されたサンプルのピーク幅は、スピン-スピン緩和時間のみによって支配されます。
しかし、印加された磁場B0は、サンプルとプローブの存在によって不均一になります。その結果、ピークが広がり、側波帯が無関係になり、分離能が低下します。
磁場の不均一性は、シミングと呼ばれるプロセスによって修正されます。
シミング中、電流はプローブを囲む一連のシムコイルを通過し、サンプル近傍のB0を増強または反対する小さな磁場を生成します。
これらのシムコイルの磁場は、サンプル全体で最も均一な磁場が得られるように操作されます。
シミングは、最適化されたパルスシーケンスパラメータと適切なサンプル調製とともに、良好なピーク形状、高いS/N比、および最大の分解能を保証します。

nmr 分光計: 分解能と誤差補正

NMR Spectrometers: Resolution and Error Correction

サンプル内の磁性核が共鳴を達成して緩和を受けると、NMR で検出される信号はほぼ指数関数的な自由誘導減衰になります。指数関数的減衰をフーリエ変換すると、周波数領域でローレンツ ピークが取得します。 NMR スペクトルはのローレンツ ピークは、その振幅、半値全幅、および位置によって定義され、ピーク幅はスピン-スピン緩和時間のみによって決まります。しかし、実際の実験では、サンプルとプローブの存在によって印加された磁場が不均一になり、その結果、ピークが広がり、無関係な側波帯が生じ、分解能が低下します。

これらの不均一性は、スペクトルはが記録される前に、シミングと呼ばれるプロセスによって補正されます。一連のシム コイルがプローブを取り囲み、そこに流れる電流に応じて小さな磁場を示す。これらの場は、サンプル付近の B0 を強化したり、B0 に対抗したりする可能性があります。シミングには、コイル磁場を操作してサンプル全体に最も均一な磁場を取得し、不均一性を修正することが含まれます。シミング、最適化されたパルスシーケンスパラメータ、および適切なサンプル前処理により、良好なピーク形状、信号対雑音比を高め、最大の分解能を保証します。

NMR 分光計: 分解能と誤差補正

試料中の磁核が共鳴して緩和を受けると、NMRで検出される信号は、ほぼ指数関数的な自由誘導となります。

nmr-spectrometers-resolution-and-error-correction

Education

JoVE Core

Analytical Chemistry

分析化学

Unit 1

ユニット1

Principles of Nuclear Magnetic Resonance

核磁気共鳴の原理

KEY TERMS AND CONCEPTS

主要な用語と概念

nmr peak shape and magnetic field shimming

NMR peak shape depends on the signal created after magnetic nuclei resonate and relax. The detected free induction decay is approximately exponential. When that decay is Fourier transformed, it becomes a Lorentzian peak in the frequency domain.

A Lorentzian peak in an NMR spectrum is described by its amplitude, full width at half maximum, and position. Its width is governed by the spin-spin relaxation time alone. In real experiments, the sample and probe can make the magnetic field inhomogeneous. This can broaden peaks, add side-bands, and reduce resolution.

These field problems are corrected before spectra are recorded by shimming. Shim coils surround the probe and produce small magnetic fields based on the current applied to them. Their fields can either enhance or oppose B0 near the sample. By adjusting the coil fields, shimming makes the magnetic field more uniform across the sample.

Good shimming works best with optimized pulse sequence parameters and proper sample preparation. Together, these steps help produce good peak shape, low signal-to-noise ratio, and maximum resolution in NMR spectra.

NMR Peak Shape and Magnetic Field Shimming

nmr-peak-shape-and-magnetic-field-shimming

nuclear magnetic resonance (nmr): overview

NMR Spectra and MRI Imaging

Nuclear magnetic resonance, or NMR, is a phenomenon that certain nuclei can show when they absorb radio frequency energy under the right conditions. Scientists use it in molecular spectroscopy and in medical imaging. In both cases, the sample or the person being studied is placed in a magnetic field and exposed to radio frequency energy. In NMR spectroscopy, the result is a spectrum. This spectrum plots the absorption frequencies of the sample against peak intensity. The pattern gives detailed...

atomic nuclei: nuclear spin

Nuclear Spin and NMR-Active Nuclei

Atomic nuclei have nuclear spin, which comes from the spins of their protons and neutrons. These particle spins are intrinsic angular momentum, and each electron, proton, and neutron has a spin value of 1/2. In nuclei, proton spins pair with other protons, and neutron spins pair with other neutrons. They do not pair with each other, so the final nuclear spin depends on which nucleons are left unpaired. Even numbers of protons do not add to the nuclear spin, and even numbers of neutrons do not...

atomic nuclei: nuclear magnetic moment

Nuclear Spin States and Magnetic Alignment

Atomic nuclei can act like tiny magnets when they have a nonzero spin. Because they are positively charged and rotating, they generate a magnetic field and a magnetic moment. In the absence of an external magnetic field, this magnetic moment points in random directions. When an external magnetic field, B0, is applied, the nuclear magnetic moments can line up with the field or against it. These two directions create 2I + 1 orientations. A hydrogen nucleus is a proton, and it has spin 1/2, so it...

atomic nuclei: nuclear spin state overview

atomic-nuclei-nuclear-spin-state-overview

Zeeman Splitting in Nuclear Spin States

NMR-active nuclei have spin states that describe the orientation of their nuclear magnetic moments. Without a magnetic field, these moments point in random directions, and the spin states are degenerate, meaning they have the same energy. When an external magnetic field is applied, the nucleus can only take on 2I + 1 orientations, where I is the nuclear spin. A proton with spin 1/2 has two possible orientations. A quadrupolar nucleus with spin 1 has three possible orientations. The...

atomic nuclei: nuclear spin state population distribution

atomic-nuclei-nuclear-spin-state-population-distribution

Nuclear Spin Populations and NMR Signal

Nuclear spin populations help explain why NMR signals appear. In a magnetic field, nuclei can occupy a lower energy spin-up state or a higher energy spin-down state. Near absolute zero, most nuclei stay in the lower energy state. As temperature rises, thermal collisions spread the spins more evenly between the two states. The balance between the spin −1/2 and spin +1/2 populations is described by the Boltzmann distribution equation. In this equation, ΔE is the energy difference, k is the...

atomic nuclei: larmor precession frequency

atomic-nuclei-larmor-precession-frequency

Magnetic Field and Nuclear Precession

Magnetic field strength controls nuclear precession in NMR. A spinning nucleus has a magnetic moment, and that moment turns around an external magnetic field directed along the z-axis. This motion is called Larmor precession, and its angular frequency is the Larmor frequency. The Larmor frequency is directly proportional to the applied magnetic field, B0. The proportionality constant is the gyromagnetic ratio, gamma. This value is fixed for each nucleus because every nucleus has its own ratio...

atomic nuclei: magnetic resonance

Nuclear Spins and Resonance in a Magnetic Field

Nuclear spins can line up in a magnetic field and create nuclear magnetic resonance. In the lower energy state, slightly more spins are aligned than in the higher energy state. That small difference gives the nuclei a net magnetization along the z axis.

atomic nuclei: nuclear relaxation processes

atomic-nuclei-nuclear-relaxation-processes

Nuclear Spin Alignment and Relaxation

Nuclear spin alignment and relaxation describe how nuclear spins respond to a magnetic field and then return to equilibrium. Without an external magnetic field, nuclear spin states are degenerate, meaning they have the same energy, and the spins are randomly oriented. When a magnetic field is applied, the spins begin to precess, or wobble, and orient themselves either along the field in a lower energy state or against it in a higher energy state. At equilibrium, a slight excess of spins...

atomic nuclei: types of nuclear relaxation

atomic-nuclei-types-of-nuclear-relaxation

NMR Relaxation Paths in Atomic Nuclei

Nuclear relaxation in atomic nuclei helps restore the balance between energy levels in NMR systems. It occurs through two main paths: spin-lattice relaxation and spin-spin relaxation. Both are first-order exponential decay processes. Spin-lattice relaxation is also called longitudinal relaxation. In this process, excited spins give energy to the surrounding lattice as they move back to the lower energy level. Magnetic dipolar interactions are an important part of this mechanism. A common...

nmr spectrometers: overview

How NMR Spectrometers Record Signals

NMR spectrometers record signals from samples that contain NMR-active nuclei. The system uses a strong magnet, a radiofrequency transmitter, and a detector connected to a computer console. These parts work together to measure and display the spectrum of the sample. Early NMR instruments were continuous-wave spectrometers. In a frequency-sweep method, the magnetic field stays fixed while the rf signal is swept. In a field-sweep method, the radiofrequency signal stays fixed while the magnetic...

nmr spectrometers: radiofrequency pulses and pulse sequences

nmr-spectrometers-radiofrequency-pulses-and-pulse-sequences

NMR Pulse Angles and Signal Collection

NMR pulse angles and signal collection explain how radiofrequency energy starts a measurement. A pulse is a short burst of radio waves spread across a range of frequencies. It excites the nuclei in the sample at the same time. When a radiofrequency pulse is applied along the x-axis, nuclei absorb energy that matches their Larmor frequencies and reach resonance. The net magnetization vector moves away from the z-axis and toward the transverse plane. This rotation angle is called the flip angle.

nmr spectrometers: resolution and error correction

NMR peak shape depends on the signal created after magnetic nuclei resonate and relax. The detected free induction decay is approximately exponential. When that decay is Fourier transformed, it becomes a Lorentzian peak in the frequency domain. A Lorentzian peak in an NMR spectrum is described by its amplitude, full width at half maximum, and position. Its width is governed by the spin-spin relaxation time alone. In real experiments, the sample and probe can make the magnetic field...

diamagnetic shielding of nuclei: local diamagnetic current

diamagnetic-shielding-of-nuclei-local-diamagnetic-current

How Electron Density Shields Nuclei in NMR

Electron density around a nucleus affects how strongly it is shielded in NMR spectroscopy. When an applied magnetic field acts on a molecule, the electrons begin to circulate. This creates a local diamagnetic current within the molecule. The circulating sigma-bonding electrons produce a local magnetic field, B local, that opposes the applied field, B0. The nucleus then feels an effective magnetic field, which is the difference between the applied field and the local field. This effect is...

key terms and concepts

Learn how NMR peak shape forms, why field inhomogeneity broadens signals, and how shimming improves resolution with better magnetic field uniformity.

NMR Peak Shape and Magnetic Field Shimming | JoVE K12

JoVE Core 分析化学 は、複雑な化学原理をアニメーションで視覚化することで、SI 単位から NMR 分光法を含むトピックをよりわかりやすくします。実験を撮影した実写動画では、これらの概念が世界中の研究所でどのように応用されているかを取り上げています。

NMR Peak Shape and Magnetic Field Shimming

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