[核磁共振波谱学讲义]第三章—NMR实验技术基础(1NMR仪器)知识讲解

第三章 NMR 实验技术基础
1 NMR 仪器
如图，现代超导核磁谱仪的主要组成部分包括：
1. 超导磁体Magnet 包括Field Lock ，Shim Coils
2. 探头Probe 内有RF Coils ，Gradient Coils
3. 脉冲编程器及射频放大器
4. 接收器
5. 数据采集及处理计算机
At the top of the schematic representation, you will find the superconducting magnet of the NMR spectrometer. The magnet produces the Bo field necessary for the NMR experiments. Immediately within the bore of the magnet are the shim coils for homogenizing the Bo field.
Within the shim coils is the probe. The probe contains the RF coils for producing the B1 magnetic field necessary to rotate the spins. The RF coil also detects the signal from the spins within the sample. The sample is positioned within the RF coil of the probe. Some probes also contain a set of gradient coils. These coils produce a gradient in Bo along the X, Y , or Z axis. Gradient coils are used for for gradient enhanced spectroscopy, diffusion, and NMR microscopy experiments. The heart of the spectrometer is the computer. It controls all of the components of the
spectrometer. The RF components under control of the computer are the RF frequency source and pulse programmer. The source produces a sine wave of the desired frequency. The pulse programmer sets the width, and in some cases the shape, of the RF pulses. The RF amplifier increases the pulses power from milli Watts to tens or hundreds of Watts. The computer also
controls the gradient pulse programmer which sets the shape and amplitude of gradient fields. The gradient amplifier increases the power of the gradient pulses to a level sufficient to drive the gradient coils. The operator of the spectrometer gives input to the computer through a console terminal with a mouse and keyboard. Some spectrometers also have a separate small interface for carrying out some of the more routine procedures on the spectrometer. A pulse sequence is
selected and customized from the console terminal. The operator can see spectra on a video display located on the console and can make hard copies of spectra using a printer.
1. 超导磁体Magnet
Magnet主要要求：
a 高磁场强度,分辨率与B0成正比，而灵敏度与B032成正比，故750MHz较600MHz的分辨
率提高25%,而灵敏度提高40%
b 高均匀性,目前可达10-9
c 高稳定性
The NMR magnet is one of the most
expensive components of the nuclear
magnetic resonance spectrometer system.
Most magnets are of the superconducting
type. A superconducting magnet has an
electromagnet made of superconducting
wire. Superconducting wire has a
resistance approximately equal to zero
when it is cooled to a temperature close to
absolute zero (-273.15 C or 0 K) by
emersing it in liquid helium. Once current
is caused to flow in the coil it will
continue to flow for as long as the coil is
kept at liquid helium temperatures.
(Some losses do occur over time due to the
infinitesimally small resistance of the coil. These losses are on the order of a ppm of the main magnetic field per year.) The length of superconducting wire in the magnet is typically several miles. This wire is wound into a multi -turn solenoid or coil. The coil of wire and cryroshim coils are kept at a temperature of 4.2K by immersing it in liquid helium. The coil and liquid helium are kept in a large dewar. This dewar is typically surrounded by a liquid nitrogen (77.4K) dewar, which acts as a thermal buffer between the room temperature air (293K) and the liquid helium.
The following image is an actual cut -away view of a
superconducting magnet. The magnet is supported by three
legs, and the concentric nitrogen and helium dewars are
supported by stacks coming out of the top of the magnet. A
room temperature bore hole extends through the center of the
assembly. The sample probe and shim coils are located within
this bore hole. Also depicted in this picture is the liquid
nitrogen level sensor, an electronic assembly for monitoring
the liquid nitrogen level.
Going from the outside of the magnet to the inside, we
see a vacuum region followed by a liquid nitrogen reservoir.
The vacuum region is filled with several layers of a reflective
mylar film. The function of the mylar is to reflect thermal photons, and thus diminish heat from entering the magnet. Within the inside wall of the liquid nitrogen reservoir, we see another
vacuum filled with some reflective mylar. The liquid helium reservoir comes next. This reservoir houses the superconducting solenoid or coil of wire.
Taking a closer look at the solenoid it is clear to see the coil and the bore tube extending through the magnet.
Field Lock
In order to produce a high resolution
NMR spectrum of a sample, especially one
which requires signal averaging or phase
cycling, you need to have a temporally
constant and spatially homogeneous magnetic
field. Consistency of the Bo field over time
will be discussed here; homogeneity will be
discussed in the next section of this chapter.
The field strength might vary over time due to
aging of the magnet, movement of metal objects near the magnet, and temperature fluctuations. Here is an example of a one line NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. The field lock can compensate for these variations.
The field lock is a separate NMR
spectrometer within your spectrometer. This
spectrometer is typically tuned to the
deuterium NMR resonance frequency. It
constantly monitors the resonance frequency
of the deuterium signal and makes minor
changes in the Bo magnetic field to keep the
resonance frequency constant. The deuterium
signal comes from the deuterium solvent used
to prepare the sample. The animation window contains plots of the deuterium resonance lock frequency, the small additional magnetic field used to correct the lock frequency, and the resultant Bo field as a function of time while the magnetic field is drifting. The lock frequency plot displays the frequency without correction. In reality, this frequency would be kept constant by the application of the lock field which offsets the drift.
On most NMR spectrometers the deuterium lock serves a second function. It provides the reference. The resonance frequency of the deuterium signal in many lock solvents is well known. Therefore the difference in resonance frequency of the lock solvent and TMS is also known. As a consequence, TMS does not need to be added to the sample to set reference; the spectrometer can use the lock frequency to calculate reference.
Shim Coils
The purpose of shim coils on a spectrometer is to correct minor spatial inhomogeneities in the Bo magnetic field. These inhomogeneities could be caused by the magnet design, materials in the probe, variations in the thickness of the sample tube, sample permeability, and ferromagnetic materials around the magnet. A shim coil is designed to create a small magnetic field which will oppose and cancel out an inhomogeneity in the Bo magnetic field. Because these variations may exist in a variety of functional forms (linear, parabolic, etc.), shim coils are needed which can create a variety of opposing fields. Some of the functional forms are listed in the table below.
Shim Coil Functional Forms
Shim Function
Z0
Z, Z2, Z3, Z4, Z5
X, XZ, XZ2, X2Y2, XY , Y , YZ, YZ2
XZ3, X2Y2Z, YZ3, XYZ, X3, Y3
By passing the appropriate amount of current through
each coil a homogeneous Bo magnetic field can be
achieved. The optimum shim current settings are found by
either minimizing the linewidth, maximizing the size of the
FID, or maximizing the signal from the field lock. On most
spectrometers, the shim coils are controllable by the
computer. A computer algorithm has the task of finding the
best shim value by maximizing the lock signal.
2. 探头Sample Probe
The sample probe is the name given to that part of the spectrometer which accepts the sample, sends RF energy into the sample, and detects the signal emanating from the sample. It contains the RF coil, sample spinner, temperature controlling circuitry, and gradient coils. The RF coil and gradient coils will be described in the next two sections. The sample spinner and temperature controlling circuitry will be described here.
The purpose of the sample spinner is to rotate the NMR
sample tube about its axis. In doing so, each spin in the sample
located at a given position along the Z axis and radius from the Z
axis, will experience the average magnetic field in the circle
defined by this Z and radius. The net effect is a narrower spectral
linewidth. To appreciate this phenomenon, consider the following examples. In picture an axial cross section of a cylindrical tube containing sample. In a very homogeneous Bo magnetic field this sample will yield a narrow spectrum. In a more inhomogeneous field the sample will yield a broader spectrum due to the presence of lines from the parts of the sample experiencing different Bo magnetic fields. When the sample is spun about its z -axis, inhomogeneities in the X and Y directions are averaged out and the NMR line width becomes narrower.
Many scientists need to examine properties of their samples as a function of temperature. As a result many instruments have the ability to maintain the temperature of the sample above and below room temperature. Air or nitrogen which has been warmed or cooled is passed over the sample to heat or cool the sample. The temperature at the sample is monitored with the aid of a thermocouple and electronic circuitry maintains the temperature by increasing or decreasing the temperature of the gas passing over the sample.
RF Coils
RF coils create the B1 field which rotates the
net magnetization in a pulse sequence. They also
detect the transverse magnetization as it precesses in
the XY plane. Most RF coils on NMR spectrometers
are of the saddle coil design and act as the
transmitter of the B1 field and receiver of RF energy
from the sample. You may find one or more RF coils
in a probe.
Each of these RF coils must resonate, that is
they must efficiently store energy, at the Larmor
frequency of the nucleus being examined with the
NMR spectrometer. All NMR coils are composed of an inductor, or inductive elements, and a set of capacitive elements. The resonant frequency, , of an RF coil is determined by the inductance (L) and capacitance (C) of the inductor capacitor circuit. RF coils used in NMR spectrometers need to be tuned for the specific sample being studied. An RF coil has a bandwidth or specific range of frequencies at which it resonates. When you place a sample in an RF coil, the conductivity and dielectric constant of the sample affect the resonance frequency. If this frequency is different from the resonance frequency of the nucleus you are studying, the coil will not efficiently set up the B1 field nor efficiently detect the signal from the sample. You will be rotating the net magnetization by an angle less than 90 degrees when you think you are rotating by 90 degrees. This will produce less transverse magnetization and less signal. Furthermore, because the coil will not be efficiently detecting the signal, your signal -to -noise ratio will be poor.
The B1 field of an RF coil must be perpendicular to the Bo magnetic field. Another
requirement of an RF coil in an NMR spectrometer is that the B1 field needs to be homogeneous over the volume of your sample. If it is not, you will be rotating spins by a distribution of rotation angles and you will obtain strange spectra.
Gradient Coils
The gradient coils produce the gradients in the Bo magnetic field needed for performing gradient enhanced spectroscopy, diffusion measurements, and NMR microscopy. The gradient coils are located inside the RF probe. Not all probes have gradient coils, and not all NMR spectrometers have the hardware necessary to drive these coils.The gradient coils are room temperature coils (i.e. do not require cooling with cryogens to operate) which, because of their configuration, create the desired gradient. Since the vertical bore superconducting magnet is most common, the gradient coil system will be described for this magnet.
Assuming the standard magnetic resonance coordinate
system, a gradient in Bo in the Z direction is achieved with an
antihelmholtz type of coil. Current in the two coils flow in
opposite directions creating a magnetic field gradient between
the two coils. The B field at the center of one coil adds to the
Bo field, while the B field at the center of the other coil
subtracts from the Bo field.
The X and Y gradients in the Bo field are created by a
pair of figure -8 coils. The X axis figure -8 coils create a
gradient in Bo in the X direction due to the direction of the
current through the coils. The Y axis figure -8 coils provides
a similar gradient in Bo along the Y axis.
3. 脉冲编程器及射频放大器
包括频率综合器，放大器及有关的电子器件。

通常一台谱仪有若干通道，分别工作在不同的核的共振频率上，在实验中可以同时施加作用于不同核的射频脉冲。

脉冲编程器控制脉冲的时序，长度，幅度，相位甚至形状。

4. 接收器
包括前置放大器，相敏检波器，模数转换器
由于核磁信号很弱，前置放大器总是尽量靠近探头，其性能决定了仪器能达到的信噪比
相敏检波器从射频信号中检出低频
的核磁信号（相当于旋转坐标系中的信
号），并完成正交检波
模数转换器或称ADC 完成模拟信
号的数字化，一般谱仪考虑到转换速度
和动态范围，配置的为16bit ADC ，其动
态范围为-28到218
-即（-32,768 to
32,767），因此一方面送到ADC 的信号
不能超出这个范围，否则要失真出现截
断效应；另一方面信号也不能太小，若小于0.5bit，则无法被确定地记录，此时噪声确定是否记录，因此需要长时间累加，而且由于量化噪声还会产生严重的基线畸变.
Digital Filtering
Many newer spectrometers employ a combination of oversampling, digital filtering, and decimation to eliminate the wrap around artifact. Oversampling creates a larger spectral or sweep width, but generates too much data to be conveniently stored. Digital filtering eliminates the high frequency components from the data, and decimation reduces the size of the data set. The following flowchart summarizes the effects of the three steps by showing the result of performing an FT after each step.
Let’s examine oversampling, digital filtering, and decimation in more detail to see how this combination of steps can be used to reduce the wrap around problem.
Oversampling is the Array digitization of a time
domain signal at a
frequency much greater
than necessary to record the
desired spectral width. For
example, if the sampling
frequency, fs, is increased
by a factor of 10, the sweep
width will be 10 times
greater, thus eliminating
wraparound. Unfortunately
digitizing at 10 times the
speed also increases the
amount of raw data by a factor of 10, thus increasing storage requirements and processing time.
Filtering is the removal of a select band of frequencies from a signal. For an example of filtering, consider the following frequency domain signal. Frequencies above fo could be removed from this frequency domain signal by multipling the signal by this rectangular function. In NMR, this step would be equivalent to taking a large sweep width spectrum and setting to zero intensity those spectral frequencies which are farther than some distance from the center of the spectrum.
Digital filtering is the removal of these frequencies using the time domain signal. Recall from Chapter 5 that if two functions are multiplied in one domain (i.e. frequency), we must convolve the FT of the two functions together in the other domain (i.e. time). To filter out frequencies above fo from the time domain signal, the signal must be convolved with the Fourier transform of the rectangular function, a sinc function. This process eliminates frequencies greater than fo from the time domain signal. Fourier transforming the resultant time domain signal yields a frequency domain signal without the higher frequencies. In NMR, this step will remove spectral components with frequencies greater than +fo and less than -fo.
Decimation is the elimination of data points from a data set. A decimation ratio of 4/5 means
that 4 out of every 5 data points are deleted, or every fifth data point is saved. Decimating the
digitally filtered data above, followed by a Fourier transform, will reduce the data set by a factor of five.
High speed digitizers, capable of digitizing at 2 Mhz, and dedicated high speed integrated circuits, capable of performing the convolution on the time domain data as it is being recorded, are used to realize this procedure.
5. 数据采集及处理计算机
一般包括谱仪内部的采样计算机和外部的主计算机，前者控制实验中整台谱仪各个部件的各种操作，后者则提供人－机接口，及各种软件完成数据处理。

第三章NMR实验技术基础。