第三章NMR实验技术基础

1、granmr06.doc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（图示）, you will find the superconducting magnet of the NMR spectrometer. The magnet produces the Bo

2、field necessary for the NMR experiments. Immediately within the bore of the magnet are the shim coils for homogenizing the Bo field（匀场）. 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

3、 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（梯度）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

4、microscopy experiments. 图示的顶端是NMR谱仪的超导磁体。它产生NMR实验必需的Bo场。超导磁体的核心是用于匀场的垫片线圈。其中有探头，包括产生用于旋转的B1电磁场的RF线圈。RF线圈同样探测样品的旋转信号。样品由此定位。一些探头中包含一系列的梯度线圈。这些线圈在Bo沿X，Y，Z方向产生梯度场。梯度线圈是被用来产生加强光谱，散射和其他NMR微尺度实验的梯度的。The heart of the spectrometer is the computer. It controls all of the components of the spectrometer. The R

5、F 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 （扩音器）increases the pulses power from milli Watts

6、 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 i

7、nput 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 spe

8、ctra on a video display located on the console and can make hard copies of spectra using a printer. 频谱仪的核心是计算机。它控制着频谱仪的所有组件。被计算机控制的RF组件是RF频率源和脉冲程序。源负责产生需求频率的正弦波，脉冲程序则设置RF脉冲的宽度，某些情况下也设定形状。RF放大器把脉冲能量从百万分之一瓦特放大到几十甚至几百瓦特。计算机同时控制用来设置梯度场形状和振幅的梯度脉冲程序。梯度放大器把梯度脉冲能量提升到可以带动梯度线圈的层次。操作者通过控制终端输入计算机。一些频谱仪也有独立的界面用以

9、执行一些常用的程序。脉冲序列是由控制终端被选择和控制的。操作者可以在屏幕上看到控制台的设置并打印出来。1. 超导磁体MagnetMagnet主要要求：a 高磁场强度,分辨率与B0成正比，而灵敏度与成正比，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10-9c 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 supercondu

10、cting 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 (-273.15C or 0 K) by emersing it in liquid helium. Once current is caused to flow in the

11、 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 th

12、e 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 ni

13、trogen (77.4K) dewar, which acts as a thermal buffer between the room temperature air (293K) and the liquid helium. NMR磁体是整个核磁共振频谱仪中最贵的组件之一。大多数磁体是超导的。超导磁体拥有一块用超导线制作的电磁石。超导材料的性质是通过把它浸入液氦中降到接近绝对零度时它将有接近0的电阻。只有一致保持在液氦中的温度那么开始其中导入的电流将一直运行下去。磁体中超导线的长度典型的有几英里。它被绕成螺线管或线圈。线圈被浸入在保持4.2K的液氦中，液氦则放置在杜瓦瓶中。杜瓦瓶中装的是

14、液氮，77.4K,作为和外界的隔热层。The following image is an actual cut-away view of a superconducting magnet. The magnet is supported by three legs, and the concentric（同中心的） nitrogen and helium dewars are supported by stacks coming out of the top of the magnet. A room temperature bore hole extends through the cent

15、er 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. 下面的图像是超导磁体的横截面图。磁体被同中心的液氮和液氦杜瓦瓶包围。一个室温通道孔延伸到装置的中心。样品管和垫片线圈就位于这个孔中。图中还显示了一个液氮层传感器用

16、来控制液氮层。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 mag

17、net. 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. 磁体从外到内，可以看到在液氮层后存在一个真空区域。真空区域是有几层反射性薄膜组成。其作用是反射热声子，从而减少进入磁体的热量。在液氮池的内侧有另外

18、一个填充着反射性薄膜的真空区域，紧接着就是液氮，可以覆盖超导螺线管或线圈。Taking a closer look at the solenoid it is clear to see the coil and the bore tube extending through the magnet. Field LockIn order to produce a high resolution NMR spectrum of a sample, especially one which requires signal averaging or phase cycling, you need to

19、 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 th

20、e 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.

21、 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

22、 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 f

23、requency without correction. In reality, this frequency would be kept constant by the application of the lock field which offsets the drift.锁场是NMR谱仪中一个单独的部分，主要用来调整NMR谐振频率，监控氘信号的谐振频率并改变Bo场以维持谐振频率稳定。氘信号用来准备样品。窗口中包含了氘谐振频率的图像，用来修正锁场频率的小磁场以及Bo场。锁场频率表现并不是对频率的修正。实际上，该频率压迫保持恒定以修正场的漂移。On most NMR spectromete

24、rs 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 t

25、o the sample to set reference; the spectrometer can use the lock frequency to calculate reference. 大多数NMR谱仪氘锁定有第二种功能。它提供出参考数据。大多数锁场溶剂中氘信号的谐振频率是已知的，因此锁场溶剂的谐振频率的不同和TMS也是已知的。所以TMS不需要对样品设置参数，谱仪可以利用锁场频率计算参数。 Shim CoilsThe purpose of shim coils on a spectrometer is to correct minor spatial inhomogeneities

26、 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（渗透性）, and ferromagnetic materials around the magnet. A shim coil is designed to create a small magnetic field which will

27、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. 垫片线圈的作用是修正B

28、o场的空间不均匀性。不均匀性可能是以下因素造成：磁体设计，探头材料，样品管的厚度变化，样品渗透性，以及磁体周围的铁磁材料。垫片线圈就是产生一个抵消不均匀性的小磁场。而可能存在一维的，二维的等不同形式的偏差，所以垫片线圈也需要能产生多样的场。Shim Coil Functional FormsShim Function Z0 Z, Z2, Z3, Z4, Z5 X, XZ, XZ2, X2Y2, XY, Y, YZ, YZ2XZ3, X2Y2Z, YZ3, XYZ, X3, Y3By passing the appropriate amount of current through each c

29、oil 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 algori

30、thm has the task of finding the best shim value by maximizing the lock signal. 在每个线圈中通过一定量的电流便可以产生均匀的Bo场。 理想的电流设置通过最小化线宽，最大化FID尺寸，或者最大化锁场信号得出。一般谱仪这些都是由计算机控制的，通过最大化锁场信号得到最佳的shim值。2. 探头Sample ProbeThe sample probe is the name given to that part of the spectrometer which accepts the sample, sends RF en

31、ergy 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

32、will be described here. 样品探头是指接受样品，给样品传送RF能量以及检测样品信号的部分。它包括RF线圈，样品旋转器，温度控制电路和梯度线圈，这里先讨论样品旋转器和温度控制电路。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 ex

33、perience 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 magneti

34、c 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 direc

35、tions are averaged out and the NMR line width becomes narrower. 样品旋转器的作用是把样品管可以绕它的轴旋转。净影响是狭窄的光谱宽度。为了得到这个现象，考虑下面的例子，包含样品的圆柱管轴向截面。在非常均匀的Bo场中样品将产生一个狭窄的光谱。当样品绕z轴旋转时，X和Y方向的不均匀性就体现出来导致NMR线宽变窄。Many scientists need to examine properties of their samples as a function of temperature. As a result many instrum

36、ents 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 m

37、aintains the temperature by increasing or decreasing the temperature of the gas passing over the sample. RF Coils 许多科学家需要检验样品与温度的函数关系。因此许多仪器都包含低于或高于室温的样品温度。可以用已加热或冷却过的空气或氮气通过样品来加热或冷却样品。样品温度可以通过包含可以改变通过样品的气体温度的热电偶或电子电路实现。RF coils create the B1 field which rotates the net magnetization in a pulse sequ

38、ence. 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. RF线圈在脉冲序列中产生一个旋转净磁

39、化的B1场。由于他在XY平面的进动也可以验测到横向磁化。大多数NMR的RF线圈都是鞍形的并可以作为B1场的传导器及RF能量的接受器。可以在一个探头中找到一个或多个RF线圈。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 induc

40、tive 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

41、 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 efficient

42、ly 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

43、 be efficiently detecting the signal, your signal-to-noise ratio will be poor. 每个RF线圈都应该是谐振的，因为他们要有效率的在被检验的核子的拉莫尔频率下储存能量。所有的NMR线圈都是由一个传感器件和一系列电容器件组成。谐振频率是由LC电路的电感和电容决定的，对每个特殊的样品都要进行调试。一个RF线圈都有一个带宽和特定的共振频率范围。当在RF线圈中放入一个样品时，样品的导电率和介电常数就影响了谐振频率。如果这个频率和正在研究的核子的谐振频率不一样的话，线圈不会效率的建立B1场或者探测样品信号。旋转90度时其实你并没有

44、真正旋转到90度。进一步说，由于线圈不能有效地探测到信号你所得到的信噪比将非常低。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 dist

45、ribution of rotation angles and you will obtain strange spectra. B1场的RF线圈和Bo场必须垂直。另一个要求是B1场在样品体积上是均匀的。不这样的话你旋转的角度将有偏差并得到奇怪的光谱。Gradient CoilsThe gradient coils produce the gradients in the Bo magnetic field needed for performing gradient enhanced spectroscopy, diffusion measurements, and NMR microsco

46、py. 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 conf

47、iguration, create the desired gradient. Since the vertical bore superconducting magnet is most common, the gradient coil system will be described for this magnet. 梯度线圈在Bo场中产生梯度以提高光谱，散射测量和NMR的显微精度。梯度线圈位于RF探头。并不是所有的探头都有梯度线圈而且并不是所有的NMR谱仪都有必要的硬件来驱动梯度线圈。梯度线圈是在室温下产生需要的梯度。由于垂直钻孔超导磁体是最常见的，梯度线圈就在这种此题上描述。Assu

48、ming 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

49、 Bo field, while the B field at the center of the other coil subtracts from the Bo field. 设想一个标准的磁共振坐标系，Bo在Z方向的梯度可以有一个反亥姆霍兹型线圈实现。两个通以相反电流的线圈将产生一个磁场梯度。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 du

50、e 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. Bo场的X和Y方向的梯度是由一对8字形线圈产生的。X方向的8字形线圈依据通过其电流的方向产生一个Bo在X方向的梯度，Y轴同理。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

51、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，其动态范围为到即（-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.Digi

52、tal Filtering 数字滤波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 hig

53、h 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. 许多新型的谱仪都用过密采样，数字滤波和批量舍去的方法来消除人为的影响。采样过密可以得到更大的光谱和扫描宽度，但产生的大量数据不易于储存。数字过滤淘汰了数据中的高频部分，批量舍去

54、减少了数据设置的数量。下面的流程图总结了三个步骤对结果的影响。Lets 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. 让我们更细致的检验过密采样，数字滤波和批量舍去来看看到底是怎么实施的。Oversampling is the digitization of a time domain signal at a frequency mu

55、ch 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

56、 factor of 10, thus increasing storage requirements and processing time. 过密采样是比需要的采样宽度而收集更过的信号。例如采样因子是10则扫描宽度也将是10倍大。不幸的是存储要求和采样时间也将大量增加。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