NMR試驗(yàn)技術(shù)基礎(chǔ)

1、granmr06.doc如圖，現(xiàn)代超導(dǎo)核磁譜儀的主要組成部分包括：1. 超導(dǎo)磁體 Magnet 包括 Field Lock ， Shim Coils2. 探頭 Probe 內(nèi)有 RF Coils ， Gradient Coils3. 脈沖編程器及射頻放大器4. 接收器5. 數(shù)據(jù)采集及處理計(jì)算機(jī)At the top of the schematic representation （圖示） , you will find the superconducting magnet of the NMR spectrometer. The magnet produces the Bo field nece

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

3、 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 micro

4、scopy experiments.圖示的頂端是 NMR 譜儀的超導(dǎo)磁體。它產(chǎn)生 NMR 實(shí)驗(yàn)必需的 Bo 場。超導(dǎo)磁體的核 心是用于勻場的墊片線圈。其中有探頭，包括產(chǎn)生用于旋轉(zhuǎn)的 B1 電磁場的 RF 線圈。 RF 線圈同樣探測樣品的旋轉(zhuǎn)信號(hào)。樣品由此定位。一些探頭中包含一系列的梯度線圈。這些 線圈在 Bo 沿 X，Y，Z 方向產(chǎn)生梯度場。 梯度線圈是被用來產(chǎn)生加強(qiáng)光譜， 散射和其他 NMR 微尺度實(shí)驗(yàn)的梯度的。The heart of the spectrometer is the computer. It controls all of the components of the spe

5、ctrometer. 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 （擴(kuò) 音器）increases the pulses power

6、 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 spe

7、ctrometer gives input to the computer through a console terminal with a mouse and keyboard. Some 第三章 NMR 實(shí)驗(yàn)技術(shù)基礎(chǔ)1 NMR 儀器spectrometers also have a separate small interface for carrying out some of the more routine procedures on the spectrometer. A pulse sequence is而靈敏度與 B03 2成正比，故750MHz 較600MHz 的分辨gra

8、nmr06.docselected 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.頻譜儀的核心是計(jì)算機(jī)。它控制著頻譜儀的所有組件。被計(jì)算機(jī)控制的RF 組件是 RF頻率源和脈沖程序。源負(fù)責(zé)產(chǎn)生需求頻率的正弦波，脈沖程序則設(shè)置 RF 脈沖的寬度，某些 情況下也設(shè)定形狀。 RF 放大器把脈沖能量從百萬分之一瓦特放大到幾十

9、甚至幾百瓦特。計(jì) 算機(jī)同時(shí)控制用來設(shè)置梯度場形狀和振幅的梯度脈沖程序。梯度放大器把梯度脈沖能量提 升到可以帶動(dòng)梯度線圈的層次。操作者通過控制終端輸入計(jì)算機(jī)。一些頻譜儀也有獨(dú)立的 界面用以執(zhí)行一些常用的程序。脈沖序列是由控制終端被選擇和控制的。操作者可以在屏 幕上看到控制臺(tái)的設(shè)置并打印出來。1. 超導(dǎo)磁體 MagnetMagnet 主要要求： a 高磁場強(qiáng)度 ,分辨率與 B0 成正比，率提高 25%,而靈敏度提高 40% b 高均勻性 ,目前可達(dá) 10-9 c 高穩(wěn)定性granmr06.docThe NMR magnet is one of the most expensive componen

10、ts 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 z

11、ero (-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 lossesdo occur over time due to the infinitesimally small resistance of the coil. These losses are on the o

12、rder 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 ofwire and cryroshim coils are kept at a temperature of 4.2K by immersing it in liquid helium. The coil and

13、 liquid helium are kept in a large dewar. This dewar is typically surrounded by a liquidnitrogen (77.4K) dewar, which acts as a thermal buffer between the room temperature air (293K) and the liquid helium.NMR 磁體是整個(gè)核磁共振頻譜儀中最貴的組件之一。大多數(shù)磁體是超導(dǎo)的。超導(dǎo)磁體 擁有一塊用超導(dǎo)線制作的電磁石。超導(dǎo)材料的性質(zhì)是通過把它浸入液氦中降到接近絕對零 度時(shí)它將有接近 0 的電阻。只

14、有一致保持在液氦中的溫度那么開始其中導(dǎo)入的電流將一直 運(yùn)行下去。磁體中超導(dǎo)線的長度典型的有幾英里。它被繞成螺線管或線圈。線圈被浸入在 保持 4.2K 的液氦中，液氦則放置在杜瓦瓶中。杜瓦瓶中裝的是液氮，77.4K, 作為和外界的隔熱層。granmr06.docThe 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 ar

15、e 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 monitor

16、ing the liquid nitrogen level.下面的圖像是超導(dǎo)磁體的橫截面圖。磁體被同中心的 液氮和液氦杜瓦瓶包圍。一個(gè)室溫通道孔延伸到裝置的中 心。樣品管和墊片線圈就位于這個(gè)孔中。圖中還顯示了一個(gè)液氮層傳感器用來控制液氮層。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

17、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 su

18、perconducting solenoid or coil of wire.磁體從外到內(nèi)，可以看到在液氮層后存在一個(gè)真空區(qū)域。真空區(qū)域是有幾層反射性薄 膜組成。其作用是反射熱聲子，從而減少進(jìn)入磁體的熱量。在液氮池的內(nèi)側(cè)有另外一個(gè)填 充著反射性薄膜的真空區(qū)域，緊接著就是液氮，可以覆蓋超導(dǎo)螺線管或線圈。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

19、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 th

20、is chapter. The field strength might vary over time due to aging of the magnet, movement of metal objectsnear 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.Thefield lo

21、ck can compensate for these variations.granmr06.docThe 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

22、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

23、 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.鎖場是 NMR 譜儀中一個(gè)單獨(dú)的部分，主要用來調(diào)整 NMR 諧振頻率，監(jiān)控氘

24、信號(hào)的諧 振頻率并改變 Bo 場以維持諧振頻率穩(wěn)定。 氘信號(hào)用來準(zhǔn)備樣品。 窗口中包含了氘諧振頻率 的圖像，用來修正鎖場頻率的小磁場以及 Bo 場。鎖場頻率表現(xiàn)并不是對頻率的修正。實(shí)際 上，該頻率壓迫保持恒定以修正場的漂移。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. There

25、fore 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.大多數(shù) NMR 譜儀氘鎖定有第二種功能。 它提供出參考數(shù)據(jù)。 大多數(shù)鎖場溶劑中氘信號(hào) 的諧振頻率是已知的，因此鎖場溶劑的諧振頻率的不同

26、和 TMS 也是已知的。所以 TMS 不 需要對樣品設(shè)置參數(shù)，譜儀可以利用鎖場頻率計(jì)算參數(shù)。Shim CoilsThe 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

27、, sample permeability (滲透性) , 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.Becausethese variations may exist in a variety of functional forms (linear, parabolic, etc.

28、), shim coils are needed which can create a variety of opposing fields. Some of the functional forms are listed in the table below.墊片線圈的作用是修正 Bo 場的空間不均勻性。 不均勻性可能是以下因素造成： 磁體設(shè) 計(jì)，探頭材料，樣品管的厚度變化，樣品滲透性，以及磁體周圍的鐵磁材料。墊片線圈就 是產(chǎn)生一個(gè)抵消不均勻性的小磁場。而可能存在一維的，二維的等不同形式的偏差，所以 墊片線圈也需要能產(chǎn)生多樣的場。granmr06.docShim Coil Functiona

29、l FormsShim FunctionZ0Z, Z2, Z3, Z4, Z5X, XZ, XZ2, X2Y2, XY, Y, YZ, YZ2XZ3, X2Y2Z, YZ3, XYZ, X3, Y3By 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, maximizin

30、g 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.在每個(gè)線圈中通過一定量的電流便可以產(chǎn)生均勻的Bo 場。 理想的電流設(shè)置通過最小化線寬，最大化 FID尺寸，或者最大化鎖場信號(hào)得出

31、。一般譜儀這些都是由計(jì)算機(jī)控制的，通過最大化鎖場信 號(hào)得到最佳的 shim 值。granmr06.doc2. 探頭 Sample ProbeThe 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

32、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.樣品探頭是指接受樣品，給樣品傳送 RF 能量以及檢測樣品信號(hào)的部分。它包括RF 線圈，樣品旋轉(zhuǎn)器，溫度控制電路和梯度線圈，這里先討論樣品旋轉(zhuǎn)器和溫度控制電路。7B1 場的傳導(dǎo)器及 RF 能量的接受器。gr

33、anmr06.docThe 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 ef

34、fect 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

35、 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.樣品旋轉(zhuǎn)器的作用是把樣品管可以繞它的軸旋轉(zhuǎn)。凈影響是狹

36、窄的光譜寬度。為了得 到這個(gè)現(xiàn)象，考慮下面的例子，包含樣品的圓柱管軸向截面。在非常均勻的 Bo 場中樣品將 產(chǎn)生一個(gè)狹窄的光譜。 當(dāng)樣品繞 z軸旋轉(zhuǎn)時(shí)， X 和 Y 方向的不均勻性就體現(xiàn)出來導(dǎo)致 NMR 線寬變窄。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

37、 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

38、 passing over the sample.RF Coils許多科學(xué)家需要檢驗(yàn)樣品與溫度的函數(shù)關(guān)系。因此許多儀器都包含低于或高于室溫的樣品溫度。可以用已加熱或冷卻過的空氣或氮?dú)馔ㄟ^樣品來加熱或冷卻樣品。樣品溫度可 以通過包含可以改變通過樣品的氣體溫度的熱電偶或電子電路實(shí)現(xiàn)。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 pl

39、ane. 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 線圈在脈沖序列中產(chǎn)生一個(gè)旋轉(zhuǎn)凈磁化的B1 場。由于他在 XY 平面的進(jìn)動(dòng)也可以驗(yàn)測到橫向磁化。大多數(shù) NMR 的 RF 線圈都是鞍形的并可以作為 可以在一個(gè)探頭中找到一個(gè)或多個(gè) RF 線圈。E

40、ach 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 coi

41、l 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 samplegranmr06.docbeing studied. An RF coil has a bandwidth or specific range of frequencies at which it resonates. When you place a sampl

42、e 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. Yo

43、u 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 poo

44、r.每個(gè) RF 線圈都應(yīng)該是諧振的， 因?yàn)樗麄円行实脑诒粰z驗(yàn)的核子的拉莫爾頻率下儲(chǔ) 存能量。所有的 NMR 線圈都是由一個(gè)傳感器件和一系列電容器件組成。諧振頻率是由 LC 電路的電感和電容決定的， 對每個(gè)特殊的樣品都要進(jìn)行調(diào)試。 一個(gè) RF 線圈都有一個(gè)帶寬和 特定的共振頻率范圍。 當(dāng)在 RF 線圈中放入一個(gè)樣品時(shí)， 樣品的導(dǎo)電率和介電常數(shù)就影響了 諧振頻率。如果這個(gè)頻率和正在研究的核子的諧振頻率不一樣的話，線圈不會(huì)效率的建立 B1 場或者探測樣品信號(hào)。旋轉(zhuǎn) 90度時(shí)其實(shí)你并沒有真正旋轉(zhuǎn)到90 度。進(jìn)一步說，由于線圈不能有效地探測到信號(hào)你所得到的信噪比將非常低。The B1 field o

45、f 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

46、 spectra.B1場的 RF線圈和 Bo場必須垂直。另一個(gè)要求是 B1 場在樣品體積上是均勻的。不這 樣的話你旋轉(zhuǎn)的角度將有偏差并得到奇怪的光譜。Gradient CoilsThe 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 pr

47、obe. 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 v

48、ertical bore superconducting magnet is most common, the gradient coil system will be described for this magnet.梯度線圈在 Bo 場中產(chǎn)生梯度以提高光譜，散射測量和 NMR 的顯微精度。梯度線圈位 于 RF 探頭。 并不是所有的探頭都有梯度線圈而且并不是所有的 NMR 譜儀都有必要的硬件 來驅(qū)動(dòng)梯度線圈。梯度線圈是在室溫下產(chǎn)生需要的梯度。由于垂直鉆孔超導(dǎo)磁體是最常見 的，梯度線圈就在這種此題上描述。Assuming the standard magnetic resonance coo

49、rdinatesystem, 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 cente

50、r of the other coil subtracts from the Bo field.設(shè)想一個(gè)標(biāo)準(zhǔn)的磁共振坐標(biāo)系， Bo 在 Z 方向的梯度可 以有一個(gè)反亥姆霍茲型線圈實(shí)現(xiàn)。兩個(gè)通以相反電流的線 圈將產(chǎn)生一個(gè)磁場梯度。The X and Y gradients in the Bo field are created by a pair of figure-8 coils. The X axisgranmr06.docfigure-8 coils create a gradient in Bo in the X direction due to the direction of the

51、 current through the coils. The Y axis figure-8 coils provides a similar gradient in Bo along the Y axis.Bo 場的 X 和 Y 方向的梯度是由一對 8字形線圈產(chǎn)生的。 X 方向的 8 字形線圈依據(jù)通過 其電流的方向產(chǎn)生一個(gè) Bo 在 X 方向的梯度， Y 軸同理。3. 脈沖編程器及射頻放大器 包括頻率綜合器，放大器及有關(guān)的電子器件。通常一臺(tái)譜儀有若干通道，分別工作在 不同的核的共振頻率上，在實(shí)驗(yàn)中可以同時(shí)施加作用于不同核的射頻脈沖。脈沖編程器控 制脈沖的時(shí)序，長度，幅度，相位甚至形狀。4.

52、 接收器 包括前置放大器，相敏檢波器，模數(shù)轉(zhuǎn)換器 由于核磁信號(hào)很弱，前置放大器總是盡量靠近探頭，其性能決定了儀器能達(dá)到的信噪比相敏檢波器從射頻信號(hào)中檢出低頻 的核磁信號(hào)(相當(dāng)于旋轉(zhuǎn)坐標(biāo)系中的信 號(hào))，并完成正交檢波模數(shù)轉(zhuǎn)換器或稱 ADC 完成模擬信 號(hào)的數(shù)字化，一般譜儀考慮到轉(zhuǎn)換速度 和動(dòng)態(tài)范圍， 配置的為 16bit ADC ，其動(dòng) 態(tài)范圍為 28 到 28 1即( -32,768 to 32,767)，因此一方面送到 ADC 的信號(hào) 不能超出這個(gè)范圍，否則要失真出現(xiàn)截 斷效應(yīng)；另一方面信號(hào)也不能太小，若 小于 0.5bit ，則無法被確定地記錄，此時(shí)噪聲確定是否記錄，因此需要長時(shí)間累加，而

53、且由 于量化噪聲還會(huì)產(chǎn)生嚴(yán)重的基線畸變Digital Filtering 數(shù)字濾波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 filte

54、ring 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.許多新型的譜儀都用過密采樣，數(shù)字濾波和批量舍去的方法來消除人為的影響。采樣 過密可以得到更大的光譜和掃描寬度，但產(chǎn)生的大量數(shù)據(jù)不易于

55、儲(chǔ)存。數(shù)字過濾淘汰了數(shù) 據(jù)中的高頻部分，批量舍去減少了數(shù)據(jù)設(shè)置的數(shù)量。下面的流程圖總結(jié)了三個(gè)步驟對結(jié)果 的影響。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.讓我們更細(xì)致的檢驗(yàn)過密采樣，數(shù)字濾波和批量舍去來看看到底是怎么實(shí)施的。10Oversampling is the digitization of a time doma

56、in 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

57、thegranmr06.docamount of raw data by a factor of 10, thus increasing storage requirements and processing time. 過密采樣是比需要的采樣寬度而收集更過的信號(hào)。 例如采樣因子是 10 則掃描寬度也將 是 10 倍大。不幸的是存儲(chǔ)要求和采樣時(shí)間也將大量增加。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 beremoved from this frequency domain signal by multipling the signal by this rectangular function. In NMR, this step would be equivalent to taki