Nuclear Magnetic Resonance (NMR)

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I would like you to write me 3 things in one world document:

1st, I want you to write me the history of the Nuclear Magnetic Resonance (NMR), Moreover, I would like you to write me everything related to the history of NMR. Moreover, write me what it the goal of Nuclear Magnetic Resonance.

2nd, I want you to write for me some of the methods that we need to use in the NMR.

3rd, I would like you to answer the question on the "Magnetic resonance techniques file". you will need data. However, I just want you to answer the questions more generally. The file called "12.5 Manual" has the sections 10.1 to 10.9 that we need to look at when the file "Magnetic resonance techniques" maintained a section.

I am looking for 9 - 10 pages.

Formal result should be in the form of a paper for publication in the American Journal of Physics (AJP). List all sources that you used as references for this work. The references should have complete details so that if the reader of your paper wants to find them, he/she should be able to. Book references should have title, authors, edition, publisher, year and page numbers. Journal articles should have title, authors, Journal name, volume, year and page numbers. Web site references should be complete and correct. These references must also be cited at appropriate places in your paper. No plagiarism. Show me a good quality work. you have enough time to finish all of that easily.

NOTE: you have to follow what I asked for, if you do not meet the requirement then I will not accept it.

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NMR and ESR Continuous Wave Spectrometer CWS 12-50 operating and experimental manual Page 2 Page 3 TEL_Atomic Incorporated PO BOX 924 ● Jackson ● MI 49204 www.telatomic.com 1-800-622-2866 Operating and experimental manual NMR and ESR Continous Wave Spectrometer Model CWS 12-50 Version 1.0 Updated: 04.19.2006 File: CWS 12_50 manual v1.0 Page 4 Page 5 Page 6 TABLE OF CONTENTS 1 2 INTRODUCTION ...........................................................................................9 SPECTROMETER MAIN FEATURES .........................................................11 2.1 Console ................................................................................................11 2.2 Spectrometer control and data acquisition............................................11 2.3 Data processing....................................................................................11 3 INSTALLATION ...........................................................................................13 3.1 Shipment check ....................................................................................13 3.2 Spectrometer location and environmental requirements ......................13 3.3 Electrical requirements .........................................................................14 3.3.1 Computer considerations...............................................................15 3.3.2 Using program with LCD monitor...................................................15 3.3.3 Using computer USB port..............................................................15 3.3.4 Software installation ......................................................................16 4 HARDWARE CONNECTION.......................................................................17 4.1 Unit connections ...................................................................................17 4.2 Configuration for NMR experiments .....................................................18 4.3 Configuration for ESR experiments ......................................................19 5 SHIPPING ITEMS........................................................................................21 5.1 Picture tour ...........................................................................................21 5.2 Itemized Shipping List ..........................................................................23 6 SPECTROMETER BLOCK DIAGRAM ........................................................25 7 SPECTROMETER SPECIFICATIONS ........................................................27 8 SOFTWARE DESCRIPTION .......................................................................29 8.1 Setup and Data Acquisition Page .........................................................29 8.2 Data Processing ...................................................................................34 9 MISCELLANEOUS ......................................................................................39 9.1 Sample preparation and positioning .....................................................39 9.2 Changing configuration file ...................................................................41 10 EXPERIMENTS .......................................................................................43 10.1 Continuous wave NMR experiment in rubber .......................................43 10.2 Examples of other NMR spectra...........................................................47 10.2.1 Acrylic............................................................................................47 10.2.2 Delrin .............................................................................................48 10.2.3 Glycerin .........................................................................................49 10.2.4 Fluoroboric acid (HBF4) .................................................................50 10.2.5 Teflon ............................................................................................51 10.3 CW ESR in TCNQ ................................................................................53 10.4 ESR in other samples ...........................................................................55 10.4.1 DPPH ............................................................................................55 10.5 Nuclear magnetogyric ratio measurement with CW NMR.....................57 10.5.1 Magnetogyric ratio of protons (1H nuclei) ......................................59 10.5.2 Magnetogyric ratio of 19F nuclei.....................................................61 10.5.3 Field/frequency factor ....................................................................62 10.6 Angle dependence of 1H NMR spectra in gypsum monocrystal ...........64 Page 8 10.7 Determining Earth’s magnetic field with ESR experiment.....................70 10.8 Measurements of a static magnetic field with a Tesla meter (Smart Magnetic Sensor) ............................................................................................74 10.8.1 Angle dependence of the readings of the Tesla meter. .............76 10.8.2 Measuring magnetic field remanence in an electromagnet. ..........78 10.8.3 Helmholtz coils ..............................................................................79 10.9 2nd modulation of magnetic field and line broadening ...........................82 Page 9 1 INTRODUCTION Historically Electron Spin Resonance1 (ESR) and Nuclear Magnetic Resonance2 (NMR) were discovered in a series of simple experiments in which a magnetic field was swept over the sample containing uncompensated magnetic dipoles (electron and nuclear spins) exposed to electromagnetic radiation. Absorption of this radiation was detected at a certain resonant field, as predicted by earlier theories. Because the source of electromagnetic radiation (electromagnetic wave) was operating in a continuous, uninterrupted way, this kind of technique was named continuous wave (CW) to distinguish it from pulsed techniques which apply short bursts of powerful pulses to excite spins polarized by a constant magnetic field. As technology and experimental techniques developed, continuous wave methods were later replaced by pulsed NMR and partially by pulsed ESR. However, continuous wave spectroscopy is perfectly suited for teaching purposes. Students witness the same original experiment - product of human ingenuity from the forties. This more simple technique which is not encumbered by elaborate instrumentation or sophisticated mathematics aids the student in learning the physical laws governing NMR. TEL-Atomic Inc. introduces a desktop, state of the art continuous wave spectrometer the CWS 12-50 that allows for demonstrations of NMR and ESR experiments through a two-in-one integrated autodyne probehead. Although designated for teaching, the CWS 12-50 hardware and software provides a convenient means for NMR spectroscopy experiments on 1H and 19F nuclei at a magnetic field of 320 mT and ESR spectroscopy at a field of 20 mT and frequency of 50 MHz. This manual consists of two parts: • operating part: Chapters 1-10 • experimental part: Chapter 11 The purpose of the operating part is to provide the user with comprehensive information about the spectrometer: • Installation • Hardware • Control Program 1 E. K. Zavoisky, Supplement to thesis, Kazan State University, Russsia, October 1944 2 E. M. Purcell, H. C. Torrey and R. V. Pound, “Resonance Absorption by Nuclear Magnetic Moments in Solids”, Physical Review, 69, 37-38 (1946). F. Bloch, W. W. Hansen and M. E. Packard, “Nuclear Induction”, Physical Review 69, 127 (1946) Page 10 All experiments have been performed using an off-the-shelf CWS12-50 NMR/ESR spectrometer and only originally acquired data are presented. The list of experiments include: • Acquiring NMR and ESR spectra from factory provided samples • Determination of magnetogyric ratio for 1H and 19F nuclei • Measurement of Earth’s magnetic field • Observation of NMR line split in gypsum monocrystal due to its rotation • Mapping electromagnet and Helmholtz coil with Hall effect Tesla meter This list is not closed. More experiments will be developed and included later. Page 11 2 SPECTROMETER MAIN FEATURES 2.1 Console • • • • • • • • • Modes of operation: -1H NMR- electromagnet 3,200 Gs/14.0 MHz -19F NMR- electromagnet 3,200 Gs/13.9 MHz -ESR- Helmholtz coils 20 Gs/50 MHz Magnetic field sweep and frequency sweep in NMR mode Magnetic field sweep in ESR mode Integrated NMR/ESR probe with high-sensitive autodyne generator Synchronous phase detection Adjustment of 2nd Modulation Field 5 mm sample holders Phase Lock Loop for stable frequency generation 10-bit signal digitizer 2.2 Spectrometer control and data acquisition • • • • • • • • • • Automatic recognition of electromagnet or Helmholtz coils and switching for NMR or ESR mode Multiple displays of current and previous experiments Accumulation to improve signal-to-noise signal Saving data in binary file to reduce occupied space Saving experimental details in a setup file Loading setups for designed experiments Alarm sounds for the status of experiment (start of sweep, end of sweep, end of accumulation) Vertical (amplitude and field/frequency) and horizontal (amplitude) measurement cursors Determination of line width Displayed status of hardware and of experiment 2.3 Data processing • • • • • • • View acquired binary data Store binary data in a text format (for processing with other programs like Excel) 1st integration of first derivative signal to obtain absorption (spectra) 2nd integration to obtain value of area under absorption line Calculate spectra 2nd and 4th Moment Calculate spectra line width Extract experimental details from old experiments and save in a setup file (to repeat experiments under same conditions) • Page 12 Print spectra and calculated parameters Page 13 3 INSTALLATION The installation of the CWS spectrometer requires only a #2 flat screwdriver. Please read this chapter before attempting to connect the spectrometer. 3.1 Shipment check Check the contents of the shipment against the enclosed Itemized Shipping List. Inspect all parts for any signs of damage that may have occurred during shipment. Immediately report any visible damage or incomplete delivery to your distributor. 3.2 Spectrometer location and environmental requirements The spectrometer should be placed on a solid table or bench, preferably wooden. Try to eliminate the presence of iron beams or any other ferrous components in the electromagnet proximity that can disturb its homogeneity. Avoid a vibrating environment: elevators, frequently used doors, etc. A clean, dust free, low humidity environment is recommended. bWarning: The magnet is protected by a process known as “bluing”. This is the same process by which gun barrels are protected. Therefore handle the magnet only by the handles since water or skin oils can cause corrosion to occur. Do not expose the magnet to water or high humidity. Store the magnet in a low humidity environment.!!! At least twice a year use gun oil or WD-40 to wipe the surface of the magnet. It is important to keep oil from getting into the magnet’s coils and the probehead therefore DO NOT SPRAY OIL OR WD-40 DIRECTLY ONTO THE MAGNET, rather saturate a piece of soft cloth or patch with the oil or WD-40 and wipe the magnet’s surface thoroughly with this. Page 14 3.3 Electrical requirements Before you turn on the console, make sure that: • The line voltage selector label matches the voltage mains supply. The label is located on the top right corner of control unit cover. 115 V label for USA market is shown. • Ensure that the AC power source meets the requirements specified Figure 1. 115V label for USA market. in Table 1. bNote: 115/220V voltage selector is located inside the control unit and should be set by authorized personnel only! Verify that the power cable is not damaged, and that the power source outlet provides a protective earth ground contact. The working fuse is located above the power cable receptacle on the CWS 12-50 back panel. Nominal AC Line Power AC Line Power Fuse Setting Voltage [V] Frequency [Hz] [A] 115 V 100 – 122 45-100 2.0 220 V 200-230 45-100 1.0 Table 1. CWS 12-50 power requirements and fuses. Page 15 Computer requirements and software installation 3.3.1 Computer considerations For proper operation, data storage and display, the spectrometer CWS control program requires an IBM PC AT VGA or compatible computer with 1GHz clock. The program and factory created files occupy less than 1MB of hard drive total space. Average binary data files with spectra first derivative and experimental parameters need only about 3 kb of space, but expand when converted into text files. 3.3.2 Using program with LCD monitor The control program supports displays with 4:3 aspect ratio of 1024x768 pixels resolution without distortion. To work with LCD (laptop) change display resolution to 1024x768 pixels and DPI settings to Normal size (96dpi): -Control Panel -Settings -Screen Resolution: 1024x768pix Advanced: DPI setting: Normal Size (96dpi). Users of Wide Screens: If display driver does not support this resolution or you do not see the whole program window, find the closest screen resolution that displays the whole window on the monitor with the lowest possible distortion. 3.3.3 Using computer USB port If no COM port is available use a USB/COM port adapter. In the spectrometer control program remember to select the proper COM port number. -Tools -Spectrometer -Communication port Select one of COM1-COM4 ports Recommended and tested USB/COM port adapter vendor/model: Vendor: www.sewelldirect.com Model: USB to Serial Adapter part #: SW-1301; price $17.95 Page 16 3.3.4 Software installation To install the software copy the CWS file from the provided compact disk into the root directory of “c:” hard drive of your computer. Keep the directory structure as factory created. For more information about program files structure refer to Table 2. Type *.exe *.ini *.cfg *.dcw *.txt *.wav *.* Folder c:\cws c:\cws c:\cws\setup c:\cws\acq c:\cws\proc c:\cws\audio c:\cws\temp Description control program initialization setup acquired data data in text format audio file temporary Default cws.exe cws.ini standard.cfg Table 2.CWS program files and files location. After copying, check files/directories and make sure that in attributes the readonly box is unchecked. • • • • right click on cws folder left click on Properties left click on General uncheck Read-only box Page 17 4 HARDWARE CONNECTION Arrange the electromagnet, electronic unit and the computer on the desk, according to space availability and convenience. Remember that the keyboard and monitor are the most used devices. As samples will be frequently replaced and repositioned in the probehead keep the electromagnet and Helmholtz coil close to your hand and eyes. 4.1 Unit connections • Connect the computer and electronic unit power supply cords to the same power line to avoid unwanted ground currents. • Connect the console to the probehead. • Depending on the mode, connect the electromagnet for NMR experiments (see Chapter 4.2) or to the Helmholtz coils for ESR experiments (see Chapter 4.3), using the provided cables. bNote: Before switching from the electromagnet to the Helmholtz coils, exit the control program and turn off the console. Page 18 4.2 Configuration for NMR experiments Page 19 4.3 Configuration for ESR experiments Page 20 Page 21 5 SHIPPING ITEMS 5.1 Picture tour Page 22 Page 23 5.2 Itemized Shipping List 1 2 3 4 5 5a 5b 5c 5d 6 Item Control unit Probehead Electromagnet Helmholtz Coils Cables Shipped Received RS 232 Electromagnet Probehead Power cord Samples glycerin rubber acrylic delrin HBF4+H2O Teflon TCNQ DPPH 7 8 9 10 Allen hex socket wrench Fuse CD with program and manual Manual Page 24 Page 25 6 SPECTROMETER BLOCK DIAGRAM Electromagnet Power Supply Sample Magnetic Field Source Electromagnet - 3,400Gs Helmholtz Coils - 19 Gs B0 Amplifier 12-50 MHz NMR-14MHz ESR-50MHz Autodyne Generator Phase Lock Loop Amplifier 38 Hz Programmable Frequency Divider Personal Computer 38 Hz Amplifier 0-255 [a.u.] RS 232 Generator of 2nd Modulation 38Hz/0.05-2.0Gs A/D Converter 10 bits µP controller Page 26 Page 27 7 SPECTROMETER SPECIFICATIONS Mode Operational Frequency Frequency Stability Electromagnet - magnetic field - maximum current - coil - gap - pole diameter - homogeneity - field stability Helmholtz Coils - magnetic field - gap - coils diameter - homogeneity Modulation Field - frequency - amplitude Sweep of magnetic field (NMR and ESR) - range - time Sweep of frequency (only NMR) - range - time RF Probehead - solenoid coil dimensions - mode Receiver - gain - detection - phase adjustment - signal filter - DC offset converter A/D Converter - resolution - number of samples Weight and dimensions WxDxH - electronic unit - probehead - Helmholtz coils (ESR) - electromagnet (NMR) Power Consumption Communication Port Computer Required Software NMR (1H, 19F), ESR NMR 1H-14.0MHz, NMR 19F-13.2MHz, ESR50MHz ≤ 1 PPM/ 1h 320 mT 0.7A 2,000 turns 10.5 mm 50 mm ≤ 10-4/ sample volume ≤ 10 µT/ 1hr 195 µT 15 mm 70 mm 10-5/sample volume 38 Hz 0.1-20 µT 0.5 mT- 10.0 mT 0.5 min – 30 min 20 Hz – 400kHz 0.5 min – 30 min ID= 5.8 mm; L= 12 mm Automatically tuned for NMR or ESR 0-48 dB (2 dB step) phase-sensitive 0-360o, step 1.5o sweep controlled automatic 10 bit Min 512 per sweep 3.5 kg, 350x135x85 mm 0.4 kg, 35x210x70 mm 0.5kg, 50x80x110 mm 10.5 kg, 175x100x160 cm 110V/220 V; 50/60 Hz; 40 W two way RS 232C IBM PC, min 750 MHz, VGA color or compatible MS Windows operated Page 28 Page 29 8 SOFTWARE DESCRIPTION Control program for the CWS 12-50 spectrometer consists of two pages: • Setup and Acquisition - for experiment preparation and data acquisition • Processing - for acquired data processing 8.1 Setup and Data Acquisition Page Figure 2. Setup and Acquisition page INFORMATION BAR ƒ ƒ ƒ ƒ Hardware name: CW NMR/ESR Spectrometer Unit Serial #: Page Name: SETUP and ACQUISITION Setup Name: (default is standard.cfg) Page 30 MAIN TOOL BAR File • • • • • • Save Data As Saves acquired data in a file if name was not declared earlier in Acquisition/Store in File box Open Setup Loads setup file with saved experimental parameters Save Setup Saves setup file with experimental parameters with current name Save Setup As Saves setup file under new name. Name standard.cfg is reserved for CWS program use. This file is loaded during program initialization along with cws.ini. About Information about control program Exit Terminates control program Spectrometer • Communication Port Depending on availability of serial port chose between COM1, COM2, COM3, COM4 for communication between your PC and console. • Connect Connects computer to NMR/ESR console Page 31 Tools • Accumulation Shows trace of accumulated signal (white color) • File Location Defines location of different files (See Table 2 for factory created structure. Users have the freedom to create their own file names and structure) • Audio Defines location of audio files (use any *wav format sounds) • Data Processing Links to DATA PROCESSING page • Service Only for service people use. Locked by password! AUXILIARY TOOL BAR • V Switches to vertical cursor. Returns signal amplitude and sweep values on cursor • H Switches to horizontal cursor. Returns current cursor position • Pass Display: Displays first derivative of spectrum from experimental passages ƒ 1, current pass only (yellow) ƒ 2, current pass and one before ƒ 3, current pass and two before ƒ 4, current pass and three before ƒ 5, current pass and four before ƒ off, no signal displayed (white) • Acc Shows the trace of an accumulated signal • DB Determines acquired spectrum line width and returns this value in the box next to cursor coordinates • Proc Links to DATA PROCESSING Page CONTROL WINDOW Mode • NMR 1H Parameter setup for Hydrogen nuclei NMR • NMR 19F Parameter setup for Fluorine nuclei NMR • NMR 1H&19F Parameter setup to acquire signals from Hydrogen and Fluorine Nuclei Page 32 • ESR Parameter setup for Electron Spin Resonance mode Detection • B0 Magnetic field magnitude [Gs] • F Autodyne generator frequency [kHz] • Gain Total gain of the receiver (0-255 [a.u.]) • Phase Relative phase of the detector reference signal [deg] Modulation • Field Sweep Select for magnetic field sweep • Frequency Sweep Select for frequency sweep • 2nd Mod Amplit Amplitude of second modulation • Sweep Time Sweep time of magnetic field or frequency Acquisition • Loop Program operates in a loop: accumulates and displays a signal, but does not store in a file • Single Program performs given number of accumulations and saves data in the chosen file name • Acc Number of accumulations • Store in File Type the name of file in which you want to store acquired data • Comment Type your comment, sample name, etc • Start Starts data acquisition • Abort Stops data acquisition and returns spectrometer to initial state • Hold/Continue Stops acquisition allowing for adjustment of certain parameters. Press again to Continue acquisition Page 33 STATUS BAR • Acc Shows the number of the current sweep in accumulation experiment • Last Data Save in: Displays file name of last saved data • Spectrometer: Shows status of the spectrometer and prompts an action: ƒ Not Connected/Connected ƒ Experiment in progress- please wait ƒ Experiment Aborted- please wait. ƒ Experiment Aborted- please wait ƒ Experiment on Hold • Magnetic Field Source: Program automatically detects what source is connected to the console ƒ Electromagnet ƒ Helmholtz coil • DB Displays line width calculated from 1st derivative (min-max) Page 34 8.2 Data Processing Data Processing page allows one to: • • • • • • • • Load a binary data file with first derivative of the absorption signal from the disk and display on the data display Export original and unchanged signal amplitudes as text for further processing with independent software: Origin, Matlab, Mathematica, Excel, etc. Correction of line base of first derivative Integration of first derivative to obtain absorption Calculating integral value of absorption within limits, Calculating 2nd moment, 4th moment and line width of first derivative and absorption lines (for NMR mode only) Saving processed data Printing data Figure 3. Data Processing page with absorption tools. Page 35 MAIN TOOL BAR File • • • • Open Loads binary data file with extension *.dcw and displays first derivative of the spectrum on the data window with experimental parameters Export ASCII Exports binary file as a text file with extension *.txt Save Setup As Extracts and saves experimental setup to file with extension *.cfg Exit Terminates the program TOOL BAR 1 • Open File Loads binary data file with extension *.dcw and displays on the data • Export ASCII Exports binary file as a text file with extension *.txt • 1st Derivative Loads processing window with signal 1st derivative • Absorption Integrates first derivative signal and loads processing window with absorption curve (spectrum) • 1st derivative base line correction cursor. Any change of base line position is instantly transferred to processing window. ƒ Arrow Down Shifts base line down ƒ Arrow Up Shifts base line up ƒ Arrow Diagonal Up Shifts base line right limit down ƒ Arrow Diagonal Up Shifts base line right limit down • Setup & Acquisition Link to Setup & Acquisition Page DATA DISPLAY Displays loaded binary data file with 1st derivative of the resonance signal EXPERIMENTAL SETUP Displays complete list of experimental parameters used in the experiment Page 36 TOOL BAR 2 • HZ Horizontal zoom ƒ Left click on Zoom button ƒ Left click on left limit and release ƒ Drag courser to right limit ƒ Left click to expand marked area • • UZ Left click to unzoom VertExp Vertical expansion to full screen • SDB Returns numerical value of the line width of 1st derivative Definition: distance between line maximum and minimum Figure 4. Calculating line width from signal of 1st derivative. • HDB Returns numerical value of the line width of absorption Definition: line width at line half-height Page 37 Figure 5. Calculating line width from absorption curve. M2 Tools to calculate 2nd and 4th moment of 1st derivative and absorption (see Figure 6, note that the identical tools are used to calculate moments of 1st derivative) • LC Left cursor position for M2 limit • MC Middle cursor position for M2 • RC Right cursor position for M2 limit Figure 6. Calculating 2nd (M2) and 4th (M4) moments of absorption line. M2L and M2R are 2nd moments of left and right part of the absorption line, respectively, M2=1/2(M2L+M2R). nd th Page 38 Numerical values of 2 and 4 moments are calculated on the fly and displayed in processing parameters window on the right. 2nd Integration tools Tools to calculate integral value of absorption line (AI=absolute integral) within given limits • LC Left cursor position for spectrum integration • RC Right cursor position for spectrum integration Figure 7. Calculating integral of absorption. PROCESSING DISPLAY Displays 1st derivative of the resonance signal or its 1st integral, depending on the action taken in TOOL BOX 1. PROCESSING PARAMETERS LIST Displays list of calculated parameters (M2, M4, integral value, line widths, limit cursors positions) Page 39 9 MISCELLANEOUS 9.1 Sample preparation and positioning Introduction The NMR/ESR signal originates from a sample located between poles of the electromagnet or inside Helmholtz coils. To avoid magnet/coils contamination and possible field homogeneity degradation due to corrosion of iron alloy and the poles use only glass vials to keep liquid samples isolated. The spectrometer probehead incorporates an ID=5.5 mm sample holder that safely accepts standard OD=5 mm NMR tubes. We recommend glass NMR tubes from WILMAD3. The important dimensions of the sample holder design are shown in Figure 8. Figure 8. Probehead and sample Liquid samples should be torch sealed. For routine studies make the sample 1520 mm long so that it will fill the whole volume of the 12 mm long RF coil. For higher resolution NMR studies only small samples of 2-3 mm length are recommended, but expect the Signal-to-Noise to drop dramatically. As dimensions slightly vary from probehead to probehead individually adjust the sample position by observing the NMR resonance signal. 3 WILMAD/Lab Glass, PO Box 688, 1002 Harding Highway, Buena, NJ 083100688, USA, tel. 856-697-3000, for order 800-220-5171, www.wilmad.com, cs@wilmad.com. We suggest 5 mm student NMR tube: borosilicate WG-5mm Thrift Page 40 Solid samples like rubber, acrylic or wood can be placed directly in sample holder. They should be cylindrically shaped and no more than 5 mm OD and a minimum 20 mm length. Glue sample to glass or plastic rod for easy sample insertion and removal . bNote: Do not use samples that fit sample holder too tight! Sample positioning Carefully insert the tube into the probehead and gently push it to feel resistance. The center of the sample should be in the area of the most homogeneous constant magnetic field and oscillating RF field. This area is the magnet isocenter. In this particular design the center of the RF coil is about 35 mm from the holder entrance. Since during experiments samples are held in a horizontal position, low viscosity liquids have a tendency to leave the bottom of the vial and stick to the vial wall. This will significantly lower the signal!!! For storage keep all liquid samples in an upright position and check to assure that the sample is at the bottom of the glass. If adhesive forces are to small to keep sample on the bottom, use of a larger amount of the substance is appropriate. An alternative is to “lock” the liquid on the bottom with little plug. WILMAD offers so called vortex plugs that can be used for this purpose. They are made of Teflon, which does not contain a proton, so the proton spectra are not contaminated with an extra signal from Teflon. Unfortunately Teflon contains a lot of fluorine nuclei, that can contribute to 19F NMR signal so do not use this Teflon as plug with 19F NMR spectroscopy Page 41 9.2 Changing configuration file The control program can be started without any initialization and configuration files. Follow instructions if you want to create a new standard.cfg setup file or change parameters in an existing standard.cfg file. • Start the control program. • Establish communication with the spectrometer by Spectrometer/Connect. • Modify elements of the Setup and Acquisition page that you want to appear when the program starts. • Save setup by File>Save As with new name (new.cfg). • Exit program. • Rename standard.cfg as standard. old, delete or move to another directory. • Rename previously saved setup new.cfg as standard.cfg. • Start program again and check if these changes you introduced appear on the Setup page. Notes: Any stored *.cfg can later be used for fast experimental setup modification by selecting File/Open Setup. If you suspect that cwne.ini or standard.cfg are for some reason corrupt delete them before starting the control program. File cwne.ini will be recreated with current parameters after the control program is closed. Configuration file can be created following the above procedure. Page 42 Page 43 10 EXPERIMENTS 10.1 Continuous wave NMR experiment in rubber Objective Preparation and execution of a field sweep and a frequency sweep NMR continuous wave experiment. This will serve as a template for other NMR experiments and will produce the 1st derivative of an NMR absorption signal. Experimental setup • Connect electromagnet to console. Program will automatically switch to NMR mode. • Slide probehead into electromagnet and then insert a rubber sample in the probehead. • Start control program. • Activate console connection to the computer by Spectrometer/Connect. Procedures Field sweep • Fill parameter boxes with values shown in Figure 9. Figure 9. Experimental setup for acquiring NMR signal in rubber by magnetic field sweep. • To find NMR signal quickly in Modulation change: Page 44 ƒ ƒ ƒ • • • • Field Sweep=300Gs; to cover widest sweep range, 2nd Mod Amplit=1Gs; to obtain strong signal, Sweep Time select=0.5min; to acquire preliminary result fast. Begin an experiment by clicking on Start. Look at the acquired signal and adjust the following: ƒ Magnetic field B0 to position signal on the display window center, ƒ Receiver Gain to fill at least half of the display window vertical scale, ƒ Reduce Field Sweep to cover about ¼ of horizontal scale by resonance signal, ƒ Detector Phase to get maximum signal or to chose between +/- or /+ pass, ƒ Measure line width by DB function available on auxiliary tool bar and lower 2nd Mod Amplit to reduce line broadening. Higher value of 2nd modulation increases signal-to-noise, but broadens the line. Find compromise between low line broadening and low noise amplitude, ƒ Increase Sweep Time to find if signal increases. Samples with long relaxation times may require longer sweep time. If signal is still weak, select number of accumulation Acc higher than 1. Note that signal-to-noise ratio increases as square root of number of accumulations. Repeat adjustments to obtain satisfying results. Store experiment in the file by File/Save Data As, or fill Acquisition/Store in file with file name and repeat experiment to store data automatically Page 45 Frequency sweep For frequency sweep experiment in Modulation, select Frequency Sweep with widest Frequency Sweep available 1,000 KHz and repeat whole procedure described above. Remember to reduce Frequency Sweep to conduct final experiment. Usually 50-100kHz sweep is enough. Follow parameters’ setting from Figure 10. Note that the signal acquired with frequency sweep is affected by limited frequency sweep resolution (frequency synthesizer limit) and therefore is less smooth than the signal acquired with field sweep. Figure 10. Experimental setup for acquiring NMR signal in rubber by frequency sweep. Page 46 Page 47 10.2 Examples of other NMR spectra 10.2.1 Acrylic Objective Finding 1H NMR resonance in solid-like sample characterized by wide line width. Experimental setup and analysis Figure 11. Experimental setup for 1H NMR in an acrylic sample. Figure 12. Absorption line and its line width at half-height in acrylic. Page 48 10.2.2 Delrin Objective Collecting 1H NMR spectra containing narrow and wide components. Setup and analysis Figure 13. Experimental setup for 1H NMR in delrin sample Figure 14. NMR absorption line in delrin showing two components: narrow and wide. Page 49 10.2.3 Glycerin Objective Collecting 1H NMR spectra in liquid-like sample. Setup Figure 15. Experimental setup for 1H NMR in glycerin sample Figure 16. Absorption line in glycerin and its line width at half-height. Notice that line is only 0.17Gs or 723Hz wide. Page 50 10.2.4 Fluoroboric acid (HBF4) Objective Simultaneous observance of 1H and 19F spectra in Fluoroboric acid (known as Tetrafluoroboric acid, Hydrogen tetrafluoroborate, Hydrofluoroboric acid). Setup Figure 17. Setup for simultaneous observation of NMR resonances on 1H and 19F nucleus in HBF4 Analysis Data from this experiment were used for calculation of γH/γF ratio. For details see Chapter 10.5.3. Page 51 10.2.5 Teflon Objective 19 F NMR spectrum in solid state-like sample Setup and analysis Figure 18. Experimental setup for NMR signal acquisition 19F nuclei in Teflon. Figure 19. Absorption line and line width at half height in Teflon. Page 52 Page 53 10.3 CW ESR in TCNQ Objective Preparation of a CW ESR experiment and data acquisition. Introduction TCNQ stands for 7,7,8,8-tetracyanoquinodimethane. This compound can be crystallized to the form that contains paramagnetic centers detectable as a strong and narrow line in an ESR experiment. Experimental Setup and Procedure • With the console off connect Helmholtz coil to console’s Magnet/Coils output. • Slide probehead into coils (horizontal slot from the side opposite the cable) and then insert a sample in the probehead from the opposite side. • Start control program. • Activate console link to the computer by Spectrometer/Connect. Program recognizes coils and automatically switches to ESR mode. • Follow instructions from Chapter 10.1 for CW NMR experiment. Use parameters from Figure 20 for initial settings. Figure 20. Setup page for ESR experiment in TCNQ. Page 54 Analysis Analysis tools for ESR signal are limited to: • Viewing of 1st derivative stored in binary file. • Exporting binary data as text file. • 1st integration of 1st derivative to obtain absorption • 2nd integration to calculate absolute integral value (AI) under absorption line within given limits (LC- left side limit, RC- right side limit). Integral value is proportional to the number of spins in a sample. • Calculation of line width: from 1st derivative (SDB) and from absorption (HDB). • Calculation of g-factor (G). Figure 21. Processing page for data acquired in ESR experiments. Page 55 10.4 ESR in other samples 10.4.1 DPPH DPPH (2,2-Diphenyl-1-Picrylhydrazyl) is an organic free radical that shows a strong line due to “free electrons” associated with one of the nitrogen atoms. Setup and analysis. Figure 22. Setup parameters for an ESR experiment in DPPH. Figure 23. Absorption line and its width at half height in DPPH. Page 56 Page 57 10.5 Nuclear magnetogyric ratio measurement with CW NMR Objective To determine nuclear magnetogyric ratio of protons (1H) and 19F nuclei. Introduction The nuclei possess a magnetic moment µ which is proportional to its spin I µ=γ Ih 2Π Eq. 1 The constant γ is called the magnetogyric ratio and is a fundamental nuclear constant which has a different value for every nucleus, h is Planck’s constant. Magnetogyric ratio can easily be determined by measurement of the resonant frequency for different magnetic field magnitudes and performing a linear regression analysis knowing that γ is slope in the Bloch equation: ω0 = γ IB0 Eq. 2 Experimental setup There are many ways to conduct this experiment. The basic idea is to get several (10-20) data points of NMR resonances at different magnetic fields with corresponding frequencies. Examples: • Operate in a narrow frequency and field range to see changes of resonances on the same screen (Figure 24). Method used to determine magnetogric ratio of 1H in glycerin sample as described on page 59. ƒ Keep the Field Sweep of 50 Gs and set B0 field to see resonance signal on the right margin of the screen ƒ Decrease Frequency by 10.0 kHz and perform field sweep. ƒ With Pass Display set for 5 observe how resonance moves towards lower magnetic filled (left side of the screen). Record f0 and corresponding B0 at which resonance occur. • Page 58 Operate in wider frequency and field range. Method used to determine magnetogyric ratio of 19F in HBF4 sample (see page 61). ƒ With Sweep of only 10 Gs (helps to measure magnetic field very accurately) change field by about 25 Gs ƒ Adjust frequency to see signal visible on the screen. If necessary, temporarily expand Sweep Width to localize the line. Change frequency to shift the line to the center of the screen and reduce Sweep Width. ƒ Perform final experiment without saving to the file. Record f0 and corresponding B0 at which resonance occurs. Figure 24. Experimental setup for determination of 1H NMR resonance frequencies for different magnitudes of magnetic field in glycerin sample. 10.5.1 1 Page 59 Magnetogyric ratio of protons ( H nuclei) Analysis Linear regression analysis (using Excel statistic tools) of experimental data (see Figure 25) returns following: • • • intercept =-268.9 [Gs] slope = 4.3464 f0 [kHz] = (4.3464B0 – 268.9) [Gs] 14,000 y = 4.3464x - 268.86 13,980 Resonant Frequency [kHz] 13,960 13,940 13,920 13,900 13,880 13,860 13,840 13,820 13,800 3,230 3,240 3,250 3,260 3,270 3,280 3,290 Resonant Magnetic Field [Gs] Figure 25. Plot of resonant frequency versus resonant magnetic field Using formula ω0 = 2Πf0 and knowing that 1[T]=104[Gs] one can calculate that experimental value of magnetogyric ratio for proton is: -1 -1 4 Page 60 γp = 2.731 [s T ] . This value differs from more accurate measurements available in literature5: γp = 2.675 [s–1 T–1] 2% relative error originates from limited accuracy of the reading of the magnetic field magnitude due to magnetic properties of the magnet yoke like magnetic hysteresis and magnetic remanence (see remanence measurement in electromagnet on page 78) Accuracy of calculation can be significantly improved if in regression analysis the intercept value is set for zero: • • • 4 5 slope = 4.2639 γp =2,679 [s-1T-1]. relative error = 0.15% [s-1T-1] =[kg-1sA] CODATA Bull., 1986, 63, 1 Page 61 Magnetogyric ratio of 19F nuclei 10.5.2 Analysis 13,800 y = 4.1181x - 306.58 Resonant Frequency [kHz] 13,600 13,400 13,200 13,000 12,800 12,600 3,150 3,200 3,250 3,300 3,350 3,400 3,450 Resonant Magnetic Field [Gs] Figure 26. Data points and linear regression of resonant magnetic fields and corresponding resonant frequencies on 19F in HBF4. Regression analysis of data from Figure 26 returns: • intercept = -306.6 [Gs] • slope = 4.118 • f0 [kHz] = (4.1181B0-306.6) [Gs] Source f0 = 4.1181B0-306.6 f0= 4.0257B0 Literature γ [s-1T-1 2.588 2,529 2.518 Relative error [%] 2.8 0.44 Table 3. Calculated magnetogyric ratios for 19F nuclei and literature comparison. Page 62 10.5.3 Field/frequency factor Measurements performed on resonance signals acquired during the same magnetic field sweep are not tinted with a hysteresis effect and can provide a very accurate value of the field factor- the relative parameter describing rate of magnetic field amplitudes at which resonances occur. Assuming constant operating frequency of spectrometer ω0, NMR resonances for 1H and 19F nuclei will occur at BH0 and BF0 : ω0 = γ HBH0 = γ FBF0 . Setup • • • Refer to Chapter 10.2.4 which describes how to acquire simultaneously resonances on 1H and 19F nuclei in water solution of fluoroboric acid. Load saved data on Processing page. Zoom area around particular resonance and using vertical cursor read field magnitude for resonance (when 1st derivative crosses zero) 1 19 H F Figure 27. Simultaneously acquired NMR resonances in 1H (left) and in 19F (right) in water solution sample of HBF4. Analysis Table 4 shows summary of calculated field BH0 / BF0 and frequency ωH0 / ωF0 factors. Frequency factor is reciprocal of field factor and is equal γH/γF. Note very low relative error of field factor measurement. BH0 [Gs] BF0 [Gs] BH0 / BF0 ωH0 / ωF0 Lit BH0 / BF0 6 Relative error [%] 3,186.12 3,385.02 0.9412 1.0624 0.9409 0.04 Table 4. Resonant magnetic fields of 1H and 19F nuclei at constant frequency f0=13,580.0 KHz and literature comparison (in red) of field and frequency factors. 6 BRUKER Almanach, 2000 Page 63 10.6 Page 64 Angle dependence of H NMR spectra in gypsum monocrystal 1 Introduction It has been seen from the study of oriented crystals, that 1H NMR spectra of solid samples can give structural information that X-ray crystallography cannot deliver due to poor X-ray scattering on the hydrogen single electron. This observation was first published by G. E. Pake in the early years of NMR7. He observed the splitting of the NMR line from water protons in a hydrated gypsum (CaSO4•H2O) monocrystal and powdered samples. The splitting originates from the interacting of magnetic dipoles µ in a static magnetic field B0. In crystalline solids these interactions produce an additional local magnetic field Bloc which contributes to the effective magnetic field acting on each spin. In less rigid substances, (mostly gases and liquids) fast molecular motion averages this local magnetic field to zero. Since dipole-dipole interactions decrease as the inverse cube of the dipoles distance, nuclear moments of protons in water molecules of hydrations are predominantly in the local field of its neighbor. Thus protons in water (spin ½) can achieve two positions with regard to the static magnetic field B0. Some spins will be located in higher fields (when the neighboring spin is parallel to B0) and some will be in lower fields (when the neighboring spin is anti-parallel to B0). In this simplified model, two NMR lines appear symmetrically located along the resonance at B0. Of the hydrous sulphates, hydrous calcium sulphate, of the chemical formula CaSO4•H2O, known as gypsum, is the most important. (The average American house contains around 5 tons of gypsum construction material!). The gypsum structure consists of parallel layers of (SO4)-2 groups bonded to Ca+2. Sheets of water molecules separate consecutive layers of strongly bonded ions. The bonds between water molecules in neighboring sheets are rather weak causing the crystal to break when it is a subjected to stress on a plane parallel to the sheets. This property is known as perfect cleavage in the (010) plane. One can determine proton-proton distance in a water molecule by Pake’s method, that is from the angle dependence of NMR line splitting. Assuming a certain angle of H-O-H obtained from crystallographic analysis the value of proton-oxygen distance can be calculated (Figure 28). 7 G.E. Pake, The Journal of Chemical Physics vol.16, p. 327-336, 1948, “Nuclear Resonance Absorption in Hydrated Crystals: Fine Structure of the Proton Line” Page 65 H 8A 5 o 1. 8 10 H 0.98A O Figure 28. Atoms distances and angle in a water molecule according to Pake. HH distance of 1.58D was calculated from spectra splitting. H-O distance was calculated from assumption of 108o angle of H-O-H bond. Objective The purpose of this experiment is the observation of the splitting of the NMR line originating from water protons located in different local magnetic fields of the gypsum monocrystal. This experiment can illustrate high-resolution NMR spectra in solids. Figure 29. Sample cut from gypsum monocrystal and its orientation with regard to external magnetic field B0. The sample has a cylindrical form of approximately 5mm diameter and 6mm long. It was cut from a large gypsum monocrystal as shown in Figure 29. The long axis of the sample is perpendicular to a crystal perfect cleavage (plane (010). Page 66 Setup • Prepare home-made goniometer (Figure 30): ƒ From thick cardboard cut two discs of 4” and 2” diameter. ƒ With sharp blade cut 5mm holes in the centers of both discs. ƒ Divide big disc into 16 segments 360/16=22.5o apart and tape it to the magnet’s side. It will serve as an angle marker. ƒ On the small disc mark a radius with a thick pen and slide it on the end of the gypsum sample glass. It will serve as a dial. Tape or glue the dial and the glass together. a) b) Figure 30. Cardboard angle marker (a) and dial (b) as elements of a home made goniometer. • Carefully insert crystal with attached dial in a probehead as shown on Figure 31 Figure 31. Home-made goniometer for gypsum monocrystal study attached to the electromagnet. Page 67 • • On Setup and Acquisition page prepare experimental setup similar to one on Figure 32. Note the large accumulation number equal to 16. It is of utmost importance to set this number at least 16 because of very week signal from water trapped between gypsum crystal layers. Perform and save experiments for crystal orientations that differ at least by 90o (45o recommended). Figure 32. Setup for observation of NMR signal from gypsum monocrystal. Activate Acc button, that displays accumulated (white trace) signal along with currently acquired pass (yellow trace). Analysis On the Processing page load previously saved data of signal first derivative acquired with different crystal orientations. Click on Absorption to see spectrum. Check all orientation to find line split. Line split visible on Figure 34 is equal 3.70Gs or in frequency units 15.7 kHz. Page 68 Figure 33. Absorption line in a gypsum monocrystal without splitting. 3.7 Gs Figure 34. Split of absorption line due to rotation of crystal by 90o with regard to orientation that produced spectrum on Figure 33. Variations: • Acquire a large gypsum crystal, make another cut and repeat measurements. Note You may purchase gypsum monocrystals from: Great South Gems & Minerals 38 Bond Drive, Ellenwood, GA 30294 1-888-933-4367 www.greatsouth.net • Since cutting gypsum is a difficult task, crush the monocrystal to powder and repeat measurements with a polycrystalline sample. • Page 69 Place small amount of dry plaster composition that contains predominately gypsum. Acquire NMR signal of dry powder. Then add a drop of water and repeat the experiment several times when mixture hardens. 10.7 Page 70 Determining Earth’s magnetic field with ESR experiment Objective Estimation of the magnitude of the Earth’s magnetic field in different environments using Electron Spin Resonance in TCNQ sample. Introduction Local magnitude of the Earth’s magnetic field changes with time and position. In an undisturbed environment it varies from 0.3 Gs to 0.6 Gs depending on latitude. This small magnetic field value can be easily measured with the CWS NMR/ESR spectrometer by recording the resonance field shift in an ESR experiment caused by different orientations of the Helmoltz coils with regard to magnetic North-South direction. When B0 field originating from the Helmholtz coil is parallel to Earth’s magnetic field BEarth, both fields add and an effective magnetic field is B0 + BEarth .When B0 is antiparallel both fields subtract and an effective magnetic field is reduced to B0 − BEarth . This ESR experiment allows for easy measurement of these effective fields by determination of ESR resonant fields. The difference between resonant returns doubled value of Earth’s magnetic field. Experimental setup ƒ Connect Helmholtz coils to the console for ESR measurements. ƒ Insert probehead in the Helmholtz coil and place both on a piece of cardboard that can be easily rotated by 360o. Keep coil/probehead assembly close to console to have enough room for rotation. ƒ Get a standard compass for determination of magnetic directions. Procedure Using the compass orient the probehead-Helmholtz coil assembly to have B0 field parallel to magnetic South-North direction8 (see Figure 35). ƒ Prepare setup to acquire ESR signal from TCNQ sample. Set Field Sweep to minimum value of 2Gs (follow values from Figure 36). ƒ Run field sweep experiment ƒ Rotate probehead-Helmholtz 8 Figure 35. Initial orientation of Helmholtz coils-probehead assembly relative to SouthNorth direction. Helmholtz coils produce magnetic field along coils opening. Page 71 ƒ ƒ coil assembly to have B0 field anti-parallel to South-North. Run Field Sweep experiment Repeat experiments with two remaining orientations of Helmholtz coils: East-West and West East. Figure 36. Experimental setup for determination of Earth magnetic filed using ESR in TCNQ sample. Analysis ƒ Display results of all four experiments using Display passes/4. ƒ With vertical cursor measure the field when first derivative crosses zero for orientations. Figure 37. Shift of resonance magnetic field in an ESR experiment with free radicals in TCNQ sample for different Helmholtz coils orientation. Dark blue- B0 and BEarth anti-parallel, olive- B0 and BEarth parallel. Red- B0 is oriented East-West and yellow- B0 is oriented West-East. Note perfect overlapping yellow and red, showing that for these orientations Earth magnetic field is not giving any contribution to effective field acting on electron spins. Total magnetic field shift is 0.46Gs and BEarth = 0.23Gs. B0 is of the range of 17.8Gs. Page 72 ƒ ƒ ƒ Calculate ∆B Earth magnetic field is half of the ∆B In the presented experiment BEarth=0.23Gs is significantly lower than the expected 0.5 Gs because of strong shielding originating from steel construction of the building where experiments were conducted. Variations ƒ Do not use compass, but repeat field sweeps for multiple B0 orientation while recording the resonant field Bres . Plot Bres =f(orientation) and find field extreme values to determine ∆B. ƒ Bring spectrometer to “iron free” environment (field, park) and repeat measurements. This configuration can serve as a very accurate magnetometer for extremely low magnetic field. Page 73 Page 74 10.8 Measurements of a static magnetic field with a Tesla meter (Smart Magnetic Sensor) Introduction TEL-Atomic Inc., sales a new pocket-sized Tesla Meter Model 2000 equipped with Hall probes that cover the measurements of a magnetic field in the range of 0.01 to 1999 mT. This Tesla meter can be used in a series of experiments with a CWS 12-50 electromagnet and Helmholtz coils to measure the magnetic field inside and outside the magnet and to illustrate properties of the Hall effect magnetic sensors. Figure 38.TEL-Atomic Inc.Tesla Meter Model 2000 An electric current flowing through a conductor located in a magnetic field experiences a transverse force called the Lorentz FL magnetic force. This force is defined as a vector product: r r r FL = qv × B = qvB sin Θ q - carrier charge v - velocity of the carrier B - magnetic induction Θ - angle between vectors v and B Eq. 3 Page 75 VH B FL I VB Figure 39. Lorentz force and separation of flowing electric charge (+/-) by an external magnetic field B. Eq. 3 implies the following: • The magnetic force is perpendicular to both the current I and the magnetic field B • The magnitude of the magnetic force FL is zero when charges move parallel to the magnetic field (or when the charges are stationary) and reaches a maximum ±(qvB) when the charges move perpendicular to the magnetic field The Lorentz magnetic force separates moving charge carriers (Figure 39). The separation effect was named Hall effect after E.H. Hall who discovered it in 1879. The charge separation produces transverse voltage between two sides of the conductor that is linearly proportional to the magnetic field B and is used to measure magnetic field. Page 76 10.8.1 Angle dependence of the readings of the Tesla meter. Before starting experiments prepare the spectrometer electromagnet and Tesla meter probe. • For all measurements use the Tesla meter axial probe type SMS102. The probehead should be removed to give free access to the space between electromagnet poles. • Wrap the Tesla meter sensor in the middle 2-3 times with ¼” paper tape. • Cut a 1” diameter disc from cardboard 1/8” thick. • With a sharp blade cut a rectangular shape in the center that will fit the Tesla meter probe. Draw an arrow extending from the probe. • Slide the Tesla meter probe in the slot. The arrow will be useful in angle measurements while the edge of the paper tape gives a convenient reference in magnetic field mapping (see Figure 40a). a) b) c) Figure 40. Measuring magnetic field in electromagnet. • From the same cardboard material cut large 4” disc and divide it into 16 equal segments (Figure 40b). Cut 6mm hole in the disk center and tape it on the top of the magnet (Figure 40c) Page 77 • Attach probe to the Tesla meter and turn it on, zero and calibrate meter and insert probe in a magnet. For angle dependence of Hall sensor indications place probehead in electromagnet center and record the Tesla meter reading while rotating the probe. For axial mapping move the probe vertically and read field every 5 mm. For proper readings keep probehead surface parallel to electromagnet’s poles. • • Analysis • Plot Tesla meter readings for different angle orientations and vertical positions of the probehead (Figure 41). 300 350 200 300 100 250 B0 [Gs] 400 B0 [Gs] 400 0 200 -100 150 -200 100 -300 50 0 -400 0 90 180 270 angle [deg] 360 -60 -50 -40 -30 -20 -10 0 450 a) 10 20 30 40 50 60 Position [mm] b) Figure 41. Angle (a) and vertical axis (b) dependence of magnetic field reading with Hall effect type Tesla meter. • • Keeping in mind Eq. 3 fit experimental points (Figure 41a) to a sine function. From vertical axis dependence (Figure 41b): ƒ Determine regions of most uniform (homogeneous) magnetic field. ƒ Calculate magnetic field gradient close to poles’ edges. ƒ Explain increase of magnetic field on poles’ edges. ƒ Analyze influence of magnetic field uniformity on the resonance signal. Page 78 10.8.2 Measuring magnetic field remanence in an electromagnet. A magnetic field in an electromagnet is produced by a direct current that flows through its coils. The amount of magnetization the electromagnet retains at zero driving current (field) is called remanence. It must be driven back to zero by a current (field) in the opposite direction. One can see remanence of the CWS 12-50 magnet in the following experiment. M remanence B(I) coercivity With the console off (no current) insert the Tesla meter probe between poles. While rotating the Figure 42. Remanence field on the probe measure the magnetic field amplitude. hysteresis curve. Usually remanence varies between 2-3 mT. Note that the magnitude of the Earth’s magnetic field is two orders lower and cannot significantly contribute to the measurement. Page 79 10.8.3 Helmholtz coils Introduction Helmholtz coils are a simple source of a relatively spatially uniform magnetic field obtained by use of a pair of circular coils on a common axis with equal currents flowing in the same sense. For a given coil radius the most uniform central field is obtained when coils separation is equal to the radius of the coils (a slightly larger separation improves the field uniformity). A cylindrical region extending between the centers of the two coils and approximately 1/5 of their diameter has a nearly homogeneous magnetic field. Helmholtz coils design is very simple and does not require a heavy or expensive yoke. Unlike electromagnets they can not produce strong magnetic field. CWS 12-50 Helmholtz coils produce a 20 Gs magnetic field, compared to the 3200 Gs produced by an electromagnet. Axial mapping of coil’s magnetic field and angle dependence of the readings of Tesla meter. Make an experiment following instructions for electromagnet in Chapter 10.8. Monitoring magnetic field sweep during ESR experiments. • Wrap the Tesla meter probe with thick tape that will hold probe inside the Helmholtz coils (Figure 43). a) b) Figure 43. Measuring magnetic field in Helmholtz coil. • • • • • • • Page 80 Connect Helmholtz coils to console. Insert spectrometer probehead in Helmholtz coils. Place TCNQ sample in spectrometer probehead. From the top insert Tesla meter axial probe in the Helmholtz coils and carefully locate it as close as possible to spectrometer probehead. For high accuracy of measurements remember to keep the Tesla meter probe axis parallel to B0 axis! On spectrometer Setup and Acquisition page prepare ESR experiment as described in Chapter 10.3 . For the observation of magnetic field changes choose Sweep Time=4min. Run experiment and see changes of magnetic filed during different phases of experiment. Perform Hold, Abort functions and check what happens. Page 81 Page 82 10.9 2nd modulation of magnetic field and line broadening Introduction Physics of magnetic resonances requires very slow passage through resonance line to fulfill the so called adiabatic conditions, when energy of nuclear spins do not change fast. Therefore the received signal is a very slowly changing alternate signal, which is almost direct current, of the amplitude of single microvolt, very difficult to amplify. Because alternate signals can be easily amplified and linearly detected in wide dynamic range, a DC-like resonance signal coming from the probehead must be somehow modulated and convert to alternate one. This is done by an additional modulation of the magnetic field (see Figure 44) during field sweep called 2nd modulation (because sweep of the magnetic field is called 1st modulation). Another benefit of applying modulation to the magnetic field is the possibility of using phase sensitive detector synchronized with 2nd modulation characterized by high linearity and for filtering of coherent noise. Look at the spectrometer block diagram at page 25 for details. An awkward consequence of this 2nd modulation is that the signal under detection is not the absorption signal, but its 1st derivative and it is artificially broadened, depending on 2nd modulation amplitude. So properly designed experiment requires finding the right 2nd modulation amplitude as a tradeoff between the gain from the resonance signal amplitude and the deteriorating natural line shape. Figure 44. Modulation of the magnetic field: sweep as a 1st modulation and sinusoidal as 2nd modulation. Page 83 Objective Studying influence of the 2nd modulation on the line width and on signal amplitude. Setup • • • • • • Connect electromagnet to console Insert glycerin sample in probehead On Setup and Acquisition page prepare experimental setup similar to one on Figure 45. Run experiments for different 2nd Mod Amplitude. Measure width of 1st derivative by pressing on DB. It calculates line width as difference between line minimum and maximum. For qualitative comparison display simultaneously 5 field sweeps for 5 different 2nd modulation amplitudes by Pass Display/5 Figure 45. Experimental setup and results from the study of 2nd influence on signal amplitude and line width. modulation Page 84 Analysis Table 5 shows summary of experiments for 5 different 2nd modulation amplitudes. Note continuous increase of line width, while signal amplitude reaches maximum for 0.50 Gs. It will be highly recommended to chose 0.10 Gs for final experiment when 50% gain in signal intensity is penalized only by 12% line broadening. # 2nd mod [Gs] Line width [Gs] Amplitude [a.u.] 1 0.05 0.17 216 2 0.10 0.19 325 3 0.20 0.30 433 4 0.50 0.58 477 5 1.00 1.02 451 Table 5 . Line width and signal amplitude for diffrent 2nd modulations. PHYS342 V. N. Smolyaninova Magnetic resonance techniques Magnetic resonance is a selective absorption of electromagnetic waves of a certain frequency ω due to change of direction of magnetic moments (magnetic moments of nuclei or electrons). Magnetic resonance is used for studies of structure of solids and liquids, motion of the magnetic moments, internal magnetic fields, etc. Nuclear magnetic resonance (NMR) has become a powerful tool in organic chemistry and biochemistry, where it is used for the identification and structure determination of complex molecules. NMR is a basic principle behind a major medical diagnostic technique, magnetic resonance imaging (MRI). Before starting this lab, the physics of magnetic resonance should be thoroughly understood. Start with reviewing the concepts of spin, magnetic moment, nuclear magnetic moment. To better understand the dynamics of magnetic moment in magnetic field, solve Example 1.1 from Blundell (the solution should be attached to the lab report). Learn about dynamical magnetic effects associated with the spin angular momentum (Kittel, Melissinos and Blundell have chapters on NMR and ESR). All necessary components of theory of magnetic resonance should be reflected in the lab report. Understand main principles of operation of NMR spectrometer (Kittel, Melissinos). Brief discussion of these principles and the experimental set up should be included in the lab report. Experiments 1. Familiarize yourself with experimental set up and its software by performing Experiment 10.1 Continuous wave NMR in rubber. Each final spectrum should be printed and attached to your report. 2. Measure 1H NMR in acrylic and glycerin (Experiments 10.2.1 and 10.2.3). Compare the line widths. Explain (See Kittel Ch. 13). Measure NMR of 19F in fluoroboric acid and in teflon. Does the line width follow the same tendency? 3. Measure nucleargyromagnetic (magnetogyric) ratio of 1H and 19F (Experiment 10.5). Find γH/γF. Determine γH/γF by method used in Experiment 10.2.4 (details in 10.5.3). Which method gives more precise γH/γF. Why? 4. Measure ESR in TCNQ. Before switching from the electromagnet to the Helmholtz coils, exit the control program and turn off the console. Determine g-factor. 5. Determine Earth’s magnetic field with ESR (Experiment 10.7). 6. What is 2d modulation of magnetic field and why it is used in magnetic resonance techniques? Study 2d modulation of magnetic field and line broadening (Experiment 10.9). (See Melissinos Ch. 7.5.3). References 1. Introduction to Solid State Physics, Charles Kittel, Eighth edition, John Wiley & Sons 2. Stephen Blundell, Magnetism in Condensed Matter, Oxford University Press, 2003 3. Wilson, Buffa, Lou, College Physics, Prentice Hall
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Nuclear Magnetic Resonance (NMR)
Name
Affiliation
Date

1

2

I.

History of the Nuclear Magnetic Resonance (NMR)
The NMR turned into the first portrayed and estimated in molecular beams via in the year

1938, by broadening the stern– Stern–Gerlach. In 1946, Varian associates developed the essential
NMR unit known as NMR hr-30. Purcell had chipped away on the development of radar amid
international WII on the Massachusetts Institute of era's radiation laboratory. His paintings amid
that assignment at the era and region of radio recurrence manage and on the ingestion of such
control by means of difficulty set up the framework for Rabi's revelation of NMR (Barluenga,
Ananthoju, García-Martín, & Angew, 2001).
Its signal demonstrates an extensive reverberation layout because of static intra and
intermolecular, homonuclear and heteronuclear, dipolar collaborations superposed on special
cooperation, for instance, anisotropic compound pass and circuitous spin– flip connections (these
are on the complete second request tensor associations whose anisotropic parts are determined
the center fee to zero by the way of sub-atomic motion in fluid examples). Within the most
important decade and that's simply the beginning, NMR line form designs had been breaking
down to deduce some primary facts and information on inner motions in the solids. It changed
into then, and, perceived that if a cylindrical sample situated at a point t as for the outer angle T
is pivoted correctly and approximately by the field, the motion arrived on the midpoint of dipolar
coupling vanishes if T is picked with the cease which aim that (3cos2t 1) = zero. This estimation
of T (= 54.7o ) is alluded to as the angle, and the method for line narrowing as angle spinning'.
Because the angular factor happens for all 2nd rank tensor collaborations, the relating phrases in
each connection such affiliation are neutralized.
➢ High Resolution
Its spin-½ nuclei in truthful diamagnetic fluids advanced, grew to become out to be

2

3

obtrusive that the road-width of the signals changed into basic because of the homogeneity ('B0 )
inside the static appealing area of B0 , and the commonplace line-width is probable to a small
quantity of one HZ, even as the NMR operating recurrence is one hundred MHz or more. A need
to accumulate spectra transferring toward ordinary line-width conditions provides a prerequisite
on the partial inhomogeneity and soundness of B0, and at the energy of the RF hardware utilized
for recording the spectra, to be in the scope of one section in 108 or 109. The Varian buddies in
Palo Alto, which had on its exploration group of workers NMR stalwarts like Anderson, Ernst,
and Freeman, built up the instrumentation that propelled excessive willpower of the NMR
spectroscopy. Inhomogeneity over the example in a completely lots deliberate magnet became
drop (or limited) by way of superposing bendy area slopes (of inverse sign) made from an
arrangement of loops that have been designed for this reason. The cylindrical sample turned into
spun approximately its hub to moreover diminish the lingering angles by means of spatial
averaging.
A formal comprehension of these spectra requires composing the turn Hamiltonian for an
atom comprising of different twists as:

.. With this, as part of the

history, the w1 is the substance circulate, because of this the impact of the concoction (electronic)
circumstance at the NMR recurrence of the center i; the second term is an attractive connection
between turns i and j interceded through their digital condition, jij is the roundabout spin– flip
coupling steady (known as circuitous to understand from coordinate via-space dipole– dipole
cooperation) in gadgets of hz. The major time period is...


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