NMR and ESR
Continuous Wave Spectrometer
CWS 12-50
operating and experimental
manual
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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
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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
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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
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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)
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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.
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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)
•
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Print spectra and calculated parameters
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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.
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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.
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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
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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
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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.
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4.2 Configuration for NMR experiments
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4.3 Configuration for ESR experiments
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5 SHIPPING ITEMS
5.1 Picture tour
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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
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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
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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
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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
Hardware name: CW NMR/ESR Spectrometer
Unit Serial #:
Page Name: SETUP and ACQUISITION
Setup Name: (default is standard.cfg)
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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
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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
1, current pass only (yellow)
2, current pass and one before
3, current pass and two before
4, current pass and three before
5, current pass and four before
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
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•
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
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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:
Not Connected/Connected
Experiment in progress- please wait
Experiment Aborted- please wait.
Experiment Aborted- please wait
Experiment on Hold
• Magnetic Field Source:
Program automatically detects what source is connected to the console
Electromagnet
Helmholtz coil
• DB
Displays line width calculated from 1st derivative (min-max)
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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.
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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.
Arrow Down
Shifts base line down
Arrow Up
Shifts base line up
Arrow Diagonal Up
Shifts base line right limit down
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
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TOOL BAR 2
• HZ
Horizontal zoom
Left click on Zoom button
Left click on left limit and release
Drag courser to right limit
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
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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
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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.
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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:
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•
•
•
•
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:
Magnetic field B0 to position signal on the display window center,
Receiver Gain to fill at least half of the display window vertical
scale,
Reduce Field Sweep to cover about ¼ of horizontal scale by
resonance signal,
Detector Phase to get maximum signal or to chose between +/- or /+ pass,
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,
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.
Keep the Field Sweep of 50 Gs and set B0 field to see resonance
signal on the right margin of the screen
Decrease Frequency by 10.0 kHz and perform field sweep.
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).
With Sweep of only 10 Gs (helps to measure magnetic field very
accurately) change field by about 25 Gs
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.
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):
From thick cardboard cut two discs of 4” and 2” diameter.
With sharp blade cut 5mm holes in the centers of both discs.
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.
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
Connect Helmholtz coils to the console for ESR measurements.
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.
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).
Prepare setup to acquire ESR
signal from TCNQ sample.
Set Field Sweep to minimum
value of 2Gs (follow values
from Figure 36).
Run field sweep experiment
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
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
Display results of all four experiments using Display passes/4.
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
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
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.
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):
Determine regions of most uniform (homogeneous) magnetic
field.
Calculate magnetic field gradient close to poles’ edges.
Explain increase of magnetic field on poles’ edges.
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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