Experiments Using The CWS12-50 NMR/ESR

 
All experiments have been performed using an off-the-shelf CWS12-50 NMR/ESR spectrometer and only originally acquired data are presented. This list is not closed. More experiments will be developed and included later.

All experiments have been performed using an off-the-shelf CWS12-50 NMR/ESR spectrometer and only originally acquired data are presented. 
This list is not closed. More experiments will be developed and included later.

  • Acquiring NMR and ESR spectra from factory provided samples
  • Comparison of NMR spectra from liquid- and solid-like samples
  • Determination of magnetogyric ratio for 1H and 19F nuclei
  • Measurement of electronic g-factor
  • 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
 
 
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Acquiring NMR And ESR Spectra From Factory Samples:   

Objective: Continuous wave NMR experiment in rubber 

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

1.   Fill parameter boxes with values shown in Figure 1.

Figure 1. Experimental setup for acquiring NMR signal in rubber by magnetic field sweep.

Figure 1. Experimental setup for acquiring NMR signal in rubber by magnetic field sweep.

2.   To find NMR signal quickly in Modulation change:

o    Field Sweep=300Gs; to cover widest sweep range,

o    2nd Mod Amplit=1Gs; to obtain strong signal,

o    Sweep Time select=0.5min; to acquire preliminary result fast.

3. Begin an experiment by clicking on Start. Look at the acquired signal and adjust the following:

1.   Magnetic field B0 to position signal on the display window center,

2.   Receiver Gain to fill at least half of the display window vertical scale,

3.   Reduce Field Sweep to cover about 1/4 of horizontal scale by resonance signal,

4.   Detector Phase to get maximum signal or to chose between +/- or -/+ pass,

5.   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,

6.   Increase Sweep Time to find if signal increases. Samples with long relaxation times may require longer sweep time.

7.   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.

8.   Repeat adjustments to obtain satisfying results.

9.   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

   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 2.

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 2. Experimental setup for acquiring NMR signal in rubber by frequency sweep

Figure 2. Experimental setup for acquiring NMR signal in rubber by frequency sweep

 

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

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:

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:

1.   Operate in a narrow frequency and field range to see changes of resonances on the same screen (Figure 1). Method used to determine magnetogric ratio of 1H in glycerin sample as described on page 59.

o    Keep the Field Sweep of 50 Gs and set B0 field to see resonance signal on the right margin of the screen

o    Decrease Frequency by 10.0 kHz and perform field sweep.

o    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.

2.   Operate in wider frequency and field range. Method used to determine magnetogyric ratio of 19F in HBF4 sample (see page 61).

o    With Sweep of only 10 Gs (helps to measure magnetic field very accurately) change field by about 25 Gs

o    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.

o    Perform final experiment without saving to the file. Record f0 and corresponding B0 at which resonance occurs.

Figure 1. Experimental setup for determination of 1H NMR resonance frequencies for different magnitudes of magnetic field in glycerin sample.

Figure 1. Experimental setup for determination of 1H NMR resonance frequencies for different magnitudes of magnetic field in glycerin sample.

 

1.1.1 Magnetogyric ratio of protons (1H nuclei)

Analysis
Linear regression analysis (using Excel statistic tools) of experimental data (see Figure 2) returns following:

  • intercept =-268.9 [Gs]
  • slope = 4.3464
  • f0 [kHz] = (4.3464B0 - 268.9) [Gs]
Figure 2. Plot of resonant frequency versus resonant magnetic field

Figure 2. Plot of resonant frequency versus resonant magnetic field

Using formula ω 0 =2πf 0 and knowing that 1[T]=10 4 [Gs] one can calculate that experimental value of magnetogyric ratio for proton is:

γp = 2.731 [s -1 T -1 ]. [1]

This value differs from more accurate measurements available in literature [2] :

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 77)

Accuracy of calculation can be significantly improved if in regression analysis the intercept value is set for zero:

  • slope = 4.2639

  • γp =2,679 [s-1T-1].

  • relative error = 0.15%

 

1.1.2 Magnetogyric ratio of 19F nuclei 

Analysis

Figure 3. Data points and linear regression of resonant magnetic fields and corresponding resonant frequencies on 19F in HBF4. 

Figure 3. Data points and linear regression of resonant magnetic fields and corresponding resonant frequencies on 19F in HBF4. 


Regression analysis of data from Figure 3 returns:

  • intercept = -306.6 [Gs]
  • slope = 4.118
  • f0 [kHz] = (4.1181B0-306.6) [Gs]




Source                              γ[s-1T-1]                Relative error [%]

f0 = 4.1181B0-306.6       2.588                          2.8

f0= 4.0257B0                    2,529                         0.44

Literature                         2.518

 

Table 1. Calculated magnetogyric ratios for 19F nuclei and literature comparison.         

 

1.1.3 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 B0H and B0F : 

ω0 = γHB0H = γFB0F.

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)
Figure 4. Simultaneously acquired NMR resonances in 1H (left) and in 19F (right) in water solution sample of HBF4.

Figure 4. Simultaneously acquired NMR resonances in 1H (left) and in 19F (right) in water solution sample of HBF4.

Analysis

Table 2 shows summary of calculated field B0H / B0F and frequency ω0H / ω0F factors.

Frequency factor is reciprocal of field factor and is equal γH/γF. Note very low relative error of field factor measurement.

[Gs] : 3,186.12

[Gs] : 3,385.02

B0H / B0F : 0.9412

ω0H / ω0F : 1.0624

Lit B0H / B0F [3] : 0.9409

Relative error [%] : 0.04

Table 2. 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.

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 anti-parallel 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 360°. Keep coil/probehead assembly close to console to have enough room for rotation.
  • Get a standard compass for determination of magnetic directions.


Procedure

Figure 1. Initial orientation of Helmholtz coils-probehead assembly relative to South-North direction.

Figure 1. Initial orientation of Helmholtz coils-probehead assembly relative to South-North direction.

Using the compass orient the probehead-Helmholtz coil assembly to have B0 field parallel to magnetic South-North direction[4] (see Figure 1).

 

  • Prepare setup to acquire ESR signal from TCNQ sample. Set Field Sweep to minimum value of 2Gs (follow values from Figure 2). Figure 1. Initial orientation of Helmholtz coils-probehead assembly relative to South-North direction.
  • Run field sweep experiment
  • Rotate probehead-Helmholtz 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 2. Experimental setup for determination of Earth magnetic filed using ESR in TCNQ sample. 

Figure 2. 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 3. 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.


  • Calculate ΔB
  • Earth magnetic field is half of the ΔB
  • 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 ΔB.
    • 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.
 

1[s-1T-1]=[kg-1sA]

2CODATA Bull.,1986,63,1

3BRUKER Almanach, 2000

4Helmholtz coils produce magnetic field along coils opening