Pulsed/CW NMR

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Pulsed NMR and CW NMR
NMR Signals from two types of nuclei: Hydrogen (protons) and Fluorine
0.49 Tesla Magnetic Field (21 MHz Proton NMR)
Magnetic Field Stability to 1 part/106 over 20 minutes
Envelope and Quadrature Phase Sensitive Detection
Measurement of T1 (spin-lattice relaxation time) and T2 (spin-spin relaxation time)
Spin Echo and Free Induction Decay (FID)
Carr-Purcell, Meiboom-Gill Pulse Sequences
Signals from Soft Solids Enhanced by 10µs Receiver Recovery Time
Direct Detection of Inequivalent Fluorine Nuclei (Chemical Shifts) in the FID Signal
Four Independently Controlled Gradient Coils for High Homogeneity
Known Gradients for One Dimensional Imaging and Measuring Diffusion
Lock-In Detection Module for CW Signals from Solids with Wide Lines
Built-In Magnetic Field and RF frequency Sweeps for CW Resonance
Digital Clock Stability in both RF Frequency and Pulse Width Synthesis
Modular Construction
Research Grade Data for Advanced Student Projects
Data Taken on Your Oscilloscope

Introducton

Introduction

In 1994 TeachSpin launched PS1-A, the first Pulsed NMR spectrometer designed specifically for teaching. Currently, over 150 institutions world-wide are using TeachSpin Pulsed NMR systems to introduce their students to this important technology. (The very first instrument delivered is still in continuous use by students in Carnegie Mellon's advanced physics lab.) When key electronic components became obsolete, TeachSpin decided that the time had come to create an entirely new system, incorporating some of the capabilities most often suggested by our "constituency." These include higher fields, higher homogeneity and gradient coils to enable students to study diffusion and one dimensional imaging. A proprietary temperature control system allows for field stability of one part in a million over a twenty minute interval.

In addition to samples such as mineral oil and glycerin, which contain hydrogen nuclei, TeachSpin provides a set of safe liquids containing fluorine nuclei. You need only supply the oscilloscope and your students will be ready to learn the fundamentals of both CW and pulsed nuclear magnetic resonance spectroscopy. And, having mastered the fundamentals, students can use this Pulsed/CW NMR spectrometer to obtain research grade data on the magnetic resonance properties of not only liquids but also soft solids containing hydrogen or fluorine nuclei.

TeachSpin PS2-B continues in the tradition of PS1 in providing an opportunity for students to manipulate directly all aspects of their measurements. Automated, computer mediated processes, while efficient for making some kinds of measurements, do not allow students to develop an intimate understanding of the interrelations between the components or to make the mistakes that often lead to the most memorable learning. Designed by university faculty who have dedicated their careers to both experimental physics and advanced laboratory instruction, PS2-B is the ideal introduction to both Pulsed NMR and CW NMR. And its capability for research grade measurements allows students to use this apparatus for wide ranging independent explorations.

With the current explosion of research and medical applications of NMR, it is crucial that students be given an opportunity for developing a deep understanding of the basic principles of this unique and important spectroscopy. Our next generation of experimentalists, from physicists to chemists, and from biologists to electrical engineers will be empowered by the conceptual and quantitative insights gained from hands on explorations with PS2-B.

The Designers
PS2-B is the result of two year development collaboration by the designers of the original PS1-1. Jonathan Reichert, President of TeachSpin and Professor Emeritus of SUNY Buffalo converted his original crossed coil sample probe into its new single coil design. Using a single coil both improved field homogeneity over the sample and provided space for shim/gradient coils. Dr. George Herold, TeachSpin Senior Scientist and Dr. Reichert designed the magnet structure and temperature control system which gives the world's first "wooden magnet" a field stability of one part per million over 20 minutes. Professor David Van Baak of Calvin College, and a TeachSpin Collaborating Physicist, designed the field gradient coil system.

The electronics system, a true tour-de-force, was designed by Dr. Norman Jarosik of the Princeton University Physics Department. His 1992 design for the electronics of TeachSpin's flagship PS1 series is responsible for its well deserved world-wide reputation for reliability, as well as sensitivity. Norman is a staff scientist in the "gravity Group" and the chief engineer of WMAP, the satellite that has been sensing and mapping the anisotropies in the microwave radiation left over from the Big Bang of the early universe. It should come as no surprise to owners of PS1 that the satellite has far exceeded its expected lifetime.

Instrument

instrument

TeachSpin's Pulsed/CW NMR Spectrometer, PS2-B, is a versatile, sensitive, rugged and reliable nuclear magnetic resonance spectrometer designed specifically for instruction. Any exploration of the capabilities of this apparatus easily explains the title of our introductory newsletter, 'Pulsed NMR on STEROIDS.'

Overview of the PS2-B System
High-Field, High Homogeneity Permanent Magnet
0.49 Tesla, 21 MHz proton resonance
field stabilized to 1 part in 106 over a 20 minute interval.

RF sample probe with
single coil and a 50 ohm input impedance; a matched unit.
four independently adjustable gradient coils (x, y, z, z2) available to:
* enhance homogeneity
* provide known gradients for diffusion measurements and one dimensional imaging

PS2 Controller containing
dedicated current regulated supply for each of the four gradient coils
magnet temperature controller

Mainframe with Four Electronic Modules
pulse programmer
synthesizer including both oscillator and pulse amplifier
receiver with both envelope detector and quadrature phase sensitive detector
lock-in/field sweep with detector

Hands-On Advantages
The specifications of this unit rival any research grade unit in this frequency range in terms of sensitivity, stability, capabilities and state- of-the-art electronics. PS2-B was, however, designed from the outset for student instruction. Students, and not a computer, adjust the shim coils and set all of the experimental parameters. They can make many informative mistakes including miswiring the spectrometer and incorrectly setting any and all of the parameters without damaging the unit. For the one case where incorrect wiring might do some damage, the unit has special connectors which make those connections impossible.

Analog Outputs Will Never Be Obsolete
All of the data is presented in analog form for examination on a digital storage oscilloscope or a computer. Students (and faculty) can then choose how they wish to perform data reduction or analysis. No proprietary software programs are needed to operate the unit, so no software updates are needed.

TeachSpin is convinced that the "volt" will never go out of style, and that the mode of data storage and analysis is a choice best left to the individual user. Although data storage and analysis hardware and software is changing so rapidly that what is "hot" today may be obsolete tomorrow, your TeachSpin spectrometer, with its analog output signals, will be capable of taking research grade data long into the future.

Single Coil Sample Probe
Unlike our original PNMR spectrometer, PS2-B is a single coil, matched sample probe instrument. There are many advantages to a single coil unit; including lower power required for RF pulses, smaller size, smaller magnet gap, and an impedance matched sample probe.

For a student, however, operation of the single coil system may appear to be pure "magic". There are many ways to accomplish this "magic", but we have chosen a system that can be readily understood by a beginner. The block diagram is shown below.

The instruction manual devotes several pages, with several diagrams, to explain carefully and clearly how this single coil system works for both pulsed and CW resonance. The "magic" turns out to be clever electronics.

Using the Apparatus for Pulsed NMR

NMR Signals of both Hydrogen and Fluorine Nuclei
Using PS2-B, students can observe NMR signals from two types of nuclei, hydrogen (proton) and fluorine. This opens up the possibility of studying an entirely new collection of molecules in liquids and some solids. Fluorine nuclei are particularly interesting because they typically exhibit large chemical shifts in various compounds. In our original spectrometer, PS1-A, where only proton NMR could be studied, the small chemical shifts were not observable. This was a deficiency, especially for the chemistry faculty. One might even say that chemical shifts are the bread and butter of organic chemistry's NMR analysis. Now, with this unit, chemical shifts in fluorine compounds are easily observable and some can even be seen in proton compounds.

The reason these "splittings" are observable in PS2-B is not just the fact that it can detect fluorine NMR, but also that the new unit has a larger and more homogeneous magnetic field. The proton resonant frequency is 21 MHz and we have observed T2* as long as 25 milliseconds in some magnet using the electric gradient field coils.

Gradient Field Coils Optimize Homogeneity
Four sets of gradient field coils are built into the RF sample head, each of which is controlled by its own current regulated power supply. Because they offer control of the x, y, z, and z2 gradients, students can adjust these coils to increase, significantly, the homogeneity of the magnetic field over the sample.

The screen capture on the right shows two free induction decays for the same distilled water sample.

The upper trace was taken with the gradient coils off while the lower trace was taken with the gradient coils adjusted

for optimum homogeneity.

Proprietary Temperature Controller
Not only has the magnetic field been increased in magnitude and homogeneity, it has also been greatly enhanced in stability. Magnetic field stability has been a problem with permanent magnets because the magnetization of the NdFeB alloys is extremely temperature dependent. Our original spectrometer needed to be retuned every five to ten minutes and thus RF phase sensitive detection was impractical. After its forty-five minute warm-up period, PS2-B has a field stability of one part in 106 over fifteen minutes. This is accomplished with a proprietary magnet design and PID temperature controller. Both the temperature controller and the current regulated supplies for the gradient coils are housed in the hardwood case that is shown on top of the Mainframe of the instrument.

With such outstanding field stability, students can take advantage of data from the two channel quadrature RF phase sensitive detection. The reference RF has a full 360° phase shift with one degree resolution. There is also an envelope detector as well as a buffered output of the direct RF magnetic resonance signal. Again, because of the field stability, students can perform long term signal averaging with the phase sensitive detector signals, to extract weak signals from noise. Such signal averaging is now a standard feature on almost all digital oscilloscopes.

Investigations Using Fast Fourier Transforms (FFT)
This new high-homogeneity, high-field magnet allows students to use the modern form of spectroscopy Fast Fourier Transforms (FFT) for both fluorine and proton samples. The oscilloscope screen shows the FID signals of a liquid called Fluorinert, FC-70, with the envelope detector (on resonance) and a RF phase sensitive detector slightly off resonance.

This sample shows a dramatic beat structure, indicating the presence of at least two inequivalent fluorine sites in the sample.

This screen capture shows the FFT of the phase detector signal, which has been signal averaged over 16 pulses. The FFT spectrum indicates the presence of three inequivalent fluorine sites and gives their relative splittings.

The spectrometer is shipped with a set of four different fluorine liquids, all of which are safe for student use.

Pulse Programmer/Spin-Echoes
The Pulse Programmer Module of the Mainframe provides a wide variety of pulse sequences. All pulse widths and time delays are digitally generated for stability and accuracy. A simple two pulse π/2 - π spin-echo combination for light mineral oil is shown. The narrow spike is the π pulse which then leads to the single spin echo. Students can vary not only the pulse width but also the delay time between pulses and the repetition time for the sequence.

Students can determine spin-lattice relaxation time (T1) in any sample by systematically varying the delay time between the two pulses of a π - π/2 sequence.

Multi-pulse combinations are available in both Carr-Purcell and Meiboom-Gill (MG) sequences. A toggle on the Pulse Programmer module allows students to switch the MG on and off so that they can see how using this sequence enhances their data. The figure at the right shows a thirty p-pulse MG sequence used to measure spin-spin relaxation time (T2) in light mineral oil.

NMR of "Soft" Solids
Our new unit has a significantly reduced recovery time after the pulse. This allows the spectrometer to measure signals from many "soft" solids which have T2 longer than ten microseconds. Here we see the FID signal for fluorine nuclei in Teflon. Rubber, greases, plastics and other solid materials with either hydrogen or fluorine nuclei can be studied. With a recovery time of the order of 15 µs, an entire class of solids can now be added to the long list of candidates for exploration.

Using the Apparatus for Continuous Wave (CW) NMR
Over the years, we have consistently had requests for an instrument that could detect magnetic resonance signals not only using pulsed NMR but also using the original continuous wave (CW) spectroscopy, so that students could study the relationship between them. That is exactly what we have accomplished. This unit is capable of observing NMR signals for either hydrogen or fluorine nuclei using a continuous RF field and sweeping the magnetic field through resonance.

A built-in two channel fixed modulation frequency lock-in amplifier is used for enhancing the CW signals for broad linewidth signals. It allows students to gain first hand experience with lock-in detection in NMR. The lock-in module also provides the current for the magnetic field sweep coils as well as an output analog voltage proportional to the field sweep. Various sweep times, sweep ranges, field offsets, voltage amplification, and low-pass time constants are available in the unit.

FID for Fluorinert FC-70

FFT for Fluorinert FC-70 showing three inequivalent fluorine sites.

Initial FID and spin-echo of Light Mineral Oil for a π/2 - π pulse sequence.

Thirty p-pulse MG spin echo signal

This oscilloscope screen capture shows the CW resonance signals for the fluorine FC-70 Fluorinert liquid. This signal came from the RF phase sensitive detector with no attempt to separate the real and imaginary part of the nuclear susceptibility using the phase shifter. However, the magnetic field splittings observed in the CW spectrum can now be compared to the frequency splittings detected in the FFT spectrum of the free induction decay. After performing both experiments, it should become clear to the students why the modern spectrometers all use the FFT analysis of the FID.

FID for Fluorine Nuclei in Teflon

FC-70 Fluorinert spectrum taken with CW NMR.

Experiments

experiments

PS2-B is a research grade Pulsed/CW NMR spectrometer capable of carrying out a wide variety of experiments and research projects. In this brief section, we are listing, in no particular order, experiments that instructors might wish to consider for their laboratory programs. This is certainly not an encyclopedic list; rather, it is a collection of experiments that will provide a challenging experience to both undergraduate and graduate students. All of the experiments require a careful reading of the literature to obtain a thorough explanation of the physics behind the experiments.

Representative data for some of these experiments can be found under the Instrument section of this page.

Measure T1 and T2 of water doped with paramagnetic ions over a wide concentration range. Paramagnetic ions that dissolve in water include CuS04 and Fe(NO3)3.
Measure T1 and T2 in glycerin and water mixtures.
Glycerin and water mix in any ratio. The motion of the protons in glycerin is significantly changed by the change of the liquid viscosity with the addition of water. The relaxation times can be correlated with the viscosity of the liquid, as well as with the water concentration.
Measure T1 and T2 in mineral oil with solvents. The relaxation times of protons in mineral oil diluted with organic solvents show the effects of diffusion and correlation times.
(see Pulse Programmer experiment)
Measure T1 and T2 in Petroleum Jelly
Vaseline is not a solid. The two relaxation times indicate fast molecular motion which is characteristic of a liquid. Samples can be heated and T1, as well as T2, can be estimated as the sample cools to room temperature. Other organic greases with sufficient proton concentrations can also be studied.
Biological Materials
Most biological materials have protons, usually in water molecules. Measurements of T1 and T2 in biological materials give detailed information about the local environment of these water molecules. This area of exploration is wide open. This might be an area appropriate for an undergraduate research participation project.
Discover inequivalent fluorine nuclei in the various fluorine liquids that come with the spectrometer; HT-110, FC-43, FC-70, and FC-770
Look up their chemical structure and identify the various fluorine sites.
(see Investigations Using Fast Fourier Transforms (FFT) experiment)
"Watch" epoxy cure. Study T1 and T2 of various slow curing epoxies and explain the data.
Rubber is a peculiar substance. Use various pencil erasers as samples. Study T1 and T2 as an eraser cools. You will have to estimate the temperature, since it is difficult to measure the temperature when the eraser is in the sample coil.
Can you observe inequivalent protons in ethyl alcohol? What about in an echo sequence?
Measure spin diffusion in distilled water.
There are well known fluorine greases. Measure T1 and T2 in those salt solids.
Examine virgin and recycled TEFLON. Can you distinguish them from their NMR data?
(see NMR of "Soft" Solids experiment)

You may also enjoy taking a look at our Magnetic Torque apparatus which is frequently used to introduce, quantitatively, a definition of the gyromagnetic ratio and to demonstrate the classical analog of a pulsed NMR spin flip. There, you will find a student manual from the University of Chicago where Magnetic Torque is used in conjunction with their ESR experiment and a description of how the apparatus is used to demonstrate the spin-flip.

Additional Resources

additional resources