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Pulsed/CW NMR 
TeachSpin's Pulsed/CW NMR Spectrometer, PS2-A, 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-A System Hands-On Advantages Analog Outputs Will Never Be Obsolete Single Coil Sample Probe NMR Signals of both Hydrogen and Fluorine Nuclei Gradient Field Coils Optimize Homogeneity Proprietary Temperature Controller Investigations Using Fast Fourier Transforms (FFT) Pulse Programmer/Spin-Echoes NMR of "Soft" Solids Using the Apparatus for Continuous Wave (CW) NMR Overview of the PS2-A System
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-A 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. Back to list 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. Back to list Single Coil Sample Probe Unlike our original PNMR spectrometer, PS2-A 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
Back to list Using the Apparatus for Pulsed NMR NMR Signals of both Hydrogen and Fluorine Nuclei Using PS2-A, 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-A 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
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-A 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. Back to list Investigations Using Fast Fourier Transforms (FFT)
Pulse Programmer/Spin-Echoes
NMR of "Soft" Solids
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.
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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
FID for Fluorine Nuclei in Teflon