Michael H. Proffitt and W. C. Gardiner, Jr. The university of Texas Austin
I Instructional NMR Instrument
The importance of nuclear magnetic resonance to chemical research has led to the incorporation of elementary discussions of NMR theory and practice in undergraduate courses. Providing a lecture demonstration or laboratory experiment in NMR is a harder job than fitting the material into the lecture program. I n large and well-to-do chemistry departments the presence of a research NMR spectrometer does not provide much of an answer, since these delicate instruments are unwillingly given into the hands of neophytes and are so complex that the principle of the experiment can hardly be discerned in the maze of refinements required to provide resolution and sensitivity adequate for research purposes. Circuits for simpler NlMR detectors that might be used for instructional purposes have been known for many years; these have the reputation, however, of requiring considerable sophistication in electronics to construct them and considerable operator skill to keep them operating satisfactorily. We undertook to develop an inexpensive circuit that could be painlessly amembled by interested but inexperienced people, could be handled by undergraduates with adequate safety for both instrument and students, and could provide an NMR signal detectable in relatively weak and nonhomogeneous magnetic fields. We desired to obtain enough signal intensity to allow studies at various frequencies and concentrations, in the presence or absence of paramagnetic salts, and for at least one nucleus other than hydrogen. The instrument described in this article meets these requirements and has proved its instrnctional usefulness in the laboratory part of the
junior physical chemistry course a t The University of Texas. The radio frequency oscillator and audio frequency amplifier circuit (Fig. I ) is a development of one proposed by Thomas.' It has proved to he sensitive to NMR signals and insensitive to varying technique in assembling the components. The coil around the sample is connected to the R F oscillator via a short length of flexible twin-lead shielded cable; we find that this arrangement is satisfactory electronically and allows separate mountings for two of the parts of the instrument requiring student adjustment. For our laboratory experiment we use a set of fixed capacitors to obtain known oscillator frequencies; a dual variable capacitor may also be used. The 6CW4 oscillator provides strong resonance signals over a wide range of settings of the regeneration control potentiometer, and is furthermore not disturbed by coupling the resonance signal into the 2N1302 audio amplifier. The signal-tonoise ratio with this type of circuit is such that additional amplification would only make the audio signal larger without making it more informative. We experience no difficulty in obtaining 0.1-volt resonance signals for display on an ordinary oscilloscope. It would he an easy matter to sweep the external magnetic field through resonance with 60-cycle sinewave modulation coils. The NiMR signal is then traversed twice each cycle, however; and if the field This work was supported by the National Science Foundation. 'THOMAS,H. A,, Electraies, January 1952, p. 114. The circuit was adapted for 6CW4 operation by F. C. Maseles of the Mass Spectrometry Laboratory, The Univerjity of Texas.
+IPVDC
OUT
Figure 1 .
152
/
Oscillator and audio amplifier circuit diogrom.
Journol of Chemicol Educofion
Sweep Coil
Figure 2.
Sawtooth generotor circuit diagram
sweep current is uscd for the osailloscol~ehorizontal sweep, two resonance signals nppcar in a symmetric pattern which is apt to confuse students. This situation would be poorly suited for instructional applications. One remedy would be to obtain single sigrmls by using the oscillosrope's own time-base, t,riggered only once from thc GO-cycle line. This remedy works but lacks flexibility in i;ignal display capability. We chose to add a simple sawtoot,h-sweep gencrator (Fig. 2) to the instrunlent. A unijunction relaxation oscillator provides the sa.wtoot.h wave form, and a power trausistor capable of handling the required current drives the sweep coils. We use 3. variable flux pcr~nanentmagnet with 3-cm pole pieces a t 1-cm gap to provide fields in the range 1.5-3.0 kilogauss. lIuch cheaper magnet,s, (for cxarnple, used mag~ielrounmgnet,~)will also do if iron pole pieces are ad(lcd to make thc field uniform. The sweep
coils are would upon the pole pieces in either case. A photograph of the instrument is shown in Figure 3. I t may be noted that an automobile battery and two 46volt dry cell batteries serve as power supplies. Figures 4 arid 5 show resonance signals from protons in Cu(N0J2 solution and fluorine nuclei in trifluoroacetic acid, both a t 11.2 megacycles. A single oscilloscope trace was photographed for each resonance signal. The students' laboratory assignment presently involves locating resonance signals a t a set of known frcquencies by varying the magnetic field, examining the signal form for various solutions, executing simple variations on the experimental conditions, and preparing interpretation and discussiori of the observations. Our experience is that the students find the experiment tractable and profitable, and that the instrument is rugged and elect,ronically foolproof enough to survive ordinary mistreatment.
Figure
4.
Proton [lower)
scope settings. 0.1
megacycler.
Figure 3.
Photograph of complete instrument
acid.
a n d fluorine ( u p p e r ) rerononce
v/cm verticol, 5 mrec/crn horizontal.
rignolr.
Orcillo-
Frequency. 11.2
Sampler were O.05M CvlN031r ~olvtion a n d tritlvoroocetic
Volume 43, Number 3, Morch 1966
/
153
Figure 5 . Relamtion oxillotionr in 0.05 M CulNO,I2 solution. O~cillo. scope setting, 0.05 v/cm verticol, 1.0 mrec/cm horizontal. The potentiome ter d the RF arcillotor is odjvrted to maximize the number of orcillotionr.
154
/
journal of Chemical Education
Our intention throughout this developmental work was to come up with a design which could be copied and brought to successful operation with a minimum of electronics experience and equipment. To this end we have arranged that the probe assembly and printed circuit cards for the sawtooth sweep generator and RF oscillator be made available commercially. The only remaining mechanical work consists of drilling holes in chassis for mounting electronic components. We have left the choice of magnet and oscilloscope open, since we anticipate that most potential users may already have suitable ones on hand. We have prepared a lengthy report. on this instrument, including detailed instructions for constructing it. Copies are available on request.
