Low Cost, Simple Operation Expand NMR Uses New model brings nuclear magnetic resonance spectroscopy within the budget and capabilities of most laboratories High resolution nuclear magnetic resonance spectroscopy is about to emerge from the esoteric realm of the highly qualified research specialist to become a widely useful tool for the organic chemist. Last week, at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Varian Associates, Palo Alto, Calif., introduced its new model A-60 NMR. With the A-60, the cost of NMR equipment is cut by about half, and size, weight, and operating complexity are cut by far more than that. Price tag of the new model is $23,750 off the shelf, including a supply of charts, an internal standard, and the like. By comparison, direct cost of the research model depends on optional equipment, can run anywhere from $35,000 to around $50,000. In addition, there's the indirect cost of an air-conditioned room. The physical contrast between the A-60 and its predecessors is just as impressive. The research model NMR consists of a number of separate units—
a control console, a recorder, a magnet housing, and a large separate power supply. An integrator is available as an accessory. All these fill a small room which must be air conditioned. The A-60, on the other hand, combines control console, recorder, and integrator in one unit; magnet and power supply are in a second enclosure. Since the heat dissipation of the A-60 is only about one fourth that of the earlier model, and since it can tolerate ambient temperature changes of as much as 1° C. per hour, air conditioning is normally not required. The big research spectrometer requires the services of a well qualified and specially trained scientist. He must have a thorough grasp of all the principles involved and he must be able to interpret the output of the machine during the warm-up period as well as during the run. The organic chemist who is interested in the information NMR could give him has to be pretty well grounded in physics and electronics if he is to get useful re-
NMR Spectrometer. Varian says its new A-60 nuclear magnetic resonance spectrometer can be operated routinely by a technician with only brief training 52
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sults from this type of equipment. Simplicity. With the A-60, however, a technician can get usable spectra with very little training. Yet performance has not been sacrificed to gain this simplicity. A high magnetic field strength, for example, helps to get high resolution and high sensitivity. Varian designed a magnetic field strength for the A-60 as high as that of the research unit—14,100 gauss. And resolution with the new model is 1 part in 10 8 . A feedback loop which makes the spectrometer self-correcting is a major factor in making the operation simple. Previously, magnetic field strength and applied radio frequency had to be stabilized separately. But the key to accurate, reproducible, and meaningful spectra is the stability of the ratio between field and frequency. The A-60 gets a stable ratio through reference to a water standard placed in the magnetic field. This stability leads directly to another advantage of the A-60. Previously, calibration of the spectra was difficult. Precalibrated charts just couldn't be used. But with the reproducibility of the A-60 output, a precalibrated chart becomes feasible. The zero point on this chart is established by a chemical reference, typically the sharp line from tetramethylsilane dissolved in the sample. A turn on the zero-adjust knob brings this line to the zero point on the paper. With the ability to make reproducible spectra on calibrated charts, it will now be possible to build up libraries of spectra and to exchange data conveniently between laboratories. The A-60 is designed for use only on proton spectra, since these have accounted for over 90% of high resolution NMR spectroscopy work in the past. The research type instruments will still be produced for work on other nuclear species as well as protons. Operation. While operation of the research type spectrometer is complex, the theory of nuclear magnetic resonance is relatively simple. For example, a nuclear proton, since it is spinning, will precess like a gyroscope
The right combination to safeguard any OXygen level. Oxygen determinations in any range, from 0 to 100%, are dependably monitored by either the Beckman Model G-2 or F-3 Continuous Analyzers. In addition, trace analyses as low as 0-5 ppm, or up to 500 ppm, are accurately provided by the Model 80. And for portable applications Beckman offers Laboratory Models C, D and E. S3- For rugged, round-the-clock service, paramagnetic sensitivity of the G-2 and F-3 is simple, fast, specific, and directly linear. Greater accuracy assures you complete protection: for inert gas blanketing control...maintaining safe oxygen levels in combustibles... catalyst regeneration control..· feed stock heater efficiency... or wherever critical oxygen control is a vital process requirement. H Check with your nearest Beckman Sales Engineer for application assistance; or, for fall details on this most complete line,write for
Data File 13-10-08. B e c k m a n / Scientific and Process Instruments Division I Beckman Instruments, Inc. J Fullerton, California
Cenco-Petersen
MOLECULAR MODELS can be quickly hand assembled with out special tools for three-dimensional study of stereochemical and conforma tional factors and reaction mecha nisms. Designed to overcome many shortcomings of earlier model sys tems, they • are made up of accurate bond lengths (1A = 5 cm). • present precise angles of distor tion (to 30°). • can be locked or left free to rotate under stress. • return p e r f e c t l y to a preset angle. The stereochemist can now directly predict inter-atomic distances and bond angle distortion using these trueto-scale models. Polyvalent atoms are colored neoprene balls with threaded aluminum inserts placed at the theoretical bond angles. Monovalent atoms are poly styrene with the appropriate bond permanently attached. The bonds themselves may be disassembled and rebuilt with other lengths to provide different scale factors. The complete No. 71306 CencoPetersen Set contains 55 monovalent atoms, 59 polyvalent atoms, 70 bonds and fittings plus extra parts, all in a partitioned metal case. Send for Book let 315 for detailed information. $295.00
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A Division of Cenco Instruments Corporation 1700 Irving Park Road · Chicago 13, Illinois Mountainside, N. J, Montreal Santa Clara Somerville, Mass. Toronto Los Angeles Birmingham, Ala. Ottawa Vancouver Houston Cenco S.A., Breda, The Netherlands Tulsa
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when a force in the form of a strong magnetic field is imposed. If a radio frequency magnetic field is added per pendicular to the first and tuned so that it exactly matches the rate of pre cession, the nucleus will absorb and emit energy. It is the frequency and amount of this energy that the spec trometer records. The rate of precession of the proton, and hence the resonance point, de pends on the magnetic environment of the nucleus. And the amount of energy absorbed is proportional to the number of protons in the particular en vironment. Thus, ethanol gives a spectrum with three distinct p e a k s one peak for the hydroxyl hydrogen, one for the methylene hydrogens, and one for the methyl hydrogens. And since there are, respectively, one, two, and three hydrogen atoms m each of these environments, the areas· under the peaks will be in the ratio 1:2:3. Basically, then, NMR spectroscopy is a technique which enables the chem ist to look at hydrogen atoms. He can tell the number of different environ ments there are in the molecule and the number of hydrogen atoms in each. And in many cases the position of a peak alone is sufficient to establish the nature of the environment responsible for producing it. Applications. In many cases, NMR is a convenient, possibly the most con venient, analytical tool. It can be used for structure determination of a wide variety of compounds. And it can be used to count the number of protons in complex molecules. One typical analytical problem might be to determine the composi tion of a mixture of hydrocarbons such as naphthalene, tetralin, and n-hexane. The NMR spectrum will show three groups of peaks, and the integrated curve will give the relative areas under these. One group of peaks comes from the naphthalene hydrogens and from the aromatic hydrogens in tetralin. The second group represents the four tetralin hydrogens that are alpha to the aromatic ring. And the third group is made up of contributions from the four tetralin hydrogens that are beta to the aromatic ring plus the 14 hexane hydrogens. Solution of three simple simultaneous equations gives the com position of the mixture. In structure determination, NMR spectroscopy can frequently provide an easy method for deciding between sev eral alternative structures. A chemist might be uncertain, for example,
whether a particular complex natural product was an amine or an imine. From the integrated curve of the spec trum he could read off the relative number of equivalent hydrogen atoms. Since the amine would have two equi valent amine hydrogens while the alternate structure would have only one imine hydrogen, the ratios ob tained from the spectrum would have to correspond unequivocally with one structure.
BRIEFS Mass flow measuring system is un affected by viscosity, pressure, tem perature, or piping configuration, ac cording to the manufacturer, Fischer & Porter Co., Warminster, Pa. It com bines a magnetic flow meter and a radiation density detector (both obstructionless) to measure volume flow rate, prints readings on a single tran sistorized chart recorder. The system is accurate to within ± 1 . 5 % , ac cording to F&P. Ε 1
High-speed infrared spectrophotome ter, Model 137-B, that provides auto matically recorded spectra in 3 min., has been developed by Perkin-Elmer Corp., Norwalk, Conn. The double beam instrument uses a NaCl crystal, presents its recording on notebooksize paper. When high resolution is required, or where quantitative and qualitative accuracy is more impor tant than time-saving, a full 12-min. scan is available. Ε 2
Vacuum evaporator accesories now available from Vacuum Technology Inc., Van Nuy, Calif., consist of a series of collars which fit in a vacuum system between bell jar and base plate. Feedthroughs can be connected to ports in the collars. Among the feed throughs currently available from VTI are high and low voltage and current leads, cooling water, RF heating leads, and discharge and thermocouple gages. Ε 3
Further useful information on keyed Equipment items men tioned is readily available . . . Use handy coupon on page 88
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