Topics in..
. Chemical Instrumentation
Edifed b y S. 2. LEWIN, New York University, New York 3, N . Y . proton's N-UR is a t 42.6 mc/s, vs. 6.54 mo/s for the deuterium nucleus. Based on the observed magnetic transition frequency for a known nucleus, such as that of the proton, an NMR sample can be used to probe a magnetic field with great accuracy. This system ia useful in cahbrating the field strengths of large mgnets. For a short introduction to magnetie resonance spectroscopy, the reader can consult review articles (1, 2) and monographs (3), as well as other more comprehensive texta (4, 5,6). The operation of a NMR or ESR spectrometer is essentially as shown in Fig. 19. A coil lhat is part of the tank
These articles, most of which are to be contributed by guest authors, are intended to serve the readers of this J o m by~catling aUenliun to new developments in the theory, design, or availability of chemical laboralmy instrumentation, oi by presenting useful insights and ezplanations of t o e t h ~ are t of practical importance to those who use, or teach the use of, modern instrumentalion and instrumental leehniques.
XVI. Instrumentation Teaching Equipment Part Three:
Miscellaneous
LOW
FREO.
Leonard Eirner, Borner Engineering Cornpony, Sfomford, Conn. Magnetic Resonance Spectroscopy The previaua two parts of this swim have dealt with optical devices, and elertronics and nucleonics, respectively. Some familiarity with all three of these disciplines is a desirable prerequisite for anyone planning to work in mirrowave nr radio-frequency speetmseopy. When atoms or nuclei are placed in a static magnetic field, and irradiated with radiofrequenciw or microwave radiation, they show rhsracteristic absorption spertra. The sample may he in the form of n solid, liquid, gas or even s n atomic henm. Absorption occurs when there is synchronism between the frequency of the electromagnetic radiation and the Larrnor precession frequency u of the atomic or nuclear magnetic moments in the static field. The spectrum may also be regarded as an indication of the energy h u required t,o cause transitions between the quanbiaed ~nagneticenergy levels of the atoms or nuclei. The Zeeman effect is a closely related phenomenon, that may be similarly interpreted on either a classical or quantum-mechanical basis. When n light source is placed in a. strong magnetic field, the spectral lines split up because of shifts of the electronic energy levels by the small amount hv. The Iwrmor precession frequency is proportional to: the static magnetic field H; the magnetic moments of the atomic or nuclear system involved; and a. constant g called the "gyromagnetic ratio" or "spectroscopic splitting factor." Electrolls, being much lighter than protons and neutrons, spin faster and have a larger magnetic moment than the latter. Accordingly, they also precess much more rapidly in a given magnetic field. For this reason, in a field of rt few thouaand gauss, nuclear magnetic resonance (NMR) occurs a t radiofrequencies (a few megn-
cycles/sec) whereas electrons sbsorh a t microwave frequencies (around 101° cps). Atoms, ions, and free radicals tlrat possess a net electronic angular momentum also ahsorb in the microwave region. The designation "electron spin resonance" (ESR) is commonly used for this class of phenomena, hut some call it "electron paramagnetic resonance" (EPR). In some atoms and molecular groups, the electron has a magnetic moment that is due not only to its spin but also t o its orbital motion. The ratio of the total magnetic moment to the spin value is the gyromagnetic ratio g. I t differs in various atoms and environments. For a free electron g = 2. Its vilhe in other cases gives information ahoot the rhemieal composition of the subst,anre involved. ESR spectroscopy has beeu used to study: ions of the transition elements (Mn, Fe, Co, Ni): free radicals; rolrw centers; radiation damage sit,=; impurit,ies in semiconductors; elect,rons in unfilled conduction bands; triplet elertronir states; etc. Biochemical npplicnbions have also proved fruitful. I n contrast to ERR, NMR spectra are relatively independent of the chemical hondmg of the substance. For example, the proton magnetie resonance may be detected in water as well nS ice: in solid, liquid, and gaseous hydrogen; and in hydrocsrbans and nitrogen compounds generally. And the magnetic resonance of the sodium nucleus may he detected in m e t d i e sodium as well as sodium salts or solutions of the latter. Usually about 10'9 nuclei are required, i.e., ahout 1 mg of sample. Different isotopes of the same nucleus have different magnetic moments and hence resonate a t different frequencies. For example, in s. field of 10,000 gauss, the
1
,
: '
DE""
AMPLIFIER
Figure 19.
Diagram of a mogndls resonance
spectrometer, showing sorption line.
a
typlcol resonant
ab-
circuit of a ratio oscillator surrounds the sample in a magnetic field. A detector coil leading to a high-gain amplifier is also coupled to the sample so that when the latter absorbs energy i t will cause an impedance change in the detector coil. Reaonanee may be observed either by changing the magnetic field or the rf frequency. The former is commonly done, by superimposing a sinuaoidally varying eomponent. As a result the sample goes in and out of resonance, and the ahsorption line can be displayed on an oscilloscope whose horizontal input is in
(Continued a page A608)
Vol. 41, No. 9, Sepfember 1964
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A607
phraae with the varying magnetic field. For satisfactory performance, the sample coil must be formed precisely, and kept rigid. The detector must have extremely low noise and the anrulifier must have high gain. Unless the field is hon~ogeneous, different Darts of an NMR samole will resonate a t different frequencies, and this will degrade the sharpness of the observed spectra. In the erne of permanent magnets, the pole faces must be very flat and parallel. When electromagnets are used, the power supply must be extremely well regulated. This is hut one illustr~, tion of the exacting design requirements that result in relatively high costs for NMR and ESR eqnipment of research caliber. Nevertheless, within the past few yeam, several N41R and ESR spectrometers have appeared on the market with prices within the reach of educational budgets. In the past, separate speetron~eterswere required for NMR and ESR because of the different ranges of operating frequencies. Manufacturers have, however, succeeded in making combination instruments. This was accomplished by Atomic Laboratories, 3100 Craw Canyon Road, San Ramon, California, for example, by the design of wide-range rf oscillators, operating over a broad range of frequencies, and by developing a line of relatively low-cost, yet good-quality, electromsgnets. ESR resonances may be made to occur a t rsdiofrequencies, instead of microwave irequencies, by appropriate reduction of the magnetic field strength. Atomic Laboratories' combint~tion NMR-ESR spectrometer (No. 71913, $495) includes separate samples and holden for each type of experiment, as well as a magnetic field sweep circuit and transistor power aupply. Other necessary equipment, that is not included, is an electromagnet (e.g., No. 79641-3 $255) and appropriate power supply, oscilloscope, and supporting hardware. Alpha Scientifio Laboratories, 940 Dwight Way, Berkeley, California, has also recently brought out their model AL675 combined EPRNXIR unit ($495)
Figure
A608
20. A1 675 Student NMR-ESR
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Chemical Education
spectrometer.
(Fig. 20) based on the design of their ALGi NMR gaussmeter. The price does not include the necessary auxiliaries, such as magnet, power supply etc., required to make s, complete NMR or EPR spectrometer. This system can also he used with standard interchangeable probes t , ~ rmeasure magnetic fields between 3W and 150,000gauss, with an accuracy of 1%. .41pha Scientific Laboratories in the past has offered ESR equipment operating in the X hand (10,000 mc/s). By appmpriate reduction of the magnetic field, their model AL60 now gives ESR spectra in the 32G350 mc/s range, and the model AL55 in the 3:&65 mc/s range. Although these instruments were designed primarily for univenity teaching, they may be used in some less demanding research applications a t low magnetic fields. The sensitivity of an ESlt spectrometer ran be improved significantly by the use of a "lock-in" or "phsse-sensitive" detertion system. Such a system is useful generally where one is dealing with very weak signals in the presence of noise. The principle of operation is to provide a filter with clase-to-the-optimum transfer properties, namely, a transfer characteristic which is proportional to the Fourier t,ransform of the signal voltage. I n practice, the closest to this optimum filter system is sceomplished by a lock-in detector a.hich departs only slightly from the theoretical optimum. A system of this type e.g., Alpha Scientific Labvratories' AL340PD for $785, and Driver Model ALl05PD for $700 provides extremely stable low-noise signals from an electron spin resonance Bystem. An ESR spectrometer operating a t 50 mc/s ran be made a t a lower cost than one requiring microwave component,s. The reduction in operating frequency is accompanied, however, hy a drop in sensitivity (by 8. factor which may vary from the inverse square root to the inverse square of the frequency). This partly explains why, for example, with the AL60, resonanre can be observed in rides of a given sample, vs. 10-4 lnoles for the ALB5. In order to eom-
pensate for the reduced sensitivity at lower frequencies, the rf power may be increased, but this may result in u n d e s i ~ able saturation effects. If not excessive, the latter are not considered objectionable in eouipment intended for . ~edapoaienl . .~ ~
~~
"Be.
Separate NMR and ESR units are now supplied in kit form by the British manufsrt.urer "Scientifica" (148 St. Dunstan's Avenue, Acton, London, W.3, England). 1)istribution in the U.S. is solely through Barnes Engineering Co., Stamford, Conneet.icut. The kits include all the required bits and pieces (down to screws and nuts), step-by-step instructions, and a mmual of experiments. A noteworthy feature of Seientficrs's NMR kit (No. .4.11/2) is the small size of the detachable probe head. The sample volume is less than 0.01 ml. Besides enabling the measurement of magnetic fields in small gaps, this also allows NMR experiments to be performed in fields of poorer homogeneity. The probe d m not contain field modulation coils, but mndulation is supplied by a thin auxiliary coil placed in the gap of a permanent magnet. A 4300-gauss permanent magnet, which gives high-field homogeneity (1 part in 108 over the NMR sample) is available. I t can be supplied either unealibrated (No. A.7) or calibrated (No. A.8). Either
