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Chemical Instrumentation
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Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles are intended to snve the readere opmrs JOURNAL by calling attention to new developmenla in the themy, design, or milability of chemical laboratory inatrumenlation, or by presenting useful insighla and ezplanations of topics t h t are of practical importance to those who use, or leach the use of, modern instrumenlation and instnrmatal techniques. The editor invites correspondence from prospective contributors.
LVII. Nuclear Magnetic Resonance Spectrometers Part One: Instrument Designs DARRYL G. HOWERY, Deportment of Chemistry, Brooklyn College of the City University of New York, Brooklyn, N. Y. 11210 Introduction The performance and t.he flexibility of commercial nuclear nmgnetic resonance (nmr) spectrometers have improved dramatically during the past decade. The main objectives of this two-part article are: in Part One to discuss the essential aspects of the instrumentation of current high-resolution nmr spectrometers, and in Part Two to describe nmr models now sold in the United States. The discussion will concentrate upon highresolution, continuous-wave (hr-cw) nmr spectrometers. Though the instrument* tion of pulsed-wave, Fourier-transform pulsed-wave rtnd wide-line spectrometers will not be detailed, each of these modes of nmr o~erationwill be discussed briefly in Part TWO. The reader is assumed to be familiar with nmr bnt unfamiliar with the details of nmr instrumentation. Chapters on nmr instrumentation can he found in most recent monographs on nmr ( 1 ) . A recently published discussion of advances in nmr instrumentation (8) supplements several of the topics to be presented in this article. Histmieal Backround. The principles of nmr were first demonstrated on bulk matter in 1946, the discovery being hastened by the rapid strides made in radiofrequency technology during the 1930's and early 1940's. Nmr spectroscopy received great impetus in 1951 when its immense potential for furnishing detailed information about the electronic environment of protons in organic molecules was realized. The earlier nmr spectrometers were difficult to operate and to maintain, so that until the late 1950's nmr research was conducted almost entirely by s rel* tively small number of physicists and chemical physicists.
Varian Associates introduced the fimt commercial nmr spectrometer in 1953. The marketing of the Varian A-60 in 1961 was of special significance since for the first time there was available an nmr spectrometer which could be fairly readily operated and maintained. Thereafter the use of nmr became routine in many chemical research laboratories. By the mid1960's several other companies had entered the U.S. market. At present the producers of nmr spectrometers sold in the U S . are Bruker Scientific, Inc.; JEOLCO (U.S.A.), Inc.; NMR Specialties, Inc.; Perkin-Elmer Corp.; and Varian. Revinu of Basic Principles. Before going into the instrumentation of nmr spectrometers, let us summarize some of the main aspects of nmr spectroscopy. I n the presence of a magnetic field a nucleus possessing a magnetic moment has two or more nuclear spin enerm states. The difference in energy between these energy levels increases with au iurrease i u rhe r;rr~ngrhof the applied nlawetic tield so that the mscnetic field ceurrares the energy level spacings. For field strengths of the order of kilogauss (kG), the resonance energy required to raise a nucleus from a lower to a, higher nuclear spin state usually is found experimentally to be in the megaherta (MHe) or radio-frequency region of the electromagnetic spectrum. The basic equation describing the nuclear magnetic resonance phenomenon, the Larmor relationship, can be written v = 1 - 2 . The resonance frequency of the nucleus undergoing the transition is v, H is the strength of the applied magnetic field, and r and n ere constants for the specified nucleus. The magnitude of the magnetogyric ratio r is a constant for a given isotope hut varies considerably from isotope to isotope. For a given nucleus the relstively narrow hut impor-
Dr. Darryl G. Howery is an Associate Professor of Chemistry at Brooklyn College of The City University of New York. A Virginia native, he received his BS in chemistry from Roanoke College in 1958and his PhD in physical chemistry from the University of North Carolina in 1963. He spent two year8 as a postdoctoral chemist a t the University of California, Berkeley, %fore joining the faculty of Brooklyn Zollege. His main research interest is the >hysical chemistry of ion exchange, nith current emphasis on proton magletic resonance studies of ion exchange ,esius. Dr. Howery twice has been a tpeaker a t the Gordon Research Conerence on Ion Exchange. He has )uhlished work on continuous elecrophoresis and on the pmr spectra, the tqnilihrium properties and the elecrical conductivity of ion exchangers. tant range of resonances depends on the chemistry of the system, i.e., the value of the shielding constant c depends on the electronic environment ahout the nucleus. To produce a spectrum of energy absorbed versus frequency, continuous-wave nmr can be performed in two equivalent ways involving either (a) frequency sweep or ( b ) field sweep. If the magnetic field is held constant (which keeps the nuclear spin energy levels constant), then the rf signal can be swept (varied continuously over a.range) to determine the frequencies at which energy is absorbed. Alternatively, if the rf signal is held constant, then the magnetic field can be swept (which varies the energy levels) to determine the magnetic field strengths which produce resonance. A high-resolution nmr spectrum may have many, often overlapping, peaks. N i t only does each chemicslly different type of nucleus have its characteristic resonance frequency, but also the nuclei can interact (Catinued a page AS88)
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Chemical lnstnrmentrrtlon with each other (spin-spin coupling). A high-resolution nmr spectrum furnishes two types of useful information: ( a ) resonance frequencies in Hs (or ppm) relative to some arbitrarily chosen resonance frequency, and (b) the broadness of each peak. By integrating the nmr signal, the relative numbers of each type of nucleus can be determined in many cases.
Maintenance of homogeneity is an absolute must even for routine nmr work; expensive magnets are required to meet the rigid specifications. I3esolution is determined from the width a t half height in Hz of the fifth and 16th peaks in the spectrum of o-dichloro-
better, equivalent to distinguishing a few parts in loq. The attainment of the more ideal resolution of 10-10 is a far from solved problem at present. Stability. An nmr spectrometer is said Performance Criteria to have adequate stability if it can reproduce spectra accurately. The main causes When one wishes to evaluate an nrnr of in&hility are variations in temperature soectrometer. the follawine are of soecial or line voltage which induce flucturttions and drifts in the maenetic field. Stahilitv Sens lit ily. S m r is :t relatively i-en,i- should be expressed in terms of both spectral reproducibility and resolution riw nunlytirnl tevlmiqtte. Fr.tu ~ >he ~ dt-tertrd nww w d i l y thin refers to the reproducibility of chemical any other common nucleus, yet fo; satisshifts, is most commonly expressed as the factory detection on a routine basis the average deviation in Hi; for five scans of molar concentration of the proton being approximately 5 min duration. The specobserved must generally he 0.01 or greater. tral reproducibility of commercial units is The sensitivity of an nmr spectrometer, usually about 0.2 Ha. Resolution stabili.e., its capability for measuring a resoity refers to the extent of degradation of a nance signal above the electrical backpeak width during a specified time. The ground noise, is dependent upon the inresolution stability of commercial specherent intensity of the nmr signal and trometers generally is better than 0.5 upon the ability of the spectrometer to Ha/16 hr, meaning that, for example, a process the signal. The intensity of the peak of width 0.4 Ha will haves. width not nrnr signal depends upon roughly the greater than 0.9 Hz 16 hour later, for the square of the strength of the magnetic same instrument settings. field and upon the magnetogyric ratio and Spectral reproducibility is a measure of the molar concentration of the nucleus short-term stability; resolution stability being observed. becomes crucial fo
