Signal Averaging... Principles and Practices NMR Fourier Spectroscopy In Number 3 of this series we discussed how and why Fourier transform techniques are used in infrared spectroscopy and said that the same principles could be extended to Nuclear Magnetic Resonance (NMR) spectroscopy. We recently applied our 1070 System to a Bruker Scientific, Inc. (Elmsford, N.Y.) spec-
trometer and the above ,:!C interferogram and spectrum are early results of this joint effort. The interferogram is only the first 1024 data points of a 4096 point free induction signal and is the average of 512 sweeps taking a total of approximately 200 seconds to accumulate. The Fourier transformed high resolution spectrum is shown below it. The equivalent sweep width is 5 KHz. The significance of this technique lies in the time savings that can be achieved and the fact that this time savings can be put to good use in improving sensitivity or signal-to-noise ratio. The above spectrum of naturally low abundance l:iC, if recorded by conventional (CW) NMR techniques, would have taken 10 to 12 hours instead of 3 to 4 minutes. While these data demonstrate approximately a 200:1 time savings, much longer savings are theoretically possible. Ernst and Anderson1 have shown that the theoretical time reduction increases in proportion to the spectral resolution. The relaxation times (T-[) of the nuclear spin systems have been considered a limiting factor in approaching these theoretical values. However, recent experiments2 indicate that the Tj limitation can be overcome by newly developed techniques and that savings in the order of 10,000:1 may soon be practical. If part of used for sensitivity improvement these time savings were through signal averaging, signal-to-noise ratio improvements of one to two orders of magnitude could be achieved. Interested? Why not call or write to arrange to see a FabriTek 1070 System. References· 1. R. R. Ernst and W. A. Anderson, Rev. Sci. Instr., Vol. 37, 93 (1966). 2. E. D. Becker, J. A. Ferretti, and T. C. Farrar, J. Am. Chem. Soc. (tentative publishing date Jan. 1970).
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may go on; if not, he must give up or he may reduce his wishes (analytical task) and start anew. This is precisely the point to which we are carried by discussing the required and available amount of in-
formation. But from here on, new problems raise their heads: Is the money of the prospective builder good money, can he buy the proper building materials and services (type of informations with respect to contents) ? It will be clear that these are questions of another level. In daily life, the builder will study a lot of technical questions (and also the market) to find the best solution. What is the corresponding procedure in chemical analysis? At present there is no systematic approach to answer the question— which one may be the best analytical procedure to solve a given analytical problem. We are relying on the knowledge and experience of the analyst, we are scanning the literature, we are dependent on habits and fashions. Isn’t that muddling
through? I dare to sketch an idea of how a more systematic approach might be achieved: First, a set of independent parameters (coordinates) must be selected to describe analytical problems by numbers. Such parameters may be correlated to the following items: (a) Elements or compounds to be detected and determined and respective ranges of concen-
trations
(b) Type, composition, homogeneity, variety of the samples to be analyzed. (c) Quantity of sample available (in particular restricted or practically nonrestricted) (d) General requirements (precision, accuracy, limits of detection) (e) Special and practical requirements (local analysis, microanalysis, production
control) Rd., Madison, Wis., 53711. Phone: 608/271-3333 Sales and Service: In Canada by Ahearn & Soper, Limited, 844 Caledonia Road, Toronto 19, Ont. Tel. (416) 789-4325 Telex 02-2757. Branches in In Europe by BrukerMontreal, Vancouver, and Ottawa Physik, 7501 Karlsruhe Forchheim, Postfach 40, 10, West 5225 Verona
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(f) Restriction with respect to costs, laboratory space, time,
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
The scales for the different pa-
