COMMENTARY Ordered Fluids and Liquid Crystals
by Ralph H. Müller
Analytical chemists will be grateful for Professor Low's outline and discussion of infrared Fourier Transform spectroscopy. In so carefully outlining the advantages as well as the present limitations of the interferometric approach to infrared spectroscopy, we believe the conclusion is inescapable that these techniques will seriously rival spectrometers operating on the principle of dispersion, however elegant and highly developed the latter class of instruments happens to be. We are happy to note Dr. Low's observation of the "connection between the spread of infrared spectroscopic techniques and the increased availability and improved quality of commercial spectrometers." We think it is clearly demonstrable that in all ages, the amount of scientific information was at all times governed by the techniques, resources, and instruments available for the problem. At present, the vast amount of publications is ample proof that most information has been obtained with elegant instruments, most of them automatic or semi-automatic. It is probably demonstrable that a smaller proportion of all investigators who publish today are concerned with the development of new methods, new techniques, and new instruments than some thirty years ago. The information explosion does not run parallel to the population explosion—it exceeds it and not by working overtime at time-anda-half remuneration. We suspect that sharp inflections in the growth curve arise about once every twenty years. Someone then gets a Nobel Prize and during the new cycle publications increase at a rate proportional to the improvement in equipment developed for the new phenomenon. We are reluctant to dwell upon the boredom which sets in after the 1961st paper on the phenomenon has appeared. Considering the question of improved resolution, we note that this is a function of the length of the sweep of the interferometer mirror and the constancy of its velocity. To achieve the highest precision is an undertaking requiring the best resources of physics and engineering, and the effort has been going on for about three quarters of a century. When Henry Augustus Rowland, at Johns Hopkins, built his first engine to rule diffraction gratings, he spent several years in perfecting the lead screw used to advance the diamond cutting tool. This was done by running 108 A ·
ANALYTICAL CHEMISTRY
a split-nut, lapped with fine abrasive, back and forth along the screw in order to eliminate periodic errors in pitch. It is not quite certain who first suggested the use of an interferometer (counting of interference fringes) as a criterion of perfection. As a rough benchmark of the dimensions which are involved, the distance between fringes for the median wave length of the sodium D lines (5893 Â) is about 11 microinches. Several decades ago, Dean George Harrison at MIT developed the Compander, an ingenious system for the automatic and continuous correction of tool position for second order correction of residual screw errors. With an interferometer mirror placed on the cutting tool carriage, the moving fringes were detected photoelectrically —giving rise to a sinusoidal voltage which was constantly compared with a similar voltage derived from the rotation of the feed screw. The two sinusoidal voltages were adjusted to equal amplitude but for corrections, the phase relations were utilized. A slight error in pitch would cause a phase shift and this difference, after amplification, was used to advance or retard the cutting tool position momentarily. One hundredth of a fringe displacement could be measured with this system, which corresponds to a mechanical displacement of λ/200. The source of light for the interferometric monitor was the 5461 Hg line obtained from an arc using the 208Hg isotope. This had been developed by Meggers, and in other aspects, the line is of such high spectral purity that it may well serve as an absolute alternate standard of length. It would not surprise the chemist to be told that the achievement and mainte nance of such high metrical precision required the control of the partial pres sure of C0 2 and water vapor in the air surrounding the dividing engine because these determine the refractive index of the air and, therefore, the velocity of light in the measuring arm. These fac tors, along with temperature fluctua tions and vibration, determine the pre cision which can be attained. Present day resources in electronics, computing, and in engineering are vastly better than when these requirements were established. Probably the technique will be competitive or superior to con ventional methods when it can provide equal or greater precision per unit time, cost, and operation ease. The principles of interferometry have other, including unexplored, uses.
Fffil&i ADVANCES IN CHEWISrflY StRIES
63
Ordered Fluids and Liquid Crystals ADVANCES IN CHEMISTRY SERIES No. 63 Twenty-two studies on characteriza tion, properties, and occurrence of these phenomena in many substances, including: • the polymorphism of tristearin • characterization
of
mesomorphic
phases by nmr spectroscopy • interfaces in nematic liquids • liquid crystals as ordered components of living substances • liquid crystalline nature of phospho lipids • the structure of synthetic polypep tides in solution by polarization of fluorescence • order and structure in concentrated polymer solutions and gels • field dependence of the magnetic sus ceptibility of the liquid crystal phase of p-azoxyanisole 3 3 2 pages with index cloth bound (1967) $9.50 postpaid in U.S. and Canada; plus 20 cents foreign and PUAS. Set of L.C. cards free with library orders.
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