EQUIPMENT
ORD Instruments Enter Market Applied Physics, Bendix join veteran maker 0. C. Rudolph & Sons Two new instruments to measure optical rotatory dispersion made their debuts in Atlantic City last week at a meeting of the Federation of American Societies for Experimental Biology. The latest candidates for this highly sophisticated task are the Cary Model 60 recording spectropolarimeter, made by Applied Physics, and the Bendix spectropolarimeter. Meanwhile, veteran manufacturer of ORD instruments, O. C. Rudolph & Sons, is modifying a member of its well known line. All three manufacturers are pushing the range of the instruments to shorter wave lengths—an important advantage for many applications. The Applied Physics instrument is rated at 1850 to 6000 A., while the Rudolph will have a range of 1900 to 6500 A. Although the Bendix prototype now goes from 2000 to 6000 A., the company plans to have the production model go down to about 1800 A. Optical rotatory dispersion—the change in optical rotation with wave length—has blossomed during the last decade into one of the most valuable research tools that the chemist has. ORD is remarkably sensitive to very slight changes in structure, conformation, or absolute configuration of molecules (C&EN, Aug. 2 1 , 1961, page 88). It has been especially important in the field of natural products. Before 1954, fewer than 100 ORD curves in the ultraviolet were recorded. Since then, investigators have run thousands. Chief reason for this surge in activity was the introduction of the Rudolph manual spectropolarimeter and, later, the Rudolph recording instrument. Until now, the Rudolph instrument has been the only commercially available one. Both Applied Physics and Bendix Instruments now
ORD ADJUSTMENTS. Whitelaw Wright, Applied Physics' product line manager, makes adjustments on Cary's instrument. The unit, one of two recently introduced, measures optical rotatory dispersion, is rated at 1850 to 6000 A.
hope to carve out a place in this highly specialized market. In addition, two other manufacturers have spectropolarimeters in the development stage. Besides pushing the wave length range down to 1850 A., Applied Physics, Monrovia, Calif., has put as much flexibility as possible into the $35,000 Cary instrument. The operator can program the slit width by setting manually a series of potentiometers. There is also a continuously variable control allowing scan speeds from 0.25 to 35 A. per second. Thus, scan time for the entire wave length range varies from 4 1 / 2 hr. down to two minutes. Because the scan and chart drive are coupled, the scan speed can be varied during operation without affecting the wave length-scale expansion. A seven-step adjustment allows fullscale ranges of 2.0° to 0.02° over the 11-in. chart width. It is also possible to shift a curve while the machine is operating to bring back on the paper a peak going off the chart. Sensitivity of the instrument is about 1.3 millidegrees at 2000 A. Applied Physics uses a double monochromator in the Model 60 to cut
down on stray light, one of the chief causes of error that results in shifting, broadening, or lowering peaks. The recording circuit includes a multipot, a series of potentiometers that allows the operator to make corrections at 33 points throughout the wave length range, with linear interpolation between. This gives a straight baseline, produces a curve that can be published without tedious corrections. For kinetic studies, an auxiliary motor provides synchronous chart speeds. How It Works. In operation, radiation from an Osram 500-watt xenon arc lamp passes through the monochromator, a Rochon polarizer, the sample cell, and then a Faraday cell. A 60 cycle-per-second alternating current in a coil surrounding the Faraday cell cyclically displaces the beam's plane of polarization. The beam then goes to the Rochon analyzer, oriented perpendicular to the polarizer, and then to a photomultiplier. If the sample has not rotated the polarized plane, light reaching the photomultiplier is equally strong on each side of the zero transmission point. If the sample is optically active, light reaching the Faraday cell is already rotated to a certain degree. APRIL
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The cycling action of the Faraday cell, then, causes the pulses to the photomultiplier to reach a maximum when the Faraday cell rotates the light in the same direction as the sample did, a minimum when the Faraday effect is in the opposite direction. The alter nately small and large pulses at the photomultiplier are filtered, amplified, and used to drive a motor that ro tates the polarizer to the null position. The polarizer angle, which compen sates for the sample rotation, is re corded by a follow-up servo driving the recorder pen. The entry by Bendix Instruments, Cincinnati, Ohio, is made in England —the electronic components by Ben dix Ericsson U.K., Ltd., and the optical assembly by Bellingham & Stanley. Workers at England's National Phys ical Laboratory worked out the basic design. The instrument has a single scan speed of about 23 min. The only moving parts are the scan mirrors. Noise level is 0.2 millidegree, referred to mercury green in the spectrum. The Bendix automatic instrument has several other features. Functions of the double monochromator and the polarimeter have been combined. Use is made of two synthetic quartz crystal line prisms, which not only disperse the light but also polarize it. The Faraday effect is used to modulate the light as well as to compensate for rotation of the specimen, Bendix says. The Rudolph instrument uses a 450watt xenon arc source. Like the Cary Model 60, it has a double monochro mator. Scanning time varies from a few minutes to 48 hr. Rudolph hasn't said when its model will ap pear, but indications are that it will be soon. Whitelaw Wright, product line man ager at Applied Physics, feels that there is still a considerable market for ORD instruments and that it is prob ably growing. The total market for instruments in the top price range is about 150 to 200 over the next three years, he says. Applied Physics al ready has several firm orders for the Cary. Because production rate for instru ments as complex as the spectropolarimeters is generally low—perhaps four or Bve a month—the market is big enough for the three present con tenders. Nevertheless, there is likely to be stiff competition on the basis of specifications and other features of the different instruments.
