Robert E. Lyle and Gloria G. Lyle University of New Hampshire Durham
1I A. 7rief B... History of Polarimetry
T h e polarimeter has played as celehrt~teda role in the development of theoretical organic chemistry as its essential pasts, the Nieol prism and the quartz plate, have done in theoretical optics (I).
Polarimetry, along with refractometry, provided the earliest methods for the organic chemist to investigate structure without the destruction of the material under examination. The first rate expression for a chemical reaction resulted from the investigation of the inversion of cane sugar by Wilhelmy (3) in 1850, providing the initial example of the application of polarimetry to kinetic investigations. The elegant research of Pasteur (3) on the stereochemistry of tartaric acids opened a segment of organic chemistry which has provided the understanding of tne fine structure of molecules and which is largely dependent upon polarimetry as the tool for research. The uses and analytical applications of polarimetry in industry have kept pace with the theoretical discoveries. Thus polarimetry was adopted very soon after birth by the sugar industry, has been applied to fermentation processes, and has been graduated into the manhood of circular dichroism by the pharmaceutical industry. Any consideration of the history of the development of polarimetry (4) must have as its origin the discovery of plane polarized light, which in turn is connected in time with the early investigations of Iceland spar. An explanation of the unusual double image produced by the spar and the fint description of what was later called "polarized light" was given by Huygens in 1678 although the properties of Iceland spar had been re~ortednine years earlier by Erasmus Bartholinus (6j. One hundred and thirty years. passed before further optical discoveries related to the polarimeter were made. I n 1808 Malus (6),whiie observing the rays of the sun reflected by the windows a t Luxembourg Palace through an Iceland spar crystal, discovered that light with the same properties as those described by Huygens could be produced by the reflection of light from a polished surface of a transparent solid such as glass, marble, or water. The reflected ray was completely polarized if the angle the ray made with the plane of the mirror were proper. This angle was later shown by Brewster (7) to be proper when the refractive index of the material composing the mirror was equal to the tangent of the angle of the incident ray. Malus, in the description of his apparatus, related the orientation of the mirrors and the ray of light to the poles of the compass. Presented as psrt of the Symposium on History of Equipment and Instrumentation before the Division of History of Chemistry at the 145th meeting of the American Chemical Society, New York, N.Y., September, 1963.
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Thus the ray of light was directed from the south and allowed to strike the lower mirror a t the polariziig angle and with the lower mirror oriented to direct the reflected beam upward. A second mirror was arranged above and parallel to the first, thus facing north. Under these conditions nothing unusual occurred and the reflection of the light was observed. If, however, the upper mirror were oriented to face the east or west, no light was reflected from the second mirror. Only when the upper mirror faced north or south was a light beam transmitted by the two mirrors to the screen (Fig. 1). As Malus said (8): Giving to these sides the nsmerr of the poles, I will describe as pofmized the modification which gives to light its properties relatively to these pales.
Figure 1. Polorired light b y reflection. When the polarized light p o s e r bemeen two mirrors placed parallel to each other, os a t the left, it is reflected from the second mirror and con be observed normally. If the mirrors .re not porollel, or ot the right, the beom is absorbed b y the mirror or lost as indicated b y the dotted line and sonnot b e observed Moivr described the light as "polarized" (see text).
First Use of Prisms
The polarization of light by reflection was inefficient, and the precise determination of the plane of polarization was difficult, if not impossible; however, the simplicity of construction and the qualitative accuracy in determining the plane of polarization caused this method to be widely used for both polarizer and analyzer in early investigations of optical properties of crystals (see device of Duboscq, Fig. 2). The ordinary and extraordinary rays produced by the spar crystal were not sufficiently separated to allow the unmodified crystal to be used as the "polarizer"; however, the lack of destruction of one of the rays presented no difficulties when the spar crystal was used as the ' aanalyzer." The devices used by Fresnel, Arago, and Biot in their early work on polarized light used this system and the zero point of the instrument was taken as the location of the spar crystal a t which the image due to the extraordinary ray was just extinguished. The introduction of any kind of substance which
caused the rotation of the plane or polarized light between the polarizing mirror and the analyzing spar crystal would cause the reappearance of the image from the extraordinary ray. The angle of rotation of the spar crystal required to extinguish the extraordinary image was then taken as the angle of rotation.
These early papers of Biot also contained reports of the optical rotation of many organic compounds and sugar solutions. It is not surprising, therefore, that when reports of the startling separation of racemic acid (paratartaric acid) by Pasteur into two compounds which differed only in their effect on the direction of rotation of plane polarized light (S), Biot requested this young student a t the Ecole Normale Superieure to demonstrate this separation in Biot's laboratory. Pasteur repeated this highly significant experiment under the watchful eye of Biot and used the first instrument constructed as a polarimeter (Fig. 4), designed by Biot (11) using the principles of earlier polarizing instruments, to check the optical activity of the resolved tartrates. Nicol Prisms
Figure 2. A polariscope of D u b o f ~ q for the study of the polorimtion of light by reflection. IPoloir de lo DCcauverte, Parir).
Figure 3. An eorly polarimeter of Biot. Note vertical form. lPhotogroph by Mu& Porteur. Paris.)
Figure 4. Poiorimeter of B i d with which he checked the mtotion of the lPhotogroph by MvrCe tartaric acid resolved by Porteur in 1848. Pmteur, Parir.1
Since the magnitude of the angle of rotation caused by a substance is dependent upon the wavelength of the polarized light as well as the thickness of the sample, the extraordinary ray image could not be completely extinguished when white light was used with this device. Arago (9) had reported that the introduction of a quartz plate between the polarizer and analyzer caused the extraordinary ray image to become unextinguishable and that rotation of the spar crystal caused the extraordinary ray to assume the colors of the spectrum. The investigation of this phenomenon by Biot (10) using quartz plates of varying thicknesses with the instrument in Figure 3 allowed him to develop the basic quantitative equations of polarimetry, the concept of optical rotatory dispersion (Law of Inverse Squares), and optical superposition. These relationships were all necessary for the development of saccharimetry (see below).
The development of an accurate polarimeter capable of analytical precision required the modification of an Iceland spar crystal in such a way as to separate the ordinary and extraordinary rays allowing the modified crystal to be used as the polarizer. Nicol (IS) reported such a modification as early as 1828; however, perhaps due to poor transmission properties or reaction to change, the Nicol prism was not introduced into a polarimeter as the polarizer until about 1842 (IS). As late as 1858, a polarimeter utilizing reflecting mirrors as both polarizer and analyzer in an instrument similar to that of Malus called the Norrenberg polarimeter, was exhibited a t Carlsruhe (4e, p. 180) and was listed in catalogues as late as 1866 (14). The modification of Iceland spar devised by Nicol to separate the two plane polarized rays was very effective and with only slight modification is the type of prism used in modern instruments. The calc spar rhombic crystal with the long dimension about three times that of the other two axes was cut on the ends to reduce the angle of the faces from 72 to 68", and a second cut was made in a plane perpendicular ~ ~ ~ ~ c a t i o ~ to the plane containing the optical cdc soor crystal for axis and perpendicular t o the small ~ ~ o ~ $ ' ~ t ~ , e face of the crystal (Fig. 5 ) . The i, .t fl,t .t the ends new faces were and re- and 'hen diagonally lengthwise or incombined using Canada balsam, a dicated by the substance of refractive index be- lines, polirhed, and with tween those of the two polarized rays. The combination of two Nicol prisms to provide the polarizer and analyzer of a polarimeter was reported by Ventzke (15) in 1842. The instrument was used with white light and the zero point was reached by rotation of the analyzer. The field a t the critical rotation was a bright red due to the total extinction of the yellow component of the white light. A similar apparatus using monochromatic light was reported by Mitscherlich (16) in 1844. Crossed Nicols of large field do not show complete blackness but a dark band a t the center of the field a t an angle bisectmg the angle of the principal planes of the two Nicols with a series of small lines on each side of the dark band. Centering this band by rocking the analyzer through a small angle gave the maximum accuracy of the instrument. The instrument could be used with white light to measure small angles of rotation, for under these conditions Volume 47, Number 6, June
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the dark band represented the total extinction of the yellow light and was bounded on one side by a red field and on the other by a blue field. At larger angles the boundaries were very diffuse, leading to inaccuracies in reading. The potential of the polarimeter for analysis of solutions and raw materials for sucrose content had been recognized by the sugar industry (17). The evaluation of raw sugar to determine sale price and duty required a convenient, rapid, and accurate method of analysis for sucrose. The determination of the sucrose content of unprocessed sugar beets for the purpose of setting the price presented similar requirements. The polarimetem of Ventzke and M i t scherlich met the requirements except for accuracy. The highly colored solutions of raw sugar required strong light sources which could be obtained only with white light, a source usually leading to the largest errors in analyses. Thus the next major developmentg were designed to increase the sensitivity of the instrument by providing a convenient and accurate method for determining the zero and critical point8 of the polarimeter.
Anolyrer rotated from clockwise zero point (+I Blue 1-1 Red
Critical Point; Teinte d e Pasloge; red-violet color
Anolyrer rotated counterclackwire from zero point I+) Red 1-1 Blue
chromatic light was uncertain. Values for conversion to a, varied from 1.105 to 1.111 to 1.128 times mD (4f) Color-blind persons obviously could not detect the zero point, and colored solutions caused further confusion. Saccharimeter
Soleil (20) introduced a modification in the polarimeter of considerably greater importance by designing the first quartz wedge compensator and thereby the first saccharimeter. Biot had found that the rotatory dispersion of sucrose solutions was nearly identical with that for quartz. Thus compensation of the rotation of a sucrose solution by introducing a quartz plate of the proper thickness allowed the more intense white light to be used without complications from the rotatory dispersion of the optically active solution. A convenient instrument built on this principle required that a method for introducing quartz of various thicknesses be devised. Soleil provided this in the form of two quartz wedges, one from (+)quartz and the other from (-)-quartz a t least one of which was movable along the direction of the long side of the right triangle of the prism. The analyzer and polarizer were fixed a t 90°, the zero point was obtained by moving the quartz wedge or wedges to increase or decrease the thickness interposed in the light beam, and the scale for determining the sucrose concentration was attached to the movable quartz wedge. Because of the fact that a complete sucrose analysis required the determination of the optical rotation after inversion and therefore measurement of a negative rotation, a second movable wedge was necessary for the compensator in the later instruments (Fig. 7).
Figure 6. Appearance of fleld in eyepiece using biquorh plote of Soleil. For the color of the teinte de pmrroge see color plotesin ref. (13).
Soleil (18) prepared a divided field device for detecting the zero point of the instrument by cutting two semicircles of quartz 3.75 mm thick, the thickness determined by Biot as necessary to cause the rotation of the yellow rays of light exactly 90". The two semicircles, one half of (+)-quartz and the other half of (-)-quartz, were cemented together and placed between the polarizer and analyzer in an instrument constructed by Robiquet (19). This instrument was used with white light and a t the zero point or critical rotation the yellow light which had suffered a -90' and +90° rotation by the two halves of the biquartz plate (Fig. 6) would be extinguished by the Nicol analyzer, and the field would show a uniform tint of red-violet due to the smaller or larger rotation of the plane polarized light of shorter or longer wavelength than the yellow. At any small angle to the right or left of the zero point, however, the wavelength of light which intercepted the analyzer after passing through the (+)-quartz would be different from that passing through the (-)-quartz. Thus on either side of the zero point one half of the field would be blue while the other was red. With this optical device the instruA number ment achieved a sensibility of about *4'. of disadvantages were inherent in this instrument, however. Only white light could be used, and the conversion of the specific rotations determined by this method to those recorded on instruments using mono310
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Single wedge cornpenrotor
Double wedge compensator
Figure 7. Rotofions caused b y solutions of sugar were compensated b y moving the (+)-quartz wedge to the left for podtive rotations of sugars. The dolted p r i m shows the location ot the rero point, and the solid prLm indicote~campenratianfor a dextrorotmtory substance.
This basic saccharimeter was modified by Duboscq (21) in an effort to increase the sensitivity of the instrument with the highly colored solutions often encountered in sugar analysis. A second Nicol prism was added to the polarizer which by adjustment extinguished some of the colors in the field and allowed a sensitive tint to be established. The mechanism for this adjustment is evident in the SoleilDuboscq-Scheibler saccharimeter (Fig. 8). Due to a very slight diierence in the rotatory dispersion of sucrose and quartz with blue light, a dichromate solution was added as a filter to remove light of green and shorter wavelengths. A device described by de Senarmont (22) in 1850 offered some advantages but it required monochromatic light which a t that time could not be produced in an intense form. De Senarmont used two or four wedges of quartz arranged so that the light of the polarimeter passed through one wedge of (+)-quartz and another
wedge of (-)-quartz. Thus a t the zero point the light would pass through an equal thickness of (+)- and (-)quartz only a t the center of the two wedges, and this point would be determined by the appearance of the black extinction line a t the center of the field. The use of four wedges arranged as shown in Figure 9 produced two black lines which were contiguous only when there were equal but opposite rotations in the quartz wedges. It is difficult to understand why this system did not receive more attention. Another unique optical devioe for detecting the critical rotation was described by Wild (23) who used a Savart prism just before the analyzer producing an interference pattern which disappeared from the center of the field and was symmetrically arranged at the outer edges a t the zero or critical point. This is the most accurate single-field polarimeter and is interesting in that the polarizer rather than analyzer is rotated t o obtain the zero point. To avoid the decreased intensity and slight diffraction accompanying the use of the extraordinary ray as the polarized beam from a Nicol prism, Jamin (24) in 1869 reported a polarizer which utilized the ordinary ray. A thin sheet of Iceland spar cut parallel to the optic axis was immersed in a liquid of refractive index near that of the ordinary ray and the sheet was inclined a t a n angle of about 70' to the beam of unpolarized light. Jamin used carbon disulfide as the liquid medium, but an improved polarizer was reported by Brace using bromonaphthalene ($5). These devices provided the most sensitive polarimeters a t the turn of the century; however, the polarizers were so fragile that their routine use was impractical.
Figure 8. A Soleil rocchorimeter with quartz wedge cornpemotor from Porteur'.Iaborotory. IPhotogroph by Mu& Porteur, Paris.)
Half-shadow Instruments
A series of photometric devices was developed which had in common the division of the polarized field into two or more parts in which the planes of polarization were a few degrees inclined to one another. These components rapidly replaced the single field polarirneters because of the increased precision in deterrnining the zero points. They are commonly called "halfshadow" polarizers from the colloquial translation of the German, halbschatten, which better defines the uniform gray appearance midway between the extinctions of the two halves of the field.
The first half-shade device was described by the Reverend William Jellett (26) in 1858. A spar crystal about 2 in. long was squared a t the ends. A cut was made through the prism parallel to the long axis but the plane of the cut was a t a slight angle to the long diagonal of the end. The two pieces were reversed with respect to one another and cemented together as shown in Figure 10. Thus the plane of polarization was inclined in one-half with respect to the other by the angle that the cut made with the long diagonal.
Figure 9. A diagram of d e SenormonPs prisms for deterrninins the zero point ldoned line) of o polmimeter. The polarized light passer through equal ienglhr of (+I- and (-I-quartz only ot the center of the prism. When the analyzer is not crossed with the polorizer, the dark band is dir. continuow to the right or left of center.
Figure 10. Diagram showing ronversion of Iceland spar crystal ltopl to Jellett's half.lh.de onolyrer Ibottoml. See text for descrip tion.
Cornu ($7) modified the Jellett half-shade prism so that the crystal would produce the effect of the Nicol prism as well as the half-shade effect. Thus the JellettCornu prism was the complete polarizer. This device had a fixed "half-shadow" angle and could not be used with highly colored solutions or a t the highest sensibility interchangeably. This led to the adjustable half-shade device of Laurent (28) which was a thin layer of quartz covering one-half of the field, cut parallel with the optical axis to a thickness which would retard the extraordinary ray one-half wavelength as compared to the ordinary ray. I n passage the plane of polarization was altered slightly giving the "halfshadow" angle. This procedure could be used only with monochromatic light for a given quartz plate. Thus it was of limited value in saccharimetry. The most versatile and useful half-shade device was that introduced by Lippich (29) which could be used with white light and allowed the "half-shadow" angle to be varied. This device was a second Nicol prism in the polarizer. The second prism was small and only covered one-half of the field. Rotation of the large prism slightly gave the "half-shadow" angle which was adjustable. Since the zero point of the instrument was determined by a uniform field, the change in absorption by the small prism on changing the "halfshadow" angle caused the zero point to change with any rotation of the large prism. This was overcome by a gear arrangement for changing the position of the analyzer as the "half-shadow" angle was varied (&, p. 41). The Lippich device can be used with monochromatic light or, with a compensator, it can be used with white light. As later modified by Lippich to produce a triple field, this polarizer is used in most modern polarimeten. The problem of the change of zero with "halfshadow" angle was solved by the Hilger modification (SO) of the Lippich devioe in which two small Nicol Volume 41, Number 6 , June 1964
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prisms were placed after the polarizer. The twin Nicol prisms were positioned a t a slight angle to one another causing the half-shade. These prisms can be rotated, but the rotational device causes a simultaneous and symmetrical rotation of both prisms about the vertical axis. Thus a change in the "half-shadow" angle does not change the zero point of the instrument. Adaptions for Analysis
During this later period of refinement of the halfshade devices polarimetry was finding application in a variety of industrial analytical procedures ( 4 4 as well as in academic research (31). The majority of the industrial applications were related to the determination of the concentration of sucrose in various products. In the fermentation industries the hydrometer and refractometer were analytical tools for the determination of the composition of the alcohol-water mixtures of distilled spirits, but the composition of the complex mixture of beer presented a third unknown, dextrose. The concentration of the dextrose could be determined by polarimetry allowing the analysis, for tax purposes, of the other two components of importance to be determined by hydrometry. In those industries concerned with the production of food products containing carbohydrates, polarimetry was a most important control instrument. The determination of the sugar content of jams and jellies and the control of the carbohydrate percentage of condensed milks are but two examples. The discovery of the pharmaceutical importance of natural products such as steroids and alkaloids caused a phenomenal increase in research in this area of organic chemistry. In addition to the usual structural problems associated with organic compounds, these natural products introduced the question of the determination of the relative and absolute configurations of the many asymmetric carbons in these complex structures. Theoretical and empirical approaches to the solution of this problem by comparing the magnitudes of the optical rotation a t a given wavelength of light or by comparing the magnitude and direction of changes in rotation with solvent or structural changes met with limited success. Accompanying these attempts was the investigation of the optical rotation of organic compounds a t varying wavelengths (optical rotatory dispersion). The discovery that Biot's relationship (Law of Inverse Squares) was not adequate (32) provided the incentive for the development of instruments of greater sensitivity. The early optical rotatory dipersion instruments placed a spectroscope at the eyepiece of the polarimeter. As the analyzer was rotated, a dark band corresponding to the wavelength of the light extinguished by the analyzer a t that position moved across the spectrum produced by the spectroscope. The first improvement was to place the spectroscope (monochromator) between the light source and polarizer allowing a measurement to be determined a t a specific wavelength. Optical Rotary Dispersion
In order to measure rotations a t wavelengths outside the region of sensitivity of the human eye, the use of sensitized photographic plates was initiated by Joub'in in 1889 to measure magneto-optic effects (S),and 3 12 / Journal of Chemicol Education
improvements in the monochromator and polarizer allowed Lowry (4e, p. 218), Bruhat (34), and Kuhn (36) to extend the range of wavelengths for measurements of the optical rotatory power. The development of the photo tube provided a more convenient method for detecting the critical point of rotation and was utilized by Rudolph in the first commercially available visible and ultraviolet spectropolarimeter (36). The relationships of ultraviolet absorption, circular dichroism, and optical rotatory dispersion have been recognized since the experiments of Cotton (37) demonstrated the unequal absorption of left and right circularly polarized light in the region of absorptiou. This phenomenon is called a "Cotton effect" and is evidenced in optical rotatory dispersion studies by the production of anomalous curves. The rapid progress in the correlation of such rotatory dispersion curves with absolute and relative configurations of organic compounds with the assistance of the Rudolph instrument permitted Djerassi and his collaborators to advance the understanding of optical activity and its theoretical bases (38). A renaissance of interest in optical activity has accompanied the developments in this area during the past 10 years. The importance of optical rotatory dispersion measurements for the qualitative and quantitative analyses of natural products, in particular the steroids, created a keen interest by the pharmaceutical companies in a rapid and convenient method of instrumentation. The recording spectropolarimeter of Rudolph was the first such instrument; however, the mechanical method used for the determination of this property possessed several inherent difficulties, most prominent of which was the difficulty in obtaining readings in the region of ultraviolet absorption of the compounds. The utilization of the Faraday effect (39)in more recent instruments promises to overcome this difficulty. The effect of an electromagnetic field on an optically inactive liquid such as water produces an optical displacement that may be used to compensate the rotation of plane polarized light caused by a solution of an optically active material. The instruments recently developed by Bellingham-Stanley and Bendix in England and Applied Physics Corporation of California use the Faraday principle. Paralleling the improvements in the essential parts of the polarimeter, the polarizer and analyzer, have been developments in the accessories, the light source, monochromator, and polarimeter tubes. The tubes have been almost unchanged since the days of Pasteur except for mechanical improvements and the nature of the materials from which they are prepared. The light source, however, has undergone extensive modification in order to provide sufficient light to penetrate a highly absorptive solution. A phenomenon closely related to optical rotatory dispersion is circular dichroism which has been investigated extensively only recently with the use of a Dichrograph developed by Velluz and Legrand (40) of the Roussel-Uclaf pharmaceutical industry in collaboratiou with Jouan Instruments of France. The greater versatility of circular dichroism for the qualitative and quantitative analyses of steroids, alkaloids, and optically active synthetic pharmaceuticals and the lack of a recording instrument for the measurement of
( 4 ) Smne nl;ptr.ls of the histmy of pdurimefry are rewnwed in lo LANDOLT, H., "The OptlrsI Rotating Pwmr of Or-
this property encouraged this pharmaceutical company to initiate the development of the Dichrograph. Circularly polarized light was first described by Fresnel (dl), hut its exploration was delayed for almost a century until the investigations of Cotton ($7). Rapid developments in this field should now be possible with the use of the Dichrograph. The availability of recording instruments for the rapid determination of optical rotatory dispersion and circular dichroism in the visible and ultraviolet regions of the spectrum has facilitated greater understanding of the nature of optical rotatory power and correlation of optical activity with structural and conformational features of molecules. I n Pasteur's lectures on molecular asymmetry delivered just a hundred years ago, he said (4.9):
uarziv Sulwmws." ('hemid Pulhlaineu ('
