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Chemical instrumentation Edited by GALEN W. WING, Seton Hall University, So. Orange, N. J. 07079 T h e s e articles are intended to serve the readers OJTHIS JOURNAL by calliny attention to new developments in the theory, design, or availability of chemical laboratory instrumentation, or by presenting useful insights and ezplanations of lopics that are of practical importance to those who use, m Leach the use of, modern insbumentation and instrumental techniques. The editor invites correspondence from prospective contn'butors.
LXXXI. Optical Rotatory Dispersion and Circular Dichroism
(Continued)
Kin-Ping Wong Department of Chemistry,,University of South Florida, Tampa, Florida 33620
Ord Measurements. The basic requirements for ORD measurements are: (1) a means of producing an intense linearly polarized beam of monochromatic light a t various wavelengths, and (2) a detection system with its associated electronic circuitry. A schematic diagram of the basic instrumental components is shown in Fig. 7. An intense light source, L,
H L
M
P
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A
D
these are always associated with absorption bands. The requirement for a broad and continuous spectrum is particularly essential for recording spectropolarimeters so that there is no need for repeated changing and focusing of the lamp. For most ORD and CD measurements, a high-pressure, 450 W, xenon arc lamp, such as the Osram XBO 450 WIP, has been proven t o he useful. This source can provide high-intensity, continuous emission through the range 185-600 nm. However, i t does possess a few disadvantages, such as the need for nitrogen purging, the instability of the arc, and the large amount of stray light produced. Recently a new lamp has been reported (7). Although not explicitly stated, this lnmn ammars t o be a modification of the standard 45u \Vnrr Ocrnm xenon arc lnmp. I t was de\&ped jointly with I'KK. Inr., at >unnyvalr. Cnl~fmnra.The arc ipxin:: uf this lamp has been reduced t o 1.3 mm for compact arc geometry and goad stability. Highly UV-transmitting quartz was used to construct the envelope in order to increase emission below 230 nm. This results in about 10 times the luminosity given by the standard Osram xenon arc a t 200 nm. However, the increased intensity must be paid for by even larger amounts of stray light. Future development of a continously tunable laser source applicable to spectropolarimetrv will be a desirable solution. I ~ . n . , c h r o m o r u r .rkmble quart7 prism tlr grating monorhrumaters arc gcncrdlv u;cd in modern commercial spectropolarimeters in order to reduce stray light which will affect the apparent rotation detected by the analyzer (since some of this stray light may be polarized). A douhle monochromator will also increase the wavelength resolution of the instrument. In most automatic recording spectropolarimeters, the slits can he programmed for constant energy a t all wavelengths. Linear polwirer. Any device which ren~
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Fig. 7. Optical block diagram of a simplified spectropolarimetsr far OR0 measurements. L, light source; M, monochromator; P, polarizer; S, S a m ple cell: A, analyzer; D, phatomubplier.
which covers a wide spectral range; a manochromator M, and a linear polarizer, P, constitute the essential components for producing linearly polarized light. The light beam passes through an optically active sample, S, and the ORD spectrum is then obtained by the detection system consisting of an analyzer, A, a photomultiplier. D, and the associated electronic and electrical circuits, and is finally displayed hy a recorder. Each of these components will he discussed in the following paragraphs. Light source. A high intensity lamp which emits a broad and continuous spectrum is required. It is necessitated by the fact that the light beam has t o pass through a polarizer and an analyzer, both being rather poor light transmitters, particularly a t short wavelengths. The other reason is that one is usually interested in measuring the ORD in the Cotton effect region in which the sample absorbs strongly. This is especially so in CD measurements as
feature ders ordinary light linearly polarized is called a linear polarizer. Three different types of linear polarizers are commonly used. Reflection polarirers produce linearly polarized light hy reflecting ordinary light externally from a dielectric surface with a n incidence angle equal to the Brewster's polarization angle, 4 = tan-' n, where n is the index of refraction. These are seldomly employed because of a number of drawbacks. Dichroic polarizers, such as Polaroid, are widely used for visible radiation, but have rather limited use in the ultraviolet region. The type which is most suitable for ORD and CD measurements in the ultraviolet and visible region, is the birefringent polnrirer. It produces light within a highly defined plane of polarization and possesses high transmittance a t a wide range of wavelengths. This type of polarizer is normally made from doubly refracting uniaxial crystals such as calcite ar quartz. Several designs of hirefringent polarizers are in common use. Most speetropolarimeters employ Rochon type polarizers which are made from two prisms cut from a uniaxial crystal with their optic axes perpendicular t o one another and one of them parallel to the direction of the incident light. Roehon polarizers made of quartz are of particular interest because of their high transmittance in the far ultravialet dawn to 175 nm. Some recent designs of polarizers using specially cut crystal quartz prisms that permit the prism t o he simultaneously used as a monochromator have been employed in spectropolarimeters. The advantage is the elimination of an additional optical component, resulting in better transmission and thus more intense polarized light. Such combined monoehromator-polarizers have been applied t o the Bendix Polarmatie Spectropolarimeter and is also used in the new JASCO 5-40 series of circular diehrameters.
Modulation When a polarized light beam passes through an optically active sample, its plane of vibration is rotated by an angle which can he masured by the use of an analyzer. In visual polarimeters, constant light intensity reaching the eye is used t o determine the closeness to crossed position of the polarizer and the analyzer. In modern commercial recording spectropolarimeters the symmetrical modulation method for automatic high-precision measurement of the plane of vibration is utilized. The method involves oscillating the polarizer through a few degrees about a mean angle, and results in extinction when the mean angle is equal to zero. This method is essential for the application of mechanical and electrical techniaues with the ootieal
(Contmnued onpage AlO) Volume 52. Number 7. January 1975
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Chemical instrumentation cal means are used in modern commercial instruments to modulate the plane of polarization of the light beam. The mechanical oscillating polarizer method is the simplest, conceptually. I t involves driving a servo system that rotates the analyzer in response to the signal detected by the photomultiplier, so that a symmetric signal is obtained. All JASCO speetropolarimeters use this method. The other technique for symmetric modulation inwlves the use of a device called a "Forodoy eell" to rotate the plane of the polarized beam so as to eompensate for rotation due to an optically acLive sample. This device is made from a silica cylinder surrounded by an electric eoil. ~ c c o i d i to n ~the Faraday effect mentioned earlier, this cell produces a rotation that varies linearly with the current passing through the eoil. When the polarized light traverses an optically active sample, the resultmr: arymetric nr current in the photomultiplier is detected and used to drive the polnrlner to the null position. The Cary 6J and the Bendix ~ ~ e c t r o p o l a r i m e t e r s u s e this method for modulation.
Photomulfipliers. In addition to the standard phototubes, a photomultiplier with a working range of 18L800 nm is available. It is an end-on type (R 376, S-20 surface) manufactured by Hamamatsu. Its advantage is its sensitivity in the red region together with UV performance similar to the standard tubes.
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Commercial Spectropolarimefers. Cory Instruments Division of Varion As. saeiotes. Cary Instruments manufactures the Cary Model 60 spectropolarimeter. This is probably the most popular instrument of its kind, particularly in the hiochemical field. The detailed features of the Cary 60 have been described (8). Figure 8
YD Fig. 8. Optical block diagram of the Cary 60 ORD spectropolarimeter. 1, light source; M, monochra mator: P, linear polarizer. S, sample cell: F, Faraday cell: A, analyzer: D, phdomultiplier.
is the optical block diagram of this instrument. An intense light-beam from a 450 W xenon arc source L is monochromatized by a fused-silica, douhle-prism monoehromator M, and then linearly polarized by a Rochon type polarizer P. The polarized beam then passes through the sample cell S and reaches the Faraday modulator eell F. The alternating magnetic field generated in the eell cyclically rotates the plane of polariza-
tion from its null position. The rotation is doubled via refledion of the light beam by a mirror on the rear surface. The beam then passes through a linear analyzer A of the Seramont type, and reaches the photomultiplier, D. The spectral range is from 185 to 600 nm, but can he extended to 800 nm by using a wide range detector. The RMS noise level (without sample) corresponds to 3.3 X degrees of rotation a t 200 nm with a 3-second Den and 1 . period . nm apwrral band width. T h r Hochon pul a r k r and Serarnunt analyrer are fnhricatrd trom ammonium dihydrogen phosphnte. and are sealed. The J a p a n Spectroscopic Company. This company makes several spectropolarimeters: the JASCO J-5, the JASCO J-15, and the most recent model 5-20. The first model can also be used as an absorption spectrophotometer. The J-15 can be used as a spectrophotometer as well as a circular dichrometer. The 5-20 is the most up-todate model for ORD and CD measurements. Only the ORD instrumentation of the 5-20 will be described here. The ORD mode of the other models basically follow the same design. The simplified optical block diagram of the 5-20 is the same as that of Fig. 7. Monochromatic light emerging from the fused silica double-prism monochromator M is linearly polarized by the Rochon auartz polarizer P. The polarizer is rocked bv a 'mechanical drive and oscillates through an angle of i 1 ° at a frequency of 12 Hz, so that the plane of polarized light is alternately rotated to the left and right of (Continued onpage AI2J
Chemical Instrumentation the mean. The pulsed polarized beam then passes through a sample cell containing an optically active material and its plane of polarization is accordingly rotated either to the left or right. Thus unequal light pulses reach the photomultiplier and the resultant output is used to rotate the analyzer t o a null position, through a servo system. This angle through which the analyzer must be turned is equal to the optical rotation of the sample, and is transmitted to the recorder. Bellmgham and Stanley, Ltd., and Bendir Electronics, Ltd. A spectropolarimeter manufactured by these firms is known as the Polarmatic 62. I t is based on the novel design of Gillham and King (9). A simplified optical block diagram is shown in Fig. 9. The navel feature, as previously mentioned. is the use of crvstal auartz ~ r i s m s for the simultaneous dispersion and polarization of light. The monochromatic linear polarized beam emerging from the prism passes through the first Faraday cell F I , which is used to compensate for sample rotation, and then the second Faraday cell F z , which modulates the direction of vibration of the light a t 380 Hz. The light beam traverses the sample S and is rotated thereby. I t is then dispersed by the second quartz prism P p , which simultaneously acts as a linear analyzer. Finally the light beam reaches the photomultiplier D and is converted to electrical signals. The 380 Hz signal passes through a phase-sensitive detector. The resulting signal is then used to provide a compensating current to the Far-
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CD Measurements.
Fig. 9.Optical block diagram of the Bendix Polarmatic 62. The light source is designated L; P, and P2 are crystal quartz prisms tor bath dispersing and polarization of the light; Fz and FI are Faraday cells which are used to modulate the polarization and to compensate tor sample rotation respectively; Sis the sample cell: and D i s the photomultiplier.
aday cell F I ,whieh is a measure of the optical rotation of the sample and can be recorded. This instrument has a spectral range of 181-600 nm and its ohvious advantages are the few optical elements and moving parts. In addition to the three speetropolarimeters discussed above, there are several other instruments whieh are basically similar. The Rudolph recording spectropolarimeter is similar t o the JASCO 5-5 but with a lower precision. However, it can he used for very large rotation measurements up to 200° without linear error. The PerkinElmer Corporation manufactures a Model P22 spectropolarimeter whieh is also similar to the JASCO 5-5 except that a douhle grating monochromator is used. The Zeiss Company makes a spectropolarimeter which is similar t o the Cary 60.
As described earlier, two types of measurements can be obtained for CD, (1) the difference in absorbance of the left- and right-circularly polarized light, and (2) a direct ellipticity measurement. The measurement of ellipticity, which involves the extent to whieh a sample converts the linearly polarized into elliptically polarized light, is rarely used in modern commercial eirculsr dichrometers. All precision circular dichrometers utilize the direct measurement of the difference of absorption of the left- and right-circularly polarized beams. Most of the major components in a circular diehrometer such as the light source, the monochromator, the linear polarizers and the photomultipliers, are the same as in ORD spectropolarimeters described in the previous section. Thus we only need t o discuss the components for the production of left- and right-circularly polarized light. The typical method for the production of circularly polarized light is illustrated in Fig. 10. Linear polarized light emerging from a polarizer is resolved into two eomponents polarized parallel to the fast, f, and slow, s, axis of a device called a "quarter-wave phase retarder." The polarized components emerge from the retarder with a quarter-wave phase difference and then recombine to produce circularly polarized light. Phase retarders based on total internal reflection and the electro-optic palarization modulator based on the Pockels effect are commonly used in CD instrumentation. All presently available commercial (Continued on page A141
Chemical Instrumentation
Fig. 10. Prcd~ctionof circularly polarized light.
circular dichrometers employ the latter device, called the "Pockels cell." I t is constructed from a Z-cut uniaxial crystal of the dihydrogen phosphate type sandwiched between two electrodes, as shown in Fig. 11. The crystal hecames biaxial upon application of a n electric field. If the field is applied in the direction of the crystal Z-axis, the induced optic ares x' and y' will he inclined a t 145' to the crystallographic xand y-axes shown in the inset of Fig. 11. For this system a t a fired wavelength, the induced retardation is directly proportional t o the voltage of the applied field. The voltage is almost a linear function of wavelength. Thus if a voltage which will cause the crystal t o act as a quarter-wave plate is applied, linearly polarized light will be eonverted to left- and right-circularly polarized radiation. Despite the versatility of the electro-optic modulator, some inherent drawbacks should be noted. The presence
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of the electrodes causes loss of optical transmission. This loss is further increased by the material used in sealing the crystal against damage by atmospheric moisture. Furthermore, the potassium dihydrogen phosphate crystal has heen reported to exhibit eleetro-luminescence a t 327 nm. Although the Poekels cell is used almost exclusively in all commercial CD instruments, some new approaches have been described. For example, the use of a n elastooptic modulator has been reported (10).
Commercial Circular Dichrometers. There are basically two types of instruments available for CD measurements. The first type, an accessory to a spectrophatometer, gives relatively low precision but gives absolute measurements of differences in absorption. The Cary Model 1401, the Reo"0th Model CD-LD-HC, and the Shimadzu CD attachment belong t o this class. The other types are either an attachment to or modification of a n O R D spectropolarimeter, utilizing an electro-optical modulator for production of circularly polarized light. The Raussel-Jouan CD 185, the JASCO J10,J-15,J-20, and 5-40, as well as the Cary 6001 attachment and the Cary 61 all belong t o this latter class. They have much higher precision, and extend to wider spectral ranges. Roussel-Jouon. The Roussel-Jouan Dichrograph Model CD 185, manufactured in France by Jouan-Quetin, is available in this country from the Farrand Optical Company, Inc. I t is the first commercial circular dichrometer based on the navel design of
Fig. 11. Diagram of an slectrooptic modulator. Inset, a uniaxial crystal of the dihydrogen phosphate type. x, y, 2, and x', y', 2' are the crystallographic and eiactrooptically induced axes, respectively.
Velluz, Legrand, and Grosjean (3). An optical block diagram of this instrument is shown in Fig. 12. Linearly polarized light, emerging from a quartz Rochon polarizer P, passes through an electro-optic modulator E. The state of polarization is then modulated a t the same frequency as the driving voltage, alternating between leftand right-circular polarization. The ordi(Contimed on page A161
Chemical Instrumentation
L M P E 5 D Fig. 12. Optical block diagram of the Roussel Jouan CD dichrograph 185. L, light source: M. monochromator: P. linear polarizer: E, electrooptic modulator: S, sample cell: D, photomultiplier. narv lieht beam traverses a s a m ~ l ecell. S
consisting of a dc and an ac component a t the terminals of the photomultiplier. The ae component is amplified and detected in phase with the voltage applied to the eketro-optic modulator E. The voltage of the dc component is a function of the absorption intensity and is applied t o the recorder terminsls. The ratio of these two voltages is a linear function of the absorption difference between the left- and right-circular components of the modulated light. This instrument allows CD measurement be-
lar dichrometers have designs which are analogous to this instrument. Japan Spectroscopic Company. This company manufactures several models of circular dichrometers. The J-10 is combined with an ultraviolet spectrophotometer. The J-15 is basically similar, hut also has ORD capability. The more recently marketed Model 5-20 has both ORD and CD capability and is basically an improved version of J-10 and J-15. JASCO has also introduced a new series of circular dichrameters, the Model J-40's which have some novel features. The 5-20 is similar in design t o the Roussel-Jouan Diehrograph. No technical specification of the J-40's are available a t the time of this writing. From the limited information available, its design appears to be an improvement of the original Grosjean-Legrand instrument as reported by Velluz and Legrand (11). A synthetic crystal quartz double monochromator is used in place of the single monochromator and polarizer. Some other improvements have also been reported. A stepping motor is used t o drive the monochromator and recorder and no mechanical gearing is required for operation. The three models available differ in the spectral rsnges covered. The 5-40 A has a range of 185700 nm, whereas the 5-40 B a n d 5-40 C extend to 800 and 1000 nm, respectively. Other designs are basically the same as the 5-20, Cary Instruments. Cary makes three devices for CD measurements. The model 1401 is an accessory to the Cary 14 doublebeam spectrophotometer. It is based on the design of Holzwarth (12). The device works as follows. A quarter-wave plate made of ammonium dihydrogen phosphate and a Fresnel rhomb are placed in both the sample and reference compartments such that the reference beam is rendered right-cireulady polarized and the sample beam is leftcircularly polarized. Identical solutions are A16
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placed in the two beams, and the difference in absorption is recorded. Although the device is of low recision and has limited wavelength range, it is simple and inexpensive, and it provides absolute CD. measurements. Cary also makes a CD attachment, Model 6001, to the Cary 60 spectropolarimeter. This attachment is similar to the Roussel-Jouan CD 185 using the Pockels cell t o generate circularly polarized light and has about the same precision and spectral range. The recent Model 61 is a circular dichrometer with some improved features, based on the same design. Other Instruments. The Rehovoth Instrument Company also manufactures a CD attachment to the Cary 14 or other double beam spectrophotometer. Its basic design is similar to the Cary 1401 CD attachment except that quartz multi-wave plates are used.
Magnetic ORD and CD Measurements. The instrumentation for MORD and MCD measurement require a good recording spectropolarimeter and circular dichrometer in which a uniform magnetic field can be applied through the sample, parallel t o the optical axis of the instrument. Usually MCD measurements are made with the light beam propagating in the positive direction of the magnetic field. The sign of the magnetic optical activity is the same as that of natural optical activity with this arrangement. For modern soectro~olarimetersand dic hromrtrrc, all that i* rcquirrd is the iim plr sddilirw (1 n magnet r i l l ) stlff~rienr t ~ e l ditrtngth and acme sm.all nereswrie. A Varian compact, variable temperature, horizontal bore, superconducting magnet is available for MORD measurement with the Cary 60 and for MCD with Cary 61. The Durrum-JASCO 5-20 and the newer 5-40 series circular dichrameters also have a spacious sample compartment and magnetic CD measurements have been performed with a 49.5 kG superconducting magnet built by Lockheed Palo Alto Research Lahoratories (Model OSCM 103). A 5,000 Gauss permanent magnet is also available far the Durrum-JASCO instruments. The location of the light source in the Cary instruments is in close proximity to the sample compartment where the superconducting magnet is installed. This appears to cause some problem in the stability and life of the xenon arc lamp. Such problems appear to have been remedied by shielding the light source from the large magnetic field nearby. The lamp housing far the JASCO instruments is located farther away from the magnet and does not cause the same problem.
LITERATURE CITED 7. N.S.Sirnmnns,et al..Biopolymerr. 7,275119681. 8. H. Cary,et al.,Appl. Oplrcr. 3.329 119641. 9. E. J. Gillham and R. J. Kin& J. Sci. Indrum.. 38.21
\."".,. ,,oa,,
10. R. H. Breeze and B. Ke. An.1 Rirmhem., 50. 281 (19721. 11. L. V e l l ~ sand M. Legrand. I n s e m . Chem.. Internal. Ed. (EnglJ. 4.818 119651. 12. G . M. Hoizwarth, Rru. Sei. Inarum.. 36.59 119651.
