An Automatic Recording MagnetoopticalRotation Spectropolarimeter

J. G. Forsythe, R. Kieselbach, and V. E. Shashoua

The design and construction of an automatic recording spectropolarimeter for measuring magnetoopticalrotations (Faraday effect) in a magnetic field is described. The instrument has two opposed 5000 G mag-netic fields (one for solution and one for solvent) to give solute rotation directly as a function of wave-length (250-750 mIA) to a precision of 4-0.001' in the visible and 140.0030 in the uv region of the spectrum.A description of the instrument performance and typical spectra is given.

Introduction

A number of spectropolarimeters have been pre-viously described for measuring the optical rotationof polarized light for a naturally optically activecompound as a function of wavelength. For example,Gilham and King' describe the basic elements for suchan apparatus and, more recently, Cary et al.2 havedemonstrated that it is possible to record automaticallyangular rotations of polarized light to a precision of0.0010 in an automatic recording instrument. In thislaboratory, we have been interested in studying themagnetooptical rotation (MOR) spectra3 of substancesas a function of molecular structure. This type ofspectroscopy' requires a magnetic field to induce opticalrotations in molecules which are otherwise opticallyinactive (Faraday effect). A recording of the inducedmagnetic rotation as a function of wavelength gives aniVIOR spectrum. MOR spectroscopy offers the possi-bility of extending the field of the optical rotatorydispersion (ORD) technique 4, which is confined tonaturally optically active substances, to all substancesirrespective of whether they possess any naturaloptical activity.

The general characteristics of MOR spectra have beendescribed elsewhere.' Anomalous dispersion featuresare observed in the regions of spectral absorption, asthey are with naturally optically active substances.The requirements for an instrument for IVIOR spectro-scopy are quite similar to those for ORD spectroscopy.Basically, the instrument must be able to measure theangular rotation of plane polarized light within strong

All authors were with Engineering Physics Laboratory, E. I.du Pont de Nemours and Co., Wilmington, Delaware 19898,when this work was done; V. E. Shashoua is presently with theNeurosciences Research Program, MIT, 280 Newton Street,Brookline, Massachusetts.

Received 28 September 1966.

absorption bands to a precision of at least 0.0010 withthe sample held in a uniform magnetic field parallel tothe direction of light transmission. With this in mind,the instrument described in this paper was designedand constructed for use in application studies of AMORspectroscopy.

Discussion

Description of the InstrumentFigure 1 shows an optical diagram of the MOR spec-

tropolarimeter. The light source is a 150-W dc xenonarc (Osram), whose image is focused by lens L-1 at theentrance slit of a double monochromator (Cary Model14). This provides the means for wavelength selectionwith the required low level of stray light, down to lessthan 1 part/million. A lens L-2 at the exit slit of themonochromator provides a parallel beam of mono-chromatic light to the quartz Rochon polarizer of theinstrument. The polarized light beam then passesthrough two magnetic fields in series, containing thesample and solvent compensation cells, and then to aFaraday modulator' consisting of a 20-cm long quartztube filled with water mounted along the axis of an air-core solenoid. This modulates the light beam to givethe required frequency for the angular sensing andmeasuring sections of the instrument. The light fromthe Faraday coil is then passed through an analyzerand onto a mirror which reflects the beam to the radia-tion detector, a photomultiplier.

In SAOR spectroscopy, unlike ORD spectroscopy,every substance rotates the plane of polarized light.This includes the sample cell windows and the solventsused in a measurement. The instrument describedherein is designed to compensate automatically for therotations of the solvent and sample cell. The diagramat the bottom of Fig. 1 illustrates how this feature isachieved by the use of two opposing magnetic fields.

April 1967 / Vol. 6, No. 4 / APPLIED OPTICS 699

XeARC L1 SOLVENT MAGNET

SOLUTO FARADAY COIL

-* = MAGNET WATER CELL

-_* _ | __1i- 2 g N ANALYZER

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e -e

Fig. 1. Optical diagram of MOR spectropolarimeter.

UASEMLY SCLUTION SOLENET FARADAY

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Fig. 2. MOR spectropolarimeter.

The plane polarized light beam (with electric vectorvertical) from the polarizer passes through the first5000-G magnetic field to give a rotation of a + 0 (soluteand combined effect of solvent and cell) and then intothe second magnetic field to give a rotation of -0 (forpure solvent and cell). Since the magnetic fields arepolarized in opposite directions, this arrangement pro-vides the means for cancelling out rotations owing tothe solvent and the sample cell, which are common toboth magnetic fields. The net rotation a of the soluteis thereby measured directly by automatically reposi-tioning the polarizer to the null (or zero transmission)setting. To accomplish this, the Faraday modulatorapplies an oscillating 60-c/s magnetic field of 1000 Gto the water-filled cell mentioned above. As a result,the rotation of the water oscillates at the same fre-quency between limits of 0bmax. The value of omax

varies with wavelength from 0.5° at the red limit to160 at the uv limit of operation. The light beam nextpasses through the analyzer set approximately at thecrossed position relative to the polarizer and to a mirrorthat directs it to the photomultiplier. If the angle ofpolarization of the light reaching the analyzer doesnot correspond to the condition of minimum trans-mission (null), there is a 60-c/s component in thephotomultiplier current resulting from the action of theFaraday modulator. The phase of this component issensitive to the direction of analyzer position away fromnull. This component serves as the signal to operatea conventional servo system to reposition the analyzerto null. The null position of the polarizer is recorded asa function of wavelength to give an iIOR spectrum.

Figures 2 and 3 show a photograph and block diagramof the instrument. The mechanical stability of the

optical system is provided by the use of a granite block(182 cm X 46 cm X 13 cm) as the optical bench. The5000-G magnetic fields for the solution and solvent com-partments are provided by two water-cooled Harvey-Wells magnets (agnion, Inc., 144 Middlesex Turn-pike, Burlington, Massachusetts). Their d supplieseach provide an adjustable output current of 35 A maxi-mum, regulated to within 0.1%, short term. Themagnets are clad with a layer of 1.3-cm thick soft ironto isolate stray magnetic fields from the optical com-ponents and the sample. The observed stray field isabout 2 G at a distance of 2.5 cm from the solenoids.The core of the solenoid of the Faraday modulator iswater cooled to allow good temperature control of thewater cell. The light source is powered by a commer-cial regulated dc power supply (American InstrumentCo., Silver Spring, laryland) designed for this applica-tion and with filters modified to minimize output ripple.The Faraday cell magnet is powered by a simple Variacand stepdown transformer.

Fig. 3. Block diagram of electrical system of MOR spectro-polarimeter.

Fig. 4. Diagram of polarizer housing and angle readout device.

700 APPLIED OPTICS / Vol. 6, No. 4 / April 1967

+0.01

.' -0.01

Z0

0

300 400 500

WAVELENGTH, mix

600 700

+0.01 I I II

0 -ZERO FIELD

-0.01

Fig. 5. Instrument zero lines with and'without magnetic fields.

z

0

aJQ1

0

CL

250 300 350 400 450 500 550 600 650 700 750WAVELENGTH, my,

Fig. 6. Maximum optical density vs wavelength for indicatednoise level. - - - 0.5-mm slit (1.5 mg) -1.0-mm slit (3.0 mu).

The electronics and servo system of the instrumentare similar in principle to those of Cary et al.2 The out-put current of the photomultiplier contains, for a givenlight flux incident on the polarizer, a dc componentproportional to the total light reaching the photo-cathode, a 120-c/s component of amplitude propor-tional to the square of rocking angle of the Faradaymodulator, and a 60-c/s signal proportional to theproduct of the rocking angle and off-null optical errorsignal. The relationships derived by Cary et al. applyhere. In addition, of course, are random noise andphotomultiplier dark current.

The photomultiplier is an EMI Type 9558Q with anS-20 surface (EMI Electronics Co., 151 W. 46th Street,New York 36, New York) and a quartz envelope with arange of sensitivity from 750 mu down to 220 mu. Avoltage proportional to the photomultiplier anodecurrent is developed across a 100-m52 load resistor, andconverted to a low-impedance signal by a feedback-stabilized, unity-gain, direct-coupled electrometer pre-amplifier. The ac component of the signal is passed bya coupling capacitor to a twin-tee 120-c/s rejectionfilter, and then to a 60-c/s synchronous demodulator.The demodulated signal, reflecting the optical errorsignal, is passed through an RC low-pass filter to aHoneywell servo motor, which, in turn, positions thepolarizing prism. No tachometer feedback is requiredfor stability of this system.

The light level reaching the photomultiplier varieswidely, but constant over-all servo gain is approximatedby control of the photomultiplier high-voltage dynode

supply through the AGC amplifier so as to maintain aconstant dc signal level. This level is adjustable byadjustment of the positive dc supply to the photomulti-plier load resistor.

The angle of rotation of the polarizer is measured bythe use of linear voltage differential transformer(Schaevitz Type 100-XS-AL-Schaevitz, Inc., Camden,New Jersey) to determine the linear deflection of a beamsupporting the polarizer. Figure 4 shows a diagramof the polarizer housing and angular readout mechanismwith the differential transformer. The 60-c/s outputvoltage of the transformer-a linear function of polar-izer angle-is demodulated and passed through spanand zero controls to a zero-center Honeywell recorder.A switch attenuator selects recorder spans of 4 10,+ 0.3°, -i-0.10, and -4-0.03°. Another adjustment pro-

vides recorder zero suppression of up to 4t 10, in incre-ments of 0.010.

Method of MeasurementIn the determination of an MOR spectrum, the solu-

tion and solvent under investigation are placed in

+0.025000 GAUSS FIELD

+ 0.01

£ 00 ~~~~ZRO ,-0.01 FIELD

-0.02 I

250 310 370 430 490 550WAVELENGTH m,

Fig. 7. Typical trace of a MOR spectrum (0.016% solution ofpotassium ferricyanide in water).

300 350 400 450WAVELENGTH, my

500 550

Fig. 8. Typical ORD spectrum obtained with the MOR spectro-polarimeter (vitamin B-12 in water-0.02% solution).

April 1967 / Vol. 6, No. 4 / APPLIED OPTICS 701

- I- - - I - I I I 1�5 -

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quartz sample cells and mounted into the water-cooledcell holders of the instrument. These are inserted inthe air cores of the solenoid magnets of the apparatus(see Fig. 3). The temperature of the cells is maintainedconstant by a flow of water from a thermostatted waterbath through the sample cell holders. A typical experi-ment consists of zeroing the instrument by recordinga reference base line at no magnetic field, followed by asecond, or calibration base line, with the magneticfield on and with solvent in both magnets and, finally, ameasurement with solution in magnet 1 and solventin magnet 2. Figure 5 shows a typical recorder charttrace for the two types of instrument base lines obtainedas a function of wavelength. These curves representthe residual uncompensated error and must be sub-tracted from the spectra obtained.

Performance Characteristics

The essential feature provided by the MOR spectro-polarimeter is a scanning system for measuring andrecording the magnetic rotation of a sample as a func-tion of wavelength with direct and continuous com-pensation for the magnetic rotation of the solvent.For most measurements, the full-scale span of 0.1 isused. Figure 6 summarizes the performance character-istics of the instrument. This shows a plot of thesample optical density and the wavelength that com-bine to give the listed magnitudes of recorded noise level(in degrees). Results are given for two monochromatorslit settings of 1 mm and 0.5 mm. Thus, at a slit widthof 1 mm (3.5 mu bandpass), one can obtain a spectrumwith a noise level of 4 0.0010 or less for the wavelengthrange 400-500 mu at a sample with an optical densityof 1.5. The curves of Fig. 6 combine such factors asthe light intensity distribution of the source, absorptioncharacteristics of the polarizer and analyzer, and giveover-all performance characteristics for the instrument.The mechanical and electrical stability of the servoinstrument is such that, at high light intensity, thepolarizer position can be recorded with an error of<0.00020.

Error Sources in Measurement

There are a number of factors which can contributeto instrumental errors in the measurement of an MORspectrum. These include the alignment of the optics,the spectral purity of the light, and temperature effects.The temperature effects can arise from several sourcesincluding: (1) convection currents induced in the sampleand solvent compartments; (2) convection currentsinduced in the air columns of the solenoid cores of themagnetic fields when the cooling water for the magnetcannot cope with the heat generated; and (3) tempera-ture effects produced by convection currents in thewater cell inside the Faraday coil. These temperatureerror sources tend to increase noise level and reducethe speed of response of the instrument.

Typical ResultsFigure 7 shows a recorder chart trace of a typical

MOR spectrum measured with the instrument. Thechart illustrates the results for solutions of potassiumferricyanide at two absorption band regions, 420 mgand 350 m, with and without magnetic field.

Figure 8 portrays the results for a naturally opticallyactive compound, vitamin B-12, without magneticfield, demonstrating the ability of the instrument tomeasure ORD, as well as MOR, spectra.

We would like to thank C. D. Reilly, W. M. Trippeer,and D. J. Troy of our Laboratory for many helpfuldiscussions.

References1. E. J. Gillham and R. J. King, J. Sci. Instr. 38, 21 (1961).2. H. Cary, R. C. Hawes, P. B. Hooper, J. J. Duffield, and K. P.

George, Appl. Opt. 3, 329 (1964).3. V. E. Shashoua, J. Am. Chem. Soc. 82, 5505 (1960).4. C. Djerassi, Optical Rotatory Dispersion (McGraw-Hill

Book Co., Inc., New York, 1960).5. V. E. Shashoua, J. Am. Chem. Soc. 86, 2109 (1964).6. H. Takasaki, J. Opt. Soc. Am. 51,462 (1961).

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702 APPLIED OPTICS / Vol. 6, No. 4 / April 1967