RESULTS AND DISCUSSION
Microphotography of the starting material and of the three cuts were taken, and a calibrated scale was photographed and enlarged LO the same magnification (1 :200). An average diameter was determined b y miwuring the distances between opposite sides of a particle on a line which bisects the pro,ected area of the particle. From
an analysis of these measurements, the following results were obtained. The percentages of the three cuts present in the initial sample were: less than 20 microns, 10%; 20 to 40 microns, 30%; and, greater than 40 microns, 60%. The 20- to 40-micron cut was 90% in the desired range. The above procedure can be to obtain the uniform 20- to 4-micron cut of ion exchange resin used in the
automatic chromatographic analysis of amino acids. The same procedure, with modifications, can be used for the fractionation of other fine particlesized micropondered resins. LITERATURE CITED
(1) Hamilton, B. P., ANAL. CHEM.30, 5 (1958) ( 2 ) Spackman, D. H , , Stein, W. H , Moore, S., Federation Proc. 15, 358 (1956j.
Improved Rudolplh Spectropolarimeter John G. Foss, Department of Biochemistry and Biophysics, Iowa State University, Ames, Iowa HE INTRODUCTIOS of the Rudolph Tspectropolarimeter has permitted optical activity and rotatory dispersion measurements to be made routinely in the ultraviolet and visible regions. However, in practice it is a very troublesome instrument to use and for this reason a spectropolarimeter was modified in a manner suggwted by Gillham (3) in 1957. The purK0.e of this paper is to s h m that such modifications can be readily made and tha; they lead to a great improvement in the Rudolph polarimeter. The principle of oiieration iy dem-ibed in w w a l places (1-3). Briefly, it depend. on the fact that if the azimuth of a plane polarized light lieam iq rocked at a frequencyf before entering a polarizer, the rmerging light modulated with components having frequencies f and 2f. +wept when the rocking is exactly ~yinmetrical about the extinction po5ition. In that case there n ill be only a si@nal of frequency 2f modulating the light beam. Thus the absence of the f component can be used as a criterion for havii g a croised analyzer and polarizer. I n practice. the mo.1 convenient way to rock the polarized beam is by use of a Faraday modulator. This modulator simply consiqts of a tranqparent substance-e.g., water-( ontained in a solenoidal electromagnet energized by an alternating current. plane polarized beam of light traveling along the axis of the colennirl nil1 alternately be
set screws to a 3/4-inch copper rod to complete the secondary of a transformer. The primary consists of two Superior Electric Go. Flexiformers connected in parallel. Sormally, the modulator is energized with 400 amp.. and cooling water is forced through the coil since about 200 watts of heat must be dissipated. The modulator is now completed by immersing the electromagnet in a tube of water. A brass tube approximately 6 em. in diameter and 13 em. long is used with fused silica windoti s mounted with silicone grease in recesses on the inside of the endplates. (In this way the slight water pressure tends to tighten the seal.) A snug-fitting plastic lid with two holes for the copper tubing reduces water evaporation, but it is convenient to have provisions for easily refilling the water without having to dismantle the entire modulator. K i t h this electromagnet, approximately 7 5 volts gives 2 amp. in the primary (or 1 amp. per Flexiformer), which means that 400 amp. flows in the secondary. Under these conditions, the primary can be energized indefinitely with only slight heating, but according to the manufacturer the Flesiformers can be operated at double this load for short periods of time. S o details will be given of the arrangement used to mount the modulator in the light beam since our polarimeter had a specially made support on it, intended originally for another use.
rotated clockwise and counterclockwise as the direction of the magnetic field changes. The rocked beam passes through the analyzer, onto a photomultiplier, and the signal is fed into an amplifier which only amplifies the frequencyf. When the analyzer is adjusted to give a minimum amplifier output the analyzer and polarizer are in a crossed position. Figure 1 illustrates the overall optical arrangement used. THE FARADAY MODULATOR
To obtain a sufficiently large magnetic field to rock the light beam several degrees, i t should be possible to either use a low current electromagnet having many turns of wire or a high current electromagnet with relatively few turns per unit length. Both methods were tried but the latter proved more satisfactory. I n its present form the electromagnet consists of a tightly wound, single layer. 20-turn helix of heavyu alled 'I4-inch copper tubing. Quarter inch copper tubing with only a l/r-inch diameter opening must be used in order to have a sufficiently lot7 d.c. resistance. The solenoid was made by heating the copper tubing with a n acetylene torch and n-inding it onto a l/r-inch steel mandrel slowly rotating in a lathe. The oxide film on the copper apparently provide. enough insulation between turns of the helix to permit close winding. The copper tubing is fastened by
A\
I
1
+
c
+ 180 v
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The brass tube with the modulator was fastened to a n adjustable, three-legged table which permitted it to be oriented correctly in the light beam. However, with a constant temperature trough of the type manufactured by Rudolph, it might be possible to make a satisfactory modulator more easily. If the '/r-inch copper tubing mere snugly wound by hand around a n ordinary 10cm. glass polarimeter tube, the modulator would be automatically centered properly when placed in the trough. Regardless of what changes are made in the modulator, it is important to keep the copper tubing as short as possible in order to reduce its d.c. resistance. FILTER AMPLIFIER
T o minimize the 60-cycle signal on the photomultiplier, it is necessary to have an amplifier which will not only amplify 60 cycles but also will strongly reject 120-cycle signals. This is important since the 120-cycle signal is a t a maximum when the analyzer and polarizer are crossed. There are a number of amplifiers which could be used for this purpose, but the simplest seems to be the one shown in Figure 2. The filtering action of this simple circuit depends on the use of the tTvo sharply tuned (40 db. attenuation per octave) band pass klters. Elimination of stray 60-cycle signals from this amplifier was a difficult problem. The amplifier and 12 90-volt batteries for it and the photomultiplier are housed in a galvanized iron box with a tight fitting lid. It is, of course, necessary to use shielded cable between the photomultiplier and filter amplifier, but, in addition, another layer of stranded shielding has to be used around the cable. The cable between the photomultiplier and filter amplifier is about 4 feet long, which permits increasing the distance between the modulator and amplifier. The output from the amplifier goes to a n y sensitive a x . vacuum tube voltmeter-e.g., the Heathkit audio vacuum tube voltmeter which has a maximum full scale sensitivity of 0.01 volt. A sensitive oscilloscope-e.g., a Tektronix 503-could also be used for this purpose, but i t is easier to minimize the 60-cycle signal on a meter. LIGHT SOURCE
It is important to have a bright light source since the sensitivity of the polarimeter depends in part on this. One which has proven very satisfactory is the Osram 450-m-att xenon arc (MacBeth Co., Nemburgh, K. Y.). If a d.c. generator is available which can supply 70 volts at 23 amp. i t can be used to power the lamp. However, such a generator was not available in this laboratory so the simple, inexpensive, 3-phase supply shown in Figure 3 was assembled. Two 20-amp. autotransformers are used as part of a n L filter since they are readily available and can be put to good use if the lamp 1330
a
ANALYTICAL CHEMISTRY
Re
3
m
START
C h 1.2
Figure 3.
Power supply for Osram XBO-450 xenon arc
T = Superior Electric Co.
3-phase autotransformer number 1 3 6 - 3 Rec = Westinghouse 6 - 3 - 1 B size K (4"X 12") set of selenium rectifiers Ch 1,2 = 2 0 ampere autotransformers Re = Potter Brumfield PR3AY relay. 230-volts, 60-cycle coil, rated a t 2 5 amp. a t 1 10 volts C1, 2 = Mallory H C l 5 0 1 0 capacitors. 1000 mfd., 1 5 0 volts R1 = Cenco number 8 2 9 6 5 water cooled rheostat, 2 5 to 5 ohms F = 30-amp. fuse A = 30-amp. meter
supply should ever be dismantled. (For the same reason it might be expedient to use three separate 20-amp. autotransformers rather than one set of ganged transformers. It mould be slightly more difficult to adjust the three transformers independently, but the adjustment is rarely necessary since minor changes in the lamp current are normally made with the water-cooled rheostat.) While the lamp is being, started the chokes must be momentarily removed from the circuit. This is accomplished by shorting them out with the relay Re which is energized a t the same time the lamp starting s&ch is closed. The lamp is mounted with large fuse clips attached to standoff ceramic insulators and is housed in a sheet metal box approximately 6 X 6 X 12 inches with several holes for ventilation in the top and bottom. A removable side of the box is made of Transite and the electrical leads are taken through it. These precautions are necessary since a very high voltage is used in starting the lamp. MONOCHROMATOR
A series of zero readings were made on the polarimeter using Beckman DU prism and Bausch and Lomb 250-mm. grating monochromators. No important differences were observed between the two monochromators under these conditions. The standard deviations for both mere approximately 10.002 between 240 and 600 mp but rose sharply outside these spectral limits. No effort has been made to learn why the lower limit cuts off so suddenly at 240 mp. The instrument used is one of the earliest Rudolph spectropolarimeters so it is quite possible that the polarizing and analyzing prismq absorb strongly in this region. METHOD O F OPERATION
It is first necessary to align the optical train. As an initial step, remove the modulator and adjust the light source, monochromator, and polarimeter to
get the brightest possible image on a white card placed in front of the analyzer. Replace the modulator and adjust it to once again get the brightest image on the card. The optical system should now be approximately aligned. Final adjustments of the optical train can be made by maximizing the 60-cycle signal for a given angular error in setting the polarizer and analyzer a t crossed positions. To make measurements, first turn on the modulator, filaments, and photomultiplier voltage. Sormally from 360 to 540 volts are used for the latter. Adjust the analyzer until a minimum is observed on the readout meter and read the angular scale. To obtain a minimum reading normally takes only a few seconds. T'ihen an absorbing solution is in the light beam the sensitivity decreases just as it must for any type of polarimeter. For strongly absorbing solutions, the minimum in the voltage-angle settings becomes flatter. This problem can be partially overcome by plotting the meter readings 21s. the angular settings a t some convenient steps in the angular scalee.g., 0.100 or 0.250 degrees-and extrapolating the tn-o linear portions of the curves to their intersection. Saturally this is more troublesome and less precise than direct readings, but it still takes only a fer!- minutes to obtain and plot about 10 points. As described, the polarimeter is used as a null instrument. But since there is a rather linear relationship betvieen the angular sett,inge and the meter readings, it should also be possible to use it as a direct reading instrument over small angular intervals. Thus the amplifier output might be fed into a chart recorder for kinetic measurements of short duration. POSSIBLE IMPROVEMENTS
The modifications described are the ones currently used on a Rudolph Spectropolarimeter in this laboratory. There are a number of powible improvements n-hich might be made. Since these may be of uqe to ana-one dupli-
cating our modifications, a few of the more obvious ones will lie included in this description. A 6-volt storage battery was ujed to operate the filaments ir. the original amplifier because of t8he€;reat difficulty experienced in eliminating stray 60cycle pickup. It is a nui::ance to maintain and it could probabIy be replaced with a rectified d.c. source. When the photomultiplier is operat'ed a t high voltages the noi5e level becomes quite significant. A simple integrating circuit can be added n-hi~~b reduces the meter fluctnat'ions. d t one t'ime in the development of this instrument this iva3 done by inserting some large capacitors in the d.c. out'put of the Heathkit meter.
At the present time no integration is used. The analyzer', divided circle is good to approximately O.O0lo and this is what determines how small an angle can be measured. I n wavelength regions n ith adequate light, the sensitivity of the null point mag permit measurements to 0.0001". It would be a relatively simple matter to include a d.c. Faraday cell to compensate sample rotations as small as this. The main drawback of u4ng such a compen.ator iq that i t would have to be calibrated a t every wavelength wed. (One of the very ingenious features of Gillham's 1957 polarimeter design was the use of a feedback loop to a d.c. Faraday compensator, making it a direct-reading inqtrument.)
ACKNOWLEDGMENT
The author thanks John A. Schellmau for the use of his laboratory facilities during the early development of the instrument, L. s. Bartell for the loan of a Rudolph polarimeter, and Joel Hail and Brian hIyhr for assistance in constructing and testing the instrument in its final form. LITERATURE CITED
(1) Fopiano, P. J., Tragerer, W. B., Ph.D.
thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1951. ( 2 ) Gates, J. W., Chem. Ind. (London)
1958, 190. ( 3 ) Gillham, E. J., J. Sci. Inst. 34, 435 (1957). WORK supported by Research Grant Kumber A6067 from the National Institutes of Health, Public Health Service.
Determination of Total Nitrogen b y a Combined Dumas-Gas Chromatographic Technique B. A. Stewart, L. K. Porter, and W. E. Beard, Soil & Water Conservation Research Division, Agricultural Research Service, USDA, Ft. Collins, Colo. ETERZIIXATION of nitrogen by the D D u m a s method has in many cases yielded high values, especially when materials containing long aliphatic carbon chains were analyzed. The high values for these materials h a r e been shown (1, 4, 5) to be due, at least in part, to incomplete combustion of the sample. Rather than csarbon dioxide and nitrogen being the only gases produced, methane, and ;ossibly other gases, escape into the nitrometer and are measured along with the nitrogen gas. Carbon monoxide and nitrogen ovides also have been known to escape into the nitrometer, although these gases generally occur cnly when the coiiper layer has been exhausted or is maintained a t too high a temperature. The possibility of using a gas chromatograph for measuring the nitrogen gas produced by the Dumaq combustion was investigated so that any other gases, if produced, could be separated from the nitrogen.
recorder capable of registering 1 millivolt full-scale. The column was 4 feet long and packed with 5 h Alolecular Sieves (32- to 60-mesh). The cabinet of the chromatograph was maintained at 34.5 + 1' C. Helium was used as the carrier gas with the flow maintained a t 90 cc. per minute. The cur-
rent to the detector cell was maintained a t 250 ma. SITROUETER AND GAS-TRANSFER EQUIPMENT.A diagram of the nitrometer and gas-transfer equipment is presented in Figure 1. The nitrometer mas attached to the combustion apparatus and replaced the nitrometer
--T pf 4-ray
s ~ o p c o c ~ =
-
T O Chromatograph
ne
Serum Cap Port
3 - w a y Teflon Stopcock
To A t m o s p h e r e
From
Combustion
50 % Aqueoui KOH
EXPERIMENTAL
Apparatus. ~ o h 1 s u s - ~ I o r id. Colemail automatic nitrogen analyzer (Model 29, Coleman Instruments, Inc., Maywood, Ill.) W E used as t h e combustion apparatus. Details of this instrument can be found in t h e operating manual of t h e manufacturer. T h e instrument waq used as recommended in t h e mxnual with the excc.ptions t h a t t h e n trometer was replaced and the micros,-ringe was not used. The description of this replacement is included below. GAS CHROUATOGRAPII. The chromatographic equipment consisted of a Beckman GC-1, equipped ivith a Brown
10 c c . Syringe
Figure 1. ment
Diagram of nitrometer and gas-transfer equip-
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