solvent nhich, in addition to increasing the bulk of a sample, may lead to undesirable side reactions or contamination. As a result of the heat provided by the preheater, augmented by direct heating of the crucible itself, the sample is very rapidly volatilized and, therefore, more likely to enter the column as a plug than would otherwise be the case. Manipulation of the spoon can be readily achieved with a little practice and the average size of the sample judged fairly accurately. As described above, the actual amount introduced
into the column can be ascertained by weighing; however, because of the possibility of adherence of particles to the interior of the spoon, the weighings are not very accurate. The method is, however, suitable for qualitative or preparative work. The dryness of the apparatus may be tested with niobium pentachloride, which turns from yellow to white if the least trace of moisture is present. The apparatus is inexpensive, easy to construct, and may be readily cleaned. As designed, the sample port is intended for attachment to a horizontal column.
A device for introducing a dilute solution of a volatile solvent onto gas chromatography columns, which aho makes use of a small mounted spoon. has recently been described by Renshaw and Brian ( 2 ) . LITERATURE CITED
(1) Keller, R . H., J . Chromatog. 5, 225 ( 1961). (2) Renshaw, -4.) Brian, L. A . , Ibid., 8,
343 (1962).
A Research Fellowship, awarded by the South African Atomic Energy Board, enabled this work to be undertaken.
Fractionation of Fine Particle-Sized Ion Exchange Resins Basil Vassiliou and Robert Kunin, Rohm & Haas Co., Philadelphia, Pa.
T
THE
A CG-120
PRESEKT
time, Amberlite
(Mallinckrodt Chemical Works, Cat. No. 3339) is employed extensively for the purpose of obtaining the necessary 20- to 40-microns cut required by the Spackman, Stein, and Moore (2) procedure for the automatic analysis of amino acids. Although the procedure is widely used, difficulty has been experienced in obtaining the required particle-sized resin in quantity. Since the screening of such small particles is practically impossible, a n elutriation process was developed which is essentially a modification of the method devised by Hamilton (1). The procedure has the advantage of eliminating much of the time previously required and offers the possibility for obtaining different particle sized fractions simultaneously. By using a combination of two separatory funnels with different maximum diameters in series, one can employ the Hamilton procedure and obtain the desired cut in a single step. The design of the apparatus is given in Figure 1. The calculation of the input flow rate was made by using Stoke's law
VBtcke
Input flow rate (cc./minute)
Figure 1. Diagram of elutriation apparatus used for fractionation of resin
case in which the particles n-ith diameter greater than 40 microns are to be retained in the loner vessel. In this study, the loiver vessel had a maximum radius of 7.65 em.
rR2max
X
Vltake
where R,,, refers to the maximum radius of the separatory funnel. The input flow rate was first calculated for the ANALYTICAL CHEMISTRY
(2
2(1.27 - 1.0) X x 10-312 x 980 x 60 9 x 0.00s94
-
= * R 2 x VStokea 3.14 X (7.65)2 X 1.573 = 289.9 = 290 cc./minute
=
9 X 0.00894 0.395 cm./niinute
Tr
EXPERIMENTAL WORK
T-tnput
and the relation
2 X (1.27 - 1.0) X (1 X 10-s)2 X 980 X 60 -
I
1 . 5 2 cm./minute
V : settling velocity a: radius of particle d l : density of particle d0: density of medium 7: viscosity of medium g: gravitational constant
=
and
v8toke =
where
1328
fraction of 20 to 40 microns a t the above flow rate was calculated as follows:
A.R. 400- to 600-Mesh
I n other n-ords, a t a flow rate of 290 cc. per minute, particles greater than 40 microns will be retained in the lower vessel and those smaller in size will pass up into the upper vessel. The greatest diameter of the upper vessel required to retain a desired particle size
Using the above data, the apparatus described in Figure 1 was constructed. At a point just above the stopcock of the lower vessel, a sintered-glass plate (coarse porosity) was inserted for the purpose of supporting the resin and to reduce the axial streaming. This made it possible, for all practical purposes, to assume a linear uniform velocity, Vup. The apparatus was clamped vertically and the input was connected with a source of deionized water. A flow meter was used to measure the flow rate. The output was filtered (medium-porosity filter paper) to separate the outflow fine (less than 20 microns) resin which enabled one to recycle the water. A centrifuge pump was used to recycle all the water in the system. The desired flon- of 290 cc. per minute M-as arranged by adjusting the capacity of the pump. The resin [Amberlite CG-120 A.R. 400- to GOO-mesh (Rlallinckrodt)] was pretreated by suspending the material in water to hydrate the resin and breakup any clumps that were present. For each run, a sample of 350 cc. of resin was loaded into the lower vessel and water was passed upflow a t the calculated flow rate. After 90 minutes, the effluent water of both vessels was clear and the run vias terminated.
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
-
VOL. 35, NO. 9, AUGUST 1963
1329
