A Simple Differential Refractometer S. J. Gill and D. B. Martin, University of Colorado, Boulder, Colo.
differential refractometer high sensitivity and precision cari be advantageously assembled on a standard 1-meter optical bench. Light is passed through a differential prism cell twice by a simple mirror lens combination (4). Although the system has high sensitivity, excellent stability is achieved from reasonably compact construction. The arrangement has been particularly useful for solubility determination of slightly soluble compounds (3). I n this instance, and for others requiring temperature control, a thermostated sample cell has been used. SIMPLE
A of
The optical bench arrangement is shown in Figure 1. An AH-4 mercury vapor lamp, 0, illuminates a 546-mp interference filter, F , and adjustable vertical slit, S. Light passing through the slit is focused at infinity b y the lens, L (82-cm. focal length), so that parallel light passes through the differential prism cell, C. This light is reflected from mirror XI back through the cell and lens, L , which focuses an image of the slit a t some point 82 cm. from the lens. T o observe this image a second mirror, Jf2, is placed to reflect the returned light t o the focal plane of a filar micrometer, R, which is placed perpendicular to the axis of the bench. The end unit of filter, slit, mirror M2, and filar micrometer are mounted on a single unit, which can be displaced horizontally and perpendicularly t o the bench by an adjustable mount, H . All optical components are firmly attached to the optical bench by rods and clamps. The cell for holding the liquid sample is a critical part of the apparatus. For sample purpose9 we have used a rectangular differential prism cell (1 inch x 1 inch x 3 inches) (constructed by Quaracell Products, Inc.. Nen. York 13, N. Y,). This cell was held in a firmly mounted U-shaped aluminum block, J , which could be thermostated by circulating water. The temperature control in the measuring cell was +0.lo C. from 20" to 50" C. The cell and thermostat block were positioned on a heavy Bakelite plank so that a magnetic stirrer could drive a stirring bar in the center compartment of the double prism cell. This was particularly useful for solubility measurements. Replacing this cell in the thermostat block produces small changes in the slit image position. A small leaf spring from shim stock, placed between cell and thermostat. will give highly reproducible replacement positions.
118
ANALYTICAL CHEMISTRY
o'rcm.l
82 cm
. U
IOOcm
Figure 1.
c
Schematic diagram of optical components.
In order to minimize cell replacement error the use of a reference slit image is suggested ( 5 ) . A cell which combines the advantages of a reference beam path, good thermostating, and low evaporation was developed and is shown in Figure 2. The cell was constructed from glass tubing of indicated diameter.
Parts are labeled in text
The center prism was cut to give a 120' prism angle. End windows and center prism windows were made from microscope slide material and cemented in place with epoxy resin. The inner and outer compartments are filled from joints I and P , which are then closed iyith Teflon plugs. I n the optical path this cell gives rise to two slit images when a difference in refractive index exists between the two solutions. Center prism windows are oversized so that the reference beam image traverses nearly the same optical components as the sample slit image. Error effects of position and bench changes are reduced to a minimum. A set of test measurements for a series of replicate fillings and replacement of cell is shown in Table I.
I 0
tFigure 2. Differential prism cell. outer cell compartment 0
Thermostat jacket
C
25 m-T--
plus inner prism cell I;
Table 1.
Replicate Determinations on a 1.034%
Slit image positions, mm. Reference Solution image image 0.000 9.882 0.114 9.996 0.149 10.037 0.181 10.069 0.181 10.065 0.199 10.089 0.144 10.027 0.121 10.002 0.202 10.086
Slit image difference, mm. 9.882 9.882 9.888 9.888 9.884 9.890 9.883 9.881 9.884
Sucrose Solution a t
30" C.
Remarks Room temp., C. 26 24 22.5 22.5 22 24 24 Same sample after 12 hours
Each value is an average of six consecutive readings. The error in reading is about ~ t 0 . 0 0 5mm.
on the ratio of the deflection and focal length of the primary lens, and the angle of the central prism. When this ratio is less than 1/100 and the central prism angle is 120°, linearity is theoretically correct within 0.3% (6). ACKNOWLEDGMENT
This ivork was supported in part by a research grant (RG-8264) from the United States Public Health Service. We thank George Unger, the university's glass blower, for his fine work in the construction of the new double cell. LITERATURE CITED
(1) Bates
The range of refractive index measurement is determined ( 2 ) b y the prism angle of the center cell, the optical path length, and the field of the filar micrometer (in our case 14 mm.). For our first cell we observed that a deflection of 1 mm. corresponded t o 2 X lo-' refractive index units. The unit was calibrated with sucrose solu-
tions ( 1 ) . The image deflection is measurable well within =kO.O1 mm., SO that one has a precision to about 2 X 10-6 refractive index unit. The maximum image deflection corresponds t o about 3 x 10-3 refractive index unit. T o the first order linearity between image deflection and refractive index difference is theoretically dependent
and associates, S a t . Bur. Std., Circular C 440 (1946). (2) . , Debve, P. P., J . i i p. p. l . Phvs. 17, 392 (1946 j. ' (3) Gill,.S. J., Hutson, J., Clopton, J. R., Downing, M.,J . Phys. C'hem. 6 5 , 1432, f\ -l F- -)-AI -l \ .
(4) Hadow, H. J., Sheffer, H., Hyde, J. C., Can. J . Research 27B, 791 (1949). ( 5 ) Korosy, F., Nature 174, 269 (1954). (6) Martin, D. B., 1I.S. t#hesis,UniversitJy of Colorado, 1962.
Gas Chromatographic Analysis of liquids Containing Nonvolatile Viscous Materials
L. J.
Myers, Humble Oil & Refining Co., Linden, N. J.
THE
GAS
CHROMATOGRAPHIST OCCB-
l sionally is required to analyze ma-
terials which contain high percentages of nonvolatile liquids for lorn percentages of volatile materials contained therein. Where temperatures high enough to volatilize the high-boiling components of such samples cannot be tolerated, because of thermal decomposition or other effects, ordinary gas chromatographic techniques are frequently unworkable; the nonvolatile components of the sample accumulate in the sample introduction sections of the equipment. Upon repeated analysis, such accumulation prevents the instantaneous volatilization of the lighter components when t h e sample is injected, with the result that the desired plug of gases is not delivered t o the column. Tailing of peaks can result and, in extreme cases, peak heights or areas will grow progressively smaller when the same sample is run repetitively. This effect is presumably due t o the retention of a portion of the volatiles dissolved in the accumulated nonvolatiles in the introduction system, with subsequent evolution so slow t h a t it merely raises the base line slightly. Such samples could be handled b y preliminary fractionation t o remove the nonvolatiles, but this leads t o other disadvantages. A simple device is described whereby such accumulation of nonvolatiles can
Figure 1. Assembly for cleaning sample introduction cell of gas chromatograph A. 8. C. D.
E. F. G.
H.
Carrier gas inlet Thermistor detector block Sample introduction cell with septum U-tube with tee Valve Four-way valve Gas chromatography column Exhaust
be cleaned from the system in a very short time without disassembly. I n the accompanying simplified diagram (Figure l), the C-tube, D, between the sample introduction cell, C, and the 4-way valve, F , is made of l/s-inch copper or stainless steel tubing, and is as short as possible. It is arranged so that it has a lob7 point, where a tee connection of the smallest possible volume is made with 'js-inch tubing using silver solder. The side arm of the tee is carried out of the case of the instrument where a valve, E , is connected in a convenient location. The side arm is arranged so that it has a continual downward slope. A sample containing nonvolatiles is injected with a syringe through the septum a t C in the usual manner with valve E closed. Sonvolatiles collect in the sample introduction cell and the nearby portion of the L?-tube. After all the desired volatiles h a r e been eluted from column G, valve E is opened and a relatively large volume of solvent (10 to 20 ml.) is injected through the septum with a large syringe. The carrier gas is allowed to continue flowing. The solvent selected should dissolve the nonvolatiles and have as low a retention time as possible on the column used. The solvent will be forced out valve E under the pressure of the carrier gas carrying the nonvolatiles with it. Solvent will be prevented from entering column G to any great extent because of the column's back pressure. After a few seconds, the solvent will be completely evaporated from the sample introduction cell and VOL. 35, NO. 1, JANUARY 1963
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