EXAMPLES
The reproducibility of permeability constants that can be obtained on dif-
ferent samples of the same material is demonstrated below for a polyethylene film.
Sample 1 Sample 2 7.50 750 Atmospheric pressure, mm. Hg .- Cm./min. moved by slug 0.827 0.847 Volume of gas permeating, cc./min. 0.00233 0.00239 Temperature, C. 24.85 f 0.04 24.74 =!= 0.05 Area of film, sq. cm. 1.60 1.60 Time, seconds 60 60 Thickness of film, cm.0 0.00432 0.00450 Initial helium pressure, cm. Hg 53.8 54.0 p - C, em. Hg 31.8 32.0 Permeability constants, cc.-cm./sec.-sq. cm.-cm. Hg 3.11 X 10-e 3.03 X a Average of six readings on American Instrument Co. Magne Gauge. O
ACKNOWLEDGMENT
Thanks are due to Herman Braun for having built and helped in the design of the film holder. LITERATURE CITED
Amerongen, G. J. van, J. Polymer Sci. 5 , 307-32 (1950).
Barrer, R. N., “Diffusion 3iffusion in and through Solids,” Macmillan, New
York, 1941. (3) Brubaker, D. W.,, Kammermeyer, K., ANAL. CHEM.25. 2 5 , 424-6 (1953). (4) Cartwright, L. C., Ibid., 19, 393 f19471. (5) ShLman,. A. C., IND. ENG. CHEX., h A L . ED.16, 58 (1944).
Fraction Cutter for Gas Chromatography Allan Weinsteinl, Radiological Warfare Division, U.
S. Army,
Dugway Proving Ground, Dugway, Utah
A
vapor phase or gas chromatography is a powerful tool for separation and identification of complex mixtures of unknown composition, other analytical procedures must often be used as adjuncts for the complete elucidation of such mixtures. For instance, the high intensity gamma radiolysis of ethyl alcohol produced mixtures that could be analyzed conveniently only through mass spectrometric examination of the separate fractions resulting from gas chromatographic separation. It was thus necessary to devise some form of apparatus for collection and removal of the fractions as they emerged from the sensing element of the gas chromatograph. Of the fraction cutters described (1-3, 5-7), two are designed for this purpose (2, 5 ) . They are a multiple series of fixed U-tubes, through vhich the sample vapors are directed by stopcocks. I n both, the samples must be transferred within a short time to storage vessels, or the array of U-tubes must be long enough to accommodate the number of fractions expected The device shown in Figure 1 has several advantages over these cutters. LTHOUGH
Figure
1.
Fraction cutter
A . Top view B. Side view, tilted
It is a compact unit, in which all four collecting traps (Figure 2) can be accommodated in a single Dewar flask. This eliminates the need for a long series of traps, with danger of breakage and inconvenience of operation. As many fractions can be collected as there are collecting tubes, without altering or enlarging the unit. While one tube is used to collect a sample, the others may be preflushed with carrier or any other gas. This is especially important when oxygensensitive compounds are being collected; Present address, Department of Fuel Technology, The Pennsylvania State University, University Park, Pa.
Figure 2.
Collecting tube
condensation of oxygen is prevented in the tubes prior to their use with a liquid nitrogen bath. The samples, after collection, may be stored directly in the tubes, and later transferred and the carrier gas removed, through the ground-glass joint, or through the simple apparatus, A , shown in dotted lines in Figure 2. A may be attached conveniently anywhere on a n independent vacuum line, or directly to a pump via the right-hand side arm. The spectrometric or other vessel is attached to the ground joint of A . After suitable freezing and evacuation, the sample may be transferred into the new vessel by immersing the vessel in a freezing bath and subsequently warming the collection tube. The collecting tubes may be converted into sample vessels for direct use with the spectrometer by blowing a vacuum stopcock at B, Figure 2 . As an alternative, ordinary spectrometric sample flasks may be used after blowing on an additional stopcock and bending to a U shape. Such flasks, however. would add much undesirable bulk to the unit. The time interval between emergence of a fraction from the sensing element and its entrance into the collecting tube is the same for all four positions. This is convenient for closely emerging fractions. This time interval may be determined by a number of simple methods, such as measurement of the volume between the sensing element and joint A of the fraction cutter, and the flow rate of the gas. I n other forms of cutters, this interval is different for each unit. K i t h chromatographs having provision for introduction of gas or vapor samples via a U-tube, collecting tubes may serve also as sample tubes. The chief value of such a n arrangement, aside from saving of materials, is the ease with which collected fractions may be reintroduced on the same or a different column. The unit is simple to operate. K i t h the stopcocks in positions shown in VOL. 29,
NO. 12, DECEMBER 1957
1899
Figure 1, A , the collecting tubes a t positions 2, 3, and 4 are being preflushed, while that a t position 1 is being flushed by the carrier gas flowing through the column. The tubes are then immersed in the freezing bath, and the determination is begun. After the first fraction has been collected a t 1, and, if necessary, after the appropriate time interval, the stopcock a t 1 is rotated 90" to the right with the right hand, while the left hand is turning stopcock 2 to the left. The next fraction will be collected a t 2. The collecting tube a t 1 then may be removed, and a new one inserted. Two ball joints may be blown on the apparatus, as shown in dotted lines in Figure 1, B, so that the apparatus may be swiveled on the central joint to the next position. The male member of the other joint is then turned manually, to accommodate the twisting of the attached rubber tubing.
If two succeeding fractions emerge closely together, a part of the first may remain in the side arm and contaminate future fractions. By suitable geometry this can be made negligible. The stopcocks of the cutter should be blown fairly close together, so that the length of the side arm from A to each stopcock is from 2 to 3 cm. In this case, the inside diameter of the tube being 2 mm., approximately 0.1 cc. of gas may be trapped in the arm. As the minimum flow of gas through a column is usually about 10 cc. per minute and the fraction rarely passes through the sensing element in less than 1 minute, a maximum of about 1% of the sample may be trapped in the arm. It is probably much less, for the tail end of the sample, the portion trapped, contains a very low concentration of the organic compound. Such contamination will not affect calculations based on the chromatogram-e.g., weight per cent. EXPERIMENTAL
To demonstrate the "cleanness" of cut of the unit, a mixture of equal parts by weight of diethyl ether, diisopropyl ether, cyclohexane, and benzene was analyzed by using a 6-foot copper column, inch in outside diameter, containing %yo by weight of D. C. 703 silicone fluid on 35-60 mesh C-22 firebrick (Johns Manville). The chromatograph was similar to that described by Dimbat, Porter, and Stross (4, except that the stopcock-type sample injection system of Tenney and Harris (8) was used. The flow rate of helium was 40 ml. per minute, the temperature 70" C., and the sample size 0.01 ml. All joints and stopcocks of the fraction cutter were lubricated with Apiezon grease N, which is soluble in all components of the mixture. I n the first run, the fractions were collected (dry iceacetone) through a single arm of the cutter. The receivers were changed a t the points marked by arrows on Figure 3, I n the second run, the fractions were
1900
ANALYTICAL CHEMISTRY
TIME, MINUTES
Figure 3.
Chromatogram of test mixture
Ordinate is in arbitrary units of voltage
collected in turn through each arm, the stopcocks being turned a t the same points as above. The fractions were analyzed with the chromatograph, and compared to standards similarly obtained. A T-tube was inserted into the helium line a t the tank. Its free end was connected via rubber tubing to a stopcock and a male 3-12/30 joint, and the system was flushed with helium prior to use. To use the collecting tubes as sample tubes, the stopcock end (Figure 2) was inserted into the neoprene O-ring of the sample injector, and the retaining nut was tightened. The stopper on the tube was removed, and replaced with the Tconnected joint above. All stopcocks were opened, and the sample was flushed into the chromatograph for 100 seconds, the time found necessary from previous calibration to remove over 95y0 of the sample from the tube. The recorder was set for 4-mv. full scale, beyond which the adverse signal-to-noise ratio prevented improvement in detection. The parent peaks only were recorded a t 64-mv. full scale, so that their exact weight could be checked by previously obtained calibration curves. RESULTS A N D DISCUSSION
that contamination of one fraction by the other is beyond the detection sensitivity of the gas chromatograph. As the mass spectrometer has a detection limit of the same order as the chromatograph, the same conclusion may be drawn for this instrument. A fraction cutter containing three or even two collecting units may be constructed, depending on its use. The unit may be simplified by omitting the ring seals and connecting each of the multiple joints to the appropriate outlets. Such a modification is more rigid than the one described, and cannot be rotated, but it is much more easily constructed. ACKNOWLEDGMENT
The author wishes to acknowledge with sincere appreciation the efforts, advice, and constant encouragement of Quentin Klingler and Andrew T. Jacobsen, Dugway Project, University of Utah. LITERATURE CITED
Berridee. K. J.. Watts. J. D.. J. Sci.
The calibration runs showed that 2 y of each component gave a clearly
defined peak a t 4-mv. full scale, ranging X 3/4 inch (height X length) from for diethyl ether to X 13,'4inches for benzene. Because of loss of sample during removal of the stopper and other manipulations, each peak would have corresponded to approximately 0.2% contamination of the major fraction. However, the chromatograms of all the fractions showed no trace of any other component. Thus it may be concluded that the cutter will separate a normal size sample of two components in a state of purity such
Dimbat, M., Porter, P.. E., Stross, F. H., ANAL.CHEM.28, 290 (1956), Drew, C. M., McNesby, J. R., Smith. S. R., Gordon, A. S., Ibid., 28, 979 (1966'1.
E. hf., Tatlow, J. C., J . e. 1955, 1184.
hhols, J. A., Dijkstra,
ti., f i e c . trav. chim. 75, 965 (1956).
Tenney, H. XI., Harri CHEW 29, 31'7 ( 1
The opinions or assertions are those of the author, and are not to be construed as reflecting the views of the Department of the Army.
