In the t; inset are the corresponding decay data for the 0.51rays from target and catcher. Again the differMeV gamma E ence in curves is striking. For the target, the ratio of activities I*FpGa = 2, whereas -98% of the catcher activity is ‘*F (unlike the 14-MeV case, the catcher at 20 MeV apparently contained a small amount of “Ga, as well as the l8F). These experiments indicate that by takins advantage of differences in recoil properties of different nuclides, one may achieve satisfactory separations of radioactive products. This conclusion has potential use in activation analysis, especially for the determination of surface impurities, like oxygen or carbon, on medium-weight elements. It may prove especially useful in situations where the activities produced in the matrix are much more intense than those from the sought impurities. The technique is similar to that of the usual activation analysis, and is simple, not requiring the more sophisticated apparatus needed for the counting of particles emitted during irradiation. But it is clear that such characteristics of the recoil method as
variations of recoil loss from the target with changes in surface depth, target thickness, and bombarding energy will have to be considered in detail before the technique can be applied to actual analyses. Indeed, in very recent work (12),recoils from the laO(t,n)‘SFreaction were used to determine surface oxygen on aluminum and lithium. ACKNOWLEDGMENT The generous cooperation of the operating staff of the ORIC is appreciated. RECEIVED for review April 21, 1967. Accepted October 12, 1967. Research sponsored by the U. S. Atomic Energy Commission under contract with the Nuclear Division, Union Carbide Corp. (12) P. E. Wilkniss and H. J. Born, Infern. J. Appl. Rodiaiion Isotopes, 18, 57 (1967).
Very Lightly Loaded Textured Glass Beads as Support for Gas-Liquid Partition Chromatography Herbert L. MacDonell Technical Staffs Dioision, Cornins Glass Works, New York 14830
THEUSE of glass beads as a solid support for gas-liquid partition chromatography is well established. Glass beads were first used by Callendar and Cventanovits (I) in an effort to reduce the tailing often associated with porous support materials. Modification of solid glass heads by surface etching has also been investigated and has resulted in improved column efficiencies (2, 3), although mechanical abrasion is (1) A. B. Callendar and R. J. Cventanovits, Can. J. Chem., 33, 1256 (1955). (2) R. W. Ohline and R. Jojola, ANAL.CHEM., 36, 1681 (1964). (3) F. A. Bruner and G. P. Cartoni, Ibid., p. 1522.
Figure 1. Acid-treated, silanized, glass beads showing surface texture detail at 6 7 X
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Bead size -60 80 mesh. Coated with 0.50% Dow Corning 710 silicone oil No “puddling” or other evidence of B mating is noticeable
essentially ineffective (4). The purpose of such treatments has been to increase the surface area of the beads and thereby to extend their loading capacity. According to Wilkinson this is desirable because a stationary phase and Gibson (3, loading of over 0.10 to 0.25% on smooth glass beads frequently renders them too “tacky” for convenient packing. (4) R. A. Dewar and V. E. Maier, J. Chromatog., 11, 295 (1963). (5) 0.E. Wilkinson and J. H. Gibson, ANAL.CHEM., 38, 1972 (1966).
Figure 2. Smooth (unetched), silanized, glass beads coated with silicone oil (67X) Bead size -60 +SO mesh Note the “puddling” between beads at points of contact even though the loading is only 0.06Z The glass composition of these beads is the same as those shown in Figure 1 VOL 40, NO. 1, JANUARY 1968
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Figure 3. Separation of C6 through Czo even-numbered saturated hydrocarbons Column: l/&ch X 6-foot glass filled with 60/80 mesh surfacetextured glass beads Coating: 0.01 DC-710 silicone oil Carrier gas: nitrogen, 40 psi: 66 ml/minute Sample: 0.04 pl of hydrocarbon mixture Attenuation: 104/1 Temperature: programmed from 30’ C to 175 C at 30O C/minute O
Some of these surface modifications have been moderately beneficial in improving the packing characteristics of glass beads. Hishta and Bomstein (6) have described the use of new surface-textured glass beads of an optimized glass composition. This glass reduces adsorptive tailing by removing metallic ion impurities that are found in most glass beads. The present work describes the results using high-purity glass beads that that have been textured to produce desirable surface characteristics. EXPERIMENTAL
Support Material. Alkali-silicate glass beads of the approximate composition 80 % Si0220 % NazO were used during this investigation. They were treated with a mixture of mineral acids to produce a surface texture as illustrated in Figure 1. Additional details describing the production of a textured surface on glass beads are reported by Filbert and Hair in a pending publication (7). The rough surface on beads of this type is desirable, because it more effectively holds the partition phase in a film or coating of uniform thickness. (6) C. Hishta and J. Bomstein, “Advances in Gas Chromatography,” A. Zlatkis, Ed., Preston, Evanston, Ill., 1967, pp. 39-44. (7) A. M. Filbert and M. L. Hair, J. Gas Chromatog., in press.
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Figure 4. Endrin response on a low-loaded, textured-glass bead column Column: ‘/?-inch X 6-foot glass filled with 60/80 mesh surfacetextured glass beads Coating: 0.38x DC-710 silicone oil Carrier gas: nitrogen, 40 psi: 66 ml/minute pl of endrin in I/Z ml of chloroform Sample: Attenuation: 10/10 Temperature: 200” C Retention time: 5.45 minutes
As a result, “puddling,” as seen at the contact points of smoother beads in Figure 2, is essentially eliminated. Likewise, peak tailing and symmetry are improved to whatever extent they may be affected by a gradient coating thickness on smooth glass beads. Coating. Glass beads used in this investigation were silanized in a 5 dichlorodimethylsilane in toluene solution, dried, and coated with Dow Corning #710 silicone oil. The latter step was accomplished by adding 50 ml of a 2 x solution of silicone oil in toluene to 65 grams of glass beads in a fluidizing bed column, agitating with nitrogen for 10-15 minutes, draining with vacuum, and finally drying at 200” C as a fluidized bed, using a nitrogen purge to maintain an inert atmosphere. The beads thus prepared were used to pack a 1/4-inch X 6-foot glass column. Vibration and tapping were employed to ensure uniform packing. Silanized glass wool plugs were placed in the ends of the column. Analysis of the coated beads disclosed that a loading of about 0.15-0.20% was achieved when the above procedure was followed. Loadings down to 0.008% were obtained when the coating solution was reduced to 0.4z silicone oil. Apparatus. A Microtek MT-220 gas chromatograph, having dual-flame ionization detectors, was used during this study. The recorder was a 1-mV Esterline Angus, having a response to 0.5 second. Specific operating conditions are in: cluded with each illustration. RESULTS
One of the initial separations made on the textured glass bead column is shown in Figure 3. This chromatogram resulted when a mixture of eight saturated hydrocarbons was injected into the apparatus. The total time for this separa-
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Column: l/Anch X 6-foot glass filled with 60180 mesh surfacetextured glass beads Coating: 0.01% DC-710 silicone oil Carrier gas: nitrogen, 40 psi; 21 ml/minute Sample: 0.7 p g of each pesticide in 0.4 pl of hexane Attenuation : 10/8 Temperature: 160' C
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Figure 7. Endrin HETP values us. percentage of smooth beads in surface-textured beads
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Figure 6. Pesticide mixture separation on textured-glass beads ; programmed temperature Temperature: Programmed from 130"C to 185" C at 5 " C/minute Other conditions: Same as in Figure 5
tion was 10 minutes. Excellent resolution is evident for each component. Hexadecane, whose boiling point is 280"C, was eluted in only 6.5 minutes. Figure 4 shows the excellent results that are obtained when endrin is passed through a lightly loaded column under isothermal conditions. This compound is considered a good test for a chromatographic column, because endrin frequently produces poorly shaped peaks as a result of its decomposition on many column support materials. The HETP value calculated from this peak is 0.29 mm; asymmetry is 1.03. Additional chromatograms showing other pesticides are presented in Figures 5 and 6 , which depict isothermal and programmed temperature results, respectively. These curves were obtained on relatively large samples, using the flame ionization detector and not electron capture. The effectiveness of surface texturing was determined by preparing and evaluating columns containing mixtures of both textured and smooth glass beads. As would be expected, the retention time for a given compound was decreased with an increase in percentage of smooth beads. Likewise, the loading that resulted from a standard coating procedure also decreased as more smooth beads were used. A plot of HETP values us. percentage of smooth beads is shown in Figure 7. VOL. 40, NO. I , JANUARY 1968
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The decrease in column efficiency is evident as the concentration of smooth beads increases. Nevertheless, the column efficiencies are not seriously affected until 50% of the beads are the smooth untextured variety. The use of lightly loaded textured glass beads permits gas chromatographic separations of steroids and other high molecular weight compounds at lower temperatures than are possible when more porous supports are used. For example, endrin was eluted from a 6-foot column in 5 minutes a t 135” C and 22 ml/minute, when textured glass beads were loaded with 0.05% DC-710. Conventional supports require from 1 to 2 hours, or longer, to permit elution of endrin a t this temDerature. Loadings of up to 0.6% have consistently maintained
HETP values of less than 1.0 mm. Optimum loading, however, was found t o be between 0.1 to 0.35% with most columns loaded at about 0.15%. CONCLUSION
Surface-textured glass beads of optimized composition have desirable properties for gas chromatographic application. Some of the advantages they offer over conventional smooth glass beads are: reduced tailing due to the inert nature of the glass; greater surface area; uniform distribution of partition phase and, as a result, less “puddling” a t the points of contact between beads. RECEIVED for review July 21, 1967. Accepted Oct. 16, 1967.
Preparation of Very Pure Hexafluorobenzene (>99.99%) by DirectionaI Freezing F. David Evans,’ Michael Bogan, and Rubin Battino Chemistry Department, Wright State University, Dayton, Ohio 45431 HEXAFLUOROBENZENE of higher purity than the commercially
available material was required for gas solubility measurements of a precision of * 0 . 3 x . We considered that a final purity of greater than 99.5% would be satisfactory, Because hexafluorobenzene is costly, we felt that the yield of purified material should be a t least 80%. Minimum batch quantities of 300 ml were required for our studies so that the purification procedure was designed to start with 500 ml of the commercial material. Directional freezing has been used to purify benzene ( I ) (we confirmed this experimentally) and gives a high yield of purified material. The impure fractions can be collected and refrozen so that loss of material is very small. The method can be automated so that little actual working time is required. For these reasons directional freezing was chosen for the purification of hexafluorobenzene. EXPERIMENTAL
The method used was to lower seven tubes containing 70 cc each of hexafluorobenzene into an acetone bath kept at 0 ” C. Antifreeze solution from a refrigerating unit was circulated through a copper cooling coil to maintain the cold bath temperature. Temperature gradients in the bath were avoided by bubbling air continually through the liquid. The sample tubes were 2 cm i.d. and 40 cm long. By using tubes with a small cross-sectional area, and by employing a slow rate of lowering, it was not necessary to stir the unfrozen hexafl uorobenzene. The apparatus used for the controlled lowering was primarily intended for zone refining of solids. A reversible 1rpm synchronous motor was connected through a gear train and pulleys of different sizes to the sample tubes, which could be raised past heaters (one tube in this case) or lowered into Present address, Marchwood Engineering Lab., C.E.G.B., Southampton, U. K.
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IMI Figure 1. Driving section of the apparatus
a cooling bath. The drive section of the apparatus is shown in Figure 1. The following parts were obtained from the Boston Gear Works (Quincy, Mass.): Gl-#G254, G2#G268, G3-#G487YP, G4-&G487YG, G5-#G486YP, G6#G486YG; P-four pulleys 21214, 1216, 1218, 1220; M-Hurst type DA-I, 1 rprn, 120 inch-ounce, reversible synchronous motor; S-Detroit solenoid replacement coil, No. 294 (Harry Alter Co., Chicago, Ill.). The length through which the sample was raised or lowered was controlled by means of microswitches, which activated a relay and, thence, a solenoid. The laiter controlled a decoupling mechanism in the gear train. The control circuit was a modified version of one used by Herington et ( I / . ( 2 ) . The rate of lowering was 1.5 cm hr-’, and a single frcczing required 17-20 hours. The end point was pre\ei a b o u t 2 cm below the hexafluorobenzene meniscus. I n practice, it was found that 13x of the commercial material would not freeze at 0 ” C , and this was removed. As the piiritb. increased, it became possible to freeze an increasing proportion of the hexafluorobenzene until after the sixth freezing, 17; of the original volume was .. (2) E. F. C. Herington. “Zoiie Xlelting of Organic Compounds,” Wiley, New York. 1963. p. 18. ~~
(1) J. D. Dickenson and C. Eaborn, Chem. hid. (London), 1956, 959.
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