Preparation of very pure hexafluorobenzene (> 99.99 percent) by


endrin was eluted from a 6-foot column in 5 minutes at. 135° C and 22 ml/minute, when textured glass beads were loaded with 0.05% DC-710. Conventiona...
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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.

SP

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.

224

ANALYTICAL CHEMISTRY

Table I.

Conditions for Gas-Liquid Chromatography Analyses of Hexafluorobenzene Column temperature,

Thermal conductivity detector temperature, C

Column

Stationary phase

Solid support

Column dimensions, inches

1

9.6% bis(2 ctnql) hex41

Chromosorb W 6 8 0 mesh

Length, 45 id.,

58

200

2

4 w< polyeth) lcne gl>col

Anatron 60-80 mesh

Length, 72 i.d.,

63

150

tetrachlororhthalate

removed as impure fraction. At this stage, almost all the liquid could have been frozen at 0" C. The hexafluorobenzene was purchased from the Imperial Smelting Corp., Avonmoath, U. K. The purity, as determined by gas-liquid chromatograph) (GLC) by the manufacturer, was 9 7 . 6 z (3). The final treatment to achieve this was fractional distillation. Our analysis of the commercial product indicated a purity of 98.25 i 0.1 %. In view of the assumptions made in quantitative analysis by GLC, when samples are not available for rigorous calibration this discrepancy is not unreasonably large. We used two stationary phases with very different ch:irncteristics in the analysis by GLC. The conditions are stiiiiniurized in Table I. The carrier gas was helium at 16-psi inlet pressure and a flowrate of 15 cc min-1. Representative results from the two columns are listed in Table 11. The quantitative estimation of purity was made by cutting out and weighing the peaks. The usual assumption that the total area under all peaks was equivalent to 100% was made. The inherent assumption that the detector response was equally proportional to a particular property of all the compounds present is an obvious source of uncertainty. It has previously been shown that the relative areas under peaks obtained using a thermal conductivity detector is reasonably well correlated with the weight percentages of structurally related compounds ( 4 ) . In studying small amounts of impurities of type similar to the main component, the assumption of direct proportionality between the peak areas and the weight percentage probably does not introduce a very significant error. In terms of following the progress of the purification by comparison of analyses, the above discussion is not important as only relative values are necessary. It would probably have been possible to achieve some purification of the starting rv,citcrial by careful fractional distillation but the relati\rIy ll~wyield and the small, though real risk of loss of material ;!tie to breakage, were disadvantages in this method. hfcm importantly, it seemed desirable t o employ a method dependent on a property not exploited during the preliminary purification.

O

-

..~.

(3) M. W. Buxton, Imperial Smelting Co., Avonmouth, England, personal communication, 1967. (4) A. E. Messner, D. M. Rosie, and P. A. Argabright, ANAL. CHEW,31, 230 (1959).

C8.3

Samde Com-

mercial 3rd pass 6th pass 3rd pass 6th pass product (at 0" C ) (at 0" C) (at 3' C) (at 3" C ) Result, column 1 column 2

98.18 98.32

99.40

99.77 99.92

99.993

o'ociY 9 8.5

~

AT 0°C

RESL'LTS

-

O

Table 11. Results of Analyses as Wt.

v

The results are summarized in the graph, Figure 2. It is clear that the purification limits of the method, using the cold bath at 0" C, are reached by the fifth freezing. The yield of the purified material before any retreatment of rejected fractions was 69:;. N o overall loss was recorded. The purity of the final product was 99.7,",, as determined by the previously described GLC method. The density of the final product a t 25.00" C was 1.60595 -c 0.00001 grams cc-1,

C

Figure 2. freezings

FREEZIN~S AT 3°C-

Per cent hexafluorobenzene us. number of

compared with 1.60678 grams cc-l, reported by Counsell et al. (5). The rejected fractions were subjected to further freezings and a yield of 50-60x was obtained with a purity of 99.5%. The rejected fractions from this were refrozen again with a yield of 20-3Ox of 99.6% of purity. The overall yield of purified material was 87%. ( 5 ) J. F. Counsell, J. H. S . Green, J. L. Hales, and J. F. Martin,

Trans. Faraday Soc., 61, 212 (1965).

VOL 40, NO. 1, JANUARY 1960

225

The product was of a satisfactory purity for our purposes but we repeated the treatment on a 70-ml sample of product using a cold bath temperature of 3" C. The purity of the product from this was 99.99az after 6 freezings. The density was 1.60688 i. 0,00001 grams cc-l at 25.00" C. This represents an increase of 0.00010 gram cc-l over the literature value (5). The purity of the hexafluorobenzene on which the earlier determination was made was 99.97 i 0.01% (5). The result reported now confirms the higher purity of our material and provides a density result for very pure (>99.99x) hexafluorobenzene.

ACKNOWLEDGMENT

We express our appreciation for the practical contributions of George W. Allison during the construction of the apparatus. Thanks are also due to Dennis Gere for making available his GLC apparatus and to Theodore J. Neubert for many useful suggestions.

RECEIVED for review July 20, 1967. Accepted October 20, 1967. Work supported by The Public Health Service Grant NO. GM 14710-01.

Advantage ob Multidiameter Separation Column in Gas Chromatographic Analysis of Organics J. Q. Walker McDonnell Douglas Corp., St. Louis, Mo. 63/(

1

HIGHRESOLUTION

GAS CHROMATOGRAPHIC TECHNIQUES used in preparative scale and trace analyses have been studied by a number of workers (1-3). Dal Nogare and Juvet (2) mentioned several problems in accurately measuring trace compounds eluting after a major component. Two practical cases frequently encountered in the gas chromatographic analysis of a minor component comprising less than 5% of a sample are shown in Figure 1. In case, 1A the trace compound is eluted before the major compound, and in case lB, it is eluted after the major component. In both chromatograms a large sample is employed to make the trace component evident. Case A permits accurate measurement of the trace compound because it is well resolved from the matrix and is eluted early, therefore presenting a well-defined peak. Case B presents a situation in which accurate measurement of the trace compound is difficult and in which improved resolution is required for satisfactory analysis (4). Rarely can one elute all the minor components from a column p r i v to the major compound. The column liquid phase may - changed and thereby change the component relative elution order. For example, the analysis of high purity ethylene for acetylene and ethane impurities is a difficult problem. The order of elution from a column containing the polar stationary liquid phase benzyl ether would be ethane, ethylene, and acetylene. Acetylene appears as a shoulder on the large ethylene peak, The separation order with a nonpolar stationary liquid phase such as squalane would be acetylene, ethylene, and ethane. Ethane appears as a shoulder on the ethylene peak. An intermediate polarity column would result in no separation. The separation of high purity ethylene is relatively simple with the two-column technique.

(1) J. Q. Walker, Hydrocarbon Process. Petrol. Refiner, 43, 154

(1964).

(2) S. Dal Nogare and R. S. Juvet, "Gas-Liquid Chromatography," Interscience, New York, 1962, p. 300. (3) A. Zlatkis and H. R. Kaufman, Nuture, 184, 2010 (1959). (4) A. Zlatkis and J. Q. Walker, Abstracts, Pittsburgh Conference

on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1962, p. 50. 226

ANALYTICAL CHEMISTRY

A

I I "

0 Time

Figure 1. Typical chromatograms of a trace component in an essentially pure sample

Large sample sizes of the order of 0.2 gram may be separated by increasing the length of an analytical column. However, any increase in column length results in a significant increase in time of analysis. EXPERIMENTAL Apparatus. An F & M Scientific Model 5750 gas chromatograph equipped with a dual thermal conductivity detector was used for these experiments. The chromatograms were recorded on a 0- to 1-mV Moseley strip chart recorder. Aluminum columns containing 20 % bis (2-ethyl hexyl) adipate (Distillation Product Industries) on Chromosorb W, 60/80 mesh, were prepared in 5-, lo-, 15-, and 20-foot lengths with diameters of lis, 1/4, 3/8, and inch. Aluminum columns containing Polypak No. 1, (F & M Scientific), 60/80 mesh, were prepared in 4-, 8-, and 12-foot lengths with lip- and 3/s-inch diameters. Stainless steel columns containing 10% Carbowax 20M on Chromosorb W, 60/80 mesh, were prepared in 14- and 3-foot lengths with diameters of 1/4 and 3/8 inch, respectively. Helium was used as the carrier gas. Columns were operated, isothermally at 70" or 80" C. The injection block temperature was maintained at 270" C. The thermal conductivity current was 150 mA. Liquid samples were injected directly with a 2.5-ml syringe (Hamilton No. 1002) equipped with a 6-inch stainless-steel needle for