Thiourea as Substrate for Gas Chromatography of Fluorocarbons SIR: We report here that thiourea, but not urea, is a good substrate in gas chromatography for the separation of the unsaturated compounds perfluorocis- and 2-methylpentene-2 and trans-perfluoro-2-met hylpentene-3 from perfluorohexanes and from one another. The use of urea and of thiourea was an attempt to makte use of the canal complexes that these chemicals form with numerous coimpounds ( 1 ) . If this phenomenon Rhould occur in a chromatographic column, a separation of isomers would be possible resulting in the elution of the most highlybranched structure first and the normal isomer last. With both of these substrates the saturated perfluorohexanes appear in an order which is the reverse of that found with n-hexadecane as substrate ( 2 ) . With the latter substrate the isomers appear strictly in order of increasing boiling point which is the order for increasing complexity of molecular chain branching in CsFla. On urea and on the n-hexadecane substrates the unsaturated molecules are not separated from the saturated ones.
EXPERIMENTAL
Sources for the perfluoroalkanes have been cited ( 2 ) . The perfluoroalkenes were prepared in this laboratory, purified by preparative-scale gas chromatography, and identified by nuclear magnetic resonance. The urea and thiourea packings were prepared from a water solution evaporated on a steam bath in contact with acid-washed Chromosorb-P. The packed 1/4-inch 0.d. columns were dried by helium flow at room temperature. Chromatograms were obtained in conventional apparatus, and the retention volumes were standardized for pressure corrections and void volumes. The carrier gas was helium. Hydrogen reacts with thiourea and destroys the separating ability of the column. RESULTS
Thiourea is a solid at temperatures below 180' C. This work was carried out at 25' C. and thus may be an example of gas-solid chromatography. Chromatogram peaks were symmetrical with no tailing for the sample sizes used, 1 to 2 ml. of liquid. The following standard retention volumes in cc.
of helium a t 1 atm. and 25" C. were obtained at 25' C. on 15.2 meters of 1/4-inch copper tube containing 31.2 grams of thiourea on 124.6 grams of Chromosorb-P, 35- to 80-mesh, with helium carrier at 34.7 p s i inlet and 16.8 p.s.i. outlet pressure flowing a t 29.9 cc. per minute when metered a t 1 atm. and 25' C . : perfluoro-2,3dimethylbutane, 344; perfluoro-2methylpentane, 368; perfluoro-3methylpentane, 385; perfluoro-nhexane, 410; trans - perfluoro - 2 methylpentene-3, 323; cis-perfluoro-2methylpentene-3, 377; perfluoro-2methylpentene-2, 451. LITERATURE CITED
(1) Brown, J. F., Jr., Sci. Am. 207, 82 (July, 1962). ( 2 ) Reed, T. M., 111, J. Chromatog. 9, 419 (1962).
JAMES C. MAILEN T. M. REEDI11 JOHN A. YOUNG Department of Chemical Engineering University of Florida Gainesville, Fla. WORKsu ported in part by the Atomic Energy &ommission and the National Science Foundation.
Infra red Mic r ocel I Richard M. Chrenko,, General Electric Research Laboratory, Schenectady, N. Y.
A
of fixed thickness microcell suitable for the measurement of infrared absorption spectra of liquids is described. . Its sandwich construction makes this microcell a scaled-down version of commercially available macrocells, except for several important features. I t has been used in the PerkinElmer 4-1 double-beam spectrophotometer with the Perkin-Elmer 4-1 beam condenser, and with the Beckman IR-7 double-beam spectrophotometer with the Beckman beam condenser. This microcell was designed to contain the liquids collected in (capillary tubes from gas chromatography columns. These liquids were volatile and reactive in air. Hence, the primary design requirements were based on the volumes that were obtained from the columns, the cell thickness needed to obtain meaningful NEW TYPE
spectra of neat samples, and the need to fill and seal the cell in a drybox. Typical samples were of the order of 5 pl. and, because of the volatility of the samples, the cell had to have approximately this total volume. This was necessary since the infrared beam illuminates and heats the central section of the cell, and if one did not have a completely filled cell, volatile samples would distill out of this section. However, the active volume of this central section of the actual cell was less than 0.2 pl. 4 constant and reproducible cell thickness of 1/2 mil was deemed appropriate for obtaining meaningful spectra. A number of secondary requirements also led to this design; namely, the cell had to be easy and quick to construct; easy to fill; capable of allowing an operator to observe the entire liquid volume during and after
filling; easy to seal; and easy to clean. A number of microcells are available commercially (Perkin Elmer Corp.; Beckman Instruments; Connecticut Instrument Corp.), or are described in the literature (1-S), that enable one to measure the infrared absorption spectra of small-volume liquid samples. However, each cell has a drawback that makes it unsuitable for the liquid samples from chromatography capillary tubes. These cells will not be described here. EXPERIMENTAL
Apparatus. T h e microcell can best be described by discussing first the liquid container and then the mechanism for sealing the filling holes. A blown-up view of the cell is shown in Figure l a . The dimensions given VOL. 36, NO. 9 , A U G U S T 1964
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