Silberstein, O., Proc. Am. SOC.Hort. Sci. 63, 359 (1954). Spencer, M. s., Stanley, W. L., J . Agr. Food Chem. 2 , 1113 (1954). Tamsma, A. F., J . Dairy Sci. 38, 284, 487 (1955).
Underwood, J. C., Lento, H. G., Jr.,
Willits, C. O., Food Research 21, 589 (1956).
(34) Winteringham, F. P. W., Science 116, 452 (1952). (35) Winteringham, F. P. W., Harrison, A., Bridges, P. M., Biochem. J . 61, 359 (1955).
RECEIVED for review August 10, 1957. Accepted November 5, 1957. Mention of specific commercial materials or equipment does not constitute recommendation for their use above similar materials and equipment of equal value.
Use of Mixed Stationary Liquids in Gas-Liquid Chromatography W. H. McFADDEN Chemistry Research Branch, Atomic Energy of Canada limited, Chalk River, Ont., Canada
b The use of Tween 60 and silicone oil to separate mixtures of monoand dibromoalkanes i s illustrated and the equivalent behavior of a twostage chromatographic column and one prepared from an intimate mixture of the two packings is shown. Mixing the two liquids before adsorption on the firebrick matrix also gives identical separations. With silicone oil, the monobromoalkanes behave as one class of compounds and the dibromoalkanes as two classes. With the more polar Tween 60 as the stationary liquid, both the monoand dibromoalkanes behave as several classes, depending on the carbonbromine skeletal structure.
S
and analysis of many coniplev mixtures of volatile liquids can be achieved by passing them, in a carrier gas, through a nonvolatile stationary liquid supported on a matrix, such as crushed firebrick or Celite. The components of the sample are partitioned between the nonvolatile solvent and the carrier gas according t o their relative volatilities and relative solubilities in the stationary liquid. Frequently one encounters a complex mixture with a t least two components t h a t cannot be separated by a given immobile phase. Khen a stationary liquid is found that separates these two, the more strongly retained compound is often unresolved from a third component in the mixture. To overcome this perversity, Keuleman, Kwantes, and Zaal (6)suggested the use of a twostage column containing appropriate lengths of each of two suitable column packings. These are chosen so t h a t one is considerably more polar than the other and as a consequence has stronger retention for the more polar components of the unknown. The desired length for each section of column can be determined experimentally. This technique has been used b y Fredericks and Brooks (2) for separating complex
L w
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EPARATIOX
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Figure 1.
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Chromatogram of 1 1 -component mixture of alkyl bromides using
GE silicone oil SF-96(40)
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Figure 2. Tween 60
, 10
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- MINUTES Chromatogram of 1 1-component mixture of alkyl bromides using TIME
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Figure 3. Chromatogram of 1 1 -component mixture of alkyl bromides using a mixed substrate of 90% GE silicone oil and 10% Tween 60 b
mixtures of saturated and unsaturated hydrocarbons and is now coming into general use wherever mixtures containing molecules of different functionality are encountered. I n connection with a study of hotatom chemistry in alkyl bromides, a convenient modification of this technique was developed. It was desired to separate alkyl bromide mixtures containing mono- and dibromo compounds. The need for a two-stage column was indicated, but for convenience a column was prepared by intimately mixing the two packings. This blend gave the same separations as the corresponding two-stage column. Mixing the solvents before they are combined with the supporting material also gives a column with the same characteristics as the two-stage column.
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1,5- D I B R O M O P E H T A N E
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MONOBROMOALKANES
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~.S-DIBROMOHEXANE 1.4-DIBROMO~ENTANE l,4- D I B R O M O B U T A N E
DIBROHOALKANES
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P R O P Y L BROMIDE
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L O G RETENTION T I M E
Figure 4. Characterization of GE silicone oil SF-96(40) as stationary liquid for mono- and dibromcalkanes
RESULTS AND DISCUSSION
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Figures 1,2, and 3 illustrate the analysis of an 11-component alkyl bromide mixture using columns of silicone oil, Tween 60, and 90% silicone oil10% Tween 60, respectively. With silicone oil ( 1 ) as the liquid phase, 1,1-dibromoethane was not resolved from 1-bromobutane and 1,2dibromoethane was not separated from 2-bromopentane. With Tween 60, both
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EXPERIMENTAL
The analyses of synthetic mixtures of mono- and dibromoalkanes were performed with a Burrell Kromo-Tog, K2. Helium was used as a carrier gas a t a flow rate of 50 ml. per minute. This flow rate, measured a t room temperature, was maintained by manual adjustment of the helium reducing valve to compensate for changes in gas viscosity as the temperature of the column was increased. Columns, 2.5 meters long, were prepared from packings that contained 40 grams of stationary liquid to 100 grams of crushed firebrick (Johns-Manville, C-22), sieved to 40 to 60 mesh after mixing. The liquids used were General Electric silicone oil SF-96(40) and Tween 60 (polyoxyethylene sorbitan monostearate, Atlas Powder Co.). During a run, the temperature was increased from 40" to 125" C. a t a rate of approximately 5" C. per 2 minutes.
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b Figure 5. Characterization of Tween 60 as stationary liquid for mono- and dibromoalkaner
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ANALYTICAL CHEMISTRY
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pairs are separated, but lJ2-dibromoethane and 1,2-dibromopropane are A mixture containing equal not. amounts of these two packings gave poorer separations than either individually, but a 90% silicone oil-lOyo Tween 60 mixture was satisfactory. Using this ratio, three columns were prepared : a n intimate mixture of the two packings, a packing made by mixing the two liquids before adsorbing them on the firebrick, and a two-stage column. All three columns gave identical separations within small experimental variations. Use of mixed solvents has been employed frequently to modify the adsorptive properties (3, 4) or to decrease the volatility (5) of the stationary liquid. This work demonstrates that the properties of a chromatographic column prepared from a mixture of two stationary liquids can be equivalent to those of a two-stage column prepared in the same ratio. This can be expected for any combination of solvents that do not interact chemically. The order of elution with respect to boiling point for several mono- and dibromoalkanes is shown in Figure 4 for silicone oil and in Figure 5 for Tween 60. The upward curvature is a result of increasing the temperature during the determination. K i t h silicone oil, the mono- and dibromoalkanes behave as three classes of compounds. At a given boiling
point level, the monobromoalkanes are retained longer and their differences in chemical structure have no significant effects. The gem- and uic-dibromoalkanes are grouped together, while the remaining dibromoalkanes (Figure 5 ) constitute the third class. With the Tween 60 substrate, which is more polar, the mono- and dibromoalkanes constitute several distinct classes depending upon the carbonbromine skeletal structure. At a given boiling point level, the monobromoalkanes are retained considerably less than the dibromo compounds. Because the bromine atom is partly shielded by the surrounding methyl groups, 2bromopropane and 2-bromo-2 methylpropane are retained less than would be predicted from the retention time of other members of the series. This effect is slightly apparent with 2bromobutane and 1-bromo-2-methylpropane, but with 2-bromopentane it is not observed. The importance of structure is more apparent with the dibromoalkanes. Using Tn-een 60, 1,ldibromoethane is retained less than the lower boiling dibromomethane (boiling point difference = 11" C.) and 1,2dibromoethane is eluted simultaneously with lJ2-dibromopropane (boiling point difference = 12" C.). In general, with lower members of the dibromoalkane series, the effect of haying the same carbon-bromine skeletal structure will compensate for a 10" C. difference
in boiling points. This causes the dibromo compounds to behave as several classes, each with the general formulation 1,l-dibromo C,, lJ2-dibromo Cn+l, l,&dibromo Cn+l, etc., but which converge in the region of the 1,4-dibromoalkanes. This is illustrated in Figure 5 for the series starting with dibromomethane and 1,l-dibromoethane. The retention value for 1,2-dibrornobutane indicates that it may be the second member of the next series. This phenomenon may be attributed partly to the importance of similar carbon-bromine structures and partly to the shielding effect of the additional methyl group. The relatively low retention times observed for 1,2-dibromo-2-methylpropane and 2,5-dibromohexane may also be attributed to shielding of the bromine atoms by the methyl groups. LITERATURE CITED
(I) Evans, J. B., Willard, J. E., J. Chem. Soc. 78, 2908 (1956). (2) Fredericks, E. M., Brooks, F. ANAL.CHEM.28, 297 (1956). (3) James, A. T., Martin, A. J. Biochem. J . 50, 679 (1952). (4) James, A. T., Martin, A. J. Howard Smith, G., Ibid., 238 (1952). ( 5 ) Keuleman, A. I. M., Kwantes, Zaal, P Anal. Chim. Acta
357 (1955).
RECEIVEDfor review July 29, 1957. Accepted December 2, 1957.
Polarograph with Direct Recording of Electrode PotentiaI DONALD T. SAWYER Division of Physical Sciences, University o f California, Riverside, Calif. ROBERT L. PECSOK and KARL K. JENSEN Department of Chemistry, University of California, los Angeles 24, Calif.
,A new polarograph has been developed which utilizes an X-Y recorder for direct measurement of the electrode potential or applied voltage. Use of a third electrode provides automatic correction for the lR drop in the cell. The instrument is compact and versatile. Data are provided in a convenient and easily cataloged farm. The performance of the instrument indicates an accuracy within A2 mv.
T
HE accurate recording of polarograms from which precise values of half-wave potentials can be read
directly is an important problem in many applications of the polarograph. The accuracy of some commercial polarographs and an improvement for the electronic control of the span voltage have been recently discussed (6). Three sources of error are inherent in the design of these instruments: The chart drive must be synchronized with the span drive with respect to both speed and starting time; the chart records time along the X-axis, which can be interpreted as voltage applied to the cell only if the span voltage is accurately known and precisely linear; and, a t best, it is the cell voltage that is read
from the chart rather than the potential of the polarized electrode. A new polarograph utilizes a modified X-Y recorder for the direct recording of current us. either the cell voltage or (with the use of a third electrode) the potential of the polarized electrode. Thus, the sources of instrumental errors have been greatly reduced, and the accuracy for the direct reading of potentials is limited only by the accuracy of the recorder, itself. The instrument offers many advantages. It can be used conventionally with two electrodes, or with a third reference electrode; when a third electrode is used, the VOL. 30, NO. 4, APRIL 1958
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