ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
661
AIDS FOR ANALYTICAL CHEMISTS Separation of Perfluorinated Hydrocarbons by Gas-Solid Chromatography Ronald R. Shields" IBM Systems Products Division, East Fishkill Facility, Hopewell Junction, New York 12533
John A. Nieman ISM System Products Division, Poughkeepsie, New York 12602
Perfluorinated hydrocarbons are of considerable interest to investigators in fields as diverse as medicine and electrical engineering. However, for those investigators working with these compounds, t h e separation and identification of multicomponent isomeric mixtures have been a difficult matter. Reed (1) studied the gas-liquid chromatographic separation of C,, C7,and C, fluorocarbons on six stationary phase liquids. H e reported that fluorocarbon and chlorofluorocarbon stationary liquid phases gave better resolution of fluorocarbon mixtures than hydrocarbon phases, but that they could not separate closely boiling isomers as effectively. Reed then reported the separation of the C6F14isomars (2)and later the separation of C7FI6isomers ( 3 ) by use of n-hexadecane on chromosorb-P. However, it was necessary to use a column t h a t was 1 7 meters in length to accomplish the separation. The use of this column for the separation of higher molecular weight fluorocarbon isomers is limited by the fact the nhexadecane has a narrow operating range (20-50 "C). Green and Wachi ( 4 ) reported the separation of certain low molecular weight perfluorocarbons (C2-C4) by use of gas-solid chromatography on a temperature programmed silica gel column. Later, Wright and Askew (5) showed that the use of silica gel could be extended to separate the perfluoroalkanes into molecular weight classes from CF, to but the separation of isomers was not realized. Certain tests performed in our laboratory required the use of a column for analyzing perfluorocarbon isomeric mixtures ranging from C4 to Cg. Therefore, a column other than those found in the literature was necessary. The use of graphitized carbon black has been reported by various investigators (6-10) as an efficient method for separating hydrocarbon isomers. T h e objective of this study was to compare the general separation characteristics of selected hydrocarbons and perfluorocarbons on this adsorbant.
EXPERIMENTAL The graphitized carbon black (GCB) used in this study was 80/100 mesh Carbopack C (Supelco, Inc., Bellefonte, Pa.), which has a surface area of 1 2 m2/g. A moderately polar stationary phase, SP-1000 (0.170 w/w), prepared from Carbowax 20M and a derivative of terephthalic acid, was used to modify the surface of the Carbopack C. The columns used were 6 ft X ' / 4 in. 0.d. X 4 mm i.d. and made of glass. They were purchased from Supelco, Inc., prepacked with the above GCB and modifier. The columns were conditioned before use for 14 h at 175 "C. The gas chromatograph on which retention data were measured was a Hewlett-Packard series 700. Thermal conductivity, with helium as the carrier gas, was used for detection. The carrier flow rate was measured with a soap-bubble flowmeter at the column exit. Oven temperature was measurable to 50.5 "C by the use of an external thermocouple system more sensitive than the one on the HP-700. The sample size used for GC analysis was, in each case, 0.5 pL. Each sample was approximately a 2:l mixture of perfluorocarbon (or hydrocarbon) to n-hexane. 0003-2700/78/0350-0661$01 OO/O
The hydrocarbons used were reagent grade chemicals obtained from Aldrich Chemical Co., Milwaukee, Wis. Commercial samples of n-perfluoropentane and n-perfluorohexane were obtained from the 3M Co. Other perfluorocarbons were obtained from Pierce Chemical Co., Rockford, Ill., with the exception of perfluoro2-methylpentane, which was a gift from Farbwerke Hochst, Germany. Commerical samples of C5F,,, CPl4,and C7FI6,when analyzed, produced many impurity peaks. Subsequently, these samples were analyzed by gas chromatography-mass spectrometry (HewlettPackard 5982A) to confirm the elution order of the compounds of interest. Chemical ionization with methane and electron impact ionization modes were used to assign empirical formulas to eluting components. Structural assignments were made by spiking mixtures with the known compound whenever possible. A separate 6 f t X 1/4 in. 0.d. X 4 mm i.d. glass column, with 0.170 SP-1000 on Carbopack C, was used for the GC/MS work. Sample spiking with known compounds was employed to confirm that the elution order of compounds on both columns reported in this study was the same.
RESULTS AND DISCUSSION A preliminary evaluation of unmodified graphitized carbon black columns showed that the elution order of components was not affected by the presence of the modifier described in the Experimental section. Rather, retention times were considerably longer and peak shape was poorer because of tailing. Increasing the percentage of the liquid modifier had the effect of decreasing retention time and eventually resulted in a loss of separation. Because of the above behavior, only a column with a low level of modifier was employed in this study. The retention behavior of selected hydrocarbons and perfluorocarbons is tabulated in Table I, which gives the values for adjusted retention time and for retention time relative to an internal standard (n-hexane) at 120 "C. When homologous compounds are being separated, retention time increases with increasing molecular weight. Spherical molecules elute with shorter retention times than straight chain molecules having the same molecular weight, and the isomer having the greatest spherical geometry has the shortest retention time. Additionally, it was observed that for all compounds tested, the fluorocarbon has a shorter retention time than the corresponding hydrocarbon, indicating a weaker interaction with the adsorbant surface. The similarity of the elution sequence observed for the hydrocarbons and perfluorocarbons in Table I indicates that the nature of the basic interaction with the adsorbant surface is the same for both classes of molecules. Likewise, the slopes of the lines obtained from plots of the log of the capacity ratio h'vs. the inverse temperature indicate that for both classes of compounds, the temperature dependency of the interaction between the sample and the adsorbent decreases in the order alkenes 2 alkanes 2 cyclic structures. Since the fluorine atom is larger than the hydrogen atom, C 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
Table I. Adjusted Retention Time and Retention Time Relative to n-Hexane of Perfluorocarbon and Hydrocarbonsa Retention data Adjusted retention timeb ( t $ ) s Hydrocarbon Per fluorocarbon
Compound 2,2-Dimethylpropane 2-Methylbutane n-Pentane 2,2-Dimethylbutane 2,3-Dimethylbutane 3-Methylpentane 2-Methylpentane n-Hexane 3,3-Dimethylpentane 2,2-Dimethylpentane 2,4-Dimethylpentane 2,3-Dimethylpentane 3-Methylhexane 2-Methylhexane n-Heptane 1-Heptene Methylcy clohexane Cyclohexane Cyciohexene Methylcyclopentane
52.9 94.0 134.2 182.2 259.2 312.6 333.5 490.0 569.2 589.1 672.7 769.7 1045.1 1164.5 1788.7
...
...
...
172.4 204.3
Relative retention time ( r , l s ) Hydrocarbon Perfluorocarbon
42.1 65.7 86.7 121.5 141.8 185.6 206.0 288.9
0.108 0.192 0.274 0.372 0.529 0.638 0.681
...
1.162 1.202 1.373 1.571 2.133 2.371 3.650
0.086 0.134 0.177 0.248 0.289 0.379 0.420 0.590
...
... ...
515.4 639.8 994.5 630.2 261.0 88.9 137.4 73.8
... ... ... ...
1.174 1.306 2.030 1.286 0.533 0.181 0.280 0.151
... ...
...
0.352 0.417
a Column temperature, 120 "C; Sample size, 0.5 mL; He flow rate, 30 cm3/min; Column, 6 ft x Carbopack C coated with 0.1%SP-1000. Based on an elution time of 34.8 s for the air peak.
in. 0.d. glass; Packing,
Table 11. Column Selectivity ( a )as a Function of Hydrocarbon and Perfluorocarbon Pairs Perfluorocarbon n-Hexane/2methylpentane 2-Methylpentane/3methylpentane 3-Methylpentane/2,3dimethylbutane 2,3-Dimethylbutane/2,2dimethylbutane
Hydrocarbon
1.40 i 0.05
1.45
?:
0.04
1.11i 0.02
1.07
i
0.02
1.31 i 0.01
1.21 r 0.01
1.18 i 0.01
1.41 * 0.02
it is possible to have steric hindrance effects with perfluorocarbons which are not observed in the corresponding hydrocarbons. These steric effects can influence the separation behavior by preventing certain perfluorocarbons from assuming confirmations that maximize the interaction of the molecule with the solid surface. This effect is shown in Table I1 where the calculated column selectivity ( a ) is given for sequentially eluting pairs of hydrocarbon and perfluorocarbon isomers of hexane. The data indicate that perfluoro-2,3dimethylbutane experiences a weaker interaction with the adsorbant than would be predicted from the behavior of the corresponding hydrocarbon. This behavior has the effect of increasing the column selectivity for the separation of the perfluoro-3-methylpentane/perfluoro-2,3-dimethylbutane pair while decreasing the selectivity for the separation of the
perfluoro-2,3-dimethylbutane/perfluoro-2,2-dimethylbutane pair. A typical perfluorocarbon separation obtained on the above modified graphitized carbon black column is shown in Figure 1. The chromatogram represents a synthetic sample prepared from reference standards. The various commerical samples employed in this study contained many partially fluorinated species and other compounds which were not of interest. The chromatogram in Figure 1does not contain all the compounds listed in Table I, since those retention values were derived from the above-mentioned complex mixtures. An examination of the chromatogram in Figure 1 indicates good peak shape and a reasonable separation time. In conclusion, the use of a graphitized carbon black column allows separation of perfluorocarbons to be accomplished
0
2
4
6
8
10
12
!4
16
I8
ELUTION TIME linin)
Figure 1. Typical chromatographic separation of perfluorocarbons. Sample size, 0.5 pL; temperature, 120 OC;He flow rate, 30 mL/min; column, 6 ft X 'I4 in. 0.d. X 4 mm i.d. glass; column packing material, 0.1% SP-1000 on 80/100 mesh Carbopack C
without the use of low or subambient temperatures. Furthermore, the column is capable of separating isomeric mixtures as effectively as does n-hexadecane ( 2 , 3 )but over larger molecular weight ranges and with shorter analysis times. Additionally, a large temperature operating range extending from subambient t o greater than 200 "C is possible with this packing. The only major disadvantage of graphitized carbon black columns relative to n-hexadecane columns is their rather low sample capacity, which poses no problems, however, for analytical separations.
LITERATURE CITED (1) T. M. Reed 111, Anal. Chem., 30,221 (1958) (2) T. M. Reed 111 J . Chromatogr., 9, 419 (1962). (3) W. C. Askew and T. M. Reed 111, J . Chromatogr., 69, 359 (1972). (4) S.A. Green and F. M. Wachi, Anal. Chem.. 35, 928 (1963). (5) J. R. Wright and W. C . Askew, J . Chromatogr. Sci., 9, 651 (1971). (6) F. T. Eggertsen, H. S.Knight, and S. Groennings, Anal. Chem.. 26, 303
(1956). (7) I. Halisz and C. Horvath., Nature (London), 197, 71 (1963). and I. H a l s z , Anal. Chem.,36,1533 (1964). (8) W. Schneidec, H.Ed&&, (9) A . Di Corcia and R. Sarnperi, J . Chromatogr., 107, 99 (1975). (IO) A. Di Corcia and R. Sarnperi, Anal. Chem., 47, 1853 (1975).
RECEIVED for review June 21, 1977. Accepted December 1, 1977.
