Retention Volumes of Isomeric Hexenes and Hexanes in Cas liquid Partition Chromatography Using Phthalate Esters as Liquid Phase LLOYD J. SULLIVAN, JOHN R. LOTZ, and CHARLES 6. WILLINGHAM Department of Research in Physical Chemistry, Mellon Institute, Pittsburgh, Pa.
flow from the exit of the column and the time necessary to remove the maximum concentration zone of a component. For routine application of this techniquc, it is often more convenient to determine TT: relative to some internal standard. If Equations 1 and 2 are expressed as ratios for Components 1 and 2, the operating and column variables disappear. From this the identity is found that
By gas partition chromatography the retention volunies for the isomeric hexenes and hexanes have been determined on a colunin of given dimensions, using a thermal conductivity cell for detection and a 10-mv. range stripchart recorder. Liquid substrates were di-n-decylphthalate and ditetrahydrofurfurylphthalate impregnated on Celite 545, with nitrogen gas as the carrier. Comparisons are given for the retention volumes (pressure corrected) of the olefins on both substrates using several combinations of temperature and flow rate. The corrected retention volumes for the isomeric hexanes at one temperature and flow rate used for the olefins have been determined and are compared with the olefins. Retention volumes for all the isomers are also given relative to n-pentane, because this compound may be used as an internal standard.
If, however, any of the operating variables are changed, the proper correction factor must be inserted into the equation. This may, but does not nece,Csarily, pertain t o a change in substrate. EXPERIMENTAL
The apparatus is a modification of that of Patton, Lewis and &ye (9). It is shown diagrammatic:rlly in Figure 1. Exit pressure was taken as barometric pressure. This system differs from that of Pstton and others in the use of twin columns. R'ithout a reference column it was found that pressure fluctuations caused both an instantaneous and a delayed detector response with resulting base line instability. The niodified system increases the stability of the base line, because flow variations are nearly in phase.
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N RECENT years several reports on the use of gas partition chromatography as an analytical tool have appeared in the literature. Separations of hydrocarbons (8, IO); aldehydes, ethers, and alcohols (IO); fatty acids ( 4 , 5 ) ; esters ( 2 , IO); amines (3,6);and mixed systpms (7') have been described. With proper operating conditions almost any material with an appreciable vapor pressure can be handled by this method. James and Martin ( 4 , 5 ) give a theory for the partition column. The present work was undertaken to study some of the applications of this method and theory to a difficult analytical problem, such as the separation of the li isonieric hexenes with a boiling range of only 32" C. The isomeric hexanes were also examined because it was expected that, if some of the close boiling olefin pairs could not be distinguished, their hydrogenation products could be separated.
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THEORY
James and Martin in a theory for gas partition chromatography have set forth the basic equation for the retention volume of a component of a mixture as
Figure 1.
where V Ris the volume of carrier gas necessary to remove the concentration maximum of a given component from the column; a is the area occupied by the gas phase in any cross section of the column; L is the length of the separating section; R, is the ratio of the movement of the zone of maximum concentration of the component to the movement of the carrier gas (the ratio of linear velocities); PI is the pressure of the carrier gas a t the column inlet; and Po is the pressure of the gas a t the column exit. The retention volume V Ris dependent on the ratio, [(P1/Po)3- 11/ [(PJPO) ~ 11, for a given column and temperature. James and Martin have shown that a limiting value for the retention volume can be calculated from
Schematic diagram of apparatus
Controlled pressure gas eource E . Differential manometer C. Reference and measuring columns, 0.5 om. in inside diameter and 110 om. long. These are packed t o give nearly equal flow rates. D. Double arm thermal Conductivity cell (7) E. Wet test meter F . Controlled temperature b a t h A.
The stationary phases consisted of 40% by weight of di-ndecylphthalate ( Eastman Chemical 6447-P) for the first substrate, and ditetrahydrofurfurylphthalate (Eastman Chemical 3048-P) for the second substrate. Both were impregnated on diatomaceous earth (Johns Mansville Celite No. 545). The carrier gas used for both series was nitrogen. 'The hexanes mere Phillips Pure Grade. The experimental plan was divided into four parts: A. To test the reproducibility of Equation 2, retention volumes for the 17 isomeric olefins were determined a t each of two (Pl/Po)values. I n all experiments n-pentane Fas run as a reference standard before and after each olefin. B. Retention volumes rvere measured at each of two temperatures to determine the effect of temperature.
T o calculate V i , the terms on the right-hand side of Equation 2 must be evaluated. P I and PO,the inlet and exit pressures, are readily measured. VR,the retention volume, is the product of the 495
ANALYTICAL CHEMISTRY
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C. The olefins were also run on the second substrate (ditetrahydrofurfurylphthalate) and the retention volumes determined a t one temperature and one flow rate. D. The retention volumes of the isomeric hexanes were determined on both substrates a t one temperature and flow rate. Sample size in all cases was approsimately 10 p1. Samples were introduced into the column through a rubber serum cap by means of a hypodermic syringe. RESULTS
I n Table I the isomeric hexenes and hexanes are listed separately in the order of increasing boiling point a t 760 mm. of mercury (1). The hexane to which each hexene is hydrogenated is also shown. The spread in boiling point is 32' C. for the I i hesenes and 19' C. for the hexanes. For the hexenes, with exception of the highest and lowest boiling isomers and their first neighbors, the maximum difference in boiling point between neighboring pairs is 2.8' C. and the average difference is only 1.2' C. In the following discussion, the hexenes and hexanes are referred to by the numbers shown in Table I. In the other tables and the figures, all retention volumes are the limiting values, Vi, for the column used as calculated from Equation 2. Different values would be obtained for a colnmn of different diniensions. Jn Table I1 are given the retention volumes and relative reteno~efin/T$ for the liexcnes as determined tion volumes
using the di-n-decylphthalate substrate. I n column I, with the exception of threo olefin pairs (4 and 5, 9 and 10, 13 and 14), the average retention volumes for the two flow rates follow the same order as the boiling points. The retention volumes calculated from Equation 2 give good agreement for the two flow rates, the average deviation from the mean for the 15 olefins being less than &3%, which is the deviation found for repacking the column. Relative retention volumes for the two flow rates (column 2) show the same agreement as the retention volumes.
2000,
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40
Table I.
NO.
1 2
Hexenes and Hexanes in Order of Increasing Boiling Point
Hexene 3,3-Diniethyl-l-butene 4-Methyl-I-pentene 3 hlet liyl- 1-p en t ene 2,3-Diinethyl-l-butene 4-Methyl-cis-2-pentene 4-Methyl-trons-2-pentene 2-Methyl-I-pentene I-Hexene 2-Ethyl-1-butene Lis-3-Hexene frons-3-Hexene 2-hfethyl-2-pentene 3-Methyl-trans-2-pentene trans-2-Hexene cis-2-Hexene 3-hlethyl-cis-2-pentene 2,3-Dimethyl-Z-butene
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Boiling Point,
c.
41.2 53.9 54.1 55.7 50.3 58.6 60.7 63.5 64.7 66.4 67.1 07.3 67.6 67.9 88.8 70.5 73.2
Hydrogenation Product 2,S-Dimethylbutane 2-Methylpentane 3-Methylpentane 2,3-Dimethylbutane 2-Alethylpentano 2-Jf ethylpentane 2-hlethylpentane n-Hexane 3-hlethylpentane n-Hexane n-Hexane 2-hlet hylpentane 3-Nethylpentane n-Hexane n-Hexane 3-hf ethylpentane 2,3-Dimetliylbutane
45
50
55 60 BOlLlNG POINT, %.
65
70
75
Figure 2. Logarithm of retention volunies ( V i ) at 20' C. with respect to boiling point for hexenes and hexanes
In columns 3 and 4 of Table I1 are found the retention volumes and relative retentlion volumes for a flow rate of 24 ml. per minute, but a t a column temperature of 25" C. Comparison of columns 1 and 2 with columns 3 and 4 shows that the higher temperature gives lower values for the retention volumes; hut there is almost 10 11 no change in relative retention volumes for the two temperatures. 12 The percentage change in Vi for the temperature interval of 13 14 5' C. is 20% on the average. The maximum cliange is 26yGfor 15 16 3-methyl-cis-2-pentene, while the minimum change is 1670 for 17 3,3-dimethyl-l-butene. Table I11 gives the retention volumes for the olefins and the Hexane retention volumes relative to n-pentane determined on a ditetra1 2,2-Dimethylbutane 49.7 2 2,3-Dimethylbutane 58.0 hydrofmfurylphthalate column a t a flow of 27 ml. per minute GO 3 3 2-Methylpentane at 20" C. This is the same column operated at the same pres4 3-Xlethylpentane 63 3 5 n-Hexane 68.7 sure ratio used for the di-ndecylplithalate substrate. The _ retention volumes in each case Table 11. Retention Volumes for IIexenes Using Di-n-decylphthalate Column are lower by a factor of almost 1 ' 2 3 4 four than those determined Retention Volume, Relative Retentioii Volume, netention ~ ~ lnetention ~ ~ i ~ . ~ f o r t h e di-n-decylphthalate v:, 200 c. G o l e t i n / G n-penti.ne. 20' C. \'olume, Volupe, Hexenes in Order of Increasing Flow R a t e Flo~yR a I L Vi V i oiei;n/VR n-pont?ne column. The results otherwise No. Boiling Point 24 ml./min. 43 nil./niin. 24 ml./niin. 43 ml./min. 25O C.; Flow Rate. 24 Ml./JIin. are very nearly identical for 3,3-Dimethyl-l-butene 365 370 1.20 305 the two substrates, as may be 21 4-Methyl-I-pentene 625 025 21.20 .05 2.00 515 21..0250 3 3-hlethyl-I-pentene 050 630 2.10 2.05 520 2.03 seen by comparing the relative 4 2 3-Dimethyl-1-butene 730 710 2.40 2.30 575 2.30 2.25 2.30 560 2.25 r e t e n t i o n v o l u m e s for the 5 4l~lethyl-cis-Z-pentene 690 705 6 4-hIethyl-trans-2ditetrahydrofurfurylplithalate pentene 730 720 2.40 2.30 580 2.30 subst.rate a t 20" C. with the 7 2-Methyl-1-pentene 945 940 3.10 3.05 770 33.10 .05 8 1-Hexene 976 920 3 20 3 .00 775 9 2-Ethyl-1-butene 1110 1120 3.60 3.65 865 3.45 relative retention volume for 10 cis-3-Hexene 1080 1100 3.50 3.55 890 3.50 11 trans-3-Hexene 3.50 3.80 875 3.45 the di-n-decylphthalate sub1090 1170 12 2-Methyl-2-pentene 1100 1170 3.85 3.80 940 3.70 strate a t the same tempera-' 13 S-hIethyl-trans-24.00 4 . 1 5 975 3.90 ture. The average change in pentene 1230 1270 14 trans-2-Hexene 1140 1130 3.70 3.65 920 3.60 15 cis-2-Hexene 1250 1250 4.10 4.00 1000 3.95 relative ret,ention volumes was 10 3-Metliyl-cis-2-pentene 1390 1410 4.55 4.55 1080 4.30 17 2,3-Dimetliyl-2-butene 5.40 5.35 1260 5.05 37,, which is within esperi1640 1640 mental error. 3 4 5 6 7 8 9
497
V O L U M E 28, N O . 4, A P R I L 1 9 5 6
volumes for the hexanes follow the order of increasing boiling point except for the inversion of 3-methylpentane and n-hexane on the ditetrahydrofurfurylphthalate substrate.
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DISCUSSIOmN OF THE RESULTS
The logarithms of the retention volumes (limiting value) given in the tables for the olefins and hesanes for 20" C. are plotted in Figure 2 as a function of boiling point. I n general, within a class the logarithm of the retention volume is a linear function of boiling point, for both substrates used. For individual oases, however, some deviation from exact linearity is found. The slopes are very nearly equal for the substrates used in this study. The di-ndecylphthalate column is preferable because it gives time differences between olefin pairs TIME IN ;M N .I greater by a factor of four over the time Figure 3. Composite drawing of experimental curves of hexenee on di-ndifferences for the ditetrahydrofurfuryldecylphthalate column at 20" C. phthalate column. However, because the hesanes are different bv a factor of sis on the two substrates, the ditetrahydrofurfurylphthalate column might have some advantage in certain cases. Table 111. Retention Volumes for Hexenes Using Ditetrahydrofi~rfurylphthalateColumn In Figure 3 is a composite drawing of the curves for all 17 Flow Rate, 27 XI1. per Minute of the hesenes. The numbers within the peaks correspond to a t 20° C . the number in Table I. The olefins within the composite fall Retention Relative volume, retention into three groupings designated by Roman numerals I, 11, and Hoxenes in Order volume, of Increasing V;; I11 in the figure. Group I consists of a Bingle olefin and is NO. Boiling Point ml. o l e i i n / G n-pantaw readily identified. Group I1 contains olefins which can be identi1 3,3-Dimethyl-l-butene 2 4-Methyl-1-pentene fied only as a unit, because the thermal conductivity detector 3 3-;\lethyl-l-pentene would produce a nearly symmetriral composite 01 additive peak. 4 2,3-Dirnethyl-l-butene 5 4-Methyl-cis-2-pentene Group I11 would be observed as a main peak, including nine G &Met hyl-trans-2-pentene 7 2-hIethy1-1-pentene olefins, and two shoulders. The shoulders consist of olefins 16 l. -. H_ ~.. x. e..n e 8 . and li, which could be identified if present in significant amounts. 9 2-Ethyl-1-butene 10 cis-3-Hexene Thus, for a sample containing equal amounts of the 17 olefins, 11 trans-3-Hexene 12 2-bIethy1-2-pentene five gronps can be identified by a single iun of the mixture. I
13 14 15 16 17
Jrz
3-Methyl-trans-2-pentene trans-2-Hexene cis-2-~'3"a"o -u-b..u 3-Met hpl-cis-2-pentene 2,3-Dimethyl-2-butene
The retention volumes of the hesanes using the two substrates are given in Table IV. These values were determined a t a flow rate of approximately 27 nil. per minute and a temperature of 20" C. The retention volumes and the relative retention
Figure 4. Composite drawing of experimental curves of hexanes on di-n-deeylphthalate column at 20' C.
'I'alde IV.
Retention \701umes for Ilevanes
(27 ml. per minute flow rate at 20' C.) Hexanes in Order MI. vi hexane/l'; of Increasing NO. Boiling Point Col. '1 CdYb Col. 1s 1 2.2-Dirnethylbutsne 430 70 1.45 620 100 2.10 2 2,3-Dimethylbutane 3 2-Rlethplpentane 635 100 2.15 4 3-hfethylpentane 770 140 2.60 5 hexane 920 135 3.10 a Di-n-deoylphthalate substrate. b Ditetraliydrofurfurylphthalate substrate.
v&
n-penbm
Col. 2b 1.15 1.135 1.65 2.30 2.20
From any misture of Group 11, i t would not be possible to distinguish whether a peak represented a single olefin or a misture. For olefins 9, 10, 11, 12, and 14, som13 information might be obtained if all of the olefins were not present in significant amounts. In such a case the olefin pair 7 and 8 could be distinguished from the pair 13 and 15, if the bulk group 9, IO, 11, 12, and 14 were missing or only present in very small amount. Thus, from a single run of n mixture, olefins 1, 16, and 17 could be identified, and in certain cases olefin pairs 7 and 8 and 13 and 15 could be distinguished as pairs. A composite drawing of the curves for hexanes is given in Figure 4. Of the four peaks, only B represents more than one component. Thus three of the hesanes are distinguishable and the other two occur as a pair. If a hesene fraction is run on a column and olefin groupings
ANALYTICAL CHEMISTRY
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Figure 6. Experimental and calculated curves for olefins 15 and 16 on di-rt-decylplithalate column at20"C.
are identified, hydrogenat i o n a n d r u n n i n g aa hexanes could be used to give additional . informaFigure 5 . Experiniental and tion, Some examples of calculated curves for olefins how this information can 3 and 5 on di-n-decylphthalbe used are illustrated in atc column at 20' C. Figures 5 to 7. In Fimre 5 are shown the individual pure olefin determinations for olefins 3 and 5 from Table I, together with the summed or calculated curve and the experimentally determined curve for the mixture of the pair. From this figure it is found that the two olefins give a single peak as predicted by adding the individual curves. Thus, identification cannot be made, other than that they belong t o Group I1 (Figure 3). Hydrogenation of these olefins gives 3-methylpentane and 2-methylpentane. Figure 4 shows that these hexanes give two distinct peaks, 4 and 3, respectively. I n Group 11, however, there remain three other olefins, 2, 4, and 6. Numbers 2 and 6 give 2-methylpentane (3 in Figure 4), while number 4 gives 2,3-dimethylbutane (2 in Figure 4). Thus, from the example given, the presence or absence of one olefin can be established. By this technique the other portion of the mixture cannot be characterized beyond a possible one or all of four olefins. Another substrate, a longer column, or an additional analytical technique would be needed to resolve them. In Figure 6 are the experimental curves for the individual olefins 15 and 16, a calculated composite, and the curve for the mixture, which is considered to be an unknoTvn. Comparison of the experimental curve in Figure 6 with Figure 3 shows that the sample mixture consists of Group 111 olefins, From Figure 3 it is apparent that this mixture consists of 16 and possibly 9, 10, 11, 12, 13, 14, and 15. Because of symmetry, it can also be shown that olefins 9, 10, 11, and 14 could be present only in small amounts. Hydrogenation of the mixture gives 3-methylpentane (4 in Figure 4) for 16. Olefins 9 and 13 also give 3methylpentane, while 10, 11, 14, and 15 give n-hexane ( 5 in Figure 4),and 12 gives 2-methylpentane (3 in Figure 4). Hydrogenation of this sample, therefore, gives a mixture of %methylpentane and +hexane, a separable mixture by Figure 4. The mixture is then olefin 16 and possibly olefins 9, 10, 11, 13, 14, and 15. However, if quantitative samples of both hexenes and hydrogenated products were used, a comparison of areas under
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Figure 7 . Experimental and ealcuIated curves for olefins 8 and 10 on cli-n-decylphthalatecolumn at 20' C.
the euives makes it apparent that 0 and 13 cannot be present in quantities greater than the error in measwement of areas. Also, considerations of symmetry show that olefin 15 is the major remaining component. The importance of symmetry consideration is brought out in Figure 7, which includes single experimental determinations on olefins 8 and 10, the calculated composite curve, and an experimental curve. This figure shows that the shape of the curve for a mixture and the position of the maximum may vary with composition, apparently producing peak shifts. This means that an asymmetric peak or a shifted peak indicates two or more components, or some other interfering phenomenon, CONCLUSIONS
Complete analysis of a complex mixture of narrow boiling range is not likely to be achieved by any single method. However, much useful information can be obtained by the technique of gas partition chromatography. A combination of this method with other chromatographic techniques and other analytical processes should furnish sufficient data to characterize qualitatively even as complex a mixture as the isomeric hexenes. ACKNOW LEDGRlENT
The hexenes were furnished through the courtesy of I
