results from the graphite rod atomizer measurements usually falling within the S.O.A.P. standard deviations. Although the precision obtained in sequential atomization (relative standard deviations of 4-7 %) is not as satisfactory as can be achieved by separate determinations, there may be occasions when the time-saving outweighs the loss of precision. With more sophisticated apparatus-e.g., a rapid-scanning monochromator-the principle of sequential atomization from the graphite rod may be useful in determining other pairs of elements in microliter samples. Elements such as arsenic, lead, and silver all appear to be completely atomized at temperatures where elements such as copper, nickel, iron, and chromium remain o n the graphite.
R . D. REEVES’ C. J. MOLNAR J. D. WINEFORDNER~ Department of Chemistry University of Florida Gainesville, Fla., 32601 RECEIVED for review March 27, 1972. Accepted May 26, 1972. This work was supported by AF-AFOSR-70-1880H. 1 On leave, Department of Chemistry and Biochemistry, Massey University, Palmerston, North, New Zealand. 2 Author to whom reprint requests should be sent.
Molecular Design-Tetrachloroterephthaloyl Liquid Phases for Gas Chromatography SIR: Stationary phase design in gas-liquid chromatography is of special interest because the principles involved are potentially important in the design of liquid materials for extraction and in understanding operative forces in solution generally. Here, we describe the design and choice of several tetrachloroterephthaloyl oligomers as liquid phases from molecular structural considerations, and principles and concepts described earlier (I-3). These materials are noteworthy because of their selectivity over a wide termperature range, their thermal stability, and their well defined chemical structure. Such properties make them useful in the range of 100 “C to more than 200 “C. Many selective liquid phases available heretofore are volatile in this temperature range ( 4 ) . Other materials with only moderate selectivity are polymers of poorly defined composition. The tetrachloroterephthaloyl nucleus was chosen as the basic structural unit because of its potential participation in P type charge transfer interaction (3,5,6) with aromatics and olefins, the tendency for formation of linear, and therefore controlled, structures with para isomers (7, s), and the relative thermal and chemical stability ( 9 - / I ) of the tetrahalogenated aromatic nucleus. The non-selective intermolecular void (1) S . H. Langer, ANAL.CHEM., 39,524 (1967). (2) S . H. Langer and R. J. Sheehan, “Progress in Gas Chromatography,” J. H. Purnell, Ed., Interscience-Wiley, New York, N.Y., 1968, pp 289-323. (3) S . H. Langer, B. M. Johnson, and J. R. Conder, J. Phys. Chem., 72, 4020 (1968). (4) M. J. S . Dewar and J. P. Schroeder, J. Amer. Chem. Soc., 86, 5235 (1964). ( 5 ) S. H. Langer, C. Zahn, and G. Pantazopolos, Chem. Ind. (London), 1958, 1145. ( 6 ) S. H. Langer, C. Zahn, and G. Pantazopolos, J. Chromatogr., 3, 154 (1960). (7) V. V. Korshak and S . V. Vinogradova, “Polyesters,” translated by B. J. Hazzard, Pergamon Press, London, 1965 pp 19-23. (8) R. W. Lenz, “Organic Chemistry of High Polymers,” John Wiley, New York, N.Y., 1967, p p 67-72 (9) A. H. Frazer, “High Temperature Resistant Polymers,” Interscience-Wiley, New York, N.Y., 1968, pp 18, 24, 118, 124
(l0,J. H. Golden, SCI (SOC.Chem. Znd. London) Monogr., 13, 231 (1961). (11) J. M. Cox, B. A. Wright, and W. W. Wright, J. Appl. Polym. Sci., 9, 513 (1965).
Oligomers as
volume of simple dialkyl tetrachloroterephthalates (3) was reduced by employing oligomers, thus increasing the concentration of the selective tetrahalogenated nuclei per unit volume ( I , 2). This conforms with our principle of “increasing the concentration of the selectively interacting groups in the solvent” to improve selectivity, provided group interaction does not interfere ( I , 2). Oligomer formation allows a decrease in ratio of methylene t o tetrahaloterephthaloyl groups “to minimize solvating (attractive) interactions which may act counter to the desired separation” o r do not contribute to it ( I , 2). Shortening the esterified alkyl groups with simple tetrachlorinated diesters to accomplish this gives materials with increased volatility and higher melting points (3, 12). Both properties are undesirable for gas chromatographic applications. Furthermore, oligomer melting points and liquid phase temperature ranges should be controllable by molecular design and blending. We prepared the following: ROXO(CHz),OXOR I. R = Butyl (a) n = 2, m p 183-184 OC (b) n = 3, m p 108.3-109.7 O C (c) n = 4, m p 184-185.5 “C 11. R = Propyl n = 3, m p 127-128 “C 111. ROXO(CHz)~OXO(CHz)~OXOR R = Propyl, m p 148-149.5 O C
As expected, the melting point is a function of end group, R , and decreases with increase in group length (I3-/6). The lower melting points of compounds containing a n odd number of carbons in the connecting chain between tereph(12) S . H. Langer, P. Mollinger, B. M. Johnson, and J. Rubin, J. Chem. Eng. Data, in press, 1972. (13) L. Mandelkern, “Crystallization of Polymers,” McGraw-Hill, New York, N.Y., 1964, pp 122-125. (14) R. Hill and E. E. Walker, J. Polym. Sci., 3, 609 (1948). (15) Reference 7, pp 311-336. (16) Reference 8, pp 91-95 and references therein.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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Table I. Separation Factors for p-/m-Positional Isomers on Tetrachloroterephthaloyl Stationary Phases Di-n-propyl Oligomer Oligomer mixture, tetrachlorocompound Comp. I1 - 6Oz, terephthalate Oligomer compound Ib Oligomer compound 11 111 - 40% Comp. 111___ Temperature 100 "C 110 "C 130 "C 110 "C 130 "C 150 "C 110 "C 130 "C Xylenes 1,039 1.048* 1.042 1.052< 1.047 1 . 044d 1.06Oe 1.052 Methylethylbenzenes 1.057 1.07 1.062 1.072 1.068 1.068 1.079 1.069 Chlorotoluenes 1.062 1.066 1.06 1.057 1.052 1.074 ... Dichlorobenzenes 1.132 1.12 1.12 ... 1.12 1.11 1.14 ... VyT,retention volume per gram of liquid phase at column temperature for toluene at 110 "C is: 147 ml estimated from references 3 and 6; * 1.03.6 ml; 80 ml; 52.6 ml extrapolated from higher temperatures; e 70.6 ml. I
thaloyl groups here and earlier (13-16) led to the choice and synthesis of a propylene-connected tetrachloroterephthaloyl trimer, bis(4-carbopropoxy-2,3,5,6-tetrachlorobenzoyloxy-3propyl) tetrachloroterephthalate (111), for further study and testing as a gas chromatographic liquid phase. Separation factors for some difficult meta-para aromatic separations o n several oligomers are compared with those for a tetrachloroterephthaloyl diester in Table I. The increase of about 0.02 in several instances is especially significant since a change in separation factor, CY,from 1.04 to 1.06 reduces the plates necessary for practical separation in a column from lo4 to about 4 X lo3 (2, 17, 18). The latter approaches the capability of conventional laboratory columns. The two- t o threefold reduction in column length permitted by these phases could make them useful for production-scale separation of aromatic isomers. The oligomer mixture of Table I was used in a relatively stable supercooled state a t 110 "C after initial heating above the melting point of 111. Tetrachloroterephthaloyl oligomers tended to stay liquid o n supercooling, especially in mixtures. A richer mixture in I11 (45.573 than that of Table I gave CY of 1.065 for the xylenes a t 100 "C. For p-/m-diethylbenzene, another challenging mixture, CY is 1.105on I11 a t 150 "C. The tetrachloroterephthaloyl oligomers thus rival liquid crystals in meta-para selectivity (4,19-22) and may be used over a wider temperature range. Other selective liquid phases ( 4 , 23) are significantly more volatile. Table I includes some specific retention volumes for toluene. The decrease in retention volume for this aromatic compound parallels a n increase in selectivity. Trimer (111) could not be used neat at 110 "C, but results from its mixtures indicate impressive selectivity a t this temperature if this were possible. Compounds I1 and I11 have been used at temperatures as high as 210 "C and found to be stable and effective. This will be important for gas chromatographic kinetic studies since there are few types of selective liquids with defined com(17) J. H. Purnell, J. Chem. SOC.(London), 1960, 1268. (18) B. L. Karger, ANAL.CHEM.,67 (8), 24A (1967). (19) E. Glueckauf, Trans. Faraday SOC.,51, 34 (1955). (20) H. Kelker, Fresetzius' 2.Anal. Cliem., 198, 254 (1963). (21) H. Kelker, Ber. Bunseges, Physik. Chem., 67, 698 (1963). (22) H. Kelker, B. Scheurle, and H. Winterscheidt, Anal. Clzim. Acta, 38, 17 (1967). (23) L. C. Case, J. Clzromatogr.., 6, 381 (1961).
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.
.
position for use as solvents a t temperatures above 150 "C (24, 25). Differential scanning calorimetric studies of oligomers reported here have shown them to be stable to 320 " C or higher. By comparison, the ester of the ortho acid, di-n-propyl tetrachlorophthalate (26, 27), was found to begin decomposition around 250 "C in agreement with results ( 2 8 ) o n the decomposition of polyesters of tetrachlorophthalic anhydride. The tendency for ortho polymers to be less thermally stable than those from meta and para diacids is well documented ( 2 9 , 3 0 ) . Control of the melting point and the low volatility of tetrachloroterephthaloyl oligomers makes them attractive materials for potential use as plasticizers (especially to impart fire resistance) and, with similar materials, candidates for use as electron acceptors with polymeric o r oligomeric electron donors (31, 32). Further investigation of tetrachloroterephthaloyl oligomers and polymers is continuing. ACKNOWLEDGMENT
The author thanks the dozen o r more students who assisted in this project over a period of years and J. A. Koutsky for DSC data. STANLEY H. LANGER
Department of Chemical Engineering University of Wisconsin Madison. Wis. 53706 RECEIVED for review April 7, 1972. Accepted June 19, 1972. The work was supported by the Wisconsin Alumni Research Foundation, t o whom patent rights are assigned. (24) S. H. Langer, J. Y. Yurchak, and J. E. Patton, Itid. Eng. Cliem., 61(4), 10 (1969). (25) G. L. Pratt and S. H. Langer, J. Phys. Cliem., 73, 2095 (1969). (26) S. H. Langer and H. Purnell, ibid., 70, 904 (1966). (27) S . H. Langer, C. Zahn, and M. H. Vial, J . Org. Cliem., 24, 423 (1959). (28) N. A. Ghanem, M. A. El-Azmirly, and M. I. Aly, Eur. Polym. J., 6, 443 (1970). (29) A. H. Frazer, "High Temperature Resistant Polymers," John Wiley and Sons, New York, N.Y., 1968, especially pp 93-96. (30) R. A. Dine-Hart, B. J. C. Moore, and W. W. Wright, J. Polym. Sci., Part B, 2, 369 (1964). (31) T. Sulzberg and R. J. Cotter, J . Polym. Sci. Part A-Z, 8, 2747 (1970). (32) T. Sulzberg and R. J. Cotter, Macromo/ec:r/es,2, 150 (1969).
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