Anal. Chem. 1994,66, 4043-4053
Effects of Solvent Composition on Polymer Retention in Thermal Field-Flow Fractionation: Retention Enhancement in Binary Solvent Mixtures Richard M. Sissont and J. Calvin Giddings' Field-Flow Fractionation Research Center, Department of Chemistry, University of Utah, Salt Lake City, Utah 84 112
The thermal FFF retention of 11 linear polystyrene polymer standards in the molecular weight range 9000-1,860,000 Daltons in 15 single-component solventsand 30 binary solvent mixtures has been measured and compared. The results have been converted by theory into the calibration constants $0 and n describing the molecular weight dependence of retention and into values of the thermal diffusion factor a,which specify the strength of the underlying thermal diffusion phenomenon. Retention is found to be enhanced considerably for certain binary mixtures. The binary solvent consisting of 70% (by volume) tetrahydrofuran (THF) and 30% dodecane yields retention parameters -60-65% as large as those measured in pure THF; the increased retention is theoretically capable of extendingthe working range of thermal FFF to polymers having only half the molecular weight of the lower molecular weight limit found for THF.
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Thermal field-flow fractionation (ThFFF) is a chromatographic-like elution technique that is commonly applied to the separation and characterization of lipophilic Like other field-flow fractionation techniques (sedimentation FFF, flow FFF, electrical FFF), ThFFF entails the differential elution of macromolecular components from a ribbon-shaped channel, a thin open structure containing no packing. Differential retention in FFF is induced by an externally imposed field or gradient acting perpendicular to a stream of liquid flowing through the channel. The field, by forcing different components into the different stream laminae of the nearparabolic flow, causes differential migration, which is followed by elution of the separated components into a detector. By using an applied field in this perpendicular configuration, separation is achieved in one phase without the need for a separate stationary phase. The hydrodynamic and kinetic complications of column packing materials and interfacial mass transport are thus avoided. Present address: Battelle Tooele Operations, 11650 Stark Rd., Tooele, UT 84074. (1) Thompson, G. H.: Myers, M. N.;Giddings, J . C.Anal. Chem. 1969,41,12191222. (2) Giddings, J. C.; Martin, M.; Myers, M. N. J. Polym. Sci.: Polym. Phys. Ed. 1981, 19, 815-828. (3) Gunderson, J. J.; Giddings, J . C . Anal. Chim. Acta 1986, 189, 1-15. (4) Kirkland, J. J.; Rementer, S.W.; Yau, W. W. J . Appl. Polym. Sci. 1989, 38, 1383-1 395. ( 5 ) Schimpf, M. E. J. Cfiromatogr. 1990, 517, 405-421. (6) Myers, M. N.; Chen, P.; Giddings, J . C. In Chromatography of Polymers: Characterization by SECand FFF; ACS Symposium Series 521; Provder, T., Ed.; American Chemical Society: Washington, DC, 1993; pp 47-62. (7) Nguyen M. Y.; R. Beckett, R. Polym. Int. 1993, 30, 337-343.
0003-2700/94/0366-4043$04.50/0
0 1994 American Chemical Soclety
In the case of ThFFF, the "field" is a temperature gradient imposed across the channel by flat heat-conducting bars positioned on each side of the channel. One bar is heated and one is cooled, thus establishing a temperature drop ATtypically in the range 30-100 K but in some cases as low as 5 K and as high as 150 K. The channel, cut from a plastic spacer and sandwiched between the bars, is usually only 75-1 00 pm thick; because of this thinness, temperature gradients often reach 10 000 K/cm. This strong gradient drives macromolecular components toward one wall (usually the cold wall), where they form equilibrium distributions only a few micrometers thick, depending upon molecular weight and composition. Separation then takes place because of the differential flow near this accumulation wall. The principles of F F F generally, and ThFFF specifically, have been more fully elaborated in the literature.'-I0 For most FFF techniques, retention can be predicted as a function of primary polymer or particle properties: mass, density, hydrodynamic diameter, diffusivity, charge, and so on.lo This predictability provides a basis for the rational design of experiments and for analytical optimization. ThFFF is more complicated. Retention in ThFFF is a result of thermal diffusion and can be related to various thermal diffusion parameters such as the Soret coefficient, the thermal diffusion factor a, and the thermal diffusion coefficient &. (The ordinary diffusion coefficient D also influences retention.) However, the thermal diffusion of macromolecular materials is poorly understood, lacking clear theoretical definition." Thus the thermal diffusion parameters that control retention cannot be theoretically related to more sought-after physicochemical parameters such as molecular weight and hydrodynamic diameter. Because of the lack of a suitable theoretical framework, this relationship must be established empirically. ThFFF has become the most widely used technique for exploring these relationships. 1-16 Thermal diffusion trends (8) Caldwell, K. D. Anal. Chem. 1988, 60, 959A-971A. (9) Martin, M.; Williams, P. D. In Theoretical Aduancementin Cfiromorography and Related Separation Techniques; NATO AS1 Series C: Mathematical and Physical Sciences 383; Dondi, F., Guicchon, G., Eds.; Kluwer: Dordrecht, The Netherlands, 1992; pp 513-580. (10) Giddings, J . C. Science 1993, 260, 1456-1465. (11) Schimpf, M. E.; Giddings, J. C. J. Polym. Sci.: Polym. Phys. Ed. 1989, 27, 1317-1332. (12) Giddings, J . C.; Hovingh, M. E.; Thompson, G. H. J . Phys. Chem. 1974, 74, 4291-4294. (13) Brimhal1.S. L.; Myers, M.N.;Caldwell, K. D.;Giddings, J.C.J.Polym.Sci.: Polym. Phys. Ed. 1985, 23, 2443-2456. (14) Schimpf, M. E.; Giddings, J. C. Macromolecules 1987, 20, 1561-1563. (15) Schimpf, M. E.; Giddings, J . C. J. Polym. Sci.: Parr B: Polym. Phys. 1990, 28, 2673-2680.
Analytical Chemistty, Vol. 66, No. 22, November 15, 1994 4043
can be found by observing the experimental retention of various polymer standards. Most importantly for present purposes, retention has been found to depend not only on molecular weight but also on polymer and solvent composition. The influence of solvent type has been examined in various studies going back to 1974.11317-18 In the initial study,17 the retention of linear polystyrenes was found to vary over a 3040% range for a group of eight common organic solvents. However, more polar macromolecules such as blue dextran, albumin, and hemoglobin experienced negligible retention in dilute aqueous media, although blue dextran was well retained in DMSO. In the first study of solvent mixture effects in ThFFF, it was shown that blue dextran was negligibly retained in H20-DMSO mixtures up to 50% DMSO, after which increasing retention was observed as the DMSO content was increased to 100%.17 In a subsequent study, Kirkland et a1.18 showed that polymer retention could be enhanced by using mixtures of organic solvents, particularly of “good” and “poor” solvents. The purpose of this study is to further examine the effects of both single solvents and binary solvent systems on the retention of polymers. The motivation of this study was the possibility that one might observe an increment in driving force for solvent mixtures that would not be found in pure solvents. This extra driving force could in principle lead to enhanced retention. The expectation for such a driving force was based on the following reasoning. Thermal diffusion is best known as a phenomenon causing the partial segregation of low molecular weight mixture^.^^.^^ It was shown as early as 1940 that a partial separation of liquid mixtures could be obtained in a Clusius-Dickel type thermogravitational apparatus.2* Such separation was further confirmed by Butler and Turner using a thin flow channel that allowed the measurement of the enrichment/depletion of liquid components in hot and cold regions of the channel.22 Because of this proven solvent segregation effect, a mixed solvent introduced into a ThFFF channel can be expected to partially separate with one component becoming enriched at the cold wall and another at the hot wall. A polymer introduced into the system will have more affinity for one solvent than the other and should thus experience an incremental driving force (added on to the normal driving force of thermal diffusion) in the direction of enrichment of the preferred solvent. (This driving force would be analogous to that proposed earlier for the ThFFF of polymers in supercritical fluids where strong density gradients would translate into an affinity gradient.23) The incremental driving force would either reinforce or partially cancel the thermal diffusive driving force, which is usually directed toward the cold wall. Thus one could expect to observe either enhanced retention or
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(16)Schimpf, M. E.; Wheeler, L. M.; Romeo, P. F. In Chromatography of Polymers: Characterization by SECand FFF; ACS Symposium Series 521; Provder, T., Ed.; American Chemical Society: Washington, DC, 1993; pp 63-76. (17) Myers, M. N.; Caldwell, K. D.; Giddings, J. C. Sep. Sci. 1974, 9, 47-70. (18) Kirkland, J. J.; Boone, L. S.;Yau, W. W. J . Chromotogr. 1990,517,377-393. (19) Haase, R. Thermodynamics of Irreversible Processes; Addison Wesley: London, 1969. (20) Tyrrell, H. J. V. Dvfuusion and Heat Flow in Liquids; Butterworths: London, 1961. (21) Korsching, H.; Wirtz, K. Eer. Drsch. Chem. Ges. 1940, 73, 249-269. (22) Butler, B. D.; Turner, J. C. R. Trans. Faraday SOC.1966, 62, 3114-3120. (23) Gunderson, J. J.; Giddings, J. C. Anal. Chem. 1987, 59, 23-27.
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Analytical Chemistry, Vol. 66, No. 22, November 15, 1994
diminished retention by using solvent mixtures, depending upon the thermal diffusive behavior of the components. Unfortunately, thermal diffusion among low molecular weight solvents is not well understood either. Experimental studies are needed to establish rational guidelines. This subject is addressed in the following paper by Rue and S ~ h i m p f . 2 ~ The enhancement of retention, if it could be systematically realized, would be useful in polymer analysis. ThFFF is a very effective tool for the fractionation of ultrahigh molecular weight polymers, in some cases ranging up to or beyond 10 or 20 million molecular weight.25 However, polymers below a few thousand molecular weight are difficult to retain and thus to analyze; AT must be driven to extreme values, up to 100-1 50 K, to properly deal with these components.26 Enhanced retention would provide a means for reducing the lower molecular weight limit without the need for large ATs, which is experimentally inconvenient. In these studies, the retention of a series of linear polystyrene standards of different molecular weights was measured using a group of binary solvent systems in varying mixture ratios. The three principal binary systems examined were cyclohexane/ 1,2-dichloroethane,cyclohexane/dioxane, and dodecanel tetrahydrofuran. For reference, the retention of the polystyrene standards in 15 single-component solvents was also measured under similar experimental conditions. (The solvents and polymer standards employed in this study are listed in Tables 1 and 2.) Experimental retention times were used to calculate thermal diffusion factors, thus converting our observations into basic physicochemical parameters describing the underlying thermal diffusion process. Figure 1 illustrates the strong solvent effects found in this study. This figure shows the superposition of fractograms (detector response curves) for 207 700 (207.7K) molecular weight polystyrene in three single-component solvents and three binary solvents with the temperature drop AT set at 40 OC. The three single solvents are perchloroethane (PCE), tetrahydrofuran (THF), and ethylbenzene (ETB). The three binary solvents, each providing enhanced retention, consist of 30% (by volume) dodecane and 70% T H F (A), 75% cyclohexane and 25% dichloroethane (B), and 70% cyclohexane and 30% dioxane (C). As shown by Figure 1 and confirmed by the more comprehensive studies reported below, the binary combination of 30% dodecane/70% tetrahydrofuran yielded significantly better retention for the polystyrene standards than any other single solvent or binary solvent combination examined. (This mixture has the further advantage of UV transparency, making UV detection practical.) The immediate implication of this work is that select binary solvents can be used for the characterization of lower molecular weight polymers at lower temperature drops than previously possible.
THEORY The retention ratio R of a given polymeric species is defined as the ratio of the velocity up of the migrating polymer band for that species to the mean cross-sectional velocity (0)of the (24) Rue, C. A.; Schimpf; M. E. Anal. Chem. 1994,66, 4054-4062.
(25) Gao, Y. S.;Caldwell, K. D.; Myers, M. N.; Giddings, J. C. Macromolecules 1985, 18, 1272-1277. (26) Giddings, J. C.; Smith, L. K.; Myers, M. N. Anal. Chem. 1975, 47, 2389-
2394.
Table 1. Information on Solvents and Solvent Components Utlllzed In Thermal FFF solvent bpo ('C)
A. solvents for polystyrene benzene (BEN) carbon tetrachloride (CTC) cyclohexane (CYH) decalin (DEC) 1,2-dichloroethane (DCE) dioxane (DOX) ethyl acetate (ETA) ethylbenzene (ETB) ethyl malonate (ETM) methyl ethyl ketone (MEK) o-dichlorobenzene (ODB) perchloroethylene (PCE) tetrahydrofuran (THF) toluene (TOL) xyleneb (XYL) B. solvent componentsC acetonitrile (ACN) dodecane (DOD) ethylene glycol (ETG) isopropyl alcohol (IPA) poly(propy1ene glycol), M = 1200
80.1 76.75 80.7 196 83.5 101 77 136.2 199.3 79.6 181 146.2 66 110.6 -138 81-82 216.3 198 82
source
grade
EM Science Burdick & Jackson EM Science Kodak Baker EM Science EM Science EM Science Kodak EM Science Burdick & Jackson Mallinkrodt EM Science Fisher Shell
HPLC HPLC HPLC technical reagent HPLC HPLC reagent reagent HPLC HPLC reagent HPLC HPLC technical
Burdick & Jackson Phillips 66 Texaco J. T. Baker Matheson
spectrograde technical technical HPLC technical
Source: Burdick, Jackson, Hi h Purity Solvent Guide; 1904. Handbook of Chemistry and Physics, 56th ed.; CRC Press: Cleveland, OH, 1975-1976. GC/MS analysis yielfed composition of 23%p-xylene, 53% m-xylene, 23% o-xylene, and
