Improvements in Polymer Characterization by Size-Exclusion

Improvements in Polymer Characterization by Size-Exclusion Chromatography and Liquid Chromatography at the Critical Condition by Using Enhanced-Fluidi...
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Anal. Chem. 1998, 70, 3298-3303

Improvements in Polymer Characterization by Size-Exclusion Chromatography and Liquid Chromatography at the Critical Condition by Using Enhanced-Fluidity Liquid Mobile Phases with Packed Capillary Columns Hao Yun and Susan V. Olesik*

Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 Edward H. Marti

Ashland Chemical Company, Technical Center, Dublin, Ohio 43017

Microscale chromatography has found numerous applications in liquid chromatography. The combination of enhanced-fluidity liquid mobile phases with packedcapillary LC is evaluated for polymer characterization using size-exclusion chromatography (SEC) and liquid chromatography at the critical condition (LCCC) phase. Separations of polystyrene polymers and copolymers are completed using liquid chromatography at the critical condition. The critical conditions of polystyrene polymers were approached by changing the concentration of CO2 in the mixture combined with temperature and pressure variation. Because the solvent strength of enhancedfluidity liquid mixtures is affected by temperature and pressure variation, the solvent strength could be finetuned to accurately find the critical condition. Long packed capillaries could be used in this application because the enhanced-fluidity mobile phases have low viscosities. High efficiencies resulted. The performance of packed-capillary and analytical-scale analytical columns containing the same packing material was compared for a challenging separation at the critical condition. The molecular weight and functionality distributions are two important parameters used to characterize polymers. Molecular weight distributions (MWDs) are often measured using sizeexclusion chromatography (SEC). Gradient elution liquid chromatographic methods have been developed to separate polymers according to chemical composition.1-4 However, polymer functionality distributions or copolymer compositions are best determined using liquid chromatography at the critical condition (LCCC), which was developed by Gorshkov et al.5-7 For example, to characterize the functionality distribution of a polymer by LCCC, (1) Mori, S.; Uno, Y. Anal. Chem. 1987, 59, 90. (2) Zimina, T. M.; Fell, A. F.; Castledine, J. B. J. Chromatogr. Sci. 1993, 31, 455. (3) Glo ¨ckner, G.; van den Derg, J. H. M. J. Chromatogr. 1986, 252, 511. (4) Glo¨ckner, G.; Wolf, D.; Engelhardt, H. Chromatographia 1994, 38, 749.

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the mobile phase condition is first identified where oligomers with different molecular weights of the nonfunctionalized polymer have the same retention time (i.e., ∆G of transfer between the mobile phase and stationary phase is zero). At that critical condition, the functionalized polymer is separated. The separation achieved is then based solely on the interaction of the functionality with the stationary phase. In other words, the chromatogram at the CC displays the functionality distribution. The exact solvent condition that corresponds to the critical condition is sometimes nontrivial to find and maintain with common liquid mobile phases. The use of enhanced-fluidity liquid mixtures of tetrahydrofuran (THF) and carbon dioxide (CO2) to obtain critical condition was recently reported.8 The addition of 50 mol % CO2 to THF lowers the viscosities by approximately 50%.9 The advantages of enhanced-fluidity liquid mixtures as mobile phases for LCCC include the ability to approach the critical condition through pressure control, increased efficiency, and a lower pressure drop across the column due to the low viscosity of the mobile-phase mixture. SEC is generally carried out on 25-50-cm × 4.6-mm-i.d. packed columns with typically 30 000 theoretical plates per meter. Microscale LC is an alternative configuration that has advantages, such as low solvent usage and high efficiency. Jinno and Nishibara first demonstrated packed-capillary SEC using a 30-cm × 250-µm-i.d. microcolumn packed with TSK gel G3000 for the characterization of MWDs of polyesters.10 Ishii and Takeuchi11,12 illustrated that the high permeability of microscale columns made highly efficient separations (100 000 plates) possible by SEC. (5) Gorshkov, A. V.; Evreinov, V. V.; Entelis, S. G. Zh. Fiz. Khim. 1985, 59, 958. (6) Pasch, H.; Much, H.; Schulz, G.; Gorshkov, A. V. LC-GC Int. 1992, 5, 59. (7) Gorshkov, A. V.; Much, H.; Becker, H.; Pasch, H.; Evreinov, V. V.; Entelis, S. G. J. Chromatogr. 1990, 523, 91. (8) Souvignet, I.; Olesik, S. V. Anal. Chem. 1997, 69, 66. (9) Yuan, H.; Olesik, S. V. Unpublished data. (10) Jinno, K.; Nishibara, M. Anal. Lett. 1980, 13, 673. (11) Ishii, D.; Takeuchi, T. J. Chromatogr. 1983, 255, 349. (12) Takeuchi, T.; Ishii, D. J. Chromatogr. 1983, 257, 327. S0003-2700(97)01152-9 CCC: $15.00

© 1998 American Chemical Society Published on Web 06/17/1998

Others have illustrated useful applications of microscale SEC13-15 and discussed optimum instrumentation for the technique.16 Due to the small volumes involved in microscale separations, the extracolumn contribution to band broadening must be carefully controlled. The concentration-based detection limits are also high for on-column UV absorbance detection in microscale chromatography because of the short optical path length. Microscale UV detectors may be used for a limited range of polymers that have UV-absorbing chromophores. A microscale, universal-type detector is, therefore, preferred for packed-capillary SEC. A refractometric detector with minimal cell volume was previously reported for microcolumn SEC.17 However, it is not easily interfaced to enhanced-fluidity LC because of the high pressure maintained at the column outlet. Evaporative lightscattering detection has shown promise as a universal detector for polymer analysis.18-20 However, the commercially available ELSD is dedicated to the detection of effluents from conventional LC columns (4.6 mm i.d.). Herein, the use of microscale SEC and microscale LCCC is evaluated for the characterization of functionalized polymers or copolymers. A low-dead-volume interface that couples an evaporative light-scattering detector (ELSD) to microscale SEC is also described. The length of the column in packed-microcolumn SEC is ultimately controlled by the upper pressure of the mobile-phase pump. The low viscosities of enhanced-fluidity mixtures are advantageous for separations involving long packed columns.21-23 Therefore, liquid mixtures of CO2 and THF were the mobile phases used for microscale SEC and LCCC. The critical condition for polystyrene was determined. Using this critical condition, a separation of mono- and dicarboxy-terminated polystyrene polymers was accomplished, as well as separation of poly(styrenemethyl acrylate) acrylate copolymers. Accordingly, the ability to obtain both molecular weight and functionality information by SEC and LCCC with this chromatographic system was illustrated. EXPERIMENTAL SECTION Instrumentation. The chromatographic system included an Isco LC-260D syringe pump (Isco, Lincoln, NE), a Valco W-series high-pressure injection valve with an injection volume of 60 nL (Valco Instruments, Houston, TX), and a 2-m × 250-µm-i.d. column packed with 5-µm diameter, 200-Å pore size Betasil particles (Keystone Scientific Inc., Bellefonte, PA). Two types of detectors were used. For polymers with a chromophore, a Spectra System UV1000 UV-visible absorbance detector was employed. A 250(13) Keve, J. J.; Belenkii, B. G.; Gankina, E. S.; Vilenchik, L. Z.; Kurenbin, O. I.; Zhmakina, T. P. J. High Resolut. Chromatogr. Chromatogr. Commun. 1981, 4, 425. (14) Keve, J. J.; Belenkii, B. G.; Gankina, E. S.; Vilenchik, L. Z.; Kurenbin, O. I.; Zhmakina, T. P. Vysokomol. Soedin. Ser. B 1982, 24, 403. (15) Keve, J. J.; Belenkii, B. G.; Gankina, E. S.; Vilenchik, L. Z.; Kurenbin, O. I.; Zhmakina, T. P. J. Chromatogr. 1981, 207, 145. (16) Alexandrov, M. L.; Belenkii, B. G.; Gotlib, V. A.; Kever, J. E. J. Microcolumn Sep. 1992, 4, 379. (17) Alexandrov, M. L.; Belenkii, B. G.; Gotlib, V. A.; Kever, J. E. J. Microcolumn Sep. 1992, 4, 385. (18) Carraud, P.; Thiebaut, D.; Caude, M.; Rosset, R. Lafosse, M.; Dreux, M. J. Chromatogr. Sci. 1987, 25, 395. (19) Hoffmann, S.; Greibrokk, T. J. Microcolumn Sep. 1989, 1, 35. (20) Demirbuker, M.; Anderson, P. E.; Blomberg, L. G. J. Microcolumn Sep. 1993, 5, 141. (21) Cui, Y.; Olesik, S. V. Anal. Chem. 1991, 63, 1812. (22) Cui, Y.; Olesik, S. V. J. Chromatogr. A 1994, 691, 151. (23) Lee, S. T.; Olesik, S. V. Anal. Chem. 1994, 66, 4498.

µm-i.d. fused silica tube (Polymicro Technologies, Phoenix, AZ) was connected to the outlet of the column; the polyimide coating of the tube was removed, and that section was used as the detection cell. A Setra 204 series pressure transducer (Setra Systems, Acton, MA) was connected in-line after the detector by using a tee connector (SGE, Austin, TX). A capillary linear restrictor (5-60 cm × 10 µm i.d., Polymicro Technologies) was also connected at this tee. For universal detection of polymers, an evaporative light-scattering detector (ELSD IIA, Alltech, Deerfield, IL) was modified with a microflow nebulizer (model SB-30A0.5, J. E. Meinhard Associates Inc., Santa Ana, CA) and interfaced to a packed-capillary column. A 30-cm × 20-µm-i.d. fused silica capillary was used to connect the end of the column to the nebulizer. This piece of capillary tubing functioned both as a restrictor and as a connection to the detector. Enhanced-fluidity liquid mobile phases were prepared by using one Isco 260D pump filled with CO2 at 211 atm (3000 psi). The required amount of CO2 was added from this pump and combined with THF in another Isco 260D pump. Chromatographic data were collected with a Pentium 100-MHz computer equipped with the EZChrom chromatography data acquisition system (Scientific Software, Inc., San Ramon, CA). The data were analyzed using Peakfit v. 4.0 for Windows (Jandel Scientific, San Rafael, CA). Peak widths at half-height were determined by fitting the chromatographic band to a Gaussian distribution. Plate heights were calculated from the measured peak widths at half-height. The packed-capillary columns were prepared using fused silica tubing (250 µm i.d. and 360 µm o.d., Polymicro Technologies) by using a CO2 slurry-packing method. The method is similar to the supercritical fluid packing method previously published.24 At the end of the column, the packing material was supported by a stainless steel frit (2-µm pores and 1/16 in. diameter) that was contained in a zero-dead-volume union (1/16 in., Valco Instruments, Houston, TX). To reduce the dead volume between the fused silica capillary and the union, the capillary column was connected to the union with a fused silica adapter. The adapter consisted of a PEEK tube as a liner that slides over the fused silica tubing. To pack the Betasil particles into the column, liquid CO2 at 352 atm (5000 psi) was used to flush packing material from a reservoir into a fused silica capillary column. Materials. The polystyrene samples (Mw ) 59 500, Mw/Mn ) 1.06; Mw ) 18 700, Mw/Mn ) 1.03; Mw ) 12 864, Mw/Mn ) 1.03; Mw ) 5120, Mw/Mn ) 1.08; Mw ) 590, Mw/Mn ) 1.07) were purchased from Scientific Polymer Products, Inc. (Ontario, NY), while the polystyrene samples with Mw ) 4136, Mw/Mn ) 1.04, and Mw ) 2360, Mw/Mn ) 1.08 were purchased from Aldrich Chem. Co. (Milwaukee, WI). Poly(methyl methacrylate) samples (Mw ) 12 700, Mw/Mn ) 1.08; Mw ) 9400, Mw/Mn ) 1.1; Mw ) 1850, Mw/Mn ) 1.06) were purchased from Polymer Laboratories (Amherst, MA). The copolymers of poly(styrene-methyl acrylate) were provided by Dr. K. Matyjaszewski from Carnegie Mellon University. Some of the properties of the copolymers are listed in Table 1. Monocarboxy- and dicarboxy-terminated polystyrene polymers with approximate molecular weight of 50 000 were obtained from Aldrich Chemical Co. and were used as models to test the separation of functionalized polymers. Figure 1 shows the structures of the carboxy-terminated polymers. All (24) Malik, A.; Li, W.; Lee, M. L. J. Microcolumn Sep. 1993, 67, 647.

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Table 1. Molecular Weight of Poly(styrene-methyl acrylate) Obtained from the SEC Chromatographic System with Enhanced-Fluidity Liquid Mobile Phase of 30% CO2 in THF

a

copolymer of poly(styrene-methyl acrylate)

structure

molecular weight, Mw, of poly(styrene-methyl acrylate)

measured molecular weight, Mw, of poly(styrene-methyl acrylate)

0622-8 0519-2 0626-12 0526-2

random block gradienta gradienta

5 000 20 000 12 000 16 000

5 130 ( 311 21 000 ( 1273 12 500 ( 758 16 500 ( 1000

Reference 32 provides more detail on the structure of these polymers.

Figure 1. (A) Structure of monocarboxy-terminated polystyrene. (B) Structure of dicarboxy-terminated polystyrene.

polymer samples were dissolved in THF (0.1-2 mg/mL). This concentration range did not affect the measured chromatograms. Tetrahydrofuran (THF) (stabilized, 99.9% purity) was purchased from Fisher Scientific (Pittsburgh, PA). SFE/SFC-grade CO2 was used as purchased from Air Products (Allentown, PA). RESULTS AND DISCUSSION Nebulizer. The evaporative light-scattering detector has three major sections: the nebulizer, the drift tube, and the lightscattering cell. The properties of each compartment influence the response. Upon entering the detector, the mobile phase is converted to droplets by the nebulizer; next, the droplets evaporate, which leaves polymer particles, and finally the light scattered by the particles is measured. Commercially available ELSDs are designed for the effluents from analytical-scale (4.6 mm) columns, and the nebulizers are for high volumetric flow rates (mL/min of liquid). These nebulizers are not readily compatible with the low flow rates (2-6 µL/min) of packed-capillary LC. Therefore, the ELSD nebulizer was replaced by a high-efficiency, low flow rate nebulizer. This nebulizer is most commonly used to add small volumes of liquid into an inductively coupled plasma (ICP). We had previously used a similar nebulizer to interface capillary electrophoresis to ICP.25 Since the mobile phase flow from the packed-capillary column is relatively small, the solvent evaporates quickly as it moves through the drift tube at room temperature. Therefore, no additional heat was applied to the drift tube. Lowtemperature operation of the drift tube (room temperature) often provides increased average drop size and increased sensitivity.19 One problem often observed when ELSDs are used for analytical-scale HPLC or SFC detection is that the detector’s response is strongly dependent on the flow rate of the mobile phase.19 This problem was not observed when the detector was used with a packed-capillary column. The response of ELSD is also commonly affected by the nebulizer gas flow rate or gas pressure. No variation in solute peak area was observed when using N2 pressure that was varied from 1.8 to 2.8 atm (25-40 psi). The nebulizer gas pressure was maintained at 2.5 atm (35 psi) throughout the experiments. (25) Olesik, J. W.; Kinzer, J. A.; Olesik, S. V. Anal. Chem. 1995, 67, 1.

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Figure 2. Pressure drop of 2-m × 250-µm-i.d. 5-µm silica gel packed column at different flow rates with two mobile phases at 30 °C. (b) THF as mobile phase; (9) 54% (mole percent) CO2 in THF as mobile phase, UV detector. Relative standard deviation of the data is 5%.

Pressure Drop. The viscosities of commonly used liquid mobile phases limit the use of longer and narrower-bore columns. Alternatively, CO2/THF mobile phases have much lower viscosities, which allows the use of long capillary columns. The pressure drop across the capillary column was compared using THF at 25 °C and 54 mol % CO2/46 mol % THF at 25 °C as the mobile phase. The pressure ahead of the column was monitored using the pump’s pressure transducer, while the pressure at the exit of the column was monitored using the Setra pressure transducer. The Setra transducer was certified by the manufacturer; the pump’s pressure transducer was calibrated against the Setra transducer before use. Figure 2 shows the pressure drop across the 2-m × 250-µm-i.d., 5-µm silica-packed column at different linear velocities with 54 mol % CO2/46 mol % THF and in pure THF. At the same flow rate, the pressure drop across the column decreases 2-fold when using 54 mol % CO2/46 mol % THF instead of THF. Separation of Polymers by Size-Exclusion Chromatography. Previously, most microcolumn SEC was carried out in 3050-cm-long and 250-500-µm-i.d. column.26,27 Short columns were chosen in order to minimize back pressure. In this system, since the mobile phases have low viscosities, the pressure drop across the column decreases. Therefore, a 2-m-long capillary column was produced and used to increase resolution and total column

Figure 3. Reduced plate height for benzene versus the linear flow rate for two mobile phases at 21 °C. (0) Pure THF, (2) 54% CO2 in THF. Column conditions: 2-m × 250-µm-i.d. silica (200-Å pore size, 5-µm particle size) packed column; UV detector.

efficiency. Figure 3 shows the variation of reduced plate height versus linear velocity for benzene obtained from a 2-m × 250-µmi.d. silica (200-Å pore size, 5-µm particle size) packed column using a 54 mol % CO2/46 mol % THF mixture and pure THF as the mobile phase at 25 °C. Benzene was used as a test solute to eliminate contributions to band dispersion caused by the polymer distribution. The minimum plate height for the polystyrene standard with Mw ) 12 864 was approximately h ) 20, for both columns. Efficiencies increased substantially when the CO2/THF mixture was used. In addition to the higher efficiency, the reduced plate height vs linear velocity curve is flatter and has a higher optimum velocity when the CO2/THF mixture was used compared to that when THF was used. For benzene at a linear velocity of 3.5 cm s-1, the measured efficiency was 100 000 theoretical plates, with 54 mol % CO2/46 mol % THF as the mobile phase. The viscosity of liquids decreases approximately exponentially as the temperature increases.28 In enhanced-fluidity liquid mobile phases, an elevated temperature can result in substantially lower viscosity. Temperature control may be another parameter that optimizes the performance of the chromatographic system.29 Figure 4 shows size-exclusion chromatograms using 54 mol % CO2/46 mol % THF as the mobile phase at constant pressure with different temperatures. The chromatographic efficiency and the flow rate increased as the temperature was raised from 30 to 90 °C. To allow a fair comparison of the performance of analyticalscale and packed-capillary SEC using enhanced-fluidity solvents, the chromatography on an analytical-scale, 25-cm-long × 4.6-mmi.d., Betasil-packed column (200-Å pore size, 5-µm particle size) was studied. Chromatograms of the polystyrene standards were compared on the two types of columns using a 40 mol % CO2/60 mol % THF mobile phase at 25 °C and 260 atm (3700 psi). The (26) Barth, H. G. In Chromatographic Characterization of Polymers: Hyphenated and Multidimensional Techniques; Provder, T., Barth, H., Urban, M., Eds.; ACS Advances in Chemistry Series 247; American Chemical Society: Washington, DC, 1995; p 3. (27) Revillon, A. J. Liq. Chromatogr. 1994, 17, 2991. (28) Reid, R. C.; Prausnitz, J. M. The Properties of Gas and Liquids, 4th ed.; McGraw-Hill: New York, 1987; p 439. (29) Trones, R.; Iveland, A.; Greibrokk, T. J. Microcolumn Sep. 1995, 7, 505.

Figure 4. Chromatograms of polystyrene standards with 40% CO2 in THF at different oven temperature: (A) 30, (B) 70, (C) 90 °C. Conditions: 1.8-m × 250-µm-i.d. silica (200-Å pore size, 5-µm particle size) packed column; column pressure 260 atm (3700 psi), microELSD. Peaks are polystyrene: Mw ) (1) 59 500, (2) 18 700, (3) 5120, (4) 590.

plate counts were determined from the peak widths as described in the Experimental Section. For the same separation time, the packed-capillary column provided higher total efficiency than the analytical-scale column (N ) 34 884 for packed-capillary compared to N ) 8617 for analytical-scale column using polystyrene MW ) 590). However, the analytical-scale column had more plates per meter (34 466 plate/m) than the long packed-capillary column (17 443 plates/m). The packing material in the long capillary column is more loosely packed than that in the analytical-scale column, which may have caused the lower number of plates per meter for the capillary column. Another possible reason for the lower number of plates per meter of the capillary may be that the pore volume per meter of packed-capillary column is smaller than that of the analytical-scale packed column. The molecular weights of the poly(styrene-methyl acrylate) copolymers were measured using the packed-capillary system. Figure 5 is the calibration curve of polystyrene standards using a 1.8-m packed-capillary filled with 250-µm silica (200-Å pore size, 5-µm particle size) and using 30 mol % CO2/70 mol % THF as the mobile phase at 70 °C. The SEC chromatograms of the polyAnalytical Chemistry, Vol. 70, No. 15, August 1, 1998

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Figure 5. Calibration curve of polystyrene. Conditions: 1.8-m × 250-µm-i.d. silica (200-Å pore size, 5-µm particle size) packed column; 30% CO2 in THF mobile phase; column pressure 260 atm (3700 psi); micro-ELSD; 70 °C.

(styrene-methyl acrylate) samples were obtained at the same chromatographic conditions. Table 1 shows the molecular weights of poly(styrene-methyl acrylate) obtained from the calibration curve. The measured molecular weights correspond well to those previously measured.8 Elution of Polymers at the Critical Point. Liquid chromatography at the critical condition (LCCC) can be achieved by using enhanced-fluidity liquid mobile phases.8 For example, upon increasing the proportion of CO2 in THF, the solvent strength lowers. If the solvent strength lowers substantially, the retention mechanism will change from size exclusion to adsorption. As mentioned earlier, the critical condition corresponds to the chromatographic condition where ∆G of transfer ) 0.30 This condition is typically found by gradual composition variation of a mobile-phase mixture. Figure 6 illustrates the calibration curves for polystyrene under SEC conditions, at the critical condition, and under adsorption conditions using the packed-capillary column. At the critical condition for the backbone polymer, the functionality distribution can be determined for macromolecules with terminal groups.31 Figure 7 illustrates the separation of a mixture of mono- and dicarboxy-terminated polystyrene at the critical condition for polystyrene. At the critical condition, the polymer chain or monomer unit in the chain (polystyrene in this case) is made to elute at the same retention time. The separation depends only on differences in functionalities that can generate a ∆G of transfer different than zero. The retention mechanism of the functionalized polymers can be based on either size-exclusion or adsorptive interactions. Figure 7 illustrates that, in this case, (30) Entelis, S. G.; Evreinov, V. V.; Gorshkov, A. V. Adv. Polym. Sci. 1987, 76, 129. (31) Gorshkov, A. V.; Much, H.; Pasch, H.; Becker, H.; Evreinov, V. V.; Entelis, S. G. J. Chromatogr. 1990, 523, 91. (32) Paik, H.-J.; Matyjaszewski, K. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1996, 37, 569.

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Figure 6. Plots of the polystyrene molecular weights versus the retention time at different mobile phase conditions: (2) 50% CO2 in THF, (b) 54% CO2 in THF, (9) 60% CO2 in THF.

Figure 7. Chromatograms of polystyrenes with mono- and dicarboxylic acid terminal group. Conditions: 1.8-m × 250-µm-i.d. silica (200-Å pore size, 5-µm particle size) packed column; 54% CO2 in THF mobile phase; column pressure 260 atm (3700 psi); micro-ELSD 70 °C. Peaks are (1) polystyrene with dicarboxylic terminal group, approximate MW 50 000; (2) polystyrene with monocarboxylic terminal group, approximate MW 50 000.

the separation is in the SEC mode. Polystyrene polymers with two carboxy terminal groups eluted first, followed by polystyrene with a single carboxy terminal group. The relative proportions of the functionalities present in the mixture can be readily determined by comparing the integrated area of the chromatographic bands. Critical chromatography can also be used to understand the components of copolymers. At the critical condition for polystyrene, Figure 8 shows the separation of a copolymer containing

size, 5-µm particle size) packed column was also used to try to separate the components of the poly(styrene-methyl acrylate). The difference in the retention times for the components was not large enough to achieve a separation. In other words, the total plate number of the analytical-scale column was not large enough to separate the two components. The 2-m-long packed-capillary column at the same chromatographic conditions (Figure 8) offered higher total efficiency for the separation at critical point.

Figure 8. Chromatograms of polystyrene and poly(styrene-methyl acrylate). Conditions: 2-m × 250-µm-i.d. silica (200-Å pore size, 5-µm particle size) packed column; 54% CO2 in THF mobile phase; column pressure 268 atm (3800 psi); micro-ELSD; 21 °C. Peaks are (1) polystyrene, Mw ) 12 864, (2) poly(styrene-methyl acrylate), Mw ) 12 000.

poly(styrene-methyl acrylate) 0626-12 with an approximate MW of 12 000 and 45 wt % polystyrene in the copolymer. The polystyrene portion of the copolymer eluted first (at the critical condition for polystyrene), followed by the methyl acrylate portion. The intensity of the first band increased when the polystyrene standards were mixed with the copolymer, which verifies the identity of each chromatographic band for the copolymer components. If a detector responds quantitatively to all parts of the polymer and the components of a copolymer are separated (as shown in Figure 8), then the relative amount of each component of the copolymer can be determined by integrating the chromatographic band for each component. We have not yet verified that the ELSD’s response is quantitative under the conditions described and, therefore, did not integrate the bands. For the same solvent conditions (critical condition for polystyrene), an analytical-scale, 25-cm × 4-mm-i.d., Betasil (200-Å pore

CONCLUSION Packed-capillary columns can be used for polymer separation with enhanced-fluidity liquid mobile phase both in SEC mode and at the critical condition. With the addition of a small-volume nebulizer, the commercial ELSD can be used as a universal detector for polymer analysis with the microcolumn liquid and enhanced-fluidity liquid systems. The enhanced-fluidity liquid mobile phases allowed the use of very long packed-capillary columns. The increased resolution and efficiency permitted separations based on both molecular weight and functionality differences with the same chromatographic system at different mobile-phase conditions. The advantages of this combined system include low consumption of mobile-phase solvent, fast separations, high resolution and efficiency, and use of small quantities of calibration standards and samples. ACKNOWLEDGMENT This work was supported by the NSF with Grant CHE-9503284 and Ashland Chemical Co. We thank Keystone Scientific Inc. for supplying packing material. We also thank Jerry Pausch from B.F. Goodrich, Brecksville, OH, and K. Matyjaszewski from the Department of Chemistry, Carnegie Mellon University, for supplying copolymer samples.

Received for review October 20, 1997. Accepted May 6, 1998. AC971152O

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