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Determination of Critical Conditions of Adsorption for Chromatography of Polymers Jana Falkenhagen* and Steffen Weidner BAM, Federal Institute for Materials Research and Testing, Richard-Willstaetter-Strasse 11, D-12489 Berlin, Germany Liquid chromatography (LC) at critical conditions of adsorption was used to separate various poly(ethylene oxides), poly(propylene oxides) and their copolymers. For the first time, the determination of the critical conditions by means of Ultra Performance Liquid Chromatography (UPLC) coupled to Electrospray Ionization Time-of-flight Mass Spectrometry (ESI-TOF MS) is reported. In contrast to established, mostly laborious routines to find suitable chromatographic separation conditions, this coupling enables a very fast adjustment of parameters. Similar to LC Matrix-assisted Laser Desorption/Ionization Time-ofFlight Mass Spectrometry (LC/MALDI MS) coupling, a two-dimensional analysis of homo- and copolymers regarding its functionality type and molecular weight distribution, as well as copolymer composition, can be performed simultaneously. Furthermore, there is no need for using polymer standards for the determination of critical conditions or Size Exclusion Chromatography calibration. Aside from a molecular weight distribution, polymers almost always exhibit a variety of additional heterogeneities like a functionality distribution, for example, different end groups, a composition distribution (copolymers) and/or a topology distribution, that is, different structures (cyclic, linear, or comblike structures). Size Exclusion Chromatography (SEC) can only be applied for the determination of the polymer molecular weight because its principle is based on a separation of dissolved polymers with different hydrodynamic volumes. For a functionality-based separation, the principle of Liquid Adsorption Chromatography (LAC) has gained significant importance within the last years. In this regard a very special separation mode also referred to as LAC at “critical conditions” (LACCC) is of particular interest. At these conditions polymers with different molecular weights and identical repeat unit structure elute exclusively according to their end group structure.1-4 A detailed thermodynamic description of this separation state was given by Gorshkov et al.5 Meanwhile, * To whom correspondence should be addressed. E-mail: jana.falkenhagen@ bam.de. Phone: +493081041632. (1) Pasch, H.; Trathnigg, B. HPLC of polymers; Springer Laboratory: Berlin, 1998. (2) Entelis, S. G.; Evreinov, V. V.; Gorshkov, A. V. Adv. Polym. Sci. 1986, 76, 129–175. (3) Gorshkov, A. V.; Much, H.; Becker, H.; Pasch, H.; Evreinov, V. V.; Entelis, S. G. J. Chromatogr. 1990, 523, 91–102. (4) Skvortsov, A. M.; Gorbunov, A. A. J. Chromatogr. 1990, 507, 487–496. (5) Gorshkov, A. V.; Prudskova, T. N.; Guryanova, V. V.; Evreinov, V. V. Polymer Bull. 1986, 15, 465–468.
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“critical conditions” for many different polymers have been determined.6-14 The principal approach for any search for “critical conditions” can be described by two basic steps. First, various polymers with identical structure and end groups and different molecular weights have to be dissolved in a thermodynamically “good” solvent. Usually, at least three polymer standards are necessary. At these conditions SEC chromatograms will be recorded, showing an elution beginning with higher masses. In a second step, a thermodynamically “poor” solvent will be added stepwise, until the separation mode changed from SEC toward adsorption. When this mode is reached, a “fine-tuning” by a cautious changing of the solvent mixture composition is necessary to achieve “critical” separation. These conditions are achieved when a co-instantaneous elution of all standards at the same retention time is obtained. However, for many polymers with practical importance standards are not available. Moreover, if the LACCC principle is extended for a separation of copolymers another problem arises. Copolymers show an additional distribution in terms of composition. Aside from the necessity to find two “critical points” (for each repeat unit in the copolymer) the solubility of repeat units might differ drastically. Thus, in an arbitrary chosen chromatographic system one polymer structure may elute in SEC mode, whereas the second structure is separated in LAC mode or, even worse, shows irreversible retention. An alternative very accurate method to determine critical conditions of adsorption and to characterize synthetic polymers was developed by Chang et al. This method was called temperature gradient interaction chromatography.15 Comprehensive results about the composition of a complex polymer sample can be achieved by a two-dimensional coupling of LACCC and SEC. Information on the structural heterogeneity and the mean value of the molecular weight and molecular weight (6) Braun, D.; Esser, E.; Pasch, H. Int. J. Polym. Anal. Charact. 1998, 4, 501-+. (7) Falkenhagen, J.; Much, H.; Stauf, W.; Muller, A. H. E. Macromolecules 2000, 33, 3687–3693. (8) Gancheva, V. B.; Vladimirov, N. G.; Velichkova, R. S. Macromol. Chem. Phys. 1996, 197, 1757–1770. (9) Kruger, R. P.; Much, H.; Schulz, G. J. Liq. Chromatogr. 1994, 17, 3069– 3090. (10) Kruger, R. P.; Much, H.; Schulz, G.; Wachsen, O. Macromol. Symp. 1996, 110, 155–176. (11) Mengerink, Y.; Peters, R.; deKoster, C. G.; van der Wal, S.; Claessen, H. A.; Cramers, C. A. J. Chromatogr. A 2001, 914, 131–145. (12) Pasch, H.; Augenstein, M. Makromolekulare Chemie-Macromol. Chem. Phys. 1993, 194, 2533–2541. (13) Pasch, H.; Gallot, Y.; Trathnigg, B. Polymer 1993, 34, 4986–4989. (14) Sauzedde, F.; Hunkeler, D. Int. J. Polym. Anal. Charact. 2001, 6, 295–314. (15) Chang, T. Y.; Lee, H. C.; Lee, W.; Park, S.; Ko, C. H. Macromol. Chem. Phys. 1999, 200, 2188–2204. 10.1021/ac8019784 CCC: $40.75 2009 American Chemical Society Published on Web 11/25/2008
Figure 1. Chromatograms of HO-PEO-(CH2)18-H; 700 g/mol (composite plot) recorded using different A/B ratios; (water (A) - acetonitrile/ methanole (B)).
distribution can be simultaneously obtained.7,16-19 Again, the availability of SEC standards and the required time are limitations. A substitution of SEC by Matrix-assisted Laser Desorption/ Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) can overcome these problems and offers some advantages.20,21 MALDI-TOF MS is a fast analytical method for the identification of chromatographically separated fractions. Because of the determination of absolute masses in mass spectrometry, the use of standards is not necessary. The coupling can only be carried out by off-line spraying or spotting by means of different commercially available interfaces. Nevertheless, sample preparation in MALDITOF MS is crucial for good spectra quality. The evaporation of solvents during transfer of fractions from chromatography either by spraying or by spotting can result in a formation of inhomogeneous spots. The substitution of MALDI-TOF MS by Electrospray (ESI) MS can be avoid such problems. Thus, on line LC/ ESI-MS is always preferred in those cases where samples are soluble in electrospray compatible solvents and do not exceed a certain mass region. Since the first studies of Fenn and co-workers, ESI-TOF MS has become an interesting tool for the characterization of synthetic polymers22,23 The first SEC/ESI-TOF MS coupling for the analysis of different polymers was reported by Simonsick and Prokai in the early nineties.24,25 Later Nielen et al. coupled different LC (16) Adler, M.; Rittig, F.; Becker, S.; Pasch, H. Macromol. Chem. Phys. 2005, 206, 2269–2277. (17) Kilz, P.; Kruger, R. P.; Much, H.; Schulz, G. Chromatogr. Charact. Polym. 1995, 247, 223–241. (18) Pasch, H.; Adler, M.; Rittig, F.; Becker, S. Macromol. Rapid Commun. 2005, 26, 438–444. (19) Adrian, J.; Esser, E.; Hellmann, G.; Pasch, H. Polymer 2000, 41, 2439– 2449. (20) Weidner, S.; Falkenhagen, J.; Krueger, R. P.; Just, U. Anal. Chem. 2007, 79, 4814–4819. (21) Weidner, S. M.; Falkenhagen, J.; Maltsev, S.; Sauerland, V.; Rinken, M. Rapid Commun. Mass Spectrom. 2007, 21, 2750–2758. (22) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Mass Spectrom. Rev. 1990, 9, 37–70. (23) Whitehouse, C. M.; Dreyer, R. N.; Yamashita, M.; Fenn, J. B. Anal. Chem. 1985, 57, 675–679. (24) Prokai, L.; Simonsick, W. J. Rapid Commun. Mass Spectrom. 1993, 7, 853– 856.
modes with ESI-TOF MS.26,27 Recent investigations on surfactants by LC/ESI-TOF MS were presented by van Leeuwen et al.28 Recently, a new type of chromatography was introducedsthe so-called Ultra Performance Liquid Chromatography (UPLC).29 This method provides an ultrafast separation and reduces the sample amounts drastically. By using smaller particles (particle size
