Liquid chromatography at the critical condition

As has been pointed out, both entropic and enthalpic interactions affect the chromatographic behavior of macromolecules. They are adjusted to the required type of separation by selecting appropriate stationary and mobile phases. In a third mode of liquid chromatography of polymers, liquid chromatography at the critical condition (LCCC) (Entelis etal., 1985,1986 Pasch, 1997), the adsorptive interactions are fully compensated by entropic interactions. This mode is also referred to as liquid chromatography at the critical point of adsorption. Hence, TAS is equal to AH and therefore, AG becomes zero. K is 1 irrespective of molar mass and, consequently, homopolymer molecules of different molar masses coelute in one chromatographic... 

Phillips, S. L., and Olesik, S. V. (2003). Initial characterization of humic acids using liquid chromatography at the critical condition followed by size-exclusion chromatography and electrospray ionization mass spectrometry. Anal. Chem. 75,5544—5553. 

In tile technique of liquid chromatography at the critical condition (LCCC), macromolecules of different sizes are eluted at the same time (Figure 10.19). ° This peculiar elution behavior is achieved making use of columns in which the macromolecules are at the adsorption-elution transition, and Figure 10.19 reports the calibration curve for LCCC along with the curves for adsorption and size exclusion chromatography, respectively. ... 

Liquid chromatography at the critical condition (LCCC) is performed at the elution-adsorption transition. It can be used to separate macromolecules with different functionalities such as chains with different chain ends or to separate linear chains from cycles. LCCC was used [77] to separate cycles from linear chains in poly(bisphenol-A-carbonate) PC. Figure 45.20 contains the LCCC trace. The trace is bimodal, with two bands, Z1 and Z2. The MALDI spectrum of Z1 displayed a large number of peaks, ranging approximately from 2.0 to 10 kDa, due to PC chains terminated with n-butyl on one side or on both sides. The MALDI spectrum of Z2 was far less crowded. It is made of cycles and one can note the systematic absence of linear chains. This implies that the LCCC separation is perfect. 

Cho D, Park S, Kwon K, Chang T, Roovers J. Structural characterization of ring polystyrene by liquid chromatography at the critical condition and MALDl-TOF mass spectrometry. Macromol 2001 34 7570. 

According to Gorbunov and Skvortsov [18], triblock copolymers of the ABA type may be analyzed by liquid chromatography at the critical point of adsorption similar to the analysis of diblock copolymers. The two possible cases for this type of investigation, i.e. (a) the analysis with respect to the inner block B using the critical conditions of the outer block A, and (b) the analysis of the outer block A using the critical conditions of the inner block B, will be discussed briefly. 

The use of enhanced-fluidity liquid mixtures of THF and carbon dioxide for Uquid chromatography at the critical condition was examined. For PS polymers, the molec.wt. range that could be analysed at the critical condition was... 

One of the popular methods used for the separation of polymer mixtures is the liquid chromatography at the chromatographic critical condition (LCCC). At the critical condition of one component, the other component, which is not under critical condition, can be separated in SEC mode [27-31, 33]. It is necessary to choose a proper combination of stationary and mobile phase considering the interaction strength to elute the component to be analyzed in the SEC mode. For example, Pasch et al. showed in the analysis of mixtures of methacrylate-based polymer blends that, at the critical point of PMMA, less polar methacrylates can be analyzed in the SEC mode using silica gel as the stationary phase and a methyl ethyl ketone/cyclohexane mixture as the eluent. For the analysis of polar polymethacr-... 

The chromatographic separation of polymers by liquid chromatography under critical conditions (LCCC), also referred to as liquid chromatography (LC) at the critical point of adsorption, LC in the critical range or LC at the point of exclusion-adsorption transition, has attracted significant attention within polymer community. Russian scientists using TLC [1-3] and later LC [4,5] have been the first experimentally identify critical conditions. At the critical conditions polymers of a given kind are eluted independently from their molar mass (for example. Fig. 1 [6]). 

Lee W, Cho D, Chang T, Hanley KJ, Lodge TP. Characterization of polystyrene-b-polyisoprene diblock copolymers by liquid chromatography at the chromatographic critical condition. Macromol 2001 34 2353. 

A number of 2DLC applications have attempted to use liquid chromatography at critical conditions (LCCC) and are discussed in Chapter 17. This mode of operation is useful for copolymer analysis when one of the functional groups has no retention in a very narrow range of the solvent mixture. However, determining the critical solvent composition is problematic as it is very sensitive to small changes in composition. 

The principle of the liquid chromatography under critical conditions (LC CC) was elucidated in Section 16.3.3. The mutual compensation of the exclusion—entropy and the interaction—enthalpy-based retention of macromolecules can be attained when applying in the controlled way the interactions that lead to either adsorption or enthalpic partition. The resulting methods are called LC at the critical adsorption point (LC CAP) or LC at the critical partition point (LC CPP), respectively. The term LC at the point of exclusion-adsorption transition (LC PEAT) was also proposed for the procedures employing compensation of exclusion and adsorption [161]. It is anticipated that also other kinds of enthalpic interactions, for example the ion interactions between column packing and macromolecules can be utilized for the exclusion-interaction compensation. 

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