Critical chromatography

It is interesting to note that there have been very few specific analytical applications reported, which demonstrate that super-critical chromatography gives results superior to either gas chromatography or liquid chromatography alone. As a consequence, considering the added complexity of the super-critical chromatograph, its value in analytical chemistry must be considered questionable. 

The first study that made it possible to estimate the critical length of a column in gradient HPLC of proteins was presented by Belenkii and co-workers in 1993 [53]. Their approach was based on the concept of critical chromatography of synthetic polymers. They introduced the concept of a critical distance, X0, after which the protein zone travels with the same velocity as the mobile phase (similarly to what has been shown previously by Yamamoto et al. [60]). The equation for the critical distance at which the zone velocity v(x) becomes virtually identical to the displacer velocity, u, is defined as ... 

Entelis et al. (20) found that homopolymers of diflFerent molar masses show exactly similar retention behavior on silica if a special eluent mixture was used. They found that under criticaP conditions the sorbent did not see the polymeric nature of the chain. The separation was dependant only on the enthalpic interaction of the sample-sorbent pair [so-called critical chromatography or liquid adsorption chromatography under critical conditions (LACCC)] (20-24). 

Table II. Quantitative Analysis of Various End Groups of Adipic Acid-Hexane Glycol Polyesters as Determined by Critical Chromatography...

The use of liquid chromatography at the critical point of adsorption (CC, critical chromatography) for the determination of the functionality type distribution of telechelics was demonstrated by Evreinov and co-workers [2,11-15]. Meanwhile, a significant number of investigations on functional polyolefines, oligoethers, polyesters, and epoxy resins were conducted showing the usefulness of this new technique. 

In order to compare the results of critical chromatography with results of an independent method, SEC with coupled density (D) and refractive index (RI) detection was used, which has been shown to be very useful for the characterization of copolymers with respect to their chemical composition [39,40]. The MMD curve for one of the block copolymers and the mass distribution curves of the components are shown in Fig. 18. From these the overall chemical composition may be calculated. An excellent agreement between the results of critical chromatography and the SEC experiments was obtained. 

Fig. 20. Two-dimensional separation of a triblock copolymer of EO and PO, critical chromatography vs supercritical fluid chromatography. (From [41] with permission)...

While SEC is the main technique used for the separation of macromolecules based on their size, a new technique called critical chromatography has been developed which separates macromolecules based on functionality, architecture, or composition. In order to utilize this technique a two-step... 

The solubility parameter (o) reflects interactions between the solvent and the polymer in question. Usually, the polymer is dissolved more easily when the solubility parameter of a polymer and solvent are similar (o(solvent) - o(polymer) <2 ). Solubility parameters are based on experimental data and are tabulated for many of solvents and polymers [163,164]. Philipsen et al. [79] and Baranetal.[70] have observed, after studying the critical conditions with few different solvent mixtures, that the solubility parameters were close, indicating that the critical chromatography mode could occur at a critical solubility parameter of the eluent. In Fig. 2A,B,C the dependence between solubility parameters of eluents at critical conditions on various type of sorbents are shown. 

Fujii T. Quadrupole mass spectrometry in combination with lithium ion attachment for sampling at atmospheric pressure possible coupling to a superfluid critical chromatography. Anal Chem. 1992 64 775-8. 

The separation shown in Fig.l0(a) has proven to be quite robust. However, this is not usually the case for critical separations. Indeed, for carboxyl-fimctionalized poly(n-butyl acrylates) it proved much more difficult to achieve genuine critical chromatography [19]. Critical liquid chromatographic separations are not always easy, but they can be highly rewarding, especially for determining FTDs. 

FIGURE 9.24 Separation of adipic acid-1,6-hexanediole oligo-ester end-groups by critical chromatography. 

A Gmonomer, = 0 for One particular monomer (critical chromatography), it is impossible to find conditions that are critical for several different monomers simultaneously. Therefore, critical chromatography is much more useful for determining the FTDs of functional polymers than for determining the CCDs of copolymers. In the latter case, two options are open. One is to find conditions at which the separation is critical towards one type of monomer, while the second monomeric unit does not show any interaction, so that it is eluted under SEC conditions. Such conditions have been applied to block copolymers. The block for which critical conditions are maintained is made invisible and the separation reflects the block-length distribution of the second block. An example is shown in Figure 7.3. 

Sinha, P. Hiller, W. Pasch, H.Characterization ot Blends ot Potyisoprene by On-line Hyphenation of Critical Chromatography and Nuclear Magnetic Resonance Spectroscopy. Polychar-18 Siegen, Germany, 2010. 

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