Structural information on humic acids is difficult to obtain because of the heterogeneity of the acids. Herein liquid chromatography at the critical condition, LCCC, is used to provide a sorting mechanism for the diverse types of molecules contained in humic acids. The critical condition of polymers that are believed to model some subunit of the humic acid is determined. Humic acids from three different terrestrial sources (soil, compost, and peat) are then separated under these chromatographic conditions. The portion of the humic acid that has structure similar to that of the model polymer elutes at the retention volume of the critical condition of the model. Next, fractions are collected and further characterized. This detailed characterization includes high-efficiency size-exclusion chromatography and electrospray mass spectrometry. The size-exclusion chromatograms of the fractions were found to be markedly different from that of the original humic acid sample. This is strong evidence that the LCCC separation mechanism is different from size fractionation. The mass spectra of the humic acid fractions were also markedly different from those of the bulk humic acids previously reported. The mass spectra of specific fractions collected had repeating clusters of m/z values, which is more evidence that the critical condition separation is a powerful sort function.

The improvement in the analysis of telechelic polymer matrixes continues to be a pursuit for many scientists of varying disciplines. This quest for a new technique has led to the continued development of liquid chromatography at the critical condition (LCCC) or liquid chromatography at the critical adsorption point (LC-CAP). LCCC allows for the isolation of one area of the polymer matrix so that other areas of the polymer can be probed with size-exclusion or adsorptive chromatographic modes. Although this technique has been successfully applied to the analysis of telechelic polymers, the practice of LCCC can be difficult. These difficulties include finding and maintaining a solvent system appropriate for the practice of LCCC as well as deterioration of peak shape once the system is operating at the LCCC mode. Because of the specificity of the mobile phase required for the practice of LCCC, the work is routinely practiced by premixing solvents. Previous work with enhanced-fluidity liquid mobile phases demonstrated that these mobile phases removed many of the aforementioned challenges associated with working at the LCCC mode. These mobile phases utilize both pressure and temperature variation in order to maintain the specific solvent strength necessary for the LCCC work. This work studies the coupling and optimization of enhanced-fluidity, EF, liquid mobile phases for LCCC. Several EF-LCCC systems, differing in mobile phase composition, temperature, and pressure, were routinely established, resulting in the effective practice of critical chromatography. The practice of LCCC with on-line mobile phase preparation is demonstrated using commercially available instrumentation. Finally, EF-LCCC is used to analyze triblock and diblock copolymers.

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Fast and efficient chromatography can result from the practice of chromatography with a mobile‐phase system that has the solvent strength of a liquid but the density near that of a gas. This approach to supercritical fluid chromatography (SFC) is achieved by utilizing a static or dynamic mobile‐phase system that is a combination of two or more fluids, one of which is a low‐viscosity fluid such as carbon dioxide. These ‘enhanced’ fluids are realized for chromatographic applications by applying the appropriate engineering controls. There are few limitations on the combination of mobile phase, stationary phase, and analyte chemistries as long as the mobile phase is maintained as a single fluid. Liquefied carbon dioxide can be exploited as either a thermodynamic or a kinetic variable when utilized as a mobile‐phase solvent. These subcritical fluids can attain a modest pH and polarity range in order to achieve a wide range of separation mechanisms. Ternary solvent systems consisting of carbon dioxide, alcohol, and water in combination with a polar stationary phase can result in novel chromatographic applications. Significant advances in the engineering of the hardware and software must be made in order for SFC to reach its full potential.