Supercritical fluid chromatography and two-dimensional liquid chromatography : From new applications to technical innovations

Abstract: Lignin depolymerisation can be utilized to produce value-added aromatic compounds. One of the major challenges in lignin depolymerisation study is the development of effective analytical tools to unravel the huge complexity of the depolymerisation product mixture. Traditional gas chromatography (GC) and high- performance liquid chromatography (HPLC) methods applied for this purpose are often in lack of a high resolving power to separate the various components, which exist in a large number in samples of this type. This PhD work started with the development of a 6-minute fast supercritical fluid chromatography method for the analysis of lignin-derived phenols from alkaline cupric oxide oxidation, which demonstrated for the first time the potential of supercritical fluid chromatography in the lignin type phenol analysis. As a continuation of this work to enable analysis of more model compounds with structural elucidation, a supercritical fluid chromatography with quadrupole-time-of-flight mass spectrometry method was developed. Considering the complexity of processed lignin sample, a two-dimensional liquid chromatography × supercritical fluid chromatography system was constructed with trapping column assisted modulation. The system exhibited high degree of orthogonality. A trapping column interface also significantly shorten the run time with higher detectability of the compounds compared with a traditional loop based interface. Large volume and water injection in supercritical fluid chromatography often lead to poor separation and peak shape, due to strong solvent effect and viscous fingering. Multiple injection technique was investigated in this PhD study to enable the injection of relatively large volume to enhance the detectability. The signal-to-noise ratio enhancement was found to be strongly dependent on retention. Multiple injection provided better enhancement of signal-to-noise than one-time injection of large volume, with comparable repeatability and reproducibility. Water as sample diluent was investigated to study its effect on retention and peak shape. The influence of water in sample diluent varies with varying stationary phases, injection volumes and sample diluent water contents. The study proved that it can be advantageous to use sample diluent containing water, especially when polar stationary phases are used. Monitoring a complete dynamic extraction process is not possible with any set-up reported so far, but coupling extraction with chromatography with a parallel sampling interface can be a potential solution. As a proof of concept, an on-line pressurized hot water extraction-liquid chromatography system with parallel sampling was built imitating the two-dimensional chromatography philosophy. The system was used in a kinetic study of extraction of curcuminoids from turmeric. Compound-specific extraction curves were obtained and thermal degradation of curcumin could also be demonstrated.