New materials for separation and preconcentration in open tubular capillary‑based separations

The preparation of microbore capillary columns with higher resolving power, better selectivity, and greater durability is necessary to develop compact and more efficient chromatographic systems. The efforts devoted to address this issue have seen innovation in new sorbent materials, notably the discovery of novel materials for stationary phases (SPs) that are efficient, stable, resilient, and selective. Open-tubular microcolumn formats are viewed as potentially favourable, due to reducing band broadening by eliminating eddy diffusion and improving mass transfer using thin layer coatings, demonstrating increasingly greater chromatographic performance. Both soft materials (i.e., surfactants) and hard materials (i.e., nanoparticles) are well-known SP materials used for various chromatographic separations, and as such are of great interest in the search for next-generation stationary phase materials, particularly for open-tubular column formats
Additionally, the need to reduce the use of organic solvents in labs is rising. Thus, downsizing and advancement of sample handling by exploring alternative greener approaches are also a topic that various scholars are now addressing.
The development of diverse SPs is vital to broadening the scope of chromatographic analysis. Therefore, this project aims to progress this field of research through (i) the study of the aggregation of amphiphilic molecules at liquid-surface interfaces in an open tubular capillary and evaluation of their separation and sample preconcentration performance at the admicellar region, and (ii) to prepare SPs with large surface areas by in-situ synthesis of silica nanospheres on the inner walls of fused silica capillaries and functionalize them with different functional groups, and characterise as new SPs for capillary chromatography.
Surfactant-based separation techniques are widely used in micellar electrokinetic chromatography (MEKC) as a rapidly growing area of separation and sample preconcentration. It is also well known that surfactants are used above their critical micelle concentration (cmc) in open-tubular liquid chromatography (OT-LC) and open-tubular capillary electrochromatography (OT-CEC). Herein, the molecular aggregation of two cationic surfactants at critical surface aggregation concentration (csac), below cmc was expolited. Electroosmotic flow (EOF) and retention factor measurements were used to estimate the csas points of two cationic surfactants, didodecyldimethylammonium bromide (DDAB) and tridodecylmethylammonium chloride (TDMAC). The admicelles served as a stationary pseudophase for open-tubular admicellar liquid chromatography (OT-AMLC) and admicellar electrochromatography (OT-AMEC), allowing the separation of neutral and charged analytes. The retention mechanisms were identified by testing the effect of changing the mobile phase (MP)/background solution (BGS) conditions (e.g., organic solvent, ionic strength, pH, and added salt) on the chromatographic retention of the analytes.
Moreover, both pseudophases were used to perform in-line sample enrichment. A coated capillary with DDAB admicelles was used as a stationary pseudophase after performing micelle to cyclodextrin stacking for OT-CEC separations. When applied in OT-LC, the attempted separation of cations at low pH mobile phases (MPs) showed low retention, while MPs with high pH showed the opposite effect. Accordingly, it was possible to establish a pseudophase microextraction (PPME) method to separate cations at high pH followed by CE separation at low pH in the same column. Additionally, significant retention for basic analytes was obtained utilising a high pH MP in a pH-assisted PPME strategy coupled with CZE, providing 2-3 orders of magnitude increase in the sensitivity.
Chapter 4 of this thesis discusses a dynamic in-situ sol-gel method for the formation of bonded-phase silica nanospheres on silica capillary inner walls, to prepare a highly porous and large accessible surface area stationary phase for OT-LC and OT[1]CEC applications. The surface of SNSs was further functionalized with different functional groups for the separation of a wide range of compounds, i.e., non-steroidal anti-inflammatory drugs, phenones, alkenylbenzenes, and enantiomers of chlorophenoxy herbicides. The synthesis and process conditions were optimized and the synthesised stationary phases were characterized using scanning electron microscopy (SEM).