Miniaturised liquid chromatography

Underlying trends towards portable analytical instrumentation over the last few decades have not been equally reflected in the development of miniaturised and portable liquid chromatography (LC) instrumentation for rapid on-site or in-field measurements. Difficulties in accessing appropriate components have inhibited the design and fabrication of effective miniaturised LC systems. Additionally, the current small market for portable LC in comparison with some other miniaturised and portable analytical devices creates impediments to their commercial development. Therefore, the overall objective of this project has been to develop a new approach in designing a miniaturised portable LC based on a modular design that would harness the advantages of integrating different technologies offering flexibility in its configurations and use. In this process, it is necessary to explore available technologies, as well as develop new technologies, and integrate them in a miniaturised capillary LC system. This miniaturised capillary LC system with gradient elution capability was designed and the performance of each of the miniaturised component modules was tested and optimised. The design was based on a flexible modular system using primarily off-the-shelf components to ensure wide accessibility to the broader analytical community. A miniaturised capillary LC system was assembled on a breadboard, containing syringe pumps and switching valves, with an injection valve and on-capillary detectors, all controlled by a PC. Four miniaturised syringe pumps, with 5, 20 and 100 ˜í¬¿L syringe options, formed the basis of the pumping system. Two pairs of pumps were used for each mobile phase to create gradient elution capability. The two pairs of syringe pumps were linked by two microfluidic switching valves and connected directly through a zero void volume cross-connector, thus providing a low hold-up volume for gradient formation. Sample was injected by a 20 nL nano LC injection valve or a newly developed miniaturised LC injection valve. The electrically actuated valve features a very small size (65 x 19 x 19 mm) and light weight (33 g), and therefore can be easily integrated in a miniaturised modular capillary LC system suited for portable and/or field analysis. The internal volume of the injection valve was determined as 98 nL. The novel conical shape of the stator and rotor and the spring-loaded rotor performed well up to 32 MPa (4641 psi). A range of chromatographic columns suitable for operation under medium pressure range were selected and characterised. The range of selected columns covered anion-exchange, cation-exchange and reversed-phase separations. The retention of 15 biogenic amines and amino acids with three capillary cation-exchange columns IonPac CS19, CS12A and CS17 (250 x 0.4 mm ID, all from Thermo Fisher Scientific) with medium, medium low and ultra-low hydrophobicity and either carboxylic or mixed carboxylic/phosphonic acid functional groups was investigated. A new deep UV-LED-based photometric detection system for miniaturised LC was developed to complement the injector system. The development of the detection system included the study of new generation aluminium nitride (AIN) substrate based deep UV-LED, the development of a new 235 nm on-capillary photometric detector and the development of a deep UV Z-typed flow cell photometric detector, with an LED under 250 nm featured in analytical chemistry for the first time. The performance of the miniaturised LC system was evaluated theoretically and experimentally, including the maximum operating pressure, gradient mixing performance, and the performance of the detectors. The 5 ˜í¬¿L syringe pump offered the best performance, with typical maximum operating pressures up to 11.4 ¬¨¬± 0.4 MPa and gradient pumping reproducibility of between 4% to 9% for gradients between 0.10 % s\\(^{-1}\\) and 0.33 % s\\(^{-1}\\). The RSD (relative standard deviation) values of tR and peak areas of 6 successive isocratic runs were 0.5%-0.7% and 1.8%-2.8% for the separation of biogenic amines, respectively, and 0.1 %-0.2% and 2.1%-3.0% for the separation of cations, respectively. Test solutes of charged and uncharged dyes, and pharmaceuticals showed typical RSD values of 0.1 ‚Äö-0.2 % and 0.6 -1.0 % in isocratic mode and 1.2 ‚Äö- 4.6% and 3.2 ‚Äö- 6.4% in gradient mode, respectively for tR and peak area reproducibility. The overall performance of the miniaturised modular LC was found in most parameters to be comparable or superior to most other reported miniaturised LC systems, with a clear potential to be portable. The potential of the platform for on-site analysis in future has been demonstrated.