Flow-through evaporative solvent removal and applications with chromatography

This thesis describes the development of a device to address solvent issues in analysis techniques and improve automation. These include the removal of solvent for preconcentration or selective removal of organic solvent in mixtures to address solvent incompatibilities. In chapter I, a review of preconcentration techniques and applications by solvent removal is presented. Preconcentration is typically the first part of analytical method development covering the need to improve detection sensitivity. This review gathers presented strategies to accomplish preconcentration by solvent removal. Evaporation and partitioning in an immiscible fluid has been reported in a variety of forms with good control of the interfaces and accurate results. A comprehensive comparison of different approaches is presented, as well as an indication on the research needs in this area. In addition, an overview on solvent incompatibility problems for instrumental coupling is provided, which can potentially be addressed by similar solvent removal techniques. In chapter II, the development of a temperature controlled membrane evaporation concentrator for continuous flow conditions is described. An exponential relationship was observed between temperature and concentration factor and a theoretical model was described to better understand the performance of the concentrator. Caffeine was the analyte used to construct the model which was employed to predict the concentration performance of three target analytes at different conditions. Experimentally, a 30-fold concentration can be attained in less than 60 min whilst maintaining solute integrity under different sub-ambient pressure conditions and mild temperatures. While using the model it was determined that a 10-fold concentration (±0.5) can be performed at 56.72 ± 0.07°C and at a flow rate of 10 μL min\(^{-1}\). Altogether, the model provides a better understanding of the process and its applicability to analytical methods. This work demonstrates that it is possible to obtain high concentration factors with a continuously flowing fluid when temperature is precisely controlled and in times that are reasonable compared to existing evaporation concentration procedures. Chapter III describes a further development of the membrane evaporator as an evaporative membrane modulator (EMM) to address solvent incompatibilities in two-dimensional liquid chromatography. A feedback control mechanism was used to precisely control evaporation, adjusting temperature automatically in real time to address the need for high temperature precision described in Chapter II. In addition, the automated interface can keep evaporation constant regardless of the changing solvent content in the feed. It reduces the volume after \(^1\)D elution by a pre-determined factor, regardless of the separation solvent gradient. This volume reduction ensures that the injection volume in the \(^2\)D is appropriate for the second column, avoiding the detrimental effects of overloading. In addition, the fraction solvent composition is constant over the length of the separation increasing reproducibility of \(^2\)D separations. The evaporative membrane modulator was demonstrated with a 10-fold reduction, reducing the fraction injection volume from 50 to 5 μL. A consequence of the EMM device is a reduction in the capacity of the first dimension, which is decreased by a factor of 2.4, but the peak width at half maximum was reduced by up to 22% in the second dimension. When band broadening is considered, the corrected peak capacity with the modulator was only 10% lower than that without the modulator, but with a gain in peak height of 2-3. Also a decrease in retention time between subsequent peak-slices reduced from 4 s to be negligible. Avoiding loss of performance in the second dimension when coupling two-dimensional liquid chromatography systems shows potential to facilitate peak identification and quantitation in more complex applications. In chapter IV, the evaporation process across the membrane is fully characterized with water/solvent mixtures showing organic solvent removal capabilities. The system allowed continuous methanol, ethanol and acetonitrile removal from samples containing up to 80% organic solvent. The use of the device is then demonstrated for on-line sample reconstitution after solvent extractions. Removal of organic solvent from sample extracts is required before analysis by RPLC to preserve chromatographic performance and allow for bigger injection volumes, boosting sensitivity. An automated on-line extraction evaporation procedure is integrated with HPLC analysis, applied to analysis of the antibiotic chloramphenicol in milk samples. Sample reconstitution and HPLC injection is performed in less than 10 min and can be executed simultaneously to HPLC analysis of the previous sample in a routine workflow, thus having minimal impact on the total sample analysis time when run in a sequence. The developed device shows great potential to aid automation and address instrumental coupling issues arise from solvent incompatibilities. Advantages and further development options for this technology are discussed