Development of a microfluidic device using hot embossing for generating electrospray ionization process

This thesis presents a PhD research program undertaken at Faculty of Engineering and Industrial Science (FEIS), Swinburne University of Technology (SUT), Melbourne, Victoria, Australia between the years 2007 and 2010. This thesis reports on the investigation and development of a high performance polymer microfluidic chip with reduced complexity appropriate for mass manufacture for electrospray ionization mass spectrometry (ESI-MS) analysis of fluid samples. This thesis investigates the effect of chip tip-to-counter electrode spacing (gap), channel size aspect ratio, and onset voltage on electrospray ionization (ESI) performance of embossed polymer microfluidic chips. The aim was to develop a polymer microfluidic chip design for electrospray which can be fabricated by hot embossing for mass manufacture. A further aim was to characterize the embossing process in terms of material test data. An extensive literature review indicated that there is a rising demand for more reproducible mass fabricated devices for ESI, to minimize system complexity along with reduction in manufacturing cost and time and to eliminate dead volume related to external connections. Therefore, to fabricate an ESI-MS chip with single material with no dead fluid volume is significant to the ESI field. Planar polymer substrates were hot embossed using an electroformed tool and a laser machined tools. The hot embossed samples were produced at low cost and of high quality. The embossed microchips were tested for successful ESI demonstration with minimum sample consumption and high throughput. Stable electrosprays were generated at microchannel exit of replicated microchips without cover using a high voltage difference between a positive stainless steel electrode in the reservoir and a negative aluminum plate. Factors influencing the stability of electrospray were analyzed by a set of experiments. Theoretically and experimentally, it was observed that the distance of the microchannel tip to the counter electrode directly relates to the onset voltage applied. The onset voltage is proportional to the microchannel dimensions. Furthermore, the overall pattern observed is the decrease in Taylor cone size with a decrease in channel dimensions. The size of the Taylor cones was analyzed as a function of onset voltage and aspect ratio. The total ion current was analyzed as a function of time and onset voltage at various gaps. Five electrospray modes namely, drop mode, pulse mode, Taylor-cone jet mode, multi-jet mode and oscillating jet mode recorded at the open channel tips were identified and analyzed. These results are in good agreement with the theory and findings of other researchers. Furthermore, a numerical model was validated with experimental results. The model was applied to the experimental data and with some limitations was found to make an improvement to the reported electrospray ionization onset voltages. Higher spray current and lower electrospray jet diameter indicates that the device can perform equivalent to nanospray emitters while using microscale dimensions. This allows higher sample throughput and eliminates potential clogging problems inherent in nano-capillaries. The hot embossing process can potentially lead to the construction of inexpensive microfabricated devices with complex designs and advanced functionalities. One of the conclusions of this research was that the open channel microchip design combined with the hot embossing process provides an improved approach to reduce impurities and simplify manufacture. The results represent a successful step toward the construction of simple, high speed microfabricated ESI system. The ES performance of the novel emitter was evaluated with respect to its geometry, operating conditions and ESI parameters such as onset voltage and current. The results indicate that the aspect ratio of the microfluidic chips is one of the most important design factors that affect the ES performance. The results also indicate that the lower the chip-tocounter electrode spacing requires a lower ‘onset’ potential. The correlation results were compared with the literature. The results obtained from this research encourage deeper investigation of the ESI interfaces and characterization of embossing master tools and replicas. Implications for the industrial application of these findings are discussed and directions for further research presented in this thesis.