Microfluidic Devices For In Line Sample Preparation Of Biological Mixture For Mass Spectrometry Based Downstream Analyses

For a majority of analytical chemistry studies, effective sample preparation is tantamount to the ability to answer "What is it?" and "How much is there?". In academia and industry, methods of sample preparation traditionally aim to make a sample "cleaner" so as to minimize interference from other species in the mixture and to do so in a timely, cost-effective manner. Ultimately, a rapidly produced "clean" sample allows for lower limits of detection for the analyte and quicker sample turnover. The goal of our work was to expand the knowledge of experimental microfluidics (also known as Lab-on-a-chip), explore designs to better exploit microfluidic phenomena, and develop microfluidic technologies readily integrated to mass spectrometers, all while focusing towards improving sample preparation and assay procedures. Through our efforts we created three novel devices that accomplished these goals. First, we designed an H-shaped microfluidic channel (H-cell) laden with gold internal electrodes which induced a continuous electrophoretic separation transverse to the direction of bulk fluid flow. When combined with a surface coating to suppress electroosmotic flow, the H-cell with internal electrodes removed approximately 96% of a peptide from a synthetic mixture of nucleic acid and peptide in less than 10 seconds. Once integrated with on-chip electrospray ionization functionality, we explored the dynamics of H-cell separations and improved the limit of detection for the peptide while integrated on-line with a mass spectrometer. Second, the original H-cell was modified by replacing the internal electrodes with a series of channels. In the place of electrodes, sub 100 nanometer channels were studied as well as covalently bound cross-linking polyacrylamide gel membranes. With these modifications, we achieved more challenging separations within high conductivity solutions. More specifically, we achieved continuous-flow electrophoretic separations in solutions containing 6.25% v/v formic acid or 1% w/v sodium dodecyl sulfate. In addition, we used these modified H-cell devices to fractionate a sample with analyte molecules of the same charge; essentially, these microfluidic platforms can operate as a continuous flow low-pass/high-pass fluidic mass filter (membrane H-cell) or a band-pass fluidic mass filter (nanochannel H-cell). Finally, we designed a microfluidic device using the concept of concentration polarization at the interface of a microchannel/nanochannel junction to explore sample preconcentration phenomena with a dilute analyte. Integrating this device with a mass spectrometer, we reduced the limit of detection by over two orders of magnitude in less than two minutes compared to detecting the analyte without the microfluidic device.