Chemical Vapor Deposition of Aminopropyl Silanes in Microfluidic Channels for Highly Efficient Microchip Capillary Electrophoresis-Electrospray Ionization-Mass Spectrometry

Abstract

We describe a chemical vapor deposition (CVD) method for the surface modification of glass microfluidic devices designed to perform electrophoretic separations of cationic species. The microfluidic channel surfaces were modified using aminopropyl silane reagents. Coating homogeneity was inferred by precise measurement of the separation efficiency and electroosmotic mobility for multiple microfluidic devices. Devices coated with (3-aminopropyl)di-isopropylethoxysilane (APDIPES) yielded near diffusion-limited separations and exhibited little change in electroosmotic mobility between pH 2.8 and pH 7.5. We further evaluated the temporal stability of both APDIPES and (3-aminopropyl)triethoxysilane (APTES) coatings when stored for a total of 1 week under vacuum at 4 °C or filled with pH 2.8 background electrolyte at room temperature. Measurements of electroosmotic flow (EOF) and separation efficiency during this time confirmed that both coatings were stable under both conditions. Microfluidic devices with a 23 cm long, serpentine electrophoretic separation channel and integrated nanoelectrospray ionization emitter were CVD coated with APDIPES and used for capillary electrophoresis (CE)-electrospray ionization (ESI)-mass spectrometry (MS) of peptides and proteins. Peptide separations were fast and highly efficient, yielding theoretical plate counts over 600,000 and a peak capacity of 64 in less than 90 s. Intact protein separations using these devices yielded Gaussian peak profiles with separation efficiencies between 100,000 and 400,000 theoretical plates.

Top-down proteomic analysis of protein pharmaceuticals, mixtures of protein complexes, and ribosomal protein extracts by capillary zone electrophoresis-mass spectrometry

“…One example involves commercial microchips sold through 908 Devices (Boston, MA). A number of publications involving this interface have been reported [71][72][73]. A distinct advantage of such systems includes administration of capillary coating in the gas phase through chemical vapor deposition [71].…”

“…A number of publications involving this interface have been reported [71][72][73]. A distinct advantage of such systems includes administration of capillary coating in the gas phase through chemical vapor deposition [71]. Utility of CZE for the analysis of charged biological macromolecules, such as intact proteins, depends in part on the use of appropriate coatings preventing non-specific adhesion to the separation channel walls.…”

“…Utility of CZE for the analysis of charged biological macromolecules, such as intact proteins, depends in part on the use of appropriate coatings preventing non-specific adhesion to the separation channel walls. While capillary coatings for CZE are to be discussed later, it should be mentioned that the utility of chemical vapor deposition for coating administration results in considerable improvements in coating homogeneity, thereby resulting in diminished longitudinal diffusion of analytes represented by a smaller B-term in Equation 4 [71]. Microchip design, shown in Figure 1-4, was based on a cross geometry and contained four functional elements: (1) an injection reservoir, (2) a BGE reservoir, (3) a waste reservoir, and (4) a serpentine separation channel.…”

“…Microchip design, shown in Figure 1-4, was based on a cross geometry and contained four functional elements: (1) an injection reservoir, (2) a BGE reservoir, (3) a waste reservoir, and (4) a serpentine separation channel. Connection between the CZE component and the ESI-MS junction was enabled by administration of a sheath-liquid, delivered electroosmotically to the ESI outlet via a separate channel within the microchip, thereby linking the sheath liquid reservoir with the ESI outlet [71]. Fundamentally, the microchip design represents a sheath-liquid interface between CZE and ESI-MS with limited dilution of analytes at the junction enabled by electroosmotic delivery of sheath liquid to the ESI outlet.…”

Three-Dimensional Electro-Sonic Flow Focusing Ionization Microfluidic Chip for Mass Spectrometry

“…Various materials have been employed to fabricate microfluidic chips, such as silicon [22,23], polymers (SU-8) [24][25][26], polymethyl methacrylate [27], glass [28][29][30][31] and poly (dimethylsiloxane) (PDMS) [1,6,32,33]. Koster et al [14] conducted a thorough summary of the materials used for microfluidic chips.…”