Reversed-phase liquid chromatography on a microchip with sample injector and monolithic silica column

Alternate Soaking Technique for Micropatterning Alginate Hydrogels on Wettability-patterned Substrates

Techniques for patterning hydrogels are important for fabrication of cell culture, analytical, and actuator devices at the micro- and nanometer length scales. In this study, we fabricated alginate hydrogels cross-linked by divalent cations on wettability-patterned substrates by alternate soaking of precursor solutions of sodium alginate and divalent cations. The wettability-patterned substrates were fabricated on hydrophilic glass plates modified with hydrophobic self-assembled monolayers of hexamethyldisilazane followed by exposure to an ultraviolet/ozone atmosphere through a metal mask. The film thickness of alginate gels with a width and length of 0.1 and 4 mm were tuned stepwise from 30 nm to 200 nm by adjusting the precursor conditions, including the pH, type of divalent metal ions, and sodium alginate concentration, and the alternate soaking conditions, including the dipping/withdrawal speed and number of alternate soaking cycles. This technique can be applied to other functional gels and will contribute to fabrication of hydrogel devices at the micro- and nanometer scales in the future.

Chromatographic performance of microfluidic liquid chromatography devices: Experimental evaluation of straight versus serpentine packed channels

We prepared a series of planar titanium microfluidic (μLC) columns, each 100 mm long, with 0.15, 0.3 and 0.5 mm i.d.'s. The microfluidic columns were packed with 1.8 μm C18 sorbent and tested under isocratic and gradient conditions. The efficiency and peak capacity of these devices were monitored using a micro LC instrument with minimal extra column dispersion. Columns with serpentine channels were shown to perform worse than those with straight channels. The loss of efficiency and peak capacity was more prominent for wider i.d. columns, presumably due to on-column band broadening imparted by the so-called "race-track" effect. The loss of chromatographic performance was partially mitigated by tapering the turns (reduction in i.d. through the curved region). While good performance was obtained for 0.15 mm i.d. devices even without turn tapering, the performance of 0.3 mm i.d. columns could be brought on par with capillary LC devices by tapering down to 2/3 of the nominal channel width in the turn regions. The loss of performance was not fully compensated for in 0.5 mm devices even when tapering was employed; 30% loss in efficiency and 10% loss in peak capacity was observed. The experimental data for various devices were compared using the expected theoretical relationship between peak capacity P_c and efficiency N; (P_c-1) = N^0.5 × const. While straight μLC columns showed the expected behavior, the devices with serpentine channels did not adhere to the plot. The results suggest that the loss of efficiency due to the turns is more pronounced than the corresponding loss of peak capacity.

A simple microfluidic chip design for fundamental bioseparation

A microchip pressure-driven liquid chromatographic system with a packed column has been designed and fabricated by using poly(dimethylsiloxane) (PDMS). The liquid chromatographic column was packed with mesoporous silica beads of Ia3d space group. Separation of dyes and biopolymers was carried out to verify the performance of the chip. A mixture of dyes (fluorescein and rhodamine B) and a biopolymer mixture (10 kDa Dextran and 66 kDa BSA) were separated and the fluorescence technique was employed to detect the movement of the molecules. Fluorescein molecule was a nonretained species and rhodamine B was attached onto silica surface when dye mixture in deionized water was injected into the microchannel. The retention times for dextran molecule and BSA molecule in biopolymer separation experiment were 45 s and 120 s, respectively. Retention factor was estimated to be 3.3 for dextran and 10.4 for BSA. The selectivity was 3.2 and resolution was 10.7. Good separation of dyes and biopolymers was achieved and the chip design was verified.

Parameters Governing the Formation of Photopolymerized Silica Sol-Gel Monoliths in PDMS Microfluidic Chips

Although polydimethylsiloxane (PDMS) microfluidic chips provide an alternative to more expensive microfabricated glass chips, formation of monolithic stationary phases in PDMS is not a trivial task. Photopolymerized silica sol-gel monoliths were fabricated in PDMS based microfluidic devices using 3-trimethoxysilylpropylmethacrylate (MPTMOS) and glycidyloxypropyltrimethoxysilane (GPTMOS). The monolith formation was optimized by identifying a suitable porogen, controlling monomer concentration, functional additives, salts, porogen, wall attachment methods, and rinsing procedures. The resulting monoliths were evaluated using scanning electron microscopy, image analysis, differential scanning calorimetry, and separation performance. Monoliths functionalized with boronic acid ligands were used for the separation of cis-diol containing compounds both in batch mode and in the microfluidic chip.

Liquid chromatography on chip

LC is one of the most powerful separation techniques as illustrated by its leading role in analytical sciences through both academic and industrial communities. Its implementation in microsystems appears to be crucial in the development of mu-Total Analysis System. If electrophoretic techniques have been widely used in miniaturized devices, LC has faced multiple challenges in the downsizing process. During the past 5 years, significant breakthroughs have been achieved in this research area, in both conception and use of LC on chip. This review emphasizes the development of novel stationary phases and their implementation in microchannels. Recent instrumental advances are also presented, highlighting the various driving forces (pressure, electrical field) that have been selected and their respective ranges of applications.

Performance of HPLC/MS microchips in isocratic and gradient elution modes

We analyzed the chromatographic performance of particle-packed, all-polyimide high-performance liquid chromatography/mass spectrometry (HPLC/MS) microchips in terms of their hydraulic permeabilities and separation efficiency under isocratic and gradient elution conditions. The separation channels of the chips (with ca 50 microm x 75 microm trapezoidal cross-section and a length of 43 mm) were slurry packed with either 3.5 or 5 microm spherical porous C18-silica particles. A custom-built holder enveloped the chip during packing to prevent channel deformation and delamination from high pressures. It is shown that the packing conditions significantly impact the packing density of the HPLC/MS chips, which determines their performance in both, isocratic and gradient elution modes. Even with steep solvent gradients, peak shape and chromatographic resolution for the densely packed HPLC/MS chips are much improved. Our data show that the analytical power of the HPLC/MS chip is limited by the quality of the chromatographic separation.

Microchip electrospray performance during gradient elution with bulk conductivity changes

This work identifies dynamic changes in bulk conductivity during reversed-phase HPLC gradient elution as a major source for spray mode changes and instabilities observed in ESI-MS. A commercial microchip-HPLC/ESI-MS configuration was modified to enable electrospray diagnostics based on frequency analysis of the microchip emitter current combined with spray imaging. This approach facilitated detection of different spray modes together with their onset potentials. Water/acetonitrile mixtures containing formic acid were selected as the electrosprayed solutions to represent typical conditions in reversed-phase HPLC. Experimental data are complemented by computational fluid dynamics simulations, treating the electrosprayed solution as leaky dielectric fluid, to address the influence of bulk conductivity and applied potential difference on the developing cone-jet morphology and stability.

Microfluidic chips for mass spectrometry-based proteomics

Microfluidic devices coupled to mass spectrometers have emerged as excellent tools for solving the complex analytical challenges associated with the field of proteomics. Current proteome identification procedures are accomplished through a series of steps that require many hours of labor-intensive work. Microfluidics can play an important role in proteomic sample preparation steps prior to mass spectral identification such as sample cleanup, digestion, and separations due to its ability to handle small sample quantities with the potential for high-throughput parallel analysis. To utilize microfluidic devices for proteomic analysis, an efficient interface between the microchip and the mass spectrometer is required. This tutorial provides an overview of the technologies and applications of microfluidic chips coupled to mass spectrometry for proteome analysis. Various approaches for combining microfluidic devices with electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) are summarized and applications of chip-based separations and digestion technologies to proteomic analysis are presented.

Polymer microchips integrating solid-phase extraction and high-performance liquid chromatography using reversed-phase polymethacrylate monoliths

Polymer microfluidic chips employing in situ photopolymerized polymethacrylate monoliths for high-performance liquid chromatography separations of peptides is described. The integrated chip design employs a 15 cm long separation column containing a reversed-phase polymethacrylate monolith as a stationary phase, with its front end seamlessly coupled to a 5 mm long methacrylate monolith which functions as a solid-phase extraction (SPE) element for sample cleanup and enrichment, serving to increase both detection sensitivity and separation performance. In addition to sample concentration and separation, solvent splitting is also performed on-chip, allowing the use of a conventional LC pump for the generation of on-chip nanoflow solvent gradients. The integrated platform takes advantage of solvent bonding and a novel high-pressure needle interface which together enable the polymer chips to withstand internal pressures above 20 MPa (approximately 2900 psi) for efficient pressure-driven HPLC separations. Gradient reversed-phase separation of fluorescein-labeled model peptides and BSA tryptic digest are demonstrated using the microchip HPLC system. Online removal of free fluorescein and enrichment of labeled proteins are simultaneously achieved using the on-chip SPE column, resulting in a 150-fold improvement in sensitivity and a 10-fold reduction in peak width in the following microchip gradient LC separation.

Structure-transport analysis for particulate packings in trapezoidal microchip separation channels

This article investigates the efficiency of particulate beds confined in quadrilateral microchannels by analyzing the three-dimensional fluid flow velocity field and accompanying hydrodynamic dispersion with quantitative numerical simulation methods. Random-close packings of uniform, solid (impermeable), spherical particles of diameter d(p) were generated by a modified Jodrey-Tory algorithm in eighteen different conduits with quadratic, rectangular, or trapezoidal cross-section at an average bed porosity (interparticle void fraction) of epsilon = 0.48. Velocity fields were calculated by the lattice Boltzmann method, and axial hydrodynamic dispersion of an inert tracer was simulated at Péclet numbers Pe = u(av)d(p)/D(m) (where u(av) is the average fluid flow velocity through a packing and D(m) the bulk molecular diffusion coefficient) from Pe = 5 to Pe = 30 by a Lagrangian particle-tracking method. All conduits had a cross-sectional area of 100d(p)(2) and a length of 1200d(p), translating to around 10(5) particles per packing. We present lateral porosity distribution functions and analyze fluid flow profiles and velocity distribution functions with respect to the base angle and the aspect ratio of the lateral dimensions of the different conduits. We demonstrate significant differences between the top and bottom parts of trapezoidal packings in their lateral porosity and velocity distribution functions, and show that these differences increase with decreasing base angle and increasing base-aspect ratio of a trapezoidal conduit, i.e., with increasing deviation from regular rectangular geometry. Efficiencies are investigated in terms of the axial hydrodynamic dispersion coefficients as a function of the base angle and base-aspect ratio of the conduits. The presented data support the conclusion that the efficiency of particulate beds in trapezoidal microchannels strongly depends on the lateral dimensions of the conduit and that cross-sectional designs based on large side-aspect-ratio rectangles with limited deviations from orthogonality are favorable.

Electroosmotic flow-based pump for liquid chromatography on a planar microchip

An electroosmotic flow (EOF)-based pump, integrated with a sol-gel stationary phase located in the electric field-free region of a microchip, enabled the separation of six nitroaromatic and nitramine explosives and their degradation products via liquid chromatography (LC). The integrated pump and LC system were fabricated within a single quartz substrate. The pump region consisted of a straight channel (3.0 cm x 230 microm x 100 microm) packed with 5-microm porous silica beads. The sol-gel stationary phase was derived from a precursor mixture of methyltrimethoxy- and phenethyltrimethoxysilanes and was synthesized in the downstream, field-free region of the microchip, resulting in a stationary-phase monolith with dimensions of 2.6 cm x 230 microm x 100 microm. Fluid dynamic design considerations are discussed, especially as they relate to integrating the EOF pump with the LC system. Pump and separation performance, as characterized by flow rate measurements, injection, elution, separation, and detection, point to a viable analytical chemistry platform that encompasses all of the benefits expected of portable, laboratory-on-chip systems, including reduced sample requirements and small packaging.

Towards an integrated microfluidic device for spaceflight clinical diagnostics Microchip-based solid-phase extraction of hydroxyl radical markers

A microchip-based solid-phase extraction method for biological fluid small molecule analysis has been developed. Using a commercially available copolymer packed into a microchip channel, extraction and preconcentration of 2,3-dihydroxybenzoic acid (DHBA) and 2,5-DHBA from saliva was achieved. The metabolites, formed from salicylic acid by reactive oxygen species, can be used as markers of oxidative stress. The results show high recovery of both metabolites (>90+/-15% for spiked saliva) with an 80-fold concentration enhancement possible. The eluent is directly analyzed using capillary electrophoresis, with good resolution for the two metabolites. This study demonstrates the feasibility of future integrated microdevices for spaceflight small molecule biomarker analysis.

Electrically actuated, pressure-driven liquid chromatography separations in microfabricated devices

Electrolysis-based micropumps integrated with microfluidic channels in micromachined glass substrates are presented. Photolithography combined with wet chemical etching and thermal bonding enabled the fabrication of multi-layer devices containing electrically actuated micropumps interfaced with sample and mobile phase reservoirs. A stationary phase was deposited on the microchannel walls by coating with 10% (w/w) chlorodimethyloctadecylsilane in toluene. Pressure-balanced injection was implemented by controlling the electrolysis time and voltage applied in the two independent micropumps. Current fluctuations in the micropumps due to the stochastic formation of bubbles on the electrode surfaces were determined to be the main cause of variation between separations. On-chip electrochemical pumping enabled the loading of pL samples with no dead volume between injection and separation. A mobile phase composed of 70% acetonitrile and 30% 50 mM acetate buffer (pH 5.45) was used for the chromatographic separation of three fluorescently labeled amino acids in <40 s with an efficiency of >3000 theoretical plates in a 2.5 cm-long channel. Our results demonstrate the potential of electrochemical micropumps integrated with microchannels to perform rapid chromatographic separations in a microfabricated platform. Importantly, these devices represent a significant step toward the development of miniaturized and fully integrated liquid chromatography systems.