A microfluidic distributor combining minimal volume, minimal dispersion and minimal sensitivity to clogging.

A new type of microfluidic flow distributor (referred to as the mixed mode or MM-distributor) is proposed. Its performance characteristics are determined using computational fluid dynamics (CFD), both in the absence and the presence of clogging, which is an important problem in microfluidic systems. A comparison is made with two existing, well-performing distributor types: the bifurcating (BF) distributor and an optimized diverging distributor, the so-called radially interconnected (RI) distributor. It was found that, in the absence of clogging, the MM-distributor produces only a little more dispersion than the bifurcating (BF) distributor, but much less than the radially interconnected (RI) distributor. The dispersion in an MM-distributor also follows a similar dependency on its width (power ≅ 2) as the BF-distributor. The dispersion in the RI-distributor on the other hand displays a very disadvantageous 4th-order dependency on its width, prohibiting its use to distribute the flow across wide beds (order of millimeters or centimeters). These observations hold independently of the flow rate. With increasing degree of clogging, the MM-distributor rapidly becomes advantageous over the BF-distributor, owing to the fluid contact zones that are provided after each bifurcation step. This means that overall, and when the occurrence of clogging cannot be excluded, the MM-type distributor seems to offer the best possible compromise between the ability to cope with local clogging events and the dispersion in the absence of clogging.

Chip-Based Multicapillary Column with Maximal Interconnectivity to Combine Maximum Efficiency and Maximum Loadability.

On the basis of our previous work on the design of pillar array columns for liquid chromatography, we report on a new pillar array design for high-efficiency, high volumetric loadability gas chromatography columns. The proposed pillar array configuration leads to a column design which can either be considered as a packed bed with perfectly ordered and uniform flow paths or as multicapillary columns (8 parallel tracks) with a maximal interconnectivity between the flow paths to avoid the so-called polydispersity effect (dispersion arising from the inevitable differences in migration velocity between parallel flow paths). Despite our relative inexperience with column coating, and most probably (not supported by data) suffering from the same problem of stationary phase pooling in the right-angled corners of the flow-through channels as other chip-based GC devices, the efficiencies obtained in a L = 70 cm long and 75 µm deep and 6.195 mm wide chip for, respectively, quasi-unretained and retained components (k = 7) went up to N = 60 000 and 12 500 under isothermal conditions using H2 as carrier gas and a downstream restriction. Under programmed temperature conditions (Ti = 80 °C, Tf = 175 °C at 30 °C/min, and a H2 flow of 0.4 mL/min), a peak capacity of 170 was obtained in 3.6 min. For retained compounds, the optimal flow rate is found to be on the order of 0.4 mL/min, achieved at an operating pressure of 2.3 bar. Intrinsically, the column combines the efficiency of a 75 µm capillary with the volumetric loadability of a 240 µm capillary.

Numerical study and theoretical performance limit of interconnected multi-capillary gas chromatography columns with perfectly ordered pillar patterns.

We present the results of a theoretical and numerical study of the chromatographic performance of a novel type of microfabricated GC column. The column consists of an array of rectangular flow diverters (pillars), creating a network of perfectly ordered, interconnected and tortuous flow-through paths. Using van Deemter and kinetic plots of simulated band broadening data, we could demonstrate that the proposed column structure performs as a bundle of parallel open-tubular capillaries with rectangular cross-section, connected by a regular pattern of channel-intermixing points that allow compensating for inevitable channel-to-channel differences in migration velocity without adding any significant dispersion themselves. The established kinetic plots also allowed to propose design rules for the optimal distance between the pillars as a function of the desired separation efficiency and the available column pressure. The simulations also allowed establishing an expression for the plate height as a function of the velocity of the carrier gas. Results are also compared to the results of a recent experimental study.

Kinetic plots for programmed temperature gas chromatography.

The applicability of the kinetic plot theory to temperature-programmed gas chromatography (GC) has been confirmed experimentally by measuring the efficiency of a temperature gradient separation of a simple test mixture on 15, 30, 60 and 120m long (coupled) columns. It has been shown that the temperature-dependent data needed for the kinetic plot calculation can be obtained from isothermal experiments at the significant temperature, a temperature that characterizes the entire gradient run. Furthermore, optimal flow rates have been calculated for various combinations of column length, diameter, and operating temperature (or significant temperature). The tabulated outcome of these calculations provide good starting points for the optimization of any GC separation.

Kinetic plots for gas chromatography: theory and experimental verification.

Mathematical kinetic plot expressions have been established for the correct extrapolation of the kinetic performance measured in a thin-film capillary GC column with fixed length into the performance that can be expected in a longer column used at the same outlet velocity but at either the maximal inlet pressure or at the optimal inlet pressure, i.e., the one leading to an operation at the kinetic performance limit of the given capillary size. To determine this optimal pressure, analytical solutions have been established for the three roots of the corresponding cubic equation. Experimental confirmation of the kinetic plot extrapolations in GC has been obtained measuring the efficiency of a simple test mixture on 30, 60, 90 and 120m long (coupled) columns.

Design and performance evaluation of a microfluidic ion-suppression module for anion-exchange chromatography.

A microfluidic membrane suppressor has been constructed to suppress ions of alkaline mobile-phases via an acid-base reaction across a sulfonated poly(tetrafluoroethylene)-based membrane and was evaluated for anion-exchange separations using conductivity detection. The membrane was clamped between two chip substrates, accommodating rectangular microchannels for the eluent and regenerant flow, respectively. Additionally, a clamp-on chip holder has been constructed which allows the alignment and stacking of different chip modules. The response and efficacy of the microfluidic chip suppressor was assessed for a wide range of eluent (KOH) concentrations, using 127 and 183µm thick membranes, while optimizing the flow rate and concentration of the regenerant solution (H2SO4). The optimal operating eluent flow rate was determined at 5µL/min, corresponding to the optimal van-Deemter flow velocity of commercially-available column technology, i.e. a 0.4mm i.d.×250mm long column packed with 7.5µm anion-exchange particles. When equilibrated at 10mM KOH, a 99% decrease in conductivity signal could be obtained within 5min when applying 10mM H2SO4 regenerant at 75µL/min. A background signal as low as 1.2µS/cm was obtained, which equals the performance of a commercially-available electrolytic hollow-fiber suppressor. When increasing the temperature of the membrane suppressor from 15 to 20°C, ion suppression was significantly improved allowing the application of 75mM KOH. The applicability of the chip suppressor has been demonstrated with an isocratic baseline separation of a mixture of seven inorganic ions, yielding plate numbers between 5300 and 10,600 and with a gradient separation of a complex ion mixture.