Microchannel devices

The microchannel or microfluidic devices used for flow switching in GC are new developments which became available due to progresses made in precise laser machining capabilities of metal films and appropriate metal surface deactivation solutions. The features required for these microchannels devices are laser cut into metal sheet (shims) with thicknesses from 20 to 500 pm. The resulting channel dimensions are similar to the conventionally fused silica 

The metal surface deactivation provides flow chaimels that are inert even to reactive analytes at trace levels. The inertness achieved is substantially better than can be achieved on conventional inlet liner glass surfaces and is similar to the inertness of deactivated fused silica columns. 

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Flow is typically laminar in microchannel devices, although not always rigorously so. Correlations for fully developed laminar flow in perfectly rectangular microchannels have been validated in the literature [33-35]. Transition and turbulent flows in a microchannel have no such consistent treatise, and are highly dependent upon channel shape, aspect ratio, and surface characteristics [36, 37]. 

Furukawa, K, Nakashima, H.. Kashimura, Y. and Torimitsu, K (2006) MicroChannel device using self-spreading lipid bilayer as molecule carrier. Lab on a Chip, 6, 1001-1006. 

Figure 26-39 MicroChannel device for replication and analysis of DNA. Channels are 7 10 p.m deep and 40-45 wide. [From J. Khandurina, T. E. McKnlght, S. C. Jacobson, L C, Waters. R. S. Foot, and J. M. Ramsey, "Integrated System for Rapid-PCR-Based DNA Analysis in Microfiuidic Devices." Anal. Chem. 2000, 72.2995.]... 

FIGURE 5.11 Schematic representation of a dialysis chip that is formed by sandwiching a dialysis membrane (MW cutoff 8000) between the sample channel (160 pm wide) and buffer channel (500 pm wide) after alignment of the microchannel device [811]. Reprinted with permission from the American Chemical Society. 

Photocatalytic reduction using a Ti02-coated microchannel device was reported by Ichimura et al. [44], By using a quartz microreactor (microchannel, 500 pm wide, 100 pm deep and 40 mm long) and a 365-nm UV-LED light source, benzaldehyde was reduced to benzyl alcohol (yield of 11%) and p-nitrotoluene to p-toluidine (yield of 46%) after 1 min in the presence of ethanol (Scheme 4.30). 

Adsorption processes can benefit from microchannel devices, because the apparent adsorption kinetics in conventional reactors are often much slower than intrinsic adsorption kinetics as a result of thermal and diffusional resistances. Microchannels provide short transfer distances for both mass and energy. 

Fig. 7.15 Adsorption and desorption processes fora temperature swing adsorption (TSA) system. By using microchannel devices, the thermal mass is low and heat transfer is rapid, resulting in a small device [195].

Most work in microchannel extraction focuses on improving the extraction efficiency or the phase separation or system development through adding multiple imit operations on a single chip or by scaling up. Because laminar flow exists in the microchannel devices, the intimate mixing of turbulent flow in traditional contactors is not present. Most studies have shown that the dissimilar phases flow parallel to each other with movement of solute molecules caused by molecular diffusion only. Thus, extraction is governed by contact time between phases [202]. 

Phase separation improvements are based on either surface modification, fluid property control, or physical separation. Studies have shown that organic liquid membranes can be developed in a microchannel device using surface modification [207,208]. An organic liquid membrane consists of an organic phase with aqueous phases on either side. An analyte can be extracted from the aqueous phase, into the organic phase and then back-extracted into the second aqueous phase. These three phases can flow stably within a single microchannel, but better separation of the three phases is possible with surface modification of the organic phase channel (Fig. 7.16). 

A microchannel device has been developed for coalescence of dispersed droplets after solvent extraction [211], Results of the study showed that the microchannel depth must be smaller than the diameter of the oil droplets for coalescence to occur. By using rectangular plates and controlling the contact angle of the liquid at the surface of the wall, the speed of the dispersed phase can be slowed with respect to the continuous phase, allowing coalescence [212],... 

Even studies involving cell shape and function [283] and cell membrane potential [284] are best performed in a microchannel device. The microchannels allow the cells to elongate and be imaged without high shear flow. The microchannel devices for measuring flow are used to minimize the amount of reagent and control its mixing. 

Several uses have been developed for electric helds in microchannel devices ... 

Okubo Y, Toma M, Ueda H, Maki T, Mae K (2004) MicroChannel devices for the coalescence of dispersed droplets produced for use in rapid extraction processes. Chem Eng J 101 (l-3) 39-48... 

Yamada YMA, Watanabe T, Torii K et al (2010) Palladium membrane-installed microchannel devices for instantaneous Suzuki-Miyaura cross-coupling. Chem Eur J 16 11311-11319... 

Cell responses to physical or chemical cues are measured in microfluidic devices primarily via optical or electrochemical means. Huorescence is the most widely used optical detection technique, because absorbance detection (commonly used for macroscale assays) is of limited value in microchannels because of the short path lengths. Fluorescence detection, characterized by its unparalleled sensitivity, is easy to implement in microfluidic systems. Chemiluminescence and bioluminescence also offer low detection limits and have less background noise than fluorescence [8]. Electrochemical detectors are even more easily integrated with microfluidic devices and often are much less expensive than optical systems. However, fabrication of electrodes in microchannel devices is a technical challenge, and the electrical fields used in detection can interfere with on-chip processes such as electrophoresis. Electrochemical techniques include potentiometry, amperometry, and... 

Waveguides have also been integrated into microchannel devices for fluorescence detection. Silica on silicon chip technology was commonly used to form the heart of a fiber for fluorescence measurements. 

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