Microchannel chromatography

Protein toxins such as botulism, staphylococcal enterotoxin B, or ricin can be separated with gas or liquid chromatography, electrophoresis, or a combination. The pChemLab (Sandia National Laboratories Albuquerque, NM) series of instruments includes a hand-held Bio Detector. Proteins in the sample are labeled with fluorescent tags, and nanoliter volumes of samples are separated by microchannels etched into a glass chip. The separation occurs as the sample moves through the channels and identification is based on retention times. The analyses can be completed within 10 min. 

As in the case of normal chromatography both stationary and mobile phases are also required in NLC. On the other hand, in NCE hydrophilic channel walls with improved control over electroosmotic flow are required for better separation of biological samples. Briefly, the separation efficiencies and selec-tivities in NLC and NCE depend on the properties of the microchannels, and, therefore, surface modification of the microchannel is usually necessary to achieve good separation of a variety of analytes. Recently, Muck and Svatos... 

Affinity chromatography of streptavidin was performed on a PET chip. The microchannel was first filled with the dual-modified latex beads (as shown in Figure 6.3). The biotinylated beads were surface-modified with a temperature-sensitive polymer, poly(N-isopropylacrylamide (PNIPAAm, 11 kDa). When the temperature was raised above the lower critical solution temperature (LCST) of PNIPAAm, the beads aggregated and adhered to the channel wall, because of a hydrophilic-to-hydrophobic phase transition. Then streptavidin from a sample solution was captured by these adhered biotinylated beads. Thereafter, when the temperature was reduced below the LCST, the beads dissociated and eluted from the channel wall together with the captured streptavidin [203],... 

Dutta, D., Leighton, D.T., Jr., Dispersion in large aspect ratio microchannels for open-channel liquid chromatography. Anal. Chem. 2003, 75, 57-70. 

Most biocatalytic conversions are performed with the enzyme immobilized in the microreactor. Miyazaki et al. [426] developed a simple noncovalent immobilization method for His-tagged enzymes on a microchannel surface. These enzymes contain a polyhistidine-tag motif that consists of at least six histidine residues, often located at the N- or C-terminus. The H is-tag has a strong affinity for nickel and can be reversibly immobilized by a nickel-nitrilotriacetic acid (Ni-NTA) complex (Scheme 4.103), a strategy commonly used in affinity chromatography. 

An interesting article by Anderson has described the potential of AFM in chromatography and microfluidics as it could provide shear-driven pumping of fluid, a mechanism for injecting samples, imaging of the liquid surfaces in the microchannels and, finally, removal of samples for further spectral analysis. ... 

Bouffard, S. P., Katon, J. E., Sommer, A. J. Danielson, N. D. (1994) Development of microchannel thin layer chromatography with infrared microspectroscopic detection. Analytical chemistry 66, 1937-1940. 

Han, J. and Singh, A. K., Rapid protein separations in ultra-short microchannels microchip sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing. Journal of Chromatography A 1049, 205-209, 2004. 

Regnier FE, He B, Lin S, Busse J (1999) Chromatography and electrophoresis on chips critical elements of future integrated, microfluidic analytical systems for life science. Trends Biotechnol 17 101-106 Cabrera CR, Yager P (2001) Continuous concentration of bacteria in a microfluidic flow cell using electrokinetic techniques. Electrophoresis 22(2) 355-362 5. Xuan X, Li D (2005) Focused electrophoretic motion and selected electrokinetic dispensing of particles and cells in cross-microchannels. Electrophoresis 26(18) 3552-3560... 

The dominance of electrophoresis over chromatography has been a trend in separations with Lab-on-a-Chip devices. The reason is that from an engineering point of view, it is easier to apply a voltage across the terminals of microchannels than the application of a pressure difference. In CE, electrokinetic control of fluid transport eliminates the need for external components such as pumps and valves. Furthermore, miniaturization of chromatography systems involves technical challenges that are usually not necessary in CE. However, LC is the most used separation technique in conventional systems. Therefore, investigation for implementing this technique in Lab-on-a-Chip devices is an active trend. 

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