Channeling, solid-phase extraction

Pushing detection limits of nitroaromatic explosives into the parts per trillion (ppt) level requires sample preconcentration. Collins and coworkers used solid-phase extraction (SPE) of explosives from sea water which was followed by rapid on-chip separation and detection [18]. Explosives were eluted from SPE column by acetonitrile and were injected in the microchip separation channel. Lab-on-a-chip analysis was carried out in nonaqueous medium. The mixed acetonitrile/methanol separation buffer was used to produce the ionized red-colored products of TNT, TNB and tetryl [27,28]. The chemical reaction of the bases (hydroxide and methoxide anions) with trinitroaromatic explosives resulted in negatively charged products, which were readily separated by microchip... 

Tollback, J., Tamburro, D., Crescenzi, C. and Carlsson, H. (2006) Air sampling with Empore solid phase extraction membranes and online single-channel desorption/ liquid chromatography/mass spectrometry analysis determination of volatile and semi-volatile organophosphate esters. Journal of Chromatography A, 1129,1-8. 

Figure 5.8 Schematic representation of a chip-based solid phase extraction-MEKC device, (a) Layout of the entire device and (b) expanded view of the extraction region of the device. The dotted lines represent the direction of fluid flow during extraction the solid lines signify flow during elution/injection (Narrow channels are 55 pm wide, column chamber is 210 pm wide, with all channels 15 pm deep.) [87].

With the need to provide PCR-amplifiable DNA, multiple approaches for incorporation of the extraction protocol onto microchips were examined. Recent development includes the implementation of a solid-phase extraction of DNA on a microchip [49]. The extraction procedure utilized was based on adsorption of the DNA onto bare silica. The silica beads were immobilized into the channel using a sol-gel network. This method made possible the extraction and elution of DNA in a pressure-driven system. 

Both liquid-phase and solid-phase extractions have been used for sample preconcentration or cleanup. For instance, an OTS-coated glass channel was employed for liquid-phase extraction, leading to an enrichment of samples (the... 

V.M.A. Hakkinen, Analysis of chemical warfare agents in water by solid phase extraction and two-channel capillary gas chromatography, J. High Res. Chromatogr., 14, 811-815 (1991). 

Solid-phase extraction in disk format has been designed to overcome the limitations of the conventional packed-bed SPE columns. Disks exhibit minimal channeling, have small void volumes, do not require frits, have low capacity for interference (cleaner extracts are provided), and capture analytes of interest very effectively. They require lower solvent volumes, providing a faster sample processing and an increased throughput. Disk products... 

In contrast, the mode of operation of a number of other modules in microfluidic devices, such as reactors, solid-phase extraction, and separation units require interactions of molecules in solution with functionalities attached to a solid support. Obviously, this support can itself be the channel wall but, in this case, the overall surface-to-volume ratio is very small. This ratio can be increased by filling the channel with a solid phase, and the monolithic materials represent one of the options to fill the channel with a suitable stationary phase. 

These cyclohexa-(-hepta-, -octa-)amyloses are arranged in the crystal lattice of C. in such a way that open, intramolecular channels are formed in which guest molecules can be enclosed in varying amounts up to saturation ("molecular encapsulation ), e.g., gases, alcohols, or hydrocarbons. a-C. also forms a blue-colored inclusion compound with iodine in which the iodine atoms are arranged like a string-of-pearls in the channels. On account of this property C. are used in the production of food, cosmetics, pharmaceuticals, and pesticides as well as for solid phase extractions and for use as high performance separation phases for enantiomeric and diastereomeric mixtures. 

Recently, Wang et al. [9] developed a microfluidic device that could perform electroki-netic sample fractionation and solid-phase extraction simultaneously. Effluents from a fractionation were subsequently delivered into an array of 36 collection channels containing butyl methacrylate-based monolithic polymer for sohd-phase extraction. Tightly focused sample zones of protein sample could be obtained after electroki-netic fractionation in each of the 36-channel polymer beds without observed cross contamination, indicating great promise for automated operation of these devices in proteomics research. 

As illustrated by the examples given in Table 3, the application of labs-on-chips to real samples is still limited. This is partly due to the fact that the analytical assay is only the final step of the whole procedure, which includes sample pretreatment protocols such as filtration, analyte cleanup, or analyte preconcentration. However, also the integration of corresponding microfabricated elements is described. Filtration was achieved by porous membranes or arrays of thin channels preventing particulates to enter the analytical device. Analyte preconcentration in combination with removal of other sample constituents is achieved by solid-phase extraction modules, which are either capillaries or beads coated with a suitable adsorbent, such as a Cl 8 phase originating from coating with octa-decyltrimethoxysilane, from which the analyte is... 

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