On-chip SPE

Microfluidic devices, study of porosity, hydrodynamic properties and on-chip SPE... 

Microfluidics and miniaturization hold great promise in terms of sample throughput advantages [100]. Miniaturization of analytical processes into microchip platforms designed for micro total analytical systems (/i-TASs) is a new and rapidly developing field. For SPE, Yu et al. [123] developed a microfabricated analytical microchip device that uses a porous monolith sorbent with two different surface chemistries. The monolithic porous polymer was prepared by in situ photoinitiated polymerization within the channels of the microfluidic device and used for on-chip SPE. The sorbent was prepared to have both hydrophobic and ionizable surface chemistries. Use of the device for sorption and desorption of various analytes was demonstrated [123]. 

The coupling of microfabricated on-chip SPE devices to MALDI-MS has previously been presented by our group. Another approach to on-chip SPE—the microfluidic compact disk Gyro Lab MALDI SPl (Gyros AB, Sweden)— has been presented and commercialized by Gyros AB, where centrifugal force is used to transport liquids for the purpose of SPE before MALDI-MS. EWOD and electrocapture are techniques that can be used instead of SPE for purification and concentration before MALDI-MS. 

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... 

A microchip device with an attached nano-ESl emitter tip was developed to facilitate the introduction of tiyptic digests by means of nano-ESl [88-89]. Instead of off-line filling of the nano-ESl needle, the sample is transferred from a vial on the chip to the nano-ESl needle by electroosmosis. Detection limits of 2 fmol/pl were achieved for fibrinopeptide A (1699 Da). Further developments enabled sequential automated analysis of protein digests by ESl-MS [90]. On-chip sample pretreatment and desalting by either sample stacking via polarity switching or SPE prior to on-chip CE was described by Li et al. [91], and applied to the identification of 2D-GE separated proteins from Haemophilus influenzae using a Q-TOF instrument. 

Efforts toward integrating SPE onto a lab-on-a-chip device are currently being investigated by the Collins group. Two complementary approaches are being pursued. One approach is to use small-diameter, Cl8 functionalized silica beads that are packed into a microchannel to form an extraction bed [46], A sample solution containing trace levels of explosives is electrokinetically directed across the microcolumn bed, causing the hydrophobic explosive molecules to adsorb onto the stationary phase with nearly 100% efficiency. Subsequently,... 

DNA extraction and purification were traditionally accomplished using organic extraction and ultracentrifugation-based procedures, which are both time-consuming and not easily transferable to the microscale. Newer methods employ solid-phase extraction (SPE) on silica surfaces, glass fibers, modified magnetic beads, and ion-exchange resins—techniques that save time and are also more amenable to chip applications. 

Fats, oils, and lipids are common components of meats, nuts, and dairy products and manufactured goods, such as potato chips, cookies, and chocolate. They are soluble in nonpolar solvents, such as hexane and methylene chloride. The analyte, of course, should also be soluble in the extraction solvent. Typically normal-phase SPE would be used to retain a compound from this extraction solvent. A solid fat may be homogenized in a blender with hexane, filtered or centrifuged, then the solvent would be passed through a normal-phase column for retention of the solute. Another approach is the use of matrix solid-phase dispersion, where the solid would be ground into the silica and C-18 directly, then the analyte eluted directly from the ground mixture with either hexane or methylene chloride. The hexane or methylene chloride extract could then be applied directly to a normal-phase sorbent for separation. Liquid oils may be directly diluted with hexane or methylene chloride and applied to the normal-phase sorbent. Other lipid substances may be handled either as solids or liquids depending on their form. 

To make any MALDI microspot technique (e.g., the spot-on-a-chip method) to work optimal, the sample has to be devoid of contaminants. Purification and enrichment of proteins and peptides is conveniently performed by SPE. Previously, standard commercial SPE options for miniaturized SPE or tips packed with beads were used for purification and enrichment before the spot-on-a-chip ... 

The development of MIP composite materials based on thin MIP films for improving separation s performance has attracted increasing attention. Examples include tailored particles for SPE or chromatography and coated tubes or capillaries for electrophoretic and other separations. The latter formats will also gain importance in lab-on-a-chip systems [111]. 

Fig. 3 Schematic workflow of porous silicon-ISET iMALDI top (I), antibody-immobilized porous silicon was used to capture the antigen angiotensin I bottom (II), RP solid-phase extraction sample preparation protocol on the ISET chip was used to purify and reconcentrate the elution followed by MALDI MS deteetion (Yan et al. 2011) (Reprinted with permission from Analytical Chemistry, 83, Yan, Ahmad-Tajudin, Bengtsson, Xiao, Laurell, Ekstrom, Noncovalent antibody immobilization on porous silieon eombined with miniaturized solid-phase extraction SPE) for array based immunoMALDI assays, 4942-4948. Copyright 2011 Ameriean Chemieal Society)...

Screen-printed electrodes (SPEs) are devices that are produced by printing different inks on a suitable substrate. Through the application of this technique, it is possible to fabricate a chip containing working, reference, and auxiliary electrode or even multisensory array. Electrode fabrication requires an ink as a precursor, which is typically a mixture containing at least three compraients" (see also Chap. 16 of Volume I) ... 

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