Extraction microchannel

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,... 

In the ESy, a miniature FS membrane is supported by two small, identical pieces of PP plastic, constituting a miniaturized membrane unit called an ESy extraction card (see the inset in Figure 4.7), which is housed under mechanical pressure in a card holder. The two PP pieces have dimensions of 2 mm x 20 mm x 40 mm. The inner surface of each piece contains a machined groove defining a microchannel of 1.65 pL volume (0.125 mm depth x 0.6 mm width x 22 mm length). The very small piece of FS membrane (2 mm width x 22 mm length x 25 pm thickness) is fastened in... 

Separation of proteins extracted from Jurkat cell lysates was achieved on a microchannel on a PMMA chip. Pressurization has been used prior to CE to improve resolution, as described in section 6.2.2. An additional merit is that transient size separation occurred during pressurization. During subsequent CE separation, IEF occurred in the microchannel. This combination of transient size separation and IEF improved resolution, which is akin to 2D separation [619]. 

DNA extraction has been achieved using silica beads (5 pm), which were packed in a glass microchannel and held by a sol-gel [913]. This method provided more reproducible extraction than a previous method in which extraction was performed without physically fixing the beads in microchambers [639]. 

Kim, H., Ueno, K., Chiba, M., Kogi, O., Nobom, K., Spatially-resolved fluorescence spectroscopic study on tiquid/tiquid extraction processes in polymer microchannels. Anal. Sci. 2000, 16, 871. 

In order to produce oxime 199 in greater quantities, the authors subsequently evaluated the use of DMF as the reaction solvent due to the increased solubility of the nitrite precursor 277 (36 mM). In conjunction with two serially connected microreactors, each containing 16 microchannels [1,000 pm (wide) x 500 pm (deep) x 1.0 m (length)] and eight black lights, the photochemical synthesis was performed continuously for 20h at a flow rate of 250 pi min 1 (residence time = 32 min). After an off-line aqueous extraction and silica gel column, 3.1 g of the oxime 199 was obtained equating to an isolated yield of 60% and successfully demonstrating the ability to use photochemical synthesis for the scalable preparation of pharmaceutically relevant compounds. 

The electrons are accelerated across this low pressure region between the plasma chamber and the adjoining ionization chamber to typically 100 eV, the optimum for the ionization of most gases. They are focused to the input slit into ionization chamber, into which the sample gas is introduced via another capillary, alternatively via a microchannel chip, which reduces the pressure from atmospheric again to about 15 Pa, the pressure for maximum ionization efficiency for the chamber diameter of also 150 pm. Analyte ions are extracted from the ionization chamber through a second slit on the opposite side into an ion optic and are then accelerated into the mass separator to an energy of typically 100 eV. 

Figure 9 Layout of the inductively coupled plasma time-of-flight mass spectrometer (ICP-TOF-MS) with improved vacuum chamber and ion optics. SI, second-stage extraction lens / , repeller C, Faraday cup Gl, G2, TOF-MS entry grids FI, Y2, steering plates D1, D2, deflection plates G3, reflectron entrance grid / 1, deceleration grid / 2, reflecting grid MCP, microchannel plate. (From Ref. 27.)...

Fig. 2.2. Schematic diagram of a reflectron-type time-of-flight mass spectrometer and the look. The polarization of the fundamental pulse of 0.8 pm was parallel to the flight axis. Typical applied voltages for ion extraction and the microchannel plate (MCP) were 3kV and —2.1kV, respectively...

In the latter (washing) area, the m-xylene phase containing the Co chelates and the coexisting metal chelates from the former (reaction and extraction) area is interposed between the HC1 and NaOH solutions, which were introduced through the other two inlets at a constant flow rate. Then a three-phase flow, HCl/m-xylene/NaOH, forms in the microchannel. The decomposition and removal of the coexisting metal chelates proceed along the microchannel in a similar manner as described above. Finally, the target chelates in m-xylene are detected downstream by TLM. 

The advantages of this approach compared with conventional methods are simplicity and omission of troublesome operations. The acid and alkali solutions cannot be used simultaneously in the conventional washing method, but this becomes possible by using three-phase flow in the microchannel. This chemical processing corresponds to the integration of eight MUOs on a microchip, two-phase formation, mixing and reaction, extraction, phase separation, three-phase... 

Figure 16.6 A simplified schematic of a time of flight spectrometer and the principle of the ion reflector (reflectron). (1) sample and sample holder (2) MALDI ionization device by pulsed laser bombardment (3 and (3 ) ions are formed between a repeUer plate and an extraction grid (PD 5000V) then accelerated by an other grid (4) control grid (5) microchannel collector plate (6) signal output. Below, a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass but which have initially different energies. The widths of the peaks are of the order of 10 and the resolution ranges between 15 to 20 000.

Polymeric microfluidic systems coupled to a microfabricated planar polymer tip can be used as a stable ion source for ESI-MS. A parylene tip at the end of the microchannel delivers fluid which easily produces a stable Taylor cone at the tip via an applied voltage. The described device appears to facilitate the formation of a stable spray current for the electrospray process and hence offers an attractive alternative to previously reported electrospray emitters. When this interface was employed for the quantification of methylphenidate in urine extracts via direct infusion MS analysis, this system demonstrated stable electrospray performance, good reproducibility, a wide linear dynamic range, a relatively low limit of quantification, good precision and accuracy, and negligible system carryover. We believe polymeric devices such as described in this report merit further investigation for chip-based sample analysis employing electrospray MS in the future. 

In phase separation two immiscible fluids are physically separated. Microchannels offer the ability to separate phases in an orientation-independent manner, since capillary and surface tension forces are more dominant in these high-surface-area devices. Various microchannel phase separators have been developed to separate organic and aqueous phases for use in unit processes such as solvent extraction or reactions conducted at an aqueous organic interface [185-188]. The approach is to hydrophobize half of the channel with a non-polar agent so that the organic phase is constrained to the hydrophobic half and the aqueous phase to the hydrophilic half Phase separation is simply then a matter of splitting the flow at the hydrophobic-hydrophilic junction of the flow. 

In addition to electrophoresis-based collection of DNA in microchannels (see Section 7.7.3), adsorbents have been used to collect and purify DNA. In these cases, microchannels are generally used because of the small quantity of material requiring collection. Using solid phase extraction, sorbent particles of nano- and micro-silica and micro-sized octadecylsilica were immobilized using sol-gel chemistry to hU the microchannels of the microfluidic device [194]. DNA as well as several organic compoimds were evaluated for adsorption and desorption. They showed excellent adsorption, but poor recovery because they were difficult to extract. 

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]. 

A study conducted by Zhao et al. created intricate patterns of hydrophilic and hydrophobic regions down the microchannel to improve extraction [205]. This was achieved by laying down photocleavable SAM (self-assembled monolayer) and exposing with UV light to create exposed hydrophilic regions. The unexposed regions remained hydrophobic. If pressure is controlled properly, once patterned, the aqueous phase will remain separated from the organic phase. 

A microchannel contactor has been developed and tested with water and cyclohexane streams extracting cyclohexanol [199]. Using this device, the relative importance of mass transfer resistance in the flow channels versus the contactor plate was explored. Both micromachined contactor plates and commercial polymeric membranes were configured with various channel heights both on the feed and solvent sides. Data indicate that contactor plate mass transfer becomes... 

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). 

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