EOF-induced flow

The EOF-induced flow can be achieved without differential coating on the glass channel. The flow can be formed when the solution flowing in the main channel is at a high concentration and that in the side stream is at a low concentration. The induced flow is greater for a higher concentration difference [389]. The EOF-induced flow has been employed to eject or withdraw reagents via apertures (rather than side channels) for the development of an artificial synapse chip (to eject neurotransmitter molecules) [344]. 

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The EOF can induce a liquid flow in a field-free region, and this is termed as the EOF-induced flow. This flow can be generated at a T-intersection [816] or near a thin gap [385,386]. At a T-intersection, EOF was first initiated to a side arm, rather than to the main channel. However, when the side arm was coated to reduce EOF, a flow was induced in the field-free main channel, resulting in an EOF-induced flow (see Figure 3.4). This strategy has been used to maintain a stable electrospray for MS analysis [816]. 

The EOF-induced flow has been amplified by using multiple capillary channels (of width 1-6 pm), so that the multiple flow streams are combined to produce adequate hydraulic pressure for liquid pumping (see Figure 3.7) [115]. The multiple channels (100) ensure the generation of sufficient flow rate (10-400 nL/min), while the small dimensions (of depth 1-6 pm) result in the necessary hydraulic pressure to prevent pressurized backflow leakage (up to 80 psi) [115]. Based on a similar approach, a narrow-gap EOF pump was constmcted to produce 400 Pa pressure with 850-nm-deep channels cascaded in three stages to produce a 200-pm/s flow velocity [390]. Another pump was constructed with 130-nm-thin channels cascaded in 10 stages to produce 25 kPa pressure [264]. 

Flat-edge emitter, glass chip EOF induced flow 1.5 nL/s ESI ITMS Tetrabutylammonium iodide, 10 jxM N/A Nil 816... 

An active mixer based on an oscillating EOF induced by sinusoidal voltage ( 100 Hz, 100 V/mm) was devised and modeled for mixing of fluorescein with electrolyte solutions. This is termed as electrokinetic-instability micromixing, which is essentially a flow fluctuation phenomenon created by rapidly reversing the flow. Various microchips materials (PDMS, PMMA, and glass) and various electrolytes (borate, HEPES buffers) have been used to evaluate this method of micromixing [480]. 

The use of hydrothermally formed retaining frits in capillary columns packed with stationary phase particles is an accepted limitation in CEC. The introduction of the frit to hold the packed bed is vital, yet introduces problems such as EOF and flow non-uniformities, compromised frit permeability [87], capillary fragility, increased likelihood of bubble formation [88] and a thermally induced modified frit surface chemistry which can detrimentally alter the chromatography [23]. Practical aspects to be considered include the appreciable effort and skill of the analyst who is required to repeatably manufacture capillaries of a particular phase and redevelop the fritting and packing methodology for each different stationary phase type. 

The pressure/vacuum, the EOF, hydrodynamic pressure (leveling of the inlet and outlet vials), etc. may be used as a driving force for counter-mobilities in this technique [15]. An interesting remark regarding the potential of a technique similar to the above mentioned was made by Dovici et al. as early as 1990 If pressure-induced flow balances the average velocity of two closely spaced analytes, then the resolution of the analytes becomes infinite [32]. 

In addition to the general improvement of transfer in micro reactors, there is evidence that the voltage of electroosmotic flow (for EOF see [14]) in combination with the large internal surface area in glass chips can induce hydroxide ion formation [6]. Concerning catalyst loss, there is no obvious direct correlation rather, micro reactors can act as mini fixed beds fixing heterogeneous catalyst particles. 

In electrophoresis there are two main contributions to the movement of analyte ions first, the electrophoretic mobility of the analyte ion itself and, second, the speed and direction of the electroosmotic flow (EOF). The EOF may be induced to travel in the same direction as the analyte ions or in the opposite direction from the analyte ions, or it can be suppressed so that the flow is negligible. Figure 4.2 shows the classification of electrophoresis according to the contribution of the electroosmotic flow. 

The main problem encountered with PFT with a neutral selector remains the prevention of the chiral selector from entering the ionization source. This problem becomes particularly important at a high pH where EOF is important. Therefore, an acidic buffer pH or a coated capillary to minimize EOF is of the utmost importance. In addition, it has to be noted that the electrospray ionization process is pneumatically assisted when a sheath-liquid interface is used. The coaxial sheath gas induces an aspirating phenomenon in the capillary which may considerably affect the separation quality. This can be due to the decrease in interaction between analytes and the chiral selector and to a hydrodynamic flow induced by the Venturi effect at the capillary end [33, 34]. 

Cations from the bulk solution are attracted to the capillary surface. These cations are attracted toward the cathode. Since the cations are hydrated and the inner diameter of the capillary is quite small, their migration induces a bulk liquid flow (a plug flow). This means that all of the liquid and solutes in the capillary flow at the same rate towards the cathode (electroosmotic flow). The mobility of solute resulting from EOF is termed the electroosmotic mobility (/i<>s). 

EOF- or chemically-induced mobilization of the focused zones past the detector, Guillo et al. illustrated the use of on-chip pumping, generated by elastomeric diaphragm pumps in a three-layer device (one PDMS layer sandwiched by two glass layers), to mobilize the zones. The authors explored the effect of mobilization flow rates on the separation of two amino acids, L-lysine and L-histidine the electropherograms can be seen in Figure 10.12. 

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