Flow-gated injection

Figure 9.7 Schematic illustration of the flow-gating interface. A channeled Teflon gasket was sandwiched between two stainless steel plates to allow for flow into the electrophoresis capillary, either from the flush buffer reservoir or from the LC microcolumn during an electiokinetic injection.

The improved design of the gating interface resulted in precise alignment of the two capillaries. A colored dye solution was added to the HPLC eluent to allow for du cct observation of the flow gating and injection processes. Through observation of the movement of the dye through the interface, it was possible to ensure that the electrokinetic injections were performed correctly. Troubleshooting had been a... 

FIGURE 5.9 The flow gating interface from Hooker and Jorgenson (1997). The cross-flow of buffer prevents LC effluent from electromigrating onto the CE capillary until an injection is desired. This figure is used by permission of the American Chemical Society. 

FIGURE 16.6 Schematic of the clear flow gating interface. The interface was constructed in-house from a 1 in. diameter, 0.5 in. thick Lexan disk. The disk is clear, which allows direct observation of the capillaries in the stream of flush buffer. The capillaries are sleeved in 0.0625 in. o.d. Teflon tubing and this tubing is held in place by Lite Touch fittings (not shown). The cross-flow of buffer prevents LC effluent from electromigrating onto the CZE capillary until an injection is desired (reprinted with permission from Analytical Chemistry). 

A chip-based integrated precolumn microreactor with 1 nl reaction volume has been explored by Jacobson et al. [67]. The reactor is operated in a continuous manner by electrokinetically mixing of sample (amino acids) and reagent (o-phthaldialdehyde) streams. The reaction time is adjusted via the respective flow velocities. By switching of potentials, small plugs of the reaction product were injected into a 15.4 mm separation channel in a gated injection scheme (< 1.8% RSD in peak area). The separation efficiency achieved was relatively poor, however, electrokinetic control of reaction time (and yield) permitted to monitor the kinetics of the derivatization under pseudo first-order conditions. A similar integrated precolumn reactor operated in a stopped flow mode has been described by Harrison et al. [68]. 

In most cases, sample introduction on-chip is achieved using electrokinetic (EK) flow [3]. Two important EK injection modes, namely, pinched injection and gated injection, have been developed. Furthermore, some alternative injection methods are described. 

For continuous sample introduction, gated injection was adopted. With EK flow, the analyte continually flowed in parallel with a separation buffer to the analyte waste reservoir (see Figure 4.15). Injection of the sample analyte was achieved by interrupting the flow of the buffer for a short time (known as the injection time) so that the analyte stream was injected. This scheme was achieved by four reservoirs (without considering the reagent reservoir) and two power supplies [317], Gated injection has also been achieved using one power supply and three solution reservoirs [564]. 

FIGURE 4.15 Schematic diagram of flow pattern for a gated injector. CCD images of the gated injection using rhodamine B (b) prior to injection, (c) during injection, and (d) after injection into separation column with E = 200 V/cm [317]. Reprinted with permission from the American Chemical Society. 

An optically gated injection was demonstrated for the CZE separation of four amino acids labeled with 4-chloro-7-nitrobenzofurazan (NBD-F) in a one-channel chip [576] or a four-channel chip [577]. The gating beam was used to continuously photobleach the sample, except for a short time during injection by interrupting the beam (100-600 ms) using an electronic shutter. With only a sample reservoir and a waste reservoir, the sample continuously flowed electrokinetically. Six consecutive separations of the same sample mixture have been accomplished in under 30 s [576,577],... 

The voltage applied on the gate induces the n- or p-channels with the accumulation of carriers (electrons for n-channel and holes for p-channel). The bias of source/gate injects the same type carriers into the channels. The bias of gate/drain creates a field that pulls the majority of these carriers into drain. Thus the device is turned on with current flowing through the source and drain regions. 

In a further development, Schlund et al. have conceived a hydrodynamic gated injection scheme [52] similar to that developed for electro kinetic flows by Ramsey s group. In this method, an injection cross is formed by the intersection of the separation channel with a bypass channel, analogous to the analyte loading channel in 4-port CE and CFG chips. Continuous streams of sample and mobile phase are fed into the chip hydrodynamically, while at the cross the flows converge in such a way that the mobile phase stream is diverted into the separation channel, and the sample stream is forced into the bypass channel. At the confluence of the... 

Fig. 9.7.18). The external sample source was left to float electrically (no grounding necessary), which was to make a more compatible interface with real external process sampling equipment such as a microdialysis probe head for bioreactor sampling. In addition, a gated injection method was introduced to accommodate this conhguration. In this design, the buffer and sample streams flow towards each other in orthogonal directions in the microchannels, then meet at the cross in a fashion similar to Fig. 9.7.7.

Fig. 9.7.18 Another example of continuous sampling CE chip for on-line analysis, designed by Y-H. Lin s group at the National Cheng Kung University, Tainan, Taiwan. This design allows for continuous flow through the chip via the SIC, with continuous sampling from the SIC. Gated injection is used to perform analysis at will.

Easy or high flow materials are important for such applications where the parts could be large, requiring multi-gate injection, and good appearance at weld lines is desirable so that... 

Injection methods for microfluidic experiments are different, but often based on similar principles. For example, gated injections (either via a perpendicular cross-flow or via an optical gate) that are similar to their CE counterparts are used for high-speed separations on microdevices. Often these injection times are fast, less than 5 s is typical, and since the sample remains close to the injection intersection, serial injections can be easily attained. 

Analysis of amino acid neurotransmitters was achieved by online, precolumn derivatization with OPA. The reaction of OPA with primary amines occurs in less than 2 min. An online reactor was created by inserting a smaller capillary with the analyte into a larger capillary through, which the OPA was pumped. After the end of the smaller capillary, the analyte would be mixed with OPA and derivatized. A flow-gated interface was used for injection. The sample was swept to waste by a cross-flow buffer except when an injection was made. Then analyte was allowed to accumulate in the gap (30-50 ttm) between the reaction and separation capillaries. A small voltage, less than the separation voltage, was applied for only 200 ms to perform an electrokinetic injection. This allowed very efficient separations with only 30 ms wide peaks. 

A flow-gated interface allows sampling and injection from a continuous flow stream. 

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