Gated Injection

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. 

This gated injection method has allowed the sample loading to be achieved in a continuous manner whereas a pinched injection mode cannot [317], Therefore, the gated injection has also been employed for two-dimensional (2D) separation OCEC/CE [333,666] or MECC/CE [565] (see Chapter 6, section 6.4 for more on 2D separation). 

FIGURE 4.16 Image of the glass microchip used for 2D chemical separations. The separation channel for the OCEC (first dimension) extends from the first valve VI to the second valve V2. The CE (second dimension) extends from the second valve V2 to the detection point y. Reservoirs for sample (S), buffer 1 and 2 (Bl, B2), sample waste 1 and 2 (SW1, SW2), and waste (W2) are positioned at the terminals of each channel. The arrows indicate the detection points in the OCEC channel (x) and CE channel (y) [333]. Reprinted with permission from the American Chemical Society. 

FIGURE 4.17 Timing diagram for making the initial 0.5-s injection into the OCEC channel (solid line) and the subsequent 0.2-s injections into the CE channel with a cycle time of 3.2 s (dashed line) [333], Reprinted with permission from the American Chemical Society. 

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An alternative approach for injection molding is the use of a rotary table injection machine. This type of machine uses the multistation concept, each station having a single cavity mold generally using a point gating/ injection point as previously mentioned. Consider a rotary table machine with eight stations, station one would be the injection station and station seven would be the unload and load metal insert station. This type of process has been successfully used for different types of lip type rotary shaft seals where more expensive elastomers are used and waste needs to be kept to a minimum. 

Roddy, E.S., Lapos, J.A., Ewing, A.G. (2003). Rapid serial analysis of multiple oligonucleotide samples on a microchip using optically-gated injection. J. Chromatogr. A 1004, 217-224. 

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]. 

The first experimental demonstration of such a device has been published by Jacobson et al. [53]. Amino acids were injected by a gated injection scheme, separated in a 7 mm channel and subsequently labeled by controlled mixing with an o-phthaldialdehyde reagent solution at a T-intersection. The separation efficiency achieved, however, was relatively poor and the inadequate kinetics of the labeling reaction caused significant band broadening. 

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]. 

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],... 

Although stack injection has been employed previously [316,582], the benefit of stacking for sample pre-concentration was only studied in detail later [346]. With the sample buffer (0.5 mM) at a 10-fold lower conductivity than the separation buffer (5 mM), simple EK stacking using the gated injection was observed. This was applied to the separation of dansylated amino acids (dansyl-lysine, didansyl-lysine, dansyl-isoleucine, and didansyl-isoleucine) [346]. 

FIGURE 5.14 (a) Schematic of a three-dimensional gated-injection separation device consisting of two crossed microfluidic channels with a PCTE membrane interconnect, (b) Electrical bias configurations for active electrokinetic injection control. (Right) Nanocapillary array gated injections. (Left) Main channel separations [591]. Reprinted with permission from the American Chemical Society. 

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. 9.7.7 Continuous sampling hydrodynamic gated injection from LC chip designed by Schlund et at [52].

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.

The injection methods used in (iCE can either be stacking- or extraction-based methods. Stacking methods such as mediated stacking [236], gated injection [237], field amplified sample injection [238], the staggered-T configuration [239], field-amplified sample injection [238], and pressure-driven injection [10, 86] are widely adopted in pCE because they are simple to implement. Extraction-based techniques, such as membrane filtration [240, 241], SPE [32, 242], liquid-liquid extraction [243], and bioaffinity purification [53, 244] offer a more scalable platform for complete sample injection. 

For impact-modified blends (Xenoy 1200 and 5720, Stapron E), screw speed should be set at minimum, as low as possible (less than 50 rpm), just sufficient to assure a reasonable cycle time. Melt temperature should be kept as low as possible, without binding the screw. When temperature control is critical, as is the case here, the acmal melt temperature should be checked with a pyrometer or digital thermometer, rather than relying on barrel set points. The material runs best in thick parts with oversized gates. Injection speed of 25 mm/sec is suggested. When running a large shot (>70% of machine capacity), a flat or reverse barrel heat profile may help to convey the melt. 

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. 

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