Floating injection

A pinched injection is shown in Figure 13.2(c). As can be seen in the figure, the flow of analyte is pinched in the intersection by grounding reservoir 3 while applying voltages to reservoirs 1, 2, and 4 in such a way that diffusion into the buffer and separation channels is prevented [3, 14, 17]. This type of injection is an improvement on the reproducibility of the floating injection. 

FIGURE 4.5 Images of sample injection at a cross-injector in a glass microchip (a) no fluorescent analyte, (b) pinched injection of rhodamine B, (c) floating injection of rhodamine B [136], Reprinted with permission from the American Chemical Society. 

The floating injection (with no push-back voltage) was successful without sample leakage in some examples of the separations of DNA [554,925] and proteins [555], possibly because of slow molecular diffusion in the viscous gel separation media. 

In non-gel solution, if the sample channel width was narrower than the separation channel width (by five-fold), floating injection did not result in sample leakage, and no pinched injection was necessary. Moreover, a push-back voltage was not necessary during separation. In addition, a T injector, rather than a crossinjector, was sufficient to perform floating injection without leakage, but the resolution obtained using the T injector was inferior to the cross-injector [556]. If T injectors are used, the number of reservoirs in a multichannel (S) system is reduced to S + 2, which is the real theoretical limit [556], rather than S + 3 [557]. 

FIGURE 6.16 Effect of surfactant type on the microchip separation of 2 mg/L TNT, 1 mg/L TNB, and 2 mg/L tetryl. The separation buffer contained MeCN/MeOH (87.5/12.5 ([v/v])), 2.5 mM NaOH, and with (a) no surfactant, (b) 0.5 mM CTAB, (c) 1.0 mM SDS. Applied separation field strength, 506 V/cm, using a 1-s floating injection. Colorimetric detection was achieved using a green LED source (505 nm) [622]. Reprinted with permission from Elsevier Science. 

Fig. 1 Operation processes of time-based floating injection, (a) Sample reservoir, (b) running buffer reservoir, (c) unused reservoir, (d) detection reservoir...

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