Pinched 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. 

In one report, even though pinched injection was used, the actual injected amount did depend on loading time if the ionic strengths of the sample and run buffers did not match [552]. It was also found that unless the sample is strongly pinched, pinched injection will inject different volumes depending on the injecting voltage [71]. 

Even if pinched injection was used, baseline drift of repetitive pinched injections was noted. This was attributed to the meniscus surface tension (Laplace pressure) effect, even though other possible effects (evaporation, buffer depletion due to electrolysis, siphoning, Joule-heat-induced viscosity change) are minimized [553]. 

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]. 

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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. 

Sample injection in NCE is very important for reproducible results with low limits of detection. In spite of some development in NCE very little effort has been made to develop sample injection devices in this technique. Of course sample injection in NCE is a challenging job due to small volume requirement [87], The controlled injection of small amounts of sample is a prerequisite for successful analysis in NCE. Electrokinetic injection (based on electroosmotic flow) is the preferred method and Jacobson et al. [88] optimized sample injection using this approach. Pinched injection allowing injection in minute quantities [89,90] and double-T shaped fluidic channels [91] have also been used for this purpose. Furthermore, Jacobson et al. [92] used a single high voltage source to simplify instrumentation. Similarly Zhang and Manz [93] developed a narrow sample channel injector to improve... 

Figure 7.6 Electrochromatogram of noradrenaline and dopamine, respectively. Pinched injection for 50 seconds (S 1 kV SW ground B and BW 0.6 kV) [22].

A similar volume defined injection protocol where the sample is confined within two controlled side streams of buffer solution has been developed by the group of Ramsey and has been dubbed pinched injection scheme [31]. The... 

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. 

EK injection bias still exists in pinched injection, with neutral species injected in a greater amount than anionic species. However, the orthogonal nature of the loading and injecting steps reduced this bias, although it did not completely eliminate it [549]. 

Various injector geometries (simple cross, double-T, triple-T) were investigated on a glass microchip. The triple-T injector allowed for a selection of different injection volumes. For instance, a triple-T injector allowed injection of three different volumes depending on whether a cross, double-T, or triple-T configuration was used (see Figure 4.6) [529]. Pinched injection has been used consecutively to inject two samples into the same separation channel (see Figure 4.7) [557]. 

FIGURE 4.11 Microchip electropherograms of APTS-G2, -G2, -G3, and -G (2.1 x 10-7 M in water, (a) symmetric pinched injection (b) default pinched injection (c) asymmetric pinched injection Conditions 0.5% methylcellulose in 20 mM phosphate buffer (pH 6.66) field strength 300 V/cm. Sample 2.1 x 10 8 M APTS-G2, -G2,) -G3, and -G3 in water. Note G2 is maltose, G2 is cellobiose, G3 is matriose, G3 is panose [560]. Reprinted with permission from the American Chemical Society. 

In addition to the use of pinched injection for liquid samples, the method is also used for injection of an air sample, as pinched by He gas. Seven injections of air (N2) were made into a He carrier flow for GC analysis, as shown in Figure 4.14. N2 was detected by a capillary OED (by the N2 band at 337 nm) [563]. 

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). 

Owing to the EK bias in favor of faster migrating species in stack injection, the signal enhancement ranged from 31 to 8. However, stack injection produced less resolution, i.e., fewer plate numbers (N), as compared to those obtained in pinched injection. In addition, RSD (n = 6) in peak areas for stack, non-stacked, and pinched injection are 2.1%, 1.4%, and 0.75%, respectively [346]. 

Another example of field-amplified sample stacking, which occurs during pinched injection, has been described in Chapter 4, section 4.1.1. 

Back-transient ITP was carried out for temporary stacking of samples in an acrylic chip. This stacking works when there is EOF suppression (i.e., the background electrolyte contains 25 mM HEPES, pH 7.4, 1% PEO). However, this stacking effect was not satisfactory in the usual pinched injection with push-back... 

H. H., Pressure pinched injection of nanolitre volumes in planar micro-analytical devices. Labchip 2002, 2, 45-49. 

Beider and coworkers reported the fast chiral separation of acidic and basic compounds using a commercially available quartz microchip with a simple cross-tee design and linear imaging UV detection. Highly sulfated cyclodextrins were added to the separation medium to improve selectivity and samples were loaded for 60 s using the pinched injection mode. The chiral separation of norephedrine was achieved in 2.5 s, and a mixture of three basic drugs was resolved in 11 s. 

The same microchip electrophoresis system was used by the Takeda and coworkers " to separate phenolic compounds [bisphenol A, 4-nonylphenol, 4-(l,l,3,3-tetramethylbutyl)phenol and 4-tert-butylphenol] by micellar electrokinetic chromatography (MEKC). The samples were loaded for 25 s using a pinched injection scheme. fi-Cyclodextrin was added to the MEKC buffer to further improve... 

Reproducible injection for capillary electrophoresis on a microdevice/Lab-on-Chip is not easy to achieve. Different injection designs (e.g., T type, double-T type, and cross type) and different injection techniques have been applied (pinched injection, gated injection, double-L injection) within microfluidic devices. The volume and the concentration of the dispensed sample are the key parameters of this dispensing process, and they depend on the applied electrical field, the flow field, and the concentration field during the injection and separation processes. The problems and properties of the different modes are described below. 

Electrokinetic Sample Injection, Fig. 5 Pinched injection method in cross-fonn system, (a) schematic diagram of injection system, (b) injection step, (c) separation step... 

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