Sample application electrokinetic injection

Electrokinetic injection is useful when the analyte is in the presence of interfering species (with different mobilities), qualitative applications, or when viscous buffers or gels are being used. It is usually not suitable for quantitative applications since the variability caused by conductivity, microenvironments, and matrix differences significantly reduces the reproducibility. Since sample depletion can be a significant issue, it is recommended that different samples are used when repeated injections are needed. 

Application of a potential between reservoirs 1 (sample) and 4 (injection waste) electrokinetically pumps sample solution as indicated in Fig. 3. In this way, a geometrically defined 150 pm (90 pi) section of the separation channel can be filled [19]. If the injection potential is applied long enough to ensure that even the slowest sample component has completely filled the injection volume, a representative aliquot of sample can be analyzed (so-called volume defined injection). This is in contrast to electrokinetic sample injection in conventional capillaries, which is known to bias the sample according to the respective ionic mobilities [61]. These characteristic differences are shown schematically in Fig. 4. It should be noted that this picoliter sample injector is exclusively controlled by the application of electric fields and does not require any active elements with moving parts such as valves and external pumps. The reproducibility of the peak height of the injected sample plugs has been reported to be within 2 % RSD (relative standard deviation) and less [19,23]. 

In a previously mentioned work by Kirlew et al., electrophoretic separations of Se, Se As As, and dimethylarsinic acid were performed using various ultrasonic nebulizer (USN) interfaces. Using the optimized CE interface conditions and a borate run buffer at pH 8, a separation was accomplished within 10 min. Electrokinetic injections gave better sensitivities for the analytes as compared to hydrostatic sample injection. In the Kirlew study, arsenate and selenite ions had very similar migration times, but these analytes were easily resolved by the multielement capability of the ICP-MS detector. An electropherogram of this work is shown in Fig. 5. In an application to field samples. Van Holderbel ... 

The injection process introduces the prepared sample or reagent into the flowing carrier stream within the manifold. Ideally, the injector system should be designed so as to provide a high sample flow rate. Injection systems typically employ electrokinetic mobility or hydrodynamic pressure techniques. In the former systems, the sample flow into the microchannel is controlled by the application of an external electric field to the reservoir, while in the latter systems, a pressure difference is created in the reservoir using either a positive pressure (pistmi-type) technique or a suction pressure (vacuum) technique. 

Electrokinetic injection schemes for microfluidic systems are generally designed to perform one of two different injection functions, namely time-based or discrete volume-based [4]. Time-based injection (also known as gated injection) allows for the introduction of the sample into the carrier stream over a controlled period of time. This technique has several key advantages, including a straightforward control and a variable injection volume, and allows for both continuous and sequential injection for on-line measurement applications. 

Recently a decreased level of CE activity has been noticed with a shift of attention towards other separation techniques such as electrochromatography. CE is apparently not more frequently used partly because of early instrumental problems associated with lower sensitivity, sample injection, and lack of precision and reliability compared with HPLC. CE has slumped in many application areas with relatively few accepted routine methods and few manufacturers in the market place. While the slow acceptance of electrokinetic separations in polymer analysis has been attributed to conservatism [905], it is more likely that as yet no unique information has been generated in this area or eventually only the same information has been gathered in a more efficient manner than by conventional means. The applications of CE have recently been reviewed [949,950] metal ion determination by CE was specifically addressed by Pacakova et al. [951]. 

Inject sample. Sample is injected either by hydrostatic pressure or electrokinetically depending on the nature of the analyte and the sample matrix. Ideally, the injection should be followed by a second injection of separation buffer (equivalent of 1-2 s at 0.5 psi) to avoid any loss of sample into the inlet vial during the first few seconds of voltage application. This is particularly important with sample matrices that end to generate significant Joule heat. 

In HPCE, samples can be detected on-column (online). Removing the polyimide coating of the fused silica capillary in a small section allows UV or fluorescent detection. A power supply is used to apply a field strength of 0-30 kV. Injections can be made by means of pressure or electrokinetic modes that allow the application of automated injection systems. 

Sample injection involves extremely small volumes (picolitre to nanolitre range) so as to not occupy more than a few percent of the total capillary volume (a 1 m capillary with d. of 50 jxm contains a volume of 2.5 xL). The two commonly used injection methods for CE are hydrodynamic (pressure) and electrokinetic. Each involves temporarily removing the bnffer reservoir from the inlet end (the cathode in the case illustrated in Figure 4.23) and replacing it by a vial containing the solution to be analyzed. Hydrodynamic injection involves application of a pressure difference between the two ends of the capillary. The volume amount of sample solution injected can be calculated using the Poiseuille equation ... 

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