Application 2 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. 

Because the amount of analytes injected depends on both the ion mobility and the electroosmotic flow, variables that are difficult to control, electrokinetic injection is adopted only when hydrodynamic injection is not applicable even though it is theoretically superior in terms of selectivity. A potentially... 

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 ... 

A. Caspar and E. Dudds, Application of internal universal cahbration for determination of fuUy dissociated species in capillary electrophoresis using indirect UV detection and electrokinetic injection,... 

Quality assurance in micro-CE has yet to be studied in detail. Research focuses on practical applications and the transfer of separation problems to a microdevice. One problem is the chip-to-chip variation from manufacturing, another the run-to-mn performance due to reloading, the cleaning of the channel surfaces, the not perfect electrokinetic injection mode, or that instrumental setups are usually still in the trial state and have only been optimized for one purpose [2]. 

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]. 

There are two main injection strategies for CE, electrokinetic and pressure. Each type of injection mode has its advantages and disadvantages. Depending on the application, the appropriate mode must be chosen. 

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]. 

Injection Injection mechanism Electrokinetic applied voltage and time of application Hydrodynamic units of pressure or vacuum applied or height of displacement and time of application... 

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