Electroosmosis injection

Reproducibility with hydrostatic injection is on the order of 1 to 2%. The advantage of electroosmosis injection is that more sample can be introduced, improving detection limits. The disadvantage is that because of differences in ion mobilities, the sample plug that enters the capillary is not representative of flie sample. 

The gradient in IEF is a pH gradient that is generated by electrolyte systems consisting of ampholytes. The capillary wall is coated in order to avoid electroosmosis. The IEF mode is especially suitable for the separation of zwitter ionic compounds like proteins, peptides, amino acids, various drugs, etc. After injection of the sample and application of the potential, the sample constituents migrate to a pH in the capillary... 

Electroosmotic flow in a capillary also makes it possible to analyze both cations and anions in the same sample. The only requirement is that the electroosmotic flow downstream is of a greater magnitude than electrophoresis of the oppositely charged ions upstream. Electroosmosis is the preferred method of generating flow in the capillary, because the variation in the flow profile occurs within a fraction of Kr from the wall (49). When electroosmosis is used for sample injection, differing amounts of analyte can be found between the sample in the capillary and the uninjected sample, because of different electrophoretic mobilities of analytes (50). Two other methods of generating flow are with gravity or with a pump. 

Oki, A., Adachi, S., Takamura, Y., Ishihara, K., Kataoka, K., Ichiki, T., Honike, Y., Glucose measurement in blood serum injected by electroosmosis into phospholipid polymer coated microcapillary. Micro Total Analysis Systems, Proceedings of the 4th IJ.TAS Symposium, Enschede, Netherlands, May 14-18, 2000, 403-406. 

The introduction of the samples onto the capillary column can be carried out by either displacement techniques or electrokinetic migration. Three methods of displacement or hydrostatic injection are available a) direct injection, or pressure b) gravity flow, or siphoning and c) suction. The electrokinetic injection method arose from findings that electroosmosis act like a pump (80). Both methods have advantages and disadvantages. For example, a bias has been reported in electrokinetically injected... 

Cement grouts injected to replace and mix with soils eroded by high pressure water jet ("soilcrete column") Stabilizing chemicals moved into soil by electroosmosis... 

P.K. Dasgupta, S. Liu, Electroosmosis a reliable fluid propulsion system for flow injection analysis, Anal. Chem. 66 (1994) 1792. 

The sample, typically a few nanoliters, can be introduced into the capillary by hydrostatic injection (gravity, pressure, or vacuum) or by electromigration. The sample volume should generally be less than 2% of the total capillary length. For gravity introduction, the capillary sample end is dipped into the sample (which may be as small as 5 fiL) and raised for a short predetermined time to allow sample to flow into the capillary. Or, it is inserted into a pressurized vial to force sample into the capillary. Or it is drawn in by suction from the other end of the capillary. After injection, the sample vial is replaced with a buffer reservoir. Alternatively, the sample end is immersed in the sample solution and a relatively low voltage is ap-phed for a few seconds, for example, 2000 V for 10 s. This injects the small volume of sample by electroosmosis. 

For electrokinetic injection a low voltage is applied for a brief time with the inlet of the separation column located in the sample vial. Sample enters the column by the combined effect of electroosmosis and electrophoresis. If it is assumed that the conductivity of the sample solution and background electrolyte is equal, then the length of the injected zone, Ljnj, is given by... 

Figure 1 A typical basic HPCE system. The capillary is tipped into two buffer vials which are connected with two electrodes for applying the electric field. After filling the capillary with buffer, injecting the sample either hydrodynamically or electrokinetically, the applied voltage leads to a separation of ions which all migrate towards the detector due to electroosmosis. Detection is performed at a detection window where the capillary is freed from its protecting outer polymer coating. Usually, a personal computer is used to receive and store analytical data.

A high voltage is then imposed at the inlet reservoir, and the outlet reservoir is grounded. Due to electroosmosis, the electrolyte carrying the solutes of a given concentration is continuously injected from the inlet reservoir at room temperature, and the heated electrolyte is streamed from the microchannel to the outlet reservoir. 

Capillary electrophoresis involves two simultaneous processes called electrophore sis and electroosmosis. Electrophoresis is the migration of ions in an electric field. Electroosmosis pumps the entire solution through the capillary from the anode toward the cathode. Superimposed on this one-way flow are the flow of cations, which are attracted to the cathode, and the flow of anions, which are attracted to the anode. In Figure 23-14, cations migrate from the injection end at the left toward the detector at the right. Anions migrate toward the left. Both cations and anions are... 

The publications in this field indicate clearly improved performance of the physical side of the analysis, that is, shorter analysis times, better separation efficiencies and a dramatically reduced consumption of reagents. Furthermore, due to the minute volumes for internal connections, new types of combinations can be used, and small samples can be analyzed with success. TTie use of parallel [57, 70, 71] or multi-dimensional arrangements [72] would lead to even larger munbers of analyses per unit time, or to dramatically increased peak capacities (separation of > 1,000 components [73]). The trend to combine biological assays with separation methods, that is, protein protein interactions, enzymes or antibodies with CE [74, 75, 76], could lead to novel concepts for chemical sensing. Optically defined sample plugs allow for precise small volume injections for millisecond separations [55], which can be used for on-line or in-vivo monitoring experiments [77]. Novel approaches to control the flow, e.g., radial control of electroosmosis in capillaries [78] or inductive mechanical micro piunps [79, 80], will allow access to novel cyclic separation techniques. 

The development of novel miniatme modules for FIA with a fibre optic detector (Manz et al. 1991) had a positive effect on further development. In 1992, a complete system for capillary electrophoresis on a planar glass carrier was established successfully (Harrison et aL 1992). Thus, electroosmosis proved to be of value for solution transport as well as for injection operations. 

The term micro total chemical analysis systems (fi-TASs) is used now nearly exclusively for systems with a basic structure described in Chap. 9, Fig. 9.8. A liquid column moves through the device. It is driven either by a micropump or by electroosmosis. This moving column, also called the carrier stream, accepts a small sample volume increment, which is injected at the appropriate position. In the adjacent line, either a separation takes place (this is characteristic for chromatography and electrophoresis) or the sample reacts with ingredients of the carrier stream, thus forming a product which can be indicated. In both cases, at the end of the separation or reaction line, a detector is located which yields a concentration-dependent (better concentration-proportional) signal. 

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