Electroosmosis, instruments

In step 2, the migration times of the solute and the marker of the electroosmosis, such as mesityl oxide, were measured at each pH and converted to the effective mobility. When the CE instrument is equipped with a photodiode array detector, the spectrophotometric method is available simultaneously. The buffers should be exchanged every five runs, because the pH of the buffer was changed by electrolysis during CE analyses. The details of the experimental conditions are described in Ref. 20. 

For an analyte cation moving in the same direction as the electroosmotic flow, xep and xeo have the same sign, so xapp is greater than xcp. Electrophoresis transports anions in the opposite direction from electroosmosis (Figure 26-20b), so for anions the two terms in Equation 26-11 have opposite signs. At neutral or high pH, brisk electroosmosis transports anions to the cathode because electroosmosis is usually faster than electrophoresis. At low pH, electroosmosis is weak and anions may never reach the detector. If you want to separate anions at low pH, you can reverse the polarity of the instrument to make the sample side negative and the detector side positive. 

A microchip device with an attached nano-ESl emitter tip was developed to facilitate the introduction of tiyptic digests by means of nano-ESl [88-89]. Instead of off-line filling of the nano-ESl needle, the sample is transferred from a vial on the chip to the nano-ESl needle by electroosmosis. Detection limits of 2 fmol/pl were achieved for fibrinopeptide A (1699 Da). Further developments enabled sequential automated analysis of protein digests by ESl-MS [90]. On-chip sample pretreatment and desalting by either sample stacking via polarity switching or SPE prior to on-chip CE was described by Li et al. [91], and applied to the identification of 2D-GE separated proteins from Haemophilus influenzae using a Q-TOF instrument. 

The instrumental arrangement commonly employed in capillary electrophoresis is shown in Figure 12.1. With untreated silica capillaries, electroosmosis causes the buffer to flow from the anode to the cathode. Samples are introduced at the anodic end, and an on-column or post-column detector is placed at or near the cathodic end of the capillary. The high-voltage produced by the power supply and present in the anodic buffer reservoir is enclosed in a protective shield. 

The potential of macroscopic grains can be conveniently measured by electroosmosis, A few commercial instruments are available, and many home made designs have been described in literature [52], The apparatus described in Ref. [53] requires samples in form of two flat disks about 50 mm in diameter one of which has a central hole 6 ram in diameter to measure radial flow streaming potential. 

Capillary-electromigration separation techniques are a family of separation methods carried out in empty, coated or packed capillary columns with electrolyte solutions as the mobile phase. An electric field is resonsible for driving the sample and mobile phase through the column by processes dependent on electrophoresis and electroosmosis. This common arrangement allows a similar instrument platform to service all capillary-electromigration separation techniques with only minor modifications for specific applications. These methods only recently entered analytical laboratories, although in a planar format they have a long history of use in biochemical and chnical laboratories [1-5]. 

The major limitation of PALS is that no mobility distribution information can be obtained and the type of mean is not defined. In addition, the accuracy of the measured electrophoretic mobility depends on the accuracy of the scattering vector K, which can te determined quite accurately based on instrument setup, and the accuracy of A, which is often affected by experimental noise and is difficult to ascertain. Other factors, such as electroosmosis, electronic artifact and the choice of a correct field frequency can also affect the measurement accuracy, as demonstrated in Figure 6.31, in which correct electrophoretic mobihty value using PALS can only be obtained at electric field frequencies between 30-200 Hz. 

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