Electrophoresis theoretical plates

Efficiency The efficiency of capillary electrophoresis is characterized by the number of theoretical plates, N, just as it is in GC or ITPLC. In capillary electrophoresis, the number of theoretic plates is determined by... 

First, solutes with larger electrophoretic mobilities (in the same direction as the electroosmotic flow) have greater efficiencies thus, smaller, more highly charged solutes are not only the first solutes to elute, but do so with greater efficiency. Second, efficiency in capillary electrophoresis is independent of the capillary s length. Typical theoretical plate counts are approximately 100,000-200,000 for capillary electrophoresis. 

Stalcup aiid co-workers [14] adapted this method to a continuous elution mini-prep electrophoresis apparatus shown in Fig. 11-3. In this apparatus, the end of the electrophoretic gel is continuously washed with elution buffer. The eluent can then be monitored using an HPLC detector (Fig. 11-4) and sent to a fraction collector where the purified enantiomers, as well as the chiral additive, may be recovered. In this system, the gel configuration was approximately 100 mm x 7 mm, and was aircooled. The number of theoretical plates obtained for 0.5 mg of piperoxan with this gel was approximately 200. A larger, water-cooled gel was able to handle 15 mg of... 

Because HPLC and HPCE are based on different physico-chemical principles, HPCE may be expected to address areas in which HPLC has shortcomings [884]. One such area is time of separation. In terms of speed of analysis, selectivity, quantitation, methods to control separation mechanism, orthogonality, CE performs better than conventional electrophoresis and varies from HPLC (Table 4.49). CE has very high efficiency compared to HPLC (up to two orders of magnitude) or GC. For typical capillary dimensions 105—106 theoretical plates are common in CE compared to 20 000 for a conventional HPLC column and... 

JC Giddings. Generation of variance theoretical plates, resolution and peak capacity in electrophoresis and sedimentations. Separ Sci 4 181-189, 1969. 

Other sources of broadening in real systems are mentioned later.) Capillary electrophoresis routinely produces 50 000-500 000 theoretical plates (Figure 26-19), which is an order-of-magnitude better performance than chromatography. 

The main argument for making MIP CEC is to combine the selectivity of the MIPs with the high separation efficiency of CEC. This argument appears to fail, however, if the adsorption isotherm of the MIP is nonlinear, which seems to be the rule. In the case of nonlinear isotherms, the peak shapes depend mainly on the isotherm, particularly so if the separation system is otherwise very efficient (has low theoretical plate height, see Fig. 1). In the case of ionized analytes the situation is more complex. If an ionized analyte is not adsorbed at all on the MIP, then it is separated only due to electrophoresis, and its peak will not be widened due to the nonlinear effect. In this case, however, the MIP is merely behaving like an inert porous material. In intermediate cases an ionized analyte may participate in both separation mechanisms and for this case we do not have exact predictions of the peak shape. 

In chromatography or electrophoresis, what does the plate number or number of theoretical plates describe (2 marks)... 

There is much interest in high-efficiency- and high-speed separation media for liquid chromatography. The plate numbers available in practice have been in the range of 10,000-30,000 in HPLC for 20 years or so, but these are low compared to well over 100,000 theoretical plates in capillary gas chromatography or in capillary electrophoresis. This is caused by the limitation in the use of small-sized particles for HPLC, where a particle-packed column is commonly used under a pressure-drop of up to 40 MPa. An increase in column efficiency by using small particles, which is the approach taken in the past, is accompanied by an increase in the pressure-drop, as expected from Eqns. 5.2 and 5.3, below. Eqns. 5.1-3 describe the efficiency (plate height) and flow resistance of a column packed with particles [1-3], where N stands for the... 

Here we establish the number of theoretical plates and separable zones achievable in ideal zone electrophoresis by following the approach in ref. [1]. This analysis aids in system evaluation, comparison, and optimization. 

The use of exceedingly thin capillary tubes ( 50 /im) in capillary electrophoresis not only reduces convection but it also provides rapid heat dissipation. Using up to 30,000 V, on the order of 106 or more theoretical plates are realized in such tubes (see Figure 8.2). 

The lactate and acetate ions have one unit of charge and the respective electrophoretic mobilities of 13.1 x 10 5 and 16.6 x 10 5 cm2/V s in 0.1 M alkaline solution at 20°C. How far through such a solution must they travel before unit resolution is achieved using an electrophoretic field strength of 20 V/cm How long does it take and how many theoretical plates are needed Assume ideal electrophoresis (0 = 1). 

Generation of Variance, Theoretical Plates, Resolution, and Peak Capacity in Electrophoresis and Sedimentation, J. C. Giddings, Sep. Sci., 4, 181 (1969). 

At times, capillary electrophoresis is called high-performance capillary electrophoresis to reflect the high number of theoretical plates offered by this technique as a result, superior resolution is possible. The original contribu-tor(s) to high-performance capillary electrophoresis and the basis of separation in various modes are shown in Table 2.4 9... 

In capillary gel electrophoresis, one of the major contributors to band broadening, besides the injection and detection extra-column effects, is the longitudinal diffusion of the solute molecules in the capillary tube [14], The theoretical plate number (N) is characteristic of column efficiency ... 

Ruhr s group studied the separation of double- and single-stranded DNA restriction fragments in capillary electrophoresis with polymer solutions under alkaline conditions in epoxy-coated capillaries and found that at pH 11 the theoretical plate numbers exceeded several millions [96], At pH 12, single-stranded DNA molecules were still well separated in entangled hydroxyethylcellulose (HEC) solutions, but the resolution decreased significantly in dilute polymer solutions. 

There are two important equations which describe the resolution of capillary electrophoresis in terms of the number of theoretical plates, and the migration time for molecules through a column. 

J. Lunney, A. Chrambach, and D. Rodbard, Anal. Biochem., 40 158-173 (1971). Factors Affecting Resolution, Bandwidth, Number of Theoretical Plates and Apparent Diffusion Coefficients in Polyacrylamide Gel Electrophoresis. 

For enantioseparation on CSPs in CEC, nonstereospecific interactions, expressed as 4>K, contribute only to the denominator as shown in Eq. (1), indicating that any nonstereospecific interaction with the stationary phase is detrimental to the chiral separation. This conclusion is identical to that obtained from most theoretical models in HPLC. However, for separation with a chiral mobile phase, (pK appears in both the numerator and denominator [Eq. (2)]. A suitable (f)K is advantageous to the improvement of enantioselectivity in this separation mode. It is interesting to compare the enantioselectivity in conventional capillary electrophoresis with that in CEC. For the chiral separation of salsolinols using /3-CyD as a chiral selector in conventional capillary electrophoresis, a plate number of 178,464 is required for a resolution of 1.5. With CEC (i.e., 4>K = 10), the required plate number is only 5976 for the same resolution [10]. For PD-CEC, the column plate number is sacrificed due to the introduction of hydrodynamic flow, but the increased selectivity markedly reduces the requirement for the column efficiency. 

Capillary electrophoresis and its most popular hybrid technique - capillary electrochromatography - are complementary to HPLC, offering rapid analysis, low consumption of sample and solvents, and usually a higher efficiency of separation (due to a larger number of theoretical plates). Similar to HPLC, enantioseparation with the use of electrophoretic methods can be conducted by direct (chiral phase... 

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