Van Deemter curves

In Figure 7, the resistance to mass transfer term (the (C) term from the Van Deemter curve fit) is plotted against the reciprocal of the diffusivity for both solutes. It is seen that the expected linear curves are realized and there is a small, but significant, intercept for both solutes. This shows that there is a small but, nevertheless, significant contribution from the resistance to mass transfer in the stationary phase for these two particular solvent/stationary phase/solute systems. Overall, however, all the results in Figures 5, 6 and 7 support the Van Deemter equation extremely well. 

The Van Deemter curve (HETP vs fluid velocity) is estimated from the two analytical chromatograms. At a flow rate of 1 ml min the liquid velocity is 0.001 m s the number of theoretical plates associated to the second peak is about 250. The... 

The highest column efficiency will be obtained at In practice, higher values are frequently used to minimize the separation time. For gas chromatography a velocity of about 2 u, called the practical operating gas velocity, is frequently recommended [67]. Provided that the ascending portion of the van Deemter curve is... 

Current commercial silica-based columns have two important characteristics (1) they can produce efficiency similar to that of columns packed with 3.5 /tm particles and (2) they typically produce a pressure drop of half that caused by a column packed with 5 /tm particles.35 Monolithic columns have been shown to exhibit flat van Deemter curves, resulting in little loss of efficiency at high flow rates.36 As a result, high-throughput separations on conventional HPLC instruments can be achieved by increasing flow rate up to nine times (up to 9 ml/min) the usual rate in a conventional packed column. Cycle times for HPLC analysis as short as 1 min (injection-to-injection) have been reported by users of monolithic columns. Additional benefits of monolithic columns cited include... 

Figure 5.6 Van Deemter curves relating H to linear flow velocity. A, eddy diffusion term ...

FIGURE 4.2 The van Deemter curves for HPLC and SFC. Conditions (a) LC, 55 cm x 250 xm ID fused-silica capillary columns packed with 5 pm porous particles, nitromethane test solute, 25°C, UV detector (214 nm) (b) SFC, 75 cm x 250 pm ID fused-silica capillary columns packed with 5-pm porous particles, 45°C, 230 atm, carbon dioxide mobile phase, methane test solute, FID. (Adapted from Wu, N. et al. Anal. Chem. 71 5084-5092. With permission.)... 

FIGURE 13 van Deemter curve showing the relationship of HETP vs.average linear velocity.The van Deemter curve has a classical shape and is a composite plot of A, B and C terms (plotted below to show their contributions). This diagram is adapted from Reference 9. 

Figure 14 shows the experimental van Deemter curves for three columns packed with 10-, 5- and S-pm particles. 

FIGURE 4 A van Deemter curve for 10-, 5-, and 1.7-gm particles. (Courtesy ofWaters Corp.)... 

In figure 7 the Resistance to Mass Transfer term (the (C) term from the Van Deemter curve fit) is plotted against the reciprocal of the diffusivity for both solutes. 

Supposing that the HETP resulting from the van Deemter curve is the same as that resulting from the calculation which might be performed when considering Question 4, calculate the coating efficiency. 

Plate height is reduced in an open tubular column because band spreading by multiple flow paths (Figure 23-19) cannot occur. In the van Deemter curve for the packed column in Figure 23-15. the A term accounts for half of the plate height at the most efficient flow rate (minimum H) near 30 mL/min. If A were deleted, the number of plates on the column would be doubled. To obtain high performance from an open tubular column, the radius of the column must be small and the stationary phase must be as thin as possible to ensure rapid exchange of solute between mobile and stationary phases. 

Most analyses are run at carrier gas velocities that are 1.5 to 2 times as great as the optimum velocity at the minimum of the van Deemter curve. The higher velocity is chosen to give the maximum efficiency (most theoretical plates) per unit time. A small decrease in resolution is tolerated in return for faster analyses. 

Figure 4.1. Van Deemter curve. A 1.7-p.m partricle column has a large optimal chromatographic functional area even at higher flow rates.

The use of column with superficially porous packing materials based on silica particles with nonporous cores is the most recently reported strategy for improving chromatographic performance. This technology, originally developed by Kirkland in the 1990s to limit diffusion of macromolecules into the pores [85], became commercially available in 2007 [86], In comparison with totally porous particles of similar diameters, the both A and C term of the Van Deemter curve are reduced [87, 88],... 

Fig. 9.5. van Deemter curves of butylbenzoate in the HPLC, pressure supported CEC and the pure CEC mode. Reproduced from [34], with permission. 

FIGURE 6.1 The van Deemter curves for 10, 5, and sub-2-pm bonded spherical silica particles. 

Results of efficiency enhancement studies have been controversial. Increasing the temperature lowers eluent viscosity and system back pressure, leading to the nse of (1) higher flow rates (shorter cycle times) [9], (2) longer columns, and (3) smaller particles that enhance efficiency in their own right. However, efficiency is also expected to increase because high column temperatures involve (1) faster adsorption-desorption kinetics, (2) enhanced diffusivity, (3) lower mass transfer resistance (C in the van Deemter Equation 6.4), and (4) flatter van Deemter curves. 

As it could be seen from the comparison of Figure 2-2 and Figure 2-3, all dependencies of the column efficiency on the flow rate follow the theoretical Van Deemter curve. In theory there is an optimum flow rate that allows obtaining the highest efficiency (the lower theoretical plate height). 

Figure 17-3. van Deemter curves for packed (YMC Cig) and monolithic (Chromolith) 4.6- X 100-mm columns. (Reprinted from reference 16, with permission from Elsevier.)... 

Values were interpreted on the basis of an active site sorption model. Diffusion coefficients and activation energies values, calculated from the slopes of the Van Deemter curves are in general accordance with previously published values. 

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