Van Deemter’s plots

Figure 11.6 The van Deemter Equation plotted as a curve. The full expression is the sum of the three individual components. Eddy diffusion, the A term, a constant, does not change as the mobile linear flow rate changes. Bandbroadening from longitudinal diffusion steadily decreases the shorter the analyte remains on the column, in other words, the faster the flow rate is. Contributions from resistance to mass transfer of the analyte into and out of the stationary phase increase linearly with the mobile phase flow rate. The separation will be most efficient at the flow rate that minimizes plate height, in this case about 0.1 cm/s.

Fig. 1.3. (A) Three comribulion.s to the column plate height. H. according to ihe van Deemter equation (Eq. (1.10)). (B) Experimental plot of the reduced plate height, h = H /dp as a function of the mobile phase velocity, ii. for a Bio.spher Cis. -5 um. column (13.5 x 0.32 mm i.d.) for toluene in 70% aqueous methanol as the mobile phase.

Figure 6.14 Plot of the axial reduced plate height vs. the reduced mobile phase velocity, (a) Comparison of the results measured by three PFGNMR methods (PFGSE, PFGSTE and APGSTE, with S = 2 ms and A - 22 ms) and best fit of these experimental data (s3rmbols) to the Giddings equation (solid line). The individual contributions to this equation are represented by the dotted lines, (b) Best fit of a subset of these data limited to t/ < 15 to the van Deemter model and extrapolation (B = 1.35, A = 0.14, C = 0.11, n = 0). Reproduced with permission from U. Tallarek, E. Bayer, G. Guiochon, f. Am. Chem. Soc., 120 (1998) 1494 (Fig. 8). (c)1998 American Chemical Society.

While the rate theory is a theoretical concept, it is a useful one in practice. It is common to obtain a van Deemter plot for one s column in order... 

Figure 12.9 Van Deemter plots for three GC carrier gases N2, He, H2. H (height equivalent to theoretical plate) versus u (linear flow velocity, cm/s).

Figure 3.8 Experimental van Deemter plot of H (cm) vs u (cm.s ) for isocratic elution (normal phase) of hexamethylben-zene with a mobile phase of 4.8 % (w/v) ethyl acetate in n-decane. The column was 25 cm long, 9 mm in diameter and packed with 8.5 pm silica gel. The curve fitting procedure gave values for the van Deemter constants, and thus the separate contributions to the curve from the multipath dispersion, longitudinal dispersion and the resistance to mass transfer were calculated as shown. Reproduced from Scott, http //www.chromatography-online.org/, with permission.

An example of a Van Deemter plot on AI2O3/KCI fused silica is shown in Fig. 7-1. The test component was 1,3-butadiene and the oven temperature was 130 °C. Optimum carrier gas velocity for nitrogen is 24 cm/s, for helium - 45 cm/s and for hydrogen -75 cm/s. The high optimal carrier gas velocity reduces the analysis time of light hydrocarbons considerably and is therefore recommended. 

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