Van Deempter curves

The first notable point Is that the minimum plate heights are the same. This Is reasonable and predictable since the minimum plate height (HETP j n) In a van Deempter curve for a packed column Is a function of the packing particle diameter (dp) and the analyte capacity factor (k ) ... 

Van Deempter curves for packed column HPLC ( 1), packed column SFC ( 2), capillary column SFC ( 3, 6, 7). and capillary column GC ( 4, 5). Curves 1 and 2, experimental data (5) Curves 3-7. calculated (see text). 

Curve 7. The values to calculate this curve were taken from capillary SFC work by Peaden et al (14) who presented van Deempter curves obtained with pyrene in n-pentane at 210°C and 29 atm for several capillary column diameters. A value for D12 for pyrene in n-pentane was calculated from one of those curves (for a 300 pm... 

Draw a Van Deempter curve for each of the three techniques in Problem... 

If the dimensions of stationary phase coating thickness and diffusion distance to the film from the gas phase have been optimized, for further improvement of GC resolution, it becomes necessary to increase the length of the column. This is seen from the simple relation of Eq. (11.7), which indicates that for an optimal minimized value of H (the height equivalent to a theoretical plate) the number of plates, N, is proportional to the length of the column. From Eqs. (11.8) and (11.9) we note that the resolution is proportional to the square root of N. For columns packed with particles of optimal size, and operated at the optimal linear flow rate at the minimum of the Van Deempter curve (Fig. 11.3), the typical maximum pressure of 100 psi achievable from a regulated gas cylinder requires that most packed columns be less than 4-7 m long. More typically they are only 1 -2 m in length. These considerations hmit the resolution achievable in packed column GC. 

LC must contain a correction for When a van Deempter plot is applied to HPLC, there is a curvature away from the mass transfer in both the mobile and theoretical curve at high velocities. Huber pointed out that in relating plate height... 

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