Plate height minimum

Column Type Minimum Reduced Plate Height Minimum Reduced Velocity flow Resistance Parameter Separation Impedance... 

Column type Minimum reduced plate height Minimum reduced velocity Flow resistance parameter Separation impedance... 

Now t[i is a minimum when k = 2, that is, when = 3t . There is little increase in analysis time when k lies between 1 and 10. A twofold increase in the mobile-phase velocity roughly halves the analysis time (actually it is the ratio Wu which influences the analysis time). The ratio Wu can be obtained from the experimental plate height/velocity graph. 

The curves represent a plot of log (h ) (reduced plate height) against log (v) (reduced velocity) for two very different columns. The lower the curve, the better the column is packed (the lower the minimum reduced plate height). At low velocities, the (B) term (longitudinal diffusion) dominates, and at high velocities the (C) term (resistance to mass transfer in the stationary phase) dominates, as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, log (h ) is about 0.35. The optimum reduced velocity is in the range of 3 to 5 cm/sec., that is log (v) takes values between 0.3 and 0.5. The Knox... 

Further, by substituting for (uopt.) in equation (3), it can be seen that the minimum plate height (Hmin.) is given by,... 

Now, the column length (L) can be defined as the product of the minimum plate height and the number of theoretical plates required to complete the separation as specified by the Purnell equation. 

The expression that gives the minimum plate height is given by equation (29), chapter 12, and is reiterated here. 

It must be emphasized that equation (4) only applies to the separation and conditions defined in Table 1. The effect of the magnitude of the separation ratio on the minimum plate height is shown in Figure 4. 

Figure 4. Graph of Minimum Plate Height against Separation Ratio of the Critical Pair...

The optimum velocity changes linearly from about 1 cm/sec. to 800 cm/sec. and the minimum plate height from 200 pm to about 10 pm over the same separation ratio... 

Plot a graph of //versus Ti and calculate the optimum gas velocity, the corresponding minimum plate height and the maximum plate number. 

In gas chromatography the B and C terms are usually larger than the A term. A plot of plate height against velocity shows a minimum. 

Of course it is impossible to carry out a given separation at the critical pressure, APe, since the analysis time would be infinite. It is easy to show that if A is zero in Eq. (40), the pressure, AP , required to obtain the minimum plate height, hn, is given by Eq. (43) ... 

If A is different from zero it is not possible to calculate exactly the numerical coefficient whose value is somewhat larger than 2, as we will show later in this section. As a rule of thumb, the pressure necessary to perform an analysis when working at minimum plate height can be approximated by Eq. (46) (AP in atm, dp in /itn)... 

Fig. II. Miniinuin analysi time as a function of the maximum available pressure and (be panicle size. Solid lines represent constant plate numbers (I) 1000 (2) 3000 (3) 20,000 (4) 100,000 (5) 300,000 (6) 1,000,000 dotted lines represent constant particle size stated in micrometers at each line. The columns are operated at minimum plate height of h 2, and the other conditions are as follows v 3,i - 3,ij 0.4cP, < - 1 x 10", - 1 x 10" ...

I X 10 cm /sec, so that, when dp = 6 /xm, the reduced velocity for such compounds at u 0.2 m/sec is between 12 and 25. Accordingly, these columns are not operated at the maximum efficiency, i.e., the minimum reduced plate height could have a value of approximately 2. Still, with... 

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