T

As we are interested in the chromatographic process we shall consider that the initial spot has a negligible width. This is obviously not true but we can always correct for it using variance addition. In eqn [46], z is the distance of front migration and the integration variable is given by:

where Ms is the molecular weight of the solvent, an association constant depending on the nature of the solvent (1 for nonassociating solvents, 2 for strong hydrogen-bonding solvents), and Vi is the molar volume of the solute.

High molecular weight solutes will exhibit a low diffusion coefficient. Some values of Dm have been selected to cover the whole range of classical solutes (we have excepted polymers or very low molecular weight compounds that are analysed by SEC or GC, respectively) and values of A', B' and C' have been computed for a given chromatographic system (dp, 6, RF). These values are reported in Table 5.

It is clear from the above data that the B' term is the most important one when the diffusion coefficient is large; its influence decreases slowly as Dm decreases. The plate height increases rapidly with diffusion coefficient (Figure 8). With small particles, H is linearly dependent on L and thus B' is the only term that governs efficiency. This efficiency decreases sharply as L increases. In other words, solutes of high Dm will be poorly separated on small particles with a rather long migration length.

When dp = 2o |jm, the situation is completely different. Provided the development length is long enough, all solutes will be separated in the same way, u =

Table 5 Effect of changing diffusion coefficient on the coefficients of the HETP equation

Dm(m2s-1)

A'(x104)

B' (x 104)a

C' (x 106)

3 x 1 0-

4.36

25.4

0.983

2x10-9

4.99

17.0

1.47

1 x10-9

6.29

8.48

2.95

7x 10- 10

7.08

5.93

4.21

5x10-10

7.93

4.24

5.90

3x10-10

9.40

2.54

9.83

Chromatographic conditions: A = 1; C = 0.01; 10" 47.2 m s"1; z0 = 0.5 cm; RF = 0.7; y = 0.7; dp = 5 ^m. aAssuming B' = 2yDm/6dpRF.

Chromatographic conditions: A = 1; C = 0.01; 10" 47.2 m s"1; z0 = 0.5 cm; RF = 0.7; y = 0.7; dp = 5 ^m. aAssuming B' = 2yDm/6dpRF.

a situation that does not occur with small particles. The most interesting feature is that H may be smaller with 20 | m particles than with 5 | m ones. A dotted round on the curves indicates the length of development when coarse particles become more efficient than fine particles.

Influence of retention From eqn [49], RF is only involved in the B' term. If RF is the only variable and the other parameters are kept constant, then we can write:

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