Question

Consider an HPLC analysis. Which term in the van Deemter equation plays a minimal role in
determining the theoretical plate height (H)? Justify your answer.

Consider a capillary column GC experiment. Which term in the van Deemter equation is most
strongly affected by increasing the inner diameter of the column? Justify your answer.

Answer 1

.A.A plate height equation expresses the correlation between plate height and mobile phase velocity. Best known is the van Deemter equation, which describes the various contributions to plate height (H). In this equation the parameters that influence the overall peak width are expressed in three terms-

H=A+B/u +C×u

u=linear velocity

C = Resistance to mass transfer coefficient

B=longitudinal diffusion term

H= HETP (plate height)

A= eddy diffusion term

The van Deemter equation is graphically expressed in the H-u curve, which is a plot of the plate height as a function of the mobile phase velocity.The H-u curve shows that:

1.The A-term is independent of u and does not contributeto the shape of the H-u curve.

2.The contribution of the B-term is negligible at normal operating conditions. This is due to the fact that themolecular diffusion coefficient in a liquid medium is very small.

3.The C- term increases linearily with mobile phase velocity and its contribution to the H-u curve is therefore considerable. A small C-term leads to a fairlyflat ascending portion of the H-u curve at higher mobile phase velocities. This means that the separation can be carried out at higher mobile phase velocities without sacrificing separation quality.

B. The C-term relates to the mass transfer of sample components between the stationary phase and the mobile phase. This term can be divided intoa Cm-term and a Cs-term, describing the contributions to peak broadening in the mobile phase and in the stationary phase respectively

Analyte molecules are retained in a chromatographic system because they interact with the stationary phase. Since analyte molecules are present in the mobile phase, they have to move to and enter into the stationary phase in order to interact. Then they have to return to the mobile phase. Because the linear velocity of the mobile phase is lower closer to the column wall (or the stationary phase particles) than in the center (or further away from the particles), the molecules experience different velocities. This results in peak broadening in the mobile phase. This phenomenon is described by the Cm term.

The Cm-term depends on:

*Particle size dp (packed columns)

*Internal diameter (capillary columns)

The internal diameter of a capillary column affects the Cm term in the equationresulting in a larger plate height at a larger diameter. The fact that there is no packing present does not mean that the efficiency of a capillary column will become independent of othercolumn parameters. As with packed columns, the amount of stationary phase material, expressed in the film thickness, will affect the resulting efficiencyThe internal diameter of the column plays a part in the height of the Cm-term. The narrower the column, the higher the efficiency, but too narrow columns are difficultto operate due to the high pressure etc.

H=B/u+(Cm+Cs) ×u