Van Deemter equation
Van Deemter equation ( JJ van Deemter, 1956) mathematical / physical describes the separation efficiency in the gas - and liquid chromatography. The plate height H (. HETP - height equivalent to a theoretical plate, engl height equivalent to a theoretical plate) is described by the following simplified formula:
The smaller H is, the higher the separation efficiency of the system. This follows from the equation:
H / V - hyperbolic function (X = V, Y = HETP ) has a minimum at the optimal flow velocity (v), wherein the bottom head (H ) at least and thus the separation performance is the greatest.
- H: ground level
- L: length of the column
- N: number of theoretical plates
- V: speed of the mobile phase
- A: Stray diffusion / eddy diffusion ( eddy diffusion ). For packed columns, the turbulence increases this Term It is independent of the flow velocity. In capillary deleted A. It is a scattering diffusion which is caused by different flow paths of individual sample molecules through the column. These come about by changing shape and size of the particles of the substrate and their irregular packing in the column. Eddy diffusion is very small, when small particles of uniform size are homogeneously packed into the column. Term A is also dependent on the particle diameter of the stationary phase: A = 2 λ d ( λ = factor, d = particle diameter).
- B / a: Longitudinal diffusion, diffusion in the longitudinal direction of the column axis. This factor depends on the viscosity and temperature of the mobile phase. It depends on the reciprocal of the flow rate. The analyte diffuses continuously away from the concentrated center of his sample zone. B term is the diffusion coefficient of the mobile phase ( D (M ) ) as a function: B = 2 λ D (M). The higher the flow rate, the shorter the applied time and applied to the column, the lower is the contribution of longitudinal diffusion.
- C · v describes the mass transfer between the stationary and mobile phases. This term will be influenced by the nature of the mobile phase and the type and thickness of the stationary phase. He is directly proportional to the flow velocity. Term C is the particle diameter, the porosity of the particles, the column length, the column radius and the diffusion coefficient of the stationary phase dependent.