The flow and outlook
Find the best balance between purity, capacity and productivity is the goal in purification.
Increasing the elution column flow reduces the time of purification and allows to gain in productivity in a quick and easy way. Therefore, the choice of the column and of the associated instrument is essential.
Column : The selection criteria for better productivity
The efficiency of the column
The effectiveness of a column is characterized by the number of theoretical plates N.
The theoretical plate, is the stationary phase portion in which a solute is distributed balanced way between the mobile phase and the stationary phase.
Enslaved to the length of the column, the more N is larger, the better the efficiency.
Column height, equivalent to a theoretical plate.
To express the effectiveness of a column whose length is L and the number of theoretical plates N , we define the parameter H (or HETP ). It’s called Height Equivalent to a Theoretical Plate.
H = HEPT = L/N
H is a function of flow .
More H is low, the better the efficiency.
The resolution factor Rs is an important chromatographic parameter. It defines the more or less good separation of the two compounds and hence the purity of the products collected.
The value of the resolution depends on the efficiency of the column.
A resolution of 1.5 corresponds to a purity of 99.85 %. This means a return to the baseline between two peaks. For a purity of 99.99 % minimum it must a resolution ≥ 2.
Courbe Van Deemter
The graph showing the evolution of H versus flow curve is called Van Deemter :
This curve shows the existence of an optimal flow rate of the mobile phase on which the column efficiency is maximum (minimum H).
Increase or reduce the flow of the mobile phase above or below the optimal flow can greatly reduce the separating power of a column.
However, everything depends on the granulometry of the stationary phase used, as the more the particle diameter is low more the elution flow will be increased without losing on efficiency.
Caution : the fact of increasing the flow of elution on a column will necessarily lead to an increase in pressure in the system. So choose the system accordingly.
Application : Using a Prep column with the PuriFlash PF4125
To conclude, it is possible to reduce the time of purification by working at a higher flow at the optimum flow without losing resolution and therefore on purity of the collected products.
To do this, you simply need to combine a low granulometry column with a system that can withstand high pressures.
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