Open tubular columns optimum radius

The efficiency obtained from an open tubular column can be increased by reducing the column radius, which, in turn will allow the column length to be decreased and, thus, a shorter analysis time can be realized. However, the smaller diameter column will require more pressure to achieve the optimum velocity and thus the reduction of column diameter can only be continued until the maximum available inlet pressure is needed to achieve the optimum mobile phase velocity. 

Equation (13) is the first important equation for open tubular column design. It is seen that the optimum radius, with which the column will operate at the optimum velocity for the given inlet pressure, increases rapidly as an inverse function of the separation ratio (cc-1) and inversely as the square root of the inlet pressure. Again it must be remembered that, when calculating (ropt)5 the dimensions of the applied pressure (P) must be appropriate for the dimensions in which the viscosity (r)) is measured. 

In a packed column the HETP depends on the particle diameter and is not related to the column radius. As a result, an expression for the optimum particle diameter is independently derived, and then the column radius determined from the extracolumn dispersion. This is not true for the open tubular column, as the HETP is determined by the column radius. It follows that a converse procedure must be employed. Firstly the optimum column radius is determined and then the maximum extra-column dispersion that the column can tolerate calculated. Thus, with open tubular columns, the chromatographic system, in particular the detector dispersion and the maximum sample volume, is dictated by the column design which, in turn, is governed by the nature of the separation. 

It is seen that the optimum column radius for an open tubular column varies widely with inlet pressure arid the difficulty of the separation. Considering a separation of some difficulty, for example ( a = 1.02), it is seen that at an inlet pressure of lOOOp.s.f, the optimum column diameter would be about 4 micron whereas, at an inlet pressure of only 1 psi, it would be about 43 micron. The former would be quite difficult to coat with stationary phase and would demand detectors and injection systems of almost impossibly low dispersion A column of 43 micron in diameter, on the other hand, would be piactical from the point of view of both ease of coating and an acceptable system extra column dispersion. However, the lengths of such columns arid the resulting analysis times remains to be determined and may preclude their- use. 

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