Open tubular columns optimum length

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. 

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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