Page 139 - Tandem Techniques

Page 121

ml per minute and standard columns at about 5 ml/min. It is seen that the acceptable length of tubing is
considerably less than that for the same columns when operated at their optimum flow rates.

Equation (3.21) clearly indicates the procedure that must be followed to reduce the dispersion that
arises from any open tube associated with a tandem interface. However, the magnitude of the flow rate
(uo) is determined by the chromatographic system and thus, is not a variable that can be used to control
tube dispersion. In a similar manner the diffusivity of the solute, (Dm), is determined by the nature of
the sample and the nature of the mobile phase that has been specifically selected for the particular
separation. Consequently, (Dm), is also not a variable available for dispersion control. The remaining
and in fact the major parameters that are free for controlling dispersion are the tube radius and, of
course, the tube length. It is seen that the dispersion increases as the fourth power of the tube radius and
thus, a reduction in the tube radius by a factor of two will reduce the dispersion by a factor of sixteen.

Unfortunately, there is a limit to the process of reducing (r) as, from Poiseuille's equation, the pressure
drop (AP)across the tube is given by,

where (h) is the viscosity of the mobile phase, and as

It is seen from equation (3.22), that the pressure drop across an open tube increases inversely as the
fourth power of the tube radius. Thus, as it is inadvisable to dissipate significant amounts of the
available column inlet pressure across the interface, there will be a lower limit to which (r) can be
reduced in order to minimize dispersion.