Extra Column Dispersion - Dispersion in Detector Sensors Due to Newtonian Flow > Page 42

 

It is seen from figure 18 that depending on the number of theoretical plates it contain in the cell (i.e. the length of the cell) the peak can exhibit various degrees of dispersion and types of distortion. However, it must be emphasized that the curves shown in figure 18 only occur when there is true Newtonian flow through the cell (i.e., all flow rates are below the maximum where turbulent flow is initiated). If, by appropriate design, the parabolic velocity profile of the fluid flowing through the cell can be disrupted, then the dispersion and distortion arising from Newtonian Flow can be virtually eliminated.

In practice, the disruption of the normal parabolic velocity profile of the fluid flowing through the cell can be readily achieved by modifying the manner in which the conduits cause the mobile phase to enter and exit the cell. The conduit connections to the cell are oriented to produce secondary flow in the manner shown in figure 19. Mobile phase enters the cell at an angle so that fluid stream is directed at the cell window. As a result of this, the direction of the fluid flow must change by nearly 360˚ and virtually reverse its direction in order to pass through the cell, producing a strong radial mixing and disrupting the Newtonian flow.

 

Figure 19. The Design of a Modern Absorption Cell