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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

nbsp;   Figure 6 The Design of a Modern Absorption Cell   The Newtonian flow is distorted by the manner in which the inlet and outlet conduits are connected to and from the cell. Mobile phase enters the cell at an angle that is directed at the cell window. It follows, that the mobile phase flow has to virtually reverse its direction to pass through the cell producing a swirling action which introduces strong radial flow and disrupts the Newtonian flow. The effect also occurs at the exit end of the cell. The flow along the axis of the cell now must reverse its direction to pass out of the port which is accomplished by attaching the exit conduit at an angle to the axis of the cell. Employing this type of entry and exit connections eliminates dispersion resulting from viscous flow

Dispersion in Tubular Conduits The dispersion that takes place in open tubular conduits, as already described, results from the parabolic velocity profile that occurs under conditions of Newtonian flow. The distribution of fluid velocity across the tube under condition of Newtonian flow, is depicted diagramatically in Figure 8A. Due to the relatively high velocity at the center of the tube, and the very low velocity at the walls, the center of the band of solute passing down the tube will move ahead of that situated at the walls. This will result in band dispersion and this effect is depicted in Figure 8B. Figure 8.  Dispersion Due to Newtonian Flow in Connecting Tubes

smaller than the peak volume the effect will merely give the peak an apparent dispersion. However, if the sensor volume becomes of the same order of magnitude as the peak volume, then the peak profile will be distorted and resolution will be lost.  In the extreme case two peaks could coexist in the sensor at one time and only a single peak will be represented.  The effect of viscous flow on dispersion will first be considered. Dispersion in Detector Sensors Resulting from Newtonian Flow Most sensor volumes are cylindrical in shape, are relatively short in length, and have a relatively small length-to-diameter ratio. The small length-to-diameter ratio is in conflict with the premises assumed in the development of the Golay equation for dispersion in an open tube. Atwood and Golay (11) extended the theory of dispersion in open tubes to tubes having small length-to-diameter ratio. The theory is complex and not relevant here as, if appropriate cell design is

Dispersion in Connecting Conduits Dispersion that occurs in connecting tubes, or conduits, is exclusively due to the parabolic velocity profile of the mobile phase passing through it. This always occurs when the mobile phase velocity is less than the critical velocity (the velocity at the onset of turbulent flow). As the critical velocity of gases and liquids is well above those used in practical gas and liquid chromatography, this type of dispersion is always present unless the parabolic flow pattern is disrupted. The conditions where a parabolic velocity profile exists across a conduit is termed Newtonian Flow and any connecting tube between the sample valve and the column, the column and the detector, or any other cylindrical conduit will contribute to peak dispersion

1. Dispersion from Connecting Tubes(Newtonian) 2. Dispersion from Sensor Cell Volume (Newtonian) 3. Dispersion from Sensor Cell Volume ( Dilution) Each of these sources of dispersion are controllable by careful sensor design and employing appropriate cell geometry. Dispersion in Connecting Tubes The dispersion that takes place in an open tube results from the parabolic velocity profile that occurs under conditions of Newtonian flow, (i.e. when the velocity is significantly below that which produces turbulence). Under condition of Newtonian flow, the distribution of fluid velocity across the tube adopts a parabolic profile as shown in figure 1. The velocity at the walls is virtually zero and that at the center a maximum. This situation is depicted diagramatically in Figure 1. Figure 1. The Parabolic Velocity Profile of a Solute Band Passing Through a Tube Due to the relatively