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

flow through an open tube. As a consequence, the Golay equation for open tubes can not be applied to cylindrical sensor cells. As a result, Atwood and Golay [12] extended the theory of dispersion in open tubes to tubes of small length-to-diameter ratios. Details of the development of the theory will not be given here because, with proper cell design, it is no longer pertinent to practical chromatography systems. It will be seen that, if the sensor cells are designed correctly, dispersion from Newtonian Flow can be made negligible. Nevertheless, the effect of the cell on solute profiles is shown in Figure 18. J. Chromatogr. 186(1979)353 Figure 18. Elution Curves Presented as a Function of the Normalized Tube Length

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

The Effect of Viscous Flow on Dispersion in a Detector Sensor Scott and Simpson (19) also measured the overall effect of dispersion in sensor cells (that is both that due to Newtonian Flow and that due to sensor volume capacity) by simulating a UV adsorption cell used in LC from simple short cylindrical tubes. The results they obtained are shown in table 4. Table 4. Dispersion in Sensor Cells   Cell Volume Cell I.D. Cell Length Vol.Variance Equi.Peak Vol. 0.59  (ml) 0.5 mm 3.0 mm 0.45 (ml2) 2.68 (4s) (ml) 1.96  (ml) 0.5 mm 10.0 mm 1.19 (ml2) 4