Dispersion in Detector Sensors There are three sources of dispersion in LC detector sensors, 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

volume is significantly 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

Dispersion in Detector Sensors Due to Newtonian Flow The majority of sensor cells are cylindrical in shape, relatively short in length and have a small length-to-diameter ratio (small aspect ratio). Unfortunately, the small aspect ratio conflicts with the assumptions adopted by Golay in the development of the equation that describes the dispersion that takes place during fluid 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,

consists of a bypass tube with a heater situated at the center. Precision temperature sensors are placed equidistant up stream and down stream of the heater. A proprietary set of baffles situated in the main conduit creates a pressure drop that causes a fixed proportion of the flow to be diverted through the sensor tube. At zero flow rate both sensors are at the same temperature. At a finite flow rate, the down stream sensor is heated, producing a differential temperature across the sensors. The temperature of the gas will be proportional to the product of mass flowing and its specific heat and so the differential temperature that will be proportional to the mass flow rate. The differential voltage from the two sensors is compared to a set voltage and the difference used to generate a signal that actuates a valve controlling the flow. Thus, a closed loop control system is formed that maintains the mass flow rate set by the reference voltage. The device can be made

nbsp; Modern sensors and electronic systems employ fast solid state sensors and solid state electronic components. Thus, the majority of detector systems commercially available are sufficiently fast for the vast majority of chromatography applications. In general, the overall time constant of the detecting system should be less than 50 milliseconds. For special applications involving very fast separations, this value may need to be reduced to around 15 milliseconds. Sensors and electronics, with very small

The performance of the photomultiplier (representing a sensor with a fast response) is shown in the lower curves of figure 10. The time constant, determined from the slope of the log curve, was only 40 milliseconds.  A response time of 40 milliseconds is acceptable for most LC separations. Nevertheless in fast LC separations, solutes can be eluted in less than 100 milliseconds in which case an even faster response might be necessary.   Contemporary sensors and electronic systems use fast solid state sensors and solid state electronic components. Thus, most commercial detector systems are sufficiently fast for the vast majority of chromatography applications. As a general rule, the overall time constant of an LC detecting system should be less than 50 milliseconds. For specially very fast separations, a lower value of 15 milliseconds may be necessary. Fast sensors and electronics will respond to high frequency noise so the chromatographic