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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

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

. Pure carrier gas enters the center of the right hand vertical tube and splits into two streams one passing along the lower horizontal tube and the other along the upper horizontal tube. The eluent from the column enters the center of the middle tube and the flow also splits into two streams and each meets the respective flow from the right-hand tube. The flows in the two horizontal tubes finally pass up and down the left-hand vertical tube to meet at the center and then exit to waste. Flow sensors are situated in the horizontal tubes between the right-hand vertical tube and the center vertical tube. When only carrier gas is present in the system, the horizontal flows are equal and the temperature and thus the potential across the filaments of the two sensors are the same. When a solute is eluted from the column, vapor will be present in the center vertical tube and the pressure at the top and bottom of the tube will differ

to Lovelock [20], if each of these electrons can generate 10,000 metastables on the way to the electrode, the steady state concentration of metastables will be about 1010 per ml (this assumes a life span for the metastables of about 10-5 seconds at NTP). From the kinetic theory of gases it can be calculated that the probability of collision between a metastable atom andan organicmoleculewillbeabout 1.6 : 1. This would lead to a very high ionization efficiency and Lovelock claims that with sensors of more advanced sensors design, ionization efficiencies of 10% could be achieved. Ionization efficiencies of at least 0.5 % are readily obtainable, which, compared with that of the FID, is very large indeed