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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 extremely compact, is highly reliable and affords accurate control of the carrier gas flow rate irrespective of gas viscosity changes due to temperature programming

rate are shown in figure 13. The flow rate is employed as the independent variable as an alternative to the more usual linear velocity because, in practice, the flow rate is defined by the column with which the low dispersion tubing is to be used. In fact, the column flow rate is independently defined by the chromatographic characteristics of the column. The curve obtained for the serpentine tube is similar to that for the coiled tube, but the maximum value of (H) occurs at a significantly lowerflow rate with the serpentine tube. It is seen that once the flow rate exceeds about 1.5 ml/min., the dispersion is small and remains more or less constant over a wide range of flow rate range that embodies those usually employed in  LC separations. Ref (P) J. Chromatogr. 268(1978)681 Figure 13. Graph of Variance against Flow Rate for Coiled and Serpentine Tubes

a knowledge of (NP) can be used in detector design when a particular sensitivity is the objective.   Flow Sensitivity Flow sensitivity is another detector property that can have a significant effect on long term noise and, consequently, also on the detector MDC. Again it is the bulk property detectors that are the most likely exhibit high flow sensitivities (e.g., the katharometer). To reduce its flow sensitivity, the katharometer is usually fitted with a reference cell through which a flow of mobile phase also passes. The two sensors for the column flow and the reference flow are placed in the arms of a Wheatstone bridge so that any changes in flow rate are to a large extent compensated. The flow sensitivity (DQ) is defined in a similar manner to pressure sensitivity (i.e. mV/ml/min). The flow sensitivity can be used to calculate the flow change (NQ) that would  provide a signal equivalent to the detector noise (ND),                          i.e.             A

the column eluent mixes with the hydrogen flow and is burnt. The jet and the actual flame is shielded to prevent light from the flame itself falling directly on to the photo-multiplier. The base of the jet is heated to prevent vapor condensation. The light emitted above the flame, first passes through two heat filters and then through the wavelength selector filter and finally on to the photo-multiplier. The response of the detector to sulfur is fairly insensitive to changes in hydrogen flow rate. However, the response to phosphorus compounds shows a maximum at a particular hydrogen flow rate, the magnitude of which varies with the air flow. A serious problem that can occur in the FPD is the quenching or re-absorption of the light emitted by the selected species. Hydrocarbon quenching can occur when the peak containing sulfur is co-eluted with another hydrocarbon in relatively high concentration. The high concentration of carbon dioxide appears to suppress the characteristic

ethanol/water : 50/50 v/v) by heating and stirring at 40˚C. The filtered sample solution and the column were maintained at 40˚C throughout the sampling procedure and separation. 400 ml of the solution (containing 800 mg of chlorokynurenine) were pumped onto the column at 50 ml/min. for 8 minutes. The sample pump was then stopped, the solvent pump started and the solutes eluted at a flow rate of 50 ml/min. for 20 minutes. As soon as the second enantiomer began to emerge, the flow rate was increased to 60 ml/min. An actual separation is shown in figure 35. and it is seen that the separation that was obtained was highly satisfactory. The products were analyzed on an analytical Chirobiotic T column and indicated that the first enantiomer was >99% pure, and the second enantiomer was 98% pure. The mid fraction, that was collected between the two main peaks, was recycled. The total cycle took 49 minutes and it is seen that the system operated very effectively. The use

a coating block where it is wetted with the column eluent. The chain then enters an evaporator tunnel, is heated, and the solvent volatilized leaving the solute deposited on the chain. The chain then exits the tunnel into the actual flame of an FID. Sample: mineral oil and a surfactant, solvent: n-heptane/ethyl alcohol, column: 2 x 300 mm, column packing: silica gel, flow rate: 0.7 ml/min., chart speed: 24 cm/min., evaporator temperature: 150˚C, nitrogen flow: 30 ml/min., hydrogen flow rate: 25 ml/min., oxygen flow rate: 30 ml/min. Figure 42. Chromatogram Obtained from the Chain Detector