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

Flow Programmers Flow programming is a procedure where the mobile phase flow-rate is increased during chromatographic development. If the mobile phase is compressible the relationship between retention volume, flow rate and inlet pressure is given by, (1) Where (Vr) is the true retention volume of the solute, (Vr(0)) is the retention volume measured at the outlet. and

does little to explain how the efficiency of a column may be changed or, what causes peak dispersion in a column in the first place. It does not tell us how dispersion is related to column geometry, properties of the packing, mobile phase flow-rate, or the physical properties of the distribution system. Nevertheless, it was not so much the limitations of the Plate Theory that provoked Van Deemter et al  (2) (who were chemical engineers and mathematicians) to develop, what is now termed the Rate Theory for chromatographic dispersion, but more to explore an alternative mathematical approach to explain the chromatographic process. Virtually all basic chromatography theory evolved over the twenty five years between 1940 and 1965 and it was in the middle of this period that Van Deemter and his colleagues presented their Rate Theory concept in (1956). Since that time, other Rate Theories have been presented, together with accompanying dispersion equations and in due course each will be

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