Low Dispersion Tubing In order to avoid dispersion in mobile phase conduits a number of attempts to design low dispersion tubing has been reported. The first attempt was by Halasz et al. (8), who crimped and bent the tube into different shapes to interrupt the Newtonian flow and introduce radial flow within the tube. His devices had limited success and the tubes had a tendency to block very easily. In 1978 Tijssen (9), developed a theory to describe the radial flow that was induced into coiled tubes by the continual change in direction of the fluid as it flowed round the spirals (his theory will be

nbsp; Low Dispersion Connecting Tubes The ideal solution to conduit dispersion, where the sample valve and the detector sensor cell are coupled directly to the column, is, in practice, mostly impossible. Consequently, a conduit system that provides little or no dispersion would be extremely useful. In order to reduce dispersion due to Newtonian flow through an open tube, the parabolic velocity profile of the fluid must be disrupted to introduce rapid radial mixing. The parabolic velocity profile can be disturbed, and secondary flow introduced, into the tube, by deforming its regular geometry. Dispersion that occurs in geometrically deformed tubes (squeezed, twisted and coiled) has been studied by Halasz (4, 5 and 6), and the effect of radial convection (secondary flow) on the dispersion introduced in

Low Dispersion Gradient Elution Apparatus Gradient elution with high efficiency low dispersion columns, particularly small bore columns that operate at very low flow rates, present another instrumental problem for the designer of the modern chromatograph. Not only must the gradients be formed accurately and precisely, they must be also formed at very low flow rates and sometimes the total volume employed for the analysis will be less than 1 ml.  The apparatus can be basic and accommodate only to solvents systems. It usually consists of two pumps controlled by an appropriate solvent programmer special modified to cope with the small flow rates used with small volume columns.   J. Chromatogr.,185(1979)27 Figure 25 A Low Volume Mixing T

still persists. It would appear that the low dispersion serpentine tubing is the most satisfactory alternative to straight tube.   Dispersion in Unions and Stainless Steel Frits Depending on their design, unions can also be a significant source of extra column dispersion. Instrument manufacturers have been aware of the problem of union dispersion and, as a consequence, have designed low dead volume unions which are now generally available. Actual data reporting the extent of the dispersion that takes place in such unions does not, however, appear to be readily available.  Scott and Simpson also measured the relative dispersion that occurred in normal, low dead volume unions and drilled-out unions. Drilled-out unions allow the ends of the connecting tubes to butt against one another, or against the frit of a microbore column and thus reduce the union volume dead volume to virtually zero. The design of low dead volume and drilled out unions are depicted in figure 15

nbsp; It is seen that the river water and the town water contain significant quantities of soluble organic materials whereas the well water is almost devoid of organic contaminants. The level of the impurities is in the 1 to 100 ppb range. The great attraction of this type of trace analysis is it capacity for almost limitless mass sensitivity and the simple nature of the procedure. The Use for Low Dispersion Instrumentation with Small Bore Columns to Obtain Ultra High Efficiencies. Small bore columns can be connected in series to lengths of 10 or even 20 meters to provide a proportional increase in overall efficiency. This, at least in theory, can be also achieved using wide (4.6 mm I.D.) columns but, the solvent consumption for such columns would be enormous. The retention volume of a solute eluted at a capacity ratio of 20  on a 10 m column 4.6 mm I.D. would be about 2.8 liters,

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 open tube results from the parabolic velocity profile that occurs under conditions of Newtonian flow, (i.e. when the velocity is significantly below that which produces turbulence). Under condition of Newtonian flow, the