Free Books and Brochures

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

in only one commercial LC detector. It should be pointed out that any conduit system that has low dispersion will also provide very fast heat transfer rates. Serpentine tubing has been also used in commercial column ovens to heat the mobile phase rapidly to the column oven temperature before it enters the column. The serpentine tubing allows effective heat exchange with a minimum of heat exchanger volume to distort the concentration profile of the solvent gradient. The different forms of dispersion profiles that are obtained from various types of connecting tubes used in LC are shown in figure 4. Figure 4 Dispersion Profiles from Different Types of Tube   These dispersion curves were obtained using a low dispersion UV detector (cell volume, 1.4 ml) and a sample valve with a 1 ml internal loop. All tubes were of the same length and carried the same mobile phase at a flow rate of 2 ml/min. employed. The peaks were recorded on a high speed recorder. The peak from

dispersion should be allotted to the sample volume, as a large sample volume may be necessary to handle a particular sample type for successful analysis. In any event, the chromatograph should always be designed so that the dispersion from other parts of the system is kept to the absolute minimum. Dispersion in sample valves can be minimized by mechanical design (internal loop valves tend to provide the minimum dispersion). Dispersion from unions can be minimized by using drilled-out unions or low dead volume unions. Stainless steel frits provide very little dispersion and can be employed without great concern for their contribution to the overall dispersion of the system. Connecting tubes are one of the major sources of extra-column dispersion and should be kept as short as possible and the radius reduced to a minimum commensurate with the tube not becoming blocked. Tubes 0.005 in. in diameter are recommended as the minimum diameter that is unlikely to become blocked. In

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

0.030 in 0.667 0.010 in 0.338 Flow rate 20 ml/min. The data was obtained using a specially designed low dispersion chromatographic system and an electrical conductivity detector with a sensor volume of only 0.08 ml. Despite its short length, it is clear that the internal bore of the valve connection can still cause significant dispersion. In addition, it would appear that if the diameter of the connection was further reduced the dispersion would be even less. However, apertures less than 0.005 in. I.D. can readily become blocked. The absolute minimum internal diameter that is recommended is 0.003 in. and if apertures of such dimension are present in the chromatographic system all samples and the mobile phase should be filtered before use. From a theoretical point of view, it should be possible to introduce secondary flow into the conduit, increase the solute diffusivity, and consequently, reduce dispersion. The

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