Extra Column Dispersion - LC Sample Valves > Page 20
Table 2 Variance of Solute Bands Resulting from Two Different Valco Valves
|Internal Bore of Valve Connection||Variance of Peak (ml2) Mean of Three Measurements|
|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 dispersion from an exterior sample tube can be reduced by packing it with appropriately sized glass beads (which would make the sample tube more like a packed column than an open tubular column). This might be advantageous for sample tubes of very large volume similar to those used with preparative columns. Alternatively, crimping the tube (4), coiling the tube (5) or employing low dispersion tubing (6) could also help reduce dispersion in open tubes.