Extra Column Dispersion - LC Sample Valves > Page 19
A diagram showing the construction of an internal loop valve is shown
in figure 7. It is seen that the connecting union is part of the body of the valve and the microbore column enters the valve body leaving a very small aperture connecting it to the sample volume in the spigot of the valve. This is probably the ideal form of connection between the sample volume and the column, and will provide the minimum extra column dispersion. There will. however, still remain some small contribution to dispersion from the Newtonian flow through the sample volume itself and the small aperture acting as a conduit to the column.
Many sample valves are connected to the column by a sequence of union–capillary tube–union–column. Most contemporary unions, used for this purpose, are designed to have low dead volumes which has largely eliminated union-dispersion. Consequently unions no longer contribute significantly to extra column dispersion in most chromatographic systems. However, it should be pointed out that they are still not completely dispersion free.
If an external tube sample valve is employed, then the vast majority of the dispersion that takes place in it will be the same as that, which would occur in a connecting tube of equivalent length (this will be discussed below). Dispersion resulting from the internal sample volume that is formed by a groove cut in the valve spigot, together with dispersion arising in the sample valve exit ports, is difficult to treat theoretically but can be determined experimentally. Scott and Simpson (3), Measured the dispersion from two different 0.2 ml sample valves and their results are shown in Table 2.