Dispersion connecting tubes

It is clearly seen that the connecting tube dispersion of 3 p.1 will still be equivalent to 75% of the column dispersion. Ideally, the extra column dispersion, in terms of standard deviation, should only be about 1 1 but this is extremely difficult to achieve in practice. Some... 

The Golay equation [9] for open tubular columns has been discussed in the previous chapter. It differs from the other equations by the absence of a multi-path term that can only be present in packed columns. The Golay equation can also be used to examine the dispersion that takes place in connecting tubes, detector cells and other sources of extra-column dispersion. Extra-column dispersion will be considered in another chapter but the use of the Golay equation for this purpose will be briefly considered here. Reiterating the Golay equation from the previous chapter. 

Dispersion in the valve-column and column-detector connecting tubing ( a.j-) ... 

The maximum allowable dispersion will include contributions from all the different dispersion sources. Furthermore, the analyst may frequently be required to place a large volume of sample on the column to accommodate the specific nature of the sample. The peak spreading resulting from the use of the maximum possible sample volume is likely to reach the permissible dispersion limit. It follows that the dispersion that takes place in the connecting tubes, sensor volume and other parts of the detector must be reduced to the absolute minimum and, if possible, kept to less than 10% of that permissible (i.c.,1 % of the column variance) to allow large sample volumes to be used when necessary. 

Equation (11) shows that the pressure drop across the connecting tube increases inversely as the fourth power of the tube radius. It follows that, as it is impractical to dissipate a significant amount of the available pump pressure across a connecting tubing, there will be a limit to the reduction of (r) to minimize tube dispersion. 

Reducing the length of the connecting tube has the same effect on both dispersion and... 

The curve in Figure IB is probably more useful from a practical point of view. Although the standard deviations of any dispersion process are not additive, they do give a better impression of the actual dispersion that a connecting tube alone can cause. It is clear that a tube 10 cm long and 0.012 cm I.D. can cause dispersion resulting in a peak with a standard deviation of 4 pi. Now, a peak with a standard deviation of 4 pi would have a base width of 16 pi and, in practice, many short... 

The different forms of dispersion profiles that are obtained from various types of connecting tubes used in LC are shown in Figure 10. 

Extra-column dispersion can arise in the sample valve, unions, frits, connecting tubing, and the sensor cell of the detector. The maximum sample volume, i.e., that volume that contributes less than 10% to the column variance, is determined by the type of column, dimensions of the column and the chromatographic characteristics of the solute. In practice, the majority of the permitted extra-column dispersion should... 

Unfortunately, some of the data that are required to calculate the specifications and operating conditions of the optimum column involve instrument specifications which are often not available from the instrument manufacturer. In particular, the total dispersion of the detector and its internal connecting tubes is rarely given. In a similar manner, a value for the dispersion that takes place in a sample valve is rarely provided by the manufactures. The valve, as discussed in a previous chapter, can make a significant contribution to the extra-column dispersion of the chromatographic system, which, as has also been shown, will determine the magnitude of the column radius. Sadly, it is often left to the analyst to experimentally determine these data. 

The system dead volume must be reduced to an absolute minimum, particularly when using very efficient narrow-bore SEC columns. Extra column dispersion becomes a greater consideration as the column volume is reduced, and dead volume should be minimized in all parts of the system, including injection valves, connecting tubing, and detectors, if the column performance is to be realized. 

The dimensions of the exit tube from the detector are not critical for analytical separations but they can be for preparative chromatography if fractions are to be collected for subsequent tests or examination. The dispersion that occurs in the detector exit tube is more difficult to measure. Another sample valve can be connected to the detector exit and the mobile phase passed backwards through the detecting system. The same experiment is performed, the same measurements made and the same calculations carried out. The dispersion that occurs in the exit tube is normally considerably greater than that between the column and the detector. However, providing the dispersion is known, the preparative separation can be adjusted to accommodate the exit tube dispersion and allow an accurate collection of each solute band. 

The main sources of broadening are the solute dispersion in the connecting tubes, the effective working volume of the detector and the dynamics of the sensor, of the electronic circuitry and of the recording device. 

The dispersion that takes place in the sample-valve/column connecting tube and the column/detector cell connecting tube will both result from the parabolic velocity profile that occurs in open tubes and thus, will be considered together. 

Equation (5) clearly indicates the procedure that must be followed to reduce the dispersion that arises from connecting tubes. However, for maximum efficiency, the column should be operated at its optimum mobile phase velocity and consequently, the flow rate, (Q), is already defined, and cannot be used to control tube dispersion. In a similar manner the diffusivity of the... 

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