Dispersion in sample valves

Dispersion in sample valve/column and column/detector con necting tubes that can be considered to contribute a variance... 

The peaks shown were obtained using a low dispersion UV detector (cell volume, 1.4 pi) in conjunction with a sample valve with a 1 pi internal loop. All tubes were of... 

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 mass spectrometer sampling capillary or the dispersive infra-red analyzers used for continuous analysis and monitoring of the gas phase composition are situated between the reactor and the sampling valve, as close to the reactor as possible, in order to avoid any delay in the recording of changes in the composition of reactants or products. This delay should be taken into account when plotting simultaneously the time dependence of catalyst potential or current and gas phase concentration of the reactants or products. 

Where peak dispersion has not been constrained to very small volumes the external sample loop injector can be used and the external loop sample system, which employs six ports, is depicted in figure 15. In the external loop sample valve, three slots are cut in the rotor so that any adjacent pair of ports can be connected. In the loading position shown on the left, the mobile phase supply is connected by a rotor slot to port (4) and the column to port (5) thus allowing mobile phase to flow directly through the column. In this position the sample loop is connected to ports (3) and (6). Sample flows from a syringe into port (1) through the rotor slot to the sample loop at port (6). At the same... 

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 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. 

Another extremely important instrument specification is the total dispersion that takes place in the sample valves, connecting tubes and detector cell of the chromatograph. The subject of extra column dispersion has already been discussed in the previous chapter. It has been shown that the extra column dispersion determines the minimum column radius and thus, both the solvent consumption per analysis, and the mass sensitivity of the overall chromatographic system. The overall extra column variance, therefore, must be known and quantitatively specified. 

The different forms of dispersion profiles that are obtained from various types of connecting tubes used in LC are shown in figure 9. These dispersion curves were obtained using a low dispersion UV detector (cell volume, 1.4 pi) in conjunction with a sample valve with a 1 pi internal loop. All tubes were of the same length and a flow rate of 2 ml/min was employed. The peaks were recorded on a high speed... 

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