Dead volume, detectors

Testing apparatus should be designed to minimize band spreading external to the column (e.g., short, narrow connecting tubing between the column and injector and detector, low dead-volume detector flow cell, etc.). 

Equation (33) shows that the maximum capacity ratio of the last eluted solute is inversely proportional to the detector sensitivity or minimum detectable concentration. Consequently, it is the detector sensitivity that determines the maximum peak capacity attainable from the column. Using equation (33), the peak capacity was calculated for three different detector sensitivities for a column having an efficiency of 10,000 theoretical plates, a dead volume of 6.7 ml and a sample concentration of l%v/v. The results are shown in Table 1, and it is seen that the limiting peak capacity is fairly large. 

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 contribution of the equipment between injection unit and detector cell should be negligable in relation to the column for a sufficient column characterization short connections with narrow capillaries and zero dead volume unions are the precondition for reliable plate numbers. Every end fitting of a column causes additional band broadening. In the past a column type was offered that could be directly combined without any capillary links unfortunately, it has disappeared from the market. 

The detector cells normally are connected by a capillary. For the interpretation of the detector signals the volume of this connection must be known data of the following detectors have to be shifted for the delay toward the first cell. Usually this dead volume cannot be measured immediately, as it has to be determined indirectly by test analyses, e.g.,... 

It must be pointed out that (Vm) refers to the volume of mobile phase in the column and not the total volume of mobile phase between the injection valve and the detector (Vo)- In practice, the dead volume (Vo) will include all the extra column volumes (Ve) involved in the sample valve, connecting tube detector cell etc. 

Dispersion can be produced outside the column by dead volume in the injector, the detector or the plumbing. The combined effect of all these is called extra-column dispersion. Fig. 2.3c shows an example of this, in which different dead volumes are connected between the column and the detector, and Fig. 5.3b shows dispersion produced by dead volume at the top of the column. You can see from these that dead volume effects can cause a serious loss of performance. 

In terms of volume, V0 is a measure of the system dead volume from the injector to the detector. For a well designed system with low extra-column dispersion, V0 will be roughly equal to the dead volume of the column, that is, the volume of the column not occupied by the packing particles. 

We should therefore use small bore tubing in short lengths for making the injector-column and the detector-column connections, and the injector and detector must be designed so that their internal volume is as small as possible. Dead volume before the introduction of the sample should also be minimised, to facilitate rapid changes of mobile phase composition that may be required during gradient elution. 

The flow cell is the most important component of a flow injection manifold for CL measurements since maximum radiation should be generated while the solution is flowing in front of the detector. Other attributes of the flow cell are the small dead volume of the cell to allow fast and effective washing between injections... 

Speed Useful for a variety of samples Selective and efficient columns High flow rate Fast data output Variety of detectors and stationary phases Low dead-volume fittings High-pressure pumps Fast-response recorders and automatic data handling... 

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