Extra Column Dispersion - Dispersion in the Detector Sensor Volume > Page 37


From the slope of the curve the dispersion per frit could be calculated. The results obtained are shown as a curve relating dispersion to number of frits in figure 16.It is seen that a single frit contributes very little to the peak dispersion. This is due to the eddy diffusion that takes place inside the pores of the frit, which is similar to that which occurs in the interstices between the particles in a packed column. In fact, it is similar to the radial diffusion tat takes place in a serpentine tube. As the solute winds its way between the interstices of the frit, the mobile phase stream is continually changing direction, causing radial flow across the pore. This causes forced mixing and, thus, there is little or no Newtonian flow taking place within the pores of the frit. However, the small dispersion that does occur will result from a multipath type of dispersion, similar to that which occurs in a packed column (see Dispersion in Chromatography Columns). Multipath dispersion in a packed column increases with the particle size of the packing and so will increase with the porosity of the frit. However, the net contribution will still be small unless the pores are exceedingly large. As a consequence, in analytical chromatography, the contribution by the frit need not be considered a significant source of extra column dispersion in any chromatographic system.


Dispersion in the Detector Sensor Volume

Irrespective of the type of detector or whether it is for LC or GC, if it is concentration sensitive, the device that actually senses the solute contained in the mobile phase must address a finite volume of the column eluent in order to measure the concentration of the solute and sense its presence. The caveat, that the sensor is concentration sensitive is important as, if the detector is mass sensitive, the response depends on the mass per unit time passing through it and not the mass per unit volume that passes through it. This is clear when the operation of the flame ionization detector (FID) is considered. In practice, for example, the eluent from a capillary column is mixed with a hydrogen, or hydrogen/nitrogen stream, which is then burnt at a small jet and the ions produced measured by an appropriate pair of electrodes. The response of the detector will depend on the mass of solute eluted per unit time from the capillary column and, thus, will be independent of the hydrogen or hydrogen/nitrogen flow (as this will not effect the rate of solute elution, in terms of mass per unit time). In contrast, if the FID was a concentration device (like the katherometer), as the hydrogen or hydrogen/nitrogen flow dilutes the sample, the response will be directly related to the flow rate of the diluting gas. It follows, that for a mass sensitive device, the sensing volume can be extremely mall and, in fact, insignificant. However, for a device to sense a concentration change, the sensor must have a finite (albeit small) volume.