Hello, Is it possible that slurry packing methods which work very well with analytical size 4.6 mm i.d. tube tend to give much lower performance in terms of chromatographic efficiency with microbore columns such as 2.1 mm i.d? What is the phenomenological basis of this apparent discrepancy? I am interested in porous spherical silica particles.
Thanks.
Microbore columns are difficult to slurry pack because of there very small diameter i.e. 1mm id-4.5mm id, and capillary columns of 200 mu micron in diameter. Smaller size makes better capillary force with in the micro-particles packed inside the column, it enhances reproducibility, sensitivity and compatibility of columns.Capillary columns are packed using electroosmotic flow (EOF) induced by an electrical double layer on the internal wall of a fused silica capillary and the electrophoretic mobility of positively-charged or negatively-charged particles of packing material. Electrokinetic packing is also applicable to uncharged packing particles. With this electrokinetic method several capillaries can be packed simultaneously. Electrokinetically packed capillary columns are useful for the separation of test mixtures and of pharmaceuticals in both electrochromatography and micro-HPLC.
I agree that narrow bore columns have small inner diameter than analytical columns but what is the fundamental cause of poor efficiency of microbore vs. analytical columns?
My guess is that the root cause is connected to the difficulty of achieving a uniformly packed bed as the internal diameter shrinks. A symptom that clearly points in that direction is the bed permeability. There is a general trend toward increasing permeability as the ID shrinks. There are those who make the argument that this is a good thing but fundamentally this is an indication that the bed density is lower than what would be expected given the properties of the same media in larger internal diameter columns. As mentioned above the wall effect grows more significant as the internal diameter shrinks. If you look at the force vectors involved in the packing process, a portion of the force vector is directed radially toward the wall while the majority of the force vector is longitudinally directed toward the outlet of the column. In larger diameter columns the vast majority the force vector is longitudinal but the radial component increases as the column diameter decreases making it more difficult to pack a dense bed due to the sideload associated with the radial vector. In theory this should be something you can compensate for by increasing packing force or increasing the dilution factor of the resin slurry.
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