Some practical comparisons of the efficiency and overloading behaviour of sub-2 μm porous and sub-3 μm shell particles in reversed-phase liquid chromatography

At their optimum flow, sub-3 μm superficially porous or “shell” particles demonstrate similar efficiency to sub-2 μm totally porous particles. The performance of 0.21 cm i.d shell columns is however inferior to those of 0.46 cm i.d., presumably due to packing difficulties. At high flow, shell columns can give flatter Knox curves due to lower operating pressure (half or less of that of the totally porous particles) producing less frictional heating, which combined with the increased thermal conductivity of their non-porous core, gives more efficient heat dissipation. However, the effects of frictional heating for sub-2 μm columns are considerably exaggerated when using pure ACN as mobile phase, as it has a thermal conductivity 3 times less than that of pure water, leading to poorer heat dissipation. Overloading is already problematic for ionised solutes, a group which contains many pharmaceuticals and compounds of clinical relevance, on conventional columns (5 μm porous particles). However, it becomes a more serious issue for both new column types, partially as a result of their very high efficiency, which concentrates the sample as a very narrow band. The sample capacity of one type of shell particle was estimated to be 60% of that of the small totally porous particles, in line with the fraction of the particle volume that is porous. Due to overloading, it is barely possible to achieve perfect peak symmetry for ionised acids or bases with either of these new column types, even by injecting the lowest amounts of sample detectable by UV. While ammonium formate and potassium phosphate buffers gave similar results in overloading studies, use of formic acid as sole mobile phase additive is not recommended for these solutes, as its ionic strength is too low, leading to a catastrophic deterioration in efficiency when sample concentrations of even a few mg/L are injected.