When evaluating the performance of high-performance liquid chromatography (HPLC) columns, efficiency is a term most commonly calculated by measuring peak width at half height, from which N (Number of Theoretical Plates) can then be derived.  Simply, columns that cause a lot of peak broadening are less efficient while columns that produce very narrow peaks are more efficient.  Increased efficiency is desirable as it results in better resolution, higher sensitivity and increased signal to noise ratios.  Historically increases in efficiency have been achieved by reducing the particle size used to pack columns.  However, reducing particle size also results in increased back-pressure, a problem addressed by the introduction of so-called ultra high-performance liquid chromatography (UHPLC) instruments, with higher pressure limits.  Using columns packed with fully porous sub-2 μm particles on UHPLC instruments, it's possible to achieve efficiencies exceeding 260,000 plates/metre, while generating a back-pressure in excess of 400 bar, the pressure limit of standard HPLC instruments.  Despite this increase in efficiency, which allow decreased analysis time and reduced solvent usage, the widespread adoption of UHPLC has been limited predominantly by the cost of upgrading instruments. 

Another method of increasing efficiency is through the use of core-shell particles, which consist of an impermeable silica core covered by a homogenous porous shell. Various groups have shown that columns packed with 2.6 μm core-shell particles can achieve UHPLC performance on an optimised HPLC system, without exceeding the 400 bar pressure limit of these instruments.  Here we discuss the various system parameters that must be optimised to achieve these results.