Evaluation of Six Core Shell Columns Based on Separation Behaviour and Physical Properties

In this study, the separation behaviour and physical properties of Kinetex C18, Accucore C18, Cortecs C18, PoroShell EC-C18, Ascentis Express C18 and SunShell C18 core shell particle columns were evaluated. The evaluation and performance measurements were done using the Tanaka Method [1] with reference to retention factor, hydrogen bonding capacity, hydrophobicity and steric selectivity. In addition to peak shape profiles, the core shell columns were evaluated for loading capacity of amitriptyline under neutral pH condition. The stability of the core shell columns at acidic pH 1 and basic pH 10 conditions, at elevated temperatures was evaluated. In addition to determination of particle physical properties such as specific surface area, pore volume, pore diameter and carbon loading of each C18 packing material, we studied the forced degradation of alkyl chains by sintering at 600 degree Celsius for 8 hours. This study revealed significant differences for the six core shell C18 particles with regard to separation behaviour, stability and physical properties. When compared to fully porous silica C18 sorbents, this difference may be attributed to the diversity of each manufacturing method and bonding technique. Of the six columns tested, SunShell C18 showed the largest retention factor and the highest physical stability despite moderate carbon loading (7%), while the lowest retention factor and the lowest stability in regards to chromatographic performance and endurance at elevated temperature and pH could be correlated with the core shell column which had the lowest carbon loading and the lowest specific surface area.

Introduction
Columns packed with 2.6 μm and 2.7 μm core shell particles (also alternatively termed solid core, porous shell or superficially porous particles) have been widely documented for HPLC and UHPLC. These core shell columns show comparable column efficiency to 1.8 μm fully porous particles along with an approximate 50 % reduction in backpressure in comparison to sub-2 μm fully porous particle (2). Core shell particles consist of a solid core of 1.6 μm to 1.7 μm diameter and an outer porous silica layer of 0.5 μm thickness giving a larger particle measuring 2.6 μm to 2.7 μm and lower operating backpressure in comparison with sub-2 μm fully porous particle (back pressure largely being governed by particle size). The volume ratio between the porous silica layer of the core shell particle and the fully porous particle of the same particle size is 75% and does not have a significant impact on the separation performance [2].
The advantage of the core shell particles is thought to arise from the structure of the core shell particle and narrow particle size distribution [3] as the packing material leads to lower values of the A term (Eddy Diffusion), B term (Longitudinal Diffusion) and C term (Mass Transfer) in the van Deemter equation. The lower values of the A term is due to a narrow particle size distribution and dense packing methodologies leading to a minimisation of the space among particles in the column. A higher value of the A term leads to increased dispersion of the analyte due to increased differences in the varying pathways around the particles [4]. The lower value of the B term is due to the limiting of the diffusion of the solute within the space between core shell particles. The inner non porous core blocks diffusion of the solute, leading to reduction in longitudinal diffusion. The reduced diffusion path length on the thin porous silica layer lowers the C term due to more rapid analyte concentration equilibration due to lower mass transfer effects.
At present more than 15 types of core shell column are available on the market. Two types of processes are used to manufacture core shell silica particles.