In this study, a systematic evaluation of core–shell particle columns for improved analysis and separation of RNA oligonucleotide samples by IP-RPLC compared various core–shell particle columns with differing particle and pore sizes. The results showed significant improvement in peak shape and separation of closely eluting impurities for the core–shell columns when analyzing RNA oligonucleotide samples.
The comparison results for core–shell (Kinetex C18 column) and fully-porous particles (Luna C18 (2) column) showed much narrower peak width (base) for core–shell particles (0.10 min compared to 0.17 min for fully-porous particles). This resulted in significant improvement in separation when using core–shell particles, where fully-porous particles provided resolution of about 1.1 for the separation of the impurity eluting on the tail of the main peak, and core–shell particles provided much improved resolution of about 2.0.
The comparison of different core–shell C18 columns showed the best peak shape with Kinetex XB-C18 column, where peak widths (base) for the main component were 0.26 min for Ascentis Express C18 column, 0.18 min for Poroshell EC-C18 column and 0.13 min for the Kinetex XB-C18 column. The more recent sub-2 m core–shell Kinetex C18 column provided even greater separation efficiency where more peaks could be separated from the complex peak clusters for these samples.
However, the significant extra column volume contributions from the conventional HPLC instruments need to be considered. With careful modifications to the instrument to minimize the extra column volume contributions, the sub-2 m core–shell particles columns can be used with conventional HPLC instruments for improved analysis and separation of oligonucleotide samples.
All of these columns, however, showed only modest long-term stability at the operating conditions of neutral pH mobile phase and elevated column temperatures of 60 ◦C or higher. Nevertheless, the significant improvement in performance of the core–shell particles, combined with the rapid technological advances and improvements in this new class of stationary phase particles, suggest a promising future for improved oligonucleotide separations.