On the performance of conically shaped columns: Theory and practice.

The chromatographic performance (speed, efficiency, and gradient peak capacity for the same analysis time) of conical columns are investigated from fundamental and experimental viewpoints. A stainless steel, conically shaped column (2.1 mm i.d/4.2 mm i.d. × 15 cm long, 0.4° opening angle) was prepared in-house and packed with 5 μm XBridge-C18 fully porous particles. Its performance was compared to that of a conventional 3.0 mm × 15 cm cylindrical column packed with the same batch of particles. Both Giddings' theory of non-uniform columns and experiments agree and show that, irrespective of flow direction, the conical column is 15% less efficient than the conventional column. Remarkably, Blumberg's theory of band broadening in gradient elution mode predicts that conical columns may outperform conventional cylindrical columns if the ratio of their outlet i.d. to their inlet i.d. is 0.95 and 0.80 for small molecule and peptide mixtures, respectively. The maximum relative gain is marginal as it does not exceed a few percents. The theory reveals that the flow direction should be from the wide to the narrow end of the conical column in order to deliver the highest peak capacity. In agreement with the theory, the observed losses in absolute peak capacity for the same analysis time are 14.5% (narrow to wide end) and only 11.0% (wide to the narrow end) for small molecules (n-alkanophenones). They are 14.2% (narrow to wide end) and only 8.5% (wide to the narrow end) for peptide samples (bombesin). Additionally, conical columns reduce peak tailing with respect to standard columns. They are suitable column technology for ultra-fast gradient separations as they also minimize sample dispersion through the narrow i.d. outlet frit.