For a more in-depth comparison between these two columns the dependence of height equivalent to a theoretical plate (HETP or H) was evaluated versus the mobile phase linear velocity (u) for all test analytes. As an example, Fig. 4 shows the H/u curve for the first peak of thalidomide. It was found that H/u curves were quite similar for all studied compounds. The value of the optimal flow rate was higher for the core–shell material but not as much as recently reported for columns of 4.6 mm I.D. [18]. The minimum plate heights for both CSPs for all 7 analytes included in this study are listed in Table 1. These values correspond to reduced plate height values of 4–6 which are significantly higher than previously reported for reversed-phase core–shell materials packed into narrow bore and standard I.D. columns. This finding seems to support the opinion that HPLC column performance decreases with decreasing the ratio between column diameter and particle
diameter dp).