Use of Ultra-short Columns for Therapeutic Protein Separations, Part 2: Designing the Optimal Column Dimension for Reversed-Phase Liquid Chromatography.

In the first part of the series, it was demonstrated that very fast (<30 s) separations of therapeutic protein species are feasible using ultra-short (5 × 2.1 mm) columns. In the second part, our purpose was to find the appropriate column length; therefore, a systematic study was performed using various custom-made prototype reversed-phase liquid chromatography (RPLC) columns ranging from 2 to 50 mm lengths. It was found that on a low dispersion ultrahigh-pressure liquid chromatography instrument, columns between 10 and 20 mm were most effective when made with 2.1 mm i.d. tubing. However, with the same LC instrument, 3 mm i.d. columns as short as ∼5 to 10 mm could be effectively used. In both cases, it has been found to be best to keep injection volumes below 0.6 μL, which presents a potential limit to further decreasing column length, given the current capabilities of autosampler instrumentation. The additional volume of the column hardware outside of the packed bed (extra-bed volume) of very small columns is also a limiting factor to decrease the column length. For columns shorter than 10 mm, columns' extra-bed volume was seen to make considerable contributions to band broadening. However, the use of ultra-short columns seemed to be a very useful approach for RPLC of large proteins (>25 kDa) and could also work well for ∼12 kDa as the lowest limit of molecular mass. In summary, a renewed interest in the use of ultra-short columns is warranted, and additional method development will be to the benefit of the biopharmaceutical industry as there is an ever-increasing demand for faster, yet accurate assays (