A study of the re-equilibration of hydrophilic interaction columns with a focus on viability for use in two-dimensional liquid chromatography.

In recent years there has been increasing interest in the use of HILIC separations in two-dimensional liquid chromatography (2D-LC), mainly because the selectivity of HILIC separations complement that of reversed-phase separations for a variety of molecules. Historically, the re-equilibration of HILIC phases following gradient elution has been perceived as too slow to be useful in the second dimension of 2D-LC separations in particular. Recent studies of re-equilibration of HILIC phases by McCalley and coworkers using a limited set of conditions showed that highly repeatable gradient separations could be obtained with re-equilibration times as short as 4.3 min [1,2]. In this study we aimed to study re-equilibration of HILIC phases under a broader set of conditions, and at much shorter re-equilibration times, in the interest of determining whether or not HILIC separations can be generally considered as a viable option for use in the second dimension of 2D-LC separations. To this end we studied the effects of mobile phase pH, buffer concentration, and preparation method, flow rate, analyte and stationary phase chemistry, column length, and re-equilibration time on retention of a variety of small molecule probe solutes following gradient elution. In general, we have found that excellent separation repeatability can be obtained with quite short (≪10 min) re-equilibration times, even when progress toward full equilibration of the column is quite slow (≫10 min). In other words, even if the stationary phase is not fully equilibrated, as long as it is partially equilibrated in a highly precise manner, highly repeatable retention times can be obtained. Higher flow rate has a positive effect on both the rate of progress toward full equilibration and the repeatability of separation. No significant, consistent effects of eluent pH or buffer concentration on repeatability of gradient separation were observed for the stationary phases studied. Excellent gradient separation repeatability was obtained with shorter columns (30 mm length) with re-equilibration times as short as 3 s. A proof-of-concept 2D-LC separation of several small molecule probes using HILIC separations in both dimensions was performed to illustrate that re-equilibration of these columns can be fast enough for HILIC columns to be considered as a viable option for the second dimension of comprehensive 2D-LC separations.