ORIGINAL PAPER Shape selectivity in embedded polar group stationary phases for liquid chromatography Catherine A. Rimmer & Lane C. Sander Received: 5 November 2008 / Revised: 21 January 2009 / Accepted: 23 January 2009 / Published online: 17 February 2009 # Springer-Verlag 2009 Abstract Seven columns with embedded polar functional- ity were evaluated for use in liquid chromatography with a focus on molecular shape recognition. Tests based on Standard Reference Material 869b Column Selectivity Test Mixture for Liquid Chromatography and the Tanaka test indicate that only two of the phases are slightly shape selective at 20 °C. The shape recognition characteristics of the phases appear to be directly related to the density of the embedded polar ligands and the temperature of the separation, consistent with trends observed with conven- tional hydrocarbon phases. Keywords Liquid chromatography . Shape selectivity . Embedded polar group Introduction Because subtle differences in stationary-phase chemistry affect column performance, a variety of synthetic ap- proaches are utilized by column manufacturers to address separation needs. A relatively new addition to the chro- matographic arsenal is stationary phases with embedded polar groups (EPG). EPG columns are marketed as providing more symmetrical (improved) peak shape for ionizable solutes, alternate selectivity when compared with conventional C 8 and C 18 phases, and better wettability in aqueous mobile phases. As a result of the unique character- istics of EPG columns, efforts have been made to study aspects of selectivity toward charged analytes [1–5], improved peak shape [6, 7], and the solvation of the stationary phase in aqueous mobile phases [8–10]. The term “embedded polar group phase” encompasses a wide range of commercially available columns. The chemical functionality of the embedded polar group is variable and includes: amide, carbamate, ether, sulfon- amide, and urea functionalities. EPG stationary phases are prepared by either direct silanization of the silica surface or a stepwise reaction with a linking agent [11]. In the latter case, the silica is initially derivatized with a short-chain- length polar-terminated reactive silane. The modified silica is further reacted with an appropriate reagent at the polar end group. The initial silanization produces a surface with high coverage; however, due to steric constraints, not all of the initially bound species react in the subsequent step. The resulting surface is heterogeneous, containing residual silanols, unreacted polar groups, and alkyl ligands with embedded polar functionality. Stationary phases synthe- sized by this approach may be subject to hydrolysis and cleavage under relatively mild chromatographic conditions. Anal Bioanal Chem (2009) 394:285–291 DOI 10.1007/s00216-009-2649-4 Catherine Rimmer is a Research Chemist at the National Institute of Standards and Technology where she began as a National Research Council Post- doctoral Fellow. Her research efforts are currently directed to- wards an improved understanding of chromatographic processes to advance the “state of the art” in chemical metrology. C. A. Rimmer (*) : L. C. Sander Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8392, USA e-mail: catherine.rimmer@nist.gov