Orthogonality of separation in two-dimensional liquid chromatography

Two-dimensional liquid chromatography is often used to reduce the proteomic sample complexity prior to tandem mass spectrometry analysis. The 2D-LC performance depends on the peak capacity in both chromatographic dimensions, and separation orthogonality. The peak capacity and selectivity of many LC modes for peptides is not well known, and mathematical characterization for orthogonality is underdeveloped. Consequently, it is difficult to estimate the performance of 2D-LC for peptide separation. The goal of this paper was to investigate a selectivity of common LC modes and to identify the 2DLC systems with a useful orthogonality. A geometric approach for orthogonality description was developed and applied for estimation of a practical peak 2D-LC capacity. Selected LC modes including various RP, SCX, SEC, and HILIC were combined in 2D-LC setups. SCX-RP, HILIG RP, and RP-RP 2D systems were found to provide suitable orthogonality. The RP-RP system (employing significantly different pH in both RP separation dimensions) had the highest practical peak capacity of 2D-LC systems investigated.