Active Solvent Modulation: A Valve-Based Approach To Improve Separation Compatibility in Two-Dimensional Liquid Chromatography

Two-dimensional liquid chromatography (2D-LC) is increasingly being viewed as a viable tool for solving difficult separation problems, ranging from targeted separations of structurally similar molecules to untargeted separations of highly complex mixtures. In spite of this performance potential, though, many users find method development challenging and most frequently cite the incompatibility between the solvent systems used in the first and second dimensions as a major obstacle. This solvent strength related incompatibility can lead to severe peak distortion and loss of resolution and sensitivity in the second dimension. In this paper, we describe a novel approach to address the incompatibility problem, which we refer to as Active Solvent Modulation (ASM). This valve-based approach enables dilution of D-1 effluent with weak solvent prior to transfer to the D-2 column but without the need for additional instrument hardware. ASM is related to the concept we refer to as Fixed Solvent Modulation (FSM), with the important difference being that ASM allows toggling of the diluent stream during each D-2 separation cycle. In this work, we show that ASM eliminates the major drawbacks of FSM including complex elution solvent profiles, baseline disturbances, and slow D-2 re-equilibration and demonstrate improvements in D-2 separation quality using both simple small molecule probes and degradants of heat-treated bovine insulin as case studies. We believe that ASM will significantly ease method development for D-2-LC, providing a path to practical methods that involve both highly complementary D-1 and D-2 separations and sensitive detection.