Development of HPLC system that uses phase-separation multiphase flow as an eluent

We developed a new type of HPLC system that uses phase-separation multiphase flow as an eluent. A commercially available HPLC system with a packed separation column filled with octadecyl-modified silica (ODS) particles was used. First, as preliminary experiments, 25 kinds of mixed solutions of water/acetonitrile/ethyl acetate and water/acetonitrile were supplied to the system to act as eluents at 20 °C. 2,6-Naphthalenedisulfonic acid (NDS) and 1-naphthol (NA) mixture was used as a model and mixed analyte was injected into the system. Roughly speaking, they were not separated in organic solvent-rich eluents and well separated in water-rich eluents, in which NDS eluted faster than NA. This means that HPLC worked under a reverse-phase mode for separation at 20 °C. Next, the separation of the mixed analyte was examined on HPLC at 5 °C, and then after judging the results, four kinds of ternary mixed solutions were in detail as eluents on HPLC at 20 °C and 5 °C. Based on their volume ratio, the ternary mixed solutions acted as a two-phase separation mixed solution, leading to a phase-separation multiphase flow. Consequently, the solutions flowed homogeneously and heterogeneously in the column at 20 °C and 5 °C, respectively. For example, the ternary mixed solutions containing water/acetonitrile/ethyl acetate at volume ratios of 20:60:20 (organic solvent-rich) and 70:23:7 (water-rich) were delivered into the system as eluents at 20 °C and 5 °C. In the organic solvent-rich eluent, the mixture of NDS and NA was not separated at 20 °C but was separated at 5 °C, the elution of NA being faster than the one of NDS (phase-separation mode). In the water-rich eluent, the mixture of analytes was separated at both 20 °C and 5 °C, the elution of NDS being faster than the one of NA. The separation at 5 °C was more effective than at 20 °C (reverse-phase mode and phase-separation mode). This separation performance and elution order can be attributed to the phase-separation multiphase flow at 5 °C.