Temperature-dependent refractive index issues using a UV-visible detector in high-performance liquid chromatography

While studying the properties of some potential, UV-detection compatible, mobile phase modifiers for reversed-phase liquid chromatography (RPLC) we encountered serious detector baseline problems. The most severe problem was a periodic saw-tooth-shaped drift. This drift was most noticeable when using water-THF and water-dioxolane mobile phases. We demonstrate that the baseline problems are related to temperature fluctuation in a water bath used to maintain a constant column temperature. The temperature fluctuations change the refractive index (RI) of the mobile phase, which in turn affects the amount of light reaching the detector. Our results show that the magnitude of the baseline drift is related to the RI value of the mobile phases. The baseline problems can be alleviated using a different temperature controller whose feedback mechanism assures smoother operation. Since there is no single and complete source of information in the literature concerning the RI values of typical mobile phases in RPLC, we decided to measure them as a function of the % modifier (between 0 and 100%) and the temperature (between 25 degrees C and 50 degrees C). Knowing the RI values allows us to calculate the excess volume of a binary liquid system.