Determination of suitable column geometries by means of van Deemter and kinetic plots for isothermal and isocratic method development in high-temperature liquid chromatography isotope ratio mass spectrometry.

The method of high-temperature liquid chromatography isotope ratio mass spectrometry (HTLC-IRMS) is used to determine the origin or authenticity of compounds. Currently, the drawback of this hyphenation is the interface which causes pronounced band broadening due to a large extra-column volume. Therefore, the aim of this study is to determine suitable column geometries and particle sizes at different temperature and to study the effect of extra-column band broadening. The tools to assess the efficiency of columns are van Deemter and kinetic plots. By comparison of different column geometries and particle sizes, it could be shown that 3.0 mm ID columns achieve a higher performance than 2.1 mm ID columns and a particle size of 1.7 μm is advantageous over 3.5 and 5.0 μm particles when the injection volume is adjusted to 2 μL and the temperature is higher than 60 °C. Because water was the mobile phase, the retention factor could not be kept constant at different column temperatures. The lower retention factor at elevated temperatures leads to a decrease of the plate number, because of the relatively larger contribution to extra-column band broadening at lower retention factors. This is the reason why 3.0 mm ID columns should be preferred for the HTLC-IRMS hyphenation when the separation is carried out under isothermal and isocratic conditions.