Hyphenating temperature gradient elution with refractive index detection through temperature-responsive liquid chromatography

Refractive index detection (RID) is attractive because it allows approaching the benefits of universal detection with liquid chromatography, by which ideally standard independent calibration and hence compound independent quantification becomes possible. Nevertheless, the implementation of RID has remained limited as it offers poor detection sensitivity while only being compatible with isocratic mobile phases. The implementation of compositional solvent gradients has remained prohibitively challenging in commercial HPLC-RID systems due to the resulting drastic alterations in refractive index and extreme baseline drift. While the refractive index is also highly dependent on temperature, more leeway appears possible to mitigate the problem, particularly when the used temperature gradients can be limited. Temperature-responsive liquid chromatography (TRLC) allows obtaining isocratic reversed phase type of separations, whereby retention is modulated via temperature changes similar to 15 degrees C-20 degrees C above and below the polymer conversion temperature. Elution profiles, reminiscent of what can be obtained with solvent gradients in conventional RPLC, can then be obtained by enacting downwards temperature gradients on the columns. This work comprises a proof-of-principle to illustrate the possibilities of combining thermal gradient TRLC with RID. The observed baseline drift appeared thereby very minor (<5 nRIU min(-1)), and hence easily controllable. Short chain fatty acids are used as representative compounds to assess this new approach. Overlapping calibration lines are accordingly obtained for all fatty acids between butyric and decanoic acid.