Systematic comparison of sensitivity between hydrophilic interaction liquid chromatography and reversed phase liquid chromatography coupled with mass spectrometry

Hydrophilic interaction liquid chromatography (HILIC) appears as a promising strategy to increase sensitivity with electrospray ionization source (ESI/MS). In the present study, peak heights, background noises and signal-to-noise ratios (S/N) obtained with HILIC–MS/MS and RPLC–MS/MS conditions were systematically compared using a dataset of 56 basic drugs possessing diverse physico-chemical properties. Various mobile phase conditions were investigated, including different pH (3 and 6 in HILIC; 3, 6 and 9 in RPLC) and flow rates (300, 600 and 1000 μL/min). The average gain in sensitivity obtained between HILIC and RPLC was equal to 7 and 10 at pH 3 and 6, respectively. However, this value was not reliable, since it was altered by a few compounds possessing an “extreme” behaviour (gain in sensitivity from 100-fold to >8000-fold better). Then, the median gain in sensitivity, equal to 4 in our case, whatever the pH, should be considered. For about 90% of the tested compounds and analytical conditions, the best S/N was systematically attained under HILIC mode. Thanks to PCA representation, it was shown that the basic compounds with pKa between 6 and 8 generally had the best sensitivity in HILIC at pH 6, while the best sensitivity for basic analytes possessing pKa higher than 8 was usually obtained in HILIC at pH 3. As previously reported, the sensitivity gain in HILIC vs. RPLC was explained by the difference in acetonitrile concentration at elution (in average 29% ACN in RPLC and 82% ACN in HILIC at pH 6) leading to better analytes’ desolvation. However, it seems that this high proportion of solvent also favourably influenced the ionization by modifying pH and pKa. Indeed the weakest bases of our training set of compounds (pKa between 2 and 5) showed an unexpectedly strong gain in sensitivity, between 20 and 100-fold in comparison to RPLC. These results prove that the ionic character of analytes in solution (i.e., pKa and pH) and the ionization mechanism (i.e., proton transfer) also play an important role for explaining the sensitivity enhancement in HILIC.