In this study, the influence of electrospray ionization (ESI) source design on the overall sensitivity achieved in hydrophilic interaction chromatography (HILIC) and reversed phase liquid chromatography (RPLC), was investigated. State-of-the-art triple quadrupole mass analyzers from AB Sciex, Agilent Technologies and Waters equipped with brand specific source geometries were tested with various mobile phase pH on 53 pharmaceutical compounds. The design of the ESI source showed to strongly influence the gain in sensitivity that can be achieved in HILIC compared to RPLC mode. The 6460 Triple Quadrupole LC/MS system from Agilent Technologies was particularly affected by mobile phase settings. Indeed, compared to RPLC conditions, 92% of the compounds had an increased signal-to-noise ratio at a flow rate of 300 μL/min in HILIC mode at pH 6, while this percentage dropped to only 7% at 1000 μL/min and pH 3. In contrast, the influence of flow rate and mobile phase pH on the gain in sensitivity between RPLC and HILIC was found very limited with the API 5000 LC/MS/MS system from AB Sciex, as only 15 to 36% of the tested compounds showed an enhanced sensitivity in HILIC mode. With the Xevo TQ-S instrument from Waters, superior sensitivity in HILIC was noticed for 85% of the compounds with optimal conditions (i.e., pH 3 and 1000 μL/min), whereas at sub-optimal conditions (i.e. pH 6 and 300 μL/min), it represented less than 50%. The gain in sensitivity observed in HILIC was found less significant with the recent LC-MS platforms used in this study than for old-generation instruments. Indeed, the improved ESI sources equipping the recent mass analyzers allow for enhanced evaporation efficiency, mainly for RPLC mobile phases containing high proportion of water and this even at high flow rates.
Keywords: ESI; Electrospray ionization; HILIC–MS; Hydrophilic interaction chromatography; Sensitivity; Source geometry.
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