Abstract
Evaporative light scattering detectors (ELSD) are commonly used with high-performance liquid chromatography (HPLC) to separate and quantify lipids, which are typically not easily detectable by more conventional methods such as UV-visible detectors. In many HPLC-ELSD methods to analyze lipids, a volatile buffer is included in the mobile phase to control the pH and facilitate separation between lipid species. Here, we report an unintended effect that buffer choice can have in HPLC-ELSD analysis of lipids - the identity and concentration of the buffer can substantially influence the resulting ELSD peak areas. To isolate this effect, we use a simple isocratic methanol mobile phase supplemented with different concentrations of commonly used buffers for ELSD analysis, and quantify the effect on peak width, peak shape, and peak area for seven different lipids (POPC, DOPE, cholesterol, sphingomyelin, DOTAP, DOPS, and lactose ceramide). We find that the ELSD peak areas for different lipids can change substantially depending on the mobile phase buffer composition, even in cases where the peak width and shape are unchanged. For a subset of analytes which are UV-active, we also demonstrate that the peak area quantified by UV remains unchanged under different buffer conditions, indicating that this effect is particular to ELSD quantification. We speculate that this ELSD-buffer effect may be the result of a variety of physical phenomenon, including: modification of aerosol droplet size, alteration of clustering of analytes during evaporation of the mobile phase, and mass-amplification or ion-pair effects, all of which could lead to differences in observed peak areas. Such effects would be expected to be molecule-specific, consistent with our data. We anticipate that this report will be useful for researchers designing and implementing HPLC-ELSD methods, especially of lipids.