Multidimensional Mass Spectrometry Coupled with Separation by Polarity or Shape for the Characterization of Sugar-Based Nonionic Surfactants

Abstract

Mass spectrometry (MS) and tandem mass spectrometry (MS/MS) were interfaced with ultra-performance liquid chromatography (UPLC) and ion mobility (IM) separation to characterize a complex nonionic surfactant, consisting of a methylated glucose core (glucam) conjugated with poly­(ethylene oxide) (PEO_<i>n</i>) branches that were partially esterified with stearic acid to form ethoxylated glucam (PEO_<i>n</i>-glucam) stearates. Reverse-phase LC-MS afforded fast separation according to polarity into five major fractions. Accurate mass measurements of the ions in the mass spectra extracted from these fractions enabled conclusive identification of six components in the surfactant, including PEO_<i>n</i>-glucam mono-, di-, and tristearates as well as free and esterified PEO_<i>n</i> as byproducts. MS/MS experiments provided corroborating evidence for the fatty acid content in each fraction based on the number of stearic acid losses observed. With IM-MS, the total surfactant ions were separated according to charge and shape into four distinct bands. Extracted mass spectra confirmed the presence of two disaccharide stearates in the surfactant, which were undetectable by LC-MS. PEO_<i>n</i>-glucam tristearates were, however, not observed upon IM-MS. Hence, LC-MS and IM-MS unveiled complementary compositional insight. With each method, certain components were particularly well separated from other ingredients (by either polarity or shape), to be detected with confidence. Consequently, combined LC-MS and IM-MS offer a superior approach for the characterization of surfactants and other amphiphilic polymers and for the differentiation of similarly composed amphiphilic blends. It is finally noteworthy that NH₄⁺ charges minimized chemical noise in MS mode and Li⁺ charges maximized the fragmentation efficiency in MS/MS mode