Chromatographic behavior and characterization of polydisperse surfactants using Ultra-High-Performance Liquid Chromatography hyphenated to High-Resolution Mass Spectrometry

Abstract

Surfactants are widely used in various petrochemical applications. Thus, it is essential to have highly efficient analytical tools to monitor the different classes of surfactants commonly used. Three among the four known classes of surfactants were studied in the present work: anionic (i.e. alkylbenzene sulfonate, alkyl ether carboxylic acid), nonionic (i.e. alkylcyclohexyl alcohol ethoxylated, alkyl alcohol ethoxylated) and cationic (i.e. alkyl trimethylammonium). Thanks to high-resolution mass spectrometry (HRMS), a useful mass list was created including 119 m/z values (error in mass around 5 ppm). This list was the foundation of a HRMS database, which, for the sake of simplicity, will be further denoted only “database”. To avoid ion competition and streamline attribution of structural formulas for isobar molecules, a suitable chromatographic method was used before MS. The retention behavior of six surfactants (trimethyloctadecylammonium bromide, myristyltrimethylammonium bromide, Brij®C10, Triton X-100 reduced, glycolic acid ethoxylate lauryl ether, 4-dodecylbenzenesulfonic acid) was evaluated under three separation modes: reversed-phase liquid chromatography (RPLC), hydrophilic interaction chromatography (HILIC) and mixed-mode chromatography (combination of anion-exchange and reversed-phase mechanisms). In RPLC mode, six columns were tested including C4, C18, C30, polar-embedded C18, PFP and phenyl chemistries. Two HILIC columns were also tested including bare silica and urea chemistries. An anion-exchange combined with RPLC mechanism was investigated as mixed-mode mechanism. Using ammonium formate at 10 mM as buffer provided the best signal in HRMS. In liquid chromatography, acid conditions (pH 3.5) were preferred, to avoid peak tailing due to residual silanols. The mixed-mode separation mode clearly appears as the best compromise for the characterization of the three surfactants classes. Nevertheless, the orthogonality observed for the separations obtained in HILIC and RPLC modes offers some possibilities for further multidimensional separations.