Evaluating the Aging of Multiple Emulsions Using Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry.

Resonance-enhanced multiphoton ionization time-of-flight mass spectrometry was applied to measurements of multiple emulsions with no pretreatment; a method for the quantitative evaluation of aging was proposed. We prepared water-in-oil-in-water (W/O/W) multiple emulsions containing toluene and m-phenylenediamine. The samples were measured immediately following both preparation and after having been stirred for 24 h. Time profiles of the peak areas for each analyte species were obtained, and several intense spikes for toluene could be detected from each sample after stirring, which suggests that the concentration of toluene in the middle phase had increased during stirring. On the other hand, in the case of a W/O/W multiple emulsion containing phenol and m-phenylenediamine, spikes for m-phenylenediamine, rather than phenol, were detected after stirring. In the present study, the time-profile data were converted into a scatter plot in order to quantitatively evaluate the aging. As a result, the ratio of the plots where strong signal intensities of toluene were detected increased from 8.4% before stirring to 33.2% after stirring for 24 h. The present method could be a powerful tool for evaluating multiple emulsions, such as studies on the kinetics of the encapsulation and release of active ingredients.