Using Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry to Quantitatively Evaluate the Transfer Behavior of A Compound in An Oil-in-Water-in-Oil Emulsion during Preparation

In the present study, an oil-in-water-in-oil (O₁/W/O₂) emulsion during preparation was measured in real time by resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS), and the behavior of a compound in the O₁ phase could be evaluated as it transferred to the O₂ phase due to differences in the stirring speeds and preparation temperatures. By applying a two-step emulsification method to prepare an O₁/W/O₂ emulsion, the time constants for the transfer of a compound at a preparation temperature of 23 °C were calculated to be 3.6 ± 0.3 and 2.0 ± 0.1 min at stirring speeds of 300 and 500 rpm, respectively. The corresponding time constants at a preparation temperature of 7 °C became longer─7.8 ± 0.7 and 4.2 ± 0.2 min, respectively. Based on these values, the calculated rate constants were found to be directly proportional to the practical stirring speed used for the preparation of the O₁/W/O₂ emulsion. The transfer of a compound from the O₁ to the O₂ phase in an O₁/W/O₂ emulsion after preparation normally occurs because of the concentration gradient. Regarding the actual transfer of components during the preparation process, the transfer is assumed to not be driven solely by the concentration gradient from the O₁ to the O₂ phase. Rather, processes associated with preparation, such as the decrease in the size of W droplets due to subdivision, which facilitates transfer, as well as to the direct transfer (dissolution) of O₁ components to the O₂ phase during the subdivision process, are thought to play a significant role. REMPI-TOFMS enables the direct analysis of emulsions, and allows a quantitative evaluation of the component-transfer behavior that occurs during the preparation of an O₁/W/O₂ emulsion.