Using Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry to Evaluate the Movement of a Constituent in a Multiple Emulsion

Abstract

Herein, we propose a method for evaluating the movement of a constituent in a multiple emulsion while maintaining its original dispersed condition. In this study, an oil-in-water-in-oil (O 1 /W/O 2 ) emulsion was prepared using a two-step emulsification method with styrene as an analyte species in the inner phase (O 1 ). The emulsion was measured using resonance-enhanced multiphoton ionization time-of-flight mass spectrometry without pretreatment such as centrifugation. From a series of obtained mass spectra, a time profile for the peak areas arising from styrene was constructed. When the emulsion was measured immediately following preparation, a time profile composed of a base, positive, and negative signals confirmed the presence of styrene in the O 2 , O 1 , and W phases, respectively. Moreover, while a small amount of styrene was present in the inner O 1 phase, almost all of the styrene was found in the outer O 2 phase. Furthermore, the results of the obtained time profile were converted into a box plot, and a method for the selection of the base, positive, and negative signals was tentatively determined. Then, the movement of styrene among the phases could be evaluated using the time courses of these signals; the time constant of the movement of styrene from an O 1 /W droplet to the O 2 phase was calculated to be 0.8 h.

Sample preparation conditions for the real-time measurement of W/O emulsions by resonance-enhanced multiphoton ionization time-of-flight mass spectrometry

“…The inner/outer diameters and the length of the inner capillary column were 50/150 µm and 115 cm, respectively, and those of the outer capillary column were 320/450 µm and 25 cm, respectively (GL Sciences, Tokyo). In the present study, the inner diameter of the inner capillary column was larger than that used in the previous studies (25 µm) [ 14 , 15 ]. In future studies, the use of an inner capillary with a larger inner diameter should allow an emulsion containing larger droplets (e.g., water droplets in the case of a W/O emulsion or O 1 /W droplets in the case of an O 1 /W/O 2 emulsion) to smoothly pass.…”

“…If toluene droplets were formed inside a water droplet, that is, if an O 1 (toluene)/W/O 2 (cyclohexane) emulsion were obtained and were measured, the positive spikes would appear. However, special methods that include a two-step emulsification are needed to prepare such a multiple emulsion [ 15 ]. It should be impossible to obtain such emulsions under the present preparation conditions, and such emulsions were not confirmed by observation via an optical microscope.…”

“…2 ). In the present study, an inner capillary column with an inner diameter of 50 µm was applied, instead of that of 25 µm that was used in the previous studies [ 14 , 15 ]. Due to the use of the inner capillary column with a larger inner diameter, we had expected such an explosive sample introduction; as a countermeasure, a toluene solution (in cyclohexane) had been measured at the beginning of each measurement day and no spikes were detected on the time profile.…”