Characteristic Signal Behaviors for Water-in-Oil and Oil-in-Water Emulsions Measured by Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry

Abstract

Resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) was used to study the characteristic signal behaviors obtained from two types of emulsions: water-in-oil (W/O) and oil-in-water (O/W). All emulsions were prepared using phase inversion emulsification, i.e., a solution for an aqueous phase was added dropwise to an oil phase with constant stirring to obtain an emulsion. Toluene served as a detection component. When using REMPI-TOFMS to measure an emulsion, a time profile for the target component can be constructed by plotting peak areas for the corresponding component on a series of mass spectra. In the case of a W/O emulsion at a water volume fraction (f w) of 0.005, the concentration of toluene was instantaneously decreased due to the existence of water droplets, and therefore, several negative spikes were detected on the time profile while establishing a baseline. In the case of a W/O emulsion at f w = 0.3, negative peaks consisting of several plots appeared on the time profile because of the formation of aggregates of water droplets while the emulsion was flowed through a capillary column for sample introduction. An O/W emulsion at f w = 0.995 was analyzed following phase inversion, and positive peaks were detected due to the aggregates of many oil droplets. In this manner, the direct mass analysis of emulsions before and after phase inversion was achieved, and the resultant signal inversion was confirmed via REMPI-TOFMS.

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

“…A negative signal based on a base signal indicates that the concentration of styrene at an ionization point is, at the moment, lower than the concentration of styrene in the O 2 phase. In our previous paper, when a W/O emulsion was continuously measured, negative signals based on a base signal were obtained . The emulsion contained toluene as an analyte species, and therefore, negative signals indicated the existence of less-concentrated regions of toluene, i.e., water droplets.…”

“…A schematic of the REMPI-TOFMS setup used for this study is shown in Figure . The details of the instrument were reported previously, , and are only briefly described here. The TOFMS setup was developed by Kyushu University, Japan.…”

“…Resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) using ultraviolet laser pulses is a highly selective analytical method for analyzing a sample with a complex mixture. − For example, real-time analysis of volatile compounds generated from roasting coffee and of aromatic compounds in the environment have been reported. Recently, we used REMPI-TOFMS for real-time monitoring of an emulsion without pretreatment. − In that case, the time profile of a peak area of a species of analyte was obtained from a series of mass spectra. Until now, when an O/W emulsion containing a hydrophobic compound (toluene) as a species of analyte was measured, a base signal and sudden positive signals based on the base signal appeared on the time profile.…”

“…Until now, when an O/W emulsion containing a hydrophobic compound (toluene) as a species of analyte was measured, a base signal and sudden positive signals based on the base signal appeared on the time profile. When a W/O emulsion was measured, however, a base signal and sudden negative signals based on the base signal were obtained . Each base signal arose from the continuous detection of an analyte present in the respective outer phase, i.e., either the aqueous phase of an O/W emulsion or the oil phase of a W/O emulsion.…”

Quantitative Evaluation of the Creaming of Emulsions via Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry

“…[19][20][21][22] Recently, we applied REMPI-TOFMS to the analysis of an emulsion without pretreatment. [23][24][25] In this technique, an emulsion is flowed through a capillary column and is continuously introduced into TOFMS. Then a series of mass spectra are recorded, the peak areas of a target species are extracted from the series of mass spectra, and finally a time profile of the peak areas can be constructed.…”