Development of Multiphoton Ionization Time-of-Flight Mass Spectrometry for the Detection of Small Emulsion Droplets

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

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 Quantitatively Analyze the Creaming of an Emulsion

In this study, we used a quantitative analytical method to indicate creaming behavior in an emulsion. An oil-in-water emulsion was directly measured by resonance-enhanced multiphoton ionization time-of-flight mass spectrometry, and the time profiles of the peak areas of an oil component, styrene, were obtained at heights of 1, 2, and 3 cm from the bottom of a sample that had a height of 4 cm. All time profiles roughly indicated that the signal intensity increased once, then decreased, and finally settled. Moreover, we proposed a fitting equation for the time profiles by subtracting two sigmoid functions, whereby the degree of the signal increases at the initial stage, the degree of the signal decreases after the increase, and the times for continuing the higher signal intensities were all longer as the monitoring positions were raised. This method would surely provide useful information about emulsions that undergo creaming behavior.

Evaluating the Creaming of an Emulsion via Mass Spectrometry and UV-Vis Spectrophotometry

The creaming behavior of a turbid oil-in-water emulsion was observed via the processes of multiphoton ionization time-of-flight mass spectrometry (MPI-TOFMS) and ultraviolet-visible spectrophotometry (UV-vis), and the results were compared. The transmittance measurement by UV-vis showed that the turbidity of the toluene emulsion was decreased with time. However, non-negligible errors are common in the measurement of a sample with high turbidity. The online measurement by MPI-TOFMS detected many spikes in the time profile, which revealed the existence of toluene droplets in the emulsion. A smooth time profile suggested that the signal intensity had initially increased, and then decreased with time; the initial concentration of toluene was 3 g/L, which had decreased by half after 60 min. The signal behavior obtained using MPI-TOFMS differed only slightly from that obtained using UV-vis. Since a change in turbidity is not the same as a change in the local concentration of an oil component, MPI-TOFMS is useful for the analysis of a turbid emulsion and offers additional information concerning the creaming phenomenon of an emulsion.