## 3 Results and Discussion  
### Microscopic Observation and Time Profiles  
In the present study, the online measurement of an O/W emulsion was performed using an inner capillary with an inner diameter of 15 \(\upmu\)m as a sample introduction port, rather than the inner diameter of 20 or 25 \(\upmu\)m used in previous reports [13; 28; 29; 30; 31; 32]. Before the measurement, we theorized that the use of a narrowed capillary column for sample introduction would allow for the detection of spikes for much smaller oil droplets on a time profile, as follows. When considering the cross section of an inner capillary column with respect to the axis of sample flow, the ratio of the cross section of an oil droplet and the cross section of a column becomes larger when using a narrowed capillary column. In other words, the ratio of the cross section of a continuous phase is diminished. A small quantity of an oil component dissolved in a continuous phase causes background signals rather than spikes on a time profile obtained from the online mass analysis of an O/W emulsion. That is, the background signal should decrease when using a narrowed inner capillary column. As a result, the ratio of the spike signal to the background fluctuation (\(S/B\)) should increase. On the other hand, we also considered the difficulty in flowing an emulsion through a narrow capillary column via the evacuation force of a vacuum chamber. Therefore, the length of the inner capillary column was shortened; other reasons for the use of a shorter capillary were mentioned in the introduction section. However, when a conventional microscope was placed between a sample container and a TOFMS in order to monitor the flow of a sample, an inner capillary was required for adjustment to no shorter than 120 cm [32]. Therefore, a compact microscopic system was constructed in the present study, and the resultant length of the inner capillary was shortened to 55 cm. Using this system, a toluene O/W emulsion was measured online. First, a prepared emulsion was allowed to stand for 1 day to promote creaming, after which the lower portion of the resultant transparent sample was measured. Figure 2 shows a time profile of the peak area of toluene constructed from a series of obtained mass spectra. In addition, a bright-field image of a sample passing through an inner capillary column obtained by the present microscopic system is also shown. In this measurement, the flow of the sample was confirmed, but no oil droplets were found to flow. From the results of a time profile obtained by MPI-TOFMS, a constant signal for toluene was detected, although it was very small. It should be noted that, at this point, a transparent portion of an emulsion was measured as almost all toluene was moved to the upper side with only small quantities remaining in the lower transparent portion. In this manner, the oil component that remains in the transparent portion of an emulsion after creaming could surely be monitored using the present system. After measuring the transparent portion, the capillary column was removed from the sample container, and the sample was ultrasonicated in order to re-disperse the toluene. The resultant emulsion had a white turbidity and was then immediately measured again. The obtained results are shown in Figure 3. In this measurement, oil droplets of various sizes were found to have flowed, although the exact diameters of oil droplets in the capillary were difficult to determine, although it must be assumed that the size of some of the oil droplets were compatible with the inner diameter of the inner capillary. Moreover, several spikes appeared on the time profile obtained by MPI-TOFMS. These results indicate that an emulsion with white turbidity could also pass through a narrow inner capillary and be measured by the present system. It should be noted that the number of spikes was considerably fewerthan that obtained in a previous report [32]. In the present measurement, small oil droplets were found, but their aggregates were seldom confirmed.