## 1 Introduction  
An emulsion is normally a mixture of two liquids, where one is dispersed as small droplets throughout the other. The applications for emulsions include foods, drinks, cosmetics, and pharmaceuticals. Most emulsions are thermodynamically unstable, and droplet phenomena such as aggregation and coalescence are common. Moreover, according to the differences in the density between the droplets and the medium in a continuous phase, dispersed droplets can move to the top or the bottom, which is referred to as creaming or sedimentation, respectively [1]. Many studies concerning emulsions appear in the literature, including recent reports on the coalescence of emulsion droplets [2; 3], the monitoring of interfacial lipid oxidation in emulsions [4], an examination into the luminol chemiluminescence profiles of emulsions [5], and a determination of the electrostatic potential of droplets [6]. The diameter of the droplets in an emulsion influences both its texture and stability. Therefore, it is important to obtain information about the size and constituents of droplets in an emulsion. Methods used to analyze emulsions include microscopy [7; 8] and dynamic light scattering (DLS) [9; 10; 11]. [PERSON] et al. used electrospray ionization quadrupole mass spectrometry (ESI-QMS) to detect small quantities of protein in small droplets [12]. ESI, however, requires the addition of a solvent for efficient ionization, which changes the droplet size and stability of an emulsion. Recently, we developed a version of multiphoton ionization time-of-flight mass spectrometry (MPI-TOFMS) for the direct analysis of an emulsion [13]. MPI-TOFMS has characteristics such as high selectivity and robustness [14; 15] that make it suitable for the analysis of emulsions with inherently high concentrations of each constituent. MPI-TOFMS is normally applied to the detection of a gas sample [15; 16; 17]. In combination with gas chromatography (GC), this method can be applied to the comprehensive trace analysis of complex environmental samples [18; 19; 20; 21]. In addition, laser desorption and pyrolytic vaporization techniques have been used to measure solid samples [22; 23; 24; 25; 26; 27]. To measure an emulsion in a liquid sample, we developed a sample introduction technique for MPI-TOFMSthat is based on a pair of concentric capillaries. This technique allowed for the mass analysis of oil constituents in an oil-in-water (O/W) emulsion [13]. Moreover, we obtained the time profile of an analyte, which was constructed by plotting the peak areas for a series of mass spectra, and found that many spikes appeared when measuring an emulsion with white turbidity. The appearance of spikes confirms the existence of oil droplets that are vaporized into the vacuum chamber of a TOFMS and then ionized by MPI. Using this MPI-TOFMS, the existing position of an analyte could be evaluated whether it was placed inside or outside an emulsion droplet [28,29]. Moreover, this technique was applied to the detection of a water-in-oil-in-water (W/O/W) multiple emulsion [30]. This method was recently used for the quantitative analysis of an oil component in an O/W emulsion [31]. In addition, we proposed a relationship between the diameter of an oil droplet in a toluene O/W emulsion and the intensity of a spike appearing on a time profile for toluene [32]. Briefly, the signal intensity of the spike should be directly proportional to the square of the diameter of the small droplet of toluene, and based on the values of \(S_{\mathrm{max}}\) (the maximum spike intensity), \(S_{\mathrm{min}}\) (the minimum detectable spike intensity), and \(D_{\mathrm{max}}\) (the diameter of the droplet which caused the spike with \(S_{\mathrm{max}}\)), we calculated \(D_{\mathrm{min}}\) (the minimum diameter of a droplet that was detectable as a spike). In that report, a \(D_{\mathrm{min}}\) of 2.9 \(\upmu\)m was obtained when a capillary column with an inner diameter of 25 \(\upmu\)m was used for sample introduction.