Aeromicelle-A new form of liquid aerosol for delivering aqueous samples into a single-particle mass spectrometer

For applying highly sensitive mass spectrometry to chemical analysis of aqueous samples, we have developed a novel technique using a new form of liquid droplets, which we call "aeromicelle" (AM), to deliver aqueous sample solutions directly into the vacuum region of a single-particle mass spectrometer in liquid form and conduct immediate mass analysis. AMs are generated by spraying an aqueous solution containing a surfactant at a concentration significantly lower than its critical micelle concentration (CMC). When the solution is sprayed, liquid droplets containing the surfactant are formed, which gradually dry in an air flow. Upon drying, the surfactant concentration in the droplet exceeds its CMC, and consequently, the surfactant molecules begin to cover the droplet surface. Finally, the surface is expected to be fully covered with surfactant molecules such as reverse micelles. The surface coverage helps suppress the evaporation of water, thereby enhancing the residence time of the liquid droplet. Our experimental results show that the AMs retained a liquid form for at least 100 s in air and survived even under vacuum conditions for further mass analysis: each AM delivered in the vacuum region of a single-particle mass spectrometer is ablated with an intense laser pulse and then, mass analyzed. Individual AMs generated from an aqueous solution containing CsCl were analyzed using a single-particle mass spectrometer. The Cs⁺ ion peak was observed even in AMs generated from the 10 nM solution. The number of Cs atoms in each AM was estimated to be approximately 7 × 10³, which corresponds to 1.2 × 10^-20 mol (12 zmol). Meanwhile, in the mass analysis of tyrosine, both positive and negative fragmentation ions from tyrosine in AMs were observed in the mass spectrum and 4.6 × 10⁵ (760 zmol) tyrosine molecules were detected.