Sonic spray mass spectrometry.

We have developed a sonic spray ionization method, in which a methanol and water solution is sprayed from a fused-silica capillary with gas flow coaxial to the capillary. Ions as well as charged droplets are produced under atmospheric pressure, and their intensities depend on the gas flow rate (gas velocity). Positive ions produced from dilute solutions of molecules regarded as neurotransmitters, such as catecholamine, by this ionization method have been analyzed with a quadrupole mass spectrometer. The protonated dopamine molecule is detected in the spray of the 10 nM solution, and the mass spectrum is compared with that obtained by the ion spray ionization method. A comparison between the mass-analyzed ion intensity and the ion current, which represents the sum of ions and charged droplets, shows that most ions are produced from the charged droplets after spraying. Furthermore, we found that the charged droplet formation cannot be ascribed to the traditional models of friction electrification, electrical double layer, or statistical charging. An explanation is proposed based on the ion concentration distribution in a small droplet.