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 now coaxial to the capillary. Ions as well as charged droplets are pro duced under atmospheric pressure, and their intensities depend on the gas now rate (gas velocity). Positive ions produced from dilute soluti ons of molecules regarded as neurotransmitters, such as catecholamine, by this ionization method have been analyzed with a quadrupole mass s pectrometer. The protonated dopamine molecule is detected in the spray of the 10 nM solution, and the mass spectrum is compared with that ob tained by the ion spray ionization method. A comparison between the ma ss-analyzed ion intensity and the ion current, which represents the su m of ions and charged droplets, shows that most ions are produced from the charged droplets after spraying. Furthermore, we found that the c harged droplet formation cannot be ascribed to the traditional models of friction electrification, electrical double layer, or statistical c harging. An explanation is proposed based on the ion concentration dis tribution in a small droplet.