On-line analysis of aqueous aerosols by laser desorption ionization

In this work the effects of water on the laser desorption ionization mass spectra of single aerosol particles are explored. Aqueous aerosols are produced by passing dry particles through a humid environment so that they undergo deliquescent growth. Laser desorption ionization is performed at 248 nm. Water affects the mass spectra and ionization characteristics of the particles in several ways. Peaks derived from water appear and the number of cluster ions resulting from the solute decreases. In addition, aqueous particles exhibit a two- to four-fold higher threshold laser fluence, a lower total ion current at a comparable laser fluence, and less peak broadening at high laser fluences than their dry particle counterparts. Aqueous aerosols of three atmospherically important sulfur compounds are explored: methanesulfonic acid, the sodium salt of hydroxymethanesulfonate, and ammonium sulfate. These compounds can be distinguished on the basis of characteristic ions and peak area ratios.