QUANTITATION OF IONIC SPECIES IN SINGLE MICRODROPLETS BY ONLINE LASER DESORPTION/IONIZATION

Mass spectra of single micrometer-size glycerol droplets containing dissolved inorganic salts were obtained by on-line laser desorption/ionization. The peak areas of quantitation ions in the mass spectra allowed cation and anion concentrations in solution to be determined. Liquid droplets have several advantages for quantitative work including well-defined morphology, reproducible matrix composition surrounding the analyte, and straightforward preparation of calibration aerosols having known amounts of analyte. Therefore, absolute quantitation can be achieved without an internal standard. Droplet-to-droplet variations of the absolute signal intensities ranged from 14 to 58%. An improvement of N-1/2 in the signal-to-noise ratio was achieved by averaging the spectra of N droplets. Calibration plots of signal intensity versus ion concentration in solution were obtained for five test ions: Fe3+, Na+, Cl-, NO3-, and SO42-. The plots were linear over at least 2 orders of magnitude in concentration. Minimum detectable concentrations were in the low 10(-2) to high 10(-5) M range. At the minimum detectable concentrations, the absolute amounts of analyte is a single droplet were on the order of 10(-16)-10(-18) mol. Potential applications of this methodology to real-time measurements of atmospherically relevant aerosols is discussed.