Laser desorption/ionization of single ultrafine multicomponent aerosols

Laser desorption/ionization characteristics of single ultrafine multicomponent aerosols have been investigated. The results confirm earlier findings that (a) the negative ion spectra are dominated by free electrons and (b)the ion yield-to-mass ratio is higher for ultrafine particles than th at of larger ones. The smallest relative mass of KCI detected in NaCl is about 0.06%, which corresponds to 10(-20) g of KCI in a 50-nm particle. Experimental results from mixtures of KCl/NaCl and NaCl/NH4NO3 show that, by measuring the peak area ratio of certain ions in the spectrum, the total composition can be inferred. The experimental results also show that ion yields vary with composition even for ultrafine particles, making multicomponent analysis of complex particles difficult. A simple model is developed to quantify the relationship between ion yield and particle composition. Using this model, a relative ion yield of 4.9 is found for K+ from KCl over Na+ from NaCl, while 4.4 is found for Na+ from NaCl over NO+ from NH4NO3. It has been shown that analyzing particles composed of chemicals with common cations but different anions, such as NaCl/NaNO3 from positive ion spectra, is also possible. The ability of laser desorption/ionization to detect trace metals is studied. With relative mass of each element on the order of 1%, which corresponds to the absolute mass of the order of 10(-17) g in a 60-nm particle, strong peaks of Na+, Mg+, K+, Cr+, Fe+, Cu+, Zn+, Cd+, Cs+, La+, and Pb+ are observed in the spectrum. The smallest amount detectable for some easily ionizable elements, such as K and Cs, should be much lower than 1%. Although no ion yield is observed for pure (NH4)(2)SO4 particles, peaks of S+ and SO+ associated with (NH4)(2)SO4 are shown in spectrum from mixtures of (NH4)(2)SO4, NH4NO3, and several trace metals. The application of laser desorption/ionization to measuring atmospheric ultrafine aerosols is also discussed.