Relative sensitivity factors for alkali metal and ammonium cations in single particle aerosol time-of-flight mass spectra

A variety of factors have been investigated with regard to the quantitation of chemical species within individual ambient aerosol particles analyzed b y laser desorption time-of-flight mass spectrometry. Spectrum to spectrum d ifferences in the interaction of the particle with the ionization laser bea m, which affect the absolute peak areas in the mass spectra, can be minimiz ed by using relative peak areas instead of absolute peak areas in each spec trum. Whereas absolute peak areas vary by an average of 59% for a given ion peak in single particle mass spectra of a monodisperse aerosol of particle s formed from the same solution, relative peak areas in the same mass spect ra vary only by an average of 16%, Relative sensitivity factors (RSF) relat ing the mass spectral ion intensity of NH4+ and the alkali metal cations Li +, Na+, K+, Rb+, and Csi in single particle aerosol time-of-flight mass spe ctrometry to their bulk concentrations have been determined. The values for Li+/Na+, K+/Na+, Rb+/Na+, Cs+/ Naf, and NH4+/Na+ are found to be 0.14, 5.1 , 6.0, 7.9, and 0.014, respectively. The higher response for heavier cation s of the alkali metals is consistent with the periodic trends of both ioniz ation potential and lattice energies of the species of interest. The respon se factor for sodium and potassium cations has been used to accurately dete rmine the relative amounts of Na+ and K+ in sea-salt particles, by analyzin g a sample of similar to 360 ambient sea-salt particles. The relative amoun ts of Na+ and K+ are found to be 97 and 3% in articles, respectively, where as in seawater they are, on average, 98 and 2%.