Relative Sensitivity Factors for Alkali Metal and Ammonium Cations in Single-Particle Aerosol Time-of-Flight Mass Spectra

Abstract

A variety of factors have been investigated with regard to the quantitation of chemical species within individual ambient aerosol particles analyzed by laser desorption time-of-flight mass spectrometry. Spectrum to spectrum differences in the interaction of the particle with the ionization laser beam, which affect the absolute peak areas in the mass spectra, can be minimized by using relative peak areas instead of absolute peak areas in each spectrum. 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 particles formed from the same solution, relative peak areas in the same mass spectra vary only by an average of 16%. Relative sensitivity factors (RSF) relating the mass spectral ion intensity of NH4+ and the alkali metal cations Li+, Na+, K+, Rb+, and Cs+ in single particle aerosol time-of-flight mass spectrometry to their bulk concentrations have been determined. The values for Li+/Na+, K+/Na+, Rb+/Na+, Cs+/Na+, and NH4+/Na+ are found to be 0.14, 5.1, 6.0, 7.9, and 0.014, respectively. The higher response for heavier cations of the alkali metals is consistent with the periodic trends of both ionization potential and lattice energies of the species of interest. The response factor for sodium and potassium cations has been used to accurately determine the relative amounts of Na+ and K+ in sea-salt particles, by analyzing a sample of approximately 360 ambient sea-salt particles. The relative amounts of Na+ and K+ are found to be 97 and 3% in particles, respectively, whereas in seawater they are, on average, 98 and 2%.

Variation of secondary coatings associated with elemental carbon by single particle analysis

“…S3). It is due to interference of other species which are highly sensitive to the laser beam and thus generated suppressive ion peaks, such as potassium in Biomass particles (Gross et al, 2000). On average, the number fraction of EC-containing particles (Nf EC , relative to total analyzed particles in the same size range throughout the study) was highly variable with d va (Table 1).…”

“…Relative peak area (RPA), defined as the peak area of each m/z divided by the total dual ion mass spectral peak area, is related to the relative amount of a species on a particle. Compared to absolute peak area, RPA was commonly applied because it is less sensitive to the variability in ion intensities associated with particleelaser interactions (Gross et al, 2000). The EC particles were found to be extensively internally mixed with detectable sulfate (97.4%), nitrate (89.5%), oxidized organics (69.6%), and/or ammonium (80%) (Fig.…”

Measurement of Ambient Aerosol Composition During the PMTACS-NY 2001 Using an Aerosol Mass Spectrometer. Part II: Chemically Speciated Mass Distributions Special Issue ofAerosol Science and Technologyon Findings from the Fine Particulate Matter Supersites Program

“…However, these techniques suffer from drawbacks inherent in their single-step vaporization/ionization process which lead to biases in particle-size representation and in the relative response to individual chemical species (Reilly et al 1999;Gross et al 2000;Allen et al 2000;Kane and Johnston 2000), making them less than ideal for quantitative analyses. In addition, the detection in most of these instruments is limited to particles larger than about 0.2 µm due to the optical particle detection techniques required for laser triggering.…”

Comparison of three techniques for analysis of data from an Aerosol Time-of-Flight Mass Spectrometer

“…Potassium and nitrate are strongly correlated with almost every factor and 329 are the dominant species, present in the majority of the particles collected. This reflects, at least in 330 part, the very high sensitivity of the ATOFMS to these species (Gross, 2000). 331…”