Elemental Composition of Nanoparticles with the Nano Aerosol Mass Spectrometer

Abstract

The nano aerosol mass spectrometer (NAMS) irradiates individual, size selected nanoparticles with a high energy laser pulse to generate a mass spectrum consisting of multiply charged atomic ions. The elemental composition of the particle is determined from the ion signal intensities of each element, which requires deconvoluting isobaric ion signals at 4 m/z (O(4+) and C(3+)) and at 8 m/z (O(2+) and S(4+)). A method to deconvolute these ion signals using sucrose and ammonium sulfate as calibrants is presented. The approach is based on the assumption that the charge state distribution of a given element is independent of the chemical form of that element in the particle. Relative to previously reported methodology, the new approach permits accurate and precise determination of sulfur, which is crucial for interpretation of ambient nanoparticle data sets. With this approach, the differences between expected and measured elemental ratios of C, O, N, and S for a variety of test particles were generally much less than 10%, although a difference as high as 16% was observed.

Nanoparticle Chemical Composition During New Particle Formation

“…The median decrease in the C mole fraction during events was ∼25%, whereas the N mole fraction increased by ∼16%. These changes are of a magnitude greater than the uncertainty associated with atomic composition analysis (indicated by the dotted lines in Figure 2), as NAMS typically measures atomic mole ratios to within ±10%, which corresponds to an uncertainty of ∼7% for individual atomic mole fractions (Zordan et al 2010). Changes to O and S mole fractions were much smaller, and the changes may be within the uncertainty assigned to these values, with a median increase of 3% for O and 8% for S during NPF.…”

“…The "limited" mass resolving power of NAMS results in some complications in interpretation of the mass spectrum, as isobaric ion signals are present at 4 m/z (C +3 and O +4 ) and 8 m/z (O +2 and S +4 ). We have developed a method to deconvolute these isobaric ion signals (Zordan et al 2010). Figure 2 also provides a pie chart that gives the relative atomic mole fractions of all particles during this event.…”

“…Table 3 provides particle composition onand off-event for all studied events. A 10% uncertainty is assigned to the molar ratios (Zordan et al 2010).…”

“…The relative intensities of these ions give the atomic composition. For standard compounds, the atomic mole ratios obtained by NAMS are typically within 10% of their expected values (Zordan et al 2010). The sampled particle size range is selected by the frequency applied to the ring electrode of the ion trap.…”

“…Typically, unapportioned matter represents less than about 5% of the atomic content of an ambient particle dataset. Unapportioned matter and removed particles arise primarily from pulse-to-pulse variations of the laser ablation process, which cause some particle mass spectra to deviate substantially from the true chemical composition (Zordan et al 2010).…”