Size and Composition Biases on the Detection of Individual Ultrafine Particles by Aerosol Mass Spectrometry

Abstract

Aerosol mass spectrometers allow particles to be counted on the basis of size and chemical composition. In most instruments, individual particles are ablated with a pulsed laser to obtain a mass spectrum. Using this method to characterize ambient aerosols requires an understanding of biases induced by the measurement process. For particles less than 200 nm diameter, the efficiency of detection is shown to be dependent on both size and composition. These dependencies arise from the transmission characteristics of the particle inlet and the intrinsic ability of a particle to be vaporized and ionized. The relative contributions of each are determined for a series of atmospherically relevant test aerosols. Small particles are generally more difficult to detect and analyze than large particles because they are more difficult to focus through the inlet into a tight beam and because they are more difficult to ablate. Particles composed of polycyclic aromatic hydrocarbons, ammonium nitrate, and alkali metal ions are efficiently ablated by laser ablation. Aliphatic organics are less efficiently ablated, and ammonium sulfate is very difficult to detect in a positive ion spectrum. The mass spectra of ultrafine particles show extensive fragmentation, making it difficult to distinguish aliphatic and aromatic components. However, organic particles may be classified on the basis of inorganic impurities. Initial field measurements of ambient ultrafine particles are consistent with these conclusions.

Characterization of urban and rural organic particulate in the Lower Fraser Valley using two Aerodyne Aerosol Mass Spectrometers

“…Drewnick et al state that part of this discrepancy arises from differences in the particle size cuts for the colocated instruments. Based on several additional laboratory and field tests (Allan et al, 2004a), we believe that a significant fraction of the observed underestimation is due to the fact that some ambient particles are irregular in shape and a fraction of them do not reach the vapourizer due to the known lower focusing efficiency of the aerodynamic lens inlet for nonspherical particles (Liu et al, 1995b;Jayne et al, 2000;Kane and Johnston, 2000;Tobias and Ziemann, 2000). To characterise this effect, we define the particle collection efficiency, CE s , as the fraction of the sampled particle mass of a given species that reaches the AMS detector (e.g.…”

Parameters contributing to efficient ion generation in aerosol MALDI mass spectrometry

“…Surprisingly, few papers directly discuss the relationship between particle size and ion yield, but several have investigated the relationship between size and ablation efficiency [45] or size and laser-induced ion formation threshold [46,47]. In general, ablation efficiency increases as particle size increases.…”

Evaluating the influence of laser wavelength and detection stage geometry on optical detection efficiency in a single-particle mass spectrometer

“…These phenomena result in ion signals that are not proportional to the mass of the chemical species and a particle number counting bias with respect to particle type. Numerous studies have reported quantitative or semi-quantitative results by using relative sensitivity factors to account for matrix effects , accounting for transmission and hit rate bias (Kane and Johnston, 2000), and calculating scaling functions by referencing conventional particle counters (Allen et al, 2000). Quantification of particle number concentrations by these methods requires large assumptions to made about particle properties such as shape, density, and refractive index, making the application to ambient aerosol difficult.…”