Nanoparticle Mass Spectrometry: Pushing the Limit of Single Particle Analysis

Abstract

Each day we breathe in millions of sub-microscopic particles, most of them under 100 nm in diameter. Where do these nanoparticles come from? How do they affect our health and the world around us? Epidemiological studies have suggested a link between exposure to ambient nanoparticles and respiratory/cardiovascular problems in the human population.1,2 Nanoparticles may also influence global climate by increasing the number of cloud condensation nuclei.3 Exposure to manufactured nanoparticles in the workplace and its implications for worker safety are largely unknown.4 Addressing issues like these requires an understanding of size and chemical composition, preferably on a particle-by-particle basis, as these parameters indicate the source of individual particles in air and their subsequent physical, chemical, or biological impact. Robust methods for single particle analysis in the micrometer and sub-micrometer size ranges have been widely used for over a decade. Several recent reviews discuss these methods and their application to atmospheric chemistry.5–8 Unfortunately, these methods become difficult to implement in the nanometer size range because the particle mass is so small (see Table I). The purpose of this article is to briefly describe some approaches used in our laboratory to push the limit of on-line single particle analysis to lower sizes. On-line analysis complements the information that can be obtained by other techniques such as off-line analysis by electron microscopy9 or the analysis of bulk nanoparticulate matter.10 The process of analyzing single particles by mass spectrometry is analogous to a conventional tandem mass spectrometry experiment (see Fig. 1). In conventional mass spectrometry, molecular ions produced at atmospheric pressure are drawn into the mass spectrometer through an inlet. The m/z of interest is selected with a first analyzer, subjected to collision-induced dissociation, and the products are mass analyzed with a second analyzer. The product ion spectrum gives information on the composition and structure of the molecular ion m/z that was selected. In single particle mass spectrometry, particles in air are drawn through an inlet into the mass spectrometer. Individual particles are sized, ablated with a laser to produce atomic and/ or molecular ions, and the ions are mass analyzed to give information on the composition of the sized particle. While the steps associated with single particle mass spectrometry are analogous to those in tandem mass spectrometry, the technology used is quite different and changes with particle size.