Replacing Gravimetry: A Metrological Route For Aerosol Mass Calibration Using ICP-MS

Authorities including the World Health Organization (WHO) define mean particulate matter concentration limits in terms of mass, which is typically calibrated gravimetrically by weighing with a microbalance. While conceptually straightforward, gravimetric methods universally suffer from high background mass, in addition to issues of humidity sensitivity, electrostatic artifacts, and incomplete particle recovery, are the factors that limit their precision and lead to significant measurement uncertainty. The high limit-of-detection of gravimetry means that many laboratories and real-world measurements must be integrated over very long, sometimes impractical, exposure times. Thus, there remains a need for accurate and SI-traceable determination of aerosol mass concentration in aerosol metrology.
In this work, we proposed an ICP-MS-derived mass calibration method based on the combination of mass-classified particles with filter sampling. The method involved two ICP-MS modes: spectrum mode to provide total Bi mass fraction after filters digestion, and single particle mode where particle size distribution after filters dissolution was obtained. We used a solid particle generator (SPG) for the generation of reference aerosols containing bismuth nanoparticles as a model aerosol. The stream of bismuth particles was classified by a centrifugal particle mass analyzer (CPMA) and split between a filter sampler and a device under test. Here, the device under test is a condensation particle counter (CPC), which allowed the calculation of mass concentration since the particles were controlled to be singly charged. Comparison of the mass fraction and/or particle size distribution derived by ICP-MS and CPMA-CPC allowed for a rigorous evaluation of our approach. The advantages and remaining challenges of the method, such as Bi recovery and sampling reproducibility are discussed in the context of developing a robust primary measurement framework for aerosol mass metrology.