Real-time analysis of trace metals in air by microwave induced plasma time-of-flight mass spectrometry (mipTOF)

Keywords: Mass Spectrometry, Real-Time, Trace Elements, Source Apportionment, Mobile

Determination of the elemental composition of airborne nanoparticles and micro-particles is essential to understand the source(s) of these particles and also to predict potential health effects.¹ The most common approach to measure the metal content of air is to collect samples on filters and then analyze digests by ICP-MS; however, this strategy offers poor time resolution (e.g. days) and only provides bulk element composition information. To understand the spatiotemporal characteristics of the emission of metal-containing aerosols, which is key to assessing exposure, real-time analysis strategies are essential. Here, we report on the development of a microwave induced plasma time-of-flight mass spectrometer (mipTOF) used for the direct analysis of metal-containing airborne particles.

The mipTOF is a field-deployable trace-element mass spectrometer. It uses a nitrogen-sustained high-power plasma (MICAP, Radom Instruments)^{2, 3} to quantitatively vaporize and atomize aerosols with sizes from the ultrafine to PM10. Singly charged atomic ions are generated in the plasma with high efficiency (up to 99%), and then extracted into the mass spectrometer, where they are sorted according to mass-to-charge ratio and recorded. Ambient air is sampled into the plasma via a concentric pneumatic nebulizer set up as a Venturi pump⁵ at flowrates from 100-200 cm³/min. With the mipTOF, concentration LODs range from 10 ng/m³ (potassium) to 0.05 ng/m³ (lead) with a time resolution of 10 seconds. The high-sensitivity, high-speed metal-aerosol measurements possible with mipTOF enable new research into real-time spatiotemporal analysis of metals in air. We will report on the use of the mipTOF in mobile lab measurements in Switzerland and Massachusetts, USA. In these measurements, we identified several unique sources of airborne metals, including emissions from automotive brake wear, trains, metal-plating industries, cement manufacturers, and light aircraft. In addition to presenting data from these campaigns, we will discuss aspects of instrument design and operation, including power and size requirements, calibration strategies, and instrumental figures of merit.

References:

(1) Daellenbach, K. R. et al. Nature 2020, 587 (7834), 414-419.

(2) Jevtic, J.; Menon, A.; Pikelja, V. PCT/US14/24306, 2015.

(3) Schild, M. et al.  Analytical Chemistry 2018, 90 (22), 13443-13450.

(4) Nishiguchi, K.; Utani, K.; Fujimori, E. J. Anal. Atom. Spec. 2008, 23 (8), 1125-1129.