Real-Time Analysis of Metal-Containing Aerosols

Metal-containing aerosols are widespread in the Earth's atmosphere, originating from both natural and human sources. These include mineral dust and volcanic emissions, as well as industrial processes, fossil fuel combustion, and vehicle emissions.

Given their varied sources and environmental impact, accurately measuring these metallic particles is essential for understanding atmospheric chemistry and air quality.

Metal concentrations can range from ~10^-6 to 100,000 ng/m³,¹ and are influenced by local parameters like proximity to emission sources and wind velocity.²

Current Measurement Technology and Limitations

Metal aerosol concentrations are typically evaluated by collecting particles on a filter, followed by total digestion and bulk analysis using inductively coupled plasma mass spectrometry (ICP-MS).³

The time resolution of bulk analysis, typically around 24 hours, poses limitations for source apportionment and hinders the ability to perform co-variation analysis or integrate metal aerosol concentration data with highly time-resolved organic aerosol measurements. This prevents effective data fusion with techniques such as proton transfer reaction time-of-flight mass spectrometry (PTR-TOFMS) or aerosol mass spectrometry (AMS).^4-6

In recent years, field-portable X-ray fluorescence (XRF) devices have gained popularity in research and air quality monitoring (AQM).⁷ These instruments provide bulk concentrations of metal aerosols with improved time resolution (around 60 minutes), though they are less sensitive than standard collect-and-digest ICP-MS analysis.

While this is progress, the time resolution provided by field portable XRF still is not enough for accurate source apportionment analysis because data at this time scale cannot intrinsically account for environmental factors such as wind velocity.

Single-particle aerosol mass spectrometry (SPMS)⁸ and real-time mass spectrometry of volatile organic compounds (VOCs), such as PTR-MS⁹, are widely used methods in atmospheric research and AQM.

These instruments monitor ambient air with high sensitivity on a timescale of seconds. At this time resolution, wind-velocity data can be collected, considerably improving source apportionment calculations. Mobile measurements can also be used to locate aerosol occurrences in both time and space.¹⁰

Advances in real-time MS analysis of ambient air have transformed AQM, providing a mechanism to detect and enforce air pollution restrictions. However, no comparable technology for real-time quantitative measurement of metal-containing aerosols has emerged.

Development of the mipTOF for Real-Time Analysis of Metal-Containing Aerosols

In this article, TOFWERK highlights the development of a novel trace-element mass spectrometer for direct quantitative analysis of metals in ambient air.

This microwave-induced plasma time-of-flight mass spectrometer (mipTOF) has a nitrogen (N₂) sustained plasma source,^11,12 which can be used to continuously and immediately evaporate, atomize, and ionize metals and metalloids from aerosol particles in ambient air.

The source, when combined with a TOF mass spectrometer, allows for the quantitative identification of elements in individual particles with mass quantities ranging from 0.1 to 10,000 femtograms (fg) and mass concentrations ranging from 0.001 to 10,000 ng m^-3 in a 30-second examination.

This article describes the mipTOF, its performance features, and how to use the instrument for direct ambient air analysis. It also presents selected findings from a four-day continuous investigation of outside air.

Instrument Design

Figure 1 shows a schematic of the mipTOF.

The instrument includes a MICAP plasma source (Radom Corp., USA), a water-cooled differentially pumped interface, an ion mirror to redirect the extracted ion beam and remove neutral species, an RF collision cell, an RF notch filter to selectively remove abundant ions with defined mass-to-charge (m/Q) values, and an orthogonal acceleration (oa) TOF mass analyzer.

Tables 1 and 2 show typical operating settings and specifications. For instrument operation, a support unit consisting of a nitrogen supply, a rack-mount thermochiller, and a Roots pump is needed. Unlike a traditional Ar-sustained ICP source,^13,14 the MICAP can be operated with air directly injected into the plasma's central channel.

This allows for direct analysis of aerosol particles without the need for additional gas exchange or dilution devices.^15,16 The mipTOF does not require cylinder-based gas sources, making it ideal for mobile or in-field analyses. The equipment consumes 4.5-5.5 kWh of power when in operation, depending on the plasma power selected.

Figure 1. A) Schematic diagram of the mipTOF. B) Image of the mipTOF resting on top of a cart for mobile operation. Image Credit: TOFWERK

Table 1. Typical plasma operating parameters. Source: TOFWERK

Table 2. TOF mass analyzer specifications. Source: TOFWERK

Aerosol Sampling and Detection Limits

Figure 2 illustrates how a concentric pneumatic nebulizer functions as a Venturi pump, drawing ambient air directly into the MICAP. The introduction of air into the N₂ plasma does not destabilize it or affect instrument performance.

To characterize the mipTOF, microdroplets containing known element mass quantities were injected, serving as particle proxies. This approach enables absolute calibration of the mipTOF by determining the counts recorded per unit mass of the element introduced into the plasma.¹⁷

Figure 3 shows typical detection limits for a range of elements. These detection limits allow for the successful identification of main elements in ultrafine particles (diameter < 100 nm) and minor and/or trace elements in larger particles (e.g., PM2.5).

Particles up to approximately 5 µm are expected to be fully vaporized and atomized in the plasma, allowing for accurate quantification of metal content across a broad range of particle sizes.¹⁸

Figure 2. A) Schematic of ambient aerosol sampling strategy using concentric pneumatic nebulizer as Venturi pump. B) Image of MICAP source in operation with direct ambient air sampling through concentric nebulizer. Image Credit: TOFWERK

Figure 3. A) Per particle detection limits of the mipTOF. B) Bulk concentration LODs of the mipTOF measured in 10 s. Image Credit: TOFWERK

References

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2. Ramírez, O., et al (2020). Hazardous trace elements in thoracic fraction of airborne particulate matter: Assessment of temporal variations, sources, and health risks in a megacity. 710, pp.136344–136344. https://doi.org/10.1016/j.scitotenv.2019.136344.
3. Suzuki, Y., Suzuki, T. and Furuta, N. (2010). Determination of Rare Earth Elements (REEs) in Airborne Particulate Matter (APM) Collected in Tokyo, Japan, and a Positive Anomaly of Europium and Terbium. Analytical Sciences, 26(9), pp.929–935. https://doi.org/10.2116/analsci.26.929.
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5. Ruuskanen, T.M., et al. (2011). Eddy covariance VOC emission and deposition fluxes above grassland using PTR-TOF. Atmospheric Chemistry and Physics, 11(2), pp.611–625. https://doi.org/10.5194/acp-11-611-2011.
6. Pratt, K.A. and Prather, K.A. (2011). Mass spectrometry of atmospheric aerosols-Recent developments and applications. Part II: On-line mass spectrometry techniques. Mass Spectrometry Reviews, 31(1), pp.17–48. https://doi.org/10.1002/mas.20330.
7. Furger, M., et al. (2017). Elemental composition of ambient aerosols measured with high temporal resolution using an online XRF spectrometer. Atmospheric Measurement Techniques, 10(6), pp.2061–2076. https://doi.org/10.5194/amt-10-2061-2017.
8. Jensen, A.R., et al (2023). Measurements of volatile organic compounds in ambient air by gas-chromatography and real-time Vocus PTR-TOF-MS: calibrations, instrument background corrections, and introducing a PTR Data Toolkit. Atmospheric measurement techniques, 16(21), pp.5261–5285. https://doi.org/10.5194/amt-16-5261-2023.
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11. Schild, M., et al (2018). Replacing the Argon ICP: Nitrogen Microwave Inductively Coupled Atmospheric-Pressure Plasma (MICAP) for Mass Spectrometry. Analytical chemistry (Washington), 90(22), pp.13443–13450. https://doi.org/10.1021/acs.analchem.8b03251.
12. Niu, H. and Houk, R.S. (1996). Fundamental aspects of ion extraction in inductively coupled plasma mass spectrometry. 51(8), pp.779–815. https://doi.org/10.1016/0584-8547(96)01506-6.
13. Houk, R.S., et al. (1980). Inductively coupled argon plasma as an ion source for mass spectrometric determination of trace elements. 52(14), pp.2283–2289. https://doi.org/10.1021/ac50064a012.
14. Nishiguchi, K., Keisuke Utani and Fujimori, E. (2008). Real-time multielement monitoring of airborne particulate matter using ICP-MS instrument equipped with gas converter apparatus. Journal of Analytical Atomic Spectrometry, 23(8), pp.1125–1125. https://doi.org/10.1039/b802302f.
15. Cen, T., et al. (2024). Rotating disk diluter hyphenated with single particle ICP-MS as an online dilution and sampling platform for metallic nanoparticles characterization in ambient aerosol. Journal of Aerosol Science, 175, p.106283. https://doi.org/10.1016/j.jaerosci.2023.106283.
16. Mehrabi, K., Detlef Günther and Gundlach-Graham, A. (2019). Single-particle ICP-TOFMS with online microdroplet calibration for the simultaneous quantification of diverse nanoparticles in complex matrices. Environmental Science Nano, 6(11), pp.3349–3358. https://doi.org/10.1039/c9en00620f.
17. Acker, T.V., et al. (2023). Laser Ablation for Nondestructive Sampling of Microplastics in Single-Particle ICP-Mass Spectrometry. Analytical Chemistry, 95(50), pp.18579–18586. https://doi.org/10.1021/acs.analchem.3c04473.
18. Gundlach-Graham, A., et al. (2024). Introducing ‘time-of-flight single particle investigator’ (TOF-SPI): a tool for quantitative spICP-TOFMS data analysis. Journal of Analytical Atomic Spectrometry, 39(3), pp.704–711. https://doi.org/10.1039/d3ja00421j.

This information has been sourced, reviewed and adapted from materials provided by TOFWERK.

For more information on this source, please visit TOFWERK.