Microwave-Induced Plasma TOFMS for Metal-Containing Aerosol Analysis

Metal-containing aerosols are present throughout the Earth’s atmosphere, originating from both natural and human-made sources. These include everything from mineral dust and volcanic emissions to industrial processes, fossil fuel combustion, and vehicle emissions.

Given their diverse sources and significant impact on atmospheric chemistry and air quality, accurately assessing these metallic particles is essential. Their concentrations vary widely—spanning more than ten orders of magnitude, from approximately 10^-6 to 100,000 ng/m³,—and are highly dependent on local environmental factors such as proximity to emission sources and wind velocity.^1,2

Current Measurement Technologies and Limitations

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

However, the time resolution of bulk analysis—typically around 24 hours—limits its effectiveness for source apportionment. This constraint also prevents co-variation analysis and data fusion of metal aerosol concentration data with highly time-resolved organic aerosol measurements, such as those obtained from proton transfer reaction time-of-flight mass spectrometry (PTR-TOFMS) or aerosol mass spectrometry (AMS).^4-6

Recently, field portable X-ray Fluorescence (XRF) tools have become more commonly used in research and air quality monitoring (AQM) applications.⁷ These tools can offer bulk concentrations of metal aerosols with better time resolution (ca. 60 minutes), albeit with lesser sensitivity in comparison to conventional collect-and-digest ICP-MS analyses.

Although somewhat better, the time resolution provided by field portable XRF remains inadequate for precise source apportionment analysis as data at this time scale may not in itself account for environmental factors like wind velocity.

Single-particle aerosol mass spectrometry (SPMS) and real-time mass spectrometry of volatile organic compounds (VOCs), e.g., through PTR-MS, are commonly utilized methods in atmospheric studies and AQM.^8,9 These tools offer high-sensitivity ambient air assessments in just seconds.

At this level of time resolution, it's possible to generate wind-velocity-resolved data, which greatly improves source apportionment analyses. Additionally, mobile assessments can be used to pinpoint aerosol events in both time and space, making localized studies more feasible.¹⁰

Progressions in real-time MS analysis of ambient air have been a revolutionary force for AQM and offer a means to both identify and enforce regulations around air pollution. However, at present, similar tools for real-time quantitative analysis of metal-containing aerosols have yet to be developed.

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) carries a nitrogen (N₂) sustained plasma source that may be utilized to consistently and directly vaporize, atomize, and ionize metals and metalloids from aerosol particles in ambient air.^11,12

Alongside a TOF mass spectrometer, the source offers quantitative detection of elements in individual particles with mass quantities over six orders of magnitude, from 0.1 to 10’000 femtograms (fg) and mass concentrations spanning from 0.001 to 10’000 ng m^-3 in a 30 s analysis.

In this section, TOFWERK outlines the mipTOF, detailing its performance features and operation for direct ambient air analysis. To illustrate its capabilities, the paper also presents selected results from a four-day continuous outdoor air monitoring study.

Instrument Design

Figure 1 presents a schematic of the mipTOF, highlighting its key components.

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

Typical operating parameters and specifications are listed in Tables 1 and 2.

The system is supported by a unit that houses a nitrogen supply, a rack-mounted thermochiller, and a Roots pump essential for operation. Unlike a conventional argon-sustained ICP source, the MICAP plasma can operate using air directly injected into its central channel.^13,14 This enables the direct analysis of aerosol particles without requiring external gas-exchange or dilution devices.^15,16

Because the mipTOF does not rely on cylinder-based gas supplies, it is well-suited for both mobile and field analyses. During operation, its power consumption ranges from 4.5 to 5.5 kWh, depending on the selected plasma power setting.

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

Ambient air is directly sampled into the MICAP using a concentric pneumatic nebulizer functioning as a Venturi pump, as shown in Figure 2. Injecting air into the nitrogen plasma does not destabilize it or compromise instrument performance.

The initial characterization of the mipTOF was conducted by introducing microdroplets containing known quantities of various elements. These microdroplets serve as particle proxies, allowing for full calibration of the mipTOF—specifically, determining the counts recorded per unit mass of each element injected into the plasma.¹⁷

Typical detection limits for various elements are shown in Figure 3. These detection capabilities enable the effective measurement of major elements in ultrafine particles (diameter < 100 nm) and the identification of minor or trace elements in larger particles (e.g., PM2.5).

Particles with diameters up to approximately 5 µm are believed to be fully vaporized and atomized in the plasma, allowing for a quantitative assessment 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 a 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 and Further Reading

1.  Zoller, W.H., Gladney, E.S. and Duce, R.A. (1974). Atmospheric Concentrations and Sources of Trace Metals at the South Pole. Science, 183(4121), pp.198–200. https://doi.org/10.1126/science.183.4121.198.
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.
4. Drewnick, F., et al. (2005). A New Time-of-Flight Aerosol Mass Spectrometer (TOF-AMS)—Instrument Description and First Field Deployment. Aerosol Science and Technology, 39(7), pp.637–658. https://doi.org/10.1080/02786820500182040.
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.
9. Rutherford, M., Koss, A. and Joost de Gouw (2024). Mobile VOC measurements in Commerce City, CO reveal the emissions from different sources. Journal of the Air & Waste Management Association, 74(10), pp.714–725. https://doi.org/10.1080/10962247.2024.2379927.
10. Schwartz, A.J., et al. (2016). New inductively coupled plasma for atomic spectrometry: the microwave-sustained, inductively coupled, atmospheric-pressure plasma (MICAP). Journal of Analytical Atomic Spectrometry, 31(2), pp.440–449. https://doi.org/10.1039/c5ja00418g.
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.

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

For more information on this source, please visit TOFWERK.