The Role of Ambient Gases in Improving LIBS Signal Quality

A recent study led by Zhe Wang from Tsinghua University and collaborators from China National Uranium Corporation, China National Coal Group Corporation, and Qinghai University explored how laser-induced breakdown spectroscopy (LIBS) signals are influenced by ambient gas properties. Their findings, which were published in Analytica Chimica Acta, offer more insight about the signal quality and repeatability in LIBS applications (1).

Diagram of a spectrometer analyzing atomic spectra. Generated by AI. | Image Credit: © Korngor - stock.adobe.com

LIBS is an advanced spectroscopic technique that uses a short laser pulse that has several key advantages (2). Some of those advantages include allowing for broad elemental coverage and versatile sampling protocols (2). LIBS also is quick when it comes to single-spot analysis (2). LIBS spectra ultimately provide data on element concentrations and atomic structures (3). However, despite these positive attributes, LIBS still has some limitations. Because of the unstable plasma generated during analysis, LIBS delivers poor signal quality, and this signal quality is influenced by interactions with ambient gases (1).

As a result, researchers are trying to improve the value and versatility of the LIBS technique by learning more about the ambient gas processes. Wang’s team conducted a study that focused on three key ambient gas properties: specific heat ratio (γ), molar mass (M), and ionization energy (E), with the hope of acquiring more knowledge about how ambient gas properties work (1).

To isolate the effects of these properties, the research team ignored secondary gas characteristics and developed gas mixtures with varying γ, M, and E values. The mixtures were then compared to air, the most used ambient gas in LIBS experiments (1).

Then, the researchers examined how these gas properties influence plasma behavior, which would tell them about how LIBS signal quality is influenced by ambient gases. The researchers employed shadowgraph analysis, rapid imaging, and optical emission spectroscopy (OES) to conduct this work.

The study revealed that the three gas properties affected LIBS signals through three mechanisms: specific heat ratio, molar mass, and ionization energy. For specific heat ratio (γ), a higher γ led to weaker shockwave intensity and stronger back-pressing within the plasma, improving signal repeatability (1). Additionally, higher γ reduced heat loss in the plasma, resulting in stronger signal intensity (1). A lower molar mass improved signal repeatability because of faster sound speed (1). And finally, higher ionization energy decreased the intensity of the back-pressing process and elevated the plasma core position, improving signal repeatability (1).

With these findings, the researchers then discussed what scientists can do practically to improve the signal quality of LIBS. For example, to improve signal intensity and repeatability, the researchers recommend using a higher specific heat ratio (1). They also suggested that changing the molar mass could be beneficial depending on what the researchers are trying to accomplish (1).

By providing a deeper understanding of the plasma evolution process, the study represents a major step forward in LIBS research. The insights gained from this work could lead to more reliable and accurate LIBS applications in fields such as environmental monitoring, material analysis, and industrial quality control (1).

As LIBS continues to evolve, the study by Wang and colleagues offers several strategies to improve LIBS signal quality. Other researchers in other studies have also investigated this topic, including proposing new techniques, such as microwave plasma torch with LIBS (MPT-LIBS (4). By addressing the longstanding issue of signal instability, the research enhances the viability of LIBS as a cutting-edge tool for chemical analysis. With these advancements, LIBS would become a more effective analytical technique across multiple scientific and industrial domains (1).

References

1. Song, Y.; Hou, Z.; Yu, X.; et al. Exploring the Impact Mechanism of Ambient Gas Properties on Laser-induced Breakdown Spectroscopy to Guide the Raw Signal Improvement. Anal. Chimica Acta 2025, 1335, 343464. DOI: 10.1016/j.aca.2024.343464
2. Applied Spectra, What is LIBS? Applied Spectra. Available at: https://appliedspectra.com/technology/libs.html (accessed 2025-01-23).
3. McMillan, N. Laser-Induced Breakdown Spectroscopy (LIBS). Carleton.edu. Available at: https://serc.carleton.edu/msu_nanotech/methods/libs.html (accessed 2025-01-23).
4. Wei, B.; Yang, C.; Wu, S.; et al. The Signal Quality Improvement of Laser-Induced Breakdown Spectroscopy due to the Microwave Plasma Torch Modulation. Anal. Chimica Acta 2024, 1328, 343183. DOI: 10.1016/j.aca.2024.343183