Exploring the impact mechanism of ambient gas properties on laser-induced breakdown spectroscopy to guide the raw signal improvement

Background

Laser-induced breakdown spectroscopy (LIBS) has long been regarded as the future superstar for chemical analysis. However, hindered by the fact that the signal source of LIBS is a spatially and temporally unstable plasma that interacts dramatically with ambient gases, LIBS has always suffered from poor signal quality, especially low signal repeatability. Although ambient gases act as one of the most direct and critical factors affecting LIBS signals, a clear understanding on how ambient gas properties impact LIBS signals is still lacking to act as guideline for the signal quality improvement.

Results

In this work, the impact mechanism of three main ambient gas properties, including specific heat ratio ($$), molar mass ($$), and ionization energy ($$) was investigated by ignoring secondary properties, accurately proportioning gas mixtures, and experimental comparative study by applying various plasma diagnosis methods. The results indicate that these three properties impact signal repeatability by impacting the intensity of the back-pressing process within the plasma, where the sound speed determined by $$ and $$ plays an important role. The properties impact the signal intensity by impacting three energy transfer processes in the plasma, including the laser energy absorption, the energy allocation between gas and sample species, and the heat dissipation. Based on the impact mechanism, guidelines of regulating the ambient gas properties to improvement LIBS signal were further provided.

Significance

For the first time, the impact mechanism of main ambient gas properties on LIBS signals was clearly and intuitively revealed. The impact mechanism not only provided a deeper understanding of the plasma evolution process but also provided practical guidelines for improving LIBS signal quality, facilitating accurate quantification of the LIBS technique.