Achieving Low-Energy fs-LIBS with a Microwave Plasma Torch: A Feasible Method for the Development of a Portable fs-LIBS Measurement System

This study introduces an MPT-fs-LIBS system, combining femtosecond laser-induced breakdown spectroscopy (fs-LIBS) with a microwave plasma torch (MPT), to enhance the fs-LIBS performance under low laser pulse energy. By using femtosecond laser pulses and MPT-generated plasma, the system reduces the Cu/Zn alloys' breakdown threshold to 1 μJ and enhances the plasma emission intensity and stability. The spectral signals of Cu/Zn alloys measured via 30 μJ MPT-fs-LIBS exceed those from 1.5 mJ fs-LIBS, indicating reliable elemental detection at lower laser energies. Time-resolved spectra show that MPT enhances plasma behavior by extending its lifetime and raising electron temperature. The 30 μJ MPT-fs-LIBS creates smaller ablation craters than 1.5 mJ fs-LIBS, enabling near-nondestructive testing of industrial materials. The system demonstrates an excellent analytical performance for Al alloys. At the same laser energy, MPT-fs-LIBS has higher intensities for emission lines of Mg II 279.55 nm, Cu I 324.75 nm, Mn I 403.11 nm, and Cr I 425.42 nm than fs-LIBS. Compared to 1.5 mJ fs-LIBS, 1.5 mJ MPT-fs-LIBS reduces trace metals' (e.g., Mg) limit of detection (LOD) by 130 times, while 30 μJ MPT-fs-LIBS lowers it by 17 times. Glass sample tests suggest the method's wide applicability, with 30 μJ MPT-fs-LIBS outperforming 1.5 mJ fs-LIBS. The compact MPT-fs-LIBS avoids complex optical setups and specialized vacuum sample chambers. This method provides a practical way to integrate compact femtosecond fiber lasers with MPT, enabling portable devices for real-time multielement analysis while minimizing sample damage.