Direct and sensitive determination of Cu, Pb, Cr and Ag in soil by laser ablation microwave plasma torch optical emission spectrometry

Based on microwave plasma torch optical emission spectrometry combined with laser ablation, a direct solid sample detection device was developed for sensitive determination of heavy metals in soil. In the proposed laser ablation microwave plasma torch optical emission spectrometry (LA-MPT-OES) device, a new ablation chamber was designed, which the washout time and the relative standard deviation of this chamber were almost one-third of those of the conventional one, indicating that the proposed chamber had a smaller dead volume to provide efficient and stable transport of ablated sample particles. Meanwhile, to ensure a high signal intensity during a long exposure time, the moving sampling method was used to guarantee a sufficient injection amount. With the optimal experimental parameters, the limits of detection (LODs) of Cu, Pb, Cr and Ag were 0.075, 0.093, 0.068, 0.009 mg·kg−1, respectively, which was reduced by one to two orders of magnitude compared with that of laser-induced breakdown spectroscopy and X-ray fluorescence and was similar to the LODs of the digestion-required techniques (e.g., ICP-OES and MP-AES) and other LA-related techniques (e.g., LA-ICP-MS). Furthermore, the LA-MPT-OES was applied to the quantitative analysis of standard samples and actual samples, and the obtained determination results were in agreement with the standard values and that of atomic absorption spectrometry. The practicability and accuracy (relative errors were 0.95%–25.9%) of LA-MPT-OES determination of heavy metal elements were also validated.