RESUMEN

High signal uncertainty has been regarded as a critical obstacle for the quantitative analysis of laser-induced breakdown spectroscopy (LIBS). One of the most effective ways for uncertainty reduction is to directly compensate for the variation of plasma properties, especially total number density. However, reliable compensation for the variation of total number density is hard to implement. In this work, we propose a data pre-processing method, called total number density compensation (TNDC), to reduce signal uncertainty. It is established on an assumption extended from the internal standard method and utilizes a weighted sum of emission lines from all major elements to reflect the variation of total number density. The TNDC method is tested on 29 brass samples and outperforms common normalization methods based on the spectral area in terms of signal repeatability and analytical performance. For Cu, the mean pulse-to-pulse relative standard deviation (RSD) of signals is greatly decreased from 5.10% to 1.03%, which is almost the best signal repeatability that LIBS can achieve and is comparable to that of ICP-OES. The root mean square error of prediction (RMSEP) and the mean RSD of prediction are decreased from 6.56% to 0.60% and from 12.00% to 1.03%, respectively. While for Zn, the mean RSD of signals improves from 6.43% to 4.12%, and the RMSEP is reduced from 1.57% to 0.59% with the RSD of prediction from 5.41% to 4.18%. Results demonstrate that TNDC can be an effective method for LIBS analysis especially for repeatability improvement.

Asunto(s)

RESUMEN

Laser-induced breakdown spectroscopy (LIBS) is a promising multi-elemental analysis technique and has the advantages of rapidness and minimal sample preparation. In traditional LIBS measurement, sample spectra are generally collected based on a single set of fixed experimental parameters, such as laser energy and delay time. When samples have the same main components and similar component concentrations, the difference in their spectral intensities becomes less obvious. This can lower the sensitivity of LIBS measurement and pose a threat to the accuracy and robustness of LIBS qualitative analysis. In this work, we propose a new method to increase the spectral difference between similar samples, namely multiple-setting spectra. For each sample, it adopts different sets of experimental parameters and obtains a group of spectra to increase the fingerprint spectral information. The effectiveness of the proposed method is theoretically verified and then tested on 11 similar coal samples. Specifically, the sample spectra were collected with different laser energy and delay time, and processed by principal component analysis (PCA) and Davies-Bouldin index (DBI). The results show that the use of multiple-settings spectra can significantly improve the sample discrimination accuracy from 81.8% to 96.4%. In addition, the proposed method can maintain the efficiency and cost of LIBS measurement.

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RESUMEN

Repeatability is of utmost importance as it is directly linked to measurement accuracy and precision of a technique and affects its cost, utility, and commercialization. The present paper contributes to explain enhanced repeatability of the femtosecond laser-induced breakdown spectroscopy (fs-LIBS) technique, remarkably significant for its industrial applications and instrumental size reduction. A fs-laser with 7 mJ pulse energy was focused to create a transient titanium plasma, and a high-resolution spectrometer was used to study time-resolved spectra and single-shot drilling sampling repeatability. Time-resolved spectroscopy study at a delay time interval of 0-1600 ns showed 200-400 ns as the optimum delay time zone for data acquisition with 2-4% line intensity RSDs. Plasma temperature RSDs were <1.8% for the investigated delay interval and reached 0.5% at 200 ns where the temperature recorded a maximum value of 22,000 K. Electron density reached 5.7 × 1017 cm-3 at 200 ns, and RSDs were <3% with the least fluctuation of 0.7%. Shot-to-shot RSDs were 3.5-5% at 15-30 drilling shot intervals for line intensities, <2% for plasma temperature, and <6.5% for electron density. Using an uncertainty propagation formula, total number density RSDs were calculated to be 1.9-5.3% for 50 single-shot drilling scenarios. Considering physics behind results, fs-plasmas are "stable ablation sources" due to their electrostatic formation mechanisms and confined hydrodynamic evolution. The fs-laser opens up new directions for LIBS applications where accuracy is significantly enhanced.