Spatially selective excitation in laser-induced breakdown spectroscopy combined with laser-induced fluorescence

Rapid characterization of heavy metals in single microplastics by laser induced breakdown spectroscopy

Microplastics (MPs) in aquatic environment usually carry hazardous matters, including toxic heavy metals. Quantification of heavy metals in MPs is crucial for the comprehensive understanding of their ecotoxicology in field environment. However, conventional methods for heavy metal determination either are applicable only to bulk/collective samples or require strict operation environment. Here we demonstrated that laser induced breakdown spectroscopy (LIBS) is a robust tool for the characterization of heavy metals in single MPs. Single-particle LIBS selects individual MPs with specific sizes (down to tens of microns), shapes, and morphologies and analyzes simultaneously multiple elements in milliseconds without sample pretreatment. In addition to the elaborate optical design, we also used stretched thin polyethylene film as a substrate, which significantly suppress the matrix interference to the particles' spectra. The single particle LIBS was demonstrated to be a quantitative analytical method, and was applied to heavy metal analysis of the MPs collected in the seawaters of the Beibu Gulf of China. Positive correlation between the spectral intensities and the local marine pollutions as well as significant heterogeneity in the elemental compositions were observed. The results demonstrate that single-particle LIBS is a promising method for MPs characterization and is suitable for studying pollutant transportation by using MPs as vectors.

Rapid and Sensitive Analysis of Trace Leads in Medicinal Herbs Using Laser-Induced Breakdown Spectroscopy-Laser-Induced Fluorescence (LIBS-LIF)

Toxic metals in medicinal herbs are potentially harmful for people taking herbal medicines. In this work, laser-induced breakdown spectroscopy-laser-induced fluorescence (LIBS-LIF) spectroscopy was first applied to carry out rapid and sensitive trace lead analysis in medicinal herb samples. To overcome the problem of diversity on the sample size, shape, and density for different samples, original samples were pulverized to powder and then pressed into pellets for spectral analysis. A series of standard samples were self-made for building a calibration curve. As an exemplary study, lead in Rheum officinale was analyzed with LIBS-LIF spectroscopy with significantly improved analytical sensitivity. The R² of the build linear calibration curve was 0.996 and the detection limit of lead in R. officinale was determined to be 0.13 ppm. The enhancement factor on the signal-to-background ratio was >100 under low lead concentrations if compared with LIBS analysis. The lead concentrations in several original R. officinale samples were quantitatively determined. This work demonstrated that LIBS-LIF can be successfully applied to carry out rapid, sensitive, and quantitative trace lead analysis for medicinal herbs.

Determination of Pb in soils by double-pulse laser-induced breakdown spectroscopy assisted by continuum wave-diode laser-induced fluorescence

Laser-induced breakdown spectroscopy (LIBS) has attracted a lot of attention due to its potential to rapidly identify and quantify any chemical element with minimal sample preparation. Despite continuous improvements, the sensitivity of this technique still remains a challenge. In order to increase LIBS intensity, a laser-induced fluorescence (LIF) system can be coupled with LIBS to re-excite a transition of the element in the plasma by employing very expensive optical parametric oscillators (OPO). In this work, a homemade tunable continuum wave-diode laser (CW-DL) has been developed and coupled to a double pulse (DP) LIBS system to enhance the sensitivity of Pb detection in a soil sample at the transition 6s²6p²-P3₂→6s²6p7s-P3₁ at 405.78 nm. Before sample analysis, the production of no scattered light by the plasma was ascertained, and the optimal temperature of 10,000 K was estimated for this transition, feasible to be achieved in DP-LIBS systems. An increase of approximately 100% for the Pb I transition at 405.78 nm was obtained by DP-LIBS-CW-DL-LIF with respect to the DP-LIBS system alone. This result opens a new promising line of research to improve LIBS sensitivity using the CW-DL approach.

Quantitative Analysis of Cadmium in Tobacco Roots Using Laser-Induced Breakdown Spectroscopy With Variable Index and Chemometrics

The study investigated some new developed variable indices and chemometrics for the fast detection of cadmium (Cd) in tobacco root samples by laser-induced breakdown spectroscopy. The variables selection methods of interval partial least squares (iPLS), backward interval partial least squares (BiPLS), and successive projections algorithm (SPA) were used to locate the optimal Cd emission line for univariate analysis and to select the maximal relevant variables for multivariate analysis. iPLS and BiPLS located 10 Cd emission lines to establish univariate analysis models. Univariate analysis model based on Cd I (508.58 nm) performed best with the coefficient of determination of prediction (R_p ²) of 0.9426 and root mean square error of prediction (RMSEP) of 1.060 mg g^-1. We developed two new variable indices to remove negative effects for Cd content prediction, including Index1 = (I _508.58 + I _361.05)/2 × I _466.23 and Index2 = I _508.58/I _466.23 based on Cd emission lines at 508.58, 361.05, and 466.23 nm. Univariate model based on Index2 obtained better result (R_p ² of 0.9502 and RMSEP of 0.988 mg g^-1) than univariate analysis based on the best Cd emission line at 508.58 nm. PLS and support vector machines (SVM) were adopted and compared for multivariate analysis. The results of multivariate analysis outperformed univariate analysis and the best quantitative model was achieved by the iPLS-SVM model (R_c ² of 0.9820, RMSECV of 0.214 mg g^-1, R_p ² of 0.9759, and RMSEP of 0.712 mg g^-1) using the maximal relevant variables in the range of 474-526 nm. The results indicated that LIBS coupled with new developed variable index and chemometrics could provide a feasible, effective, and economical approach for fast detecting Cd in tobacco roots.

Surface elemental microanalysis with submicron lateral resolution by the laser-ablation laser-induced fluorescence technique

In order to realize surface elemental microanalysis of solid samples with submicron lateral resolution, laser-ablation (LA) combined with high sensitive laser-induced fluorescence (LIF) detection was investigated. A 532 nm or 266 nm nanosecond laser pulse with low pulse energy was used to realize submicron laser-ablation on the surface of a copper alloy, and LIF technique was used to sensitively detect a minor lead element in the ablated samples. ~344 nm and ~267 nm lateral resolutions could be achieved experimentally under 532 nm and 266 nm laser ablations under the current experimental condition, respectively. This demonstrated the feasibility of using a LA-LIF technique for surface elemental microanalysis of solid samples with submicron spatial resolution. The potentials of continually improving the spatial resolution of this technique to nanoscale were discussed.