Song-Hee Han

Laser-Induced Breakdown Spectroscopy (LIBS) of Heavy Metal Ions at the Sub-Parts per Million Level in Water:

We report a simple sub parts per million (sub-ppm) detection method for heavy metals in water by laser-induced breakdown spectroscopy (LIBS). Filter papers were used as the substrates for both transforming aqueous solutions to solid samples and for pre-concentrating dissolved heavy metal ions. The amount of sample uptake was 1.2 g by soaking a filter paper. This provides limit-of-detection (LOD) values of 2.7 and 0.36 ppm for Pb and Cr, respectively. The LODs could be improved remarkably by pre-concentrating the heavy metal ions. When a 40 g sample solution was evaporated on a filter paper, the LODs of 75 and 18 parts per billion (ppb) were obtained for Pb and Cr, respectively. Moreover, by either increasing the evaporated amount of sample solution or applying an argon gas flow, further improvement of the LODs was found to be very promising. The LIBS spectra of tap water were recorded using the pre-concentration method and are discussed in comparison with the results from inductively coupled plasma atomic emission spectroscopy (ICP-AES). We could observe strong emission lines of Ca, Mg, K, Cu, and Sr in the tap water, of which concentrations were determined to be 6.3, 1.3, 1.1, 0.64, and 0.046 ppm, respectively, by ICP-AES. Our method shows promise as a fast, reliable, water-quality monitoring tool for heavy-metal concentrations as well as for hardness.

Feasibility of Laser-Induced Breakdown Spectroscopy (LIBS) for Classification of Sea Salts

We have investigated the feasibility of laser-induced breakdown spectroscopy (LIBS) as a fast, reliable classification tool for sea salts. For 11 kinds of sea salts, potassium (K), magnesium (Mg), calcium (Ca), and aluminum (Al), concentrations were measured by inductively coupled plasma–atomic emission spectroscopy (ICP-AES), and the LIBS spectra were recorded in the narrow wavelength region between 760 and 800 nm where K (I), Mg (I), Ca (II), Al (I), and cyanide (CN) band emissions are observed. The ICP-AES measurements revealed that the K, Mg, Ca, and Al concentrations varied significantly with the provenance of each salt. The relative intensities of the K (I), Mg (I), Ca (II), and Al (I) peaks observed in the LIBS spectra are consistent with the results using ICP-AES. The principal component analysis of the LIBS spectra provided the score plot with quite a high degree of clustering. This indicates that classification of sea salts by chemometric analysis of LIBS spectra is very promising. Classification models were developed by partial least squares discriminant analysis (PLS-DA) and evaluated. In addition, the Al (I) peaks enabled us to discriminate between different production methods of the salts.

Laser-Induced Breakdown Spectroscopy (LIBS) Analysis of Calcium Ions Dissolved in Water Using Filter Paper Substrates: An Ideal Internal Standard for Precision Improvement

We report an approach for selecting an internal standard to improve the precision of laser-induced breakdown spectroscopy (LIBS) analysis for determining calcium (Ca) concentration in water. The dissolved Ca2+ ions were pre-concentrated on filter paper by evaporating water. The filter paper was dried and analyzed using LIBS. By adding strontium chloride to sample solutions and using a Sr II line at 407.771 nm for the intensity normalization of Ca II lines at 393.366 or 396.847 nm, the analysis precision could be significantly improved. The Ca II and Sr II line intensities were mapped across the filter paper, and they showed a strong positive shot-to-shot correlation with the same spatial distribution on the filter paper surface. We applied this analysis approach for the measurement of Ca2+ in tap, bottled, and ground water samples. The Ca2+ concentrations determined using LIBS are in good agreement with those obtained from flame atomic absorption spectrometry. Finally, we suggest a homologous relation of the strongest emission lines of period 4 and 5 elements in groups IA and IIA based on their similar electronic structures. Our results indicate that the LIBS can be effectively applied for liquid analysis at the sub-parts per million level with high precision using a simple drying of liquid solutions on filter paper and the use of the correct internal standard elements with the similar valence electronic structure with respect to the analytes of interest.

Soft Independent Modeling of Class Analogy (SIMCA) Modeling of Laser-Induced Plasma Emission Spectra of Edible Salts for Accurate Classification:

We report soft independent modeling of class analogy (SIMCA) analysis of laser-induced plasma emission spectra of edible salts from 12 different geographical origins for their classification model. The spectra were recorded by using a simple laser-induced breakdown spectroscopy (LIBS) device. Each class was modeled by principal component analysis (PCA) of the LIBS spectra. For the classification of a separate test data set, the SIMCA model showed 97% accuracy in classification. An additional insight could be obtained by comparing the SIMCA classification result with that of partial least squares discriminant analysis (PLS-DA). Different from SIMCA, the PLS-DA classification accuracy seems to be sensitive to addition of new sample classes to the whole data set. This indicates that the individual modeling approach (SIMCA) can be an alternative to global modeling (PLS-DA), particularly for the classification problems with a relatively large number of sample classes.