Improvement of sample discrimination using laser-induced breakdown spectroscopy with multiple-setting spectra.

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.

[Source and enrichment of heavy metals in sewage-irrigated area soil of Dagu Sewage Discharge Channel].

Concentrations of metals, total nitrogen, total phosphorus and organic matter were determined in topsoil (0-25 cm) and subsoil (25-50 cm) samples collected from 16 selected blocks in the sewage-irrigated area of Dagu Sewage Discharge Channel in Tianjin, China. Contamination factors (CF), relative enrichment factors (REF) and cluster analysis (CA) were taken to analyze the source and enrichment of heavy metals. The results show that the concentrations of Hg, Cd, Pb and Zn are above the 1st limit value of Environmental Quality Standard for Soils (China), while those of As, Ni and Cr are below the 1st limit value of the standard. The concentrations of Hg, Cd, Pb, Zn, As, Ni and Cr in topsoil are 0.323, 0.341, 44.7, 108.29, 11.2, 29.86 and 66.6 mg/kg, respectively. CF and REF of Hg in topsoil are 10.8 and 7.2, respectively, which indicates very high contamination and moderately severe enrichment. Both factors of Cd are 3.8 and 4.5, and indicate considerable contamination and moderate enrichment. Those of Pb, Zn and As are all less than three, which indicates moderate contamination and minor enrichment. As for Ni and Cr, their contamination is low and no enrichment occurs. REF for Hg, Pb and Zn in topsoil is relatively higher than that in subsoil while As in subsoil is higher. When compared with data measured in 1985, the concentrations of Cd and As increase while those of Hg, Pb and Zn decrease. Furthermore, measures, including CA and in situ investigation, are taken to identify the main sources of heavy metal contamination, and it shows that Hg and As contamination result from anthropogenic effect as well as deposition of coal combustion. Cd, Zn and Pb contamination are mainly from sewage irrigation, and atmosphere deposition also makes certain contribution. Ni and Cr contamination are attributed to natural sources.

[Concentrations and distribution of heavy metals in urban sewage discharge channel of Tianjin].

Concentrations of metals, organic matter(OM), total nitrogen (TN) and total phosphorus (TP) were determined in sediment and soil samples collected from 15 selected sites along Dagu sewage discharge channel in Tianjin, China. Relative enrichment factors and correlation analysis were taken to study the distribution and pollution level of metals. The results show that pollution level is more serious in sediment than that in soil. Hg, Cd, Zn and Pb are more enriched while Cr, Ni and As are less enriched. Relative enrichment factors (REF) of Hg, Cd, Zn and Pb in sediment are 59.3, 25.4, 14.5 and 7.5, and that in soil are 8.44, 5.19, 6.6 and 3.3, respectively. The heavy metal concentrations greatly vary among different sampling sites, which is demonstrated by the fact that coefficient of variation (CV) varies from 74.4% to 110.8% in sediment. The highest enrichments of heavy metals are found in sampling site S4 and S12, which are affected by chemical industry wastewater. There is no significant correlation between metal concentrations in sediment and those in soil. The concentrations of heavy metals in sediment are not reciprocally correlated, either. However, significant correlations exist among many pollutants in soil. Hg, Cd, Zn and Pb are positively correlated with each other, at the same time they are positively correlated with OM, TN and TP, but Cr, Ni and As are poorly correlated with OM and TP, which suggests that Cd, Zn and Pb have similar environmental geochemical properties and are contaminated by anthropogenic impact. When compared with the main rivers in Tianjin, the enrichment of heavy metals in Dagu sewage discharge channel is relatively high.