In this study, chemical replacement combined with surface-enhanced laser-induced breakdown spectroscopy (CR-SENLIBS) was for the first time applied to improve the detection sensitivities of trace heavy metal elements in aqueous solutions. Utilizing chemical replacement effect, heavy metal ions in aqueous solution were enriched on the magnesium alloy surface as a solid replacement layer through reacting with the high chemical activity metallic magnesium (Mg) within 1 minute. Unitary and mixed solutions with Cu, Pb, Cd, and Cr elements were prepared to construct calibration curves, respectively. The CR-SENLIBS showed a much better detection sensitivity and accuracy for both unitary and mixed solutions. The coefficients of determination R2 of the calibration curves were above 0.96, and the LoDs were of the same order of magnitude, i.e., in the range of 0.016-0.386 μg/mL for the unitary solution, and in the range of 0.025-0.420 μg/mL for the mixed solution. These results show that CR-SENLIBS is a feasible method for improving the detection sensitivity of trace element in liquid sample, which definitely provides a way for wider application of LIBS in water quality monitoring.
© 2016 Optical Society of America
Heavy metal pollution of water, even at very low concentrations, poses a significant health risk to humans and animals due to its toxic, neurotoxic, carcinogenic, mutagenic, and teratogenic effects [1, 2]. For example, Chromium (Cr) is a powerful carcinogenic agent that causes important chromosomic aberrations . However, heavy metal pollution of water is continuing and even increasing in some areas, especially in electroplating industry, in which the plating solution with addition of heavy metals, such as chromium (Cr), cadmium (Cd), copper (Cu), and lead (Pb) etc., is still used commonly. Therefore, a suitable method is required for real-time, in situ monitoring of heavy metals in water.
Laser-induced breakdown spectroscopy (LIBS) is one of the competitive approaches for monitoring water quality due to its attractive features, such as rapid, simultaneous multi-element detection, and in situ, real-time analysis capabilities [4–6]. However, the detection sensitivity of LIBS is still unsatisfactory when directly analyzing liquid samples because it suffers from several problems, e.g., water splashing, surface ripples, and quenching of intensity [7, 8]. To address these problems, various sampling configurations including liquid flow [9, 10], liquid jets [11–13], isolated droplets , and aerosol  have been reported. Although these sampling configurations have improved the detection sensitivity of LIBS, the complexity of analytical equipment is increased and the reproducibility of experimental results is still poor. Recently, an effective approach to improve detection sensitivity is to transform the sample from the liquid to the solid, such as freezing liquid into ice , absorbing heavy metals by using adsorbent materials [17–21], enriching the heavy metals on electrode by electrodeposition [22–24], and drying liquid samples on a non-absorbent solid surface [25–27]. For example, M. A. Aguirre proposed a new method named surface-enhanced LIBS (SENLIBS), which combined with static liquid-liquid microextraction for analysis of the manganese microdroplets prepared by drying on metallic substrates in air atmosphere . Although a limit of detection (LoD) of 6 μg/g for manganese (Mn) could be achieved, the static liquid-liquid microextraction as a pretreatment method requires additional reagents and the whole sample preparation time is longer than 15 minutes.
One hopeful way to simplify the pretreatment procedure and decrease the sample preparation time is chemical replacement method. Chemical replacement is an automatic process in which the inert metal ions in the aqueous solution were replaced by active metal  and enriched by converting the sample from liquid to solid. It is worth mentioning that there are few published reports about the chemical replacement.
In this work, chemical replacement (CR) was introduced as an automatic, simple, and rapid pretreatment method for SENLIBS (CR-SENLIBS). The proposed sample prepared method can overcome the poor detection sensitivity in LIBS when directly analyzing liquid samples, which was realized by converting the heavy metal cations in an aqueous solution to a solid replacement layer on the surface of magnesium alloy. The quantitative analysis of heavy metal elements (Cu, Pb, Cd, and Cr) in unitary and mixed solutions were also carried out by CR-SENLIBS, and then the detection sensitivities of CR-SENLIBS for detecting trace heavy metal elements in aqueous solution were investigated.
2.1 Sample pretreatment procedures
Unitary solutions (500 μg/mL of each CrCl3, CdCl2, CuCl2, and Pb(NO3)2) were prepared by dissolving the given amounts of each corresponding analytical reagent in distilled water, respectively. To compared with the LoDs of unitary solutions, mixed solution which contains all the 500 μg/mL of CrCl3, CdCl2, CuCl2, and Pb(NO3)2 was prepared. Seven standard solutions were prepared by diluting the above solutions with distilled water, respectively, i.e. the concentrations of each heavy metal elements in the standard solutions were 1, 2, 3, 4, 5, 6, and 7 μg/mL. To increase the chemical reaction rate and reduce sample preparation time, the PH value of each solution was adjusted to 1.5 using HCl and the high chemical activity metallic magnesium (Mg) was chosen for reacting with the heavy metal cations in liquid. Each liquid sample used for CR-SENLIBS was processed by the following procedures (see Fig. 1(a)): a magnesium alloy (AZ31B, 60 × 15 × 10 mm3, Mg: 95.56 wt.%, Al: 3.1 wt.%, Zn: 0.82 wt.%), that contains no Cr, Cd, Cu, and Pb elements, was chosen as a metallic substrate. Each magnesium alloy surface was cleaned by use of 240-grit SiC abrasive paper under running water and then washed with ethyl alcohol three times to remove surface impurities. Then the cleaned magnesium alloy was immersed in a 8 mL standard solution for 1 minute and taken out to be dried with a hot air blower. Finally, a solid replacement layer on the surface of the magnesium alloy that contains the heavy metal elements in the standard solution was prepared.
2.2 LIBS instrumentation
The experimental setup for the LIBS is schematically illustrated in Fig. 1(b). A Q-switched Nd:YAG laser (Quantel Brilliant B, maximum energy: 400 mJ/pulse, wavelength: 532 nm, pulse width: 5 ns) was used to ablate samples. The laser beam was reflected by a 40/60 beamsplitter and then vertically focused onto a target sample by a plano-convex lens (Len 2, f = 100 mm). The focal point of the lens was placed at 4 mm below the target surface. To provide a fresh surface for each laser ablation, the target was mounted onto a 2D motorized translation stage at a speed of 5 mm/s. The plasma emission was collected by an optical fiber (50 μm × 200 cm) through a UV-grade quartz lens (Len 1, f = 30 mm), and the other end of the fiber was coupled into an echelle spectrometer (Andor Tech., Mechelle 5000) with a wavelength range of 200-950 nm and a spectral resolution of λ/Δλ = 5000. An intensified charge-coupled device (ICCD) (Andor Tech., iStar 334T) camera triggered by the laser was used for spectra acquisition. Data acquisition and analysis were performed with a personal computer.
For elemental analysis, the higher signal-to-noise ratio (SNR) is, the lower LoD would be achieved. To obtain the highest SNR, the laser energy and gate delay time with a fixed gate width of 1 μs were optimized. In general, the analyzed solution is a mixed solution, which contains not only one element. Therefore, simultaneous multi-element analysis capability of LIBS is needed. As the results, the optimal laser energy was 60 mJ, the gate delay and gate width were set to 5 μs and 10 μs, respectively. To reduce the intensity deviation, each spectral intensity was accumulated for 100 shots and repeated for 10 times.
3. Results and discussion
3.1 The chemical replacement mechanism
In this work, inert αβ+ of each heavy metal in an aqueous solution was replaced by the highly chemically active metallic Mg. The chemical replacement reaction can be expressed as:Equation (1) indicates that concentration of each heavy metal α on the magnesium alloy surface is determined by the αβ+ concentration in the aqueous solution when the amount of Mg is enough. As a result, the concentration of each heavy metal element α on the magnesium alloy surface was proportional to the αβ+ concentration in the aqueous solution, which can be expressed as:
Figure 2 shows the time-integrated spectra in the range of 324.00-509.00 nm (a) and 516.20-521.20 nm (b) from magnesium alloy (black line) and replacement layer (red line) prepared by mixed solution (500 μg/mL of CrCl3, CdCl2, CuCl2, and Pb(NO3)2). Compared with the blank magnesium alloy, the heavy metal elements lines could be found on the replacement layer obviously. It can be concluded that heavy metal ions in an aqueous solution were converted on the magnesium alloy surface through chemical replacement.
To provide more clarity about the chemical replacement process, scanning electron microscope (SEM) micrographs of the magnesium alloy surface before and after sample preparation are shown in Fig. 3. Comparing Figs. 3(a) and 3(b), it can be seen that the surface of the magnesium alloy is covered by a solid replacement layer after sample preparation, which is obtained by chemical replacement reaction. The average thickness of replacement layers prepared by both unitary and mixed solution is about 2 μm (as shown in insets of Fig. 3(b)), which is within the range of 1-3 μm as reported in .
To test the content of the replacement layer, energy dispersive spectrometer (EDS) analysis was carried out and the spectral line of Cr element was found on the replacement layer, shown as Fig. 4(a). Moreover, the concentration of Cr in the replacement layer was about 4.50 wt.%, which is only 0.00498 wt.% (or 500 μg/mL) in the CrCl3 solution. Therefore, the preconcentration factor () for Cr is about 9000. This showed that the chemical replacement could be used as a preconcentration method for enriching the heavy metal cations on the magnesium alloy surface as a replacement layer. Therefore, the ablation amount of the magnesium alloy would affect the intensity of the analyzed element in the replacement layer. Figure 4(b) showed the variation in the relative intensity of Cr I 520.84 nm and the intensity of Mg I 516.78 nm with the laser pulse number using CR-SENLIBS. The relative intensity of Cr I 520.84 nm decreased with the intensity of Mg I 516.78 nm, which meant that the intensity of Cr reduced with the ablation amount of magnesium alloy. Moreover, the crater was produced by the laser-induced damage with only one laser pulse ablation . And the crater depth of the magnesium alloy about 6 μm (as shown in the inset of Fig. 4(b)), which is thicker than 2 μm of the replacement layer. As the result, each point on the replacement layer must be ablated with only one laser for CR-SENLIBS analysis. These above results show that the intensity of the analyzed element is truly affected by the ablation amount of the magnesium alloy.
3.2 Calibration curves and limits of detection
As we all know, the LoD of LIBS analysis of liquid sample would be affected by the liquid matrix . The quantitative analyses of both unitary and mixed solutions were performed to evaluate the effects of the content of other elements in aqueous solution on the LoD of the analyzed element by CR-SENLIBS. To estimate the LoDs of analyzed elements, the -criterion was used according to the following formula:
Figures 5(a)-5(d) show the calibration curves of Cu I 324.75 nm, Pb I 405.78 nm, Cd I 508.58 nm, and Cr I 520.84 nm for standard unitary solutions (solid red line) and mixed solutions (dotted black line). For CR-SENLIBS analysis, the ablation amount of magnesium alloy would affect the intensity of the analyzed element. To reduce this effect, the Al I 394.40 nm was chosen as a reference line with the following reasons [5, 30]: (i) has weaker intensity fluctuation than that of Mg due to its lower chemical activity and content; (ii) has the similar intensity to those analyze lines (as shown in Fig. 2); (iii) has weaker self-absorption effect due to its lower content than that of Mg. The average relative standard deviation (ARSD) of the relative intensities is in the range of 3.80-4.63% for unitary solutions and 5.61-6.63% for mixed solutions, which mean that the distribution of elements on the replacement layer is homogeneous. The relative spectral intensities for mixed solution are weaker than those for the unitary solutions, because the relative concentrations of the analyzed elements in replacement layer prepared by mixed solution are lower than those by unitary solution. All the calibration curves show a good linear fit (R2>0.96), which showed that CR-SENLIBS had the capability for good linearity of the calibration curves for liquid sample analysis. As shown in Fig. 5(a), the calibration curve is nonlinear results from self-absorption due to the resonant character of Cu I 324.78 nm chosen here. For calculated the LoD of Cu I 324.75 nm, the concentration range for establishing calibration curves was 1-3 μg/mL. Table 1 lists the background range, S, 3σB, and the calculated LoD values (according to Eq. (3)) for unitary and mixed solutions by using CR-SENLIBS. The LoDs for mixed solution are of the same order of magnitude as those for the unitary solution. This can be ascribed to the main matrix of the analyzed elements is magnesium alloy, which is due to the thickness of the replacement layer is thinner than the ablated depth of magnesium alloy (6 μm).
Table 2 demonstrates a comparison of the LoDs of Cu, Pb, Cd, and Cr acquired in present work with the results that reported in literatures with the aid of different sampling configurations (e.g., liquid surface, liquid flow, liquid droplet, and liquid jet). It was shown that, the LoDs of present method were lower than those reported with the aid of sampling configurations by SPLIBS. The LoDs of present method were lower than or comparable than those with DPLIBS and LIBS-LIF. Moreover, the conversion by the magnesium alloy through chemical replacement process can be completed within only 1 min, which is shorter than preparation time of 15 min reported by M. A. Aguirre . Furthermore, the magnesium alloy as a substrate material could be polished for reuse, which is more convenient than ice and adsorbent materials for sample preparation. And the LoDs of Cu, Pb, Cd, and Cr obtained by CR-SENLIBS are close to or lower than the level of the integrated wastewater discharge standard of China (No. GB/T 18918-2002), e.g., 0.5 μg/mL for Cu, 1.0 μg/mL for Pb, 0.1 μg/mL for Cd, and 1.5 μg/mL for Cr. These results show that the CR-SENLIBS is a feasible method to detect heavy metal elements for waste-water quality monitoring although the detection sensitivity should be further improved for drinking-water monitoring.
To improve the detection sensitivities of trace elements in the liquid sample by LIBS, chemical replacement was applied as an automatic, simple, and rapid pretreatment method to convert the heavy metal cations in liquid solution on magnesium alloy surface as a replacement layer for surface-enhanced LIBS (CR-SENLIBS). Using CR-SENLIBS, the quantitative analyses of trace heavy metal elements (Cu, Pb, Cd, and Cr) in both unitary and mixed solutions were carried out. The results show that the detection sensitivity was obviously improved comparing those with the aid of sampling configurations. All the coefficients of determination R2 of the calibration curves were higher than 0.96. The LoDs of Cu, Pb, Cd, and Cr for mixed solution were 0.250, 0.118, 0.420, and 0.025 μg/mL, respectively, which were of the same order of magnitude as those for the unitary solution. This suggests that CR-SENLIBS is a feasible method for waste-water quality monitoring through detecting the trace heavy metal elements.
This research was financially supported by the Major Scientific Instruments and Equipment Development Special Funds of China (No. 2011YQ160017), the National Natural Science Foundation of China (No. 51429501, 61575073, and 61378031), and the Fundamental Research Funds for the Central Universities of China (HUST: 2015TS075).
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