On-Site Quantitative Elemental Analysis of Metal Ions in Aqueous

Jan 5, 2015 - ... by Underwater Laser-Induced Breakdown Spectroscopy. Combined with Electrodeposition under Controlled Potential. Ayumu Matsumoto,*...
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On-Site Quantitative Elemental Analysis of Metal Ions in Aqueous Solutions by Underwater Laser-Induced Breakdown Spectroscopy Combined with Electrodeposition under Controlled Potential Ayumu Matsumoto,*,† Ayaka Tamura,† Ryo Koda,† Kazuhiro Fukami,‡ Yukio H. Ogata,§ Naoya Nishi,† Blair Thornton,⊥ and Tetsuo Sakka*,† †

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan § Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan ⊥ Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan ‡

S Supporting Information *

ABSTRACT: We propose a technique of on-site quantitative analysis of Zn2+ in aqueous solution based on the combination of electrodeposition for preconcentration of Zn onto a Cu electrode and successive underwater laser-induced breakdown spectroscopy (underwater LIBS) of the electrode surface under electrochemically controlled potential. Zinc emission lines are observed with the present technique for a Zn2+ concentration of 5 ppm. It is roughly estimated that the overall sensitivity over 10 000 times higher is achieved by the preconcentration. Although underwater LIBS suffers from the spectral deformation due to the dense plasma confined in water and also from serious shot-to-shot fluctuations, a linear calibration curve with a coefficient of determination R2 of 0.974 is obtained in the range of 5−50 ppm.

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by adding CaO to solutions35) have been investigated. Some groups have reported the LIBS analysis of deposits obtained with displacement reaction and electrochemical reaction under the application of high voltages (tens of volts) between electrodes in a solution containing heavy metal ions.36−38 Even though the techniques described above greatly enhance the signal and can detect heavy metal ions with concentrations in the part-per-million to part-per-billion range, all of them are basically performed in the gaseous phase. For the application to on-site underwater analysis, e.g., deep-sea resource exploration, we need to develop a technique to generate plasmas appropriate for LIBS measurement in a water-confined geometry. Electrodeposition can be a practical tool for on-site preconcentration of heavy metal ions in solutions for succeeding underwater LIBS. We have previously reported underwater LIBS for in situ monitoring of electrodeposition under controlled potential.39 The study aimed at the characterization of electrodeposited films. Since this technique analyzes the metal species originating in liquid phase, it can also be used for the solution analysis. Recently, Lu et al. successfully demonstrated underwater analysis of part-per-billion order

aser-induced breakdown spectroscopy (LIBS) is an analytical method that is capable of performing in situ multielemental analysis. LIBS can be used for various kinds of samples under a wide range of environmental conditions. Emission spectroscopy of a laser-induced plasma produced in a bulk solution allows us to identify the dissolved species, and high sensitivity can be achieved for group 1 and group 2 metal ions.1−4 This technique can be applied to in situ monitoring of water quality under various environmental situations. Experiments of LIBS under high-pressure environments have been performed, aiming at the application to deep-sea fluids.5−13 In the viewpoint of metal resource exploration, the survey of heavy metal ions enriched in fluids from deep-sea hydrothermal vents is especially important. For the analysis of solutions using LIBS, various techniques have been investigated to improve the overall sensitivity. Techniques using liquid surfaces,14−17 flows, 18−22 and droplets23,24 as a target avoid the plasma confinement and quenching caused by the presence of bulk water, and significant improvements in the sensitivity have been reported. Pretreatment of liquid samples and subsequent emission spectroscopy of the laser ablation plasma produced from the pretreated targets can be a powerful technique for some applications. For example, drying microdroplets on metallic substrates,25 preconcentration in absorbers (carbon,26 ion-exchange membrane,27,28 wood slice,29 paper,30,31 porous electrospun ultrafine fibers32), and conversion into solids (ice,33,34 pellet preparation © 2015 American Chemical Society

Received: September 3, 2014 Accepted: January 1, 2015 Published: January 5, 2015 1655

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Analytical Chemistry Cu2+ in CuSO4 aqueous solutions with nanosecond single-pulse LIBS of Cu deposits obtained by a displacement reaction on an Al sheet, as well as an electrochemical reaction with the application of a high voltage to the sheet electrode.40 Their work greatly extended the concentration range where underwater LIBS can be used. However, studies on this technique are very limited. There are still problems especially in quantitative aspect due to serious spectral deformation and shot-to-shot fluctuations40 that need to be solved before the technique is actually applied to on-site measurements. In the present study, we propose potential-controlled electrodeposition as an on-site sample treatment technique for underwater LIBS to improve the overall sensitivity in the analysis of the heavy metal ions in the solution. We evaluate the feasibility of quantitative analysis of Zn2+ in aqueous solution using underwater LIBS combined with the electrodeposition. Since Zn2+ concentration increases to part-per-million order in the fluid from hydrothermal vents, we focus on the concentration range from 5 to 50 ppm. The improvements in the quantitative aspect of the analysis are based on the following points. We used a single pulse laser with the pulse duration of 100 ns. To plot a calibration curve, a nonresonance emission line, whose lower level is not the ground state, is employed. A Cu plate, whose standard potential (Cu2+/Cu) is more positive than that of Zn2+/Zn, is employed as a working electrode, and the potential is electrochemically controlled during the preconcentration process and also during the LIBS measurements.

The laser was focused onto a working electrode (WE) placed in the electrochemical cell in the direction normal to the surface through an achromatic lens with a focal length of 63.5 mm (Newport, PAC043). The lens was used both for laser focusing and for the collection of the optical emission from the plasma produced on the surface. The electrochemical cell was filled with 40 mL of the electrolyte solution, and the depth of the solution was 30 mm. A cold mirror that reflects visible light (spectral range to be measured) and transmits infrared light (laser) was placed above the lens. Note that a portion of the laser is also reflected by this mirror. It is roughly estimated that the pulse energy is 2.8 mJ at the electrode surface taking into account the reflection by the cold mirror and absorption by the solution. The plasma emission reflected from the cold mirror was focused onto an entrance of an optical fiber bundle with a diameter of 1.6 mm (Oriel, 77532) using an achromatic lens with a focal length of 125 mm (Newport, PAC055). The exit of the fiber bundle was connected to a spectrograph (Bunkoukeiki, MK-302) equipped with an intensified charge coupled device (ICCD, Princeton Instruments, ICCD-1024MTDGE/1) to obtain the emission spectra. The electrochemical cell consisted of a Cu plate (Nilaco, 99.94%, 0.30 mm) WE, a Pt ring counter electrode (CE), and a Ag|AgCl (sat. KCl) reference electrode (RE) was used. A square ring was used between the WE and the cell to prevent liquid leakage. Another Cu plate was used as a current collector. A potentiostat (Hokuto Denko, HA-301) was used for the control of the potential and for the measurement of the current. The surface area of the WE is 0.78 cm2. The Pt ring (CE) was cleaned with 6.0 M HCl aqueous solution prior to the use. The electrochemical cell was mounted on an XY stage. We prepared 40 mL of ZnSO4 + 0.10 M Na2SO4 aqueous solutions ([Zn2+] = 5, 10, 15, 30, 50 ppm) as electrolyte solutions for Zn deposition on the Cu plate. Na2SO4 was employed as a supporting electrolyte. The following reagents were used: zinc sulfate, heptahydrate (Nacalai Tesque, 3701125), and sodium sulfate, anhydrous (Kishida Chemical, 00072955). Ultrapure water (Millipore, Milli-Q Gradient) was used as solvent. For all the deposition experiments, the potential and time for the deposition were set to −4.5 V versus Ag|AgCl (sat. KCl) and 1 h, respectively. A potential of −1.125 V was applied during the LIBS measurements. To plot the calibration curve, 50 spectra were recorded in a series of measurements, and the spectra were averaged to minimize the effects of shot-to-shot fluctuation. This operation was performed eight times for a single sample. The horizontal position of the electrochemical cell was moved manually for each laser shot to avoid overlapping between the irradiation spots. The experimental spectra were fitted with a theoretical spectrum,41,42 and the parameters, such as the peak height and the area of emission lines, the atomic excitation temperature, and the atomic density ratio NZn/NCu in the plasma, were obtained. Details of emission spectroscopy and the fitting procedure are described in Supporting Information.

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EXPERIMENTAL SECTION The experimental setup for in situ underwater LIBS of electrodeposits under controlled potential is shown in Figure 1. A custom-built diode-pumped Q-switch Nd:YAG laser (OK Lab. Co. Ltd.) with a wavelength of 1064 nm and repetition period of 2.75 s was used. The pulse duration was adjusted to 100 ns, and a pulse energy of 6.0 mJ was achieved using two diode-pumped Nd:YAG amplifiers. The beam profile can be approximated to a Gaussian beam with a diameter of 1.9 mm.

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RESULTS AND DISCUSSION Electrodeposition. To determine the potential for deposition and the potential for the LIBS measurements, we preliminarily performed cyclic voltammetry in a 20 mM ZnSO4 + 0.10 M Na2SO4 aqueous solution using an electrochemical analyzer (Metrohm, μAUTOLAB type III). The result is reported in Supporting Information (Figure S-1).

Figure 1. Experimental setup for simultaneous operation of electrodeposition and underwater LIBS. The setup enables us to perform in situ underwater elemental analysis of electrodeposits under controlled potential. 1656

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Analytical Chemistry For quantitative control of the deposition during the preconcentration process, the electrode potential should be electrochemically controlled. In the present deposition experiment, the potential was set to −4.5 V where convection occurs due to rigorous H2 evolution, which in turn improves the deposition rate. As we are considering application to on-site measurement, a stirrer to improve the deposition rate was not employed. Also, we did not adjust pH. The potential employed for deposition is significantly more negative than that for the onset of the diffusion-limited condition of Zn (see Figure S-1 in Supporting Information). The current during the Zn deposition in 5−50 ppm Zn2+ solutions was −40 mA, regardless of the Zn2+ concentration. After the deposition, we saw the color of Zn metal on the Cu electrode surface. When we left the sample without applying the external potential, the surface color gradually changed back to the color of Cu metal especially in the case of low-concentration solution, probably due to spontaneous oxidation of Zn. This would lower the quantitative performance of the present technique. Therefore, we held the potential at −1.125 V during the LIBS measurements. At this potential, we can avoid H2 evolution and Zn stripping (see Figure S-1 in Supporting Information), and neither bubble generation nor surface color change was observed. We consider that the deposition rate at this stage is sufficiently slow since the overpotential is small and also there is no convective flow in the solution. In fact, the current was negligibly small at this potential. Note that the standard potential of Cu2+/Cu is more positive than that of Zn2+/Zn. By employing an appropriate potential, we can avoid the displacement reaction which makes it difficult to control the start and the end of the reaction and causes unwanted deposition. Effects of Preconcentration. Figure 2 shows typical emission spectra obtained for a Cu electrode in a ZnSO4 + 0.10 M Na2SO4 aqueous solution without and with Zn electrodeposition ([Zn2+] = 5 ppm), and without electrodeposition but high concentration of Zn2+ ([Zn2+] = 50 000 ppm). The spectrum which gives the temperature closest to the average temperature obtained from eight spectra was chosen as a representative one, and is shown in Figure 2. It is known that species originating in the solution can enter the bubble produced by laser ablation of a solid target in the solution.42,43 We consider that, in the initial stage of the bubble formation, the water phase is vaporized by the heat from the surface or from the ablated species, which is in a high-temperature state. Also, the species dissolved in the water phase could be introduced in the growing bubble through the bubble−water interface. Therefore, the dissolved species are included in the ablation plasma. However, in the case without electrodeposition ([Zn2+] = 5 ppm) shown in Figure 2a, Zn emission lines from the solution cannot be observed while Cu nonresonance emission lines at 510.5537, 515.3230, 521.8197, 522.0066 nm (the values were taken from the database44) attributed to Cu atoms from the electrode surface are observed in the spectrum (the lines at 521.8197 and 522.0066 nm are overlapped due to the limited resolution). The reason why Zn lines cannot be observed is that the amount of Zn atoms in the plasma is very small due to the low concentration in the solution (5 ppm). Note that bulk breakdown does not occur on the laser focusing path before the laser arrives at the surface in the present experimental condition. On the other hand, after the deposition at the same concentration (5 ppm), we successfully obtained the Zn nonresonance emission lines at 468.0134, 472.2153, 481.0528 nm as shown in Figure 2b even in the solution with

Figure 2. Typical emission spectra (solid line) obtained for a Cu plate in ZnSO4 + 0.10 M Na2SO4 aqueous solutions (a) without and (b) with Zn electrodeposition for 1 h at the potential of −4.5 V vs Ag|AgCl (sat. KCl) ([Zn2+] = 5 ppm), and (c) without electrodeposition ([Zn2+] = 50 000 ppm). Each spectrum is an average of the spectra obtained by 50 laser shots. The best-fit theoretical spectra (broken line) are also shown in the figure. The duration and the energy of the laser pulse were adjusted to 100 ns and 6.0 mJ, respectively. The delay time and the gate width of the ICCD were set to 700 ns after the laser irradiation and 1000 ns, respectively.

Zn2+ concentration as low as 5 ppm. This is because Zn2+ in the solution was preconcentrated as a deposit on the electrode surface, and the Zn species were ablated from the deposit, rather than introduced from the solution. The contribution from the dissolved species to the Zn emission lines is very limited because of the low Zn2+ concentration in the solution. We estimated the intensity factors corresponding to the relative peak area of Zn emission lines from eq S-2 in Supporting Information using the literature values of the parameters as well as the temperature obtained as a best-fit parameter in the fitting process. The ratio of the intensity factor of the resonance line at 213.8573 nm, whose lower level is the ground state, to that of the nonresonance line at 481.0528 nm is as large as 49 at the temperature of 6330 K, which is an average temperature in the eight replicate measurements. This indicates that the use of a resonance line should give higher detection sensitivity. The intensity of Cu 511 nm line relative to the other two lines (515 and 522 nm lines) obtained without electrodeposition seems to be different from those obtained with electrodeposition, as is seen in Figure 2, parts a and b. This indicates that the temperature is affected by the presence of Zn film on the Cu electrode surface. The temperatures were 5590 ± 50 K and 6330 ± 80 K in the eight replicate measurements for the plasma obtained without and with a Zn film, respectively. We estimated the temperature by fitting the theoretical spectrum to the spectrum obtained by averaging 50 raw spectra. The standard deviation was determined from eight 1657

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Second, the density of Zn atoms in the plasma is proportional to the thickness of the deposit. Third, the plasma is in the local thermodynamic equilibrium (LTE) condition, i.e., the Boltzmann equation can be used. Forth, the plasma conditions, i.e., temperature, densities, volume, shape, temporal variation of their properties, are independent of the thickness of the deposit. If all these assumptions are satisfied, quantitative analysis of the Zn2+ concentration would be possible by a linear calibration curve using the intensity of a Zn emission line. Figure 3 shows typical emission spectra obtained for Zn films electrodeposited on Cu plates in the solutions with various

temperatures obtained by the above procedure (400 spectra were measured altogether). This results in a relatively small standard deviation of the temperature. The increase in temperature after Zn deposition is explained by the increasing number of the electrons with high energy due to the enhancement of the ablation efficiency associated with the decrease in the breakdown threshold caused by the Zn film. This effect seems to be related to the difference of thermal properties between Zn and Cu, such as melting point (Zn, 692.68 K; Cu, 1356.6 K) and thermal conductivity (Zn, 121 W· m−1·K−1; Cu, 398 W·m−1·K−1 at 300 K). A rough surface of a less-dense electrodeposition film could also be a reason for the enhancement of the ablation efficiency. Due to the increase in the temperature, we can see in Figure 2b an emission line at 486 nm which is assigned to hydrogen (Balmer Hβ line) originating in the solution. Various techniques have been suggested to enhance the LIBS signal and to improve the limit of detection (LOD). For example, De Giacomo et al. obtained greatly enhanced LIBS signal by depositing silver nanoparticles on metal samples.45 Although the intensity of Cu emission lines does not seem to be enhanced in the present case, it is possible that the electrodeposition of thin films may enhance the LIBS signal if the properties such as thickness, surface roughness, and composition of the deposit are optimized. In Figure 2c we show a typical emission spectrum obtained for the Cu electrode without electrodeposition in a 50 000 ppm Zn2+ solution. In the spectrum we can see the Zn emission lines originating in the species dissolved in the solution. Since the concentration of Zn2+ in the solution is high, a considerable amount of Zn atoms are introduced into the ablation plasma to show intense atomic emission. On the other hand, the atomic line of H is not seen in the spectrum. The estimated temperature obtained as a best-fit parameter was 5160 ± 70 K in the eight replicate measurements, and this is lower than that obtained without electrodeposition in the 5 ppm Zn2+ solution (5590 K). The change in the refractive index, viscosity, and amount of the particles produced by the former pulses might have affected the plasma. The absorption of the laser energy in the 50 000 ppm solution was the same as the 5 ppm solution within the experimental error. The peak height of the Zn emission line at 481 nm was 11 000 ± 2000 counts in the eight replicate measurements, and this is smaller than that obtained with the Zn film deposited in the 5 ppm Zn2+ solution (20 000 ± 2000 counts in the eight replicate measurements). Note that the origins of the Zn atoms are different. Consequently, we can state that the overall sensitivity over 10 000 times higher was achieved by the introduction of the electrodeposition process to underwater LIBS, although the direct comparison is difficult due to various effects mentioned above. Not only Zn emission lines, but also Cu emission lines can be seen in Figure 2b. This means that the laser energy is sufficient to ablate Cu atoms beneath the Zn film. Such an ablation of the Cu substrate occurs probably because the Zn electrodeposition film is thin and not dense. We consider that this condition is important to perform quantitative analysis as discussed in the next section. Quantitative Analysis. Under the fixed experimental conditions the intensity of Zn emission lines is expected to be proportional to the concentration of Zn2+, if the following assumptions are all valid. First, the thickness of Zn deposit is proportional to the concentration of Zn2+ in the solution.

Figure 3. Typical emission spectra obtained for Zn films electrodeposited on Cu plates for 1 h at the potential of −4.5 V vs Ag|AgCl (sat. KCl) in ZnSO4 + 0.10 M Na2SO4 aqueous solutions ([Zn2+] = (a) 10, (b) 15, (c) 30, (d) 50 ppm). Each spectrum is an average of the spectra obtained by 50 laser shots. The best-fit theoretical spectra (broken line) are also shown in the figure. The duration and the energy of the laser pulse were adjusted to 100 ns and 6.0 mJ, respectively. The delay time and the gate width of the ICCD were set to 700 ns after the laser irradiation and 1000 ns, respectively.

Zn2+ concentrations ([Zn2+] = 10, 15, 30, 50 ppm). The intensity of Zn emission lines increases with the concentration of Zn2+, while the intensity of Cu lines tends to decrease. This is explained by the dependence of the thickness on the Zn2+ concentration of the solution. The ratio of the amount of ablated species from the upper surface layer (Zn deposit) to that from the substrate (Cu electrode) would increase with increasing the thickness of the deposit under the assumption that the ablation volume is constant. This is confirmed by the result of the best-fit atomic density ratio NZn/NCu in the plasma as a function of Zn2+ concentration in the solution, as is shown in Figure S-2 (Supporting Information). An increasing trend of NZn/NCu with increasing Zn2+ concentration is clearly seen. The ratio NZn/NCu does not increase linearly since the Zn species in the plasma increase whereas the Cu species decrease with increasing the thickness of the deposit. If the deposit layer is thicker than the ablation depth, the intensity of the emission lines of Zn atoms originating in the deposit would be constant, regardless of the thickness of the deposit, and we would not observe any Cu lines in the spectra. If the thickness of the deposit is very thin, the emission line from the deposit could not be observed. Since all the spectra in Figures 2b and 3 give both the Zn and Cu signals, the present experimental conditions are appropriate for the quantitative analysis of Zn2+ solutions with the concentration range of 5−50 1658

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due to the high population density of the ground state.50,51 To obtain clear and intense emission lines, double-pulse irradiation is often used in underwater LIBS for solid samples.3,4,43,47,48,52−56 It has been clarified that the first pulse generates a cavitation bubble and the subsequent second pulse enters the expanded bubble. The plasma is generated by the second pulse in the conditions similar to those in a gaseous phase and yields higher quality signals that are more suitable for elemental analysis. However, the double pulse is not suitable for quantitative analysis of thin films deposited on an electrode, since the first pulse also ablates and disturbs the surface before the second pulse, which determines the quantitative nature of the analysis. It would be preferable to irradiate with a single pulse shot especially for the analysis of thin films. A single long pulse with a pulse duration of ∼100 ns is also known to give clear and intense emission lines, where the signal enhancement is attributed to the formation of a relatively low-density plasma with a long lifetime.46,51,57 The improvement in the signal quality when using a long pulse is explained by a mild ablation by the early part of the pulse, followed by a continuous supply of pulse energy by the later part of the pulse to the ablated species in the bubble, which has already expanded to a certain volume.58 The surface damage is very low in the case of a long pulse compared to the lasers with shorter pulse duration.59 We can observe intense emission lines with a low-energy pulse, even as low as several millijoules. Furthermore, the spectra obtained by a single pulse irradiation scheme, regardless of the pulse duration, are basically not affected by hydrostatic pressure, even at high pressures of several hundred atmospheres, which correspond to the depths where hydrothermal vents are found on the seafloor.60 Therefore, the long pulse is the best choice for the present technique. Shot-to-shot fluctuation of the emission spectra is usually a serious problem in underwater LIBS compared to that in air. However, the coefficient of determination R2 of 0.974 in the present calibration curve is acceptable for quantitative analysis for certain applications. This mainly owes to the averaging of 50 emission spectra. For example, the relative standard deviation (RSD) of the peak height in 400 laser shots before averaging were 0.33, 0.33, 0.35, 0.47, 0.53 for Zn2+ concentrations of 5, 10, 15, 30, 50 ppm, respectively. Note that the RSD values before averaging were determined without fitting, since the fitting process could give an artifact due to the noise. The peak height was obtained by subtracting the background intensity from the maximum intensity. Background intensity was obtained by averaging the intensity in the range from 474 to 479 nm. Although direct comparison of the RSD values before and after the averaging procedure is difficult due to different total number of examined spectra, the RSDs of the peak height of eight spectra, each of which is an average of 50 spectra, were 0.08, 0.05, 0.03, 0.07, 0.10 for Zn2+ concentrations of 5, 10, 15, 30, 50 ppm, respectively. The fluctuations due to various factors seem to be averaged out to a certain level simply by virtue of applying the averaging procedure. Figure 5 shows the full width at half-maximum (fwhm) of the Zn emission line at 481 nm and the temperature as a function of the Zn2+ concentration in the solution (solid circles). The temperature obtained without electrodeposition in the 5 ppm Zn2+ solution is also shown as an open circle. The fwhm is almost constant in the concentration range studied, as shown in Figure 5a. This indicates that the effect of self-absorption is negligible. The result also indicates that the difference in the amount of Zn deposit on the Cu electrode does not result in a

ppm. However, it is necessary to change an experimental condition according to the concentration range to be measured. According to the assumption that the thickness is proportional to the deposition time, we can adjust the concentration range by changing the deposition time. Figure 4 shows the calibration curve in which the peak area of the Zn emission line at 481 nm is plotted as a function of

Figure 4. Calibration curve for the determination of Zn 2+ concentration in ZnSO4 + 0.10 M Na2SO4 aqueous solutions. The peak area of Zn emission line at 481 nm was plotted as a function of Zn2+ concentration. The error bar at each concentration corresponds to the standard deviation of the eight replicate measurements. The best-fit linear function passing through the origin is shown as a solid line.

Zn2+ concentration. The error bar at each concentration corresponds to the standard deviation of the eight replicate measurements. The best-fit linear function passing through the origin obtained by using a weighted least-squares method is shown as a solid line. Here, the data fluctuation attributed to the reproducibility of the electrodeposition process is not included in the error bar, since the data of each concentration were obtained from a single sample. As shown in Figure 4, the peak area of the Zn line increases linearly. The slope S of the curve of peak area [counts·nm] versus concentration [ppm] was 3130 ± 70. A coefficient of determination R2 of 0.974 and the LOD of 0.35 ppm were obtained. The calculation of the LOD is described in Supporting Information. The averaging of spectra reduces the background noise and consequently improves the LOD. It should be noted that the peak area of the Zn line did not saturate with increasing Zn2+ concentration, which means that the self-absorption is not serious and that the Zn deposits are thinner than the depth of ablation. The small self-absorption effect is attributed to the use of a nonresonance line having a low population density of the lower level of the transition, as well as the formation of an optically thin plasma due to the mild ablation associated with the long-pulse laser irradiation. It has been suggested that a calibration curve method would not be easy to implement accurately using a single nanosecond pulse combined with the observation of resonance lines, due to the large deformation of the spectral lines.40,46 The plasma produced by irradiating a solid sample in water with a single pulse laser having the pulse duration of 20 ns or less usually gives deformed spectral lines overlapped with a continuous spectrum.40,46−48 They decay rapidly since the plasma is confined and quenched due to the surrounding water.49 In general the detection of resonance emission lines is relatively easy because of their high intensity. However, there is a disadvantage in this case that the self-absorption effect is serious 1659

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target and the solution, might make the correction more difficult in the present case. We would like to emphasize here at the end of this section that the use of H emission lines as an internal standard has an advantage that it is always abundant in underwater environments.

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CONCLUSIONS In the present study, we demonstrated the possibility of on-site quantitative analysis of heavy metal ions dissolved in a solution using the combination of electrodeposition and underwater LIBS of an electrode surface under electrochemically controlled potential. Zinc emission lines were observed after the deposition on a Cu electrode in a 5 ppm Zn2+ solution. A calibration curve was plotted in the range from 5 to 50 ppm using the Zn nonresonance line. Although underwater LIBS suffers from the spectral deformation and serious shot-to-shot fluctuations, a coefficient of determination R2 of 0.974 was achieved. The irradiation with the long nanosecond pulse, the use of the nonresonance line, and the averaging process employed in the present study are important to improve the quantitative performance. The present investigations will be useful for the application of underwater LIBS to part-permillion order elemental analysis of heavy metal ions in solutions.

Figure 5. (a) Full width at half-maximum of the Zn emission line at 481 nm and (b) best-fit atomic excitation temperature in the plasma plotted as a function of Zn2+ concentration in the solution (solid circles). The temperature obtained in the case of a Cu plate in a ZnSO4 + 0.10 M Na2SO4 aqueous solution ([Zn2+] = 5 ppm) without electrodeposition is plotted as an open circle. The error bar at each concentration corresponds to the standard deviation of the eight replicate measurements.

significant change of the plasma electron densities assuming that the line broadening is mainly due to Stark and collision effects.50,61 By virtue of a very slight change of the fwhm, we can plot a calibration curve using the peak height (R2 = 0.984, LOD = 0.35) without any significant difference from that based on the peak area (R2 = 0.974, LOD = 0.35), as shown in Figure S-3 (Supporting Information). We emphasize that the peak height can be determined easily compared to the peak area. The temperature seems to fluctuate within a certain range while a clear difference can be seen between the results obtained without and with electrodeposition, as is shown in Figure 5b. The variation of the temperature does not seem to be so serious for the purpose of obtaining a calibration curve. We consider that the change in the plasma parameters mentioned above with respect to the amount of Zn deposit is small enough for practically useful calibration curves to be drawn. An internal standard is widely used for quantitative analysis by LIBS measurement.17,21,25,28,31 The emission lines of other elements included in the sample with a constant concentration can be used. Barreda et al. greatly improved both the linearity and the repeatability using an internal standard for the normalization of emission lines.21 The background intensity can also be useful, and a study has demonstrated a strong correlation between the line and background intensities.15 The signal obtained from a reference sample is also applied to obtain calibration curves.33 In the present study, we employed a H emission line (Balmer Hβ line) as an internal standard, since the concentration of H species in the solution is nearly constant. The result is shown in Supporting Information (Figure S-4). The R2 value was improved slightly (from 0.974 to 0.989) by the normalization, suggesting that the H emission line can be used as an internal standard for quantitative analysis. Charfi and Harith have reported that the use of the intensity of a H emission line at 656 nm (Balmer Hα line) as an internal standard reduces the shot-to-shot fluctuation and improves the R2 value considerably for LIBS analysis of water surfaces.17 Dockery et al. also have used the Hα line as an internal standard to minimize the shot-to-shot fluctuation for LIBS measurements of ion-exchange polymer membranes (containing hydrogen).28 In our case, however, a significant improvement was not seen, i.e., the RSD values changed from 0.09, 0.05, 0.03, 0.08, 0.11 to 0.11, 0.11, 0.10, 0.12, 0.05 for concentrations of 5, 10, 15, 30, 50 ppm, respectively, by the normalization. The difference of the origin between Zn and H species, i.e., the solid

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ASSOCIATED CONTENT

S Supporting Information *

Details of emission spectroscopy, the fitting procedure of the theoretical spectrum, a cyclic voltammogram, the atomic density ratio NZn/NCu in the plasma obtained as a function of Zn2+ concentration, the calculation of the LOD, and calibration curves plotted using the peak height and the normalized peak area. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Nos. 23560023, 13J04184, and 14J02461. REFERENCES

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DOI: 10.1021/ac503737c Anal. Chem. 2015, 87, 1655−1661

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DOI: 10.1021/ac503737c Anal. Chem. 2015, 87, 1655−1661