In Situ Underwater Laser-Induced Breakdown Spectroscopy Analysis

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In-Situ Underwater Laser Induced Breakdown Spectroscopy Analysis for Trace Cr(VI) in Aqueous Solution Supported by Electrosorption Enrichment and Gas-Assisted Localized Liquid Discharge Apparatus Tian-Jia Jiang, Meng Yang, Shan-Shan Li, Ming-Jun Ma, NanJing Zhao, Zheng Guo, Jinhuai Liu, and Xing-Jiu Huang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b00629 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 14, 2017

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Analytical Chemistry

In-Situ Underwater Laser Induced Breakdown Spectroscopy Analysis for Trace Cr(VI) in Aqueous Solution Supported by Electrosorption Enrichment and Gas-Assisted Localized Liquid Discharge Apparatus

Tian-Jia Jiang,†,‡,§ Meng Yang,†,‡,§ Shan-Shan Li,†,‡ Ming-Jun Ma,ǁ Nan-Jing Zhao,*,ǁ Zheng Guo,*,†,‡ Jin-Huai Liua,b and Xing-Jiu Huang*,†,‡

†

Key Laboratory of Environmental Optics and Technology, Institute of Intelligent

Machines, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

‡

Department of Materials Science and Engineering, University of Science and

Technology of China, Hefei 230026, People’s Republic of China

ǁ

Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics

and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

§

These two authors contributed equally to this work.

*

Correspondence should be addressed to N.J. Zhao, Z. Guo and X.J. Huang

E-mail: [email protected] (N.J.Z), [email protected] (Z. G), [email protected] (X.J.H) Tel: 86-551-6559-1167; Fax: 86-551-6559-2420 1

ACS Paragon Plus Environment


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ABSTRACT Traditional laser induced breakdown spectroscopy (LIBS) always fails to directly detect target in aqueous solution due to rapid quenching of emitted light and adsorption of pulse energy by surrounding water. A method is proposed for the in-situ underwater LIBS analysis of Cr(VI) in aqueous solution freed from the common problems mentioned above by combining gas-assisted localized liquid discharge apparatus with electrosorption for the first time. In this approach, the introduction of gas-assisted localized liquid discharge apparatus provides an instantaneous gaseous environment for underwater LIBS measurement (That is, the transfer of sampling matrix is not needed from aqueous solution to dry state). The preconcentration of Cr(VI) is achieved by electrosorption with a positive potential applied around adsorbents, which can promote the adsorption of Cr(VI) and inhibit that of the co-existed cations leading to a good anti-interference. Amino groups functionalized chitosan-modified graphene oxide (CS-GO) is utilized for Cr(VI) enrichment, which can be protonated to form NH3+ in acidic condition promoting the adsorption towards Cr(VI) by electrostatic attraction. The highest detection sensitivity of 5.15 Counts µg-1 L toward Cr(VI) is found for the optimized electrosorption potential (EES = 1.5 V) and electrosorption time (tES = 600 s) without interference from co-existed metal ions. A corresponding limit of detection (LOD) of 12.3 µg L-1 (3σ method) is achieved, which is amazingly improved by 2 or even 3 orders of magnitudes compared to the previous reports of LIBS.

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INTRODUCTION Laser induced breakdown spectroscopy (LIBS) is worked as a powerful tool given its application in the qualitative and quantitative analysis of environmental samples within fraction of second.1 The technique can provide the information of distributed elements in samples based on the optical emission of target element.2-5 As such, LIBS has been widely applied for the determination of heavy metal ions (HMIs). Representatively, Duan et al. achieved sensitive and quick LIBS detection of Cu(II), Ag(I), Pb(II), and Cr(III) in aqueous solutions by the introduction of a 3D nano-channel substrate.6 Again, the same group realized the analysis for Cu, Ag, Mn, Cr in liquid samples by combining LIBS with metal precipitation and membrane separation.7 Hidalgo et al. proposed a method based on dispersive liquid-liquid microextraction for simultaneous LIBS detection of Cr, Cu, Mn, Ni and Zn in water samples.8 Even though the sensitivity and the detection limit were all significantly improved or increased, however, most of previous works generally concentrated on the metal enrichment using different sampling substrate/techniques. Prior to LIBS analysis, the aqueous samples were needed to transfer into solid by drying on a matrix,7 which could be called off-line metal LIBS detection in aqueous samples. This is due to that the rapid quenching of emitted light and adsorption of pulse energy by surrounding water always occurs in direct determination of the dissolved HMIs in aqueous phase. One of impressive works on direct metal LIBS analysis in aqueous solution was by Matsumoto et al., who proposed an electrodeposition as an on-site sample 3



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treatment technique for underwater LIBS detection of Zn(II) in aqueous solution. A concentration range of 5 - 50 mg L-1 was reported by such a strategy.9 It is reasonable to point out that the influences of spectral deformation and serious shot-to-shot fluctuations were not avoided absolutely due to the dense plasma confined in water, which would decrease the sensitivity of LIBS analysis.10 Obviously, the challenge still exists for underwater LIBS detection of HMIs. It is significant to find an advanced method to eliminate a series of negative effect of water on the laser for trace HMIs measurements. Gas-flow prior to laser ablation is an effective pathway to solve these problems in the LIBS detection of submersed solid target. This technique can create a sample-air interface to eliminate the influence from water and improve the quality of LIBS spectra,11 and it has been applied for the detection of samples submerged in water.12-14 Comparing with the commonly used underwater LIBS methods, such as double pulse and long pulse,15,16 gas-flow method is more suitable for the analysis of thin films modified on an electrode. It can be explained by the fact that a cavitation bubble has to be generated for plasma by an ablation induced by the first pulse or the early part of long pulse when using double pulse or long pulse LIBS system, the process will disturb the surface of sample. It is well known that LIBS suffers from the undesirable LOD (commonly at parts per million level) in analysis for environmental samples, so a preconcentration for target ions in solution is considered as an effective path to improve the LOD of LIBS. Several pretreatment methods have been investigated, such as metal precipitation,7 preconcentration by nanomaterials,6 and electrodeposition.9 Among these approaches, 4


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although some ideal results have been achieved by electrodeposition-assisted LIBS, it is accepted that such a process is time-consuming and the concentration range is not suitable for the determination of trace HMIs (parts per billion levels) in environmental samples (e.g., drinking water)9. An alternative, effective, relatively unexplored method of enrichment is electrosorption, which is defined as potential-introduced adsorption. This preconcentration process can force charged ions transferring to the electrode with opposite charge by imposing a potential.17 Compared with traditional enrichment methods (e.g., ion exchange, evaporation, adsorption and so on), electrosorption exhibits several advantages.18 Compared with adsorption, electrosorption offers more selectivity and more efficiency towards charged ions. Unlike evaporation, electrosorption is an energy-effective enrichment method.19 Besides, no membrane is required in electrosorption process, so more operation advantage is shown.20 However, the combination of LIBS with electrosorption is rarely reported so far. Hexavalent chromium (Cr(VI)) is a common pollution in water arising from the discharge of effluent from industrial process,21,22 which is hazardous and cancerogenic to human beings even at a trace content.23,24 Considering the hazard of Cr(VI), a maximum permissible level for Cr(VI) content in drinking water is recommended as 50 µg L-1 by World Health Organization (WHO).25 Nowadays, several methods have been developed for the accurate determination of Cr(VI), such as high-performance liquid chromatography, X-ray fluorescence spectrometry (XRF), inductively coupled plasma mass spectroscopy (ICP-MS) and so on.26-30 Among these conventional 5



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methods, electrochemical method has a wide application in the determination of trace HMIs in environmental samples due to the rapid analysis time and simple operation.31,32 Although electrochemical method presents a good determination performance for Cr(VI), there are still some drawbacks needed to overcome in practical analysis, such as the use of noble metal based materials and the strong acidic media required in electrochemical measurement. In this work, we propose an spectoelectrochemical device based on LIBS and electrosorption for the in-situ detection of Cr(VI) in solution supported by gas-assisted localized liquid discharge apparatus for the first time. The liquid discharge apparatus could provide a gaseous environment for underwater LIBS measurements by forcing the solution to leak out from laser channel and plasma activation cavity. And chitosan-modified graphene oxide (CS-GO) was worked as adsorbents for accumulation of Cr(VI). Such material has been applied for the removal of HMIs as an effective adsorbents.33-35 The amine groups on CS-GO can be protonated (NH2→NH3+) to make the surface exhibiting positive charge in acid condition, which can improve the adsorption capacity towards anionic metal ions by the electrostatic interaction.36,37 Electrosorption process for Cr(VI) (mainly existed as HCrO4- in pH 4.0) was carried out by holding a positive potential through working electrode, and then a positive electricity field was formed to force HCrO4- transferring from solution to the surface of adsorbents, which increased the concentration of HCrO4- near working electrode. Meanwhile, the electric field could impede the interference from the existed metal ions. The modification of CS-GO on the surface of 6


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electrode was a crucial factor for the enrichment of HCrO4-. The protonated amine groups (-NH3+) on CS-GO were worked as the adsorption sites to improve the preconcentration efficiency for HCrO4-.

EXPERIMENTAL SECTION Reagents and Chemicals. All the purchased reagents in this work were used directly and obtained from Shanghai Chemical Reagent Co., Ltd. (China). The deionized water was obtained through a water purification system (Milli Q, specific resistivity >18 MΩ cm, S.A., Molsheim, France).

Instruments and Characterization. The morphology of samples was observed by a field-emission scanning electron microscope (FESEM, Quanta 200 FEG, FEI Company, USA). The groups exposed on the surface of prepared products were characterized by Fourier transform infrared (FT-IR) spectrometer (Nicolet Nexus-670).

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Figure 1. Experimental setup for in-situ underwater electrosorption-LIBS device supported by gas-assisted localized liquid discharge apparatus. a) Schematic of in-situ spectoelectrochemical LIBS device; b) Deconstructed arrangement and c) sectional view of gas-assisted localized liquid discharge apparatus. The green rectangle in panel c is the laser channel.

In-situ Underwater LIBS Measurements. The electrosorption of Cr(VI) was performed in an electrochemical cell as shown Figure 1a, which consisted of Ag/AgCl worked as reference electrode, platinum wire used as auxiliary electrode and bare or CS-GO modified Ti plate utilized as working electrode. The parameter of electrosorption experiments was controlled by a potentiostat (ChenHua Instruments Co., Shanghai, China). The quantitative analysis of the preconcentrated Cr(VI) on the working electrode was operated by in-situ LIBS system, and the schematic of such device was presented in Figure 1a. A Q-switched Nd:YAG laser (Brilliant, Quantel) was used as the ablating source delivering 5 ns pulses with energy of 100 mJ, wavelength of 1064 nm and repetition rate of 1 Hz. The laser was focused onto the surface of working electrode (CS-GO modified Ti plate) in electrochemical cell through a convex lens (focal length = 100 mm) to ablate the preconcentrated Cr(VI). The spectrometer was triggered by the laser pulse. The delay time and gate width for the LIBS experiments were 1.2 µs and 1 ms, respectively. And each spectral data collected from the experiment was the average of 10 laser pulses. For the direct LIBS analysis underwater, designing a miniature instrument was of great necessary to collect LIBS signal accurately freed from the influence of surrounding water, such working part was named gas-assisted localized liquid 8


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discharge apparatus. This device was equipped with focal lens, gas inlet, optical fiber and plasma activation cavity, so it could be used for draining liquid, transmitting laser beam, collecting plasma emission simultaneously, and the specific positions of structural components were shown in Figure 1b. The sectional view of liquid discharge device was displayed in Figure 1c. It should be noted that all components were connected to each other tightly with a good gas seal. The liquid emptying system had to be immersed into solution during actual operation, and the instrument below N2 inlet would be filled with solution. Flowing N2 could force the solution to leak out from laser channel and plasma activation cavity leading to an instantaneous gaseous environment for further LIBS measurements, and the disturbance from the water could be avoided in laser beam propagation. The gas flow rate was about 300 cm3 min-1, and it would take 2 - 3 s to empty the instrument. Then, a laser was focused onto the surface of working electrode to form plasma (2 mm in size), and the optical emission of plasma was collected directly by an optical fiber (diameter: 0.18 mm 0.23 mm, corresponding acceptance angle: 10o - 13o) linked with a spectrometer (Avantes, AvaSpec-ULS2048 model, grating: 2400 lines/mm, slit width: 10 µm), and the wavelength range was 200 - 500 nm with a resolution of less than 0.1 nm.

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Figure 2. Preparation procedure of CS-GO and schematic of electrosorption process towards Cr(VI). Synthesis of CS-GO. Graphene oxide (GO) was synthesized by the Hummer’s method according to the previous work.38,39 CS-GO was obtained through the amination reaction between GO and chitosan (CS) as shown in Figure 2.40 In brief, 500 mg chitosan was dispersed into 50 mL 1% acetic acid solution by stirring for 12 hours and a homogenous solution was obtained. Then 100 mg GO was added into the obtained solution following by 1 hours sonication bath. And 652 mg N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC) and 782 mg N-hydroxysuccinimide (NHS) were added slowly into the mixture solution stirred continuously at room temperature for 24 hours. The final product was collected by centrifugation and washed with acetic acid solution (5%) and deionized water for several times to remove excess the chitosan, EDC and NHS. Electrosorption Experiment. The preparation of CS-GO modified Ti plate (working electrode) could be described as follow: 20 µL of the suspension contained 5 mg/mL CS-GO in water was dropped onto the surface of cleaned Ti plate (99.99%, 0.25 mm in thickness) and drying in air for further experiments. The surface area of Ti plate was 0.75 cm2. It should be noted that the main existed formation of Cr(VI) was HCrO4- at pH 4.0. The electrosorption of Cr(VI) was carried out by holding a positive potential on working electrode (bare or CS-GO modified Ti plate) to form a positive electricity field, which could promote the transfer of HCrO4- from solution to the surface of working electrode by electronic attraction. A concentration gradient of Cr(VI) would be formed around CS-GO modified Ti plate, the concentration near 10


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working electrode would be higher than that far away electrode. Finally, Cr(VI) could be fixed onto CS-GO by the electrostatic attraction between HCrO4- and protonated amino groups (NH3+) of adsorbents. The schematic of this process was described in Figure 2.

Figure 3. SEM images of GO (a) and CS-GO (b). FT-IR spectra (c) of CS, GO and CS-GO.

RESULTS AND DISCUSSION Characterization of CS-GO. To explore the morphologies and the structure of CS-GO, several methods were carried out to characterize the as-prepared product. From the SEM image of GO, it can be observed that the smooth surface of GO with obvious wrinkled edges (Figure 3a). Compared with the SEM image of CS-GO (Figure 3b), it indicates that the surface morphology of CS-GO is quite different with that of GO, CS-GO presents a clear sheet-structure and the surface is less smooth and has more wrinkles. So the introduction of CS to GO increases the roughness of surface. The groups exposed on CS-GO are further characterized by FT-IR pattern (Figure 3c). In the spectrum of CS, the bonds at 1654 cm-1 and 1596 cm-1 are assigned 11



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to the amino groups of CS.33 The curve of CS-GO is similar to that of CS and GO, and some differences can be observed among them. The amino group vibration appears at 1616 cm-1 and a little redshift can be observed compared to the curve of CS due to the electronegativity of GO. Besides, the decrease of the peak at 1708 cm-1 (relative to the bond at 1724 cm-1 at the spectrum of GO) verifies the interaction between the carboxyl groups of GO and the amino groups of CS in amination process. The obtained data indicates the successful synthesis of CS-GO. Effect of Electrosorption and Modification of CS-GO. Electrosorption is the crucial step to improve the LIBS intensity of Cr(VI), and the efficiency of preconcentration is influenced by many parameters, especially electrosorption potential (EES), electrosorption time (tES) and pH. In order to get the best analysis performance of electrosorption-LIBS with gas-assisted localized liquid discharge apparatus, these factors were optimized (the details are available in Supporting information, Figure S1). Finally, EES = 1.5 V, tES = 600 s and pH = 4.0 were chosen for the further electrosorption experiments.

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Figure 4. a) Typical LIBS intensity of 100 µg L-1 Cr(VI) on GO and CS-GO modified Ti plate after adsorption without (black column) and with (red column) a positive potential of 1.5 V in 0.1 M HAc-NaAc solution (pH 4.0). The LIBS is supported by gas-assisted localized liquid discharge apparatus. The insets are the corresponding schematics of adsorption towards Cr(VI) under different experimental conditions. b) Typical underwater electrosorption-LIBS determination for Cr(VI) with (i) and without (ii and iii) (100 µg L-1 Cr(VI) for i and ii, 1000 mg L-1 Cr(VI) for iii) gas-assisted localized liquid discharge apparatus. The substrate is CS-GO modified Ti plate. The insets are the schematics for each obtained curves under different experimental conditions.

The modification of CS-GO was critical for the accumulation of Cr(VI). As shown in Figure 4a, compared with the GO modified Ti plate, the intensity of LIBS peak is much higher on CS-GO modified Ti plate no matter with electrosorption or 13



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without it. It should be noted that no obvious signal is obtained on bare Ti plate at the same experimental conditions, and chitosan can be dissolved into HAc-NaAc solution, so we neglect bare Ti and CS/Ti plate in this comparison part. The result proves that the modification of CS-GO can improve the LIBS signal of Cr(VI) significantly. It may be contributed to the amino group exposed on the surface of CS-GO, which can be protonated to NH3+ in pH 4.0 to improve the adsorption capacity towards Cr(VI) by electrostatic attraction. So there are more adsorption sites existed on CS-GO than on GO and bare Ti plate, resulting to a better determination performance for Cr(VI). The impact of electrosorption on enhancing the peak intensity can be observed from the comparison of the LIBS responses after adsorption and electrosorption on CS-GO modified Ti plate (Figure 4a iii and iv, respectively). And the LIBS intensity after electrosorption is much higher. The founding is resulted from the formation of positive electric field in electrosorption process, which can enhance the concentration of Cr(VI) anions around CS-GO modified Ti plate as displayed in the inset (iv) of Figure 4a. The above results confirm that the modification of CS-GO and the electrosorption process play an important role in improving the determination performance of traditional LIBS. Effect of Gas-Assisted Localized Liquid Discharge Apparatus. The gas-assisted localized liquid discharge apparatus is an essential part of the proposed LIBS system for the in-site Cr(VI) analysis underwater. Its improvement for LIBS determination performance can be seen clearly from the comparison between the LIBS intensity obtained with and without gas-assisted localized liquid discharge 14


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apparatus, as shown in Figure 4b. It is well known that LIBS fails to direct analysis of the target underwater, since the energy pulse of laser would be adsorbed by water, and splashing of water droplets would appear as a laser breakdown occurred on the surface of water samples leading to the contamination of LIBS instrument and the decrease of LIBS intensity.7 It should be noted that there are several atomic emission lines in the spectral range of 200 - 500 nm for identifying Cr element according NIST spectral database, and the spectral line (Cr I 359.35 nm) is isolated and free of interference, for which it is selected for further analyses. A obvious LIBS signal is obtained for 100 µg L-1 Cr(VI) by electrosorption-LIBS with gas-assisted localized liquid discharge apparatus as displayed in Figure 4b-i. However, in the case without liquid discharge apparatus, Cr emission line cannot be observed due to the severe interference from surrounding water (Figure 4b-ii). Increasing the concentration of Cr(VI) in solution, a small LIBS peak is achieved (Figure 4b-iii). It can be explained by the fact that although the interference from surrounding water causes the decrease of LIBS intensity, the contribution from dissolved Cr(VI) to the Cr emission line is increased due to the high concentration of Cr(VI) in solution. And the LIBS signal for 1000 mg L-1 Cr(VI) without gas-assisted localized liquid discharge apparatus is still lower than that for 100 µg L-1 Cr(VI) with the liquid discharge device. Consequently, the introduction of gas-assisted localized liquid discharge apparatus improves the sensitivity of underwater LIBS determination remarkably, and the interference from surrounding water can be avoided effectively. And the electrosorption-LIBS supported by gas-assisted localized liquid discharge apparatus is utilized throughout the entire 15



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experiments, unless specially noted.

Figure 5. Determination of Cr(VI) by underwater electrosorption-LIBS supported by gas-assisted localized liquid discharge apparatus. Plots of peak intensities versus different concentrations of Cr(VI) in 0.1 M HAc-NaAc solution (pH 4.0). Inset is the LIBS peaks of Cr element over concentration range 100-500 µg L-1, tES = 600 s, and EES = 1.5 V.

In-Situ underwater LIBS Analysis of Cr(VI). In order to test the quantitative analysis of this device, different concentration of Cr(VI) ranged from 100 µg L-1 to 500 µg L-1 was measured by in-situ electrosorption-LIBS. As presented in Figure 5, the sensitivity is 5.15 Counts µg-1 L and the correlation coefficient is 0.995. The obtained LOD is 12.3 µg L-1 (3σ method), which is lower than the guideline value of 50 µg L-1 set out by the World Health Organization (WHO). It should be noted that the quenching of emitted light and splashing of water droplets are the common problems existed in the direct solution analysis by LIBS, and these drawbacks can be efficiently avoided by in-situ underwater LIBS combined with electrosorption. Table 1. Comparison of Determination Performances for Analysis of Cr(VI) by Different Methods (Representative Studies) LOD Linear range Method Substrate Electrolyte (µg Ref (µg L-1) -1 L ) 16


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EC (LSV)

Au/SPE1

EC (AM)

graphite/SPE1

EC (DPCSV)

Au microchip

EC (LSV)

graphite/SPE1

EC (EIS)

Azacrown/Au

SPE2-XRF ICP-MS ET-AAS LIBS LIBS LIBS LIBS LIBS LIBS Adsorption-LIBS Adsorption-LIBS

PVC ---------------------Ca(OH)2/CaO wood slice paper microfiltration membrane

SPE2-LIBS

0.05 M H2SO4 0.1 M H2SO4 0.1 M HCl 0.05 M H2SO4 1×10-5 M HCl ---------water water water water water water water water ----

520 - 83200

228.8

41

156 - 5.2×105

52

42

104 - 10400

46.8

43

100 - 1000

19

44

1 - 100

0.0014

45

300 - 8800 ---0.5 - 6 5×104 - 4×106 1×106 - 5×106 2×104 - 2×107 1×102 - 1×105 5×104 - 4×106 3×103 - 1.2×106 1×103 - 9×103 500 - 20000

300 0.18 0.02 6000 30000 1100 100 39000 1200 34 26

46 47 48 49 50 51 52 53 54 55 56

10 - 120

1.29

7

In-situ underwater HAc-NaAc this electrosorption-LI CS-GO 100 - 500 12.3 (pH 4.0) work BS Abbreviations: EC, electrochemical method; LSV, linear sweep voltammetry; AM, amperometry; EIS, electrochemical impedance spectroscopy; DPCSV, differential pulse cathodic stripping voltammetry; SPE1, screen-printed electrode; SPE2, solid phase extraction; XRF, X-ray fluorescence; ICP-MS, inductively coupled plasma-mass spectrometry; ET-AAS, electrothermal-atomic absorption spectrometry; LIBS, laser induced breakdown spectroscopy; PVC, poly(vinyl chloride). To date, several successful works have been reported for the determination of Cr(VI) by different methods, the representative studies and the corresponding results are presented in Table 1. By contrasting the data of electrochemical method and in-situ electrosorption-LIBS for analysis of Cr(VI), it is obvious to find that the experimental condition of in-situ electrosorption-LIBS is much milder and the obtained LOD is similar or even superior to electrochemical measurements. The other 17



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impressive work of electrochemical method for determination of Cr(VI) in solution was by our own group,45 even though the LOD is more satisfying than that of in-situ electrosorption-LIBS, the obtained determination performance of the proposed LIBS system is already sufficient for trace analysis of environmental samples. Comparing with traditional spectroscopy systems, like XRF, ICP-MS and AAS, the LOD of these works is lower than that of in-situ electrosorption-LIBS. However, these spectroscopy methods usually suffer from the need of high experimental cost and special operation staff. In contrast to LIBS systems, the combination of electrosorption with in-situ LIBS is proposed firstly, the LOD of this method is lower (2 or 3 orders of magnitude) than that of traditional LIBS systems. And electrosorption exhibits a better enrichment than traditional adsorption. Besides, the linear concentration range actually measured (100 - 500 µg L-1) of in-situ electrosorption-LIBS shows more advantages for the analysis of trace Cr(VI) than traditional LIBS. As compared to SPE-LIBS, in-situ electrosorption-LIBS has a similar determination performance needing less operation.

Figure 6. a) Interferences from various co-existed cations and anions on the 18


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underwater LIBS peak intensity of 100 µg L-1 Cr(VI). The concentration of the added ions is 500 µg L-1. b) Schematic of electrosorption for Cr(VI) with interference-free from cations. The electrosorption-LIBS is supported by gas-assisted localized liquid discharge apparatus.

Interference Study. Anti-interference is an important assessment for a method in analytical chemistry, and the co-existed ions usually present influence on the determination of target ions in some degree. So some common ions were selected to evaluate the anti-interference ability of in-situ electrosorption-LIBS in this work. Figure 6a displays that the response of 100 µg L-1 Cr(VI) in the presence of common cations and anions, the concentration of these ions is 500 µg L-1. There is no impact can be observed after adding the cations (Cd(II), Pb(II), Cu(II), Hg(II) and Zn(II)) (5.2% - 13.1% in decrease) and the responding LIBS signal is presented in the inset of Figure 6a. It can be explained by the fact that the positive potential applied in the electrosorption process could provide a positive electronic field around the working electrode, which could repel the co-existed cations by electrostatic repulsion and reduce the competition adsorption from these ions as shown in Figure 6b. In addition, the majority of NH2 on CS-GO is transformed to NH3+ at pH 4.0, which makes the decrease of NH2 loci for the combination of heavy metal cations. Thus, the interferences resulted from these cations can be ignored. The impact of Cr(III) on the response of Cr(VI) was also explored, Cr(III) was existed mainly in the form of Cr(OH)2+ cation at pH 4.0. Since the oxidation of Cr(III) to Cr(VI) is hard to occur in pH 4.0 without noble metal materials,57 so the introduction of Cr(III) shows no impact on the LIBS signal of Cr(VI). The additions of anions, such as Cl-, SO42-, PO43-, NO3-, 19



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also have no obvious interference on the response of Cr(VI) (6.5% - 13.3% in decrease) (Figure 6a). The possible reason for the result may be that these ions do not compete the surface adsorption sites with Cr(VI) on the surface of adsorbent in electrosorption experiments. Moreover, no impact is observed after addition of a mixed solution contained all the mentioned cations and anions, and the concentration for each interfering ion is 500 µg L-1. Therefore, the obtained data indicates the possible application of in-situ electrosorption-LIBS for the analysis in environmental water samples. Table 2 Determination of Cr(VI) in Real Spiked Water Sample Sample 1 2 3

Added (µg L-1) 100 200 300

Found (µg L-1) 102.6±2.5 203.1±1.9 296.1±3.1

Recovery (%) 102.6 101.5 98.7

Determination of Cr(VI) in Real Water Environment. The practical application of in-situ electrosorption-LIBS was evaluated in real water samples collected from Dongpu Reservoir, Hefei city, China. It should be mentioned that the water source is not contaminated by Cr(VI), so the samples are spiked with Cr(VI) for the application evaluation. The experiment was carried out by adding a known concentration of Cr(VI) into the samples. As shown in Table 2, the recovery of different samples is calculated to be 102.6%, 101.5% and 98.7%, respectively. The consistency between added and determined concentration verifies the prospect of electrosorption-LIBS in the analysis of environmental samples.

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CONCLUSIONS In presented work, we try to combine a gas-assisted localized liquid discharge apparatus with in-situ electrosorption-underwater LIBS for the direct determination of Cr(VI) in solution. The proposed underwater LIBS system successfully avoids the drawbacks commonly existed in the direct solution analysis by traditional LIBS, such as the splashing of water droplets, adsorption of laser energy by water, laser energy fluctuation and decrease of LIBS signal, since a liquid discharge apparatus could make an instantaneous gaseous environment for underwater LIBS measurements by flowing N2 to force the solution to leak out from laser channel and plasma activation cavity. Cr(VI) dissolved in solution is accumulated by electrosorption process, which shows more efficiency and more operation advantages compared with the common preconcentration methods for LIBS analysis. Such preconcentration step also has a contribution for the anti-interference ability of the proposed method due to the positive electricity field formed around electrode in electrosorption process, which can force Cr(VI) anions transferring from solution to adsorbents modified on the surface of working electrode and repel co-existed metal cations away from the adsorbents. Such a system could be also applied for the accurate determination of Cr(VI) in real water samples. Most importantly, this method has the superiorities in the analysis for any other charged ions in trace level in the solution samples compared to traditional LIBS.

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ACKNOWLEDGEMENTS This work was supported by the financial support from the National Natural Science Foundation of China (61474122) and the National High Technology Research and Development Program of China (863 Program; 2014AA06A513 and 2013AA065502). X.J. Huang acknowledges the CAS Interdisciplinary Innovation Team of the Chinese Academy of Sciences, China, for financial support.

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SUPPORTING INFORMATION AVAILABLE Optimization of electrosorption potential, electrosorption time, supporting electrolytes and pH value (Figure S1).

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For TOC only

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In Situ Underwater Laser-Induced Breakdown Spectroscopy Analysis

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