Ultra-sensitive detection of heavy metal ions in tap water by laser-induced breakdown spectroscopy with the assistance of electrical-deposition

Abstract

The trace heavy metal ions in tap water were electrically deposited on the surface of a high purity aluminium rod and then analyzed with laser-induced breakdown spectroscopy (LIBS). Cr³⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ in tap water samples were quantitatively analyzed and the calibration curves have been built up within the concentration range of 1–1000 μgL⁻¹. The limits of detection of these ions were determined to be 0.16–1.35 μgL⁻¹, which are improved 5–6 orders than directly analyzing aqueous solutions by LIBS. The influence of Ca²⁺ and Mg²⁺ in water at different concentrations to the quantitative analysis of trace heavy metal ions was evaluated. It is possible to realize reliable quantitative analysis by using different calibration curves according to the hardness of the analyzed water samples. LIBS plus electrochemical enrichment is a potential approach to high sensitivity detection of trace heavy metal ions in tap water and natural water samples, and it will find applications in environmental pollution monitoring.

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1. Introduction

Toxic heavy metal ions in water pose potential dangers to the health of humans, even in trace amount. Sensitive analysis or in situ monitoring of trace heavy metal ions in water is very important. In the past decades, different instrumental analysis techniques have been used to analyze the trace heavy metal ions in water, these include AAS, ICP-AES, MPT(microwave plasma torch)-AES and ICP-MS.^1–4 Other analytical techniques, such as electrochemical analysis,^5–8 fluorescence measurement,^9–11 absorption measurement,¹² X-ray fluorescence technique,^13,14 and electrolyte cathode atmospheric glow discharge (ELCAD)¹⁵ have also been developed to analyze trace heavy metals in water.

To enhance the detection sensitivity of the trace or ultra-trace heavy metal ions in water, several pre-concentration methods have been developed. These include solvent or solid extraction,^16–19 on-line pre-concentration using chelating resin,^3,20–22 and electrochemical enrichment.^13,23 Bio-monitoring techniques are an indirect method to monitor heavy metals in water, in which the contents of heavy metals in plants, cells, mussels and fish are analyzed with traditional analytical instruments.^24–27

Laser-induced breakdown spectroscopy (LIBS) can be used for a rapid analysis of heavy metals in water. The detection sensitivity of direct analysis of aqueous solutions is usually poor with single-pulse LIBS, while the laser was focused on the liquid surface or into the bulk liquid. To overcome this drawback, the liquid sample can be absorbed into different solid substrates, thus transferring liquid sample analysis to solid sample analysis.^28–31 Dual-pulse LIBS has been developed to enhance the detection sensitivity of single-pulse LIBS and can be successfully used to analyze elements in water.^32,33 However, dual-pulse LIBS needs two laser sources, this inevitably increases the complexity and cost of the measurement system. In a previous work, we combined single-pulse LIBS with electrical-deposition enrichment method to realize ultra-sensitive trace metal analysis of water samples and predicted a potential application of this technique in monitoring of environmental pollution of heavy metal in water.³⁴ In this previous work, the heavy metal ions were added into deionized water for experimental studies, thus the interference of other ions existed in natural water at relatively high concentrations, such as Ca²⁺, Mg²⁺, to the electrical-deposition process has not yet been studied.

The purpose of this work is to explore the feasibility of LIBS on sensitive detection of toxic heavy metal ions in natural water with the assistance of electrical-deposition enrichment method. A rotating cathode was designed to enrich the heavy metals in water and reduce gas bubble interference during the deposition process. The influence of Ca²⁺ and Mg²⁺ at high concentration to the deposition process of lead at low concentration was experimentally studied. The same level of detection sensitivity of heavy metal ions in tap water has been achieved as that in deionized water after taking longer deposition time.

2. Experimental

2.1 Electrical-deposition apparatus

The apparatus of electrical deposition is depicted in Fig. 1. High purity aluminium (99.999%) was chosen as cathode material. There are two reasons to do so: first, high purity aluminium is cheap and easy to buy; second, the strong spectral lines of aluminium within 200–450 nm region are rare and will not congest with the analysis lines of most metals. The aluminium rod used as cathode was cut to 42 mm long and 8 mm in diameter and was rotated around its axis by a stepping motor. The revolution speed of the stepping motor was set at 20 r.p.m. (revolutions per minute). A glass beaker was used as a deposition cell, in which 800 mL aqueous solution was contained. The beaker was put on a magnetic stirring apparatus and the solution was stirred with a magnetic stirrer at a revolution rate of 600 r.p.m. to ensure the homogeneity of the concentration of the analytes during electrical-deposition.

Fig. 1 The apparatus of electrical-deposition.

A platinum wire of 1 mm diameter was twisted to helix shape and used as an anode. The helix anode was 50 mm high and 30 mm in inner diameter and was immersed into the aqueous solution. The aluminium rod was placed at the position of the central axis of the helix. A variable DC voltage of 2–15 V was applied between the electrodes. The revolution of the aluminium rod during electrical-deposition process is helpful for throwing off the gas bubbles generated on rod surface effectively and ensuring the homogeneity of the deposited heavy metals on the rod surface. After electrical-deposition for a period, the aluminium rod was taken out and mounted on a translation stage for immediate elemental analysis by LIBS.

2.2 Chemicals and sample preparation

To prepare aqueous solutions of Cr³⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ with different concentrations, chromium(III) chloride, manganese chloride, copper chloride, zinc chloride, cadmium chloride, and lead nitrate were dissolved with deionized water to prepare aqueous solutions of the metal ions with high concentrations. Then 20 mL original solutions were diluted to 800 mL with tap water to prepare aqueous solutions for experimental studies. The chemicals were purchased from Fuchen Chemical Reagents (Tianjin, China). The purities of them were over 99.0% and no further purifications were carried out in the experiments.

2.3 Experimental setup of LIBS

The experimental setup used in our laboratory was reported in detail before.²⁸ Briefly, the fundamental output (1064 nm) of a Q-switched Nd:YAG laser was slightly defocused on the surface of an aluminium rod with a spherical lens (BK7, focal length = 150 mm). The pulse duration was 12 ns and the repetition rate was 5 Hz for the laser. Typical laser pulse energy was 50 mJ in the experiments. The aluminium rod was mounted on a specially designed translation-rotation stage that was driven by two stepping motors. During the run of the experiment, the rod moves from one end to the other end horizontally, and then rotates a small angle around its horizontal axis, and then moves back again horizontally to the starting end. The linear speed of the horizontal motion was 1 mm s⁻¹ during the experiments to ensure that the laser hit a fresh surface for each shot. The plasma spark was imaged onto the entrance slit of a 50 cm monochromator equipped with a 2400 lines/mm diffraction grating and a photomultiplier tube (PMT). The spectral resolution of the monochromator was less than 0.1 nm in UV-visible region at 100 μm widths of both entrance and exit slits.

The electrical signal of the PMT was monitored with a 250 MHz digital storage oscilloscope and the data were sent to a personal computer via a GPIB interface for further processing. To get an averaged signal of the atomic emission at analysis line wavelength and enhance the signal to noise ratio (S/N), the oscilloscope was set at 256 pulses' averaged mode and the temporal profile of the light emission was averaged for 300 shots, then the data of the profiles were recorded to calculate the line intensity. Time-resolved detection technique was applied to decrease the background contribution from the electron Bremsstrahlung emission. The optimized gate parameters for data acquisition for different atoms and their analysis line wavelengths are listed in Table 1.

Table 1 List of wavelengths of the analysis lines and parameters of data acquisition gate for each element

Element	Analysis line wavelength/nm	Data acquisition gate delay/μs	Data acquisition gate width/μs
a Two atomic lines here are irresolvable.
Cr	425.44	2.0	1.0
Mn	403.08	3.0	1.0
Cu	324.75	2.0	1.0
Zn	330.28^a	1.0	0.5
Cd	228.80	1.0	0.5
Pb	405.78	1.0	0.5

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3. Results and discussion

3.1 Optimization of parameters of electrical-deposition

The concentrations of trace heavy metal ions in natural water are usually very low. However, some cations, such as Ca²⁺ and Mg²⁺ exist in tap water with high concentrations, usually at the level of a few hundreds micro-mole per litre (μM). In addition, there exist some anions in tap water. In electrical-deposition process, all these ions will participate in electrochemical reactions on the electrodes, thus generate a current of a few to tens of milliampere, which depends on the amplitude of electrical-field (V/d) used. At the same time, lots of gas bubbles generated on the surfaces of both electrodes due to electrolyte of water molecules. Gas bubbles will hinder electrical-deposition of the interested heavy metal ions on the surface of aluminium rod. A revolution of the aluminium rod will be able to reduce this effect. In order to have a good analytical performance with the present system, the parameters of electrical-deposition were first optimized by experimental studies.

Under ideal conditions without gas bubble interference and in a limited volume of the solution, the number of deposited atoms on the surface of the aluminium rod can be described as,³⁴

N = C₀V(1 − e^−BUt) (1)

Where C₀ is the analyte's concentration at time t = 0; V is volume of the solution; B is a constant relative with the physical property of different ions; U is the voltage used for metal deposition and t is the deposition time. This predicts an increasing number of deposited atoms with the increase of applied voltage and deposition time.

Fig. 2 shows a plot of the intensity of the atomic emission of lead at 405.78 nm and copper at 324.75 nm versus the deposition voltage. The concentrations of Pb²⁺ and Cu²⁺ in the analyzed aqueous solution were 207.2 μgL⁻¹ and 63.5 μgL⁻¹ respectively, and the deposition time was 15 min. The optimized voltages determined in the experimental studies were both about 7.5 volts. Further increase of the voltage will decrease the number of deposited atoms during the same deposition time. This implies that the optimized deposition voltage is mainly determined by water matrix but not by electrochemical property of metal ions. Because the amount of gas bubbles generated on the surfaces of both anode and cathode is dependent on the applied voltage and hardness of water. Increasing deposition voltage will generate more gas bubbles and the interference of these bubbles to the deposition process will become more and more serious. Furthermore, for harder water (with higher concentrations of Ca²⁺ and Mg²⁺), the optimized deposition voltage will be expected to be lower.

Fig. 2 The relation of observed atomic emission intensities of copper and lead to the applied voltage between electrodes. The deposition time was 15 min and the concentration of Cu²⁺ and Pb²⁺ were 63.5 μgL⁻¹ and 207.2 μgL⁻¹, respectively.

Fig. 3 shows the relation of emission intensity of lead atoms to the deposition time, the voltage was 7.5 volts and concentration of Pb²⁺ was 207.2 μgL⁻¹. This is similar with the result obtained using deionized water as solvent, the number of deposited atoms increase with the increasing of deposition time. Considering both speed of analysis and sensitivity, 20-minute deposition time and 7.5 volts deposition voltage were selected for all six elements in the following experimental studies.

Fig. 3 The relation of observed atomic emission intensity of lead to the deposition time. The deposition voltage was 7.5 volts and the concentration of Pb²⁺ was 207.2 μgL⁻¹.

3.2 Quantitative analysis

Using 7.5 volts deposition voltage and 20 min deposition time, the contents of Cr³⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ in tap water were analyzed using LIBS with the assistance of electrical-deposition. All six heavy metals were artificially added into tap water for building up calibration curves. Using C₀ represents the original (background) concentration of ions and x represents the concentration of the added ions for one element, the real concentration of this element in the water is x + C₀, then the calibration curve of the corresponding element can be fitted with the following formula if considering the self-absorption saturation effects:³⁵

y = bc(1 − exp(−(x + C₀)/c) (2)

Where y is the observed atomic emission intensity, b and c are two parameters. If the data points can be well fitted with a linear equation, the self-absorption saturation effect can be ignored, thus the curve can be simply fitted with:

y = b(x + C₀) (3)

Table 2 lists all the fitted results of parameters of b, c and C₀ for six heavy metal ions in tap water.

Table 2 Fitted results according to eqn (2) and limit of detection for each element. As the content of Cr³⁺, Cd²⁺ and Pb²⁺ in tap water was so low that they are undetectable, C₀ for these three elements are fixed to zero in the fitting process

Element	b	c	C 0/μgL^−1	LOD/μgL^−1
Cr	28.40	135.66	0	0.317
Mn	42.53	179.72	18.46	0.176
Cu	138.98	129.79	5.376	0.162
Zn	18.85	—	11.79	1.35
Cd	22.85	202.47	0	0.787
Pb	9.48	700.82	0	0.570

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Fig. 4(a) shows the plots of emission intensity versus concentration of the added ions. The fitted curves do not pass through zero point of the axes. The absolute value of the intersection point of the fitted curve with horizontal axis is corresponding to the original concentration, C₀, of each element in the tap water. That is to say, the concentration of metal ions in the tap water can be determined by this way. After knowing C₀, a calibration curve can be re-plotted for each element by plotting intensity versusreal concentration of each element in the water, that is C_added + C₀ (Fig. 4(b)). Fig. 5(a) and Fig. 5(b) shows the calibration curves of copper and zinc, magnesium and chromium, cadmium and lead, respectively. Then the quantitative analysis of the trace heavy metal ions in tap water can be carried out using these calibration curves and observed spectral intensities.

Fig. 4 (a) A plot of observed atomic emission intensities versus concentrations of artificially added metal ions. The points were fitted with eqn (2). The absolute values of the intersect points on horizontal axis are corresponding to the original concentrations of Cu²⁺ and Zn²⁺ in tap water. (b) Calibration curves of copper and zinc.

Fig. 5 (a) Calibration curves of manganese and chromium; (b) Calibration curves of cadmium and lead.

The limit of detection (LOD) of one element was estimated according to 3σ rule, that is:

Where, σ_B was the standard deviation of the background; S was the sensitivity of the system. Here S can be replaced by parameter b according to eqn (2). For each element, the averaged temporal profiles of the background at the wavelength beside analysis line were recorded for ten times, the relative intensities of the background within the same data acquisition gate were obtained, and then σ_B can be estimated. According to our measurements, the relative standard deviation (R.S.D.) of both signals and backgrounds in ten measurements was about 5%, indicating a good reproducibility of measurement with the present system. The LODs of Cr³⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ in tap water were determined to be 0.16–1.35 μgL⁻¹ in this work (Table 2). These are 2–3 orders better than those that were obtained by using wood slice substrates²⁸ and at the same level of the results obtained by using deionized water samples.³⁴ This demonstrates that the method developed here is applicable to the ultra-sensitive trace metal ions analysis in natural water samples by using rotating cathode design. However, because of the inevitable interferences of other metal ions with high concentration, longer deposition time is necessary.

3.3 Influence of Ca²⁺ and Mg²⁺

In order to study the influence of the concentration of Ca²⁺ and Mg²⁺ to the deposition process of the trace heavy metal ions, different amounts of calcium chloride and magnesium chloride were added into deionized water to prepare aqueous solutions. The solutions also contained Pb²⁺ at the concentration of 207.2 μgL⁻¹. The plots of observed lead emission intensity in LIBS versus the concentration of Ca²⁺ and Mg²⁺ are shown in Fig. 6(a) and 6(b), respectively. The observed emission intensity of lead decreases with increasing of the concentration of Ca²⁺ or Mg²⁺ in the water. The points can be approximately fitted with linear lines, and the slopes of the linear lines for Ca²⁺ and Mg²⁺ are −3 and −15 L mg⁻¹, respectively. This is reasonable because Mg²⁺ is easier to deposit on the cathode than Ca²⁺ according to their electrochemical properties. Fortunately, the concentration of Mg²⁺ in natural water is usually lower than Ca²⁺ (usually Ca²⁺ : Mg²⁺ is about 2 : 1∼2.5 : 1, in mg L⁻¹), one can expect that the influence of Mg²⁺ to the electrical deposition process will be comparable to Ca²⁺ for natural water samples. We have not studied the influence of Na⁺ and K⁺ to the electrical deposition of trace heavy metals because of their lower concentrations in natural water samples in comparison with Ca²⁺ and Mg²⁺. Another reason is Na⁺ and K⁺ ions are univalent, physically, their moving speed should be slower than bivalent Ca²⁺ and Mg²⁺ ions in the same electrical field, thus the expected deposition rate of Na⁺ and K⁺ should be lower than that of Ca²⁺ and Mg²⁺.

Fig. 6 Relation of the observed intensity of atomic emission of lead to the concentration of Ca²⁺ and Mg²⁺ in the water, the concentration of Pb²⁺ in water was 207.2 μgL⁻¹, the deposition voltage was 7.5 volts and deposition time was 15 min. (a) Ca²⁺, (b) Mg²⁺.

3.4 Calibration curves for different water samples

Due to the influence of Ca²⁺ and Mg²⁺ to the process of electrical deposition, the calibration curves of one element obtained in different water samples must be different. Hardness is a parameter to indicate the amount of Ca²⁺ and Mg²⁺ in water. If a serious calibration curve of one element for different water samples with different hardness values has been established, then the method described here will be able to give a reliable analysis of the concentrations of trace heavy metal ions in different water samples, only need refer to the hardness of the water samples. As an example, Fig. 7 shows two calibration curves of lead obtained for two different water samples, the hardness of them are about 50 mg L⁻¹ (CaCO₃) and 80 mg L⁻¹ (CaCO₃), respectively. Where the former is tap water and the latter is tap water added with calcium chloride and magnesium chloride artificially. To simulate natural water samples, the concentration (in mg L⁻¹) ratio of added Ca²⁺ to Mg²⁺ in this experiment is 2 : 1.

Fig. 7 Calibration curves of lead obtained from different water samples with different hardness. The deposition time is 20 min and deposition voltage is 5 volts, which is better than 7.5 volts for harder water.

3.5 Co-deposition of different metal ions

Different metal ions present different potentials of deposition on the surface of the aluminium rod, depending on their electrochemical properties. To study the possibility of simultaneous multi-element analysis of natural water samples using this technique, trace amounts (at 2 μM) of Cd²⁺, Cr³⁺ and Pb²⁺ were added into the water samples taken from a river and then analyzed with this technique. Atomic emission from calcium, magnesium, copper, zinc, iron, cadmium, chromium and lead etc. can all be observed in the wavelength region of 200–450 nm. This observation demonstrates that different metal ions can be co-deposited on the surface of the aluminium rod simultaneously, no matter whether their concentrations are high or low. (The concentrations of Ca²⁺ and Mg²⁺ in natural water are two-orders higher than that of added heavy metal ions, such as Cd²⁺, Cr³⁺ and Pb²⁺.) Therefore, a simultaneous multi-elements analysis with this technique will be possible once their calibration curves have been established.

Usually the concentration of heavy metal ions in natural water is much lower than Ca²⁺ and Mg²⁺. In electrical deposition process, most of the deposited materials are calcium and magnesium, thus the plasma property will be mainly affected by aluminium (base material), calcium and magnesium. The matrix effect due to co-deposited heavy metals in laser-ablation and breakdown process should be negligible. However, the effect of calcium and magnesium has already been accounted for while establishing the calibration curves. One conclusion can be unambiguously made that, if a calibration curve of one element has been established for natural water samples, it is able to use this calibration curve in quantitative analysis for natural water samples with approximately equal hardness.

3.6 Technical discussion

One of the major advantages of LIBS combined with electrical-deposition enrichment is its high sensitivity for analyzing trace metal ions in water. The reasons include concentration enrichment, elimination of quenching effect of water molecules to atomic emission, and reduced continuum background due to low laser power density was required to ablate and excite the heavy metals deposited on the surface of the aluminium rod. Another major advantage is the possibility of simultaneous multi-elements analysis. If replacing the monochromator and PMT with a spectrometer and ICCD (Intensified Charged-Coupled Device), a spectrum between 180–900 nm can be obtained within just a few minutes with good S/N ratio and different trace metals in water samples can be quantitatively determined. The third advantage of this technique is that the operator does not need to pose in danger when analyzing toxic heavy metals. Such as in electrochemical analysis of cadmium and lead, a mercury electrode has to be used. However, mercury is harmful to operators and the environment.

The only drawback of the electrical deposition is that it usually needs to take a few to tens of minutes for enrichment, which decreases the analysis speed. However, this drawback can be partially compensated if simultaneous multi-element analysis is carried out. On the other hand, if the detection sensitivity improved, it is possible to reduce the deposition time. The detection sensitivity of the present system can be further improved by using a dual-pulse LIBS setup, optimizing optical detection system and optimizing the design of the electrical-deposition apparatus.

4. Conclusions

The different trace heavy metal ions in tap water can be electrically co-deposited on the surface of an aluminium rod and analyzed by LIBS with high detection sensitivity. The limits of detection of Cr³⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ in tap water were determined to be 0.16–1.35 μgL⁻¹ with present experimental setup and can be further improved. This technique is suitable for quantitative analysis of different water samples with different hardness. A sensitive simultaneous multi-element analysis will be possible using LIBS with the assistance of electrical-deposition for tap water and natural water samples, which is very important in environmental pollution monitoring.

Acknowledgements

The authors are grateful to undergraduate students, Yiwen Zhang, Qifeng Guo and Junhao Lin et al., who participated in the SRP program of South China University of Technology. Linxun Lu, a graduate student of Sun Yat-Sen University, participated in some early experiments. Qian Zhang helped do more experiments in the revision process. This work was financially supported by National Science Foundation of China (Grant No.: 10874047) and SRP program (Grant No.: x2lx-y1080360) of South China University of Technology.

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Footnote

† Present address: National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012.

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