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Analysis of heavy metals in liquids using Laser Induced Breakdown Spectroscopy by liquid-to-solid matrix conversion
Spectrochimica Acta Part B 61 (2006) 929 – 933 www.elsevier.com/locate/sab
Analysis of heavy metals in liquids using Laser Induced Breakdown Spectroscopy by liquid-to-solid matrix conversion D.M. Díaz Pace ⁎, C.A. D'Angelo, D. Bertuccelli, G. Bertuccelli Instituto de Física ‘Arroyo Seco’, Facultad de Ciencias Exactas, U.N.C.P.B.A., Pinto 399, B7000GHG Tandil, Bs. As., Argentina Received 20 April 2006; accepted 8 July 2006 Available online 22 August 2006
Abstract Laser Induced Breakdown Spectroscopy has been applied to the analysis of heavy metals in liquid samples, an issue of major importance for environmental monitoring. In this work, a new approach was developed in which liquid solutions were converted into solid pellets of calcium hydroxide by mixing with CaO. Therefore, liquid sample analysis is replaced by solid matrix analysis, overcoming the well-known difficulties and drawbacks of the analysis of liquid samples, and providing additional advantages with respect to other experimental setups. The plasma was produced in air at atmospheric pressure, by a pulsed Nd:YAG laser. Analytical results were achieved for Cr, Pb, Cd and Zn through calibration curves and limits of detection. © 2006 Elsevier B.V. All rights reserved. Keywords: LIBS; Laser; Plasma; Analytic spectroscopy
1. Introduction Laser Induced Breakdown Spectroscopy (LIBS) has been widely applied to the detection and quantification of trace elements in gaseous, solid and liquid samples [1–9]. Most investigations had been performed in solid samples that provide an uniform surface, greater sensitivity and repeatability. In those experiments, a laser pulse is focused onto a substrate to create plasma by material ablation. The plasma emission is then analysed in order to gather information about the plasma parameters, and thus the ionic and atomic composition of the target. In contrast, liquid samples analysis has been less investigated due to the fact that laser-produced plasmas in either the bulk or the surface of liquids, present several inherent drawbacks such as splashing, surface ripples, extinction of emitted intensity and a shorter plasma life-time [10–12]. All these facts affect adversely the analytical performance of the technique. In the last years, several approaches have been investigated to cope with those difficulties. For instance, there have been used laminar flows and jets [13,14], freezing samples [15], ⁎ Corresponding author. Tel.: +54 2293 444432; fax: +54 2293 444433. E-mail addresses: [email protected] (D.M. Díaz Pace), [email protected] (C.A. D'Angelo), [email protected] (D. Bertuccelli), [email protected] (G. Bertuccelli). 0584-8547/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2006.07.003
double-pulse techniques [10] and surface liquid layers evaporated onto a substrate [16]. Nevertheless, all of them are subjected to a trade off among limits of detection, cost and complexity of experimental equipment, and the possibility of carrying out many laser shots without changing the surface characteristics of the sample in order to average a big number of spectra. In addition, most studies have accounted for detection of light elements, [15–17] but only a few have investigated about heavy metals, [18,19] which are essential for environmental monitoring. In the present work, we have investigated the feasibility of laser-produced plasmas obtained at atmospheric pressure, for the determination of heavy metals in liquid samples. Liquid solutions were converted in solid-matrix samples overcoming the drawbacks of liquid analysis and accomplishing the solid target advantages. In fact, calcium oxide (CaO) was added to aqueous solution samples, which forms calcium hydroxide (Ca (OH)2) and the precipitate was then pressed into pellets. Calibration curves in the range 5–900 μg/g were constructed employing reference samples with known amounts of Cr, Pb, Cd and Zn in order to evaluate the sensitivity of the method. The curves obtained are linear at low concentrations and saturate at higher concentrations because of plasma self-absorption [20,21]. Therefore, a non-linear fitting was carried out to determine the limits of detection.
930
D.M. Díaz Pace et al. / Spectrochimica Acta Part B 61 (2006) 929–933
2. Experimental 2.1. Sample preparation The calibration samples were prepared by mixing 6 mg of CaO (Aldrich Powder 98%) with specific amounts (6 ml in our work) of aqueous solutions previously prepared with known concentrations of Cr, Pb, Cd and Zn in the range 3–1200 ppm. The wet Ca(OH)2 precipitate, was well stirred to make it homogeneous and left dry at room temperature, long enough for the non-absorbed water to evaporate completely in a place free of any possible contamination. After drying it was ground into powder and poured into a cylindrical stainless steel die. Finally, the powder was pressed at approximately 1 ton/cm2 into pellets of 3 cm diameter and 1 cm thick. This process was optimized to give samples where the analytes result uniformly distributed inside the solid matrix. It is important to remark that the concentration of the initial solutions differs from the final concentration in the solid samples as can be easily deduced knowing the amount of CaO and the volume of solution, and calculating the amount of water absorbed to form Ca(OH)2 (1.94 ml in our work). This was verified by measuring the weight of the pellets, which were close to 8 g in all the cases. 2.2. Experimental setup The experimental arrangement is sketched in Fig. 1: The plasma was generated in air at atmospheric pressure, focusing a pulsed Nd:YAG Q-switched laser (160 mJ/pulse, 7 ns pulse width, 2 Hz repetition rate, λ = 1064 nm) at normal incidence
on the sample surface, by means of a quartz lens of f = 10 cm. The pellet was fixed to a rotary holder so that every laser shot hit on a fresh site. The emitted plasma radiation was then collected perpendicular to the laser beam direction, focusing it by a second quartz lens (f = 20 cm) into the entrance slit (100 μm-wide) of a monochromator (resolution 0.01 nm). The detector was a photomultiplier (PM) whose signal is time resolved and averaged with a Box-Car. Finally, spectra were recorded and processed by a PC. 2.3. Quantitative analysis A set of reference samples with known concentrations of Cr, Cd, Pb and Zn were investigated in order to relate the measured spectral line intensities of the species in the plasma to the concentration in the target. A pellet free from those elements, was also manufactured using bi-distilled water to analyze the Ca(OH)2 substrate. As a matter of fact, it contains preexisting traces of some of the analyte elements and also others such as magnesium, iron, strontium and sodium, at concentrations below 5 μg/g, which were taken into account to make the calibration curves. The spectral region used for detection of elements was 270− 500 nm, which contains the suitable lines for the analysis. We took care that these lines were isolated and free of interference from either the substrate or other elements. The measured lines and their available spectroscopic data are listed in Table 1, according to Ref. [22]. Calibration curves were attained for all of them; each experimental point was obtained from the net peak intensity
Fig. 1. Experimental setup for LIBS measurements.
D.M. Díaz Pace et al. / Spectrochimica Acta Part B 61 (2006) 929–933 Table 1 Spectral lines employed for LIBS analysis Element
λ (nm)
Aij (108s− 1)
Ei (eV)
Ej (eV)
gi
gj
Cr I Pb I Cd I Zn I
357.87 405.78 326.10 481.05
1.48 8.90 4.06 –
0 – – –
3.46 – – –
7 5 1 –
9 3 3 –
(considered as the peak intensity of the line minus the background intensity), averaging a total of three spectra for each analyte line. The sampling time needed to perform each scan was about 2 min. 3. Results and discussion 3.1. Substrate selection Many favorable characteristics were in favor to choose CaO as the substrate material to fabricate the pellets, namely: — Inexpensive and easy to handle. — Possesses a simple chemical composition with relatively few lines, resulting in a reduced spectral interference.
931
— Reacts chemically with an aqueous solution, leading to Ca(OH)2 which can be formed up into solid pellets, remaining the existing species in the solution, homogeneously distributed inside the sample volume. 3.2. Time of measurement Because of the temporal evolution of laser-produced plasmas, their atomic/ionic populations evolve with time. Hence, the plasma spectral emission depends strongly of the observing time after the laser ablation (delay time). Initially, emission of lines, corresponding to ionic species that are significantly broadened due to the high electronic density, appears superimposed to an intense continuum that decreases with time. At later times, as the plasma expands and cools, lines from excited neutral elements dominate plasma emission. An adequate selection of delay time involves a compromise between signal-to-noise ratio and line intensities. In general, the optimal delay time depends on the laser pulse energy, the kind of sample and the surrounding atmosphere. On the other hand, its dependence on the species is not significant and, generally, a unique delay time can be chosen for all the elements [23]. In this
Fig. 2. Calibration curves obtained for the lines 357.87 Cr I; 405.78 Pb I; 481.05 Zn I and 326.10 Cd I. The dashed line corresponds to the linear behavior of an optically thin plasma.
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D.M. Díaz Pace et al. / Spectrochimica Acta Part B 61 (2006) 929–933
Table 2 Parameters of Eq. (1) obtained by fitting calibrations curves and the correlation coefficients Element
A
B
c
R
Cr Pb Zn Cd
0 0 0 0
0.02 0.001 0.0006 0.00005
34.5 334.1 131.6 695.4
0.981 0.994 0.995 0.998
work, a delay time of 7 μs, and a gate window of 2 μs were selected for spectral lines corresponding to neutral atoms. 3.3. Calibration curves Reference samples were used to construct calibration curves of the line peak intensity versus the corresponding pellet concentration for the elements under study, as shown in Fig. 2 In such curves, each point corresponds to an average of six measurements at different locations in the target, and the error bars are the relative standard deviations, calculated measuring the peak intensity of the line corresponding to each element over 100 laser shots. These values were between 5% and 10%, in agreement with those reported in the literature [4,17]. The calibration curves show a linear behavior in the lowconcentration region and saturation is reached at high concentrations, due to self-absorption of the lines by the plasma, a feature commonly observed in laser-produced plasmas at atmospheric pressure [4,20,21]. On saturated calibration curves, a non-linear function should be used to fit the experimental data. In this work we employed the expression proposed in [5], y ¼ a þ bcð1−e−x=c Þ
ð1Þ
where x represents the element concentration and y the line intensity. For low concentrations (x < 1), Eq. (1) is approximated by the straight line y ¼ a þ bx
ð2Þ
The parameter c is the concentration at which the slope of the calibration curve decreases in a factor 1 / e from its value at x = 0 (i.e. b). It can be regarded as a measurement of the degree of saturation of the line. Physically, it refers to the critical concentration at which absorption of the line becomes important and the plasma is said to become optically thick.
Table 3 Limits of detection and relative standard deviations
Table 4 Comparison of detection limits (ppm) Element
This work
Other works (literature)
Comments
Cr
1.2
Pb
20
Cd
129
Zn
21
0.1 [15] 30 [13] 40 [13] 10 [17] 0.4 [17] 2 [15] 10 [15] 190 [13] 100 [17] 1 [15] 0.1 [15] 60 [13] 120 [13] 1 [15] 120 [17]
Evaporated Aerosol Aerosol Water Water Evaporated Evaporated Aerosol Water Evaporated Evaporated Aerosol Aerosol Evaporated Water
solutions on carbon
solutions on carbon solutions on carbon
solutions on carbon solutions on carbon
solutions on carbon
Calibration curves obtained by fitting experimental data to Eq. (1) are shown in Fig. 2. The corresponding values for the a, b and c parameters are listed in Table 2. In the analysis of environmental samples, the use of intense lines is necessary in order to achieve the lowest detection limits. Therefore, some degree of saturation is expected. Comparing the c values in Table 2, it turns out that the highest level of saturation occurs for Cr and the lowest saturation is observed for Pb. 3.4. Detection limits The limit of detection depends on both the element and the spectral line. It is usually defined as the concentration that originates a net line-intensity equivalent to three times the standard deviation from the background, measured close to the line profile. It is determined by means of the expression [4] LoD ¼
3rB s
ð3Þ
where σB is the standard deviation of the background, and s is the sensitivity given by the slope of the corresponding calibration curve. Based on the calibration curves, the analytical performance of LIBS technique was evaluated for the detection of Cr, Pb, Cd and Zn in liquid samples. Results are listed in Table 3. Detection limits were calculated taking the sensitivity equal to the slope of the respective calibration curve in the low concentration region (i.e. b). In Table 4, we can see that these limits are comparable to those reported in the literature.
4. Conclusions
Element
LoD (ppm)
RSD (%)
Cr Pb Zn Cd
1.2 ± 0.4 20 ± 4 21 ± 5 129 ± 23
5.4 7.5 9.2 6.7
LIBS technique has been applied for detection of heavy metals on liquid samples, which were transformed into solid pellets of calcium hydroxide, resulting the atomic elements homogeneously distributed inside them. Detection limits obtained for Cr, Pb, Cd and Zn are in the range 1–120 ppm.
D.M. Díaz Pace et al. / Spectrochimica Acta Part B 61 (2006) 929–933
These values are comparable to those of other works. However, this new sample aggregation state transformation approach, has many potential applications in LIBS analysis. Nevertheless the samples require some minimum preparation; thereby LIBS cannot be applied to in-situ analysis. The analytical capabilities are better, in general, than those obtained with liquid samples in other different setup configurations, and allows to overcome the difficulties of the analysis of liquids. The method, in fact, provides the additional advantages inherent to solid targets, and others like simplicity, low cost, repeatability and sensitivity. Therefore, its application to trace monitoring of liquid environmental samples turns out to be practical. References [1] M. Hanafi, M.M. Omar, Y.E.E.-D. Gamal, Study of laser-induced breakdown spectroscopy of gases, Radiat. Phys. Chem. 57 (2000) 11–20. [2] C. D'Angelo, J.M. Gomba, D. Iriarte, G. Bertuccelli, Trace element analysis in water by LIBS technique, SPIE Proceeding III RIAOOPTILAS, vol. 98, 1999, pp. 534–541. [3] B. Charfi, M.A. Harith, Panoramic laser-induced breakdown spectrometry of water, Spectrochim. Acta Part B 57 (2002) 1141–1153. [4] M. Sabsabi, P. Cielo, Quantitative analysis of aluminum alloys by laserinduced breakdown spectroscopy and plasma characterization, Appl. Spectrosc. 49 (1995) 499–507. [5] C. Aragón, J.A. Aguilera, F. Peñalba, Improvements in quantitative analysis of steel composition by laser-induced breakdown spectroscopy at atmospheric pressure using an infrared Nd:YAG laser, Appl. Spectrosc. 53 (1999) 1259–1267. [6] J.M. Gomba, C. D'Angelo, D. Bertuccelli, G. Bertuccelli, Spectroscopic characterization of laser induced breakdown in aluminum–lithium alloy samples for quantitative determination of traces, Spectrochim. Acta Part B 56 (2001) 695–705. [7] M. Kuzuya, H. Aranami, Analysis of a high-concentration copper in metal alloys by emission spectroscopy of a laser-produced plasma in air at atmospheric pressure, Spectrochim. Acta Part B 55 (2000) 1423–1430. [8] E. Tognoni, V. Palleschi, M. Corsi, G. Cristoforetti, Quantitative microanalysis by laser-induced breakdown spectroscopy: a review of the experimental approaches, Spectrochim. Acta Part B 57 (2002) 1115–1130.
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[9] M. Kuzuya, M. Murakami, N. Maruyama, Quantitative analysis of ceramics by laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 58 (2003) 957–965. [10] D.A. Cremers, L.J. Radziemski, T.T. Loree, Spectrochemical analysis of liquids using the laser spark, Appl. Spectrosc. 38 (1984) 721–729. [11] G. Arca, A. Ciucci, V. Palleschi, S. Rastelli, E. Tognoni, Trace element analysis in water by laser-induced breakdown spectroscopy technique, Appl. Spectrosc. 51 (1997) 1102. [12] L. St-Onge, E. Kwong, M. Sabsabi, E.B. Vadas, Rapid analysis of liquid formulations containing sodium chloride using laser-induced breakdown spectroscopy, J. Pharm. Biomed. Anal. 36 (2004) 277–284. [13] C.W. Ng, W.F. Ho, N.H. Cheung, Spectrochemical analysis of liquids using laser-induced plasma emissions: effect of laser wavelength on plasma properties, Appl. Spectrosc. 51 (1997) 976. [14] B.T. Fisher, H.A. Johnsen, S.G. Buckley, D.W. Hann, Temporal gating for the optimization of laser-induced breakdown spectroscopy detection and analysis of toxic metals, Appl. Spectrosc. 55 (2001) 1312–1319. [15] J.O. Cáceres, J. Tornero López, H.H. Telle, A. González Ureña, Quantitative analysis of trace metals ions in ice using laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 56 (2001) 831–838. [16] R.L. Vander Wal, T.M. Ticich, H.R. West Jr., P.A. Householder, Trace metal detection by laser-induced breakdown spectroscopy, Appl. Spectrosc. 53 (1999) 1226–1235. [17] D.A. Cremers, The analysis of metals at a distance using laser-induced breakdown spectroscopy, Appl. Spectrosc. 41 (1987) 572–579. [18] P. Fichet, P. Mauchien, J.-F. Wagner, C. Moulin, Quantitative elemental determination in water and oil by laser induced breakdown spectroscopy, Anal. Chim. Acta 429 (2001) 269–278. [19] P. Yaroshchyk, R. Morrison, D. Body, B. Chadwick, Quantitative determination of wear metals in engine oils using laser-induced breakdown spectroscopy: A comparison between liquid jets and static liquids, Spectrochim. Acta Part B 60 (2005) 986–992. [20] C. Aragón, J. Bengoechea, J.A. Aguilera, Influence of the optical depth on spectral line emission from laser-induced plasmas, Spectrochim. Acta Part B 56 (2001) 619–628. [21] V. Lazic, R. Barbini, F. Colao, R. Fantoni, A. Palucci, Self-absorption model in quantitative laser induced breakdown spectroscopy measurements on soils and sediments, Spectrochim. Acta Part B 56 (2001) 807–820. [22] NIST electronic database, at http://physics.nist.gov/PhysRefData. [23] R. Wisbrun, I. Schechter, R. Niessner, H. Schröeder, K. Kompa, Detector for trace elemental analysis of solid environmental samples by laser plasma spectroscopy, Anal. Chem. 66 (1994) 2964–2975.
Analysis of heavy metals in liquids using Laser Induced Breakdown Spectroscopy by liquid-to-solid matrix conversion D.M. Díaz Pace ⁎, C.A. D'Angelo, D. Bertuccelli, G. Bertuccelli Instituto de Física ‘Arroyo Seco’, Facultad de Ciencias Exactas, U.N.C.P.B.A., Pinto 399, B7000GHG Tandil, Bs. As., Argentina Received 20 April 2006; accepted 8 July 2006 Available online 22 August 2006
Abstract Laser Induced Breakdown Spectroscopy has been applied to the analysis of heavy metals in liquid samples, an issue of major importance for environmental monitoring. In this work, a new approach was developed in which liquid solutions were converted into solid pellets of calcium hydroxide by mixing with CaO. Therefore, liquid sample analysis is replaced by solid matrix analysis, overcoming the well-known difficulties and drawbacks of the analysis of liquid samples, and providing additional advantages with respect to other experimental setups. The plasma was produced in air at atmospheric pressure, by a pulsed Nd:YAG laser. Analytical results were achieved for Cr, Pb, Cd and Zn through calibration curves and limits of detection. © 2006 Elsevier B.V. All rights reserved. Keywords: LIBS; Laser; Plasma; Analytic spectroscopy
1. Introduction Laser Induced Breakdown Spectroscopy (LIBS) has been widely applied to the detection and quantification of trace elements in gaseous, solid and liquid samples [1–9]. Most investigations had been performed in solid samples that provide an uniform surface, greater sensitivity and repeatability. In those experiments, a laser pulse is focused onto a substrate to create plasma by material ablation. The plasma emission is then analysed in order to gather information about the plasma parameters, and thus the ionic and atomic composition of the target. In contrast, liquid samples analysis has been less investigated due to the fact that laser-produced plasmas in either the bulk or the surface of liquids, present several inherent drawbacks such as splashing, surface ripples, extinction of emitted intensity and a shorter plasma life-time [10–12]. All these facts affect adversely the analytical performance of the technique. In the last years, several approaches have been investigated to cope with those difficulties. For instance, there have been used laminar flows and jets [13,14], freezing samples [15], ⁎ Corresponding author. Tel.: +54 2293 444432; fax: +54 2293 444433. E-mail addresses: [email protected] (D.M. Díaz Pace), [email protected] (C.A. D'Angelo), [email protected] (D. Bertuccelli), [email protected] (G. Bertuccelli). 0584-8547/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2006.07.003
double-pulse techniques [10] and surface liquid layers evaporated onto a substrate [16]. Nevertheless, all of them are subjected to a trade off among limits of detection, cost and complexity of experimental equipment, and the possibility of carrying out many laser shots without changing the surface characteristics of the sample in order to average a big number of spectra. In addition, most studies have accounted for detection of light elements, [15–17] but only a few have investigated about heavy metals, [18,19] which are essential for environmental monitoring. In the present work, we have investigated the feasibility of laser-produced plasmas obtained at atmospheric pressure, for the determination of heavy metals in liquid samples. Liquid solutions were converted in solid-matrix samples overcoming the drawbacks of liquid analysis and accomplishing the solid target advantages. In fact, calcium oxide (CaO) was added to aqueous solution samples, which forms calcium hydroxide (Ca (OH)2) and the precipitate was then pressed into pellets. Calibration curves in the range 5–900 μg/g were constructed employing reference samples with known amounts of Cr, Pb, Cd and Zn in order to evaluate the sensitivity of the method. The curves obtained are linear at low concentrations and saturate at higher concentrations because of plasma self-absorption [20,21]. Therefore, a non-linear fitting was carried out to determine the limits of detection.
930
D.M. Díaz Pace et al. / Spectrochimica Acta Part B 61 (2006) 929–933
2. Experimental 2.1. Sample preparation The calibration samples were prepared by mixing 6 mg of CaO (Aldrich Powder 98%) with specific amounts (6 ml in our work) of aqueous solutions previously prepared with known concentrations of Cr, Pb, Cd and Zn in the range 3–1200 ppm. The wet Ca(OH)2 precipitate, was well stirred to make it homogeneous and left dry at room temperature, long enough for the non-absorbed water to evaporate completely in a place free of any possible contamination. After drying it was ground into powder and poured into a cylindrical stainless steel die. Finally, the powder was pressed at approximately 1 ton/cm2 into pellets of 3 cm diameter and 1 cm thick. This process was optimized to give samples where the analytes result uniformly distributed inside the solid matrix. It is important to remark that the concentration of the initial solutions differs from the final concentration in the solid samples as can be easily deduced knowing the amount of CaO and the volume of solution, and calculating the amount of water absorbed to form Ca(OH)2 (1.94 ml in our work). This was verified by measuring the weight of the pellets, which were close to 8 g in all the cases. 2.2. Experimental setup The experimental arrangement is sketched in Fig. 1: The plasma was generated in air at atmospheric pressure, focusing a pulsed Nd:YAG Q-switched laser (160 mJ/pulse, 7 ns pulse width, 2 Hz repetition rate, λ = 1064 nm) at normal incidence
on the sample surface, by means of a quartz lens of f = 10 cm. The pellet was fixed to a rotary holder so that every laser shot hit on a fresh site. The emitted plasma radiation was then collected perpendicular to the laser beam direction, focusing it by a second quartz lens (f = 20 cm) into the entrance slit (100 μm-wide) of a monochromator (resolution 0.01 nm). The detector was a photomultiplier (PM) whose signal is time resolved and averaged with a Box-Car. Finally, spectra were recorded and processed by a PC. 2.3. Quantitative analysis A set of reference samples with known concentrations of Cr, Cd, Pb and Zn were investigated in order to relate the measured spectral line intensities of the species in the plasma to the concentration in the target. A pellet free from those elements, was also manufactured using bi-distilled water to analyze the Ca(OH)2 substrate. As a matter of fact, it contains preexisting traces of some of the analyte elements and also others such as magnesium, iron, strontium and sodium, at concentrations below 5 μg/g, which were taken into account to make the calibration curves. The spectral region used for detection of elements was 270− 500 nm, which contains the suitable lines for the analysis. We took care that these lines were isolated and free of interference from either the substrate or other elements. The measured lines and their available spectroscopic data are listed in Table 1, according to Ref. [22]. Calibration curves were attained for all of them; each experimental point was obtained from the net peak intensity
Fig. 1. Experimental setup for LIBS measurements.
D.M. Díaz Pace et al. / Spectrochimica Acta Part B 61 (2006) 929–933 Table 1 Spectral lines employed for LIBS analysis Element
λ (nm)
Aij (108s− 1)
Ei (eV)
Ej (eV)
gi
gj
Cr I Pb I Cd I Zn I
357.87 405.78 326.10 481.05
1.48 8.90 4.06 –
0 – – –
3.46 – – –
7 5 1 –
9 3 3 –
(considered as the peak intensity of the line minus the background intensity), averaging a total of three spectra for each analyte line. The sampling time needed to perform each scan was about 2 min. 3. Results and discussion 3.1. Substrate selection Many favorable characteristics were in favor to choose CaO as the substrate material to fabricate the pellets, namely: — Inexpensive and easy to handle. — Possesses a simple chemical composition with relatively few lines, resulting in a reduced spectral interference.
931
— Reacts chemically with an aqueous solution, leading to Ca(OH)2 which can be formed up into solid pellets, remaining the existing species in the solution, homogeneously distributed inside the sample volume. 3.2. Time of measurement Because of the temporal evolution of laser-produced plasmas, their atomic/ionic populations evolve with time. Hence, the plasma spectral emission depends strongly of the observing time after the laser ablation (delay time). Initially, emission of lines, corresponding to ionic species that are significantly broadened due to the high electronic density, appears superimposed to an intense continuum that decreases with time. At later times, as the plasma expands and cools, lines from excited neutral elements dominate plasma emission. An adequate selection of delay time involves a compromise between signal-to-noise ratio and line intensities. In general, the optimal delay time depends on the laser pulse energy, the kind of sample and the surrounding atmosphere. On the other hand, its dependence on the species is not significant and, generally, a unique delay time can be chosen for all the elements [23]. In this
Fig. 2. Calibration curves obtained for the lines 357.87 Cr I; 405.78 Pb I; 481.05 Zn I and 326.10 Cd I. The dashed line corresponds to the linear behavior of an optically thin plasma.
932
D.M. Díaz Pace et al. / Spectrochimica Acta Part B 61 (2006) 929–933
Table 2 Parameters of Eq. (1) obtained by fitting calibrations curves and the correlation coefficients Element
A
B
c
R
Cr Pb Zn Cd
0 0 0 0
0.02 0.001 0.0006 0.00005
34.5 334.1 131.6 695.4
0.981 0.994 0.995 0.998
work, a delay time of 7 μs, and a gate window of 2 μs were selected for spectral lines corresponding to neutral atoms. 3.3. Calibration curves Reference samples were used to construct calibration curves of the line peak intensity versus the corresponding pellet concentration for the elements under study, as shown in Fig. 2 In such curves, each point corresponds to an average of six measurements at different locations in the target, and the error bars are the relative standard deviations, calculated measuring the peak intensity of the line corresponding to each element over 100 laser shots. These values were between 5% and 10%, in agreement with those reported in the literature [4,17]. The calibration curves show a linear behavior in the lowconcentration region and saturation is reached at high concentrations, due to self-absorption of the lines by the plasma, a feature commonly observed in laser-produced plasmas at atmospheric pressure [4,20,21]. On saturated calibration curves, a non-linear function should be used to fit the experimental data. In this work we employed the expression proposed in [5], y ¼ a þ bcð1−e−x=c Þ
ð1Þ
where x represents the element concentration and y the line intensity. For low concentrations (x < 1), Eq. (1) is approximated by the straight line y ¼ a þ bx
ð2Þ
The parameter c is the concentration at which the slope of the calibration curve decreases in a factor 1 / e from its value at x = 0 (i.e. b). It can be regarded as a measurement of the degree of saturation of the line. Physically, it refers to the critical concentration at which absorption of the line becomes important and the plasma is said to become optically thick.
Table 3 Limits of detection and relative standard deviations
Table 4 Comparison of detection limits (ppm) Element
This work
Other works (literature)
Comments
Cr
1.2
Pb
20
Cd
129
Zn
21
0.1 [15] 30 [13] 40 [13] 10 [17] 0.4 [17] 2 [15] 10 [15] 190 [13] 100 [17] 1 [15] 0.1 [15] 60 [13] 120 [13] 1 [15] 120 [17]
Evaporated Aerosol Aerosol Water Water Evaporated Evaporated Aerosol Water Evaporated Evaporated Aerosol Aerosol Evaporated Water
solutions on carbon
solutions on carbon solutions on carbon
solutions on carbon solutions on carbon
solutions on carbon
Calibration curves obtained by fitting experimental data to Eq. (1) are shown in Fig. 2. The corresponding values for the a, b and c parameters are listed in Table 2. In the analysis of environmental samples, the use of intense lines is necessary in order to achieve the lowest detection limits. Therefore, some degree of saturation is expected. Comparing the c values in Table 2, it turns out that the highest level of saturation occurs for Cr and the lowest saturation is observed for Pb. 3.4. Detection limits The limit of detection depends on both the element and the spectral line. It is usually defined as the concentration that originates a net line-intensity equivalent to three times the standard deviation from the background, measured close to the line profile. It is determined by means of the expression [4] LoD ¼
3rB s
ð3Þ
where σB is the standard deviation of the background, and s is the sensitivity given by the slope of the corresponding calibration curve. Based on the calibration curves, the analytical performance of LIBS technique was evaluated for the detection of Cr, Pb, Cd and Zn in liquid samples. Results are listed in Table 3. Detection limits were calculated taking the sensitivity equal to the slope of the respective calibration curve in the low concentration region (i.e. b). In Table 4, we can see that these limits are comparable to those reported in the literature.
4. Conclusions
Element
LoD (ppm)
RSD (%)
Cr Pb Zn Cd
1.2 ± 0.4 20 ± 4 21 ± 5 129 ± 23
5.4 7.5 9.2 6.7
LIBS technique has been applied for detection of heavy metals on liquid samples, which were transformed into solid pellets of calcium hydroxide, resulting the atomic elements homogeneously distributed inside them. Detection limits obtained for Cr, Pb, Cd and Zn are in the range 1–120 ppm.
D.M. Díaz Pace et al. / Spectrochimica Acta Part B 61 (2006) 929–933
These values are comparable to those of other works. However, this new sample aggregation state transformation approach, has many potential applications in LIBS analysis. Nevertheless the samples require some minimum preparation; thereby LIBS cannot be applied to in-situ analysis. The analytical capabilities are better, in general, than those obtained with liquid samples in other different setup configurations, and allows to overcome the difficulties of the analysis of liquids. The method, in fact, provides the additional advantages inherent to solid targets, and others like simplicity, low cost, repeatability and sensitivity. Therefore, its application to trace monitoring of liquid environmental samples turns out to be practical. References [1] M. Hanafi, M.M. Omar, Y.E.E.-D. Gamal, Study of laser-induced breakdown spectroscopy of gases, Radiat. Phys. Chem. 57 (2000) 11–20. [2] C. D'Angelo, J.M. Gomba, D. Iriarte, G. Bertuccelli, Trace element analysis in water by LIBS technique, SPIE Proceeding III RIAOOPTILAS, vol. 98, 1999, pp. 534–541. [3] B. Charfi, M.A. Harith, Panoramic laser-induced breakdown spectrometry of water, Spectrochim. Acta Part B 57 (2002) 1141–1153. [4] M. Sabsabi, P. Cielo, Quantitative analysis of aluminum alloys by laserinduced breakdown spectroscopy and plasma characterization, Appl. Spectrosc. 49 (1995) 499–507. [5] C. Aragón, J.A. Aguilera, F. Peñalba, Improvements in quantitative analysis of steel composition by laser-induced breakdown spectroscopy at atmospheric pressure using an infrared Nd:YAG laser, Appl. Spectrosc. 53 (1999) 1259–1267. [6] J.M. Gomba, C. D'Angelo, D. Bertuccelli, G. Bertuccelli, Spectroscopic characterization of laser induced breakdown in aluminum–lithium alloy samples for quantitative determination of traces, Spectrochim. Acta Part B 56 (2001) 695–705. [7] M. Kuzuya, H. Aranami, Analysis of a high-concentration copper in metal alloys by emission spectroscopy of a laser-produced plasma in air at atmospheric pressure, Spectrochim. Acta Part B 55 (2000) 1423–1430. [8] E. Tognoni, V. Palleschi, M. Corsi, G. Cristoforetti, Quantitative microanalysis by laser-induced breakdown spectroscopy: a review of the experimental approaches, Spectrochim. Acta Part B 57 (2002) 1115–1130.
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[9] M. Kuzuya, M. Murakami, N. Maruyama, Quantitative analysis of ceramics by laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 58 (2003) 957–965. [10] D.A. Cremers, L.J. Radziemski, T.T. Loree, Spectrochemical analysis of liquids using the laser spark, Appl. Spectrosc. 38 (1984) 721–729. [11] G. Arca, A. Ciucci, V. Palleschi, S. Rastelli, E. Tognoni, Trace element analysis in water by laser-induced breakdown spectroscopy technique, Appl. Spectrosc. 51 (1997) 1102. [12] L. St-Onge, E. Kwong, M. Sabsabi, E.B. Vadas, Rapid analysis of liquid formulations containing sodium chloride using laser-induced breakdown spectroscopy, J. Pharm. Biomed. Anal. 36 (2004) 277–284. [13] C.W. Ng, W.F. Ho, N.H. Cheung, Spectrochemical analysis of liquids using laser-induced plasma emissions: effect of laser wavelength on plasma properties, Appl. Spectrosc. 51 (1997) 976. [14] B.T. Fisher, H.A. Johnsen, S.G. Buckley, D.W. Hann, Temporal gating for the optimization of laser-induced breakdown spectroscopy detection and analysis of toxic metals, Appl. Spectrosc. 55 (2001) 1312–1319. [15] J.O. Cáceres, J. Tornero López, H.H. Telle, A. González Ureña, Quantitative analysis of trace metals ions in ice using laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 56 (2001) 831–838. [16] R.L. Vander Wal, T.M. Ticich, H.R. West Jr., P.A. Householder, Trace metal detection by laser-induced breakdown spectroscopy, Appl. Spectrosc. 53 (1999) 1226–1235. [17] D.A. Cremers, The analysis of metals at a distance using laser-induced breakdown spectroscopy, Appl. Spectrosc. 41 (1987) 572–579. [18] P. Fichet, P. Mauchien, J.-F. Wagner, C. Moulin, Quantitative elemental determination in water and oil by laser induced breakdown spectroscopy, Anal. Chim. Acta 429 (2001) 269–278. [19] P. Yaroshchyk, R. Morrison, D. Body, B. Chadwick, Quantitative determination of wear metals in engine oils using laser-induced breakdown spectroscopy: A comparison between liquid jets and static liquids, Spectrochim. Acta Part B 60 (2005) 986–992. [20] C. Aragón, J. Bengoechea, J.A. Aguilera, Influence of the optical depth on spectral line emission from laser-induced plasmas, Spectrochim. Acta Part B 56 (2001) 619–628. [21] V. Lazic, R. Barbini, F. Colao, R. Fantoni, A. Palucci, Self-absorption model in quantitative laser induced breakdown spectroscopy measurements on soils and sediments, Spectrochim. Acta Part B 56 (2001) 807–820. [22] NIST electronic database, at http://physics.nist.gov/PhysRefData. [23] R. Wisbrun, I. Schechter, R. Niessner, H. Schröeder, K. Kompa, Detector for trace elemental analysis of solid environmental samples by laser plasma spectroscopy, Anal. Chem. 66 (1994) 2964–2975.