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Laser ablation in liquids as a new technique of sampling in elemental analysis of solid materials
Spectrochimica Acta Part B 64 (2009) 119–125
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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Laser ablation in liquids as a new technique of sampling in elemental analysis of solid materials☆ E.V. Muravitskaya ⁎, V.A. Rosantsev, M.V. Belkov, E.A. Ershov-Pavlov, E.V. Klyachkovskaya B.I. Stepanov Institute of Physics, National Academy of Sciences, 70, Nezalezhnastsi Ave. 220072 Minsk, Belarus
a r t i c l e
i n f o
Article history: Received 30 October 2007 Accepted 12 November 2008 Available online 27 November 2008 Keywords: Sample preparation technique Laser sampling ICP-OES Elemental analysis
a b s t r a c t Laser ablation in liquid media is considered as a new sample preparation technique in the elemental composition analysis of materials using optical emission spectroscopy of inductively coupled plasma (ICP-OES). Solid samples are transformed into uniform colloidal solutions of nanosized analyte particles using laser radiation focused onto the sample surface. High homogeneity of the resulting solution allows performing the ICP-OES quantitative analysis especially for the samples, which are poorly soluble in acids. The technique is compatible with the conventional solution-based standards. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Heating materials to the plasma state by means of focused laser beams to excite the material emission spectra finds increasingly wide application in elemental analysis of samples by the optical emission spectroscopy (OES) in different fields of human activity. Many successful examples can be found of the laser-induced breakdown spectroscopy (LIBS) application for industry, art, environmental, medical, forensic sciences, etc. [1–4]. Recently the laser ablation in liquids is also developed as applied to the synthesis of nanosized particles [5]. Moreover, the laser ablation in liquids can be used for the sample preparation of solids as well in their chemical composition analysis by the optical emission spectroscopy of the inductively coupled plasma (ICP-OES) [6–8], and further investigations are necessary to provide the technique usability in the ICP-OES elemental analysis. Nowadays, ICP-OES is one of the major spectrometry techniques for quantitative analysis of elemental composition of different materials [4,9–11]. It offers the opportunity to measure content of multiple chemical elements in a sample in a single run, and it provides a comparatively low detection limit. However, the analyzed samples are obviously introduced into the ICP-OES spectrometers in solutions, and the sample preparation is necessary before the analysis, which includes the sample destruction and mineralization [10]. So, the sample preparation is one of the most important stages of the elemental composition analysis. Most popular chemical techniques of the sample destruction and mineralization do not always allow total digestion of the ☆ This paper was presented at the Euro Mediterranean Symposium on Laser Induced Breakdown Spectroscopy (EMSLIBS 2007) held in Paris (France), 11–13 September 2007. ⁎ Corresponding author. E-mail address: [email protected] (E.V. Muravitskaya). 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.11.003
analyzed substances [12]. Many materials (especially it concerns archeological and environmental objects) have very low solubility in acids. The incomplete transition of a sample into the solution (formation of sediments) results in a partial sample loss, giving rise to the measurement total error. Sometime, concentrated and aggressive acids and alkali are used to increase the sample digestion. However, the increase of the acid content in the solution under analysis results in the analytical signal suppression [13–15]. The suppression rate depends on the acid nature, and it is proportional to the acid concentration. As far as density and viscosity of acids are higher than ones of water, the suppression is mainly related to the processes of the solution transportation. Partly it is also due to a decrease of the plasma temperature because of energy losses for atomization of the acid molecules. The common internal standard technique can not eliminate the effect. Thus, one should take into account that uncontrolled changes in the acid concentration can raise the measurement error prevailing over the instrumental ones [15]. It also should be noted that the best detection limit can be reached only at higher quantity of the sample dissolved in lower solvent volume, which enhances the sample preparation problems. As a result, the preparation of certain samples is a very complex and time-consuming procedure. In this work, an alternative technique for the sample preparation of solids in their ICP-OES elemental composition analysis is considered. The solid samples are transformed into colloidal solutions of nanosized analyte particles by means of laser ablation (LA). Nowadays, the LA technique is commonly used in the synthesis of nanoparticles having controlled dimensions [5,16,17]. The laser radiation focused onto a surface of a solid sample in a liquid medium provides sample ablation resulting in the formation of nanoparticles, their composition corresponding to that of the sample. The prepared solutions are further directly used in the common procedure of the ICP-OES measurements.
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The proposed techniques have been tested using reference standard samples with known compositions. Two modes of the sample preparation techniques have been applied: (i) the sample common
chemical destruction and (ii) the sample laser ablation in liquid. The samples obtained using the both preparation modes have been analyzed by means of the ICP-OES techniques. For this, calibration curves using
Fig. 1. General view of the laser spectral analyzer (LSA) (A) and block-diagram of the LSA setup (B). Insertions show the beaker used.
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ICP standards have been plotted. Then, the solutions prepared have been analyzed to determine their concentration from the calibration curves, and the signal intensity has been plotted versus the measured concentration. The elemental analysis results have shown that the LA in liquid can be applied as the sample preparation technique in the ICPOES elemental composition measurements, and the technique is compatible with the common solution-based standardization. 2. Experimental 2.1. ICP-OES setup In ICP-OES measurements a THERMO ELECTRON Model IRIS Intrepid II ICP optical spectrometer was used [18]. The spectrometer is equipped with Echelle optics and a Charge Injection Device (CID) solid state detector. The spectrometer provides coverage of the wavelength range from 165 to 1000 nm. Measurements are controlled by Thermo Electron Validated Analysis (TEVA) software. Principle line intensities can be corrected for the corresponding backgrounds at selectable wavelengths, and the net line intensities are automatically calculated for pre-selected integration time for analytical curves plotting. Double optics is used to decrease the negative effect of torch wandering on image stability. Emission of the middle zone of the torch column is imaged by a band light fibers onto the entrance slit of the spectrometer. The applied spectrometer makes also possible the time resolution of spectra, i.e., line intensity versus torching time plots can be made simultaneously for 15 (with single read out time of about 0.5s) elements, called “time-scans” of spectra. Spectral lines together with selected background signals (at one or both sides of the spectral lines) can be recorded, and also net signals are elucidated by the software. The timescans (non-integrated and integrated) and the analytical curves can be graphically represented together with numerical data. 2.2. Laser ablation setup Laser ablation of the samples under study has been performed using an LA optical unit of the Laser Spectra chemical Analyzer (LSA) designed for the LIBS analysis of different materials [19]. The analyzer general view and its block-diagram are presented in Fig. 1A and B, respectively. LSA includes a Q-switched Nd:YAG double pulse laser, the LA optical unit consisting of a microscope, an imaging CCD-camera, a PC-controlled sample table and a spectrometer with a CCD-arrays recording system. The laser wavelength is 1064 nm. The laser pulses duration and the repetition rate can be controlled in the intervals of 10–20 ns and 1–15 Hz, respectively. The pulses energy is also variable and can be set as high as 100 mJ. The laser can work both in the singleand double-pulse modes at the collinear scheme of the laser beams action on a sample. A time delay between the pulses can be varied from 0 to 140 µs. The laser action zone is imaged at the PC monitor and can be positioned on a sample with 0.01 mm accuracy. LSA is equipped with a 1-meter grating spectrometer. Spectral range and spectral resolution of the spectrometer are 200–800 nm and 20,000, respectively. 2.3. Model experiment The proposed technique of the sample preparation has been tested using the elemental composition of calibration standards of zinc– aluminum–copper alloys (ZAC) No 11, 13, 22, 25, elemental composition of which is presented in Table 1. The sample solutions of the reference materials for the ICP-OES measurements have been obtained using both techniques — a common chemical digestion and the laser ablation. All solutions have been prepared with MilliQ purified water (Millipore System Eschborn, Germany). Standard solutions were prepared by dilution the ICP-standards (CertiPUR, Merk, Darmstadt, Germany) in 3% (m/m) HNO3.
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Table 1 Reference data of the percentage concentration of elements in the ZAC samples El. Zn Fe Al Cu Mg
Concentration, % No 11
No 13
No 22
No 25
93.50 0.11 2.70 1.65 1.93
88.50 0.30 8.81 0.68 1.59
84.30 0.42 12.03 3.09 0.14
83.40 0.14 7.50 8.19 0.76
2.3.1. Sample preparation by the chemical digestion The digestion in chemical reagents has allowed complete destroying the material under investigation irrespective of the form in which it previously existed [9,20–22]. The chemical dissolution of the alloys has been performed in several stages. The selected mass of 200 mg of the sample has been weighted using Satogosm RU-LV-210-A balance and put into a measured vessel. Such a rather high sample quantity has been chosen to exclude inhomogeneity of the resulting solution, to increase a signal, and thus to lower the measurement errors. The concentrated nitric acid has been used, which was added to the sample in small portions. Further, the solution has been heated up to the sample complete dissolution. After the cooling, the solution has been set into a standard 200 ml vessel, deionized water has been added to obtain exactly 200 ml volume, and then has been thoroughly mixed up. 2.3.2. Sample preparation by the laser ablation In the LA sample preparation, the laser ablation in liquid was used to transform solid samples into colloidal analyte solutions. The sample has been put into distilled water in a 15 ml beaker, and the beaker has been positioned on the LSA sample table (see Fig. 1). The laser beam has been focused on the sample surface. A double-pulse mode of the laser action on the sample has been realized, because it provides up to ten times higher ablation than the single-pulse one [23]. The following working regime of the laser has been used: single pulse duration of 10–12 ns with the energy of 50 mJ, which provided a mean power density of 1010 W/cm2 at the sample surface. The double-pulse repetition rate of 10 Hz with a delay of 8 μs between the pulses has been used. The regime has been chosen to obtain practically homogeneous analyte solution, where particles of 10–15 nm prevail. The particle size was chosen in accordance with the earlier observations [17] to assure maximum solution uniformity and stability against sedimentation, which is required to increase the accuracy of ICP measurement. The upper limit of particle diameter is determined by pass capacity of the ICPOES spectrometer spray [18]. The laser radiation acted in one spot of the sample during about 3 s. Then the sample table has been moved to a new position. The sample preparation procedure took totally 80 s, leaving 27 erosion craters on the sample surface. After 80 s exposition of the laser radiation, one could observe a visible gray coloration of the solution, which indicated the nanoparticles formation. As far as the solution was not stabilized, the ICP-OES concentration measurements have been performed within an hour after the solution preparation. 2.3.3. Selection of spectral lines The TEVA software makes possible the visual inspection of the preselected spectral ranges (intensity versus wavelength scans) for the selection of the background values in either of sides of the spectral lines to be corrected. It is a general practice with this instrumentation to select 2–3 spectral lines of every element in a preliminary study of the calibration curves with regard to the crossing point on the intensity axes (ordinate). Optical Echelle scheme allows using very high orders of the spectrum from the thirty to the two hundred and even above. Some analytical lines can appear in the spectrum more than once, and the same line can have different intensities in different orders of a spectrum. Therefore not only a line, but also its order is important. For example, iron of 238.204 nm is observed in two different orders (140) (141), but in this case intensities of these are identical. In the investigation of ZAC
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Table 2 Data on the spectral lines of Zn, Fe, Al, Cu and Mg used for the measurements El./ionization state/wavelength (nm)
Order of the spectrum
Relative intensity
Al I 396.152 Al I 394.401 Cu I 324.754 Cu I 327.396 Fe II 259.940 Fe II 238.204 Fe II 238.204 Mg II 279.553 Mg II 280.271 Mg I 285.213 Zn I 213.856 Zn II 206.200
85 85 103 102 129 141 140 120 120 117 157 163
350,000 250,000 5,000,000 3,000,000 2,000,000 1,800,000 1,800,000 50,000,000 15,000,000 6,000,000 3,000,000 900,000
air, no resolidification is observed of the material around the edges of the craters after melting in a liquid. All solid material is ablated in the form of vapor and liquid-drop phases and is left in the liquid [24]. It allows calculating the evaporated material volume as the volume of the erosion crater. Depth profiles of the craters in air and in water are presented in the Fig. 3. The crater profiles in water have been approximated by a truncated cone. Averaged values of a diameter and depth of the craters are 470–490 µm and 20–21 µm, respectively. The measurement precision of the diameter and depth is estimated to be, respectively, 2.5 and 1.0 µm. From these data, the volume and mass of the evaporated materials have been calculated. The volume of the ablated material is calculated from the following equation: V = πh R21 + R22 + R1 R2 =3;
alloys, spectral lines of zinc, iron, aluminum, copper and magnesium have been considered, which are presented in Table 2. 3. Results and discussions 3.1. The morphology of the crater and depth profile The morphology of the craters has been studied using optical microscope LMA-1 (Carl Zeiss). Fig. 2 shows the craters left on the ZAC surface after its irradiation with 30 laser pulses in air and in water. There is a visible difference in the craters in air and in liquid: the craters in liquid are larger in diameter and shallower. Also there is no rim around the crater, which is characteristic of the craters in air. It is due to a difference in the laser ablation in air and in liquid. Contrary to
ð1Þ
where h is depth of the crater, R1 and R2 are the crater (truncated cone) radii at the sample surface and in the crater depth. The resulting average volume of the ablated sample matter has been found equal to 4.2 · 10− 5 cm3. For the alloys under consideration it gives the mass value of 3 · 10− 4 g. 3.2. Analyte solution study The transmission electron microscopy (TEM) was used to study particles in colloidal solutions. The average diameters of the particles formed in the solution were estimated from the TEM micrographs of the evaporated drop of the solution on the carbon-coated copper grid at the Hitachi H-800 microscope. Normally, 100 or more particles were counted to determine the size distribution of each sample. Fig. 4A shows a distribution on size of the particles formed in the liquid after the laser ablation. It follows from the figure, that the particles are rather uniform in size with a mean diameter around 10 nm. The photomicrography of particles in the analyte solution is presented on the Fig. 4B. One can see that the particles are rather uniformly distributed inside the liquid. 3.3. Preparation of standard solutions and analytical curves plotting The mass concentration of the element Ci (ppm) and the relative standard deviation (% RSD) have been defined in agreement with the analytical curves (Fig. 5). Multi-element standard solutions prepared
Fig. 2. Photomicrography of the erosion craters on the sample surface after its irradiation by 30 laser pulses in liquid (A) and in air (B).
Fig. 3. Profiles of the erosion craters on the sample surface irradiated with 30 laser pulses in liquid (squares) and in air (rhombs). R1 and R2 are the radii used at the ablated volume evaluation using the truncated cone approximation.
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the sample. The measured mass concentration of the element in 1 l of the solvent is proportional to the mass concentration that is calculated for given volume. If total concentration of all elements in the sample is
Fig. 4. Distribution of the analyte particles in size (A) and photomicrography of the nanosized particles in the colloidal analyte solution (B).
daily by dilution of ICP-standard (CertiPUR, Merk, Darmstadt, Germany) in 3% (m/m) HNO3 have been used for the device calibration. The mass concentration of elements in the standard solution is 1 g/l. For every element under measurement we have prepared three standard solutions: Zn—0–5–15–20 mg/l, Al—0–0.5– 1–2 mg/l, Cu—0–0.5–1–2 mg/l, Mg—0–0.2–0.5–1 mg/l, Fe — 0–0.02– 0.05–0.1 mg/l. Analytical curves (Fig. 5) give a relation between relative intensity values and the concentration of elements in the analyte solution. Linear fits for all data points can be characterized with the average correlation coefficient R2 = 0.9983, 0.996, 0.9996, 0.9976 and 0.9993, respectively. The data for four ZAC samples prepared using both modes are presented along with analytical curves in Fig. 5. Mass concentration of the elements (mg/l) is calculated taking into account the concentration in the standard solution. As far as a deviation of the measured values does not prevail over the measurement errors, one comes to the conclusion that the multielement standards can be applied also in the quantitative measurements of the element concentration in the solutions prepared using laser ablation. In Table 3 we compare mass concentration of the elements for the samples prepared using two modes. The values for mass concentration of zinc have been reached after the 100 times sample dilution in order to fit the signal intensity values to the analytical curves. The values obtained are proportional to mass of the sample and to volume of the analyte solutions. The mass values at the laser ablation mode are less about three orders of magnitude than ones at the chemical digestion. However, the device sensitivity allows us to obtain quantitative data for the elemental compositions with a sufficient accuracy. 3.4. Calculation of the element percentage concentration in the alloy At the ICP-OES measurements, the mass concentration Cppm of the elements is defined as the element content (mg) in one liter (l) of the solvent. Percentage elements concentration C% in the alloy is calculated taking into account volume of the solvent and mass of
Fig. 5. Calibration curves for Al, Fe, Cu, Mg, Zn and the measured relative intensity values for the ZAC samples (No 11 — squares, No 13 — triangles, No 22 — rhombs, No25 — circles) prepared by the chemical digestion (open marks) and by the laser ablation (dark marks).
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Table 3 Mass concentration of the elements determined with ICP-OES at the sample preparation by chemical digestion (1) and by laser ablation in liquid (2) Sample
No 11
(1) (2) No 13 (1) (2) No 22 (1) (2) No 25 (1) (2)
Zn
Fe
Al
Cu
213.856
259.940
396.152
324.754
mg/l
%RSD mg/l
18.68 934.0 17.72 886.0 16.86 843.0 16.7 835.0
0.91 2.35 1.02 1.89 1.16 1.93 1.61 2.27
%RSD mg/l
0.5 10.20 22.1 0.022 1.52 0.54 1.4 7.11 60.3 0.06 1.79 1.562 1.2 7.32 110.5 0.084 2.33 2.406 0.7 4.83 55.3 0.028 1.87 1.356
Mg 279.553
%RSD mg/l
%RSD mg/l
%RSD
1.15 3.33 0.55 1.79 0.69 2.27 1.41 1.73
1.91 0.73 1.29 1.78 0.58 2.13 1.32 0.83
0.65 0.46 0.95 0.89 0.74 0.50 0.49 0.12
16 0.33 7 0.136 24.6 0.618 64.6 1.57
0.386 36.8 0.318 6.6 0.028 1.5 0.152 4.5
Our comparative analysis of the obtained results shows some deviation of measured concentrations from the reference data for the samples prepared in different ways. However, the general tendency in the measured values allows to conclude that the application of the laser ablation in liquid for a sample preparation of solids for the quantitative measurements using ICP-OES is reasonable. The accuracy of the measurements for Fe and Mg can be increased by selection of special conditions, but in this work we have performed the measurements for all elements in a single run. So, we can propose the technique for quantitative measurements of the element concentration for contents of the impurities over 0.5%. For lower contents, the technique can be used for a qualitative analysis. 4. Conclusion
Table 4 Percentage concentration of the elements determined with ICP-OES at the sample preparation by chemical digestion (1), by laser ablation in the liquid (2) and the reference data (3) Sample No 11
No 13
No 22
No 25
(1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3)
Zn, %
Fe, %
Al, %
Cu, %
Mg, %
93.40 92.37 93.50 88.60 91.93 88.50 84.30 84.30 84.30 83.50 84.47 83.40
0.15 0.05 0.11 0.25 0.14 0.30 0.43 0.12 0.42 0.19 0.07 0.14
2.75 2.21 2.70 8.77 6.03 8.81 12.01 11.05 12.03 7.29 5.53 7.50
1.67 1.60 1.65 0.70 0.70 0.68 3.10 2.46 3.09 8.21 6.46 8.19
1.90 3.68 1.93 1.55 0.66 1.59 0.12 0.15 0.14 0.79 0.45 0.76
The method of sample preparation is considered in the elemental composition analysis of metal alloys using ICP-OES. The method allows quantitative measuring elemental compositions of materials that have low chemical digestions. A solid sample is transformed into a colloidal solution of nanosized (10–15 nm) particles in a liquid medium by means of laser ablation. The technique has the following advantages. There is no waste of material in the form of insoluble residues. The suppression action of acids is excluded. The particles sizes in the laser prepared solution are low enough to fit flow capacity of the ICP device. A traditional calibration of the ICP-OES device that is based on standard multi-element solutions can be used. The technique can be applied to measure elemental composition in solid materials, which have a low solubility in acids. The technique needs further investigations to extend the range of materials to be analyzed and to lower limits of the concentration to be measured. Acknowledgement
equal to 100%, then percentage concentration for a given element (С%) can be found from the equation: Ck = Cppm V ðm K Þ−1 ;
The authors are thankful to Prof. S. Gaponenko for critical reading of the manuscript.
ð2Þ
where Cppm is mass concentration measured with ICP-OES (mg/l), V is analyte solution volume (ml), (for the samples prepared by chemical dissolution V = 200 ml, for the samples prepared by laser ablation V = 15 ml), m is mass of the substance under investigation (g) (for the samples prepared by chemical dissolution m = 0.2 g, for the samples prepared by laser ablation m = 0.0003 g), K is a transformation coefficient for the measurement units, K=10,000. If one knows a reference value of the percentage concentration, the mass concentration measured with ICP-OES and the analyte solution volume, one can calculate mass of the material from Eq. (2). In this a way we have calculated also mass of the ablated material, and both results give the value of 0.0003 g. As far as the measurement accuracy of Zn, Al, Cu mass content is high enough (the RSD values are between 0.73 and 2.35%, see Table 3) the determination of the ablated material mass is high enough. The RSD values for the samples prepared by the laser ablation in liquid are between 0.46 and 3.33%. RSD is rather high in the case of Fe (4.83–10.2%), which can be related to the fact that Fe concentration in all samples is much less (lower 0.5%) than that of other elements, and relative intensity of Fe I 259.94 line has the least sensitivity (see Table 2). Quantitative data of percentage element concentration are presented in Table 4. A good agreement with reference values is observed for Al, Zn and Cu percentage concentration. However, sample mass obtained in the laser ablation that is equal to 0.3 mg does not allow to take into account the sample inhomogeneity. For example in No 11 sample, one observes an overestimated concentration of Mg, which can obviously have a non-uniform distribution in alloys. For other ZAC samples we observe more accurate conformity of the concentration.
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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Laser ablation in liquids as a new technique of sampling in elemental analysis of solid materials☆ E.V. Muravitskaya ⁎, V.A. Rosantsev, M.V. Belkov, E.A. Ershov-Pavlov, E.V. Klyachkovskaya B.I. Stepanov Institute of Physics, National Academy of Sciences, 70, Nezalezhnastsi Ave. 220072 Minsk, Belarus
a r t i c l e
i n f o
Article history: Received 30 October 2007 Accepted 12 November 2008 Available online 27 November 2008 Keywords: Sample preparation technique Laser sampling ICP-OES Elemental analysis
a b s t r a c t Laser ablation in liquid media is considered as a new sample preparation technique in the elemental composition analysis of materials using optical emission spectroscopy of inductively coupled plasma (ICP-OES). Solid samples are transformed into uniform colloidal solutions of nanosized analyte particles using laser radiation focused onto the sample surface. High homogeneity of the resulting solution allows performing the ICP-OES quantitative analysis especially for the samples, which are poorly soluble in acids. The technique is compatible with the conventional solution-based standards. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Heating materials to the plasma state by means of focused laser beams to excite the material emission spectra finds increasingly wide application in elemental analysis of samples by the optical emission spectroscopy (OES) in different fields of human activity. Many successful examples can be found of the laser-induced breakdown spectroscopy (LIBS) application for industry, art, environmental, medical, forensic sciences, etc. [1–4]. Recently the laser ablation in liquids is also developed as applied to the synthesis of nanosized particles [5]. Moreover, the laser ablation in liquids can be used for the sample preparation of solids as well in their chemical composition analysis by the optical emission spectroscopy of the inductively coupled plasma (ICP-OES) [6–8], and further investigations are necessary to provide the technique usability in the ICP-OES elemental analysis. Nowadays, ICP-OES is one of the major spectrometry techniques for quantitative analysis of elemental composition of different materials [4,9–11]. It offers the opportunity to measure content of multiple chemical elements in a sample in a single run, and it provides a comparatively low detection limit. However, the analyzed samples are obviously introduced into the ICP-OES spectrometers in solutions, and the sample preparation is necessary before the analysis, which includes the sample destruction and mineralization [10]. So, the sample preparation is one of the most important stages of the elemental composition analysis. Most popular chemical techniques of the sample destruction and mineralization do not always allow total digestion of the ☆ This paper was presented at the Euro Mediterranean Symposium on Laser Induced Breakdown Spectroscopy (EMSLIBS 2007) held in Paris (France), 11–13 September 2007. ⁎ Corresponding author. E-mail address: [email protected] (E.V. Muravitskaya). 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.11.003
analyzed substances [12]. Many materials (especially it concerns archeological and environmental objects) have very low solubility in acids. The incomplete transition of a sample into the solution (formation of sediments) results in a partial sample loss, giving rise to the measurement total error. Sometime, concentrated and aggressive acids and alkali are used to increase the sample digestion. However, the increase of the acid content in the solution under analysis results in the analytical signal suppression [13–15]. The suppression rate depends on the acid nature, and it is proportional to the acid concentration. As far as density and viscosity of acids are higher than ones of water, the suppression is mainly related to the processes of the solution transportation. Partly it is also due to a decrease of the plasma temperature because of energy losses for atomization of the acid molecules. The common internal standard technique can not eliminate the effect. Thus, one should take into account that uncontrolled changes in the acid concentration can raise the measurement error prevailing over the instrumental ones [15]. It also should be noted that the best detection limit can be reached only at higher quantity of the sample dissolved in lower solvent volume, which enhances the sample preparation problems. As a result, the preparation of certain samples is a very complex and time-consuming procedure. In this work, an alternative technique for the sample preparation of solids in their ICP-OES elemental composition analysis is considered. The solid samples are transformed into colloidal solutions of nanosized analyte particles by means of laser ablation (LA). Nowadays, the LA technique is commonly used in the synthesis of nanoparticles having controlled dimensions [5,16,17]. The laser radiation focused onto a surface of a solid sample in a liquid medium provides sample ablation resulting in the formation of nanoparticles, their composition corresponding to that of the sample. The prepared solutions are further directly used in the common procedure of the ICP-OES measurements.
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The proposed techniques have been tested using reference standard samples with known compositions. Two modes of the sample preparation techniques have been applied: (i) the sample common
chemical destruction and (ii) the sample laser ablation in liquid. The samples obtained using the both preparation modes have been analyzed by means of the ICP-OES techniques. For this, calibration curves using
Fig. 1. General view of the laser spectral analyzer (LSA) (A) and block-diagram of the LSA setup (B). Insertions show the beaker used.
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ICP standards have been plotted. Then, the solutions prepared have been analyzed to determine their concentration from the calibration curves, and the signal intensity has been plotted versus the measured concentration. The elemental analysis results have shown that the LA in liquid can be applied as the sample preparation technique in the ICPOES elemental composition measurements, and the technique is compatible with the common solution-based standardization. 2. Experimental 2.1. ICP-OES setup In ICP-OES measurements a THERMO ELECTRON Model IRIS Intrepid II ICP optical spectrometer was used [18]. The spectrometer is equipped with Echelle optics and a Charge Injection Device (CID) solid state detector. The spectrometer provides coverage of the wavelength range from 165 to 1000 nm. Measurements are controlled by Thermo Electron Validated Analysis (TEVA) software. Principle line intensities can be corrected for the corresponding backgrounds at selectable wavelengths, and the net line intensities are automatically calculated for pre-selected integration time for analytical curves plotting. Double optics is used to decrease the negative effect of torch wandering on image stability. Emission of the middle zone of the torch column is imaged by a band light fibers onto the entrance slit of the spectrometer. The applied spectrometer makes also possible the time resolution of spectra, i.e., line intensity versus torching time plots can be made simultaneously for 15 (with single read out time of about 0.5s) elements, called “time-scans” of spectra. Spectral lines together with selected background signals (at one or both sides of the spectral lines) can be recorded, and also net signals are elucidated by the software. The timescans (non-integrated and integrated) and the analytical curves can be graphically represented together with numerical data. 2.2. Laser ablation setup Laser ablation of the samples under study has been performed using an LA optical unit of the Laser Spectra chemical Analyzer (LSA) designed for the LIBS analysis of different materials [19]. The analyzer general view and its block-diagram are presented in Fig. 1A and B, respectively. LSA includes a Q-switched Nd:YAG double pulse laser, the LA optical unit consisting of a microscope, an imaging CCD-camera, a PC-controlled sample table and a spectrometer with a CCD-arrays recording system. The laser wavelength is 1064 nm. The laser pulses duration and the repetition rate can be controlled in the intervals of 10–20 ns and 1–15 Hz, respectively. The pulses energy is also variable and can be set as high as 100 mJ. The laser can work both in the singleand double-pulse modes at the collinear scheme of the laser beams action on a sample. A time delay between the pulses can be varied from 0 to 140 µs. The laser action zone is imaged at the PC monitor and can be positioned on a sample with 0.01 mm accuracy. LSA is equipped with a 1-meter grating spectrometer. Spectral range and spectral resolution of the spectrometer are 200–800 nm and 20,000, respectively. 2.3. Model experiment The proposed technique of the sample preparation has been tested using the elemental composition of calibration standards of zinc– aluminum–copper alloys (ZAC) No 11, 13, 22, 25, elemental composition of which is presented in Table 1. The sample solutions of the reference materials for the ICP-OES measurements have been obtained using both techniques — a common chemical digestion and the laser ablation. All solutions have been prepared with MilliQ purified water (Millipore System Eschborn, Germany). Standard solutions were prepared by dilution the ICP-standards (CertiPUR, Merk, Darmstadt, Germany) in 3% (m/m) HNO3.
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Table 1 Reference data of the percentage concentration of elements in the ZAC samples El. Zn Fe Al Cu Mg
Concentration, % No 11
No 13
No 22
No 25
93.50 0.11 2.70 1.65 1.93
88.50 0.30 8.81 0.68 1.59
84.30 0.42 12.03 3.09 0.14
83.40 0.14 7.50 8.19 0.76
2.3.1. Sample preparation by the chemical digestion The digestion in chemical reagents has allowed complete destroying the material under investigation irrespective of the form in which it previously existed [9,20–22]. The chemical dissolution of the alloys has been performed in several stages. The selected mass of 200 mg of the sample has been weighted using Satogosm RU-LV-210-A balance and put into a measured vessel. Such a rather high sample quantity has been chosen to exclude inhomogeneity of the resulting solution, to increase a signal, and thus to lower the measurement errors. The concentrated nitric acid has been used, which was added to the sample in small portions. Further, the solution has been heated up to the sample complete dissolution. After the cooling, the solution has been set into a standard 200 ml vessel, deionized water has been added to obtain exactly 200 ml volume, and then has been thoroughly mixed up. 2.3.2. Sample preparation by the laser ablation In the LA sample preparation, the laser ablation in liquid was used to transform solid samples into colloidal analyte solutions. The sample has been put into distilled water in a 15 ml beaker, and the beaker has been positioned on the LSA sample table (see Fig. 1). The laser beam has been focused on the sample surface. A double-pulse mode of the laser action on the sample has been realized, because it provides up to ten times higher ablation than the single-pulse one [23]. The following working regime of the laser has been used: single pulse duration of 10–12 ns with the energy of 50 mJ, which provided a mean power density of 1010 W/cm2 at the sample surface. The double-pulse repetition rate of 10 Hz with a delay of 8 μs between the pulses has been used. The regime has been chosen to obtain practically homogeneous analyte solution, where particles of 10–15 nm prevail. The particle size was chosen in accordance with the earlier observations [17] to assure maximum solution uniformity and stability against sedimentation, which is required to increase the accuracy of ICP measurement. The upper limit of particle diameter is determined by pass capacity of the ICPOES spectrometer spray [18]. The laser radiation acted in one spot of the sample during about 3 s. Then the sample table has been moved to a new position. The sample preparation procedure took totally 80 s, leaving 27 erosion craters on the sample surface. After 80 s exposition of the laser radiation, one could observe a visible gray coloration of the solution, which indicated the nanoparticles formation. As far as the solution was not stabilized, the ICP-OES concentration measurements have been performed within an hour after the solution preparation. 2.3.3. Selection of spectral lines The TEVA software makes possible the visual inspection of the preselected spectral ranges (intensity versus wavelength scans) for the selection of the background values in either of sides of the spectral lines to be corrected. It is a general practice with this instrumentation to select 2–3 spectral lines of every element in a preliminary study of the calibration curves with regard to the crossing point on the intensity axes (ordinate). Optical Echelle scheme allows using very high orders of the spectrum from the thirty to the two hundred and even above. Some analytical lines can appear in the spectrum more than once, and the same line can have different intensities in different orders of a spectrum. Therefore not only a line, but also its order is important. For example, iron of 238.204 nm is observed in two different orders (140) (141), but in this case intensities of these are identical. In the investigation of ZAC
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Table 2 Data on the spectral lines of Zn, Fe, Al, Cu and Mg used for the measurements El./ionization state/wavelength (nm)
Order of the spectrum
Relative intensity
Al I 396.152 Al I 394.401 Cu I 324.754 Cu I 327.396 Fe II 259.940 Fe II 238.204 Fe II 238.204 Mg II 279.553 Mg II 280.271 Mg I 285.213 Zn I 213.856 Zn II 206.200
85 85 103 102 129 141 140 120 120 117 157 163
350,000 250,000 5,000,000 3,000,000 2,000,000 1,800,000 1,800,000 50,000,000 15,000,000 6,000,000 3,000,000 900,000
air, no resolidification is observed of the material around the edges of the craters after melting in a liquid. All solid material is ablated in the form of vapor and liquid-drop phases and is left in the liquid [24]. It allows calculating the evaporated material volume as the volume of the erosion crater. Depth profiles of the craters in air and in water are presented in the Fig. 3. The crater profiles in water have been approximated by a truncated cone. Averaged values of a diameter and depth of the craters are 470–490 µm and 20–21 µm, respectively. The measurement precision of the diameter and depth is estimated to be, respectively, 2.5 and 1.0 µm. From these data, the volume and mass of the evaporated materials have been calculated. The volume of the ablated material is calculated from the following equation: V = πh R21 + R22 + R1 R2 =3;
alloys, spectral lines of zinc, iron, aluminum, copper and magnesium have been considered, which are presented in Table 2. 3. Results and discussions 3.1. The morphology of the crater and depth profile The morphology of the craters has been studied using optical microscope LMA-1 (Carl Zeiss). Fig. 2 shows the craters left on the ZAC surface after its irradiation with 30 laser pulses in air and in water. There is a visible difference in the craters in air and in liquid: the craters in liquid are larger in diameter and shallower. Also there is no rim around the crater, which is characteristic of the craters in air. It is due to a difference in the laser ablation in air and in liquid. Contrary to
ð1Þ
where h is depth of the crater, R1 and R2 are the crater (truncated cone) radii at the sample surface and in the crater depth. The resulting average volume of the ablated sample matter has been found equal to 4.2 · 10− 5 cm3. For the alloys under consideration it gives the mass value of 3 · 10− 4 g. 3.2. Analyte solution study The transmission electron microscopy (TEM) was used to study particles in colloidal solutions. The average diameters of the particles formed in the solution were estimated from the TEM micrographs of the evaporated drop of the solution on the carbon-coated copper grid at the Hitachi H-800 microscope. Normally, 100 or more particles were counted to determine the size distribution of each sample. Fig. 4A shows a distribution on size of the particles formed in the liquid after the laser ablation. It follows from the figure, that the particles are rather uniform in size with a mean diameter around 10 nm. The photomicrography of particles in the analyte solution is presented on the Fig. 4B. One can see that the particles are rather uniformly distributed inside the liquid. 3.3. Preparation of standard solutions and analytical curves plotting The mass concentration of the element Ci (ppm) and the relative standard deviation (% RSD) have been defined in agreement with the analytical curves (Fig. 5). Multi-element standard solutions prepared
Fig. 2. Photomicrography of the erosion craters on the sample surface after its irradiation by 30 laser pulses in liquid (A) and in air (B).
Fig. 3. Profiles of the erosion craters on the sample surface irradiated with 30 laser pulses in liquid (squares) and in air (rhombs). R1 and R2 are the radii used at the ablated volume evaluation using the truncated cone approximation.
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the sample. The measured mass concentration of the element in 1 l of the solvent is proportional to the mass concentration that is calculated for given volume. If total concentration of all elements in the sample is
Fig. 4. Distribution of the analyte particles in size (A) and photomicrography of the nanosized particles in the colloidal analyte solution (B).
daily by dilution of ICP-standard (CertiPUR, Merk, Darmstadt, Germany) in 3% (m/m) HNO3 have been used for the device calibration. The mass concentration of elements in the standard solution is 1 g/l. For every element under measurement we have prepared three standard solutions: Zn—0–5–15–20 mg/l, Al—0–0.5– 1–2 mg/l, Cu—0–0.5–1–2 mg/l, Mg—0–0.2–0.5–1 mg/l, Fe — 0–0.02– 0.05–0.1 mg/l. Analytical curves (Fig. 5) give a relation between relative intensity values and the concentration of elements in the analyte solution. Linear fits for all data points can be characterized with the average correlation coefficient R2 = 0.9983, 0.996, 0.9996, 0.9976 and 0.9993, respectively. The data for four ZAC samples prepared using both modes are presented along with analytical curves in Fig. 5. Mass concentration of the elements (mg/l) is calculated taking into account the concentration in the standard solution. As far as a deviation of the measured values does not prevail over the measurement errors, one comes to the conclusion that the multielement standards can be applied also in the quantitative measurements of the element concentration in the solutions prepared using laser ablation. In Table 3 we compare mass concentration of the elements for the samples prepared using two modes. The values for mass concentration of zinc have been reached after the 100 times sample dilution in order to fit the signal intensity values to the analytical curves. The values obtained are proportional to mass of the sample and to volume of the analyte solutions. The mass values at the laser ablation mode are less about three orders of magnitude than ones at the chemical digestion. However, the device sensitivity allows us to obtain quantitative data for the elemental compositions with a sufficient accuracy. 3.4. Calculation of the element percentage concentration in the alloy At the ICP-OES measurements, the mass concentration Cppm of the elements is defined as the element content (mg) in one liter (l) of the solvent. Percentage elements concentration C% in the alloy is calculated taking into account volume of the solvent and mass of
Fig. 5. Calibration curves for Al, Fe, Cu, Mg, Zn and the measured relative intensity values for the ZAC samples (No 11 — squares, No 13 — triangles, No 22 — rhombs, No25 — circles) prepared by the chemical digestion (open marks) and by the laser ablation (dark marks).
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Table 3 Mass concentration of the elements determined with ICP-OES at the sample preparation by chemical digestion (1) and by laser ablation in liquid (2) Sample
No 11
(1) (2) No 13 (1) (2) No 22 (1) (2) No 25 (1) (2)
Zn
Fe
Al
Cu
213.856
259.940
396.152
324.754
mg/l
%RSD mg/l
18.68 934.0 17.72 886.0 16.86 843.0 16.7 835.0
0.91 2.35 1.02 1.89 1.16 1.93 1.61 2.27
%RSD mg/l
0.5 10.20 22.1 0.022 1.52 0.54 1.4 7.11 60.3 0.06 1.79 1.562 1.2 7.32 110.5 0.084 2.33 2.406 0.7 4.83 55.3 0.028 1.87 1.356
Mg 279.553
%RSD mg/l
%RSD mg/l
%RSD
1.15 3.33 0.55 1.79 0.69 2.27 1.41 1.73
1.91 0.73 1.29 1.78 0.58 2.13 1.32 0.83
0.65 0.46 0.95 0.89 0.74 0.50 0.49 0.12
16 0.33 7 0.136 24.6 0.618 64.6 1.57
0.386 36.8 0.318 6.6 0.028 1.5 0.152 4.5
Our comparative analysis of the obtained results shows some deviation of measured concentrations from the reference data for the samples prepared in different ways. However, the general tendency in the measured values allows to conclude that the application of the laser ablation in liquid for a sample preparation of solids for the quantitative measurements using ICP-OES is reasonable. The accuracy of the measurements for Fe and Mg can be increased by selection of special conditions, but in this work we have performed the measurements for all elements in a single run. So, we can propose the technique for quantitative measurements of the element concentration for contents of the impurities over 0.5%. For lower contents, the technique can be used for a qualitative analysis. 4. Conclusion
Table 4 Percentage concentration of the elements determined with ICP-OES at the sample preparation by chemical digestion (1), by laser ablation in the liquid (2) and the reference data (3) Sample No 11
No 13
No 22
No 25
(1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3)
Zn, %
Fe, %
Al, %
Cu, %
Mg, %
93.40 92.37 93.50 88.60 91.93 88.50 84.30 84.30 84.30 83.50 84.47 83.40
0.15 0.05 0.11 0.25 0.14 0.30 0.43 0.12 0.42 0.19 0.07 0.14
2.75 2.21 2.70 8.77 6.03 8.81 12.01 11.05 12.03 7.29 5.53 7.50
1.67 1.60 1.65 0.70 0.70 0.68 3.10 2.46 3.09 8.21 6.46 8.19
1.90 3.68 1.93 1.55 0.66 1.59 0.12 0.15 0.14 0.79 0.45 0.76
The method of sample preparation is considered in the elemental composition analysis of metal alloys using ICP-OES. The method allows quantitative measuring elemental compositions of materials that have low chemical digestions. A solid sample is transformed into a colloidal solution of nanosized (10–15 nm) particles in a liquid medium by means of laser ablation. The technique has the following advantages. There is no waste of material in the form of insoluble residues. The suppression action of acids is excluded. The particles sizes in the laser prepared solution are low enough to fit flow capacity of the ICP device. A traditional calibration of the ICP-OES device that is based on standard multi-element solutions can be used. The technique can be applied to measure elemental composition in solid materials, which have a low solubility in acids. The technique needs further investigations to extend the range of materials to be analyzed and to lower limits of the concentration to be measured. Acknowledgement
equal to 100%, then percentage concentration for a given element (С%) can be found from the equation: Ck = Cppm V ðm K Þ−1 ;
The authors are thankful to Prof. S. Gaponenko for critical reading of the manuscript.
ð2Þ
where Cppm is mass concentration measured with ICP-OES (mg/l), V is analyte solution volume (ml), (for the samples prepared by chemical dissolution V = 200 ml, for the samples prepared by laser ablation V = 15 ml), m is mass of the substance under investigation (g) (for the samples prepared by chemical dissolution m = 0.2 g, for the samples prepared by laser ablation m = 0.0003 g), K is a transformation coefficient for the measurement units, K=10,000. If one knows a reference value of the percentage concentration, the mass concentration measured with ICP-OES and the analyte solution volume, one can calculate mass of the material from Eq. (2). In this a way we have calculated also mass of the ablated material, and both results give the value of 0.0003 g. As far as the measurement accuracy of Zn, Al, Cu mass content is high enough (the RSD values are between 0.73 and 2.35%, see Table 3) the determination of the ablated material mass is high enough. The RSD values for the samples prepared by the laser ablation in liquid are between 0.46 and 3.33%. RSD is rather high in the case of Fe (4.83–10.2%), which can be related to the fact that Fe concentration in all samples is much less (lower 0.5%) than that of other elements, and relative intensity of Fe I 259.94 line has the least sensitivity (see Table 2). Quantitative data of percentage element concentration are presented in Table 4. A good agreement with reference values is observed for Al, Zn and Cu percentage concentration. However, sample mass obtained in the laser ablation that is equal to 0.3 mg does not allow to take into account the sample inhomogeneity. For example in No 11 sample, one observes an overestimated concentration of Mg, which can obviously have a non-uniform distribution in alloys. For other ZAC samples we observe more accurate conformity of the concentration.
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