Laser-Induced Breakdown Spectroscopy (LIBS) Analysis of Calcium Ions Dissolved in Water Using Filter Paper Substrates: An Ideal Internal Standard for Precision Improvement Daewoong Choi,a Yongdeuk Gong,a Sang-Ho Nam,a,* Song-Hee Han,b Jonghyun Yoo,c Yonghoon Leea,* a b c

Department of Chemistry, Mokpo National University, Jeonnam 534-729, Korea Division of Maritime Transportation System, Mokpo National Maritime University, Jeonnam 530-729, Korea Applied Spectra, 46665 Fremont Boulevard, Fremont, CA 94538 USA

We report an approach for selecting an internal standard to improve the precision of laser-induced breakdown spectroscopy (LIBS) analysis for determining calcium (Ca) concentration in water. The dissolved Ca2þ ions were pre-concentrated on filter paper by evaporating water. The filter paper was dried and analyzed using LIBS. By adding strontium chloride to sample solutions and using a Sr II line at 407.771 nm for the intensity normalization of Ca II lines at 393.366 or 396.847 nm, the analysis precision could be significantly improved. The Ca II and Sr II line intensities were mapped across the filter paper, and they showed a strong positive shot-to-shot correlation with the same spatial distribution on the filter paper surface. We applied this analysis approach for the measurement of Ca2þ in tap, bottled, and ground water samples. The Ca2þ concentrations determined using LIBS are in good agreement with those obtained from flame atomic absorption spectrometry. Finally, we suggest a homologous relation of the strongest emission lines of period 4 and 5 elements in groups IA and IIA based on their similar electronic structures. Our results indicate that the LIBS can be effectively applied for liquid analysis at the sub-parts per million level with high precision using a simple drying of liquid solutions on filter paper and the use of the correct internal standard elements with the similar valence electronic structure with respect to the analytes of interest. Index Headings: Laser-induced breakdown spectroscopy; LIBS; Internal standard; Calcium; Water quality; Precision; Reference signal.

INTRODUCTION Water, a simple polar solvent, contains a wide variety of solutes such as cations, anions, and polar molecules. The water can also suspend various organic and inorganic particulates. Therefore, the quality of water we drink is becoming one of the most important health concerns of modern times. Over the last several decades, the knowledge about the biological effects of ions and molecules dissolved in water and the associated analytical techniques to monitor them have advanced greatly. These advances have allowed for the regulation, reinforcement, and revision of the maximum contamination levels of toxic species in the drinking Received 28 May 2013; accepted 27 September 2013. * Authors to whom correspondence should be sent. E-mail: yhlee@ mokpo.ac.kr, [email protected]. DOI: 10.1366/13-07163

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water.1 Today there are several sources of drinking water. Tap water is taken from lakes or dammed rivers and undergoes flocculation, filtration, chlorination, and fluoridation processes prior to human use. Tap water is sometimes further purified through a filtration unit, thereby reducing toxic heavy metals and organic matters. Many bottled drinking waters come directly from underground springs and wells, or are tap water with additional purification processes. Moreover, many ground water sources are used for direct human consumption without any treatment. For these drinking water sources, concentrations of many ions and molecules are important for safety and dietary reasons. The major contaminants in drinking water with detrimental effects on human health include mercury (Hg2þ), cadmium (Cd2þ), lead (Pb2þ), chromium (Cr3þ and CrO42), benzene (C6H6), trihalomethanes (CHCl3, etc.), and nitrates (NO3).2 The concentrations of alkali and alkaline earth metals such as calcium (Ca2þ) and magnesium (Mg2þ) in drinking water are also of interest because of their dietary benefits and their effect on water properties including hardness.3–5 Laser-induced breakdown spectroscopy (LIBS) is a rapidly growing elemental analysis technique utilizing emission spectra from laser ablation-generated plasma of the sample materials.6 Compared to conventional elemental analysis techniques such as inductively coupled plasma optical emission spectroscopy (ICPOES) and atomic absorption spectroscopy (AAS), LIBS provides several technical advantages that include little to no sample preparation, simple measurements in air, fast measurement time, simultaneous multi-elemental analysis, and spatial mapping and depth profiling analysis. In spite of these advantages, the limits of detection (LODs) and precision performance are perceived to be lower than by using conventional elemental analysis techniques. In particular, the LODs and precision can be worse for liquid samples due to rapid quenching of the LIBS plasma in the liquid medium and interaction between the laser-induced shockwave and surface of the liquid.7,8 Recently many alternative LIBS approaches for liquid samples involving transformation of the sample phase from liquid to solid have been reported.9–20 These LIBS approaches for the liquid samples using a solid absorbing matrix have been quite successful for improving the LODs.

0003-7028/14/6802-0198/0 Q 2014 Society for Applied Spectroscopy

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Precision is also an important analytical figure of merit for LIBS. In general, the precision of LIBS is mainly influenced by sample homogeneity and the ability to deliver consistent laser fluence on the sampled areas. Milling sample materials and mixing the powder can achieve a more homogeneous chemical composition of the sample. Alternatively, larger sample areas, enough to represent the bulk sample composition, and accumulation and averaging of LIBS intensity can improve the analysis precision without additional sample homogenizing processes. The laser fluence on the sample surface can vary with sample height, laser power fluctuation, accumulation of particles above the sample surface, etc. These can lead to variations of plasma excitation conditions and cause fluctuations or drifts of LIBS signal intensity. The effect due to the variation of laser fluence can be considerably mitigated using an appropriate reference signal. In general, weaker emission lines of the major matrix element, with upper-level energy similar to those of the analyte emission line, are often selected for the reference lines.21 In this work we report a measurement approach for improving the precision of LIBS analysis for Ca2þ ions dissolved in water. Previously we reported the details of the pre-concentration method using filter paper substrates for sub-parts per million (ppm) level LIBS analysis of dissolved ions in water.9 The greatest advantage of this method is greatly improved LODs down to a few tens of parts per billion (ppb) through a very simple process. Liquid samples are transformed to solids, which are favorable for more sensitive LIBS analysis. The analytes can also be deposited with large pre-concentration factors. The filter paper matrix can provide useful reference signals such as C I lines and the carbon-related molecular emission bands. In order to differentiate trace level concentrations even with improved detection sensitivity, high precision of the measurements is highly desirable. In our experiment the precision could be influenced by the following three factors: (1) fluctuation of the plasma excitation conditions such as plasma temperature and electron density, (2) homogeneity of the pre-concentrated Ca2þ ions on the filter paper, and (3) variation of preconcentration efficiency. Due to the structure of filter paper, a mat of cellulose microfibers, the shot-to-shot fluctuations of plasma temperature and signal intensity are quite large. The plasma temperature would be higher when the laser pulse hits the fiber body, while it would be lower when the laser beam is focused between the microfibers. During the pre-concentration process, the sample homogeneity can be influenced by solubility, ion-exchange interaction with the substrates, etc. The pre-concentration efficiency varies since all of the ions and particles in the water cannot be deposited on the filter paper. Thus, an ideal internal standard should (1) provide reference lines with spectroscopic parameters (upper-level energy and transition probability) similar to those of the analyte line, (2) have close chemical proximity to the analyte element, and (3) compensate for the variation of the pre-concentration efficiency. In order to find the optimum reference line, we investigated the LIBS intensity correlations between the analyte line and the several potential reference lines available in the

collected LIBS spectra, such as Sr I, Sr II, and Mn I lines from the compounds added to the sample and cyanide radical (CN) bands originating from the filter paper substrate and the ambient air.9,22 Of these investigated reference signals, the Sr II line at 407.771 nm was found to be remarkably effective for the intensity normalization of the analyte Ca II lines at 393.366 and 396.847 nm. The Ca II and Sr II line intensities showed highly positive shot-to-shot correlation. This is due to the close thermodynamic properties of the selected analyte and reference lines. Moreover, the spatial distributions of the Ca II and Sr II line intensity on the filter paper substrate were found to be very similar. The similar spatial intensity distribution of both the reference and analyte lines can be attributed to the close chemical proximity between Ca and Sr, which leads to similar chemical interaction with the filter paper substrate and more homogeneous mix of Ca and Sr crystals during the preconcentration process. The emission lines from the externally added elements, Sr and Mn, provide ways to compensate for the variation of pre-concentration efficiency. We applied this approach to the analysis of Ca in several drinking water samples. The determined Ca concentrations agree well with the results from flame atomic absorption spectrometry (FAAS) analysis. Finally, we suggest a homologous relation between the strongest emission lines of period 4 and 5 elements in groups IA and IIA. Like Ca and Sr, K and Rb would be ideal internal standards for each other because of the similar valence electronic structures of K and Rb, leading to close spectroscopic and chemical properties.

EXPERIMENTAL METHOD Laser-Induced Breakdown Spectroscopy (LIBS). We used two LIBS spectrometers: one is a homemade system, and the other is a fully integrated commercial instrument for this study. Details of the homemade system were reported previously.9,22 In brief, a second harmonic beam from a Q-switched Nd : YAG laser (Quantel, Brilliant b, 532 nm, 10 Hz, ;8 ns) was focused on filter paper through a plano-convex lens (f ¼ 70 mm). The laser pulse energy was 10 mJ. The plasma emission was collected by two lenses (plano-convex, f ¼ 70 mm) at an angle of ;458 from the focused laser beam path axis. Two spectrometers were coupled for the homemade system; one is a single-channel charge-coupled device (CCD) spectrometer (spectral coverage from 307 to 466 nm, ;0.1 nm resolution, Aurora, Applied Spectra, Inc.), and the other is a narrow-band high-resolution spectrometer (40 nm bandwidth, 50 cm, Czerny-Turner type, DM-500i, Dongwoo Optron, grating with 1200 grooves/ mm blazed at 500 nm) with an intensified CCD (ICCD) camera (Andor Technology, iStar, 1024 3 512 pixels). The single-channel CCD spectrometer was used for the simultaneous detection of Ca I, Ca II, Sr I, Sr II, and Mn I lines and CN bands for the line-intensity correlation study and the plasma temperature estimation. For the calibration curve and analysis of Ca in the tap, bottled, and ground water samples, the narrow-band highresolution spectrometer was used. The laser firing and the CCD/ICCD detector gating were synchronized by a digital delay generator (SRS, DG645). The CCD detection

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TABLE I. Typical concentrations of trace elements in the filter paper used in this work.

TABLE II. Concentrations of Ca, Mn, and Sr in the standard solutions.

Elements

Solution no.

Al Sb As Ba B Br Ca Cl Cr Cu F

Concentration (lg/g)

Elements

Concentration (lg/g)

1 ,0.02 ,0.02 ,1 2 1 8 55 0.7 0.2 0.3

Fe Pb Mg Mn Hg N K Si Na S Zn

3 0.1 0.7 ,0.05 ,0.005 260 0.6 ,2 8 ,2 0.3

gate with a 1.05 ms width was delayed by 1.3 ls from the laser pulse. The ICCD detection gate with a 5 ls width was delayed by 1 ls from the laser pulse. The commercial LIBS system (RT100-EC, Applied Spectra, Inc.) has broad spectral coverage from 190 to 880 nm. This instrument was used for precision investigation of different filter paper samples, long-term stability of the pre-concentrated samples, and intensity correlations of the emission lines of alkali metals (Li, Na, K, and Rb) and alkaline earth metals (Mg, Ca, Sr, and Ba). Commercial cellulose filter paper (hardened ashless filter paper, Whatman, 541, B 70 mm) was used as a preconcentration matrix without any pre-treatment. In Table I the nominal metal concentrations in the filter paper are listed.23 The concentration of Ca in the filter paper was ;8 ppm. This amount (3.8 lg in a filter paper) is comparable to the amount of Ca2þ in the most dilute standard solution (4.0 lg). In Table II the chemical composition of the standard solutions used for the calibration curves of the Ca2þ concentration is listed. Solutions no. 15 and 69 were for the analysis of Ca2þ in the tap and bottled water samples and the ground water sample, respectively. No other chemical reagent was added to the sample and standard solutions. The filter paper was placed in the crystallizing dish (150 mL, B 80 mm 3 H 45 mm), which was filled with 80 g of standard or sample solutions and placed in an oven at 150 8C for 1 hr to evaporate water. Since the aqueous solutions were not filtered but evaporated, both dissolved ions or molecules and suspended particles are deposited together on the filter paper. The dried filter paper was fixed on an aluminum cylinder block with a placement ring. The aluminum cylinder block was then mounted on the sample stage. The Ca2þ concentrations in the tap, ground, and bottled water samples were analyzed following the same procedure. The tap water samples were taken from the tap water source at Mokpo National University. The ground water was taken from a location in front of the Natural Science Building at Mokpo National University. For the bottled water samples, commercially available drinking water from South Korea (Samdasoo, obtained on Jeju Island, South Korea) was used. The Ca2þ concentration of the bottled water is indicated on the label as 2.2–3.6 ppm. To obtain the LIBS spectra of the standard solutions and the drinking water samples, the sample stage was translated at a rate of 16 mm/min.

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1 2 3 4 5 6 7 8 9

Ca (ppm)

Mn (ppm)

Sr (ppm)

2.0 3.0 4.0 5.0 6.0 0.1 0.2 0.3 0.4

200 200 200 200 200 20 20 20 20

10 10 10 10 10 1 1 1 1

Three hundred single-shot spectra were recorded on a single piece of filter paper and accumulated for a single measurement. Flame Atomic Absorption Spectrometry (FAAS). A FAAS spectrometer (iCE 3000 Series, THERMO) equipped with a deuterium (D2) background correction system was used for the determination of Ca in the tap, bottled, and ground water samples. An air-acetylenetype flame was used. The fuel flow rate and the burner height were 1.4 L/min and 11.0 mm, respectively. A hollow cathode lamp with an operating current of 6.0 mA was used as a radiation source at a wavelength of 422.7 nm with spectral band pass of 0.5 nm. The calibration curve for the determination of Ca was obtained using the five standard solutions (0.1, 1, 2, 5, and 10 ppm Ca). The standard solutions were prepared by diluting a commercial AAS standard solution (Ca 996 6 3 lg/mL in a 0.1% HNO3 solution, CGCA1-1, Inorganic Ventures) with ultrapure water (18 MX/cm). The linear dynamic range for the calibration curve of Ca (0.110 ppm) was obtained.

RESULTS AND DISCUSSION Figure 1 shows the typical LIBS spectra of the filter paper recorded using the single-channel CCD spectrometer prior to pre-concentration (lower) and that after preconcentration of the aqueous solution of Ca2þ (9 ppm), Sr2þ (10 ppm), and Mn2þ (100 ppm) ions (upper). The LIBS spectrum of the filter paper shows three CN band systems of which band heads are located around P 358, 388, and 420 nm, which are assigned to the B 2 þ Dv ¼ þ1, 0, and 1 transitions, respectively.24 The CN bands originate from the recombination of carbon and nitrogen atoms in the LIBS plasma.25 The carbon atoms are from the cellulose fiber of the filter paper and the nitrogen atoms from the surrounding air. The LIBS spectrum of the filter paper used for pre-concentration shows Ca I, Ca II, Sr I, Sr II, and Mn I lines along with the CN bands. These atomic and ionic emission lines originated from the compounds dissolved in the aqueous solution. The spectroscopic parameters such as the wavelength, the transition probability (Aupper-lower), the lower- and upperlevel energies (Elower and Eupper, respectively), and the statistical weights of the lower and upper levels (glower and gupper, respectively) of each line are listed in Table III. In order to find an ideal pair of analyte and reference lines available for the analysis of Ca, we investigated the intensity correlations of Ca ICN, Ca IMn I, Ca ISr I,

FIG. 1. LIBS spectrum of a filter paper used for pre-concentration of the aqueous solution of Ca2þ (9 ppm), Sr2þ (10 ppm), and Mn2þ (100 ppm) ions (upper) and that of filter paper prior to being used for pre-concentration (lower).

Ca ISr II, Ca IICN, Ca IIMn I, Ca IISr I, and Ca IISr II line-line or line-band pairs. The emission signals employed for this intensity correlation analysis are indicated by an asterisk in Fig. 1. Figure 2 shows the plots of the Ca II, Sr II, Ca I, Sr I, and Mn I line and the CN band intensities (ICa II, ISr II, ICa I, ISr I, IMn I, and ICN, respectively) obtained from all 1200 single-shot measurements. The intensities are integrated areas of the baseline-subtracted spectra around the corresponding line or band. For the IMn I values, the area under the three close-lying lines at 403.076, 403.307, and 403.449 nm were taken together since they could not be fully resolved using the spectrometer. The relative standard deviations (RSDs) of ICa II, ISr II, ICa I, ISr I, IMn I, and ICN are noted in the corresponding panels in Fig. 2. Comparing the intensity fluctuations of the Ca I and II lines, we can find that the Ca II line shows more spikes than those of the Ca I line. This difference in the number of spikes leads to the RSD of ICa II (0.53), which is larger than that of ICa I (0.35). Also, the spikes in the Ca I line intensity

coincide with those in the Ca II line intensity. It should be noticed that the Ca II line shows an intensity fluctuation pattern very similar to the Sr II line, and the same behavior is observed between the Ca I and Sr I lines. Both line pairs of Ca IISr II and Ca ISr I show an impressive shot-to-shot coincidence of their intensity spikes. Thus, the RSD of ISr II (0.52) is very close to that of ICa II (0.53). Likewise, the RSDs of ISr I (0.34) and ICa I (0.35) are approximately equal to each other. The Mn I line is found to show its intensity spikes coincidentally with the Ca II line intensity spikes. However, the magnitude of fluctuation is not as large as that of ICa II. Thus, the RSD of IMn I (0.43) is smaller than that of ICa II (0.53). The intensity correlation between the Ca II (or Ca I) line and the CN band does not seem strong as those of the Ca IISr II and Ca ISr I line pairs. To quantitatively compare the degree of line-intensity correlations, the correlation analysis for the observed line intensities was performed. Figure 3 shows the plots of ISr II, ISr I, IMn I, and ICN as functions of ICa II (ad) and

TABLE III. Spectroscopic parameters of the observed Ca I, Ca II, Sr I, Sr II, and Mn I lines. Species Ca I Ca I Ca I Ca II Ca II Ca II Sr I Sr II Sr II Mn I Mn I Mn I

Wavelength (nm)

Aupper-lower (3108 s1)

Elower (cm1)

422.673 430.253 445.478 317.933 393.366 396.847 460.733 407.771 421.552 403.076 403.307 403.449

2.18 1.36 0.87 3.6 1.47 1.4 2.01 1.41 1.26 0.17 0.165 0.158

0 15 315.943 15 315.943 25 414.40 0 0 0 0 0 0 0 0

Eupper (cm1) 23 38 37 56 25 25 21 24 23 24 24 24

652.304 551.558 757.449 858.46 414.40 191.51 698.452 516.65 715.19 802.25 788.05 779.32

glower

gupper

1 5 5 4 2 2 1 2 2 6 6 6

3 5 7 6 4 2 3 4 2 8 6 4

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FIG. 2.

Plots of ICN, ICa I, ISr I, IMn I, ICa II, and ISr

II

obtained from 1200 single-shot measurements for each emission signal.

ICa I (eh) together with the plot of ICa II as a function of ICa I (i). The Pearson correlation coefficient (q) values are noted for the corresponding line pairs in Fig. 3. From the intensity correlation analysis, we could find that ICa II and ISr II show the strongest positive correlation with qCa IISr II ¼ 0.990, which is consistent with the high coincidence of the Ca II and Sr II line intensity spikes. This indicates that the optimum pair of analyte and reference lines is the Ca II and Sr II line pair. When the Ca I line is selected as the analyte line, the reference line should be the Sr I line since the Ca I and Sr I lines show the second strongest positive correlation with qCa ISr I ¼ 0.925. Although the Ca I and Ca II lines originate from the same element, their correlation (qCa ICa II ¼ 0.797) is not as strongly positive as those of the Ca IISr II and the Ca ISr I line pairs.

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Based on the LIBS intensity fluctuation patterns and correlation analysis, one of the main causes of intensity fluctuation in our experiment can be attributed to shot-toshot variation of the plasma temperature. In principle, the increase in plasma temperature enhances atomic or ionic emission line intensities. The enhancement is more effective for the emission lines with the higher upperlevel energy or for the excited species in the higher ionization stage.26 The higher plasma temperature is also the same underlying mechanism of LIBS signal enhancement using double laser pulse excitation that leads to more than the number of excited electrons in higher upper-level energy states or formation of more excited species in the higher ionization stage.27,28 As discussed above about the intensity fluctuations of the Ca I and II lines, the number and magnitudes of the Ca II

FIG. 3. Plots of ISr II, ISr I, IMn I, and ICN as functions of ICa II (ad) and ICa I (eh) and that of ICa II as a function of ICa I (i). The q values are noted for the corresponding line pairs.

line intensity spike are larger than those of the Ca I line intensity spike, and the spikes of the Ca I line intensity coincide with those of the Ca II line intensity. This means that the ratio of ICa II/ICa I increased intermittently during the 1200 singlet-shot measurement due to the variation of the plasma temperature. We estimated the plasma temperature for several laser-sampling points among the 1200 single-shot measurements shown in Fig. 2 using the logarithmic form of the following coupled SahaBoltzmann relation:29 ! ! IIIij AImn gIm 2ð2pme kTÞ3=2 ln I II II ¼ ln Imn Aij gi ne h3 ðEion  DEion þ EIIi  EIm Þ ð1Þ  kT A, g, and E are the transition probability, the statistical weight for the upper level, and the upper-level energy, respectively. ne, me, h, T, and Eion are the electron density, the electron mass, Planck’s constant, the plasma temperature, and the ionization energy, respectively. The lowering of Eion in plasma, DEion, was ignored in this estimation. The subscripts, i, j, m, and n indicate upper and lower levels of the ionic emission line and

upper and lower levels of the atomic emission line, respectively. Figure 4 shows the estimated plasma temperatures as a function of the intensity ratio of the Ca II line at 396.847 nm to the Ca I line at 422.673 nm (ICa II/ICa I). The plasma temperatures were estimated using the Ca I lines at 422.673, 430.253, and 445.478 nm and the Ca II lines at 317.933, 393.366, and 396.847 nm. The average plasma temperature of the 1200 single-shot measurements was also estimated to be 12 100 K from the accumulated spectrum, which is indicated by the horizontal dashed line in Fig. 4. For the selected spikes of the Ca II line intensity with small or ignorable coincident spikes of the Ca I line intensity (ICa II/ICa I ¼ 3.04.5), the estimated plasma temperatures are in the range between 12 260 and 14 640 K, which are higher than the average plasma temperature. When the measurement precision is mainly affected by variation of the plasma temperature, the analyte and reference line pair should be homologous to effectively compensate for changes in the plasma excitation condition.21 Zorov et al. suggested the following conditions for the homologous line pairs from the considerations of thermodynamic criterion: (1) their excitation energies must be similar, (2) their intensities differ no

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compensating the Ca II line intensity fluctuation due to variations of both the plasma temperature and the electron density. On the basis of these considerations, we suggest that the Sr II line at 407.771 nm is the optimum reference line for the analysis of Ca when the Ca II lines at 393.366 and 396.847 nm, the strongest Ca lines observed from typical LIBS plasmas, are used for the analysis. The same conditions for homologous analyte and reference line pairs hold true for the Ca I and Sr I lines. However, the agreement of upper-level energy and transition probability between analyte and reference lines is better for the ionic line pair. Figure 5 shows the line intensity ratios of ICa I/ICN, ICa II/ICN, ICa I/IMnI, ICa II/IMn I, ICa I/ISr II, ICa II/ISr II, ICaI/ISr I, and ICa II/ISr I calculated from the 1200 single-shot values shown in Fig. 2. The intensity ratios are actually normalized values using their average calculated using the following equation: Normalized line ratio ¼

ðIA;j =IR;j Þ N X ð1=NÞ ðIA;j =IR;j Þ

ð2Þ

j¼1

FIG. 4. The estimated plasma temperatures as a function of the intensity ratio of the Ca II line at 396.847 nm to the Ca I line at 422.673 nm (ICa II/ICa I). The average plasma temperature of the 1200 single-shot measurements (12 100 K) is indicated by the horizontal dashed line.

more than ten times, and (3) they belong to the same ionization stage.21 From the spectroscopic parameters listed in Table III,30 we can find that the Sr II line at 407.771 nm would be an ideal reference line for the Ca II lines at 393.366 and 396.847 nm. The upper-level energy of the Sr II line is in good agreement with those of the Ca II lines at 393.366 and 396.847 nm with differences of 3.5% and 2.7%, respectively. Moreover, the transition probability of the Sr II line (1.41 3 108 s1) is very close to those of the Ca II lines (1.47 3 108 and 1.4 3 108 s1 for the lines at 393.366 and 396.847 nm, respectively). These conditions lead to the excellent intensity correlation between the Ca II and Sr II lines. In Fig. 3a the intensity correlation between the Ca II line at 393.366 nm and the Sr II line at 407.771 nm is shown. Strong positive shot-toshot intensity correlation was also observed for the line pair of Ca II at 396.847 nm and Sr II at 407.771 nm in our experiment (q ¼ 990). The emission line intensity can be also affected by the variation of electron density of the plasma through the Saha equilibrium.31 The Sr II and Ca II lines are from the species in the same ionization stage (Srþ and Caþ, respectively). The first ionization potential values of Ca and Sr are 6.11316 and 5.69484 eV, respectively. It should be noted that Ca and Sr have the closest ionization potentials among the alkaline earth metals. Therefore, the Sr II line would be very effective in

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The subscripts, A and R, indicate analyte and reference lines, respectively. N is the number of data, 1200; j indicates the data number. After the normalization, all of the intensity ratios plotted in Fig. 5 have the same average value, 1, and their standard deviations are equal to RSDs and able to be compared directly with each other. The RSDs are noted in the corresponding panels of Fig. 5. Among the intensity ratios, ICa II/ISr II shows remarkably small RSD (8.1%). The remarkably small RSD value at the single-shot measurement level would lead to sub-percent RSD when the single measurement is taken by accumulating a few hundred single-shot spectra. In addition to fluctuation of the plasma excitation condition, sample heterogeneity and variation of preconcentration efficiency can contribute to poor precision. In the early stage of this work, we intended to compensate for the variation of pre-concentration efficiency by adding the constant amount of MnCl2 to the standard and sample solutions since the pre-concentration efficiency variation cannot be corrected by any reference signal related to the main elements of the filter paper substrates. However, sometimes the use of the Mn I line as a reference line from the added MnCl2 was successful, and at other times not completely effective. One possible cause of ineffectiveness of the Mn I line for normalizing the analyte Ca II line could be the different spatial distribution of the standard (Mn2þ) and analyte (Ca2þ) species. An internal standard with chemical properties similar to those of the analyte would be better to compensate for the spatial distribution of the analytes on the filter paper. Sr is well known as the element with chemical properties closest to that of Ca. Due to this chemical proximity between Ca and Sr, Sr coexists with Ca in many Ca-bearing natural materials.32 Therefore, it may be more reasonable to employ Sr as an internal standard rather than other elements. To investigate this hypothesis, we investigated the spatial distribution of ICN, IMn I, ICa II, and ISr II on the filter

FIG. 5. The line intensity ratios of ICa I/ICN, ICa II/ICN, ICa I/IMn I, ICa II/IMn I, ICa I/ISr II, ICa II/ISr II, ICa I/ISr I, and ICa II/ISr I calculated from the 1200 singleshot values. The values in the parentheses are RSDs of the corresponding line ratios.

paper. Figure 6 shows spatial distributions of ICN, IMn I, ICa II, and ISr II for the filter paper. The inset in (a) is the image of the analyzed filter paper. For determining the Ca concentrations in the drinking water samples, we performed eight measurements of the filter paper. Each measurement consists of 300 single-shot ablations of the filter paper. During the 300 laser shot sampling, the filter paper was moved at a linear translation stage. Thus, there are eight lines on the filter paper numbered from top to bottom in Fig. 6a. Each line with a length of 24 mm

is spaced by 3.4 mm. The calibration curves and determined concentrations will be discussed below. The ICN, IMn I, ICa II, and ISr II values obtained at the eight positions were plotted for four pieces of filter paper used for pre-concentrating tap water sample (a), standard solutions with 6 ppm Ca (b) and 0.4 ppm Ca (c), and ground water samples (d). In all standard and sample solutions, the constant amounts of MnCl2 and SrCl2 were added. The intensity values were normalized by their maximum values. These data represent five typical

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FIG. 6. ICN, IMn I, ICa II, and ISr II values measured for eight line scans on the filter paper used for pre-concentrating tap water sample (a), standard solutions with 6 ppm Ca (b) and 0.4 ppm Ca (c), and ground water samples (d). In all standard and sample solutions, the constant amounts of MnCl2 and SrCl2 were added. The intensity values were normalized by their maximum values. The inset in (a) is the image of the analyzed filter paper. The eight line scan positions are indicated on the image.

patterns of the line-intensity distribution observed for the standard and sample solutions. The variations of ICN, IMn I, ICa II, and ISr II in Fig. 6a show similar trends. The Ca2þ, Sr2þ, and Mn2þ ions were deposited homogeneously on the filter paper during the pre-concentration process. In such cases the CN band and the Mn I line as well as the Sr II line could be a reasonable reference line for normalization of the analyte Ca II line intensity. However, IMn I, ICa II, and ISr II can show a slightly different distribution pattern from that of ICN. This indicates that the ions in the aqueous solution were accumulated unevenly on the filter paper surface. In the case shown in Fig. 6b, from position no. 1 to 8, ICN decreases, but on the other hand, IMn I, ICa II, and ISr II increase. The decrease of ICN could be due to sample height variation that leads to a decrease in the laser fluence. In spite of the decrease of laser fluence, the increase of IMn I, ICa II, and ISr II indicates that the amount of ions pre-concentrated on the filter paper increases gradually from position no. 1 to 8. For this case, the CN band would not be as effective as a reference signal as the Mn I and Sr II lines, which show a very close intensity trend to the analyte Ca II line. This trend is quite pronounced in Fig. 6c at the lower Ca concentration. Water may stagnate within a part of the bottom area in a

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crystallizing dish when most of the water has been evaporated during the pre-concentration process. The asymmetric distribution of ions on the filter paper would be due to this localized stagnating water at the last stage of pre-concentration. The results shown in Figs. 6a6c indicate that the CN bands produced by ablation of the filter paper substrate can only help to compensate for any fluctuation formed by the laser sampling, while on the other hand, the addition of the potential internal standard to the liquid samples can compensate for any problem in the whole process (pre-concentration and laser sampling steps). Furthermore, chemical properties of the potential internal standard would be important since that may have some influence on the distribution of the ions in the filter paper substrate. We found a case in which the distribution of IMn I is quite different from that of ICa II and ISr II and rather similar to that of ICN (see Fig. 6d). This intensity distribution pattern indicates that Mn2þ ions can be deposited on the filter paper surface with a different spatial pattern than Ca2þ and Sr2þ ions. However, in almost all the experiments we performed, ICa II and ISr II consistently show a very close spatial distribution. This can be rationalized by the close chemical proximity between Ca and Sr that leads to homogeneously mixed

FIG. 7. LIBS spectrum of tap water without any internal standard (a) and that of tap water with 10 ppm Sr and 200 ppm Mn as potential internal standards (b).

crystals of Ca and Sr compounds and similar interactions with the filter paper substrate during the pre-concentration process. Consequently, in our experiment the Sr II line at 407.771 nm is found to be the ideal reference line for the analyte Ca II line 393.366 nm. We applied the Ca IISr II line pair to the analysis of Ca in the drinking water samples. Figure 7 shows the LIBS spectrum of the tap water without any internal standard (a) and that of tap water with 10 ppm Sr and 200 ppm Mn (b). The concentrations of Ca and Sr in the tap water with no internal standard were determined to be 6.275 6 0.009 and 0.0460 6 0.0005 ppm, respectively, using ICP-OES.9 The concentration of Ca in the tap water with Sr and Mn as potential internal standards was determined to be 5.967 6 0.043 ppm using FAAS and 6.03 6 0.20 ppm using LIBS. The two tap water samples were taken from the same tap but at different times with an interval of one and a half years. However, the concentrations of Ca are not significantly different. The weak Sr II line at 407.771 nm was observed in the spectrum of the tap water without internal standards due to the small amount of Sr (46 ppm) originally contained in the tap water. The added amount of Sr for the internal standard is significantly larger than the originally contained amount of Sr by a factor of ;220. Therefore, the Sr

FIG. 8. The calibration curve in the Ca concentration range between 2 and 6 ppm (a) and that between 0.1 and 0.4 ppm (b), and the RSD values of ICa II, ICa II/ICN, ICa II/IMn I, and ICa II/ISr II (c).

originally contained in the tap water would not have a significant effect on the accuracy of our LIBS analysis of Ca using Sr as an internal standard. The Mn lines around 403 nm were not discernable in this spectrum. In the

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TABLE IV. Ca concentrations in tap, bottled, and ground water samples using LIBS and FAAS. Units are in parts per million (ppm). Sample Tap water Bottled water Ground water

LIBS

FAAS

6.03 6 0.20 3.07 6 0.12 0.321 6 0.026

5.967 6 0.043 3.185 6 0.018 0.365 6 0.003

spectrum of tap water with internal standards, strong Sr II and Mn I lines were observed (see Fig. 7b). Figure 8 shows the calibration curve in the concentration range between 2 and 6 ppm (a) and that between 0.1 and 0.4 ppm (b) and compares the RSDs of ICa II, ICa II/ ICN, ICa II/IMn I, and ICa II/ISr II (c). In both Ca concentration ranges, the experimental values of ICa II/ISr II are fitted well using a linear function with an R2 value close to 1 (R2 ¼ 0.9971 and 0.9930 in the higher and lower concentration ranges, respectively). The normalization of ICa II by ICN is not very effective in improving the analysis precision. The RSDs of both ICa II and ICa II/ICN are thus similarly about 13% to each other. This indicates that the variation of laser fluence across the filter paper was not significant. When ICa II is normalized by IMn I, the RSD values of ICa II/IMn I decrease to ;3.4%. This would compensate for the variation of pre-concentration efficiency. When the Sr II line is used as a reference line, we could obtain the best RSD values around 0.98%. The LOD of Ca in our method was estimated to be ;9 ppb using the following equation: LOD ¼

3r s

ð3Þ

where r and s are the standard deviation of the signal values obtained for the standard sample with a minimum concentration (0.1 ppm Ca in our work) and the slope of a calibration curve, respectively. Previously we reported the LODs of 75 and 18 ppb for lead (Pb) and chromium (Cr) ions dissolved in water using this filter paper method. The LOD of Ca is comparable to those of Pb and Cr. This remarkable improvement of LOD is apparently due to the simple pre-concentration method. The calibration curves shown in Figs. 8a and 8b were used to determine the concentrations of Ca in the tap, bottled, and ground water samples. In Table IV the determined Ca concentrations are listed together with the results from FAAS. The LIBS and FAAS results agree within 6(12)r of the LIBS measurements. The RSD values of the LIBS analysis are larger than those of FAAS by a few multiples. The precision between different pieces of filter paper becomes slightly worse than that within a single filter paper. Figure 9a shows the ICa II values and the ratios of ICa II/ISr II obtained from 10 pieces of filter paper used for the same tap water sample. For each piece of filter paper, eight measurements were performed, and their average was taken as the final value. The error bars in Fig. 9a represent the 1r error. The RSD of ICa II from the 10 pieces of filter paper is 48%. By using the ratio of ICa II/ISr II, the RSD value decreases to 2.8%. We also checked the repeatability of the ICa II/ISr II values measured for an aged sample. A piece of filter paper was used for pre-concentrating the tap water sample

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FIG. 9. ICa II and ICa II/ISr II values obtained from 10 pieces of filter paper used for the tap water sample (a) and the variation of ICa II/ISr II over 10 post-preparation days (b).

with 10 ppm Sr for the internal standard and stored in our laboratory. We checked the variation of ICa II/ISr II over ten post-preparation days. The ICa II/ISr II values from five single measurements for the same piece of filter paper left in open air are plotted in Fig. 9b. They agree within the RSD by 2.9%. Usually we can average eight measurement data, in which case thepmultiple measureffiffiffi ments improve the RSD to ;1% (2.9/ 8). This indicates that our pre-concentrated samples using pieces of filter paper stay stable during at least ten post-preparation days and thus can be stored this long. Herein, it is worth comparing spectroscopic parameters of the strongest lines of alkali and alkaline earth metals. In Fig. 10, Eupper and Aupper-lower values of the strongest emission lines of alkali metal atoms (a and b, respectively) and alkaline earth metal ions (c and d, respectively) are plotted. This predicts a homologous relation between the emission lines from the period 4 and 5 elements in group IA (alkali metals) and IIA (alkaline earth metals). For the alkali metals, the strongest emission lines typically observed from LIBS plasmas are due to the transition of neutral atoms from np12 P3/2,1/2 to ns12 S1/2 where n is the principal quantum number of valence electrons and also the number of the period to which the relevant elements belong (n ¼ 2 for Li, 3 for Na, 4 for K, 5 for Rb, and 6 for Cs). The strongest

FIG. 10. Eupper and Aupper-lower values of the strongest emission lines of alkali metal atoms (a) and (b), respectively, and alkaline earth metal ions (c) and (d), respectively. In (a) and (b), for each element, two values are plotted since the Li I (670.776 and 670.791 nm), Na I (588.995 and 589.592 nm), K I (766.490 and 769.896 nm), Rb I (780.027 and 794.760 nm), and Cs I (852.113 and 894.347 nm) lines are doublets. Likewise, in c and d, two values are plotted for the strongest doublet lines of Be II (313.042 and 313.107 nm), Mg II (279.553 and 280.271 nm), Ca II (393.366 and 396.847 nm), Sr II (407.771 and 421.552 nm), and Ba II (455.403 and 493. 408 nm).

lines are doublets since the upper level is split into two spin-orbit sublevels with total angular momentum quantum number J ¼ 3/2 and 1/2. Thus, two values for each spectroscopic parameter, Eupper and Aupper-lower, are plotted for each element (Li I at 670.776 and 670.791 nm, Na I at 588.995 and 589.592 nm, K I at 766.490 and 769.896 nm, Rb I at 780.027 and 794.760 nm, and Cs I at 852.113 and 894.347 nm lines) in Figs. 10a and 10b, respectively. As indicated in these plots, the strongest emission lines of the period 4 and 5 elements, K and Rb, respectively, have very similar Eupper and Aupper-lower values. Thus, the line pair is predicted to show the relation of a homologous line pair. For the alkaline earth metal elements, the strongest lines of the ionic species in the ionization stage II are through the transition from np12 P1/2,3/2 to ns12 S1/2 and thus are doublets due to the splitting of the upper levels via spin-orbit interaction (Be II 313.042 and 313.107 nm, Mg II 279.553 and 280.271 nm, Ca II 393.366 and 396.847 nm, Sr II 407.771 and 421.552 nm, and Ba II 455.403 and 493.408 nm). Like the alkali metals, the elements in periods 4 (Ca) and 5 (Sr) are

found to show emission lines with very similar spectroscopic parameters; the Ca IISr II line pairs are identified as the homologous line pairs. Moreover, the ionization potentials of the period 4 and 5 elements belonging to the same group are closest to each other, as shown in Fig. 11. This would lead to effective compensation of electron-density variation when the emission lines from the period 4 and 5 elements are selected as the analyte-reference line pair. Our observation of the close correlation of the Ca II and Sr II emission lines is consistent with these discussions. Based on our observation and discussion of spectroscopic parameters of the alkali and alkaline earth metals, we suggest that the period 4 and 5 elements of groups IA and IIA provide the best reference line for each other. The underlying physics of this phenomenon is the similar valence electronic structures of period 4 and 5 elements in groups IA and IIA. The 10 electrons in the inner 3d orbitals of the period 5 elements (Rb and Sr) are not effective at shielding outer electrons from the positive nuclear charge.33 This increases the effective

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FIG. 11. Ionization potentials of alkali metals (a) and alkaline earth metals (b).

nuclear charge for the valence electrons of the period 5 elements. As a result, the outermost electron shell is contracted. Under these circumstances, the spectroscopic parameters of the emission lines of period 4 and 5 elements, originating from the valence isoelectronic states, would become very close.

Finally, our prediction of homologous relations between the emission lines of period 4 and 5 elements in groups IA and IIA based on their electronic-structure similarities was confirmed by the intensity correlation analysis of the Li I (670.776 nm), Na I (588.995 nm), K I (766.490 nm), Rb I (780.027 nm), Mg II (279.553 nm), Ca II

FIG. 12. Plots of IRb I (780.027 nm), INa I (588.995 nm), and ILi I (670.776 nm) as functions of IK I (766.490 nm) and those of IBa (407.771 nm), and IMg II (279.553 nm) as functions of ICa II (393.366 nm). The corresponding q values are noted in each panel.

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II

(455.403 nm), ISr

II

(393.366 nm), Sr II (407.771 nm), and Ba II (455.403 nm) lines. Aqueous solutions of the alkali metal chlorides (LiCl, NaCl, KCl, and RbCl) and alkaline earth metal chlorides (MgCl2, CaCl2, SrCl2, and BaCl2) were prepared and absorbed into the filter paper. Then the pieces of filter paper were dried and analyzed using the commercial broadband LIBS instrument. We recorded 1200 singlet-shot spectra and obtained the intensity values. In Fig. 12, the intensity correlations of the line pairs K ILi I (a), K INa I (b), K IRb I (c), Ca IIMg II (d), Ca IISr II (e), and Ca IIBa II (f) are shown, and the corresponding q values are noted. As predicted above, the intensity correlation between the period 4 and 5 elements in each group was strongly positive. The homologous relations between the strongest emission lines of the period 4 and 5 elements in groups IA and IIA can be generalized to the other groups. Detailed study of the other group elements is in progress.

CONCLUSION We report an analytical approach for improving the LIBS precision of Ca measurement in water using an ideal internal standard, Sr. It has been found that Sr provides the ideal reference line at 407.771 nm, which has very similar spectroscopic parameters to those of the analyte Ca II lines at 393.366 and 396.847 nm. This leads to strongly positive shot-to-shot correlation and a clear linear relation between the analyte and reference line intensities. Moreover, Ca and Sr belong to the same group of alkaline earth metals. Among the alkaline earth metals, Sr is particularly well known for its very close chemical properties to those of Ca. The chemical proximity of Ca and Sr enables us to effectively compensate for the inhomogeneous distribution of analyte Ca2þ ions on the filter paper substrate using the internal standard, Sr2þ. We applied this method to determine the Ca concentrations in tap, bottled, and ground water samples. The estimated Ca concentrations in the drinking water samples are in good agreement with the results from FAAS. Also, homologous relations between the strongest valence isoelectronic emission lines of the period 4 and 5 elements in groups IA and IIA were suggested. This has been evidenced by the intensity correlations observed for the emission lines from Li, Na, K, Rb, Mg, Ca, Sr, and Ba. The spectral-line parameters and chemical properties of elements are not independent; they originate from the common factor, the valence electronic structures of elements. Our results imply that an ideal internal standard should have a valence electronic structure close to that of analyte elements. An individual spectral line with spectroscopic parameters similar to those of selected analyte lines may not be a sufficient condition for choosing the ideal internal standard. The closeness of the valence electronic structures would be helpful in correcting for the effects of both plasma-temperature fluctuation and sample inhomogeneity occurring from interactions with the substrate during sample preparation or pre-concentration processes. The improvement of precision and LODs would enable us to apply LIBS for more various kinds of trace-level liquid analysis including water. Higher precision is very important in

discriminating concentrations that are too low. The use of a filter paper substrate can be useful in the analysis of liquid that contains high contaminations and is unfit for direct sample analysis using ICP-OES and FAAS. ACKNOWLEDGMENTS This paper was supported by the National Research Foundation of Korea (2010-0009376 and 2011-0023778) and the Anti-Corrosion Research Center at Mokpo National University. Y.L. thanks Jinsu Na for the FAAS analysis. 1. U.S. Environmental Protection Agency. ‘‘Drinking Water Contaminants’’. http://water.epa.gov/drink/contaminants/index.cfm [accessed Apr 20 2013]. 2. L.P. Eubanks, C.H. Middlecamp, C.E. Heltzel, S.W. Keller. Chemistry in Context: Applying Chemistry to Society. New York, USA: McGraw-Hill, 2009. 6th ed. P. 219. 3. R.P. Heaney. ‘‘The Importance of Calcium Intake for Lifelong Skeletal Health’’. Calcif. Tissue Int. 2002. 70(2): 70-73. 4. J. Vormann. ‘‘Magnesium: Nutrition and Metabolism’’. Mol. Aspects Med. 2003. 24(1–3): 27-37. 5. R. Rylander, H. Bonevik, E. Rubenowitz. ‘‘Magnesium and Calcium in Drinking Water and Cardiovascular Mortality’’. Scand. J. Work Environ. Health. 1991. 17(2): 91-94. 6. D.W. Hahn, N. Omenetto. ‘‘Laser-Induced Breakdown Spectroscopy (LIBS), Part II: Review of Instrumental and Methodological Approaches to Material Analysis and Applications to Different Fields’’. Appl. Spectrosc. 2012. 66(4): 347-419. 7. V.N. Rai, F.Y. Yueh, J.P. Singh. ‘‘Laser-Induced Breakdown Spectroscopy of Liquid Samples’’. In: J.P. Singh, S.N. Thakur, editors. Laser-Induced Breakdown Spectroscopy. Amsterdam, the Netherlands: Elsevier Science, 2007. Pp. 224-226. 8. L. St-Onge, E. Kwong, M. Sabsabi, E.B. Vadas. ‘‘Rapid Analysis of Liquid Formulations Containing Sodium Chloride Using LaserInduced Breakdown Spectroscopy’’. J. Pharm. Biomed. Anal. 2004. 36(2): 277-284. 9. Y. Lee, S.-W. Oh, S.-H. Han. ‘‘Laser-Induced Breakdown Spectroscopy (LIBS) of Heavy Metal Ions at the Sub-Parts per Million Level in Water’’. Appl. Spectrosc. 2012. 66(12): 1385-1396. 10. R.L. Vander Wal, T.M. Ticich, J.R. West, P.A. Householder. ‘‘Trace Metal Detection by Laser-Induced Breakdown Spectroscopy’’. Appl. Spectrosc. 1999. 53(10): 1226-1236. 11. N.E. Schmidt, S.R. Goode. ‘‘Analysis of Aqueous Solutions by Laser-Induced Breakdown Spectroscopy of Ion Exchange Membranes’’. Appl. Spectrosc. 2002. 56(3): 370-374. 12. C.R. Dockery, J.E. Pender, S.R. Goode. ‘‘Speciation of Chromium via Laser-Induced Breakdown Spectroscopy of Ion Exchange Polymer Membranes’’. Appl. Spectrosc. 2005. 59(2): 252-257. 13. D.M. Dı´ az Pace, C.A. D’Angelo, D. Bertuccelli, G. Bertuccelli. ‘‘Analysis of Heavy Metals in Liquids Using Laser Induced Breakdown Spectroscopy by Liquid-to-Solid Matrix Conversion’’. Spectrochim. Acta B. 2006. 61(8): 929-933. 14. M.O. Al-Jeffery, H.H. Telle. ‘‘LIBS and LIFS for Rapid Detection of Rb Traces in Blood’’. In: R.R. Alfano, editor. Optical Biopsy IV, Proc. SPIE. 2002. 4613: 152-161. 15. P. Yaroshchyk, R.J.S. Morrison, D. Body, B.L. Chadwick. ‘‘Quantitative Determination of Wear Metals in Engine Oils Using LIBS: The Use of Paper Substrates and a Comparison between Single- and Double-Pulse LIBS’’. Spectrochim. Acta B. 2005. 60(11): 1482-1485. 16. M.A. Gondal, T. Hussain. ‘‘Determination of Poisonous Metals in Wastewater Collected from Paint Manufacturing Plant Using LaserInduced Breakdown Spectroscopy’’. Talanta. 2007. 71(1): 73-80. 17. D. Alamelu, A. Sarkar, S.K. Aggarwal. ‘‘Laser-Induced Breakdown Spectroscopy for Simultaneous Determination of Sm, Eu and Gd in Aqueous Solution’’. Talanta. 2008. 77(1): 256-261. 18. I.Y. Elnasharty, A.K. Kassem, M. Sabsabi, M.A. Harith. ‘‘Diagnosis of Lubricating Oil by Evaluating Cyanide and Carbon Molecular Emission Lines in Laser Induced Breakdown Spectra’’. Spectrochim. Acta B. 2011. 66(8): 588-593. 19. Z. Chen, H. Li, M. Liu, R. Li. ‘‘Fast and Sensitive Trace Metal Analysis in Aqueous Solutions by Laser-Induced Breakdown Spectroscopy Using Wood Slice Substrates’’. Spectrochim. Acta B. 2008. 63(1): 64-68.

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