Quantitative elemental determination in water and oil by laser induced breakdown spectroscopy

Analytica Chimica Acta 429 (2001) 269–278

Quantitative elemental determination in water and oil by laser induced breakdown spectroscopy Pascal Fichet∗ , Patrick Mauchien, Jean-François Wagner, Christophe Moulin CEA Saclay, Fuel Cycle Division, LSLA DCC/DPE/SPCP Bâtinment, 391 91191 Gif Sur Yvette Cedex, France Received 17 July 2000; accepted 27 September 2000

Abstract Laser induced breakdown spectroscopy has been used to evaluate the potentiality of the technique for the determination of trace amounts of elements in different types of liquids, in the framework of nuclear applications. A specific set-up using a pulsed laser focussed with a tilted angle on the surface of the liquid is presented. It allows on-line quantitative measurements with good reliability and reproducibility. Twelve elements (Pb, Si, Ca, Na, Zn, Sn, Al, Cu, Ni, Fe, Mg, Cr) have been studied in two different liquid matrices: water and oil. Detection limits (0.3–120 ␮g ml−1 ) and reproducibilities (ca. 3%) are reported. Moreover, the use of an echelle spectrometer for such elemental analysis is also proposed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser induced breakdown spectroscopy; Analysis of liquids; Atomic emission spectroscopy; Laser ablation; Echelle spectrometer

1. Introduction In the nuclear industry [1–3], there are still needs for suitable remote sensing or direct measurements in liquid materials in order to control processes or effluents. Thus, researches are going on to provide original techniques. In terms of requirements, the technique should allow access to difficult or hostile environments (chemical and radioactive), and it should have a fair limit of detection (␮g ml−1 range) with a large dynamic range (104 ) in order to cover different needs. Moreover, various types of solutions from aqueous nitric acid matrices to complex organic solutions, and slurry solutions and the presence of precipitates should be possible to analyse. Finally, measurements should be sufficiently reproducible for process monitoring. Laser induced breakdown spectroscopy (LIBS), due to its principle where only laser photons are sent ∗

Corresponding author.

to the target and photons emitted by the plasma are detected, can allow direct and in situ measurements of numerous elements in complex liquids. LIBS is a well-established analytical technique [4,5]. It can briefly be described as an elemental analysis based on emission from a plasma generated by focussing a laser beam on a sample. Recent reviews [4,5] concerning analytical applications of LIBS mention that experiments with liquids are rather sparse compared to those on solids. Hence, besides papers on fundamental points concerning physical mechanisms of the ablation breakdown on liquids [6–8], few papers are related to research on LIBS in the field of application for quantitative elemental determination. LIBS was applied for experiments on liquids with different types of configurations, including plasma formation on the surface [9,10], on the bulk liquid [1,11–14], on droplets [15,16], and on liquid jets [17–22]. Arca et al. [9] and Berman and Wolf [10]

0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 0 ) 0 1 2 7 7 - 0

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reported LIBS results for the determination of several elements Mg, Ca, Cr, Ni and some chlorinated hydrocarbons (CHCs) in aqueous solutions. LIBS experiments on liquids in a closed cell were also carried out for bulk analysis of aqueous solutions containing alkali metals [11,14], alkaline earth metals [11,13,14], other metals [11], rare earths and actinides [1,14]. Analytical experiments on droplets were performed for the measurements of Li, Na, Mg, Ca, Mn, Al [15,16] as well as for Na [18,20–22], K [20,21], Ba [20], Rb, Li, and Cd [22] in a liquid jet and for FeO(OH) suspensions [17,19]. Species in organic solvents have been studied by Cremers et al. [11]. The research by Nyga and Neu [23] on ablation efficiency for solid samples in aqueous solutions and Boiron et al. [24] on fluid inclusions in geological samples should also be mentioned. In this paper, an experimental set-up directly applicable to a nuclear reprocessing plant is described while retaining the main advantages of the LIBS experiment i.e. lack of sample preparation and in situ measurement. The capability of the technique for reproducible quantitative determination of trace elements in liquids of interest (aqueous and organic) was investigated. Furthermore, LIBS has been considered with respect to on-line applications and for surface analysis. The main drawbacks that affect a LIBS experiment on liquid surfaces, particularly splashing and emission variability, have led to a particular experimental set-up.

2. Experimental set-up 2.1. Apparatus A standard LIBS arrangement was used except for the laser configuration at the solution cell (see Fig. 1). The experimental conditions are summarised in Table 1. The plasma formation was attained with the aid of a Q-switched Nd:YAG laser (Quantel YG 580, Les Ulis, France) operating at 532 nm (pulse duration of 14 ns). The laser beam was transported by three reflective mirrors and focussed on the liquid surface by a 25 cm-focal length quartz lens to generate the plasma. For liquid quantitative elemental determination, the laser direction was tilted at an angle of 15◦ to the liquid surface. The last mirror placed just before the

Fig. 1. Experimental set-up.

laser-focussing lens was a reflective (at 45◦ ) quartz mirror (Optique FICHOU, Fresnes, France) in order to allow plasma recording in the same direction. The optical transmission at 45◦ of the mirror was 100% between 200 and 800 nm except at 532 nm, which was 0% with a spectral bandwidth of 30 nm. The laser beam produced a plasma on the liquid surface. The vessel full of liquid was made of PTFE rather than glass in order to avoid eventual damage. The laser had an energy of 60 mJ and a repetition rate of 1 Hz. The plasma light was focussed by a 10 cm-focal length quartz lens in a 5 m long fused-silica optical fibre. The emission of the plasma was imaged, either with two quartz lenses (10 cm focal length (optical fibre side) and 40 cm focal length (spectrometer side)) on the entrance slit (30 ␮m width) of a 1 m focal length spectrometer (THR 1000, Jobin–Yvon, Longjumeau, France) equipped with a 2400 lines mm−1 grating, or an echelle spectrometer (ESA 3000, LLA GmbH, Berlin). With the Jobin–Yvon system, an intensified CCD (ICCD) camera (Princeton Instruments, Monmouth Junction, NJ, 576 × 384 pixels) was used as a detector with its controller (ST 138 model). A spectral resolution of 0.04 nm was obtained. The Q-switch output trigger of the laser was directly connected to the input of the PG 200 enabling the triggering of the generator. With a laser frequency of 1 Hz, all the 100 spectra of an experiment (duration = 100 s) were recorded and summed separately with the data acquisition software (Winspec 2.2.3.1, Princeton Instruments). A program was developed using Visual Basic 6.0

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271

Table 1 Experimental conditions and apparatus Laser Wavelength, pulse width, repetition rate Energy used Optical fibre Numerical aperture, core, length Detection system Spectrometer Type, grating Slit width, slit height ICCD camera Controller, pulse generator Software Samples Aqueous solutions Oil solution

Q-switched Nd:YAG (YG 580 Quantel) 532 nm, 14 ns, 1–10 Hz 60 mJ/pulse on the liquid surface UV–VIS–NIR transmission 180–1100 nm Polymicro Technologies Inc. 0.22, 800 ␮m, 5 m Jobin–Yvon THR 1000 1 m Czerny Turner, 2400 lines/mm 30 ␮m, 1 cm Princeton Instruments (EEV 576 × 384 pixels (6 ph)). Dynamic range: 16 bits ST 138, PG200 Winspec 32 (version 2.2.3.1) Separate elements, 1 g in water (Titrisol, Merck) 21 Element oil standard (concentration 900 ␮g/g in base oil 75) SPEX Certiprep

(Microsoft Corp.) to read the binary files given by Winspec and to average all the spectra recorded for each calibration. The ESA 3000 echelle spectral analyser provided simultaneous detection of spectra obtained by LIBS. The standard ESA 3000EV possessed a spectral range of 200–780 nm for simultaneous detection. The detector of the system was a KAF 1000 from Kodak (Munich, Germany) and the entire device provided a resolution of λ/1λ = 10,000. With this spectrometer, ten laser shots were integrated for each replicate measurement. 2.2. Sample preparation Standard aqueous solutions (Merck Titrisol, Darmstadt, Germany), each containing one of the twelve impurities studied, were used and subsequently diluted with doubly distilled water to obtain lower concentrations between 2000 and 50 ␮g ml−1 . A standard oil solution containing 21 elements at concentration 900 ␮g g−1 (SPEX Certiprep, Metuchen) was used and diluted with the pure corresponding oil to between 900 and 50 ␮g ml−1 .

3. Results and discussions For LIBS experiment on liquids, special care, due to the nature of the matrix, should be taken in order to be able to perform reproducible measurements.

3.1. Liquid configuration for plasma formation Detection of contaminants, without sample preparation, in different types of liquid matrices that are likely to be encountered in the nuclear fuel cycle: matrices such as collo¨ıds, turbid liquids, sludge, oils, etc., requires production of a plasma at the liquid surface. Hence, bulk liquid analysis was not investigated because many different liquids under investigations are turbid which would prevent the laser beam from reaching the bulk liquid. The droplets and jet configurations were also not studied because sample preparations are required and this is not consistent with further use for on-line application. Using a classical configuration, however, as used for LIBS on solids with the laser beam perpendicular to the surface leads to splashing in the case of liquids. Splashing results in covering the focussing optics with droplets and, therefore, prevents further use of this technique. This can easily be explained by the fact that the plasma expansion at atmospheric pressure is directed perpendicularly to the surface [25]. Thus, a tilted configuration (as presented in Fig. 1) can minimise this phenomenon. Another important effect is the perturbation that takes place at the liquid surface following the laser pulse. By using a low laser frequency of 1 Hz, it was shown that measurements were more reproducible. In summary, these two parameters (tilting of the laser beam to the surface and low laser repetition rate) have minimised splashing of liquid on the optics as well as optimal surface

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configuration for analytical purposes. Moreover, for further industrial applications, (i) the use of the same axis for laser beam focussing and for plasma collection presented advantages in terms of simplicity and reproducibility, (ii) measurements were performed in air at atmospheric pressure, without the addition of buffer gases (sometimes proposed [26] in order to improve the sensitivity), and (iii) the laser wavelength was chosen as 532 nm which was a compromise between laser efficiency and fibre optic transmission [3]. 3.2. Time resolution Another important parameter in LIBS is time resolution as already [4] shown for solids analysis. Hence, time-resolved spectroscopy is essential for improving the sensitivity in any LIBS experiment (as shown in numerous papers on LIBS [3–5] applied to solids). A combination of a time delay of 500 ns and a gate width of 25 ␮s was chosen because it was experimentally proved that these values produce the optimum signal-to-noise (S/N) ratio. The emission coming from the plasma continuum does not interfere with the analytical lines of the elements of interest. A shorter time delay leads to strong background emission and a longer gate width does not improve the S/N ratio. Thus, for oil and water matrices, the same temporal parameters were used. 3.3. Spectra recording and reproducibility All experiments for aqueous and oil solutions were carried out under constant conditions (laser energy, frequency and optical configuration) and each point displayed on calibration graphs is a mean value obtained from six replicate experiments. The reproducibility for each point was calculated with a confidence of 95% which induces a Student t factor equal to 2.571. So the error bars for all mean calibration values were calculated [27,28] from the formula t ×σ √ , n

(n = number of results, σ = standard deviation)

The calibration graphs were obtained by calculation of the maximum of the emission signal of the trace element emission line divided by the background and

plotting it versus concentration (in ␮g ml−1 ). Since the simultaneous spectral range of the Jobin–Yvon system is only 4 nm wide, it is very difficult to normalise the emission line intensity by the intensity of a matrix line, such as line emission of O for water, which is sometimes proposed [29] to improve the reliability and the reproducibility of the LIBS technique. It was found by calculations that the maximum line intensity of the element divided by the background of the spectrum improves the reproducibility of the results. The background signal chosen for the calculation was the mean value of the 50 smallest values among the 576 recorded by the CCD camera for each spectrum. One possible explanation for this improvement observed might be due to the spectra acquisition conditions. With a short delay time of 500 ns used for liquid investigations, the emission background of the spectrum may represent a major part of the plasma emission. Xu et al. [29] have previously mentioned a correlation between the analytical peak and the average background signal on a pulse to pulse basis. Gornushkin et al. [30] reproduced the statistical hypothesis of Xu et al. and proved that it was incorrect. Ciucci et al. [31] reported recently the difficulties of using a standard calibration for multielemental determination and proposed a new procedure to process LIBS data, based on a patented algorithm and for a particular range of analyte concentrations. 3.4. Trace determination in water A series of measurements between 2000 ␮g ml−1 and the detection limits of the twelve elements Pb, Si, Ca, Na, Zn, Sn, Al, Cu, Ni, Fe, Mg, Cr were performed for the water matrix. As an example and for simplicity, Fig. 2 shows the difference of emission between pure water and water with 50 ␮g ml−1 Mg and 100 ␮g ml−1 Si in two different spectral region of interest (around Mg (285.213 nm) and Si (288.1579 nm)). Fig. 3 shows the corresponding calibration graphs obtained for Si and Mg. All the calibration graphs obtained have proved to be linear or parabolic (for Mg, Na and Ca). Calibration graphs displayed in Fig. 3 are composed of two distinct parts (one for the higher concentrations and one for the lower). These two parts correspond to two different gains of the CCD camera. It was shown experimentally that an increase of the gain improves the signal

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Fig. 2. Emission spectra of pure water and water with introduction of 100 ␮g ml−1 Si and 50 ␮g ml−1 Mg (100 plasma emissions are accumulated with a delay time of 500 ns and gate width of 25 ␮s).

to background ratio for the lower concentration. However, measuring concentrations near the discrepancies of the calibration plots is not an issue because the two parts correspond to a specific gain. The gain change is due to the intrinsic dynamic of the CCD detector, which is of the order of 102 . All the calibration graphs intersect the ordinate near the value equal to 1 because it corresponds to the ratio background/background. The reproducibility of the analytical results obtained by LIBS was experimentally found to be 3% (the mean value of all the data reported in the calibration plots). The results obtained for the twelve elements studied are given in Table 2. The wavelengths of the sensitive lines [32] are reported as well as the detection limits calculated by the standard method: 2σ /s, where σ is the standard deviation associated with the total noise of the system and s is determined from the calibration slope. A comparison of the detection limits found in this work and those reported in the literature are also given

in Table 2. As previously mentioned, the results in the literature are very sparse and only [6] and [7] describe experiments performed directly on liquid surfaces. Detection limits are generally of the same order of magnitude i.e. the ␮g ml−1 range (except for Na). Arca et al. [9] obtained a slightly better detection limit for Cr (283.563 nm) but the visible line (425.43 nm) of Cr was the more sensitive one for comparison with the oil results. Better detection limits were found previously for Pb and Ca [14] but plasmas were formed in the bulk liquid. This study reports better detection limits for Cu and Mg. It should also be noted that for Fe, Si, Sn and Zn, no detection limits have been reported in the literature. 3.5. Trace determination in oil The same study was undertaken for the same twelve elements in oil, in order to investigate the potentiality of the technique to analyse such matrices, which are

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Fig. 3. Calibration graphs for Mg and Si in water.

difficult to study with conventional techniques and to evaluate the difference between the LIBS elemental detection for two different common liquid matrices. From Fig. 4, it is clear that Si and Mg can be detected

(as expected) in oil at the concentration reported. The corresponding calibration graphs obtained with our optical set-up are presented in Fig. 5. All the calibration graphs are linear or parabolic (for Mg, Na and Ca)

Table 2 Results obtained for 12 elements in watera Element

Wavelength (nm)

Detection limit (␮g ml−1 ) (this work)

Detection limit (␮g ml−1 ) (literature)

Pb Si Ca Na Zn Sn Al

405.87 288.1579 393.366 588.995 334.502 283.999 309.271 396.152 324.754 341.476 373.4864 371.9935 285.213 279.55 425.43 283.563

100 25 0.3 0.5 120 100 10 10 7 20 30 35 1

12.5 [14], 100 [5]

Cu Ni Fe Mg Cr a

10 0.4

0.13 [14], 0.4 [16], 8 [22] 0.0075 [14], 0.23 [18], 2.2 [16], 0.9 [22], 0.014 [11]

5.2 [16], 10 [22] 20 [11] 50 [5] 18 [10]

1.9 [16], 3 [22] 100 [11] 100 [5] 0.1 [9]

The sensitive lines are reported, as well as the detection limits obtained compared with those of the literature.

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Fig. 4. Emission spectra of pure oil and oil with introduction of 100 ␮g ml−1 Si and 50 ␮g ml−1 Mg (100 plasma emissions are accumulated with a delay time of 500 ns and gate width of 25 ␮s).

and thus, the reliability of the LIBS method is similar to the measurements in a water matrix. The analytical reproducibility for oil was also found to be 3%. Detection limits for the same twelve elements were also determined and are summarised in Table 3. The wavelengths chosen for the twelve elements were the same sensitive lines used for water except for chromium and iron because the oil spectrum possessed an interference with the Cr line in the UV and with the Fe line at 373.4864 nm. For the purpose of this research, one can conclude that the LIBS technique has nearly the same analytical figures of merit for water and oil matrices. This can be due to the boiling points of water and oil (100 and 315◦ C, respectively) that are close compared to the melting points of different solid matrices. Hence, the melting point temperature was previously mentioned as a key factor to explain the matrix effect observed by LIBS [3]. Only Li (670.8 nm) in oil was previously studied by LIBS. Cremers et al. [11] obtained results only

for a 1.4 ␮g ml−1 Li solution in different solvents (methanol, acetone and ethanol). They mentioned that the signal obtained is of the same order of magnitude as for H2 O or even greater, but no calibration was shown and the experimental set-up was based on bulk elemental determination. 3.6. Rapid and panoramic analytical determinations One of the most promising approaches to atomic emission spectroscopy (AES) and particularly for the LIBS experiment involves the use of an echelle spectrometer [33–35]. Hence, for industrial applications, it is necessary to be able to analyse for several elements at once and, therefore, the use of an echelle spectrometer that covers simultaneously the emission spectra from the UV to visible with sufficient resolution is desirable. LIBS has been very recently evaluated on water samples with a new echelle spectrometer (ESA 3000). Ten laser shots were accumulated

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Fig. 5. Calibration graphs for Mg and Si in oil.

on the ICCD of the system to analyse a source water (trade mark: Cristaline from Aurele Source, France) which contained 98 ␮g ml−1 Ca, 4 ␮g ml−1 Mg and 4.1 ␮g ml−1 Na. As seen in Fig. 6, a large domain of

Table 3 Results obtained for 12 elements in oila Element

Wavelength (nm)

Detection limit (␮g ml−1 ) (this work)

Pb Si Ca Na Zn Sn Al Cu Ni Fe Mg Cr

405.87 288.1579 393.366 588.995 334.502 283.999 396.152 324.754 341.476 371.9935 285.213 425.43

90 20 0.3 0.7 130 80 10 5 35 20 1 20

a

The sensitive lines are reported, as well as the detection limits obtained.

the spectrum can be investigated simultaneously with an echelle spectrometer (typically 200–780 nm with a resolution of λ/1λ = 10,000); the wavelengths of interest (around 279 nm for Mg, 394 nm for Ca and 588 nm for Na) in several spectral windows are presented. A signal of 500 (in arbitrary units), corresponding to twice the background signal, was arbitrarily fixed to evaluate the detection limit. Taken into account this value, and assuming the linearity of the detection system, detection limits for Mg, Ca and Na in water were evaluated at 1 ␮g ml−1 , 0.8 ␮g ml−1 and 1 ␮g ml−1 , respectively. These values are very close to those obtained with the previous detection system. Fig. 7 shows the calibration graph for aluminium in water obtained with such system and for concentrations between 200 and 1000 ␮g ml−1 . Each point corresponds to the mean of the results of six replicate experiments. When the entire spectrum is accessible, it is possible directly to see all the elements previously mentioned with the classical system and, therefore, to normalise (as in this case) the analytical signal of interest with one of the oxygen lines (777.194 nm

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Fig. 6. Three spectrum sections of mineral water recorded simultaneously with an echelle spectrometer (ESA 3000). Ten laser shots are accumulated with a delay time of 500 ns and an opened gate of 10 ␮s.

for a water matrix). The analytical reproducibility was found to be 10% and the detection limit for Al in water was 30 ␮g ml−1 . These values are a little higher than the classical system but only 10 spectra (compared to

100) were accumulated for each data point. Also, only 10 s was required to observe all the elements. Further investigations are in progress to quantify signal to noise, reproducibility, required time analysis,

Fig. 7. Calibration graph for Al in water obtained with the echelle spectrometer.

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but it is very likely that such an echelle spectrometer will be of a great help for fast and precise LIBS elemental determination in liquids (and solids).

4. Conclusions In the present work, LIBS has been used for the measurement of twelve elements in two liquids: water and oil. With the experimental set-up proposed, a laser focussed directly with a tilted angle and at a frequency of 1 Hz onto the liquid surface, direct elemental detection without perturbation was possible. The experimental set-up was devoted to fast in situ experiment. Elemental line intensities were monitored in the laser-produced plume as a function of analyte concentration to determine detection limits. In terms of detection limit and reproducibility, no significant differences were observed between results obtained for oil and water samples. Initial results obtained with an echelle spectrometer, which allow direct panoramic analysis by LIBS, are very promising.

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