New perspectives in LIBS analysis of polluted soils · easily transferred in the food chain through...

TECHNOLOGYMARWAN IPCF-CNRAPPLIED LASERSPECTROSCOPYLABORATORY

MODÌ THE FIRST MOBILE INSTRUMENT FOR DOUBLE PULSE LASER INDUCED BREAKDOWN SPECTROSCOPY (LIBS) A N A L Y S I S O F M A T E R I A L

New perspectives in LIBS analysis of polluted soils

Laser-Induced Breakdown Spectroscopy (LIBS) is a technique for compositional analysis measurements particularly useful for environmental applications. The potential of this technique for in situ analysis could be greatly improved using innovative experimental setups. A new mobile instrument for soil analysis, developed at the Applied Laser Spectroscopy Laboratory in Pisa, is presented, and some experimental results are given. The possibility of performing soils analysis using miniaturized laser and spectrometer is also described; the LIBS spectra obtained with these innovative systems are compared. 1. Introduction

In recent years, the growing concerns on environmental issues has produced a number of regulatory actions on the quality of soils, based on the definition of acceptable upper concentration limits of potentially hazardous elements. Among the potential pollutants, heavy metals are particularly dangerous because they are not subject to bio-degradation processes and may be easily transferred in the food chain through absorption of plants or pollution of the underground waters. Analytical determination of trace metals concentration in soils is usually carried out by complex analytical techniques as ICP, AAS, etc., which require separate analysis for each element and the use of Standard Reference Material. Therefore the analysis of many samples may be lengthy and expensive. This consideration makes the LIBS technique - a method that allows the determination of all the elements with a single measurement in a few seconds, without pre-treatment of the samples - particularly interesting for environmental analysis. This technique, developed in recent years, has been applied for the determination of elemental composition in a wide range of materials in the solid, liquid or gaseous phase (Rusak et al., 1997). Moreover in LIBS measurements the samples do not need chemical treatments; therefore the risk of contamination and environmental pollution of the samples is minimized.

2. Principles of the LIBS technique

Laser Induced Breakdown Spectroscopy (LIBS), also named Laser Induced Plasma Spectroscopy (LIPS) or Laser Ablation Spectroscopy (LAS), is a technique based on the spectral analysis of the radiation emitted by a plasma generated by focusing an intense laser pulse on the sample surface. The intense laser pulse focalized on the surface sample, produces evaporation, atomization and ionization of the material; the subsequent cooling of the plasma determines the emission of atomic and ionic lines, allowing the identification of the species present in the plasma (Adrain and Watson, 1984).The intrinsic simplicity, robustness and quickness of the LIBS method make it particularly appropriate for in-situ analysis even in hostile environment. In recent years, a number of applications of LIBS technique have been proposed in the field of Material Science, Industrial Processes Control, Environmental Protection and Cultural Heritage conservation and study (Evans at al., 2003). However, one of the most stringent constraints to the application of LIBS technique to environmental samples is the use of reference calibration standards which may introduce, because of the matrix effect, important errors in the quantitative analysis of the materials of interest. To overcome this problem, the Applied Spectroscopy Laboratory of CNR has recently developed an analytical technique based on LIBS which does not depend on calibration curves or reference samples. This technique, called Calibration-Free LIBS (CF-LIBS), could be very interesting in soil analysis where the large

1

variability of the matrix analysed makes particularly problematic the use of standard reference samples, thus producing large errors in the determination of the heavy metals concentrations present in the sample.

2.1 The Calibration-Free LIBS method

The method used for the analysis of LIBS spectra has been described in detail elsewhere (Ciucci et al., 1999), therefore we will just present here the basic characteristics of this approach. Assuming the plasma in Local Thermodynamic Equilibrium (LTE) and optically thin, the LIBS line integral intensity corresponding to the transition between two levels Ek and Ei can be expressed as:

(1)

where (l, C, A , g and F are the wavelength of the transition, the concentration of the emitting ki katomic specie, the transition probability for the given line, the k level degeneracy and a constant to be determined after normalisation of the species concentrations, respectively. U(T) is the partition function for the emitting specie defined as:

(2)

Taking the logarithm of eq. (1) and substituting the following definitions:

(3)

we obtain a linear relationship between the y and x parameters

y = m x + q (4)

According to eq. (3), the slope of the curve is related to the specie temperature, while the q parameter is proportional to the logarithm of the specie concentration via the F constant factor. The F factor can be determined by normalizing the species concentrations as

(5)

The application of the above described method gives the concentration of all the components of the emissions, including the trace elements in concentrations up to the detection limit of the technique (typically 1 ppm) without the need of calibration. The minimum error in the composition determination is of the order of the concentration of the undetected elements.

3. New perspectives for in-situ LIBS quantitative analysis

3.1 Double-pulse LIBS

Although CF-LIBS solves most of the problems related to the LIBS technique, allowing the

I FCAU

lki

kik

EkTg

T

k

=-exp( )

( )

U(T) = -å gk E kTk

kexp( )

y lnki

k ki

=I

g Al

mkT

= -1

x Ek=

q ln=CF

U(T)

1=åk

kC


2

definition of reliable procedures for LIBS quantitative analysis, it is nevertheless true that the LIBS technique still suffers of a relatively poor sensitivity with respect to other analytical techniques. Even considering that this drawback is, in typical environmental application, widely compensated by the possibility of performing multi-elemental simultaneous analysis with a dynamic range going from major to trace elements (ppm) in the same measurement, an improvement of the LIBS techniques towards lower limits of detections for most of the elements of interest is necessary in view of a practical application of the technique.Among the various methods proposed for improving the LIBS limits of detection, a promising technique seems to be the excitation of the plasma through a sequence of multiple laser pulses. In particular, the double pulse technique has been recently applied to the analysis of solids samples in a gaseous environment. In its simpler version, the procedure requires two lasers emitting at the same wavelength, or a single laser emitting two separated pulses with a suitable delay. More complex experimental configurations involve the use of lasers with different wavelengths, allowing separating the process of laser ablation from the process of plasma excitation, or trains of several laser pulses.The review of the existing literature demonstrates that, even in the simplest configuration involving two identical lasers, the double pulse technique allows obtaining a significant increase in the LIBS sensitivity (corresponding to a marked improvement of the limits of detection) for several elements of interest. Sattmann and co-workers observed increased intensity and enhanced ablation when using multiple pulses from the same Nd:YAG laser, compared to the single pulse with the same energy of the two combined laser pulses (Sattmann et al., 1995). St-Onge and co-workers also used a Nd:YAG laser in double pulse mode and studied the temporal evolution of line intensity, plasma temperature and electronic density as a function of the interpulse interval on an aluminium alloy sample (St-Onge et al., 1998). They found a significant enhancement of the line intensity, and noticed that the signal-to-continuum ratio is especially improved in the double pulse mode. On the other hand, they didn't observe significant increases in the plasma temperature and electronic density values. The signal enhancement is therefore not attributed to a more efficient excitation of the pre-existing plasma by the second laser pulse, but instead to a stronger ablation which increases the number of particles in the plasma and the volume of the plasma itself. The work by Colao et al., who studied the characteristics of the plasma created on an aluminium sample by a Nd:YAG in double pulse collinear mode is in substantial agreement with the above-mentioned results. They confirmed the observation of an increase in the line intensity, an improvement of the signal-to-continuum ratio and an increase in the decay constants, compared to the case of single pulse of energy equal to the sum of the two pulses (Colao et al., 2002).

3.1.1 Experimental

At the best of our knowledge, almost all the in-situ environmental analyses reported in the literature used LIBS mobile systems based on the "classical" single pulse setup (Yamamoto et al., 1996; Theriault et al., 1998; Miles and Cortes, 1998; Castle et al., 1998; Wainner et al., 2001; Barbini et al., 2002). The possible occurrence of double or multiple pulses during the measurement has been mentioned as a consequence of the use of a passively Q-switched Nd:YAG laser, not allowing to control neither the pulses energy nor their timing (Wainner et al., 2001).Our Laboratory has recently realized a Mobile Dual-pulse Instrument (Modý) for LIBS environmental applications (see fig.1). The instrument integrates a dual-pulse laser, which emits two collinear laser pulses with energy variable between 50 and 150 mJ per pulse at a maximum repetition rate of 10 Hz and a reciprocal delay which can be set from 0 to 60 ?s. The LIBS measurements can be performed on soil samples, compressed in form of small pellets with a 5-tons hand press; inside a closed experimental chamber, equipped with a motorised table for exact positioning of the sample at the focus of the laser beams; a laser pointer and an optical microscope allow the control of the region of the sample under analysis. Alternatively, for direct measurements on the ground an articulated 5-joints arm allows the focusing of the laser outside the instrument as well as the collection of the spectral signal. A safety switch disables the laser operation if the articulated arm head is not pressed against the ground, thus preventing accidental reflections and


3

consequent possible eye damage.The LIBS signal, either produced ins ide the exper imenta l chamber or directly on the ground, is collected through an optical fibre and sent to a compact Echelle spectrometer coupled with an intensified CCD camera for spectral acquisition.The operations of the instrument are controlled by an integrated personal computer which m a n a g e s t h e s a m p l e v i s u a l i z a t i o n a n d m o v i m e n t a t i o n , t h e experimental settings of the laser (energy of the beams, delay between the pulses, repetition rate) and the spectral acquisition parameters (number o f s p e c t r a a v e r a g e d , acqu i s i t i on de lay , CCD measurement gate and gain). The LIBS spectra, after acquisition and storage, are

qualitatively and quantitatively analysed using a proprietary software (LIBS++) which implements the Calibration-Free method. A typical environmental analysis on soil samples can take between one and five minutes, according to the complexity of the LIBS spectrum produced.

3.1.2 Examples

Figure 2 shows the comparison between two LIBS spectra obtained inside the Modý experimental chamber, using double pulse and single pulse LIBS, on the same sample (PACS-1 marine sediment reference material).The composition of the sample, certified by NRC of Canada for trace elements, is reported in Table I.

Figure 1 - Side view of Modì, showing the articulated arm used for in-situ environmental analysis

Main components (percent)

Aluminium 6.5 ± 0.12

Calcium 2.1 ± 0.1

Iron 4.9 ± 0.1

Potassium 1.25 ± 0.07

Magnesium 1.46 ± 0.05

Sodium 3.35 ± 0.08

Silicium 26.0 ± 0.2

Titanium 0.42 ± 0.01

Trace metals (ppm)

Chromium 113 ± 8

Cobalt17.5

±1.1

Copper 452 ± 16

Lead 404 ± 20

Manganese 470 ± 12

Strontium 277 ± 11

Vanadium 127 ± 5

Zinc 824 ± 22


4

Figure 2 -LIBS spectrum of the PACS-1 sample, in a) single pulse configuration and b) double pulse configuration. Note that the vertical scale is the same for the two spectra.

The energy of the single pulse (160 mJ) corresponds to the sum of the energies of the two pulses (80 mJ each) which, for the acquisition of the double-pulse spectrum, were delayed by 2 ms the one with respect to the other. In both cases, the spectra were acquired after 1 ms from the second pulse, with a 500 ns integration gate on the intensified CCD. Both spectra correspond to the average of three series of 30 (couples of) laser pulses acquired at three different points on the soil sample, in order to minimize the effect of the laser-induced crater on its surface. A repetition rate of 2 Hz was used in both cases; the acquisition of the experimental spectra was thus concluded in less than one minute.

From the first visual comparison of single and double pulse spectra, it is clear that the double pulse spectrum gives in general an improvement (of the order of 5-10 times) of the LIBS signal with respect to the single pulse one. Such improvement leads to a lowering of the Limits Of Detection for all the chemical elements up to an order of magnitude in the best case. In fact, while the main components of the matrix are detected in both single and double pulse configuration, some of trace elements (Cr and Pb) are detectable only in the double pulse spectrum and not in the single pulse one (see Table II).

Table II - Comparison of chemical elements detected in single and double pulse configuration

Moreover, it is important to remark that even in the case of elements detected in both the configurations, we obtained an improvement of the signal (and of the S/N ratio) for the lines observed (e.g. an enhancement of the signal of a factor 5.7 and 4.2 for Mn II 260.5 nm and Mn II 257.6 nm emission lines, respectively) and an increase of the number of lines observed for each species. This characteristics leads to a reduction of the uncertainties of the elementary composition calculated by the CF-LIBS procedure and thus a better accuracy and precision of the results.

Finally, it should be noted that the signal improvement is not the same for all the lines. In Table III is shown a comparison of the line intensities corresponding to the main components of the

200 400 600 800 1000

0

5000

10000

15000

Wavelength (nm)

200 400 600 800 1000

0

5000

10000

15000

20000

Wavelength (nm)

Element SinglePulse

Double pulse

Cr No Yes

Co No No

Cu Yes Yes

Pb No Yes

Mn Yes Yes

Sr Yes Yes

V Yes Yes

Zn Yes Yes


5

polluted soil, along with the spectral parameters associated.

Table III - Signal enhancement observed for double pulse compared to single pulse of the same total energy (sample PACS-1)

Comparing results obtained from different soil samples, it seems that the signal improvement also depends on sample granulometry, with fine grained soils showing lower enhancement with respect to more compact samples. This effect is probably related to the presence of fine particles aerosol preventing the second pulse to properly reach the sample.

3.2 LIBS using micro-lasers and mini-spectrometers

The Modý spectrometer, presented in the previous section, represents an important step forward for in-situ LIBS analysis. However, in many situations a truly miniaturized LIBS system would be extremely useful, especially for applications in hostile environments were the use of a bulky system can be not advisable. For facing the problem of further reducing the dimensions of the LIBS instrument, our laboratory started a collaboration with Quanta System S.p.A. - an Italian firm operating in the field of laser sources and instrumentation - in the framework of the MIAO (Microsensors for Analysis in Hostile Environment) project, funded by Italian Ministry for Education and Research.The R&D team at Quanta System recently developed a diode pumped solid state source (QSWCW IR) operating in Q-switched mode with pulse energy of 80 mJ in 20 ns @ 1064 nm and a repetition rate up to 10kHz. The laser head is included in a small footprint casing together with

Element & Ionization Wavelength (A) Enhancement Ek (cm-1)

Fe II 2611.87 1.3 38660

Al I 2652.48 Not visible insingle pulse

37689

Mg I 2776.68 9.4 57874

Mg II 2790.77 1.9 71491

Ca II 3158.87 5.9 56839

Ti II 3361.20 2.4 29968

Fe I 3490.57 Not visible insingle pulse

29056

Al I 3944.00 4.4 25348

Ti I 3989.76 3.5 25227

Fe I 4045.81 3.2 36686

Pb I 7280.29 Not visible insingle pulse

35287

Sr II 4077.70 7.3 24517


42816

Ca I 4283.00 7.7 38552

Ba II 4554.02 7.5 21952

Ba II 4934.09 8.4 20262

Ti I 5064.66 Not visible insingle pulse

20126

Mg I 5183.60 5.4 41197


44677

Li I 6707.75 2.7 14904

Ba I 7280.30 Not visible insingle pulse

22947

K I 7664.91 3.5 13043

Ca II 8542.02 3.5 25414


6

the cooling system and the supply/control electronics. The casing is hermetically sealed for maximum ruggedness to external environment (figure 3).

Figure 3 - Technical drawing of Quanta System micro-laser head (dimensions in mm)

The utilization of similar micro-lasers of new generation is very promising for LIBS application, not only for their better portability, but also for the reduced damage produced on the sample surface due to the high quality (TEM00) of the laser beam, which guarantees a tighter focussing on the target surface. This characteristics can be fundamental for applications, e.g. analysis of cultural heritage samples, where the damage produced on the target is a critical parameter.In spite of these potentialities, in our knowledge, only Gornushkin et al. (2004) recently tested a similar micro-laser for LIBS applications.In the experimental setup used here, the laser radiation is focussed through a microscope objective (10x) on the soil sample; the breakdown radiation is then collected through an optical fiber and sent to the spectrometer. Two different spectrometers were tested: at first, the same Echelle spectrometer fitted in Modý was used; for further miniaturization of the apparatus, a mini-spectrometer (Avaspec Avantes Fiber Optic Spectrometer System) was also tested. The spectral range of the Avantes spectrometer was between 180 and 400 nm, with a spectral resolution of 0.2 nm over three spectral points. The repetition frequency of the laser was set to 10 KHz and the signal acquisition was made in continuous mode with exposure times of 100 ms (corresponding to the average of 1000 laser shots) with the Echelle spectrometer and 5 s (averaging of 50000 laser shots) with the Avantes spectrometer.Experiment control and data analysis were performed using a laptop computer. The whole dimensions of the miniaturized LIBS system can be reduced, through the use of the micro-laser and the mini-spectrometer, down to a few cubic decimetres, with a total weight of less than 15 Kg.

In figure 4, the LIBS spectra of the PACS-1 sample, obtained using the Quanta System micro-laser with a) the Echelle spectrometer and b) with the Avantes mini-spectrometer are shown.


7

Figure 4 -LIBS spectrum of the PACS-1 sample, obtained with the Quanta System micro-laser source and a) Modý echelle spectrometer, b) Avantes mini-spectrometer.

In both cases the LIBS emission lines are well visible and the emission lines of the main components of the polluted soil (Ca, Si, Al, Mg, Na, Ti, Fe, Pb, see fig. 5 for the identification of the LIBS lines in the spectrum) are clearly recognizable; these results are extremely encouraging towards the realization of a man-portable miniaturized LIBS prototype for environmental analysis.

Figure 5 -LIBS spectrum of the PACS-1 sample, with the identification of the main emission lines

4. Conclusion

In this paper we have shown the first application of double-pulse LIBS technique on soils, using a mobile instrument specifically realised for in-situ analysis. Improvements on LIBS signal of the order of 5-10 times, depending on the upper level energy of the transition, are typically obtained on compact soil samples, while smaller improvements, or even a reduction of the LIBS signal, are observed on fine grained soils. The enhancement of the LIBS signal obtained using the double-pulse technique corresponds to an improvement of the LIBS limit of detection for that particular transition and, in general, for

200 400 600 800 1000

0

20000

40000

60000

80000

LIB

Ssi

gnal(

a.u

.)

Wavelength (nm)

a)

200 250 300 350 400

0

2000

4000

6000

8000

10000

12000

14000

Wavelength (nm)


8

the corresponding emitting element. This effect is particularly interesting in view of quantitative analysis using standard-less procedures (CF-LIBS). Moreover, the possibility of using a micro-source for LIBS analysis of polluted soils was demonstrated. The development of the miniaturized LIBS system will continue within the activities of the MIAO project; a field prototype of the system will be tested in year 2006.

Acknowledgement

The present activity was performed in the framework of the MIAO (Microsensors for Analysis in Hostile Environment) project, funded by Italian Ministry for Education and Research under the FIRB scheme

REFERENCES

Adrain RS, Watson J Laser microspectral analysis: a review of principles and applications. J. Phys. D: Appl. Phys., 17 (1984) 1915-1940.Barbini R, Colao F, Lazic V, Fantoni R, Palucci A, Angelone M. On Board Libs Analysis of Marine Sediments Collected During the Xvi Italian Campaign in Antarctica. Spectrochimica Acta Part B-Atomic Spectroscopy 2002;57:1203-1218.Castle BC, Knight AK, Visser K, Smith BW, Winefordner JD. Battery powered laser-induced plasma spectrometer for elemental determinations. Journal-of-Analytical-Atomic-Spectrometry 1998;13:589-595.Ciucci A, Corsi M, Palleschi V, Rastelli S, Salvetti A, Tognoni E. New procedure for quantitative elemental analysis by laser-induced plasma spectroscopy. APPLIED SPECTROSCOPY 1999;53:960-964.Colao F, Lazic V, Fantoni R, Pershin S. A Comparison of Single and Double Pulse Laser-Induced Breakdown Spectroscopy of Aluminum Samples. Spectrochimica Acta Part B-Atomic Spectroscopy 2002;57:1167-1179.Evans EH, Day JA, Price WJ, Smith CMM, Sutton K, Tyson JF. Atomic Spectrometry Update. Advances in Atomic Emission, Absorption and Fluorescence Spectrometry and Related Techniques. Journal of Analytical Atomic Spectrometry 2003;18:808-833.Gornushkin IB, Amponsah-Manager K, Smith BW, Omenetto N, Winefordner J Microchip Laser-Induced Breakdown Spectroscopy: A Preliminary Feasibility Investigation, Appl. Spectrosc., 58 [7] 762-769 (2004). Miles B, Cortes J. Subsurface heavy-metal detection with the use of a laser-induced breakdown spectroscopy (LIBS) penetrometer system. FIELD-ANALYTICAL-CHEMISTRY-AND-TECHNOLOGY 1998;2:75-87.Rusak DA, Castle BC, Smith BW, Winefordner JD. Fundamentals and applications of laser-induced breakdown spectroscopy. Critical-Reviews-in-Analytical-Chemistry 1997; 27:257-290.Sattmann R, Sturm V, Noll R. Laser-induced breakdown spectroscopy of steel samples using multiple Q-switch Nd:YAG laser pulses. Journal of Physics D (Applied-Physics) 1995;28:2181-2187.St-Onge L, Sabsabi M, Cielo P. Analysis of solids using laser-induced plasma spectroscopy in double-pulse mode. Spectrochimica Acta Part B Atomic Spectroscopy 1998;53:407-415.Theriault GA, Bodensteiner S, Lieberman SH. A real-time fiber-optic LIBS probe for the in situ delineation of metals in soils. FIELD-ANALYTICAL-CHEMISTRY-AND-TECHNOLOGY 1998;2:117-125 .Yamamoto KY, Cremers DA, Ferris MJ, Foster LE. Detection of metals in the environment using a portable laser-induced breakdown spectroscopy instrument. Applied-Spectroscopy 1996;50:222-233.Wainner RT, Harmon RS, Miziolek AW, McNesby KL, French PD. Analysis of environmental lead contamination: comparison of LIBS field and laboratory instruments. Spectrochimica-Acta-Part-B-Atomic-Spectroscopy 2001;56:777-793


9

Pagina 1Pagina 2Pagina 3Pagina 4Pagina 5Pagina 6Pagina 7Pagina 8Pagina 9

New perspectives in LIBS analysis of polluted soils · easily transferred in the food chain through...

Documents

Transcript of New perspectives in LIBS analysis of polluted soils · easily transferred in the food chain through...