APPLICATION OF HIGH-POWER Nd: YAG LASERS

APPLICATION OF HIGH-POWER Nd: YAG LASERS FOR ENVIRONMENTAL MONITORING Sergey Golik*, Oleg Bukin**, Alexey Ilyin***, Vladimir Tsarev*, Pavel Saluk***, Konstantin Shmirko* *Far Eastern National University, Institute of Physics and Information Technologies, 690950, Vladivostok, Russia, Sukhanova st. 8, tel. +7 (4232) 433684, fax +7 (4232) 220315, [email protected] ** Maritime Physical-Technical Institute, Vladivostok, Russia *** Pacific Oceanological Institute, Far East Division of RAS, Vladivostok, Russia ABSTRACT

The laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF) methods were applied for element and pigment concentration detection of marine water and phytoplankton samples. The spectra of plasmas generated by focusing the first harmonic λ=1064 nm of a pulsed Nd:YAG laser on the liquid surfaces and phytoplankton samples in atmospheric pressure are described. Some of the chemical elements detecting by LIBS in the marine water (Na, Ca, Mg) and phytoplankton (Mg, Ca, Na, Fe, Si, Al) were defined. The radiation of second harmonic λ=1064 nm of a pulsed Nd: YAG laser was used for phytoplankton pigments and dissolved organic matter detection by LIF. The results of joint application of LIBS and LIF measurements for phytoplankton communities state investigations inside Okhotsk Sea are presented. Keywords: laser, spectroscopy, emission spectrum, fluorescence. 1. INTRODUCTION

The developments of new efficient methods of environmental monitoring have an increasing significance with the changes in biosphere. Usages of methods provide the operative measurements of the parameters in large spatial scales are main requirements of the modern level of environmental monitoring. The optical methods especially laser spectroscopy methods satisfy this requirements [1-3]. As a sources of excitations the Q-switched Nd:YAG lasers have a wide spreading by it reliability, high peek power, small size and simple in use, it are a very important factors of in situ measurements [4-5]. The application of Q-switched Nd: YAG lasers for marine water and phytoplankton monitoring were described in this work. Now, phytoplankton community has especially attention as a main reproducer of the organic matter in the Ocean. Most important parameters characterized a phytoplankton cells state is a chemical and pigment sells composition. As known, it is necessary of presence of number chemical elements (such as Mg, Ca, Na, Fe and other) in definite concentrations range for normal phytoplankton cells metabolism [6]. Also, phytoplankton makes accumulation of chemical elements of different contaminations during habitability. In this case, in first, phytoplankton can use as an indicator of

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High-Power Lasers and Applications III, edited by Dianyuan Fan, Ken-ichi Ueda, Jongmin Lee, Proceedings of SPIE Vol. 5627 (SPIE, Bellingham, WA, 2005) 0277-786X/05/$15 · doi: 10.1117/12.571018

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contamination and in the second, as an object of investigation of influence of different contamination on the phytoplankton cells conditions. The laser-induced breakdown spectroscopy and laser-induced fluorescence methods provide the possibility of the registration of the large data sets of the element and pigment concentration of phytoplankton in real time (in situ) conditions on a large marine areas. A joint LIBS and LIF measurements extend a number of parameters used in phytoplankton monitoring and may be applied to investigation of the influence of marine water element concentration changes on the phytoplankton pigments compositions. 2. EXPERIMENTAL

The spectra of laser spark generated on the water, phytoplankton surfaces were registered by a LIBS spectrometer. The scheme of a LIBS spectrometer is presented in a fig 1. The main parameters are indicated in Table 1. Table 1. Experimental setup parameters Laser Q-switched Nd:YAG Wavelength, 1064 nm, single pulse duration, 20 ns, total pulse duration 150 – 200 µs Total energy used up to 600 mJ Detection system Polychromator

Grating

Fig 1. Experimental LIBS setup: 1 – Nd: YAG laser; 2 – rotation prism; 3 – focusing lens ( focus 70 mm); 4 – protective glass; 5 – target; 6 – quarts lens (focus 75 mm); 7 – polychromator; 8 – camera; 9 – PC.

Camera

Flame Vision Pro System Spectra Pro 150, Action Research Corporation, GMB 1200/(2400) lines/mm DiCAM-PRO, PCO CCD IMAGIN, GMB, 12 bit, 1280х1024 elements

The special complex shape of the Nd: YAG-laser pulse is applied to plasma plume excitation [7]. Several Q-switched pulses (6-8) with duration of 20 ns (FWHM) were observed above the free oscillation background. Total pulse duration on the basis was 150-200 µs. Intervals between Q-switched pulses were 20-30 µs. Mode-lock crystal LiF was used to obtain Q-switched pulse. The very high energy level of the flash-lamps was used to obtain complex laser pulse form. Sample preparation for LIBS. The marine water was clear of zoo- and phytoplankton by special filter and put in glass cell for analyses. Phytoplankton was filtered by filter with the diameter of a cell of 0.4 microns and thickness of 0.1 mm and then this filter was used for analysis. The spectra of fluorescence of marine water and phytoplankton were registered by a LIF spectrometer. The scheme of a LIF spectrometer is presented in a fig 2. The second harmonic of Qswitched Nd: YAG laser (λ=532 nm, pulse energy 10 mJ, repetition rate up to 10 Hz) was used for LIF spectra excitation. The diameter of laser beam of 30 mm and power density about 70 kW/sm2

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supply a necessary condition of linear fluorescence response. The LIF spectra were induced in pumping water cell (volume 250 sm3), positioned above the axis of the laser beam. The scattering light is directed into optical fiber which project this light onto the entrance slit of the monochromator. A small-sized scanning monochromator is based on a MUM-2 monochromator, in which a spherical diffraction grating with 1200 groove/mm and inverse linear dispersion of 6 nm/mm is used. The wavelength scanning range is 540-700 nm. As a rule, the measurements time over the entire spectral range is 1 min at repetition rate of laser pulse of 2 Hz. This correspond to a spatial resolution of 300 m along the ship’s rout at a velocity of 10 knots. It was not necessary to prepare the samples for LIF analysis.

Fig 2. Experimental LIF setup: 1 – active laser element (Nd: YAG); 2, 3 – resonator mirror; 4 – mode-lock crystal LiF; 5, 6, 9, 10 – rotation prism; 7 – active element of amplifier (Nd: YAG); 8 – KDP crystal; 11 – glass, 12 – telescope, 13 – optical cell; 14, 18 – optical fiber, 15 – scanning monochromator; 16, 19 – photomultiplier, 17 - color filter; 20 AD converter; 21 – PC; 22 – water pump; 23 – discharge pipe. 3. RESULTS

The possibility of direct LIBS analyses of elemental composition was investigated by registering of emission spectra of marine water and phytoplankton in wave range of 190-800 nm. The emission spectra of a laser spark excited on the surfaces of distilled and filtered marine water and a pure filter and a filter contained phytoplankton samples are given in a fig. 3, 4. The wavelengths on horizontal axes are in nanometers.

Fig 3. LIBS spectra of distilled water (1), air (2) and marine water (3).

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Fig 4. LIBS spectra of pure filter (1) and filter with phytoplankton (2).

The relative intensity of emission lines on vertical axis is in arbitrary units. So, the emission lines of Mg, Ca, Na and atmospheric N, O were registered in marine water and Mg, Ca, Na, Si, Al, Zn, C in phytoplankton. Quantitative elemental analysis of marine water was carried out using of the calibration procedure of the solutions. Calibration curves for Mg, Ca, Na, Si, Al and Zn water solutions and detection limits for these elements were obtained. The examples of calibration curves for Mg, Ca, Na, Ва are performed in Fig.5. 1800

2

R = 0,9781

b)

y = 913,57x + 139,3

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R = 0,9639

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R = 0,9233

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200 0 0

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Fig 5. Calibration curves for Ca (a), Mg (b), Ba (c), Na (d) water solutions.



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Table 2. Detection limits of chemical elements water solutions. The detection limits were Element Wavelength, nm Detection limit, Relative errors, % calculated by the standard g/l method: 2σ/s, where σ is Na 588.9 0,001 17.7 the standard deviation Ca 393.4 0,0009 10.9 associated with the total Mg 279.5 0,0007 13.7 noise of the system and s is Ba 455.4 0.006 12.8 the slope determined from Fe 373.4 0,4 11.6 the calibration curves [8]. Al 396.1 0,5 14.9 The detection limits are Zn 334.5 0,6 22.2 given in Table 2. The LIF spectra of marine water with phytoplankton registered by flow-through fluorometer during the algae bloom in Okhotsk Sea is presented in fig 6. This spectra were smoothed and separated into the next biooptical compounds: the line of phycoerethrin fluorescence IPhc (centered at 580 nm) [9]; the line of fluorescence with the center on 610 nm (pigment Y); Raman scattering (IRS, 648 nm); line of fluorescence of chlorophyll-a (IChla, centered at 680 nm); line of fluorescence of chlorophyll-b (IChlb, centered at 710 nm); and fluorescence of chromophoric DOM (I FDOM). The parameters of all biooptical components were determined by the least-squares algorithm [10]. Fig 6. LIF spectra of marine water with phytoplankton. The route of marine expedition with a joint LIF and LIBS measurements is shown in fig.7. The continuous lines on a fig.7 are LIF measurements and the points on the route are the regions where the LIBS measurements were carried out.

Fig 7. The route of marine expedition with a joint LIF and LIBS measurements in 2001.

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The scattering diagram between the relative intensity of emission line of Mg (285.2 nm) measured by LIBS in marine water and intensity of LIF lines of chlorophyll – a, chlorophyll – b, phycoerethrin and pigment Y in phytoplankton are given in a fig. 8.

Fig 8. The scattering diagrams It was observed a good linear correlation (r = 0.8) between a intensity of emission line (279.5 nm) of Mg in marine water and intensity of fluorescence line of chlorophyll – b (Fig. 8 b) and intensity of fluorescence line of pigment Y (Fig. 8 d), but in the other diagrams it wasn’t find a good agreements. The analyses of such diagrams provide the possibility to establish the influence of marine water element concentration changes on phytoplankton pigments compositions. 4. CONCLUSION Thus, it was defined a round of the chemical elements detecting in laser spark generated by focusing a radiation of a pulsed Nd: YAG laser on the marine water and phytoplankton samples. The spectra of fluorescence of marine water and phytoplankton exciting by radiation of the second



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harmonic of a pulsed Nd: YAG laser were investigated. A joint LIBS and LIF measurements were made in the Okhotsk Sea. It was shown, that application of lasers give the possibility of detection of influence of marine water element concentration changes on phytoplankton pigments compositions REFERENCES 1. Barbini R., Colao F., Fantoni R., Palucci A., Robes S. Differential lidar fluorosensor system used for phytoplankton bloom and seawater quality monitoring in Antarctica // Int. J. Remote Sensing.-2001. v. 22. №2/3. p. 369-384. 2. C.M. Davies, H.H. Telle, D.J. Montgomery, R.E. Corbett Quantitative analyses using laserinduced breakdown spectroscopy (LIBS) // Spectrochimica Acta Part B.-1995.-v.50.-p.10591075. 3. Kolber Z.S., Falkowski P.G. Use of active fluorescence to estimate phytoplankton photosynthesis in situ // Limnology and Oceanography.-1993. v. 38.№8.- p. 1646-1665. 4. Exton R.J., Houghton W.M., Esais W., Harriss R.C., Farmer F.H., White H.H. Laboratory analysis of techniques for remote sensing of estuarine parameters using laser excitation // Applied Optics.-1983.-v.22.- № 1.- p. 54-64. 5. D. A. Rusak, B.C. Castle, B. W. Smith, J. D. Winefordner Recent trends and the future of laserinduced plasma spectroscopy. Trends in analytical chemistry.// v. 17. n . 8+9. 1998. p. 453-461. 6. J. H. Martin, K. H. Coale, K. S. Johnson et al. Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean // Nature.- 1994.- v. 371 p.123-129. 7. S.S. Golik, O.A. Bukin, A.A. Ilyin, V.I. Tsarev Investigation of marine water quality and monitoring phytoplankton by laser-induced breakdown spectroscopy // Proceedings of SPIE.2002.-v. 5149.- p. 223-230. 8. P. Fiched, P. Mauchien, J-F. Wagner, C. Moulin. Quantitative elemental determination in water

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