Oceanic Applications of Laser Induced Breakdown Spectroscopy: Laboratory Validation

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1 Oceanic Applications of Laser Induced Breakdown Spectroscopy: Laboratory Validation Anna PM Michel, Marion J. Lawrence-Snyder, S. Michael Angel, and Alan D. Chave Massachusetts Institute of Technology / Woods Hole Oceanographic Institution Joint Program Department of Applied Ocean Physics and Engineering Woods Hole, Massachusetts amichel@whoi.edu, Telephone: (58) , Fax: (58) Department of Chemistry and Biochemistry University of South Carolina Columbia, South Carolina 2928 Department of Applied Ocean Physics and Engineering Woods Hole Oceanographic Institution Woods Hole, Massachusetts 2543 Abstract New chemical sensors are needed for both present day expeditionary oceanography and an emerging new phase involving long term in situ ocean observations. Over the past four decades, a new spectrochemical technique, Laser Induced Breakdown Spectroscopy (LIBS), has been under development for the identification of the elemental constituents of materials. This technique uses a laser to create a spark or plasma on a sample. The plasma emission is then analyzed with a spectrometer to determine its elemental composition. Recently, LIBS has been identified as a viable tool for in situ field measurements because it is able to analyze all forms of matter (solids, liquids, and gases), can operate in a stand-off mode, and is non-invasive and non-destructive. A marine LIBS sensor would be a useful tool for studying many environments in the ocean, especially midocean ridge hydrothermal vents where in situ measurements are difficult due to the presence of high-temperature, corrosive fluids. A feasibility assessment of oceanic LIBS in the laboratory has been initiated. A high pressure chamber was designed and built for investigating the effect of realistic ocean environments on the LIBS signal. Preliminary work shows that LIBS can successfully detect Li, Na, K, Ca, Mn, and Zn in bulk aqueous solutions at pressures up to 272 atm, making LIBS a viable technique for deep ocean chemical sensing. I. INTRODUCTION In recent ocean science workshops, the development of in situ chemical sensors was identified as a major priority for understanding the ocean [1]. This is especially important since a new paradigm for study is beginning with the implementation of ocean observatories. As these facilities become the mode of future ocean study, sensors capable of long deployments will be essential. The goal of the present work is the development of a new chemical sensor capable of obtaining real-time analytical data in the ocean. Since it was reported in the literature in 1962, laser-induced breakdown spectroscopy (LIBS) has been under development as a laboratory technique and has recently been identified as a viable tool for real-time in situ geochemical and environmental field work and for work in extreme and hostile environments [2] [11]. LIBS has been used in a variety of areas including the analysis of rocks, archaeological artifacts, and paint. Several field portable systems have been developed, for example, for detecting heavy metals in soils [5], [12], and other efforts are focused on designing systems for geochemical analysis on Mars and Venus [13] [17]. The LIBS technique yields simultaneous sensitivity to virtually all elements in the parts-per-million (ppm) or better range in solids, liquids, gases, and aerosols. LIBS is effectively non-invasive and non-destructive (typically only pg to ng of material are ablated) and requires no sample preparation. LIBS can be used in a stand-off mode without perturbing the target and it requires only optical access to a sample. LIBS is essentially a real-time measurement, requiring <1s per cycle. Unlike many techniques that require collection of a sample followed by its transport to a laboratory, LIBS measurements can be made directly in the field. These characteristics are those required for in situ chemical sensing in the ocean [1], [18] [21]. LIBS is a promising in situ technique for oceanography and will have extensive applications in chemical, geological, and biological oceanography ranging from laboratory experimentation to fieldwork. LIBS could be used to determine the composition of sediments, rocks, or seawater and for studying environmental issues. To determine the feasibility of oceanic LIBS, we have focused on the detection of elements found in one extreme and hostile ocean environment, hydrothermal vent systems. Taking measurements directly in vent fluid is especially challenging due to extremes of temperature and its corrosive nature. The use of an indirect method like LIBS may enable an understanding of the chemistry of vents, and especially its temporal variability, that has thus far been difficult to achieve. II. LASER INDUCED BREAKDOWN SPECTROSCOPY LIBS utilizes a high power laser focused onto a sample to create a plasma (Fig. 1). In detail, when a short duration, laser pulse of sufficient energy density strikes the target, the surface temperature instantly increases causing the matter in

2 1) Solution is pressurized in the sample chamber. 7) A computer is used for data collection. Computer 6) An intensified charge-coupled device detects the spectrally resolved signal. High Pressure Pump 3) A spark or plasma is formed in the solution Spectrograph ICCD * Valve Sample Chamber Nd:YAG Laser nm or 532 nm Nd:YAG Laser nm or 532 nm Fiber Optic 4) The light emitted by the plasma is focused onto a fiber optic which transmits the collected light 5) A spectrograph separates the emitted light into wavelengths. All elements emit at different wavelengths. 2) High power laser(s) is focused using optics into sample chamber. If 2 lasers are used, the timing between their pulses is controlled by a timing generator. Timing Generator Fig. 1. Experimental set-up for laser induced breakdown spectroscopy studies of liquids in a pressurized chamber. the spark to be vaporized, reduced to its atomic species, and then electronically excited [22]. Emission lines at discrete wavelengths that characterize the elements present are then resolved spectrally and temporally with a spectrometer covering part or all of the ultraviolet through near infrared range (2-1 nm wavelength). A CCD (charge coupled device) or ICCD (intensified charge coupled device) serves as the detection device within the spectrometer. Each element emits at different wavelengths and therefore by examining a spectrum, the elements contained within a material can be identified from the spectral peaks present. The peak intensities provide a quantitative description of the material [1], [23]. Some researchers have successfully created plasma ablations on materials submerged in water and on liquid surfaces; yet, few researchers have attempted plasma ablation within bulk aqueous solutions [24] [41]. LIBS is capable of identifying Li, Na, K, Rb, Cs, Be, Ca, B, and Al in aqueous solutions with varying detection limits, but typically at the ppm level [24]. This work also showed that by using two laser pulses, separated by a short time delay, the detection limit for dissolved species could be improved. The first laser pulse creates a cavitation bubble and a second pulse is fired into the bubble, forming a plasma within the gaseous environment. LIBS studies have shown that plasma emission is quenched in a liquid environment, resulting in a reduction in both plasma light intensity and the length of time during which plasma emission can be observed. Liquid LIBS also displays a broadening of spectral peaks [24], [32], [35], [42]. In the development of a LIBS sensor for use at hydrothermal vents, the effect of both pressure and temperature on the LIBS signal must be quantified. A few papers have reported using LIBS in higher than atmospheric pressure environments; yet, the pressures used were less than 1 atm, well below the ambient pressures in the deep ocean [16], [43]. Molten materials have been analyzed with LIBS, demonstrating the feasibility of using the technique under high temperature conditions [44] [47]. III. SCIENTIFIC APPLICATION: HYDROTHERMAL VENT CHEMISTRY Hydrothermal venting occurs on mid-ocean ridges where seawater circulates through the fractured and permeable crust. Exit temperatures at high temperature vents range from 2-45 o C at ambient pressures of atm. Substantial changes in fluid composition occur due to interaction with the host rock, phase separation, and possibly magma degassing. For example, many alkalis (e.g., Li, Na, and Ca) and transition metals (e.g., Fe, Mn, Cu, and Zn) are leached from the host rock and concentrated to varying degrees in the fluid, while Mg and SO 4 are largely removed from the fluid. Cooling due to mixing with seawater occurs above the seafloor causing additional changes to the fluid chemistry [48]. Von Damm [49] and Butterfield et al. [5] review the chemistry of hydrothermal vent fluids. The elements that have been selected to be a focus of our work are important for understanding vent chemistry (Li, Na, K, Mg, Ca, Mn, Fe, Cu, Zn, Si, Cl and Br). Using a subset of these elements, we investigated the effect of the environmental conditions found at vent systems on the LIBS signal; more specifically, we examined the effect of high pressure, temperature, and NaCl concentration. In addition, the effect of laser pulse energy on analyte detection at high pressures was evaluated. IV. EXPERIMENTAL SET-UP The laboratory LIBS system was set up to run both single (SP) and dual (DP) laser pulse experiments (Fig. 1). For single pulse experiments, a Continuum Surelite III laser (7-ns pulse) was used for excitation. For dual pulse experiments, a Quantel Nd 58 (9-ns pulse) was used for the first laser pulse followed by a second pulse from the Surelite laser. Both lasers were Q- switched Nd:YAG lasers operated at the 164 nm fundamental wavelength with a repetition rate of 5 Hz. For dual pulse experiments, a variable clock (Stanford Instruments Model SR25) with a delay generator (Stanford Instruments Model DG535) controlled the triggering and timing of the lasers. A high pressure cell, designed to reach pressures of >34 atm, was designed and constructed of stainless steel Swagelok fittings with six 1 -ID view ports. Stainless steel tubing ( 1 8 ) connected one port to a pump (Isco Syringe Pump Model 26D, Teledyne Technologies Incorporated) which allowed aqueous solutions to be flowed into the cell and for the cell to be pressurized. A second port was equipped with the same tubing and a regulating valve for cell drainage. Two ports were affixed with sapphire windows (MSW1/125, Meller Optics Incorporated). The remaining two ports were sealed with Swagelok steel plugs. Fused silica lenses and dielectric mirrors were used for focusing the laser pulses into the cell and for focusing the plasma emission onto a collection fiber optic. When dual pulses were used, the laser pulses followed the same optical path, making the pulses collinear to each other. All optics were mounted on micrometer stages allowing precise control

3 of beam overlap and collection field of view within the high pressure cell. The plasma emission was measured using one of two different spectrometer set-ups. The first is a 196-µ-core-diameter,.51-N.A. (numerical aperture), light guide (Edmund Scientific Co. Model 2551) connected to a.25-m focal length, f/4 spectrograph (Chromex, Model 25IS/RF) with a 12-groove grating blazed at 5 nm. The data collection system was an intensified CCD detector (Princeton Instruments, I-Max 124E) with spectra taken using a computer running Win- Spec/32 software. In the alternate configuration, the plasma emission was focused onto an 8 µ diameter fiber which was connected to an Echelle spectra analyzer ESA 3 (LLA instruments GmbH), with a 1 µm slitwidth. This spectrometer was used for obtaining spectra over a broad wavelength range (2-78 nm). The spectral resolution ranges from nm over the 2-78 nm spectral range. This system consists of an Echelle spectrograph, an ICCD camera, and a control unit containing a computer, the electronics, and a water-cooling system. The detector consists of a CCD-array coupled with an MCP-image intensifier (Model 3 CP). The internal shutter is controlled by a Fast-Pulse-Generator board (Model 3 FP) and a driver board (Model 3 IS). The key LIBS timing parameters that must be optimized for analyte detection are shown in Figure 2 [35], [36]. The first and second laser pulse energies are referred to as E 1 and E 2. For dual pulse experiments the timing between the two pulse energies, the interpulse delay, is referred to as T. The gate delay, t d, is the delay after the laser pulse, or for dual pulse experiments, after the second laser pulse, before the detector is turned on. The emission is recorded by the detector for the length of time set by the gate width, t b. Sample solutions were made using NaCl, CaCl 2, LiCl, MnSO 4 H 2 O, NaBr, ZnBr, and KI dissolved in deionized water. Where noted, NaCl was added to the solutions to simulate seawater. All concentrations are listed in parts per million (ppm) by mass. V. RESULTS AND DISCUSSION A. Effect of Pressure on LIBS Signal In this work, LIBS was successfully used for the detection of Li, Na, K, Ca, Mn, and Zn at pressures up to 272 atm. LIBS yields reproducible, high quality spectra at these high pressures, and probably beyond, with no major problems from line broadening or quenching. Figure 3 shows spectra taken at 3 atm and 272 atm obtained with the broadband Echelle spectrometer, and shows the simultaneous detection of 1 ppm Ca, Mn, Na, Li, and K. It is important to note that increased pressure does not significantly affect the signal intensity and peak width of these analytes. In another study, using a low energy single pulse, the pressure effects on signal intensity for Mn, Ca, and Na were investigated (Fig. 4). Using the same conditions for all pressures, no effect was seen on the detection of Ca and Na. Increased pressure enhanced the signal intensity of Mn. This may have been a result of using the same timing parameters for all pressures. The full peak width at half of the maximum intensity (FWHM) was calculated for each analyte and did not change Fig. 2. Key timing parameters used in LIBS experiments: E 1 =energyof laser pulse 1 and E 2 = energy of laser pulse 2; T = interpulse delay, length of time between laser pulses, when dual pulses are used; t d = gate delay, the delay after the laser pulse, or for dual pulse experiments, after the second laser pulse, before the detector is turned on; t b = gate width, the length of time the detector mesasures emission. Fig. 3. Spectra of 1, ppm Ca, Mn, Na, Li, and K taken with the Echelle spectrometer taken at 3 atm and 272 atm. The full broadband spectra is shown in and a smaller wavelength region (39-43 nm) is blown-up and shown in. The minimal effect of pressure on LIBS spectra is evident.

4 Intensity (a.u.) x Intensity (a.u.) x Pressure (atm) Fig. 4. Effect of pressure on the LIBS signal using low laser pulse energies. Each data point is an average of 1 spectra each with 25 accumulations and error bars represent ±1σ. 1 ppm Na ( ), E 1 = 22 mj; 5, ppm Mn in 2,54 ppm NaCl ( ), E 1 = 14 mj; 5 ppm Ca in 2,54 ppm NaCl ( ), E 1 =2mJ);t d = 35 ns; t b =1µs, slit width = 75 µm. FWHM (nm) Intensity (a.u.) x Laser Pulse Energy (mj) mj 11 mj 88 mj Pressure (atm) Fig. 5. Effect of pressure on the FWHM of the peaks using low laser pulse energies. Each data point is an average of 1 spectra each with 25 accumulations and error bars represent ±1σ. (1 ppm Na ( ), E 1 =22mJ; 5, ppm Mn in 2,54 ppm NaCl ( ), E 1 = 14 mj; 5 ppm Ca in 2,54 ppm NaCl ( ), E 1 =2mJ);t d = 35 ns; t b =1µs, slit width = 75 µm. with increased pressure (Fig. 5). Pressure did not have a deleterious effect on signal intensity or on peak width; therefore, the significant pressures in the ocean environment should not inhibit the measurement of these analytes. B. Detection of Analytes Using Single and Dual Pulses and Various Laser Pulse Energies In these studies, readily ionized elements were found to be easily detectable at high pressure using SP LIBS, while elements whose excitation occurs at higher energy levels, for example Zn, were virtually undetectable. Using SP LIBS, the peak signal intensity for four analytes (Li, Ca, Na, and Mn) was measured at various laser pulse energies (11-91 mj) at both low (7 atm) and high (272 atm) pressures. These studies showed increased emission intensity with the use of low laser pulse energy (2-3 mj) for both detection at low and high pressures, for example as shown in Figure 6. Results from these four analytes suggest that lower laser pulse energies yield greater signal intensities than higher energies with the requirement that the energy must be above some threshold level. The optimal laser pulse energy range was also shown to be the same for solutions in low and high pressure environments. A comparison of high and low single pulse energies was completed using the Echelle spectrometer to see the effect over a large range of wavelengths (Fig. 7). This experiment again shows stronger emission intensity at lower powers Fig. 6. Effect of laser pulse energy on the LIBS signal intensity of Na ( nm) at both 7 atm ( ) and 272 atm ( ). Each data point is an average of 1 spectra each with 25 accumulations and error bars represent ±1σ. Effect of three different laser pulse energies (11 mj (middle trace), 22 mj (upper trace), and 88 mj (lower trace)) on Na spectra at high pressure (272 atm). Conditions used for both and : 1 ppm Na; t d = 35 ns; t b =1µs, slit width = 75 µm. The data for Na, Ca, Li, and Mn suggest that the laser pulse energy required for optimizing the LIBS signal is analyte dependent and that there is no significant pressure dependence on the optimal laser pulse energy. A certain minimum laser pulse energy is needed to create a plasma that emits strongly enough to record analyte emission. For the analytes studied, a relatively low laser pulse energy produced the greatest signal intensity. Dual pulse LIBS has been shown to enhance the signal intensity in bulk aqueous solutions for some analytes [24], [36]. In an effort to determine which dual pulse energy levels give the greatest signal intensity at high pressure, four conditions were compared for four analytes at a pressure of 272 atm. These conditions were two low energy pulses, two Intensity (a.u.) x Na Fig. 7. Effect of laser pulse energy ( 2 mj: upper trace, 4 mj: middle trace, 9 mj: lower trace) on the LIBS signal at 68 atm using a 8 µ diameter fiber and an Echelle spectrometer. Solution contains 1 ppm Na, 5 ppm Zn, and 5 ppm K. Note: this experiment was carried out single pulse using the Quantel laser. Conditions: 3 accumulations, t b = 2 ns, t d = 2 ns.) K 7 8

5 high energy pulses, a low first energy pulse followed by a high second energy pulse, and a high first energy pulse followed by a low second energy pulse. Each condition was tested over a range of interpulse delay times. DP LIBS experiments on Li, Na, and Ca show that two low energy pulses ( 31 mj) result in greater emission intensity as compared to the other three conditions (Fig. 8). However, Mn showed a greater emission intensity when a low energy pulse was followed by a high energy pulse ( 85 mj). For DP LIBS, the laser pulse separation determines the expansion volume of the initial bubble and the amount of energy remaining from the first laser pulse before it is excited by the second one. Our experiments have shown that the use of DP excitation provides greater emission intensity for all elements studied and allows a larger range of elements to be measured at elevated pressure. These studies all show that the T value is highly pressure dependent, decreasing to values of 5 ns for pressures of 25+ atm, while pulse separations ranging from µs work well for many elements at atmospheric pressure (Fig. 9). The dual pulse experiments studied here suggest that for several of the analytes, the dual pulse conditions that result in the greatest signal intensity, approach single pulse conditions (low pulse energies separated by a short interpulse delay). Intensity (a.u.) x1 3 Intensity (a.u.) x mj, 84 mj 15 mj, 84 mj 13 mj, 6 mj 15 mj, 6 mj mj, 6 mj 15 mj, 84 mj 15 mj, 6 mj 13 mj, 84 mj Fig. 8. Spectra of 1, ppm Ca with 2,54 ppm NaCl at 272 atm using four different dual pulse conditions. This shows the greatest emission intensity for low-low pulse energies and shows the ionic Ca peaks (393.4 nm and nm in addition to the atomic peak nm). Slitwidth = 1 µm. Spectra of 5, ppm Mn with 2,54 ppm NaCl at 272 atm using four different dual pulse conditions. This shows the highest emission intensity for a low-high pulse combination. Slitwidth = 25 µm. For and The laser pulse energies for E 1 and E 2 are shown above each spectra. T = 2 ns; t d = 35 ns, t b =1µs Intensity (a.u.) x Intensity (a.u.) x ns 3 µs Interpulse Delay (µs) Fig. 9. Effect of dual laser pulse energies on signal intensity at 272 atm for 1, ppm Ca in 2,54 ppm NaCl at various interpulse delays. t d = 35 ns. Each data point is an average of 5 spectra each with 25 accumulations and error bars represent ±1σ. Spectra of Ca showing the enhancement in signal at T = 3 ns over T =3µs. For and : E 1 =13mJ,E 2 = 6mJ;t b =1µs, slit width = 1 µm At short T, three Ca peaks (393.4 nm, nm, and nm) are visible; whereas at the longer T only the third Ca peak (422.7 nm) is present and at a much lower intensity. C. Effect of NaCl on Spectra Understanding how a high concentration of NaCl in a solution affects the detection of other analytes is important for determining the feasibility of using LIBS in the ocean, which has a Na concentration of 1.77 x 1 4 ppm and a Cl concentration of 1.95 x 1 4 ppm [51]. The effect of NaCl on peak signal intensity of Ca, Mn, and K was measured separately at pressures ranging from atm using a dual pulse configuration with a low energy pulse followed by a high energy pulse (4 mj, 125 mj). Each analyte was tested in three solutions: 1) distilled water, 2) 2,54 ppm NaCl dissolved in distilled water, and 3) 25,4 ppm NaCl dissolved in distilled water. For Ca, the addition of NaCl increased the emission intensity of the nm atomic line and both the nm and nm ionic lines (Fig. 1) with no significant difference seen between the addition of a low concentration and high concentration of NaCl. Increased atomic emission relative to ionic emission is seen which is the result of ionization suppression by the readily ionized Na atom. The enhancement from NaCl was pressure independent with only a minimal decrease in signal intensity occurring with increasing pressure. No effect from NaCl on the signal intensity of Mn or K was found. The two effects seen, enhancement of the signal or no change to the signal, suggest that the high NaCl concentration in the ocean will not have a deleterious effect on the ability to detect analytes in this environment and that its effect must be further studied. D. Temperature Effect on LIBS Spectra We investigated the effect of temperature on the peak intensity of Ca (422.7 nm) over the range o C. Peak intensities were measured for a solution of 1, ppm Ca with 2,54 ppm NaCl dissolved in distilled water at atmospheric pressure using a 37 mj single laser pulse (Fig. 11). Over this range of temperatures, no effect on peak intensity with increased temperature was seen indicating that the temperature

6 6x1 3 Intensity (a.u.) Ca + 25,4 ppm NaCl Ca ppm NaCl Ca 41 Fig. 1. Effect of the addition of NaCl on the spectra of 1, ppm Ca. E 1 =4mJ,E 2 = 125 mj, T = 46 ns, t d 1 µs, t d = 1 ns, slit width = 35 µm. ) of the fluid exiting the hydrothermal vent will not impair the ability to measure the fluid in situ. Intensity (a.u.) x Temperature (ºC) Fig. 11. Effect of temperature on the LIBS signal of Ca at atmospheric pressure. (1, ppm Ca with 2,54 ppm NaCl in DI; laser pulse energy, 37 mj; gate delay, 4 ns; gate width, 1 µs) VI. CONCLUSION In this work, the detection of Li, Na, K, Ca, Mn, and Zn at pressures up to 272 atm using LIBS was found to be feasible. In addition, an optimal range of low laser pulse energies was shown to exist for the detection of analytes in bulk aqueous solutions at both low and high pressures, suggesting that optimization of the LIBS parameters can help to improve detection limits. Using low pulse energies, no pressure effect was seen on the emission intensity for Ca and Na, and an increase in intensity with increased pressure was seen for Mn. No peak broadening due to pressure was observed for Ca, Na, or Mn. Using the dual pulse technique, it was also found for several analytes that two low energy pulses separated by a very short delay time, resulted in the greatest emission intensity. However, the results of the dual pulse work varied somewhat by analyte. The addition of NaCl enhanced the emission intensity of Ca but showed no effect on the intensity of Mn or K. Ca was found to be detectable over a wide range of concentrations at a range of pressures. In addition, temperature was shown to have no effect on emission intensity. Overall, increased pressure, the addition of NaCl to a solution, and temperature did not inhibit detection of analytes in solution. 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