Laser-induced breakdown spectroscopy of bulk aqueous solutions at oceanic pressures: evaluation of key measurement parameters

Abstract

The development of in situ chemical sensors is critical for present-day expeditionary oceanography and the new mode of ocean observing systems that we are entering. New sensors take a significant amount of time to develop; therefore, validation of techniques in the laboratory for use in the ocean environment is necessary. Laser-induced breakdown spectroscopy (LIBS) is a promising in situ technique for oceanography. Laboratory investigations on the feasibility of using LIBS to detect analytes in bulk liquids at oceanic pressures were carried out. LIBS was successfully used to detect dissolved Na, Mn, Ca, K, and Li at pressures up to 2.76 x 10(7) Pa. The effects of pressure, laser-pulse energy, interpulse delay, gate delay, temperature, and NaCl concentration on the LIBS signal were examined. An optimal range of laser-pulse energies was found to exist for analyte detection in bulk aqueous solutions at both low and high pressures. No pressure effect was seen on the emission intensity for Ca and Na, and an increase in emission intensity with increased pressure was seen for Mn. Using the dual-pulse technique for several analytes, a very short interpulse delay resulted in the greatest emission intensity. The presence of NaCl enhanced the emission intensity for Ca, but had no effect on peak intensity of Mn or K. Overall, increased pressure, the addition of NaCl to a solution, and temperature did not inhibit detection of analytes in solution and sometimes even enhanced the ability to detect the analytes. The results suggest that LIBS is a viable chemical sensing method for in situ analyte detection in high-pressure environments such as the deep ocean.

LIBS Analysis of Liquids and of Materials Inside Liquids

“…For example, the SP LIBS signal at various water pressures was the highest for low pulse energies, between 20-60 mJ [81]. In a similar experiment [82] the emission from different atomic lines was the most intense for laser energies between 20-30 mJ (Fig. 8.13) while the optimal excitation of atomic Mn lines corresponded to the pulse energy of only 11 mJ.…”

“…Efficient plasma confinement by high pressure media also increases self-absorption, which additionally contributes to broadening of the emission lines. Changes of water temperature between 27 and 99°C do not have a noticeable effect on SP LIBS signal [82].…”

“…Effects of the pressure on LIBS signal strongly depends on the laser pulse energy and on the chosen analytical line. For example, the intensities of the atomic lines from Na, Ca, Li and Mn are almost independent on the water pressure in range 1-270 bars while the intensity of Mn triplet around 403 nm increases with the pressure [82]. The plasma lifetime does not seem significantly affected by the pressure in the given range [16].…”

“…Fig. 8.13 LIBS signal of 100 ppm Na(I) at 588.995 nm: left the line intensity as a function of the pulse energy, at two water pressures b spectra taken at 276 bars for different pulse energies (spectra offset) [82] SP LIBS signal as a function of liquid pressure was studied in [16,81]. At low laser energy (30 mJ) the emission intensity from Li doublet around 670.7 nm was slightly higher at pressure of 7 bars with respect to 276 bars, and the maximum line intensity appears at delay of about 400 ns from the laser pulse [16].…”

Optical tools for ocean monitoring and research

“…Laboratory experiments have also validated the LIBS technique in simulated deep ocean environments (pressure chambers) up to 2.76×10 7 Pa with both single and double pulse techniques (Lawrence-Snyder et al, 2006;Michel et al, 2007). Current results show more promising use of single pulse LIBS in high-pressure liquids due to the fact that the high pressures cause the laser-induced bubbles to collapse faster than occurs at atmospheric pressure (LawrenceSnyder et al, 2007).…”