Detection of Low Lithium Concentrations Using Laser-Induced Breakdown Spectroscopy (LIBS) in High-Pressure and High-Flow Conditions.

This paper describes the effects of laser pulse rate and solution flow rate on the determination of lithium at high pressure for water and 2.5% sodium chloride solutions using laser-induced breakdown spectroscopy (LIBS). Preliminary studies were performed with 0-40 mg L-1 Li solutions, at ambient pressure and at 210 bar, and in static and flowing (6 mL · min-1) regimes, for a combination of four different measurement conditions. The sensitivity of calibration curves depended on the pressure and the flow rate, as well as the laser pulse rate. The sensitivity of the calibration curve increased about 10% and 18% when the pressure was changed from 1 to 210 bar for static and flowing conditions, respectively. However, an effect of flow rate at high pressure for both 2 and 10 Hz laser pulse rates was observed. At ambient pressure, the effect of flow rate was negligible, as the sensitivity of the calibration curve decreased around 2%, while at high pressure the sensitivity increased around 4% when measurements were performed in a flow regime. Therefore, it seems there is a synergistic effect between pressure and flow rate, as the sensitivity increases significantly when both changes are considered. When the pulse rate is changed from 2 to 10 Hz, the sensitivity increases 26-31%, depending on the pressure and flow conditions. For lithium detection limit studies, performed with a laser pulse energy of 2.5 mJ, repetition rate of 10 Hz, gate delay of 500 ns, gate width of 1000 ns, and 1000 accumulations, a value around 40 µg L-1 was achieved for Li solutions in pure water for all four measurement conditions, while a detection limit of about 92 µg L-1 was determined for Li in 2.5% sodium chloride solutions, when high pressure and flowing conditions were employed. The results obtained in the present work demonstrate that LIBS is a powerful tool for the determination of Li in deep ocean conditions such as those found around hydrothermal vent systems.

Suppressing the Multiplex Disadvantage in Photon-Noise Limited Interferometry Using Cross-Dispersed Spatial Heterodyne Spectrometry.

Spatial heterodyne spectrometers are members of the static Fourier transform class of spectrometers, well-regarded for their ability to acquire high-resolution, high wavelength precision emission spectra in compact, light footprint packages. In a spatial heterodyne spectrometer experiment, a Fizeau fringe is generated for every spectral feature in a given spectrum, and spatial heterodyne spectrometer records the superposition of all Fizeau fringes in the spectrum on a detector. Hence, the sensitivity of spatial heterodyne spectrometers is constrained by uncorrelated, multiplicative photon noise that limits the detection of spectral features to those that are more luminous than the square root of the total incident flux onto the detector. In essence, powerful spectral features create a rising floor of noise that wash out less luminous features. In the present work, we introduce a novel spectrometer coupling, that being an Amici prism spectrometer in series with spatial heterodyne spectrometer, that correlates photon shot noise along one axis of a detector that in turn suppresses multiplicative photon noise within each row of the interferogram image. We demonstrate that this spectrometer pairing facilitates the measurement of weak Raman spectral features that, in a traditional spatial heterodyne spectrometer measurement, would be washed out by multiplicative photon noise.