Impact of fluorescence on Raman remote sensing of temperature in natural water samples

Abstract

A comprehensive investigation into the impact of spectral baseline on temperature prediction in natural marine water samples by Raman spectroscopy is presented. The origin of baseline signals is investigated using principal component analysis and phytoplankton cultures in laboratory experiments. Results indicate that fluorescence from photosynthetic pigments and dissolved organic matter may overlap with the Raman peak for 532 nm excitation and compromise the accuracy of temperature predictions. Two methods of spectral baseline correction in natural waters are evaluated: a traditional tilted baseline correction and a new correction by temperature marker values, with accuracies as high as ± 0.2°C being achieved in both cases.

A LIDAR-Compatible, Multichannel Raman Spectrometer for Remote Sensing of Water Temperature

“…Marker sensitivities were also estimated for an ultrapure water sample, representing the percentage change in the marker values per °C. For natural water samples variations in the markers values may be associated with the presence of fluorescence from other optically active components in water, as reported in Reference [23], hence not representing the markers sensitivity to temperature only.…”

“…Raman signals were integrated within channels on both sides of the isosbestic point and temperature markers were calculated based on the ratio of integrated signal intensities for each channel. By using two-colour markers calculated from channel integrations, accuracies as high as ±0.1 °C were achieved for ultrapure water (Reverse-Osmosis) and ±0.2 °C for natural water samples [23] measured in laboratory.…”

“…Raman spectroscopy has proven to be an effective technique for determining water temperature in the laboratory with high accuracies of up to ±0.1 °C and ±0.5 °C using two-colour or depolarization markers, respectively [11,24]. The reports in [23,25,26] propose the possibility of measuring subsurface water temperature using Raman spectroscopy in combination with LIDAR methods, collecting time-resolved Raman signals in channels selected by optical filters. This is the ultimate goal of our research project.…”

Remote Sensing of Natural Waters Using a Multichannel, Lidar-Compatible Raman Spectrometer and Blue Excitation

“…That work utilized unpolarized Raman spectra, two-color markers, and Reverse-Osmosis laboratory water. When trying to conduct the same analysis for temperature predictions in natural waters, we found substantially lower accuracies, which we attributed to the overlapping of the Raman peak for 532 nm excitation and fluorescence signals (de Lima Ribeiro et al, 2019a). The commercial dispersive Raman spectrometer (RS) used in Artlett and Pask (2015) and de Lima Ribeiro et al (2019a) did not fulfill LIDARcompatibility requirements and, in order to transition from commercial equipment toward LIDAR-compatible technologies, we designed and assembled a LIDAR-compatible multichannel RS integrated to a 532 nm pulsed excitation laser (de Lima Ribeiro et al, 2019b).…”

“…Here, the main purpose of using excitation at 473 nm was avoiding Chl-a fluorescence at 680 nm, as the water Raman peak for blue excitation lies around 560 nm. However, constituents other than Chl-a exhibit fluorescence peaks around 560 nm, including DOM and other photosynthetic pigments (James et al, 1999;Lin, 1999Lin, , 2001de Lima Ribeiro et al, 2019a), and it is virtually impossible to avoid overlapping between the water Raman peak and all possible signal sources in natural waters. In de Lima Ribeiro et al 2019b, the presence of Chl-a fluorescence signals overlapping with the water Raman signals excited by green light (532 nm) led to higher signal counts and consequent higher SNRs, and lower % errors in the markers calculated for all-natural water sample.…”