Highly sensitive Raman system for dissolved gas analysis in water

Abstract

The detection of dissolved gases in seawater plays an important role in ocean observation and exploration. As a potential technique for oceanic applications, Raman spectroscopy has already proved its advantages in the simultaneous detection of multiple species during previous deep-sea explorations. Due to the low sensitivity of conventional Raman measurements, there have been many reports of Raman applications on direct seawater detection in high-concentration areas, but few on undersea dissolved gas detection. In this work, we have presented a highly sensitive Raman spectroscopy (HSRS) system with a special designed gas chamber for small amounts of underwater gas extraction. Systematic experiments have been carried out for system evaluation, and the results have shown that the Raman signals obtained by the innovation of a near-concentric cavity was about 21 times stronger than those of conventional side-scattering Raman measurements. Based on this system, we have achieved a low limit of detection of 2.32 and 0.44 μmol/L for CO₂ and CH₄, respectively, in the lab. A test-out experiment has also been accomplished with a gas-liquid separator coupled to the Raman system, and signals of O₂ and CO₂ were detected after 1 h of degasification. This system may show potential for gas detection in water, and further work would be done for the improvement of in situ detection.

Optimization of parabolic cell for gas Raman analysis

“…For this reason, the data for CO 2 are not included in Table as CO 2 in ambient air has a concentration of 400 ppm. Compared with the LODs of parts‐per‐million level reported in the literatures (LOD of 9.86 ppm‐bar [0.44 μmol/L] of CH 4 at exciting power 300 mW and exposure time of 120 s and LOD of 50 ppm‐bar of air constituents at exciting power 5 W and exposure time of 30 s), our result gives the similar LODs with higher spectral resolution (5 cm −1 , compared with 12 and 10 cm −1 ).…”

“…At present, the cavity‐enhanced techniques have been further developed, and the LODs reported by Hippler et al and Popp et al both have been improved to parts‐per‐million level. Moreover, several sample cells with special optical structures can also be found, such as the near‐confocal cavity reported by Li et al and Yang et al, by which high‐sensitivity detection with tens of parts per million for multitrace gas was achieved . The similar work was also reported by Petrov et al, which is an improved multipass optical system that reaches LODs with several parts per million for gas media by 30‐s registration time under the exciting laser power of 5 W. Normally, the key element of multipass system for signal enhancement is the design and alignment of the reflectors, so high‐precision manufacture and excellent mechanical stability are all needed.…”

A Direct Bicarbonate Detection Method Based on a Near-Concentric Cavity-Enhanced Raman Spectroscopy System

“…As shown in Figure 1 , the near-concentric cavity-enhanced Raman spectroscopy system is adapted on the basis of a formerly reported design [ 14 ]. In order to detect liquid samples, a sample cell of fused silica with 10 mm × 10 mm × 40 mm inner size and 1 mm wall thickness is used and placed at the center of the cavity.…”

“…The strong background signal resulting from SO 4 2− , which has a concentration of ~28 mmol/L in seawater [ 10 ], makes direct detection of HCO 3 − even more difficult. With a similar waveguide-based enhancement mechanism, the multi-pass cavity concept has been presented as an effective approach in gas Raman detection with desirable enhancement [ 11 , 12 , 13 , 14 ]. Taking such an approach in direct seawater Raman detection is the motivation for the work presented in this article.…”

Gas phase Raman spectroscopy : comparison of continuous wave and cavity based methods

“…Here we propose a technique called Cavity Enhanced Raman Spectroscopy (CERS). Including the already increasing range of applications, this technique would also motivate real time, low cost, small footprint in situ measurements in a broad range of other scientific and industrial fields from NEMs/MEMs systems, organic electronics and Nano/Microscale chemistry to planetary atmospheric detection, exo-meteorology, deep-sea explorations [1], human breath analysis [2], turbine power plant [3] and fermentation gases [4]. Real time analysis capabilities, and continuous Raman signals with low power diodes, makes the cavity technique none invasive and applicable for future applications.…”