Development of a compact deep-sea Raman spectroscopy system and direct bicarbonate detection in sea trials

Area of Science:

• Marine Geochemistry
• Analytical Chemistry
• Oceanographic Instrumentation

Background:

• Raman spectroscopy is valuable for deep-sea exploration but faces challenges with system size and sensitivity.
• Existing underwater Raman systems are often too large for deployment on underwater vehicles.
• Improved sensitivity is needed to detect trace components in marine environments.

Purpose of the Study:

• To develop a compact, high-sensitivity in situ Raman spectroscopy system for deep-sea applications.
• To address the deployment and sensitivity limitations of previous underwater Raman systems.
• To enable direct, in situ chemical analysis of seawater components.

Main Methods:

• Designed and constructed a compact deep-sea Raman system (60 kg) within an L800 mm × Φ258 mm pressure vessel.
• Integrated a 532 nm Nd:YAG laser, high-throughput spectrograph (8 cm⁻¹ resolution), and a TEC-cooled CCD detector.
• Evaluated system performance through laboratory experiments and in situ deployment on a remote-operated vehicle.

Main Results:

• Achieved a limit of detection of 0.4 mmol/L for sulfate (SO₄²⁻) in laboratory tests.
• Successfully detected in situ Raman signals of sulfate (SO₄²⁻), chlorophyll a (chl-a), and chromophoric dissolved organic matter (CDOM).
• Obtained the Raman signal of bicarbonate (HCO₃⁻) after spectral accumulation, representing the first direct in situ measurement.

Conclusions:

• The developed compact deep-sea Raman system effectively overcomes previous deployment and sensitivity issues.
• The system demonstrates capability for in situ detection of key seawater chemical species and organic matter.
• Future applications include long-term carbon cycling research via seafloor observation networks.