Rapid and real-time analysis of multi-component dissolved gas in seawater by Raman spectroscopy combined with continuous gas-liquid separator

Abstract

Rapid and sensitive detection of dissolved gases in seawater is quite essential for the investigation of the global carbon cycle. Large quantities of in situ optical detection techniques showed restricted measurement efficiency, owing to the single gas sensor without the identification ability of multiple gases. In this work, a novel gas-liquid Raman detection method of monitoring the multi-component dissolved gases was proposed based on a continuous gas-liquid separator under a large difference of partial pressure. The limit of detection (LOD) of the gas Raman spectrometer could arrive at about 14 ml·L−1 for N2 gas. Moreover, based on the continuous gas-liquid separation process, the detection time of the dissolved gases could be largely decreased to about 200 s compared with that of the traditional detection method (30 min). Effect of equilibrium time on gas-liquid separation process indicated that the extracted efficiency and decay time of these dissolved gases was CO2 > O2 > N2. In addition, the analysis of the relationship between equilibrium time and flow speed indicated that the decay time decreased with the increase of the flow speed. The validation and application of the developed system presented its great potential for studying the components and spatiotemporal distribution of dissolved gases in seawater.

1. Introduction

As the source and sink of global carbon oxide, the carbon storage of oceans is much greater than that in atmospheric and terrestrial storage, which plays an important role in the global carbon cycle [1,2]. Dissolved gases in seawater exhibit important research value in the fields of marine chemistry, marine biology and marine environment, etc. [3,4]. Typically, marine dissolved gases including carbon dioxide (CO2), methane (CH4) and hydrogen sulfide (H2S) are essential dissolved gases, participating in the physical, chemical, biological, and geological processes in the ocean [5]. Moreover, the concentration of dissolved gases such as CO2, O2 and CH4 in seawater is varied at different time and space, considering their interconversion at the air-sea interface [3,6]. Thus, the study of the components, concentrations, spatiotemporal distribution of the dissolved gas, and the exchange process between ocean and surface air is of great significance, providing important fundamental research for many other research areas [7,8]. Precise measurement of the dissolved gases could provide accurate data support and reliable theoretical foundation for the research of the global carbon cycle, marine ecological environment, and global climate change [9].

At present, large quantities of effective methods are successfully used for the detection of dissolved gas in seawater, including chemical titration [10], infrared spectroscopy [11], chromatography, mass spectrometry [12,13], etc. Owing to the excellent sensitivity and selectivity, detection methods of chromatography and mass spectrometry are widely used in gas detection including the dissolved gases in seawater [14]. Considering the demand for the research in the chemistry and biological process of dissolved gases, it is quite necessary to develop the rapid and real-time detection techniques for the detection. While, methods of chromatography and mass spectrometry exhibit the disadvantages of the time-consuming and the destruction to the samples [15]. In recent years, there have been more and more research reports on chemical sensors for the detection of dissolved gases, most of which are developed based on the mechanism of “extraction before detection” [16,17]. Nevertheless, each analysis method shows its advantages and disadvantages, which makes it difficult to meet all the detection requirements of multi-component dissolved gases through one technique. Therefore, many detection methods are often combined to achieve comprehensive analysis.