Development of a low-cost semi-automated system for dissolved dimethyl sulfide (DMS)

Oceanic dimethyl sulfide (DMS) plays a critical role in the marine sulfur cycle and atmospheric processes. Therefore, accurate DMS quantification is essential for understanding ocean–atmosphere feedback. Despite its importance, dissolved DMS measurement remains analytically challenging because of its high volatility. Conventional gas chromatographic methods provide high sensitivity, but they are time consuming, expensive, and bulky which makes them unsuitable for near-real–time applications. Chemiluminescence–based approaches provide improved detection and enabled field analysis; however, their performance depends on complex sampling and specialised instrumentation, which also limits routine use. Therefore, the lack of simple, compact, semi-automated, and low–cost systems has created a gap between laboratory precision and field applicability in marine research. This motivated the development of a reliable, portable, and affordable semi-automated system for DMS quantification using low–cost commercial components.

To address this need, this study developed a semi-automated electrochemical system for DMS measurement through sequential stages, from sampling design and optimisation to a fully integrated platform for semi-automated quantification. Although a chemiluminescence method was developed, low–cost amperometric sensors were used as the final configuration to align with the objectives of affordability and automation. This choice required systematic mitigation of low sensitivity at the ppb level, cross–sensitivity and signal drift in amperometric gas sensors to establish a stable and sensitive DMS detection system.

In Chapter 2 a closed–loop vapour generation (CLVG) approach was first developed to improve sensitivity by enhancing headspace concentration through gas recirculation within a closed system. When the design was coupled with ozone–induced chemiluminescence detection, a 2.6–fold improvement in signal–to–noise compared to the conventional sampling method was achieved. In the next stage, the CLVG system was used in a proof–of–concept experiment using commercial ozone and sulfur dioxide sensors for indirect electrochemical detection of DMS (Chapter 3). The findings revealed that the low–cost SO₂ sensor could directly respond to DMS. This discovery guided the next stage for repurposing the SO₂ sensor for direct DMS quantification (Chapter 4). Critical parameters were optimised and the repurposed sensor showed strong analytical performance for DMS gas analysis with a detection limit of 0.23 ppm and precision of 1.18 % RSD. Also, by implementing a simple oxidant filter, the cross–sensitivity of repurposed sensor to SO₂ gas was fully resolved. These results provided the quantitative basis for full system integration.

In Chapter 5, a semi-automated platform for dissolved DMS measurement was established by integrating the validated amperometric sensing system with CLVG sampling. The system was capable of semi-automated measurement and with continuous signal acquisition. Further improvements in instrumentation and experimental conditions made the system faster, more stable, and more sensitive, especially under saline conditions which confirmed the suitability of the system for marine applications. Overall performance showed strong precision (RSD = 6.76 %) and linearity from 346–1014 nM (R² = 0.99), with an LOD of 114 nM and an LOQ of 346 nM. Spike recoveries in natural seawater were 95–109%, showing good accuracy with only a modest matrix effect. The capability of the system was evaluated by applying the platform to real samples for the quantification of intracellular dimethylsulfoniopropionate (DMSP), the biological precursor of DMS, in macroalgal matrices (Chapter 6). A protocol was applied to convert intracellular DMSP to DMS via acid extraction followed by alkaline hydrolysis. This produced a DMS–only headspace which enabled selective quantitative analysis. Quantitative results for Ulva lactuca, Macrocystis pyrifera, Pachymenia dichotoma, and Judithia delicatissima were consistent with reported literature values. These results demonstrated that the system could measure macroalgae intracellular DMSP content rapidly (within 45 minutes) and at low cost (under USD 1,300), compared with conventional GC–based methods that require about 24 hours per sample and equipment costs over USD 60,000.

As the final stage, a slope–based transient response analysis was developed to mitigate signal drift by replacing steady–state measurement with transient–slope quantification. The method was able to detect the rise region, apply short sliding straight–line fits, verify plateau region, and perform quality checks before calculating the transient slope. The method halved the CLVG DMS measurement time, agreed with steady–state results (R² = 0.95; LOD = 89.9 nM; LOQ = 299.6 nM; linear range = 299.6 - 750 nM), and removed the need for baseline drift correction. Overall, the semi-automated DMS system provided an effective and low–cost alternative to GC–based analysis, achieving substantial reductions in both analysis time and instrumentation requirements. This work established a foundation for developing accessible, reliable sensing system that can support broader marine research and environmental monitoring applications.