Application of purge and trap‐atmospheric pressure chemical ionization‐tandem mass spectrometry for the determination of dimethyl sulfide in seawater

Abstract

We describe a method for measuring trace concentrations of dimethyl sulfide (DMS) in seawater using a commercial tandem mass spectrometer configured for atmospheric pressure chemical ionization (PT-APCI-MS/MS), coupled with a custom-built purge and trap gas extraction system. DMS was ionized through proton transfer, generating abundant [M + H] + ions. The semiautomated method analyzes samples in under 6 min, and is capable of processing up to 10 samples in a single batch. A detection limit of 0.9 pmol L −1 was determined for the analysis of 5 mL sample volumes, with a precision of 3.9% between replicates. Practical performance was evaluated during two oceanographic research cruises within the coastal waters around Vancouver Island, British Columbia. To demonstrate method utility, a series of DMS depth profiles were obtained along two transects extending from the west coast of Vancouver Island into deep water off the continental shelf. Additional depth profile sampling was conducted in Saanich Inlet, a coastal anoxic fjord with active chemotrophic sulfur cycling. This method enabled us to capture the deep-water accumulation of subnanomolar DMS in the anoxic water of Saanich Inlet, providing evidence of cryptic sulfur cycling. The method was also leveraged to facilitate stable isotope rate measurement experiments, in which the consumption of isotopically labeled DMS, dimethylsulfoxide, and dimethylsulfoniopropionate tracers was monitored in the low picomolar range. These measurements enable metabolic rate determinations using low-level tracer additions that do not perturb in situ microbial activity. Our sensitive, high throughput method helps to improve understanding of the natural marine cycling of volatile sulfur compounds.

The determination of dimethyl sulfoxide in natural waters using electrochemical reduction

“…For the analysis of 10 consecutive samples, the coefficient of variation between sequential DMS signals was 5.4% by peak area and 5.5% by peak height. This level of precision is consistent with the reproducibility provided by the P&T‐APCI‐MS/MS method used for DMS analysis (3.9% by peak area and 4.1% by peak height; McCulloch et al 2020), indicating that the introduction of the electrochemical reduction method does not substantially impact the reproducibility of the overall analysis. The level of reproducibility provided is also consistent with other purge and trap GC‐based DMS detection techniques (Kiene 1996; Zindler et al 2012; Zhang and Chen 2015).…”

“…This study demonstrates the performance and practicality of a new electrochemically driven reduction protocol, which facilitates the accurate, selective, and reproducible measurement of DMSO in aqueous solutions. When coupled with the P&T-APCI-MS/MS workflow described previously (McCulloch et al 2020), the method can detect DMSO at concentrations typically observed within both fresh and seawater samples. The new electrochemical reduction method represents an alternative to existing enzyme-linked and reagent-based chemical reduction methods for DMSO analysis.…”

“…The custom‐built purge and trap sample handling system and commercial mass spectrometer used for the detection of DMS have been described previously by McCulloch et al (2020). This detection system was coupled with a custom‐built electrochemical cell, based on a membrane separated H‐cell design (Fig.…”

“…To increase reproducibility between replicates, a fresh aliquot of H 2 SO 4 is added to the anode half‐cell prior to the analysis of each new sample. Native DMS is first sparged from the solution and measured using P&T‐APCI‐MS/MS (McCulloch et al 2020). For a 10 mL sample volume, 7‐min of sparging is sufficient to entirely remove all native DMS from the sample.…”

Potential roles of dimethysulfoxide in regional sulfur cycling and phytoplankton physiological ecology in the NE Subarctic Pacific

“…Over a period of ~ 2–5 h, we collected 5 mL subsamples from each sample bag every 30–90 min using a glass syringe with a Luer Lock port. The concentration of isotopically labeled DMS species was determined using a SCIEX API 3200‐series triple quadrupole mass spectrometer equipped with an atmospheric pressure chemical ionization source (McCulloch et al, 2020). DMS, and each of its isotopically labeled analogues were found to ionize exclusively through the proton transfer (PT) mechanism, yielding abundant [M+H] + ions.…”