A comparison study is presented in which the relative performance of a new orthogonal geometry field-free atmospheric pressure photoionization (FF-APPI) source was evaluated against both electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) for the analysis of a small panel of clinically relevant steroids, spiked within various complex biological matrices. Critical performance factors like sensitivity and susceptibility to matrix effects were assessed using a simple, isocratic, high-throughput LC-MS workflow. FF-APPI was found to provide the best performance in terms of both sensitivity and detection limit for all of the steroids included in the survey. Order-of-magnitude sensitivity advantages were realized for some low polarity analytes including both estradiol and estrone. A robust linear regression, post extraction addition method was used to evaluate the relative impact of matrix effects upon each ionization method using protein precipitated human serum, plasma and Surine (simulated urine) as standard clinical matrices. Under conditions optimized for sensitivity, both the field-free APPI and APCI sources were found to provide similarly high resistance to matrix suppression effects, while ESI performance was impacted the most dramatically. For the prototype FF-APPI source, a strong relationship was established between optimizable source parameters and the degree of ion suppression observed. Through careful optimization of vaporization temperature and nebulizer gas pressure it was possible to significantly reduce or even eliminate the impact of matrix effects, even for high-throughput LC-MS methods.

Between 2015 and 2018, we collected approximately 2,000 water column measurements of methane (CH4) and nitrous oxide (N2O) concentrations in the North American Arctic Ocean during summer and early fall. We also obtained 25 measurements of CH4 and N2O concentrations in rivers along the Northwest Passage and Ellesmere Island in midsummer 2017–2019. Our results show that N2O is generated in the highly productive Bering and Chukchi Seas and transported northeastward, producing a persistent subsurface N2O peak in the Beaufort Sea. The Chukchi and Beaufort Sea sediments are a significant source of CH4 to the water column. These sedimentary sources and associated water column consumption display significant spatial gradients and interannual variability. CH4 isotope data demonstrate the importance of CH4 oxidation across the study region. We find that rivers are not a significant source of CH4 or N2O to the Arctic Ocean at the time of year sampled. The estimated annual sea‐air flux across the study region (2.3 million km2) had a median (first quartile, third quartile) of 0.009 (0.002, 0.023) Tg CH4 y−1 and −0.003 (−0.013, 0.010) Tg N y−1. These results suggest that the North American Arctic Ocean currently plays a negligible role in global CH4 and N2O budgets. Our expansive data set, with observations at many repeat stations, provides a synopsis of present‐day Arctic CH4 and N2O distributions and their range of variability, as well as a benchmark against which future climate‐dependent changes can be evaluated.

An APPI source configuration that includes an extended field-free reaction region was demonstrated to have the potential to provide enhanced sensitivity relative to commercially available open-geometry source designs. Improved performance will no doubt lead to increased acceptance and widespread application of the technique.

Between 2015–2018, we collected ~2000 measurements of methane (CH4) and nitrousoxide (N2O) concentrations in the North American Arctic Ocean during summer and early fall from water column profiles. We also obtained 25 measurements of CH4 and N2O concentrations in rivers along the Northwest Passage and Ellesmere Island in mid-summer 2017–2019. Our results show that N2O is generated in the highly productive Bering and Chukchi Seas and transported northeastward, producing a persistent subsurface N2O peak in the Beaufort Sea. The Chukchi and Beaufort Sea sediments are a significant source of CH4 to the water column. These sedimentary sources and associated water column consumption display significant spatial gradients and interannual variability. CH4 isotope data demonstrate the importance of CH4 oxidation across the study region. We find that rivers are not a significant source of CH4 or N2O to the Arctic Ocean at the time of year sampled. The estimated annual sea-air flux across the study region (2.3 million km2) had a median (interquartile range) of 0.009 (0.002, 0.023) Tg CH4 y-1 and −0.003 (−0.013, 0.010) Tg N y-1. These results suggest that the North American Arctic Ocean currently plays a negligible role in global CH4 and N2O budgets. Our expansive dataset, with observations at many repeat stations, provides a synopsis of present-day Arctic CH4 and N2O distributions and their range of variability, as well as a benchmark against which future climate-dependent changes can be evaluated.

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.