Dataset: GP15 TDS and OCS Leg 2

Methodology:
Total dissolved sulfide (TDS) and carbonyl sulfide (OCS) was determined at sea within 8 hours of collection using the Radford-Knoery and Cutter (1993) method. This method entails hermetically transferring 300 mL of seawater sample into a 300 mL gas stripping vessel and purging with helium, acidification to a pH of 1.6 using 1.5 M phosphoric acid, and then gas stripping and cryogenically trapping the sulfide gases, and subsequent quantification using a gas chromatograph coupled with a flame photometric detector.

Sampling and analytical procedures:
Total dissolved sulfide (TDS) and carbonyl sulfide (OCS) were collected from the GTC GO-FLO bottles and the GeoFish, filtered through 0.2 μm AcroPak filter capsules and stored in 4 L polyethylene cubitainers. The cubitainers were rinsed 3 times with the sample before filling hermetically with approximately 1.5 L of seawater sample and stored in a refrigerator until analyzed at sea usually within 8 hours of collection. Upon analysis, roughly 100 mL the seawater sample was used to rinse the gas stripping vessel, it was then filled with 300 mL of seawater sample and purged with helium (120 mL/min) for 2 minutes. After 2 minutes, the cryogenic trap was immersed in liquid nitrogen followed by injecting 1.5 M phosphoric acid and stripped/trapped the gases for 20 minutes before quantifying TDS and OCS using a gas chromatograph coupled with a flame photometric detector.

This method quantifies TDS as free ions and metal-sulfide complexes with detection limits of 0.2 pmol/L for TDS and 1.3 pmol/ L for OCS for 300 mL samples (Radford-Knoery & Cutter, 1993). Dissolved sulfide water samples were mostly analyzed in duplicate, occasionally triplicate when time permitted. To ensure accuracy, the H2S and OCS gases are calibrated using permeation tubes whose permeation rates have been gravimetrically measured for 2 - 4 years. By trapping and measuring known amounts of permeated H2S and OCS over a range of times, linear calibration curves for each gas were assembled daily and applied to the unknown samples.

Instruments:
These samples were processed following the analytical apparatus first stated by Radford-Knoery and Cutter (1993). All details can be found in that publication.

A Hewlett Packard (HP) 5890 Series II gas chromatograph coupled with a HP flame photometric detector (model 19256A) was used in the quantification of TDS and OCS. The output signal from the detector was processed using a PeakSimple Chromatography Data System (model 333). Hydrogen (130 mL/min) and air (130 mL/min) are used for the flame photometric detector’s flame while ultra-high purity helium is used as the carrier gas (30 mL/min) and the stripping gas (120 mL/min). While cylinders were used as the source of air and ultra-high purity helium, a VWR hydrogen generator (model H2PEM-165) was used for the hydrogen source. A VWR circulating water bath (model 1130S) was used to maintain 40°C for H2S and OCS permeation devices (Metronics) which were used to calibrate the instrument for H2S and OCS.

Quality Flags:
The SeaDataNet scheme was used to assign data quality flags to samples. More information can be found at https://www.seadatanet.org/Standards/Data-Quality-Control.

The reported codes for flagged data are:
0 = no quality control
1 = good value
2 = probably good value
3 = probably bad value
4 = bad value
5 = changed value
6 = value below detection
7 = value in excess
8 = interpolated value
9 = missing value