The modifications included changes of the trap and temperature purge/retention program of the gas chromatograph. The traps used in both systems were Vocarb® 3000, with a trap temperature of − 5 °C for the custom-made system and ambient temperature for the Tekmar system. The desorption temperature
check details was 225 °C. The systems were connected to gas chromatographs with electron-capture detectors (Varian 3800). Separations of halocarbons were performed using an Agilent DB-624 wide-bore column (60 m, I.D. 0.32 mm, film 1.80 μm). The chromatographic conditions were a starting temperature of 30 °C at a hold time of 7.23 min, followed by an increase in temperature to 55 °C at a rate of 5 °C min− 1, raised to 69 °C at a rate of 2 °C min− 1, raised to 100 °C at a rate of 5 °C min− 1, raised to 140 °C at a rate of 10 °C min− 1 and raised to 255 °C at a rate of 30 °C min− 1, with a hold time of 1.50 min. The systems were calibrated with external standards
of CH3I (Fluka (> 99.5%), CH3CH2I (Merck, 99%), CH3CHICH2 (Fluka, > 98%), CH2Br2 (Merck, 99%), CH3CH2CH2I (Aldrich, 99%), CHBrCl2 (Fluka, > 98%), CH2ClI (Fluka, > 97%), CH3CHICH2CH3 (Fluka, > 99%), CHBr2Cl (Fluka, > 97%), CH2ICH2CH2CH3 (Fluka, > 99%), CH2BrCH2Br (unknown), CH2BrI (Fluka), CHBr3 (Merck, > 98%) and CH2I2 (Merck, > 98%) diluted from a stock click here solution in methanol (Sigma-Aldrich, suitable for purge and trap analysis) in seawater to give final concentrations of pmol L− 1 in the purge chamber. The systems were calibrated with standards every 5 days, and no drift was observed during the duration of the cruise. The absolute detection limits for the compounds are in the fmol L− 1 range (Supplementary material), and the overall precision for the biogenic halocarbons were between PDK4 1 and 5%. Halocarbon data from the OSO07 expedition is archived at the PANGEA information system, http://doi.pangaea.de/10.1594/PANGAEA.779087. Water samples
were collected for chlorophyll a, photosynthetic pigments and microscopic analysis. All filtrations were completed using low (⅓ atm) vacuum through 25 mm Whatman GF/F filters. Samples for chlorophyll were placed in 7 mL 90% acetone, extracted for at least 24 h in cold (~− 10 °C) dark conditions, the filters removed, and the extracts read before and after acidification on a Turner Designs Model 700 fluorometer (Knap et al., 1996). The fluorometer was calibrated before and after the cruise using commercially purified chlorophyll a (Sigma), which in turn was checked using high performance liquid chromatography (HPLC). Samples for pigment analysis were collected and filtered, wrapped in aluminum foil and frozen at − 80 °C. Samples were returned to the laboratory frozen and processed on Waters Millenium HPLC equipped with dual-beam photocells and a fluorescence detector.