Dataset: Halomethane concentration profile

Samples were collected at 7 depths using Niskin sampling bottles deployed on a rosette for deep samples from 0 meters to 250 meters. Samples were transferred from Niskins into acid cleaned 250 ml borosilicate serum bottles without entraining air, acidifed with 1mL 1 molar hydrochloric acid and stored in the refrigerator for up to two weeks prior to analysis. Duplicate samples were taken.

Samples were analyzed for dissolved halocarbon concentrations using a gas chromatography (GC) method adapted from Schall and Heumann (1993) and quantified relative to an internal standard. Purge-and-trap capillary column gas chromatography with electron capture detection(GC-ECD) (EPA, 1986; Schall and Heumann 1993). 5-50 mL seawater samples were purged with ultra-high purity He for 45min at a flow rate of 60mL min-1 through an in-line K2CO3 drying tube and onto a liquid nitrogen trap. The purge vessel is then rinsed with methanol and the drying trap replaced with 0.75g fresh K2CO3 between individual analyses.

Cryo-concentrated samples were introduced into an Agilent 7890A GC by means of a splitless injection with sweep pressure at 50psi for 3.5min returning to analytical column pressure of 18psi after injection. Inlet temperature was set to 60°C to facilitate cryo-focusing on the column. Initial oven temperature was 40°C for 10min increasing to 120°C by 4°C min-1 and held there for another 2min. Temperature was then ramped to a final 240°C at a rate of 5°C min-1 and held for 20min 

Calibration was carried out using 20µL of 0.5µg/L of tribromochloromethane as an internal standard
(Gonzalez-Gago et al. 2007)