Samples
Sampling was conducted aboard the CCGS John P. Tully during five cruises (2018-2020) in the northeast Pacific Ocean from Vancouver Island to Station P (50°N, 145°W).
Fisheries and Oceans Canada (DFO) Cruise numbers: 2018-40, 2019-001, 2019-006, 2019-008, 2020-001

Seawater samples for dissolved and total organic carbon (DOC, TOC) were collected from 10 L Niskin bottles into pre-cleaned 40 mL scintillation vials, with 0.125 cm Teflon low-bleed septa. The vials had been cleaned in Extran 300 and rinsed several times with Type I Ultrapure water. Vials were then soaked in 10% HCl for a minimum of four hours and rinsed several times with Type I Ultrapure water. Cleaned vials were allowed to dry and were then baked at 450°C for a minimum of 5 hours. Septa were briefly washed in 10% HCl, rinsed with Type I Ultrapure water and allowed to air dry.

Samples for TOC were collected directly from the spigot of the Niskin bottle. Samples for DOC were filtered through a Millipore Opticap XL Durapore 0.22 µm inline filter cartridge (Product No. KVGLA04HH3) attached to the spigot. Vials were rinsed three times with sample water and then filled to about three-quarter capacity, to avoid breakage during freezing. The samples were quick-frozen immediately after collection, using a stainless-steel freezer block, and then stored frozen at -20°C until analysis.

DOC and TOC were measured at the Institute of Ocean Sciences, using a Shimadzu TOC-L DOC/TOC analyzer (measurement reproducibilty of ± 1.5 %), following Standard Operating Procedure 7 from Chapter 4 in Dickson et al. (2007), with the following modifications: 1) sulphuric acid was used in place of phosphoric acid to reduce vitrification of the column with saline samples; and 2) the sparging time was increased to 150 seconds.

Standards

A 1000ppm standard of potassium hydrogen phthalate was prepared using Type 1 water (defined below). The standard stock solution was stored refrigerated for up to 2 months.
Standard dilutions were done by calibrated pipette into volumetric flasks. They were made with Type 1 water that had been further processed to reduce TOC into the 2-3 ppb range. Dilution standards have a shelf life of two weeks. The very low standard concentrations (0.1-0.5 ppm) were made in larger volume flasks to reduce the errors associated with pipetting small amounts.
Type I - Ultrapure, Type I water is defined by the American Society for Testing and Materials (ASTM) as having a resistivity of >18 MΩ-cm, a conductivity of <0.056 µS/cm and <50 ppb of Total Organic Carbons (TOC).
Low Carbon Water (LCW) is a purchased, 2 μM, reference standard used to determine the instrument blank. Miami Seawater is a purchased, 42-45 μM, reference standard used to determine when the column/instrument was running cleanly enough to measure low concentrations in seawater.