Technical Note: A fully automated purge and trap GC-MS system for quantification of volatile organic compound (VOC) fluxes between the ocean and atmosphere

Abstract

Abstract. The oceans are a key source of a number of atmospherically important volatile gases. The accurate and robust determination of trace gases in seawater is a significant analytical challenge, requiring reproducible and ideally automated sample handling, a high efficiency of seawater-air transfer, removal of water vapour from the sample stream, and high sensitivity and selectivity of the analysis. Here we describe a system that was developed for the fully automated analysis of dissolved very short-lived halogenated species (VSLS) sampled from an under-way seawater supply. The system can also be used for semi-automated batch sampling from Niskin bottles filled during CTD (conductivity, temperature, depth) profiles. The essential components comprise a bespoke, automated purge and trap (AutoP & T) unit coupled to a commercial thermal desorption and gas chromatograph mass spectrometer (TD-GC-MS). The AutoP & T system has completed five research cruises, from the tropics to the poles, and collected over 2500 oceanic samples to date. It is able to quantify > 25 species over a boiling point range of 34-180 • C with Henry's law coefficients of 0.018 and greater (CH 2 I 2 , k cc H dimensionless gas/aqueous) and has been used to measure organic sulfurs, hydrocarbons, halocarbons and terpenes. In the eastern tropical Pacific, the high sensitivity and sampling frequency provided new information regarding the distribution of VSLS, including novel measurements of a photolytically driven diurnal cycle of CH 2 I 2 within the surface ocean water.

Potential controls of isoprene in the surface ocean

“…Isoprene in seawater was analyzed from the ships' pumped nontoxic seawater supplies as well as from CTD casts from within the photic depth, using the semiautomated P&T system described in Andrews et al . [], with modifications detailed in the supporting information. Analysis was performed by GC‐MS, with regular calibrations using a premixed gas standard.…”

Segmented flow coil equilibrator coupled to a Proton Transfer Reaction Mass Spectrometer for measurements of a broad range of Volatile Organic Compounds in seawater

“…Some purge and trap systems have been automated to allow for underway measurements of halocarbons, DMS, and isoprene semi-continuously every ca. 30 minutes (Andrews et al 2015). Extracted or equilibrated air from seawater contains a large amount of water vapour, which potentially 55 affects the VOC detection.…”

A comparison of very short lived halocarbon (VSLS) and DMS aircraft measurements in the tropical west Pacific from CAST, ATTREX and CONTRAST

“…The positive pressure in the canisters was utilised for sample introduction into the instrument to avoid contamination from any potential leaks associated with reduced pressure sampling using a pump. Sample humidity was controlled using a −30 • C glass cold trap to remove water without loss of the analytes (Andrews et al, 2015;Swan et al, 2015). Two litres of sample was preconcentrated onto the cooled (−30 • C) adsorbent trap (Tenax TA) of a thermal desorption unit (TDU, Markes UNITY 2-CIA-T) and desorbed with a 2 mL min −1 helium carrier at 250 • C onto a capillary column (Restek RTX502.2, 30 m, 0.25 mm I.D., 1.4 µm film thickness) of a gas chromatograph (Agilent 7890A) held at 40 • C for 3 min and then ramped at 25 • C min −1 to 250 • C and held for 3 min.…”