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

Abstract

Abstract. Here we present a technique that utilises a segmented flow coil equilibrator coupled to a Proton Transfer Reaction-Mass Spectrometer to measure a broad range of dissolved organic gases. Due to its unique design composed of a segmented flow and a headspace-water separator, the equilibrator is highly efficient for gas exchange and has a fast response time (under 1 min). The system allows for both discrete and continuous measurements of volatile organic compounds in seawater due to its ease of changing sample intake and low sample water flow (100 cm3 min-1). The equilibrator setup is both relatively inexpensive and compact. Hence it can be easily reproduced and installed on a variety of oceanic platforms, particularly where space is limited. As a result of its smooth and unreactive surfaces, the segmented flow coil equilibrator is expected to be less sensitive to biofouling and easier to clean than membrane-based equilibration systems. The equilibrator fully equilibrates for gases that are similarly soluble or more soluble than toluene. The method has been successfully deployed in the Canadian Arctic. Here, some example data of underway surface water and Niskin bottle measurements in the sea ice zone are presented to illustrate the efficacy of this measurement system.

Underway seawater and atmospheric measurements of volatile organic compounds in the Southern Ocean

“…Thus the humidity dependence of the signal and the background was slightly different from Wohl et al (2019). Methanol and possibly acetaldehyde showed a humidity dependence for the background at 640 V. The calibration slope for isoprene was corrected for its humidity dependence at 640 V due to fragmentation in the PTR-MS (Wohl et al, 2019). The other VOCs did not show a humidity dependence in either the slope or background.…”

“…During ANDREXII, VOCs in seawater and ambient air were measured with a Proton Transfer Reaction-Mass Spectrometer (PTR-MS, High Sensitivity Model by Ionicon). To measure seawater concentrations, a segmented flow coil equilibrator (SFCE) was used to equilibrate underway seawater with "zero air" (Wohl et al, 2019). The underway seawater inlet of the JCR is situated at approximately 5-7 m depth and set flush with the hull.…”

“…A lower PTR-MS drift tube voltage of 640 V was applied during this cruise compared to Wohl et al (2019), while other PTR-MS settings were kept the same. Thus the humidity dependence of the signal and the background was slightly different from Wohl et al (2019).…”

“…A lower PTR-MS drift tube voltage of 640 V was applied during this cruise compared to Wohl et al (2019), while other PTR-MS settings were kept the same. Thus the humidity dependence of the signal and the background was slightly different from Wohl et al (2019). Methanol and possibly acetaldehyde showed a humidity dependence for the background at 640 V. The calibration slope for isoprene was corrected for its humidity dependence at 640 V due to fragmentation in the PTR-MS (Wohl et al, 2019).…”

“…Two methods were used to determine the SFCE calibration slopes for seawater methanol, acetone, and acetaldehyde concentrations during this cruise: invasion and evasion. In evasion calibrations, pure solvents were dissolved by serial dilution in MilliQ water and seawater as described previously (Wohl et al, 2019). These diluted standards were measured with the SFCE-PTR-MS system using the same procedure as for seawater samples.…”