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Reconsideration of seawater surfactant activity analysis based on an inter-laboratory comparison study

Lookup NU author(s): Dr Philippa Rickard, Dr Guenther Uher, Professor Robert Upstill-Goddard

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This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND).


Abstract

Surfactants, or surface-active substances (SAS), are amphipathic organic substances thatadsorb on aquatic phase boundaries, including the air-sea interface that covers ~70% ofEarth’s surface. SAS thus mediate all mass transfer across the air-sea interface and arecentral to planetary scale biogeochemical processes. SAS are routinely quantified inseawater and freshwater in terms of total surfactant activity (SA), using alternating current(AC) out-of-phase voltammetry with a hanging mercury drop electrode (HMDE). Althoughthis technique is well established, method modifications have been implemented anddiffering calibration procedures adopted in individual research laboratories. Increasinginterest in the environmental roles of SAS prompts a timely inter-comparison of thesevarying analytical approaches. Using sea-surface microlayer (SML: uppermost 80 μm layersampled) and sub-surface (SSW: 1 m depth sampled) seawater from Jade Bay (southeasternNorth Sea), we carried out the first inter-laboratory comparison for SA, usingmethods and calibration protocols previously established in three participating laboratories.Internal calibration protocol follows direct calibrations of individual samples against themodel surfactant Triton-X-100 during analysis, whereas external calibration producesindependent Triton-X-100 calibration curves; both protocols express SAS concentrations inTriton-X-100 equivalents (T-X-100 eq.). There was no significant difference between SAderived via internal or external calibration protocols, or by using different analyticalinstruments (range in Kruskall-Wallis and Dunn-Bonferroni post-hoc test p-values: 0.062-1.000), except where freeze/thaw degradation was suspected to have occurred during transit(p < 0.001). We recommend using discrete calibration standards during external calibration.Irrespective of any differences in SA determined by the three laboratories, the SAenrichment factor (EF: = SASML/SASSW) was not affected for any sample; the root meansquare error (± one standard deviation) between all laboratories was 0.156 ± 0. 226 (n = 45).We present and discuss recommendations for a standard analytical protocol to ensure theinter-laboratory compatibility of SAS measurements into the future.


Publication metadata

Author(s): Rickard PC, Uher G, Upstill-Goddard RC, Frka S, Mustaffa NIH, Banko-Kubis HM, Kusan AC, Gasparovic B, Stolle C, Wurl O, Ribas-Ribas M

Publication type: Article

Publication status: Published

Journal: Marine Chemistry

Year: 2019

Volume: 208

Pages: 103-111

Print publication date: 20/01/2019

Online publication date: 03/12/2018

Acceptance date: 30/11/2018

Date deposited: 04/12/2018

ISSN (electronic): 0304-4203

Publisher: Elsevier

URL: https://doi.org/10.1016/j.marchem.2018.11.012

DOI: 10.1016/j.marchem.2018.11.012

Notes: Co-authored with RC Upstill-Goddard, School of Natural and Environmental Sciences


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