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

Abstract

Surfactants, or surface-active substances (SAS), are amphipathic organic substances that adsorb on aquatic phase boundaries, including the air-sea interface that covers ~70% of Earth's surface. SAS thus mediate all mass transfer across the air-sea interface and are central to planetary scale biogeochemical processes. SAS are routinely quantified in seawater and freshwater in terms of total surfactant activity (SA), using alternating current (AC) out-of-phase voltammetry with a hanging mercury drop electrode (HMDE). Although this technique is well established, method modifications have been implemented and differing calibration procedures adopted in individual research laboratories. Increasing interest in the environmental roles of SAS prompts a timely inter-comparison of these varying analytical approaches. Using sea-surface microlayer (SML: uppermost 80 μm layer sampled) and sub-surface (SSW: 1 m depth sampled) seawater from Jade Bay (south-eastern North Sea), we carried out the first inter-laboratory comparison for SA, using methods and calibration protocols previously established in three participating laboratories. The internal calibration protocol follows direct calibrations of individual samples against the model surfactant Triton-X-100 during analysis, whereas external calibration produces independent Triton-X-100 calibration curves; both protocols express SAS concentrations in Triton-X-100 equivalents (T-X-100 eq.). There was no significant difference between SA derived via internal or external calibration protocols, or by using different analytical instruments (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 < .001). We recommend using discrete calibration standards during external calibration. Irrespective of any differences in SA determined by the three laboratories, the SA enrichment factor (EF: =SASML/SASSW) was not affected for any sample; the root mean square 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 the inter-laboratory compatibility of SAS measurements into the future.