In situ analyser for dissolved iron based on the chemiluminescence of luminol

Sebastian Steigenberger1, Alexander Beaton1, Antony Birchill2, Medya Namiq1, Daisy Tong1, Rudolf Hanz1, Matthew Patey1, Maeve C Lohan3 and Matthew C Mowlem4, (1)National Oceanography Centre, Ocean Technology and Engineering Group, Southampton, United Kingdom, (2)Plymouth University, School of Geography, Earth and Environmental Sciences, Plymouth, United Kingdom, (3)University of Southampton, Ocean and Earth Sciences, National Oceanography Centre, Southampton, United Kingdom, (4)National Oceanography Centre, Southampton, Ocean Technology and Engineering Group, Southampton, United Kingdom
Miniaturised in situ analysers for dissolved iron in coastal areas and close to hydrothermal sources have been reported previously, but these do not have the required sensitivity for measuring dissolved iron in the open ocean (e.g. picomolar to nanomolar levels). Here we report the design, construction and initial lab/field testing of a system that combines lab-on-chip (LOC) technology (previously demonstrated for in situ nutrient analysis) with the iron luminol assay which has been shown to achieve detection limits in the laboratory of ~100 pM.

The LOC system described here features an automated pre-concentration module using a chelating resin to achieve seawater matrix separation and pre-concentration of iron. After being eluted with hydrochloric acid, the pre-concentrated solution passes into a microfluidic mixing chip that uses passive micromixers to achieve mixing with ammonia, luminol and hydrogen peroxide solutions before it passes into the detection cell. The chemiluminescence detection cell consists of a transparent plastic spiral-shaped microfluidic flowcell facing a photomultiplier tube that is located in a pressure housing.

The system can operate to at least 300 m depth, has been characterised in the laboratory and integrated into a long-range autonomous underwater vehicle as part of a multi-parameter sensor suite.