The METal In Situ (METIS) analysers are compact (ca. 1 l volume) wet-chemical analysers that are designed to determine the presence and concentration of dissolved iron(II) or manganese(II) in the water column. The analysers employ a method of unsegmented continuous flow analysis. The sample stream is inoculated with a reagent, the combined solutions are mixed and pass into a cell where the intensity of colour is determined using an LED as light source and a photodiode with light to frequency converter (TAOS) as detector.
Changes in intensity of the light transmitted through the cell are proportional to the colour development and hence concentration of analyte in question. The system has a series of valves that switch the system from pumping sample
to a blank solution and a known standard. Thus, ‘onboard’ 2-point calibrations (standard and blank) can be carried out during deployment. This has the advantage that a correction for any effect that pressure and/or temperature may have on the colorimetric system can be applied to the data.
The specificity for either iron or manganese determination is achieved through the use of different chemical reagent regimes while the physical instrument is the same for both analytes. Special emphasis was put on a fast reaction time of the analyser (ca. 8 s between independent measurements) so that it can be deployed in profiling mode with concurrent CTD measurements or as payload on AUVs.
The development of the analysers was carried out at the National Oceanography Centre Southampton, UK, together with Doug Connelly. Since 2005 the IOW METIS is developed independently from the NOCS instruments but using common electronics, developed at NOCS.
Dr. Ralf Prien, Dr. David Meyer