Analytica Chimica Acta 463 (2002) 265–274 In situ osmotic analyzer for the year-long continuous determination of Fe in hydrothermal systems Thomas P. Chapin ∗ , Hans W. Jannasch, Kenneth S. Johnson Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA Received 4 December 2001; received in revised form 26 April 2002; accepted 14 May 2002 Abstract A submersible osmotically pumped analyzer (Fe-OsmoAnalyzer) has been adapted for the long-term continuous high resolution monitoring of iron(II) and (III) in deep-sea hydrothermal vents. Ferrozine is used as the colorimetric reagent. The detection limit for our analyzer is 0.1 M for a 0.7 cm pathlength with a linear response up to 50 M. The Fe-OsmoAnalyzer uses periodic injections of standards and blanks to self calibrate and can operate unattended for over a year collecting data every 15 min. The Fe-OsmoAnalyzer was field tested during a 6 week test deployment at 900 m in Monterey Bay and a year-long deployment at 1100 m off the coast of Hawaii. A second year-long deployment in a low temperature hydrothermal vent on the Juan de Fuca Ridge provided high resolution continuous monitoring of Fe and was able to distinguish tidally influenced fluctuations in Fe concentrations. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Iron determination; In situ; Hydrothermal; Seawater; Long-term; Osmotic pumps 1. Introduction The concentration of dissolved iron is a key variable controlling biogeochemical processes in hydrothermal environments [1]. Iron oxides formed by the oxida- tion of reduced Fe(II) may precipitate other trace met- als and scavenge large amounts of phosphate from the water column [1,2]. It has also been suggested that Fe speciation plays a major role in hydrothermal vent ecology by controlling the availability of free sulfide to vent animals [3]. Dissolved iron may reach con- centrations as high as 20 mM in high temperature hy- drothermal vents and recent observations have shown ∗ Corresponding author. Present address: US Geological Survey, P.O. Box 25046/M.S. 973, Denver Federal Center, Denver, CO 80225, USA. Fax: +1-303-236-3200. E-mail address: tchapin@usgs.gov (T.P. Chapin). Fe emissions are much more dynamic than first sus- pected [1]. However, it has been difficult to observe the temporal variations in this key variable. Long-duration continuous in situ monitoring of Fe is required to eluci- date the biogeochemical mechanisms that control hy- drothermal Fe concentrations and the impact of these variations on hydrothermal ecosystems. Oceanographic sensors for physical properties, such as temperature can be deployed for year-long pe- riods in hydrothermal systems but the development of chemical sensors designed for long-term (>1 month) deployments lags significantly [4]. Continuous-flow analyzers using peristaltic pumps, and colorimetric chemistries, which are capable of in situ calibration, have been developed for in situ analysis [5,6]. These instruments, which are deployed from submersible vehicles, have provided a wealth of high-resolution information on Fe, Mn, nutrient, and sulfide dynam- 0003-2670/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0003-2670(02)00423-3