Development of an in situ fiber optic Raman system to monitor hydrothermal vents Tina M. Battaglia, a Eileen E. Dunn, a Marvin D. Lilley, b John Holloway, a Brian K. Dable, c Brian J. Marquardt c and Karl S. Booksh* a a Department of Chemistry and Biochemistry, Arizona State University, Tempe, USA. E-mail: booksh@asu.edu; Fax: +1 480 965-2747; Tel: +1 480 965-3058 b School of Oceanography, University of Washington, Seattle, USA c Center for Process Analytical Chemistry, University of Washington, Seattle, USA Received 25th March 2004, Accepted 25th May 2004 First published as an Advance Article on the web 11th June 2004 The development of a field portable fiber optic Raman system modified from commercially available components that can operate remotely on battery power and withstand the corrosive environment of the hydrothermal vents is discussed. The Raman system is designed for continuous monitoring in the deep-sea environment. A 785 nm diode laser was used in conjunction with a sapphire ball fiber optic Raman probe, single board computer, and a CCD detector. Using the system at ambient conditions the detection limits of SO 4 22 , CO 3 22 and NO 3 2 were determined to be approximately 0.11, 0.36 and 0.12 g l 21 respectively. Mimicking the cold conditions of the sea floor by placing the equipment in a refrigerator yielded slightly worse detection limits of approximately 0.16 g l 21 for SO 4 22 and 0.20 g l 21 for NO 3 2 . Addition of minerals commonly found in vent fluid plumes also decreased the detection limits to approximately 0.33 and 0.34 g l 21 respectively for SO 4 22 and NO 3 2 . 1 Introduction Deep sea hydrothermal vents are one of the most extreme inhabited environments known. 1,2 Black smokers are formed when cold 2 °C sea water seeps into cracks in the sea floor and is heated to almost 400 °C near the magma chamber. The heated fluid dissolves minerals from the basalt and eventually bursts back up through the sea floor emitted as “vent fluid.” As the vent fluid is rapidly cooled by the sea water upon ejection, metal oxides precipitate forming chimneys. 2,3 Some common minerals found near black smokers include pyrite (FeS 2 ), chalcopyrite (CuFeS 2 ), pyrrhotite (FeS) and sphalerite ((Zn,Fe)S). 3 Surprisingly in these extreme environments, high temperatures (up to 350 °C), low pH, high pressures ( > 220 bar) and little oxygen, there is a diverse system of life such as tubeworms and chemoautotrophic microbes. 2,4 The metabolic pathways that sustain this vast ecology are not well known, but it is believed that if abiotic organic synthesis is occurring in the vents that the geobiochemistry could be similar to that when life originated on Earth. 5 There have been no known in situ studies of hydrothermal vents ecologies with Raman to date, the only analysis has involved snapshot measurements or collecting water and mineral samples and transporting them to land or ship labs. 1,6,7 However, this method has problems of contamination and difficulties in maintain- ing the original environment. Neither of these methods provides much insight on the dynamic fluctuations in the physical and chemical environment of the vent ecosystem or the dynamic interrelationship between the environment and the life that it supports. To date few in situ chemical sensors have been deployed in vent environments, and the ones that have either measure crude physical properties (i.e. temperature and pressure) or measure high concentration inorganics by electrochemistry (i.e. salinity by conductivity). Raman spectroscopy has been used to monitor gas hydrates at low temperature and high pressure on the sea floor previously using a 4 km fiber optic cable attached to a ship. 20 Development of a stand-alone multivariate optical chemical sensor that could withstand the harsh environment and be left near a vent for an extended period of time is key in eliminating such problems while able to continuously monitor and characterize the vent environment. Raman theory has been discussed in detail. 8,9 A monochromatic light source is used to irradiate a sample. However, only a fraction (10 26 of the incident light) actually interacts with the sample causing inelastic scattering or the Raman effect. The use of notch filters to block the elastically scattered light or the Rayleigh scattering can be used prior to detection reducing noise. 10 Water is virtually invisible to Raman detection and most compounds of interest in the sea water and surrounding vent environment have unique Raman features. Thus, Raman spectroscopy should be a good choice for monitoring the chemistry of the deep sea vents. 9,11,12 Bench top studies have used Raman spectroscopy to study sea water and various minerals. 6,13,14 The use of fiber optics has also enhanced the capabilities of Raman spectroscopy. 10,15 Choosing the right probe design and fibers can reduce past problems of decreased sensitivity and fiber background, and can allow fiber optic probes to be used in hazardous environ- ments. 8–10,12,15,16 There are many practical problems associated with using optics to monitor hydrothermal systems including the fact that hydro- thermal fluids may react and corrode most materials used to make optical components. However, sapphire (Al 2 O 3 ) lenses are resistant to corrosion at hydrothermal conditions. The use of sapphire windows for high pressure and temperature Raman studies has been documented. 17 Typically a window is placed at the end of the Raman probe, instead a sapphire ball lens will be used. The ball lens will focus the collimated laser light to a point much closer to the probe tip than a window. Although the sampling size is reduced this will allow studies to be performed in turbid solutions, like black smoker fluid. Another problem arises from the fact that the seawater is 2 °C and the hydrothermal fluid is about 350 °C at the opening of the chimney. However, 1 2 B from the vent opening the temperature of the vent fluid is 2 °C due to heat dissipation. The fiber optic probe cannot be heated above 80 °C because the optics will be damaged. In order to probe the hot vent fluid a 316 stainless steel sheath was designed that extends 6B from the probe tip, allowing 1–2B of the probe to be immersed into the hot liquid while the seawater keeps the remaining part of the probe cool. For laboratory experiments at ambient temperature and pressure a sapphire ball lens was epoxied into place. This paper will discuss the use of this sheath with a commercially modified, field portable, battery powered Raman system to look at compounds of interest in and around hydro- This journal is © The Royal Society of Chemistry 2004 DOI: 10.1039/b404505j 602 Analyst , 2004, 129 , 602–606