The future of chemical in situ sensors

Many of the sampling methods used in oceanographic sciences today date back decades, if not centuries. Although there has been a marked change in how sampling for chemical oceanography is carried out it still relies on taking samples of seawater from a research vessel in most cases. Assessing processes on small timescales as well as transient events requires higher temporal and spatial resolution of measurements; long time series stations require high duration deployments of instruments. Both requirements can only be adequately satisfied by in situ sensors; for the physical parameters off the shelf instruments are available, in the field of chemical oceanography only a few parameters, such as oxygen, are covered by off the shelf instruments, for most parameters adequate instrumentation only exists in the form of prototypes, if at all. Chemical in situ sensors are needed to gain new insights in how the oceans and the life in it works. This paper will give an overview of several technologies used for developments of chemical in situ sensors over the last five years and what potential these technologies have for future sensors. The technologies covered are: optical absorption spectroscopy (e.g. for nitrate measurements from the low micromolar to high concentrations); Raman spectroscopy (demonstrated e.g. on polyaromatic aromatic hydrocarbons with detection limits (LOD) from ng 1(-1) to mu g 1(-1)); microelectrodes (e.g. Cu(II), Pb(II) and Cd (II) with LOD in the pM range); optodes (e.g. for oceanic oxygen measurements with accuracy of 2 mu M); in situ mass spectrometry (e.g. dissolved gases, volatile organic compounds (1-5 ppb LOD)) and microelectromechanical (MEMS) analysers (that could be used in situ to measure a variety of analytes with wet-chemical methods). The paper will also evaluate how it can be assured that measurements made with new in situ sensors are consistent with proven lab analysis. (c) 2007 Elsevier B.V. All rights reserved.