Technologies for new in situ chemical 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 that meet the requirements of fast sampling and long deployment times. For most parameters adequate instrumentation only exists in the form of prototypes, if at all. More and more chemical in situ sensors are becoming available commercially but new chemical in situ sensors for a variety of measurands are needed to gain new insights in how the oceans and the life in it work. The long term challenge is to develop in situ instruments that meet the same stringent precision and accuracy standards that are reached by careful laboratory analysis. In situ instrumentation, however, has to perform the measurements in the real world environment with variations in temperature, salinity, pH and under high ambient pressure. In the near future these standards are unlikely to be met by new in situ chemical sensors. However, even with lower precision and accuracy, chemical sensors with high temporal resolution can provide valuable insights in the dynamics of concentration changes of various measurands and guide the strategy for taking water samples for subsequent analysis in the lab. This paper gives an overview of several technologies that have been used for developments of chemical in situ sensors over the last Ave years and what potential thes- e 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 hydrocarbons with detection limits (LOD) from ng l^-1 to mug l^-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 muM); 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.