Chemical sensors sensing volume

The requirements for chemical sensors suitable for use in eddy correlation direct measurements of surface fluxes are examined. The resolution of chemical sensors is examined and defined in terms of surface flux and commonly measured micrometeorological parameters. Aspects of the design and operation of sensor systems are considered. In particular, the effects of the inlet ductingy the sensing volume, and the signal processing on the ability to measure surface fluxes were analyzed. 

Sensing Volume. The sensing volume of a sensor is the volume where the air is actually monitored. The sensing volume is the reaction chamber of a flame photometric detector or a chemiluminescence device, the field of view of an open-path sensor, or the White cell of a reduced-pressure optical system. The residence time of the sample within the sensing volume ultimately limits the temporal resolution of most chemical sensors. 

Most label-free optical bio/chemical sensors belong to the category of evanescent-wave sensors [5], in which the evanescent field exponentially decays into the surrounding medium for tens to few hundred of nanometers away from the solid sensor surface. These sensors usually utilize the RI change induced by the molecular interaction with the evanescent field as the sensing mechanism. RI change is related to the sample concentration rather than the total sample mass. Therefore, in principle, an Rl-based sensor can perform sensitive detection with small sample volumes. 

Metal oxides are among the most used active materials for conductometric chemical sensors. They have a wide variety of electrical properties spanning from insulator to quasi metallic behavior. The discovery of their sensing properties was made more than five decades ago, thereafter the interest of researchers was focused on nanostructured materials. These materials may give a greater modulation of the electrical properties for the interaction with the surrounding atmosphere thanks to the higher surface to volume ratio. 

A priori, sensors should be small and be available in large volume. The origins of their development can be traced, in part, to the idea of creating technical sense organs. On the other hand, they became part of the arsenal of analytical chemistry. Considering all these characteristics from a holistic point of view, it becomes comprehensible that sensors may help to achieve objectives which would not be within reach using classical scientific tools. With the advent of chemical sensors, new ideas arose of non-classical analytical chemistry. Many of these new ideas will be of utmost importance in future. 

Precise measurement tools are necessary parts of most successful scientific and engineering enterprises. The sensing devices that we consider in this volume are such tools, capable of measuring physical, chemical, and biological quantities. What they have in common is that they all employ acoustic waves in their cq>-eration. The purpose of this introductory chapter is to provide an overview of these devices, and to answer the question why use acoustic sensors ... 

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