Flow chemical sensors

In parallel with improvements in chemical sensor performance, analytical science has also seen tremendous advances in the development of compact, portable analytical instruments. For example, lab-on-a-chip (LOAC) devices enable complex bench processes (sampling, reagent addition, temperature control, analysis of reaction products) to be incorporated into a compact, device format that can provide reliable analytical information within a controlled internal environment. LOAC devices typically incorporate pumps, valves, micromachined flow manifolds, reagents, sampling system, electronics and data processing, and communications. Clearly, they are much more complex than the simple chemo-sensor described above. In fact, chemosensors can be incorporated into LOAC devices as a selective sensor, which enables the sensor to be contained within the protective internal environment. Figure 5... 

The working principle of the sensor is simple. If the tip of the sensor, which contains the electrodes, is immersed in a liquid free of HF, an anodic oxide is formed and the anodic current decreases within a second to very low values the LED is off. For the case of a liquid containing more than 5% HF, a constant anodic current flows which is only limited by the series resistor and the LED emits with its maximum intensity. If the liquid contains between 0.5% and 5% HF the intensity of the LED becomes roughly proportional to the HF concentration. In contrast to other chemical sensors where the electrodes are very sensitive to contamination or drying, the HF sensor is quite robust. The sensor electrode can be... 

Volume14 Analytical Applications of Circular Dichroism edited by N. Purdie and H.G. Brittain VolumelS Trace Element Analysis in Biological Specimens edited by R.F.M. Herber and M. Stoeppler VolumelS Flow-through (Bio)Chemical Sensors... 

Bio)chemical sensors can be used in both the batch and the continuous mode. While this is also true of probe-type sensors, flow-through sensors can only be used in a continuous regime coupled on-line to a continuous-flow configuration. 

In broad terms, a flow-through sensor is an analytical device consisting of an active microzone where one or more chemical or biochemical reactions, in addition to a separation process, can take place. The microzone is connected to or incorporated into an optical, electric, thermal or mass transducer and must respond in a direct, reversible, continuous, expeditious and accurate manner to changes in the concentrations of chemical or biochemical species in the liquid or gaseous sample that is passed over it, whether forcefully (by aspiration or injection) or otherwise (gases). 

One possible classification is based on the type of physico-chemical phenomena that may occur in the sensor. Based on this criterion, there are passive flow-through sensors, which posses no reactive microzone and are... 

Figure 2.4 — Classifications of flow-through (bio)chemical sensors.

Active flow-through (bio)chemical sensors include a microzone where a (bio)chemical reaction, a separation or both takes place. The active microzone may be located in the flow-cell itself (Figs 2.6.B and 2.6.C) or built into a probe sensor for insertion into a continuous-flow analytical system (Fig. 2.6.A). The external appearance of a sensitive microzone can be as widely different as the type of detector and process concerned. This is discussed in greater detail in the following section. 

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