Calibration chemical sensors

M.N. Tib and R. Narayanaswamy, Multichannel calibration technique for optical-fibre chemical sensor using artificial neural network. Sensors Actuators, B39 (1997) 365-370. 

Compared with the sensors for atoms and radicals, the calibration of EEP sensors is also somewhat specific. To calibrate detectors of atomic particles, it will be generally enough to determine (on the basis of sensor measurements) one of the literature-known constants, say, tiie energy of parent gas dissociation on a hot Hlament. For the detection of EEPs when nonselective excitation of gas is taking place, in order to calibrate a sensor use should be made of some other selective methods detecting EEPs. The calibration method may be optical spectroscopy, chemical and optic titration, emission measurements, etc. 

The response curve (RC) represents the calibrated output response of a sensor as a function of the measurand/s applied to its input. For instance, in the case of a chemical sensor based on conductivity (G), it is recommended to use one of the following notations [1] for the output response ... 

The first group of sensor properties in Fig. 1.15 is concerned with the quality of results obtained in analytical processes involving a (bio)chemical sensor. All of them are obvious targets of analytical tasks [3]. As shown in the following section, the accuracy of the analytical results relies on a high reproducibility or repeatability, a steep slope of the calibration curve (or a low detection or quantification limit) and the absence of physical, chemical and physico-chemical interferences from the sample matrix. Sensors should ideally meet these essential requisites. Otherwise, they should be discarded for routine analytical use however great their academic interest may be. 

M. D. DeGrandpre. M. M. Baehr. and T. R. Hammar. Calibration-Free Optical Chemical Sensors, Anal. Chem. 1999, 71, 1152. 

The concept of order applies across the analytical field (recall the discussion of kinetics in Chapter 2). Order is also applied in classifying chemical sensors. When only one physical parameter constitutes the output of the sensor and is correlated with concentration, we call it a first-order sensor. An example is optical sensing of a component at one fixed wavelength. The concentration of the unknown sample is then obtained from the calibration curve (Fig. 10.1a) against absorbance, or by a standard addition method. For nonlinear sensors it is possible to use a linearization function /. 

Fig. 10.1 (a) First-order chemical sensor in which absorbance is uniquely related to concentration by calibration curve, (b) Second-order sensor in which absorbance is shown as a function of wavelength X. Interferant is easily identified in the spectrum, (c) Third-order sensor yielding information in 3-D space. The red dashed line shows conversion of third-order sensor to second-order sensor when the value of response R is obtained at a fixed retention time/ ... 

The most frequently used calibration procedure is based on temperature dependence of pressure of saturated mercury vapour [19,39-41]. At 25°C this pressure is of 0.0018 mm Hg height it corresponds to the vapour density of 20 pg/1. To get in the measurement cell a mercury concentration of about 10 ng/1, the saturated vapour should be strongly diluted. Instead of dilution, a lower temperature can be used however, the density of saturated vapour of 10 ng/1 corresponds to the temperature of less than —40°C. Both dilution and temperature decrease can be realized easily in laboratory conditions but their incorporation into a miniaturized chemical sensor is rather complicated. An attempt to develop such a device is reported in Ref. [41]. An additional problem in application of these techniques in portable sensing devices with integrated calibration is the necessity to have a reservoir with mercury in the device it complicates recycling of these devices and does not correspond to modern trends in technology. 

Both organic and inorganic polymer materials have been used as solid supports of indicator dyes in the development of optical sensors for (bio)chemical species. It is known that the choice of solid support and immobilization procedure have significant effects on the performance of the optical sensors (optodes) in terms of selectivity, sensitivity, dynamic range, calibration, response time and (photo)stability. Immobilization of dyes is, therefore, an essential step in the fabrication of many optical chemical sensors and biosensors. Typically, the indicator molecules have been immobilized in polymer matrices (films or beads) via adsorption, entrapment, ion exchange or covalent binding procedures. 

Another problem with the development of implantable sensors is the need to calibrate the sensor ex vivo. This requires a high enzyme stability since the sensor has to be calibrated before implantation. The longest lifetime reported for enzymes immobilized in an implanted sensor was between 6 and 10 days (Shichiri et al., 1987). Neither physical entrapment nor chemical binding and crosslinking of GOD have provided a higher stability for continuously operated glucose sensors. 

Smilde AK, Tauler R, Henshaw JM, Burgess LW, Kowalski BR, Multicomponent determination of chlorinated hydrocarbons using a reaction-based chemical sensor. 3. Medium-rank second-order calibration with restricted Tucker models, Analytical Chemistry, 1994a, 66, 3345-3351. 

The application of chemical sensors now and in the future will be oriented around real time monitoring of chemical and environmental processes [3, 4]. These processes are complex systems where a variety of chemicals are present. From a calibration point of view, sensor systems for these applications will either have to be fully selective or be able to handle multicomponent samples. Since the perfectly selective sensor is difficult to almost impossible to develop, sensors in the array or two-dimensional array format have certain advantages. 

Henshaw J M, Burgess L W, Kowalski B R, Smilde A and Tauler R 1994 Multicomponent determination of chlorinated hydrocarbons using a reaction-based chemical sensor. 1. Multivariate calibration of Fujiwara reaction products Anal. Chem. 66 3328-36... 

Fig. 3 The calibration curves w=f(p), w=/((/out), and p=f Uom) of the chemical sensor. Reprinted from (Guenther et al. 2008) with kind permission from Elsevier...

Hohnberg, M., Davide, F.A.M., Di Natale, C., D amico, A., Winquist, F., Lundstrom, L Drift counteraction in odour recognition applications Lifelong calibration method. Sensors and Actuators B Chemical 42, 185-194 (1997)... 

Calibration curve of the response of a chemical sensor towards a chemical species. 

Following the development of the electronic nose for gas analysis, the electronic tongue (an array of potentiometric chemical sensors) has been devised for the analysis of complex liquid samples. If properly configured and trained (calibrated), the electronic tongue is capable of recognizing the qualitative and quantitative composition of multicomponent solutions of different natures. The electronic tongue and electronic nose, in combination, can be used to predict the sensory characteristics and their relationship to the quality of, e.g., apple juices measured by a trained sensory panel and consumers. 

Clean and calibrate all sensors Instruments must be calibrated on a regular basis. Improper sensor readings will lead to inaccurate normalization and present a false picture as to how the RO system is functioning. Calibrate chemical feed pumps Chemical feed systems should be calibrated on a regular basis to make sure the required dosage of chemical is being fed. 

The essential component of a potentiometric measurement is an indicator electrode, the potential of which is a function of the activity of the target analyte. Many types of electrodes exist (see Table 9.1), but those based on membranes are by far the most useful analytical devices. The broader field of potentiometry has been reviewed recently (1). The potential of the indicator electrode cannot be determined in isolation, and another electrode (a reference electrode) is required to complete the electrochemical cell. Undoubtedly the best known of the potentiometric indicator electrodes is the glass pH electrode, the operation and use of which has been adequately discussed (2). Ion-selective electrodes (ISEs) are also commonplace, and have been the subject of several books (3-5) there is even a review journal for ISEs (6). Unfortunately, the simplicity of fabrication and use of ISEs has given rise to the idea that ISEs are chemical sensors. At the best this is a half-truth certainly, they can behave like chemical sensors under well-controlled laboratory conditions, but in the real world their performance leaves much to be desired. Moreover, from a manufacturing point of view important features of a sensor are that it can be fabricated in relatively large numbers, and that each device is identical to all the others. Although some ISEs can be mass-produced , many cannot, and even those that do lend themselves to this form of production invariably require calibration before use. Nonetheless, in spite of the limitations of ISEs, transducers based on potentiometric membrane electrodes have much to contribute to the field of chemical sensing. 

A complete potentiometric transducer includes an external reference electrode. This point is often ignored in the ISE literature, for it is assumed that some suitable device can be incorporated into the cell, and that calibration will overcome any problems associated with variability in this cell component. This approach is inadmissible for a chemical sensor, because the reference electrode must be included as part of the package. Too often, publication of an elegant sensor is marred by a test set-up which includes a large, conventional reference electrode. 

The thrust of chemical sensor development is to produce devices which are small, inexpensive, disposable and readily manufactured. These need not be advantageous for all (e.g. industrial) measurement scenarios. In this context, the need to calibrate each device individually before use is clearly a major... 

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