Applications, Potential and Limitations

Most publications deal with the application to foods (Table 15.3), but published studies are also available covering other products, such as tobacco, cosmetics, health diagnostics and the environment [86]. 

The feasibility and limitations of using multisensor array systems in food and aroma applications will be discussed with an application intended to discriminate hop varieties [149] byway of example 

The sensor responses generated in a measurement result from physical and/ or chemical interactions between the sensors and the volatile compounds present in the headspace above the measured sample. By using a QMB sensor system with an array of six sensors, good discrimination between three hop varieties can be observed (Fig. 15.10a). In this example only 12 measurements per sample were analysed. The distance between clusters is reduced if the data set is increased to 50 measurements per sample (Fig. 15.10b). 

The reason for this effect has to be attributed to a better and adequate ratio between sample size and array dimensionality. For a significant clustering of the patterns, with an array of six sensors a sample size of at least 18 is required [149, 184]. As a consequence, the discrimination based on only 12 measurements has poor statistical relevance. Most of the applications with sensor arrays found in the literature do not consider this fact frequently discriminations with 12-32 sensors in an array and with a sample size of three to four are described. All of them are of limited feasibility with concurrent poor validation, especially in terms of reproducibility and predictive ability. In other words, if there are not enough calibration measurements one can separate data in a predetermined way, but will fail to verify the result using independent test samples. 

A great disadvantage of MOS, MOSFET, conducting polymer and QMB sensor arrays is that system-to-system matching is not possible in practice, as can 

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Easton CJ, Lincoln SF, Barr L, Onagi FI (2004) Molecular reactors and machines applications, potential, and limitations. Chem Eur J 10 3120-3128... 

Easton et al. have discussed the applications, potential, and limitations of molecular machines and reactors [15]. They define molecular reactors as miniature vessels for the assembly of reactants at the molecular level, in order to change the nature of chemical transformations. Molecular machines consist of interrelated parts with separate functions, and perform some kind of work, at the molecular level. However, practical examples are not based on individual functions of singlemolecule devices. 

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