Chemical signals evaporation

As described above, desiccation stimulates ABA synthesis and there are high concentrations of ABA in water-stressed leaves. This relation leads to the proposal that ABA mediates the response to drought. Indeed, most ABA deficient mutants display an excessive water loss due to defects in stomatal regulation which leads to an increased tendency to wilt [28,44,99]. Characterization of ABA deficient tomato plants showed a direct relationship between stomatal closure and ABA. The stomata of these plants were unresponsive to water stress but could be closed by applying ABA. ABA synthesized in water-stressed roots may act as a chemical signal to induce stomatal closure before any desiccation of the leaves. When the roots are submitted to stress they will produce ABA that is transported to the aerial parts of the plant and induces stomatal closure to prevent evaporation. This eould be a meehanism to optimize the plant s water use under restricted availability [19]. 

One hypothesis is that this phenomenon may be due to a competition between nonvolatile matrix components and analyte ions for access to the droplet surface for transfer to the gas phase. This competition can enhance or suppress the signal depending on the environment in which the ionization and the ion evaporation processes take place therefore, the presence of a matrix can influence ion formation. Chemical-physical properties of the analytes, and in particular polarity, play a role on the degree of matrix effects that have a strong influence on the analytical precision of the method, and in particular, on the sensitivity and the limit of quantification [37]. 

An other interesting strategy is the modification of the surface of the electrodes with multiwalled carbon nanotubes (MWNTs) or single-walled carbon nanotubes (SWNTs) [13,32]. The MWNTs are grown on the electrodes covered with a nickel catalyst film by plasma-enhanced chemical vapour deposition and encapsulated in Si02 dielectrics with only the end exposed at the surface to form an inlaid nanoelectrode array [13]. In the other case, commercial SWNTs are deposited on SPE surface by evaporation [32], The carbon nanotubes are functionalised with ssDNA probes by covalent attachment. This kind of modification shows a very efficient hybridisation and, moreover, the carbon nanotubes improve the analytical signal. 

---