STANDARD SOLUTIONS BUFFERS 1 Characteristics

The inhibition and the subsequent signal detection were performed in two different solutions. First the pesticide solution was added and then after 10 min (incubation time) the sensor was moved into a new buffer solution where the substrate (5mmoll 1 acetylthiocholine) was injected and the signal measured. This procedure is particularly suitable when a complex matrix, which could pose problems for the direct measurement of thiocholine oxidation, is used. The analytical characteristics of pesticide determination in standard solutions were then evaluated. Detection limits, defined in this work as the concentrations giving an inhibition of 20%, were 30 and 10 ppb for aldicarb and paraoxon, respectively. By increasing the incubation time up to 30 min, an increase in the degree of inhibition could be observed and lower detection limits both for Aldicarb (5 ppb) and Paraoxon (3 ppb) were achieved. 

While it is usually thought that pH of an aqueous solution can easily be measured using two electrodes and a pH meter, in fact, pH can easily be defined only in dilute solutions. There are two main problems in defining pH (1) pH is a thermodynamic characteristic of an individual ion and therefore cannot be experimentally found without some assumptions that are outside of thermodynamics, (2) available electrochemical probes cannot work in many environments of practical interest (e.g., very concentrated brine) and, therefore, should be specially developed for particular applications. Generally, to measure pH of a test solution, pH,, a calibration of reliable electrochanical probe using a number of standard solutions should be carried out with an assumption that pH of the standard solutions, pH, is well defined. The standard solutions should preferably be buffer solutions (see Section 1.17) so when a small amount of acid or base is added, pH of the standard solution is not significantly changed. 

A direct solution to this problem would be a buffer layer between the pentacene channel layer and parylene passivation layer. Given that poly(vinyl alcohol) (PVA) has been used as a photo-patternable etch mask for pentacene, it can also work as this buffer layer. After PVA patterning of the pentacene TFT, 1 pm parylene was vapor deposited and patterned by standard photolithography and RIE dry etching. The TFT characteristics were measured before and after the parylene passivation. 

Figure 3. Difference in ultraviolet spectra of a standard avidin solution (10 6 M in 0.2 M ammonium carbonate buffer at pH 8.9) and a solution of avidin in which varying amounts of the specific ligand biotin have bound to avidin. The characteristic shift of the avidin spectrum to higher wavelengths upon binding to biotin is called the hyperchroic effect (Green (10)) and the maximum shift occurs at 233 nm as illustrated in this difference spectrum.

Response Characteristics. The pH electrode in the assembled apparatus was calibrated with standard buffer solutions. The electrode output... 

It is important to know the thermodynamic characteristics of redox couples in order to understand and qualitatively draw the current-potential curves . Mentioned here are some particular cases which are frequently seen in practice. When protons or hydroxide ions take part in the redox half-reaction and when the solution is buffered, one can assume that the proton or hydroxide ion activity remains constant whatever extent the reaction has reached. This is also the case when components present in large quantities take part in the half-reaction. In this instance therefore, the Nernst law is expressed by defining an apparent standard potential for this specific medium. These types of example are frequently found in systems involving highly acidic or highly basic solutions where the pH can be considered constant. Here apparent standard potentials are therefore defined, corresponding to aqueous solutions with fixed pH. 

Rgure 6 Comparison of the extraction characteristics of an analyte (morphine, ) and its first homologs (Af-ethyl, O, and Af-propyl, A). The extraction of dextrorphan ( ), used as internal standard in a published method, is shown for comparison. The extraction solution was 10 volumes butanol-i-toluene (1 -i-9) to one volume of aqueous buffer. 

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