Electroanalytical methods

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Basic Principle

Electroanalytical chemistry, as the name implies, involves the analysis of chemical species (analyte) through the use of electrochemical methods. Generally, we monitor alterations in the concentration of a chemical species by measuring changes in current in response to an applied voltage with respect to time using electrodes. According to Faraday's law, the charge is directly proportional to the amount of species undergoing a loss (oxidation) or gain (reduction) of electrons.

Q = n F e

Q is the total charge generated (coulombs)

n is the number of moles of a species undergoing oxidation or reduction

F is Faraday’s constant (96,487 C/mol)

e is the number of electrons per molecule lost or gained

Current is the change in charge as a function of time.

I = dQ / dt

Thus, the current response with respect to time (voltammogram) gives information about changes in the concentration of the species of interest.

Methods

  Electroanalytical methods usually fall into two main categories depending on what is attempted to be ascertained. Each method has its pros and cons. A summary of these is shown in the figure.

Amperometry

In constant potential amperometry, a uniform potential is applied and the change in current is monitored as a function of time. The advantage of this technique is that the time resolution is limited only by the data collection frequency of the instrument. On the other hand, the primary disadvantage is the low chemical selectivity. For example, all species with oxidation potentials below the applied voltage will be oxidized and contribute to the current.

Chronoamperometry is a square wave pulsed voltammetric technique. Limited information about the identity of the electrolyzed species can be obtained from the ratio of the peak oxidation current versus the peak reduction current. However, as with all pulsed techniques, chronoamperometry generates high charging currents, which in this case, decay exponentially with time. To measure the faradic current (the current that is proportional to the concentration of the analyte), current in the last 70-80% of each scan is integrated (when charging current has dissipated). In chronoamperometry, it takes approximately one second to complete a scan in the delayed pulse mode, the latter of which is necessary to prevent fouling of the electrode by serotonin and its oxidation products. Since the current is integrated over relatively longer time intervals, chronoamperometry gives a good signal to noise ratio.

Voltammetry

Fast cyclic voltammetry is a linear sweep voltammetry technique in which the background subtracted voltammogram gives additional information about the electrolyzed species. The current response over a range of potentials is measured, making it a better technique to discern additional current contributions from other electroactive species. FCV is a relatively fast technique with single scans typically recorded every 100 ms, however, the fast scan rates decrease the signal to noise ratio.

Differential pulse voltammetry is a hybrid form of linear sweep and pulsed voltammetries. It has found excellent usage in the identification of electrolyzed species. However, multiple pulses in the waveform make it a relatively slower technique with individual scans taking minutes to complete.

See also

Voltammetry

References

1. http://www.drhuang.com/science/chemistry/electrochemistry/polar.doc.htm

Suggested Further Reading

1. Electrochemical Methods: Fundamental and Applications. Bard AJ and Faulkner LR, John Wiley and Sons, Inc. 2nd Ed.

2. Principles of voltammetry and microelectrode surface states. Kawagoe KT, Zimmerman JB, Wightman RM. J Neurosci Methods. 1993 Jul;48(3):225-40. Review.

3. Fast cyclic voltammetry: measuring transmitter release in 'real time'. Stamford JA. J Neurosci Methods. 1990 Sep;34(1-3):67-72. Review.

4. Webpage with more informations and all of the images can be found at http://www.brain.psu.edu/ElecAnal_tech.htm.

5. Understanding Voltammetry. Compton RG and Banks CE, World Scientific, 2007.