handbook of modern sensors

522 17 Chemical Sensors
(A)
(B)
Fig. 17.17. Fourier transform infrared spectroscopy example (A) and voltammetry example (B).
Fourier transform infrared (FTIR) spectroscopy is a chemical analysis technique
that involves bombarding an analyte sample with a range of different IR radiations and
measuring the magnitude of different wavelengths of the IR that absorbed [26]. The
absorption is plotted versus wavelength to produce a very noisy spectrogram that is
then filtered via Fourier transforms (Fig. 17.17A). FTIR is still a popular experimental
technology with improvements being made regularly, but those improvements also
represent changes that make standardization and universal libraries of spectra of
limited use. The technique has not yet been miniaturized and the noise found in the
spectrum requires significant computational capability to remove it. Chemometrics
application software that includes Fourier transforms may be used to process the FTIR
spectrograms.
Voltammetry is an electrochemical measurement technique that involves applying
a changing potential across two or three electrodes in contact with a liquid or
gaseous analyte [27]. The changing potential triggers redox reactions in electroactive
species and affects the overall electrical current measurable in the system loop. The
plot (Fig. 17.17B) of the measured current versus the applied potential is called a
voltammogram [28] and it contains a significant amount of information which allows
one to identify and quantify the chemical species in the mixture or compound. The
voltammogram produced can be simple or complex depending on the complexity of
the shape of the applied potential [29]. There is a great deal of interaction between
the different electrochemical reactions, but, in general, different chemical species
have specific dissociation potentials, so the location along the potential curve of the
feature identifies the species. The size of the features is controlled by the amount
of any given species in the analyte [30]. Voltammetric analysis began in the early
part of the twentieth century [31,32]. It is an excellent technique for organic, inorganic,
metallic, and metallorganic species. Because voltammetry produces such
complex results, specialized chemometrics strategies are replacing manual analysis
of the voltammogram [33].
Several basic waveforms are effective and popular for producing the voltammetry
response. These include a simple linear sweep, a triangle sweep, stair-step sweep,
pulsed differential sweep, and a square-wave sweep. The simpler linear sweep and
triangle sweep are excellent for diagnostic capability but have somewhat poor detection
limits, often limited to 10 −3 –10 −4 M levels. Stair-step, differential pulse, and
square-wave voltammetries have detection limits as low as the 10 −7 –10 −8 M levels.