UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 211

more conveniently used as a standard than gaseous NO 2 , Saltzman (1954) also
examined stoichiometry of nitrite and concluded that 1 mole of NO 2 (gas)
produces the same color intensity as 0.72 mole of sodium nitrite (NO 2 ). This
conversion must be applied to the final determination in order to compare with
the standard nitrite solution.
UV-VIS Methods for Pollutants in Water
Table 8.5 is a list of common colorimetric methods (UV methods not included)
that comprise many of the methods employed in average labs. Key reactions are
also described for selected analytes. It is noted that although there are always
other alternative methods for the measurement, for special applications, some of
the UV-VIS methods are more widely used than their counterparts due to their
simplicity. An example is the colorimetric method for Cr(VI) analysis. At
concentration higher than 0.5 mg/L, this method is superior to other instrumental
methods for a reliable and quick analysis.
To illustrate the principles of colorimetric methods, selected chemical reactions
are listed in Figure 8.16.
8.3 INFRARED SPECTROSCOPY
Three types of IR instruments are described below: (a) Fourier transform infrared
(FTIR) spectrometers, (b) Dispersive infrared (DIR) instruments, (c) Nondis-
persive infrared (NDIR) instruments. Nowadays, the interferometric spectro-
meter coupled with Fourier transform (FTIR) is the most common of all and has
largely displaced dispersive IR instruments. For compound-specific applications
such as in environmental analysis, the nondispersive IR is also used due to its
low cost.
8.3.1 Fourier Transform InfraredSpectrometers (FTIR)
The FTIR uses the Michelson interferometer to produce the interference signals of
the sample source light, with two perpendicularly arranged mirrors, one movable
and the other fixed. The source light is split into two half beams and reflected back
by each mirror. As there are phase-lags caused by the movable mirror, an
interference pattern is produced after the combination of two beams, which is
composed of all frequency signals of sample in a time-domain . This signal is then
Fourier-transformed into frequency-domain , giving the FTIR spectrum. The
Fourier transform , named after a French mathematician and physicist Joseph
Fourier (1768-1830), is an integral transform that reexpresses a function in
terms of sinusoidal basis functions, that is, as a sum or integral of sinusoidal
functions multiplied by some coefficients (amplitude).
Major instrument components shown in Figure 8.17 are similar to UV-VIS
(Fig. 8.14), but the materials are made very differently as described below.