UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 214

spectrometer are in between the UV-VIS and FTIR instruments. The infrared source
is the same as FTIR, which is essentially the heat; so hot wires, a light bulb, or
glowing ceramics are used as infrared light sources. However, instead of Michelson
interferometer used in FTIR, a monochromator similar to those used in UV-VIS is
used to disperse infrared radiations of various wavelengths. The materials of
monochromator are different. Since glass and fused silica transmit very little
infrared light, prisms and other optics must be made from large crystals
of alkaline earth halides, which are transparent to IR radiation. NaCl can be
used in the entire region from 2.5 to 15.4 m (4000-650 cm 1 ). For longer m

m m

wavelengths, KBr (19-25 m) or CsI (10-38 m) can be used. The mono-

chromator compartment must be kept dry.

8.3.3 Nondispersive Infrared Instruments (NDIR)

This type of IR instrument is designed for target application, that is, for the
quantification of one or several compounds. The nondispersive infrared (NDIR)
spectrometers incorporate interchangeable filters to isolate a particular wavelength
for measurement. Therefore, the nondispersive infrared instruments do not record
a spectrum. Major environmental applications include analysis of CO and CO 2 from
car exhausts by recording absorbance at 2170 and 2350 cm 1 , respectively. The
commercial total organic carbon (TOC) analyzer also uses nondispersive IR.

8.3.4 Applications in Industrial Hygiene and Air
Pollution Monitoring

IR has a variety of applications as various inorganic and organic chemicals respond
to infrared light. It is estimated that, of more than 400 chemicals for which
maximum tolerable limits have been set by the OSHA, more than half appear to have
absorption characteristics suitable for determination by means of midinfrared filter
photometers or spectrophotometers. Out of 189 hazardous air pollutants or HAPs
(Table 2.7) listed by the U.S. EPA, more than 100 HAPs have their established IR
spectra (http://www.epa.gov/ttn/emc/ftir/welcome.html).
One major disadvantage of IR, however, is its limitation in measuring
compounds at low concentrations. This explains why many IR applications are
limited to high concentration detection, such as, auto exhaust emission testing,
occupational exposure testing in industrial hygiene, and measurement of aggregate
organic contaminants in environmental analysis (oil and petroleum). An exhaustive
list of such applications is impossible, but some of the major IR-related EPA
methods are summarized in Table 8.6 (air methods) and Table 8.7 (water methods).
Methods with IR coupled with GC have also been included in EPA's SW-846. For a
list of OSHA and NIOSH approved IR methods, the reader should refer to the web
sites given in Chapter 5. IR has also found special applications in tropospheric
studies on the global distribution of CO 2 ,O 3 ,H 2 O, HNO 3 , and NO 2 , and even in
stratospheric monitoring of Freon that causes ozone layer depletion.