Page 260 - Handbook of Instrumental Techniques for Analytical Chemistry

250 Handbook of Instrumental Techniques for Analytical Chemistry

Transmittance, T, is the ratio of radiant power transmitted by the sample (I) to the radiant power inci-
dent on the sample (I 0 ). Absorbance (A) is the logarithm to the base 10 of the reciprocal of the transmit-
tance (T).
A = log ( 1 T) = – log T = – log I I⁄
⁄
10 10 10 0 (15.2)
The transmittance spectra provide better contrast between intensities of strong and weak bands be-
cause transmittance ranges from 0 to 100% T whereas absorbance ranges from infinity to zero. The an-
alyst should be aware that the same sample will give quite different profiles for the IR spectrum, which
is linear in wavenumber, and the IR plot, which is linear in wavelength. It will appear as if some IR
bands have been contracted or expanded.
The IR region is commonly divided into three smaller areas: near IR, mid IR, and far IR.

Near IR Mid IR Far IR

–1
Figure 15.1 IR spectra of polystyrene film with different x-axis units. (a) Linear in wavenumber (cm ), (b) linear
in wavelength (µm).(Reprinted from R. M. Silverstein, G. C. Bassler, and T. C. Morrill, Spectrometric Identifica-
tion of Organic Compounds, 4th edition. New York: John Wiley & Sons, 1981, p. 166, by permission of John Wiley
& Sons, Inc., copyright © 1981.)