DX4040 INSTRUCTION AND OPERATING MANUAL (E2.04)
Figure 5.
A typical interferogram.
The spectrum is computed from the digitized interferogram by performing a Fourier
transform by a computer utilizing a Fast Fourier Transform (FFT) algorithm.
3.3 Quantitative Analysis of FTIR Spectra
The basic law for spectroscopic quantitative analysis is Beer's law (also known as the
Beer-Lambert law). It shows how the concentration of the sample gas is related to the
measured absorbance of the sample spectrum:
log(
/ )
log( / )
I
I
T
A
abc
0
1
=
=
=
I
o
= intensity of infrared radiation entering the sample
I = intensity of the infrared radiation that has passed through the sample
A = absorbance
T = transmittance
a = a (υ) = absorptivity (depends on wavelength)
b = optical path length
c = sample concentration
The absorptivity
a
characterizes the capacity of the molecule to absorb infrared
radiation. The value of
a
varies from a molecule to another and as a function of
wavelength, but is constant for a given molecule at a given wavelength. The quantity
b
is the optical path length, that is, the distance the infrared radiation beam traverses in
the gas sample. The quantity
c
indicates the concentration of the sample gas molecules
in the sample. If the optical path length is constant, Beer's law states that the
V
ol
ta
ge
U
(
m
v)
Optical path difference x/2 (cm
-1
)
- L
0
+ L
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