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DX4040 INSTRUCTION AND OPERATING MANUAL (E2.04)
For example, suppose we have a sample spectrum and reference spectra like those
shown in Figure 6. In this case, we know that the sample gas consists of gases
Reference 1
and
Reference 2
. We have the reference spectra available and we know
that these reference spectra represent concentrations of 10 ppm and 8 ppm,
respectively. To find out the concentration of each component in the sample gas, we try
to form the measured sample spectrum using a linear combination of the two reference
spectra. We find out that if we multiply the spectrum
Reference 1
by 5 and the
spectrum
Reference 2
by 2, and combine these two spectra we get a spectrum that is
similar to the sample spectrum. Accordingly, the sample gas contains reference gas 1
at five times the amount in the reference spectrum 1, and reference gas 2 at two times
the amount in the reference spectrum 2. The analysis indicates that the sample indeed
consists of these two reference gases. The concentration of the reference gas 1 in the
sample is found to be 50 ppm (= 5 x 10 ppm), and the concentration of the reference
gas 2 in the sample is 16 ppm (= 2 x 8 ppm).
0
0.05
0.1
0.15
0.2
0.25
0.3
Sample
Reference 1
Reference 2
Figure 6.
An example of spectra for multi-component analysis. The spectral analysis
routine in Calcmet Lite software performs all calculations automatically.
3.5 The Spectral Resolution of FTIR Analysis
The spectral resolution of the measurement indicates how accurately it is possible to
separate different wavelengths of radiation in the absorption spectrum. High resolution
allows one to detect visually the exact location of narrow absorbance peaks and
reduces spectral overlap. If it were possible to achieve infinite resolution, no deviations
of Beer's law due to wide concentration range would occur. Thus, it would seem
beneficial to have high resolution in order to get good results for the analysis. However,
increasing the resolution causes other problems. For example, spectral noise increases
and makes the analysis less precise.
Furthermore, as high a resolution as possible does not maximize the information
content of the measurement. Jaakkola
et al
. have shown
that low-resolution FTIR
spectroscopy offers some valuable advantages compared to the traditional high-
resolution FTIR gas phase spectroscopy, especially in non-laboratory environments.
1
See, for example, P. Jaakkola, J. D. Tate, M. Paakkunainen, J. Kauppinen, and P. Saarinen, "Instrumental Resolution Considerations for
Fourier Transform Infrared Gas-Phase Spectroscopy,"
Applied Spectroscopy 51
(1997) 1159 - 1169.
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Wave number (cm
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