Thermonicolet Omnic Software User's Guide 6.1 (PDF) - Charles E ...

Before searching an ATR spectrum against a library of normalized transmission
spectra or a library of corrected ATR spectra, correct the spectrum. (If you search
a library of uncorrected ATR spectra, do not correct the sample spectrum.)
ATR correction multiplies the sample spectrum by a wavelength-dependent factor
to adjust the relative peak intensities. The resulting spectrum has peaks more like
those in a spectrum collected by transmission and can be visually compared with
transmission spectra or searched against a library of transmission spectra.
Water correction
Carbon
dioxide correction
Water and carbon
dioxide correction
Use H2O to correct a spectrum for the effects of water. This requires the use of a
water reference spectrum. See “Specifying water and carbon dioxide reference
spectra” in the “Edit” chapter for a procedure for collecting one.
Use CO2 to correct a spectrum for the effects of carbon dioxide. This requires the
use of a carbon dioxide reference spectrum. See “Specifying water and carbon
dioxide reference spectra” in the “Edit” chapter for a procedure for collecting one.
Use H2O And CO2 to correct a spectrum for the effects of water and carbon
dioxide. This requires the use of a water and carbon dioxide reference spectrum.
See “Specifying water and carbon dioxide reference spectra” in the “Edit” chapter
for a procedure for collecting one.
In a typical FT-IR experiment the sample spectrum is ratioed against a background
spectrum that contains all of the spectral characteristics of the instrument. These
characteristics include absorptions due to any atmospheric water vapor and carbon
dioxide inside the spectrometer. Ratioing ensures that the sample spectrum
contains information that is characteristic only of the sample.
Since you collect the sample and background spectra separately, the water and
carbon dioxide absorptions may not be exactly the same in both spectra. This can
result in small positive (or negative) peaks in the water (3,800 and
1,600 wavenumbers) and carbon dioxide (2,350 and 668 wavenumbers) regions of
the ratioed sample spectrum. These residual peaks will cause problems when you
search the spectrum against a library.
If you use Other Corrections, the water and carbon dioxide correction involves
subtracting a reference spectrum of pure water and carbon dioxide (or of just one)
from the sample spectrum to remove the correct amount of each component. Since a
pure reference is used, only the water and carbon dioxide absorptions are affected.
If you use Automatic Atmospheric Suppression on the Collect tab of the Experiment
Setup dialog box, the effects of water vapor and carbon dioxide are automatically
suppressed through the use of a quantitative model. We recommend using this
feature instead of the water and carbon dioxide corrections available through
Correction on the Collect tab of the Experiment Setup dialog box or Other
Corrections in the Process menu, unless you find that the humidity in your facility is
too high for it to work effectively.
Process spectra using Other Corrections
1. Select the spectra.
Select the spectra. If you are performing a Kramers-Kronig (dispersion)
transformation, display the spectral region that you want corrected. Only this
region will be corrected and included in the new spectra.
206 Thermo Nicolet