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

Using Other
Corrections
Other Corrections in the Process menu lets you correct spectra for the effects of
dispersion, variations in the depth of penetration for ATR spectra, or water or
carbon dioxide absorptions. When you choose Other Corrections, a dialog box lets
you select one of the following correction options.
• Kramers-Kronig • CO2
• ATR • H2O And CO2
• H2O
Kramers-Kronig
(dispersion)
transformation
Kramers-Kronig removes the effects of optical dispersion from spectra collected
using a reflection technique such as specular reflection (SR) or diffuse reflection
(DR). These spectra may have derivative-shaped peaks (see the example below)
instead of normal (Gausian-Lorentzian) peaks because of a true specular (nonabsorbing)
component of the reflectance that is a first derivative. This component,
which produces a negative part of the peak along with the normal positive
absorption, is called optical dispersion.
Searching a spectral library is especially affected by this dispersion since none of
the library spectra exhibit it. By first applying a mathematical correction called the
Kramers-Kronig transformation to a sample spectrum, you can remove the
dispersion effect and produce a normal absorbance spectrum. You can then search
this spectrum against the spectra in a library.
Note
Note
Kramers-Kronig transformation does not work for peaks with negative absorbance
values. If you correct the baseline of a spectrum (using Baseline Correct or
Automatic Baseline Correct in the Process menu) before correcting the spectrum for
dispersion effects, be careful not to bring any data points below zero absorbance. ▲
Although data that have been corrected for optical dispersion resemble normal
absorbance data, they are not necessarily related linearly to concentration and
generally should not be used for quantitative measurements. ▲
ATR correction
Use ATR to correct spectra collected using the attenuated total reflection (ATR)
technique for the effects of variation in the depth of penetration.
In the ATR technique, the depth of penetration (that is, the effective pathlength) of
the infrared beam varies as a function of the wavelength of light: The longer
wavelengths (lower frequencies) penetrate the sample more deeply than do the
shorter wavelengths (higher frequencies). As a result, the peaks at lower
frequencies are much stronger than those at higher frequencies. This skewing of
peak intensities causes problems when you search a sample spectrum against a
library of normalized spectra collected using standard transmission techniques,
since the peaks have a different appearance.
OMNIC User’s Guide 205