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

• The absorbance values in an ATR spectrum may slope upward toward lower
frequencies, because the depth of penetration into the sample increases and
more energy is lost by reflection.
• The overall baseline of a spectral subtraction result may not be near zero
absorbance if the baseline of the reference spectrum is significantly different
from that of the sample spectrum.
What are the benefits
of correcting baselines?
You can obtain better results from searching spectral libraries, subtracting spectra or
finding peaks if you first correct the baselines of your spectra. When you search a
library, the entire sample spectrum—including baseline regions—is compared with
the library spectra. If the spectrum’s baseline is not flat and at zero absorbance (or
100 %T), the baseline regions will be considered to be significantly different from
those of the library spectra. These differences will affect the search results.
Similarly, when you subtract a reference spectrum from a sample spectrum, the
baseline regions are subtracted along with the regions that contain peaks. If the
sample spectrum’s baseline is not flat and at zero absorbance (or 100 %T), the
baseline of the subtraction result will have the same undesirable characteristics. If
you correct the baseline first, you can obtain a “clean” subtraction in which
corresponding peaks are subtracted out, without baseline problems in the result.
When you use Find Peaks in the Analyze menu to locate peaks above a specified Y
value, peaks “riding” on a baseline that is not flat and at zero absorbance (or 100
%T) may be found even though their corrected peak heights are less than the
threshold. By correcting the baseline first, you can find peaks in a more consistent
manner; that is, based on their height measured from the baseline.
A flat baseline is also helpful when you are visually comparing spectra or
performing quantitative analysis.
When you add a spectrum to a user library, any negative absorbance values are set
to zero by the normalization process. If you don’t want those values set to zero,
correct the baseline before adding the spectrum to the library.
Use Baseline Correct to correct a baseline by specifying the baseline points or
Automatic Baseline Correct to correct a baseline with the baseline points selected
automatically by the software.
Using the baseline
correction features
Correcting a baseline manually involves clicking points on (or near) the baseline
of the original spectrum displayed in the upper pane of the Baseline Correct
window. As you move the pointer to the point you want to click, the readout
below the panes shows the X and Y values of the pointer location. When you click
a point, its Y value is subtracted from the Y value of the corresponding point in
the spectrum to produce the new baseline point. The result is displayed in the
lower pane. If you click a point directly on the baseline, that point will be brought
to zero absorbance units (or 100 %T). By selecting points carefully, you can
correct the baseline without clicking a large number of points.
In general, you should click points at the beginning and end of baseline regions that
need to be corrected and “low points” above zero absorbance (or high points below
100 %T). When the clicked points are brought to zero absorbance (or 100 %T), the
points between the clicked points will also be shifted so that their vertical position
relative to the nearest clicked points is maintained.
OMNIC User’s Guide 199