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

3. If you are not satisfied with the result, try changing the subtraction factor.
The current factor was calculated using the spectral data displayed when you
chose Subtract. If you change the factor, the new difference spectrum will be
automatically displayed in the bottom pane. There are two ways to change the
factor: by using the scroll bar or by typing a new value.
• To change the factor using the scroll bar, drag the scroll box or click the
scroll arrows. The current upper and lower limits of the scroll bar range
appear near the upper and lower scroll arrows, respectively.
The factor is displayed between the limits. To increase or decrease the
range of the scroll bar, choose Coarser or Finer. Each time you choose
Coarser, the range is doubled; changes you subsequently make to the factor
by clicking the scroll arrows or dragging the scroll box will be twice as
large. Each time you choose Finer, the range is reduced by one-half and
changes you make to the factor by clicking the scroll arrows or dragging the
scroll box will be half as large. Choosing Coarser or Finer does not change
the current value of the subtraction factor displayed next to the scroll bar;
only the number of decimal places in the factor setting may be affected.
• To change the factor by typing a new value, choose Factor. In the dialog
box that appears type a new factor in the text box and then choose OK, or
choose Cancel if you don’t want to change the factor.
4. When you are satisfied with the difference spectrum, you can replace the
sample spectrum with it or add it to a spectral window.
To do this, select the desired option from the window selection box near the
top of the Subtract window and then choose Add or Replace. The title of the
spectrum will be “Subtraction Result.”
To close the Subtract window without placing the difference spectrum into a
spectral window, click its Close button.
Subtracting a spectrum from another spectrum
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The initial subtraction factor is automatically calculated from the displayed
region. If you display a different region and perform the subtraction again, the
result will probably be different because the subtraction factor changed.
When you subtract the spectrum of a pure reference material from that of a
mixture, the peaks may not subtract cleanly. This is because the reference
spectrum does not account for any changes that may occur due to molecular
interactions with the other components in the mixture or differences in
relative concentrations of components. These conditions may cause some
peaks to shift slightly or change shape.
Use a subtraction factor that lets you subtract the unwanted component
absorptions without subtracting other important spectral information.
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