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

A background spectrum is used to eliminate signals due to the spectrometer and its
environment from the sample spectrum. The background single-beam spectrum
shows how the energy of the source is distributed over the displayed frequency
range. It includes the characteristics of the environment of the spectrometer,
including the detector, beamsplitter and atmospheric conditions.
Each sample single-beam spectrum is ratioed against the background single-beam
spectrum so that the absorptions in the final spectrum are due solely to the sample.
The sample spectrum is displayed using the specified final format; for example,
absorbance or % transmittance.
When to collect a new
background spectrum
Collect a new background spectrum if...
• You have changed the hardware in your spectrometer.
• You have changed the settings of any of the following parameters in the
Experiment Setup dialog box.
On the Collect tab: Resolution, Automatic Atmospheric Suppression.
On the Bench tab: Velocity, Aperture, Sample Compartment, Detector,
Beamsplitter, Source, Accessory, Window Material, Spectral Range.
On the Advanced tab: Zero Filling, Apodization, Sample Spacing, Phase
Correction, Low Pass Filter, High Pass Filter, Single-Sided Interferogram.
• There is a change in the size of the water or carbon dioxide bands in your
sample spectra.
• You are about to collect a sample spectrum to use as a standard in a
quantitative analysis method.
• You are about to collect a sample spectrum to use to calculate the
concentrations of components in some samples.
A tip for demanding applications
You can obtain the best results by collecting sample and background spectra under
similar conditions. If your usual procedure for collecting sample spectra is to install
the sample, allow the system to purge for three to five minutes, and then choose
Collect Sample, try opening and closing the sample compartment (to let in some
room air) and then waiting three to five minutes before choosing Collect
Background. Since the spectra will have been collected under similar atmospheric
conditions, the absorptions due to gases inside the spectrometer will be cleanly
removed when the sample spectrum is ratioed against the background. ▲
Viewing the progress
Viewing the resolution
The progress of the collection is indicated visually by the gauge at the left side of
the window just below the pane. As data are collected, the gauge is gradually
filled with black from left to right. For some spectrometers the number of scans
collected so far and the total number of scans for the collection are displayed to
the right of the gauge.
The resolution of the data is displayed to the right of the total number of scans.
OMNIC User’s Guide 155