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IFS 125M User Manual Bruker Optik GmbH
Operation 5
5.7 Performing measurement
Before starting any type of measurement, the spectrometer has to be set up properly.
Make sure that the HeNe laser is stabilized, the beamsplitter inserted and the detector
cooled down (if you use a liquid nitrogen-cooled detector).
5.7.1 Types of measurement
5.7.1.1 Standard measurement using the internal source and the sample compartment
There are two types of measurement to be performed:
• sample measurement
• background measurement
A background measurement is a measurement which uses the empty gas cell or not any
sample at all, and is always performed before sample measurement. The purpose of the
background measurement is to detect how the ambient conditions, e.g. level of air
humidity, temperature etc., and the spectrometer itself have an influence on the spectro
-
scopic measurement result.
After the sample measurement has been completed, OPUS calculates the result sample
spectrum, dividing the sample spectrum (SSC
1
) by the background spectrum (RSC
2
).
The spectral bands, which result from the ambient conditions and/or the spectrometer,
are eliminated from the result sample spectrum.
5.7.1.2 Atmospheric measurement using the input port for collimated beam
This type of measurement performs a sample measurement only. If there are narrow
absorption lines on a smoothly varying background, a quasi absorption spectrum can be
obtained as follows:
• use the OPUS Spectrum Calculator command to obtain -lg SSC
3
• use the OPUS Baseline Correction command (interactive mode) to bring the
baseline down to zero
5.7.2 Measurement parameters
The measurement parameters are defined on the OPUS Measure menu. Select the
Advanced Measurement command and define the parameters on the different tabs. The
standard measuring parameters are provided in appendix
C.
1. SSC: Sample Single Channel, name of the data block added to the OPUS spectrum file
2. RSC: Reference Single Channel, name of the data block added to the OPUS spectrum file
3. Due to the nature of FT spectroscopy, absorbance values >=2 are unreliable and should not be included in
any analysis.
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