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

As an example, for a standard DTGS detector operating at a mirror velocity of
0.633 cm/s and an optical cutoff at 7900 wavenumbers, the electronic filter should
be set to 10 kHz (see the equation below).
10 kHz = 2 * (0.633 cm/s) * 7900/cm
Note
The available settings depend on your application. The 10 kHz setting used in this
example may not be available. ▲
For near-IR measurements, OMNIC automatically sets the sample spacing to
every laser crossing. Thus, the free spectral range is opened up to 15,800
wavenumbers, and the low pass filter can also be opened up to a higher value
(20,000 Hz in the case of our example). For visible spectroscopy, a sample
spacing of one-half the HeNe frequency is also available.
Finally, there are some research applications where a sample spacing of 4 is
desirable. In such cases, pay close attention to the effects of folding on the infrared
spectrum. For these sample spacing values, you must use both optical and
electronic filtering to prevent folding of spectral features and noise into the desired
spectrum. For more information, please see the excellent text by Peter R. Griffiths
and James A. de Haseth entitled Fourier Transform Infrared Spectrometry, New
York, John Wiley and Sons, 1986.
If you want the cutoff to be set automatically based on the velocity of the moving
mirror, turn on Set Filters Based On Velocity. The Low Pass Filter drop-down list
box is unavailable when the check box is on.
Note
If you use the SST software to perform an AM step scan, PM step scan, PEM,
TRS or SMM experiment, the settings of Low Pass Filter, High Pass Filter and Set
Filters Based On Velocity are ignored. ▲
Specifying the
cutoff frequency
of the high pass filter
High Pass Filter determines the frequency below which the effects of low-frequency
noise are removed from collected data. Sources of this noise include room
vibrations, fans, etc. This noise is normally a concern only when you are using a
very low mirror velocity, such as for a slow-scan photoacoustic measurement, or
when you are collecting far-IR spectra.
You should normally set this parameter to a value equal to 1 divided by the
frequency of the noise.
Note
Note
If you want the cutoff to be set automatically based on the velocity of the moving
mirror, turn on Set Filters Based On Velocity. The High Pass Filter drop-down list
box is unavailable when the check box is on. ▲
If you use the SST software to perform an AM step scan, PM step scan, PEM,
TRS or SMM experiment, the settings of Low Pass Filter, High Pass Filter and Set
Filters Based On Velocity are ignored. ▲
Collecting
single-sided
interferograms
Turn on Single-Sided Interferogram if you want to collect single-sided, rather than
double-sided, interferograms. This speeds up data collection and is useful when
you are performing GC/IR, kinetics or other series experiments in which a large
number of scans are collected during a given amount of time.
Note
The signal-to-noise ratio will be lower when you collect data with this option
turned on. ▲
140 Thermo Nicolet