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

Checking the baseline
Turn on Baseline Error in the spectrum checks if you want collected sample spectra
to be checked for baseline problems. Use this check for any type of experiment.
A baseline consists of those portions of a spectrum where there are no significant
absorptions. Ideally, the intensity in these portions of the spectrum is zero
absorbance units (or 100% transmittance). In reality, many factors can affect the
baseline, including the quality of the background spectrum, the quality of the
sample, the way the sample was prepared, the type of accessory used, the thermal
stability of the system, how well the beamsplitter is aligned and how well the system
is purged. Because of these factors, baselines may be tilted, shifted or curved. See
“Why isn’t a baseline always flat and at zero absorbance?” and “What are the
benefits of correcting baselines?” in the “Process” chapter for more information.
Specify the sensitivity for this check by using the scroll bar. The sensitivity value
indicates how readily the check finds baseline problems. If you use a low value,
these problems will be found only if they are large; if you use a high value, these
problems will be found even if they are small. The current setting appears below
the scroll bar, between the minimum and maximum values.
Checking for carbon dioxide
Turn on CO2 Levels in the spectrum checks if you want collected sample spectra
to be checked for carbon dioxide absorptions. This is a routine check that you
should normally leave turned on.
A spectrometer is normally purged to minimize the amount of carbon dioxide (and
water vapor) inside it. If the carbon dioxide level is too high, absorptions of infrared
energy by the gas will result in unwanted peaks that may obscure peaks due to
absorptions by the sample. Also, if the background contains more carbon dioxide
absorption than does the sample spectrum, negative carbon dioxide peaks will result.
Specify the sensitivity for this check by using the scroll bar. The sensitivity value
indicates how readily the check finds and reports carbon dioxide absorptions. If you
use a low value, these absorptions will be found only if they are large; if you use a
high value, these absorptions will be found even if they are small. The current setting
appears below the scroll bar, between the minimum and maximum values.
Checking for water vapor
Turn on H2O Levels in the spectrum checks if you want collected sample spectra
to be checked for water absorptions. This is a routine check that you should
normally leave turned on.
A spectrometer is normally purged to minimize the amount of water vapor (and
carbon dioxide) inside it (or it has a sealed and desiccated optics compartment to
eliminate water vapor). If the water vapor level is too high, absorptions of infrared
energy by the gas will result in unwanted peaks that may obscure peaks due to
absorptions by the sample. Also, if the background spectrum contains more water
absorption than does the sample spectrum, negative water peaks will result.
Specify the sensitivity for this check by using the scroll bar. The sensitivity value
indicates how readily the check finds and reports water absorptions. If you use a
low value, these absorptions will be found only if they are large; if you use a high
value, these absorptions will be found even if they are small. The current setting
appears below the scroll bar, between the minimum and maximum values.
Checking the spectral range
Turn on Spectral Range Consistent Among Beamsplitter, Detector, Source And
Accessory in the parameter checks if you want the software to check the
consistency and appropriateness of the spectral range used for collecting and
saving sample spectra. This is a routine check that you should leave turned on
unless you are collecting data with an unusual spectral range.
OMNIC User’s Guide 133