Department of Chemistry, Tufts University
Why Run a Background Spectrum?
The IR spectrum is usually displayed as per cent transmittance, that is how much
of the original IR intensity is left after passing through the sample. In order to
calculate this, we first need to know the IR intensity with no sample, so we run a
background.
The first thing you notice about the background spectrum is there are strong IR
peaks near 3800, 2400,1600 cm-1. These peaks are due to the O-H stretch of H2O,
the asymmetric stretch of CO2 and the H-O-H bending of H2O, respectively. The
H2O and CO2 are present in the air, which fills the spectrometer. The H2O bands
consist of many sharp peaks. These peaks are the vibrational transitions between
different rotational states of H2O. With a higher resolution instrument, the CO2
band shows similar rotational splitting, but the lines are closer together. Rotational
splitting is only observed in the gas phase. Passing a steady stream of nitrogen
though the spectrometer can eliminate the H2O and CO2 bands. This is called
purging the spectrometer.
Even without the CO2 and H2O bands, the background spectrum is much more
intense in the middle than at both ends reflecting the output spectrum of the source:
strong in the middle, but falling off at the ends.
A Typical FTIR Background Spectrum
Revised 11/3/00
D. Wilbur