|Sensitivity Issues in Step-Scan FT-IR Spectrometry
Things are not as bad as they first appear.
Step-scan measurements often involve sample modulation, such as repetitive stretching of a
polymer film. A second level of demodulation is then required to extract the spectral
response from the detector signal. Much of the noise in the single-beam spectrum is
rejected by the second demodulation, resulting in performance more competitive with
Consider a difference spectrum of a polymer in a stretched vs. relaxed
state. To perform this measurement by rapid-scan, it would be necessary to subtract a
spectrum of one state from a spectrum of the other. For a 1 hour averaging time for each
of these two spectra, the difference spectrum would have a noise level on the order of 10
to 30 microabsorbance units. Any time dependence of the spectral difference would be lost.
A 2 hour step-scan measurement will produce difference spectra with a comparable noise
level, but retaining the time-dependence of the spectral changes.