Sensitivity Issues in StepScan FTIR Spectrometry
This is the most important slide in the entire
sequence. The optical path of the infrared and laser beams through the interferometer is
the product of the physical distance and the optical index. If the temperature varies
between the two arms, a false path difference can be introduced. The refractive index of
air at 15 °C is 1.0002760, while at 16 °C is it is 1.0002749 (a difference of about 1
ppm). A typical twoway path in each of the arms is 8 cm, so up to 88 nm of error, per
degree of temperature difference, can be introduced. If the difference were stable or
homogeneous it would not be so serious. However, temperature differences are induced by
convective heat transfer and are neither stable nor homogeneous. To obtain a SNR of
10,000:1 using interferometric measurements in the midIR, the RMS error in sampling
position must be held to less than 1 nm. Hence, the effective temperature difference
between the two arms of the interferometer should be less than 1/100 of a degree. Now we
can look for experimental verification of these ideas.
