The SNR of measurements made by the use of step-scan FT-IR spectrometers has often been
lower than that of similar rapid-scan measurements. Recent work indicates that the reason
for this disparity is temperature fluctuations within spectrometers which subtly alter the
optical properties in the critical beam paths, causing a multiplex disadvantage.
Hirschfeld showed that a multiplicative variation as small as 1 part per million would
adversely affect many FT-IR measurements.
MAT has developed four accessories for stabilizing the temperature of FT-IR
spectrometers. The most important problem in most systems is the reference laser which
dissipates about 10-20 watts of waste heat into the interferometer compartment. A
water-cooling jacket is available to remove and isolate the laser heat. The second most
serious problem in most systems is scintillation produced by the source element. Typically
the source operates near 1200 degrees C, in a metal cooling jacket. The large temperature
difference drives a strong convection cell, causing refractive index fluctuations in the
critical beam path from the source. The MAT solution to this problem is the addition of a
window to the source aperture, and additional proprietary measures to control convection.
The third level of stabilization is an insulation blanket to reduce fluctuations caused by
the laboratory air conditioning system blowing on the (often metal) spectrometer cover.
The final level of stabilization is the installation of foil heating elements and a
temperature control system to hold the temperature within the spectrometer constant a few
degrees above room temperature.