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Noise Sources in Step-Scan FT-IR Spectrometry

Volume 51, Number 8 (Aug. 1997) Page 1092-1101

Manning, Christopher J.; Griffiths, Peter R.

Step-scan Fourier transform infrared (FT-IR) spectrometry has been accepted as a useful tool for obtaining vibrational spectra of a variety of time-dependent systems. Unfortunately, a significant signal-to-noise ratio (SNR) disadvantage has been associated with the step-scan mode of data collection relative to the same data collection time with conventional rapid-scan FT-IR spectrometry. The key difference between the two methods is the average mirror velocity, which alters the dynamic range of the detector signal, as well as the frequencies of its components. The SNR disadvantage is shown to be related to low-frequency multiplicative fluctuations, caused in part by temperature variations, which convolve noise with measured spectra. Refractive index variations of air or purge gas in the paths of the infrared- and reference-laser radiation can be a particularly serious temperature-induced problem. The various noise sources are described, and experiments confirming that some are related to temperature variations are reported.