Signal-to-Noise Ratio, Signal Processing, and Spectral Information in the Instrumental Analysis Laboratory

A laboratory is described that illustrates concepts related to the signal-to-noise ratio, spectral data acquisition parameters, and signal processing. The laboratory is designed to illustrate the compromises one must make between signal-to-noise ratio, spectral information, and analysis time. Using an FTIR spectrometer, the signal-to-noise ratio is obtained as a function of spectral resolution and frequency range. The S/N increases with decreasing resolution, and can be correlated with the single beam signal intensity. Digital smoothing with various numbers of smoothed points is performed to illustrate how the S/N can be enhanced. A plot of signal-to-noise ratio versus the number of scans co-added (at constant resolution) fits a power curve with an exponential very close to the theoretical value of 0.5. Single-beam atmospheric CO2 spectra are obtained at various resolutions to illustrate the loss of spectral information at low resolution. Spectra are compared after applying digital smoothing routines to a high resolution spectrum to illustrate the potential drawbacks of this S/N enhancement technique.