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Linear Wavenumber Plotting of Spectrophotometer Outputs

Volume 23, Number 1 (Feb. 1969) Page 51-54

Baker, Earl W.; Wolfe, Court L.; Noble, Frank W.

The quantity of direct interest in the mathematical analysis and correlation of spectra is the integrated absolute intensity, i.e., I = ∫εdν, where ε is the molar absorptivity at wavenumber v. Its evaluation requires that the spectra be plotted with an abscissa linear in wavenumber (energy) and the ordinate linear in absorptivity or absorbance. Paradoxically, while the convenience of the ratio recording spectrophotometer has long been available with direct plotting of absorbance, spectra linear in wavenumber have generally been obtained by either hand replotting or digitizing and replotting by computer. A simple analog device is described which obviates this slow step and performs an on-line conversion of wavelength to wave-number according to the equation: 107/mμ = cm−1. The converter, which is mechanically linked to the wavelength scroll of the spectrophotometer, provides an output voltage proportional to the reciprocal of the scroll position. This output is used to position a servocontrolled chart drive (abscissa) on the readout recorder. Electrical circuits and details of construction are given, along with examples of spectra produced.