Effect of Resolution on the Wavenumber Determination of a Putative Standard to Be Used for near Infrared Diffuse Reflection Spectra Measured on Fourier Transform near Infrared Spectrometers

Abstract

A mixture of powdered Er2O3, Dy2O3, Ho2O3 and talc has been proposed as a wavenumber standard for near infrared (NIR) diffuse reflection (DR) measurements. Previous measurements were made at a resolution of 2 cm−1 which is significantly less than the full-width at half-height of the peaks in the spectrum. Since many NIR DR spectra are measured at a much lower resolution, some of the peaks are observed to fuse and/or shift. In this paper, the behaviour of the band positions is studied as a function of resolution. The factors that allow some of the peaks to be useful as wavenumber standards for NIR DR measurements while others should not be used are discussed. The instrument should first be calibrated with certain lines in the vibration–rotation spectrum of water vapour. Because most of the lines in the water spectrum are located less than 1 cm−1 from their nearest neighbours, only two of the 3768 lines in the region between 5750 and 7990 cm−1 can be used for calibration and only then if the spectrum is converted to its second derivative and interpolated so that there are at least five data points per resolution element and the wavenumber of maximum absorption is found by fitting the line to a quartic function. For the spectrum of the putative wavenumber standard measured at a resolution of 3–10 cm−1, the band centre is best measured after baseline correction of the spectrum with a polynomial function, while lower resolution spectra are better converted to the second derivative. As might be expected, when choosing peaks as wavenumber standards for instruments operating at different resolutions, one should avoid bands that either become fused at resolutions close to the value being used or whose position varies rapidly with resolution. Peaks should be selected that, besides being in the wavenumber range of interest, can be measured with both high precision and high accuracy.