Determination of wavelength accuracy in the near-infrared spectral region based on NIST's infrared transmission wavelength standard SRM 1921

A major concern with the use of near-infrared (NIR) spectroscopy in many QA /QC laboratories is the need for a simple reliable method of verifying the wavelength accuracy of the instrument, This requirement is particularly imp ortant in near-infrared spectroscopy because of the heavy reliance on sophi sticated statistical vector analysis techniques to extract the desired info rmation from the spectra. These techniques require precise alignment of the data points between een the vectors corresponding to the standard and samp le spectra, The National Institute of Standards and Technology (NIST) offer s a Standard Reference Material (SRM 1921) for the verification and calibra tion of mid-infrared spectrometers in the transmittance mode. This standard consists of a 38 mu m-thick film of polystyrene plastic, While SRM 1921 wo rks well as a mid-infrared standard, a thicker sample is required for use a s a routine standard in the near-infrared spectral region. The general acce ptance and proven reliability of polystyrene as a standard reference materi al make it a very good candidate for a cost-effective NIR standard that cou ld be offered as an internal reference for every instrument, In this paper we discuss a number of the parameters in a Fourier transform (FT)-NIR instr ument that can affect wavelength accuracy. We also report a number of exper iments designed to determine the effects of resolution, sample position, an d optics on the wavelength accuracy of the system. In almost all cases the spectral reproducibility was better than one wavenumber of the values extra polated from the NIST reference material, This finding suggests that a thic ker sample of polystyrene plastic that has been validated with the SRM 1921 standard would make a cost-effective reference material for verifying wave length accuracy in a medium-resolution FT-NIR spectrometer.