Rare-earth glass reference materials for near-infrared spectrometry: sources of x-axis location variability Original Research Article

The National Institute of Standards and Technology (NIST) recently introduced two optical filter standards for wavelength/wavenumber calibration of near-infrared (NIR) spectrometers. Standard Reference Material®s (SRM®s) 2035 and 2065 were fabricated in lots of ≈100 units each from separate melts of nominally identical rare-earth glass. Since individual filter certification is extremely time-consuming and thus costly, economic production of these SRMs required the ability to batch certify band locations. Given the specification that the combined uncertainty for the location of the bands in a given filter should be ≤0.2 cm−1, rigorous evaluation of material heterogeneity was required to demonstrate the adequacy of batch certification for these materials. Among-filter variation in measured band locations convolves any influence of material heterogeneity with that of environmental, procedural, and instrumental artifacts. While univariate analysis of variance established band-specific heterogeneity upper bounds, it did not provide quantitative descriptions of the other possible sources for the observed measurement variability. Principal components analysis enabled both the identification and isolation of the most important NIR band location variances among the SRM 2065 filters. After correction for these variance sources, the upper bound on the material heterogeneity was determined to be 0.03 cm−1 for all bands. Since this is a small part of the measurement uncertainty, we conclude that batch analysis provides an acceptable certification approach for these and similarly fabricated rare-earth glass reference materials.