Rare-Earth Glass Reference Materials for Near-Infrared Spectrometry: Correcting and Exploiting Temperature Dependencies Steven J. Choquette, Lindsey O’Neal, and David L. Duewer* Analytical Chemistry Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8394 Quantitative descriptions of the location of seven near- infrared absorption bands as functions of temperature 5 -50 °C are presented here for three recently introduced wavelength/ wavenumber Standard Reference Materials (SRMs): SRM 2 0 3 5 , SRM 2 0 6 5 , and SRM 2 0 3 6 . For all bands in all three SRMs, locations are well described as linear models parametrized with the location at 0 °C (intercept) and the rate of location change per °C (slope). Since these materials were produced from compositionally similar melts, the slopes for each band are identical within measurement imprecision in all three SRMs; only minor differences are observed in the intercepts. Because the direction of change in location differs among the bands, it is possible to use the measured band locations to reliably estimate sample temperature. Two approaches to estimating temperature are evaluated: slope and mea- surement uncertainty-weighted means. While both meth- ods work well with measurements made under well- characterized and stable environmental conditions, the more complex uncertainty-weighted analysis becomes relatively more predictive as the total measurement uncertainties increase. The National Institute of Standards and Technology (NIST) recently introduced two rare-earth glass optical filter Standard Reference Materials (SRMs) suitable for the verification and calibration of the x-axis (as wavenumber, cm -1 , or wavelength, nm) of near-infrared (NIR) spectrometers operating in transmit- tance mode. SRM 2035 Near Infrared Transmission Wavelength Standard from 10 300 to 5130 cm -1 was issued in early 1999. 1,2 SRM 2065 UV-Visible, Near-Infrared Transmission Wavelength/ Vacuum Wavenumber Standard was issued in early 2002. 3 A third glass-based SRM suitable for use with NIR spectrometers operat- ing in diffuse reflectance mode will be available by mid-2003 as SRM 2036. These three optical standards are produced from glass of the same nominal composition and have very similar NIR transmittance properties. Figure 1 displays a representative NIR transmittance spectrum of these materials. The x-axis locations of the same seven fairly symmetric and well-resolved NIR absorption bands in the three SRMs are certified for a sample temperature of 24 ( 1.5 °C. Since the location and intensity of NIR absorption features are sensitive to sample temperature, 4-6 the relationships between filter temperature and * Corresponding author. Tel: 301-975-3935. Fax: 301-977-0685. E-mail: david.duewer@ nist.gov. (1) SRM 2035 Certificate, National Institute of Standards and Technology. Standard Reference Material 2035 Near Infrared Transmission Wavelength Standard from 10300 cm -1 to 5130 cm -1 . Standard Reference Materials Program, NIST, Gaithersburg, MD 20899, 22 February 1999. http:/ / patapsco.nist.gov/ srmcatalog/ common/ view_cert.cfm?srm)2035. (2) Choquette, S. J.; Travis, J. C.; O’Neal, L. E.; Zhu, C.; Duewer, D. L. Spectroscopy 2001 , 16 (4), 14-19. (3) SRM 2065 Certificate, National Institute of Standards and Technology. Standard Reference Material 2065 Ultraviolet-Visible-Near-Infrared Transmis- sion Wavelength/Vacuum Standard. Standard Reference Materials Program, NIST, Gaithersburg, MD 20899, 28 March 2002. http:/ / patapsco.nist.gov/ srmcatalog/ common/ view_cert.cfm?srm)2065. (4) Wu ¨ lfert, F.; Kok, W. Th.; Smilde, A. G. Anal. Chem. 1998 , 70 (9), 1761- 1767. (5) Lin, J. Appl. Spectrosc. 1998 , 52 ( 12) , 1591-1596. (6) Wu ¨ lfert, F.; Kok, W. Th.; de Noord, O. E.; Smilde, A. G. Anal. Chem. 2000 , 72 (7), 1639-1644. Figure 1. Representative FT-NIR spectrum of zirconia-stabilized, borate matrix glass of nominal rare-earth composition: 0.106 mass fraction Ho2O3, 0.042 mass fraction Sm2O3, 0.025 mass fraction Yb2O3, and 0.015 mass fraction Nd2O3. The bands with certified x-axis location are designated with the digits 1-7. Anal. Chem. 2003, 75, 961-966 10.1021/ac025969f Not subject to U.S. Copyright. Publ. 2003 Am. Chem. Soc. Analytical Chemistry, Vol. 75, No. 4, February 15, 2003 961 Published on Web 01/10/2003