E.S. Etz, W.S. Hurst (Div. 836), S.J. Choquette (Div. 839), and D.H. Blackburn (Div.837)

Objective: The development of new methods and practical standards that will allow the calibration of the Raman spectral intensity and thereby permit instrument-to-instrument transfer of Raman spectra.

Problem: Major advances in Raman instrumentation have virtually revolutionized the application of laser-Raman spectroscopy to materials characterization and analysis. For all analytical work, calibration of both the Raman intensity and frequency are important, especially for comparison of spectra and any attempts at quantifying the Raman data based on the measured intensities. While frequency calibration of Raman spectra is generally well in hand, calibration of the Raman intensity is not. The lack of accepted practices for the determination of the instrument response function has remained an obstacle to the wider adoption of Raman spectroscopy in industrial settings and has been a real barrier to its use in the regulated industries.

Approach: We are developing a series of luminescent glass standards suited for the calibration of the Raman spectral intensity. This approach rests on the adoption of a suite of fluorescent glasses with broad-band spectral output dependent on Raman laser excitation wavelength. The intensity of these luminescence spectra is calibrated versus the radiant output of a primary NIST radiometric (white light) source, thereby transferring the white light calibration onto the spectrum of the luminescent glass. Since the glass standards are measured under identical conditions in the Raman instrument as the analytical sample, their spectra are subject to the same instrument response function. The luminescence spectrum of the glass standard is described by a polynomial, and this mathematical relation is used in the correction of the measured intensity of the sample spectrum.

Results and Future Plans: We have identified suitable luminescent glasses to serve as standards for Raman work at wavelengths 488/514.5, 533, 633, 785, and 1064 nm. These glasses consist of a basic silicate matrix and are doped with either transition metal or rare earth ions that furnish intense broad-band emission spectra. They are stable and immune to photo-bleaching by the laser. The standard for 785 nm excitation is a chromium-bearing glass and furnishes the luminescence spectrum shown above, acquired on two different Raman instruments. This glass is to be certified as a Standard Reference Material (SRM) in FY-2001, while candidate glasses for other laser wavelengths are being evaluated through round robin tests coordinated with the ASTM E-13.08 Raman Subcommittee.

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