Products Solutions Services

IntroductionExploiting Raman spectroscopy as an analytical tool requires a well-developed protocol for calibrating both the wavelength and the intensity axes. The ability to quantitatively compare spectra generated on different continents becomes critical for global manufacturers. Cross-instrument comparisons require that each analyzer be intrinsically robust and properly calibrated. Kaiser Optical Systems, Inc. (Kaiser) recommends a calibration protocol that provides true cross-instrument calibration. When coupled with the intrinsic stability of an axially transmissive spectrograph, Raman becomes a powerful tool for analytical measurements.

Historically, multi-wavelength, flat-field calibration of a Raman system was a painstaking process. In many cases, researchers used laser plasma lines near the region of interest to maintain their wavelength calibration as the instrument scanned. Intensity calibrating a scanning instrument required complicated optical alignment, integrating spheres, and expensive NIST sources and was rarely attempted in practice.

Figure 1. The Kaiser Raman Calibration Accessory.

Kaiser Raman Calibration Accessory The Kaiser Raman Calibration Accessory makes wavelength and intensity calibrations fast, simple, and accurate. A tungsten-halogen source within the accessory provides a NIST-traceable calibration of the relative intensity axis. An integrated atomic line source allows wavelength calibration to well-established spectral standards. Kaiser Raman’s embedded RunTime™ technology automatically applies the flat-field correction and calibrates the instrument with 18 atomic lines (532 nm systems) across the whole spectral region. An ASTM-approved Raman shift standard allows accurate calibration of the laser operating wavelength without removal of laser notch filters, as well as guaranteeing overall system performance and calibration. There are several accessories which interface to Kaiser Raman analyzers and probes.

Calibration Procedure Three measurements can be calibrated using the Kaiser Raman Calibration Accessory: spectrometer wavelength, intensity, and laser wavelength.

In the spectrograph wavelength calibration, the CCD array detector in the spectrometer is illuminated with neon light. Because the wavelengths of neon’s emission lines are stable and well established, they can be used to determine the pixel-to-wavelength correlation of the CCD. This calibration is stored as a polynomial equation and used when generating Raman spectra.

The intensity calibration is necessitated by variations in the sensitivity of individual pixels in the CCD detector. A broadband tungsten-halogen lamp with a NIST-traceable spectral output is used to

Key Issues • Calibration of spectrometer

wavelength, intensity and laser wavelength

• Calibration transfer between instruments

• Robust, quantitative univariate and multivariate modeling of reaction systems

Kaiser Raman Calibration Accessory Protocol

illuminate the CCD detector. Because the photon flux of the tungsten-halogen lamp at each wavenumber is known, the calibration determines the response of each pixel of the CCD for the known intensity impinged upon it.

The laser wavelength must be determined empirically because two lasers that have nominally the same wavelength may differ slightly, and the wavelength of a single laser can vary significantly with temperature. The laser wavelength is determined indirectly by acquiring the Raman spectrum of cyclohexane, which has sharp, well-defined peaks across the entire spectral range, and measuring the Raman shift of a stable peak with a well-established Raman shift value. For example, if the measured Raman shift of the 801.1 cm–1 peak is 801.3 cm–1, then the true laser wavelength is 0.2 cm–1 greater than the nominal wavelength. Results Figure 2 shows cyclohexane spectra obtained on three different instruments normalized to the 800 cm–1 vibration. Cyclohexane serves as a useful sample for evaluating the calibration protocol because it has narrow bands across the entire spectral range. Any difference between the instruments’ resolutions, wavelength accuracy, and intensity calibration will be readily apparent in the cyclohexane spectrum, but these spectra are virtually indistinguishable.

When the laser wavelength is changed from 532 nm to 785 nm, the carbon-hydrogen stretch vibration scatters at a wavelength of approximately 1000 nm. At this wavelength the efficiency of silicon CCD detectors drops dramatically, as shown in Figure 3a. However, if the Raman instruments are properly calibrated the intensity ratios of the Raman bands are restored to their true values (Figure 3b).

Conclusion Cross-instrument calibration will expand the applicability of Raman spectroscopy to a new class of process monitoring applications. Calibration transfer will decrease model development costs for multiple production sites, and decrease model maintenance costs at single sites. As modeling approaches move to more complex, multivariate data analysis approaches, calibration transfer becomes essential as well as augmenting their accuracy.

Reference: Tedesco, J.M.; Davis, K.L., “Calibration of dispersive Raman process analyzers.” Proceedings of SPIE, Vol. 3537, 1998, 200.

Copyright © 2019 Kaiser Optical Systems, Inc. All rights reserved.

SD/K

-013

00/C

/61/

EN/0

1.20

Kaiser Optical Systems, Inc.

371 Parkland PlazaAnn Arbor, MI 48103USA

Tel 734 665 8083Fax 734 665 8199www.kosi.com

Figure 2. Overlaid cyclohexane spectra from three different analyzers. Spectra have been normalized to the 800 cm–1 vibration. The inset shows the entire spectrum; the body of the plot expands the region from 600 to 1800 cm–1.

Figure 3. Comparison of results from 532 nm and 785 nm analyzers. (a) Cyclohexane spectra from un-calibrated analyzers; (b) Overlaid cyclohexane spectra from calibrated 532 nm and 785 nm analyzers.