Li-Lin Tay, Metrology Research Centre, National Research Council Canada
Raman analysis faces several obstacles in meeting objective regulatory requirements — such as the lack of traceable quantification methods and absence of certified reference materials — which prevents its widespread use. Raman spectroscopy does not have an underlying metrological infrastructure, such as standardization allowing measurements to be linked to a quantity in the International System of Units (SI).
A recent survey, conducted by the Versailles Project on Advanced Materials and Standards (VAMAS) Technical Working Area 42 on Raman Spectroscopy and Microscopy, has shown that existing users do not make use of the few Raman standards regularly. The limited availability of calibrated certified reference materials hinders the support of all levels of users and instrument manufacturers. Raman shifts and intensities are two of the most important quantities. The two relevant documentary standards — ASTM E1840-96(2022) and E2911-23 — that are currently used by most manufacturers were developed by the American Society for Testing and Materials (ASTM).
The ASTM E2911-23 Standard Guide for Relative Intensity Correction of Raman Spectrometers has seen broad adoption and is supported by the use of SI-traceable certified reference materials. Because different Raman spectrometers have different instrument response characteristics, such as the spectral responsivity of different detectors and spectrometer-dependent spectral response, spectra obtained with different instruments can show significant variations in the measured relative peak intensities of the same compound. The conventional method of calibrating the spectral response of a Raman spectrometer is to use a traceable calibrated spectral irradiance source. This is challenging for routine analytical work, particularly for the users of handheld analyzers. Instead, E2911-23 uses a series of traceable transfer standards, which are luminescent glasses that are calibrated and certified at NIST for the relative intensity correction of Raman spectrometers for seven of the most common excitation wavelengths.
There is a need to develop standards for spatial resolution, confocal volume, and depth resolution. For quantitative Raman measurements, the optical probe volume or confocal volume, a dimensional quantity, is often needed to determine the total number of molecules contributing to the Raman scattered photons. The metrology of Raman spectroscopy will involve many different measurands and therefore be traceable to different SI units. For example, measurands for the quantitative Raman analysis will need to be traceable to the mole, while spatial resolution, confocal volume, and depth resolution measurands need to be traceable to the meter. Documentary and physical Raman standards will grow. But expert users need to join forces to close the gap in Raman metrology to enable standardization — especially as hand-held instruments are used by those who are not experts but rely on the their data.
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Meet the author
Li-Lin Tay is a principal research officer at the Metrology Research Centre, National Research Council Canada. She holds a bachelor’s degree in physics and a doctorate in physical chemistry, both from the University of Toronto; email: [email protected].
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