Relative Intensity Correction Standards for Fluorescence and Raman Spectroscopy

Luminescence measurements have become the methods of choice for new clinical and biochemical analyses due to their high selectivity and sensitivity. At the same time, the use of Raman measurements for chemical identification, e.g. by first responders, has also increased dramatically. These new methods are becoming increasingly quantitative and require standards to calibrate instruments and to validate the analytical methods themselves. The attendees of recent NIST workshops as well as standards, guidelines and regulatory organizations (e.g. ASTM, USP and FDA) have expressed a need for instrument calibration and performance validation standards in these two key areas of analytical spectroscopy. To work towards meeting this demand, we have developed several novel Raman and fluorescence SRMs.

Intended Impact

The Raman-based detection and identification of bio-agents and other chemical species depends on the measurement of relative intensities of Raman signal peaks with varying Raman shift. Similarly, the accuracy of fluorescence-based, quantitative clinical and biochemical assays depends on the measurement of relative intensities of fluorescence with wavelength. Both fluorescence and Raman spectroscopy yield absolute signals, i.e., they are not ratios like absorbance measurements. This means that every instrument has a unique spectral responsivity making both the spectral shape and absolute intensity of a single sample different on every instrument and even on a single instrument at different times. For all of these reasons, relative intensity standards for fluorescence and Raman instrument calibration are needed to obtain accurate and reproducible results.

Our targeted customer base is instrument manufacturers who will use these standards to calibrate their instruments both at the factory and in the field. Stakeholders also include users of these instruments. This type of instrument qualification should aid clinical, pharmaceutical, and other highly regulated industries in satisfying the quality assurance and validation requirements of national health certifiers and regulators, in addition to satisfying their own internal quality assurance standards. This type of standard is also ideal for calibrating portable instruments in the field where conventional calibration techniques are very difficult or impossible to implement.

Objective

Produce two suites of SRMs certified for relative intensity, one with a fluorescence emission wavelength range covering the near infrared (NIR), visible and near UV regions of the light spectrum, and the other covering Raman excitation wavelengths of 488 nm, 514.5 nm, 532 nm, 633 nm, 785 nm and 1064 nm with Raman shifts between 150 cm^-1 and 3500 cm^-1 

Goals

• Identify and characterize promising standards candidates.
• Certify the most promising standards candidates for relative intensity under a well-defined set of conditions using instruments that have been qualified to collect high accuracy measurements.
• Determine the uncertainties in the certified values and limitations of the standards

Research Activities and Technical Approach

Research grade commercial instruments (SPEX Fluorolog 3 for fluorescence and Renishaw for Raman) are calibrated using physical transfer standards such as a calibrated light source and atomic lamps. These instruments are used to characterize the relative signal intensity of candidate materials and to obtain certified values when appropriate. The uncertainties introduced by experimental variables, such as temperature, light polarization and wavelength are also measured using these instruments, while such variables are changed. Photostability studies are performed by irradiating each material with UV lamps or lasers for a fixed period of time and then taking fluorescence spectra of the material and comparing them to the corresponding spectra taken before irradiation. Metal-ion-doped glasses, our materials of choice due to their high photostability, are produced with a range of dopant concentrations and their fluorescence spectra measured to determine what concentration will give the most desirable intensity and spectral characteristics.

• Qualification of instruments for measuring true spectra with high accuracy
• Certification of Raman SRMs 2241-2243 (excitation at 488nm, 514.5 nm, 532 nm and 785 nm)
• Certification of fluorescence SRMs 2940-2943 (UV, blue, green and orange emission)

Associated Publications:

DeRose, P.C.; Smith, M.V.; Mielenz, K.D.; Blackburn, D.H. and Kramer, G.W., Characterization of Standard Reference Materials 2940, Mn-Ion-Doped Glass, Spectral Correction Standard for Fluorescence. J. Luminescence 2009, 129, 349-355.

DeRose, P.C.; Smith, M.V.; Mielenz, K.D.; Blackburn, D.H. and Kramer, G.W., Characterization of Standard Reference Materials 2941, Uranyl-Ion-Doped Glass, Spectral Correction Standard for Fluorescence. J. Luminescence 2008, 128, 257-266

DeRose, P.C.; Early, E.A.; Kramer, G.W., Qualification of a Fluorescence Spectrometer for Measuring True Fluorescence Spectra. Rev. Sci. Instru. 2007, 78, 033107.

DeRose, P.C.; Early, E.A.; Kramer, G.W., Measuring and Certifying True Fluorescence Spectra with a Qualified Fluorescence Spectrometer. In: Proceedings of the 5th Oxford Conference on Spectrometry (Crown, UK, 2008).