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THE IMPORTANCE OF PROPER INTENSITY CALIBRATION FOR RAMAN ANALYSIS OF LOW-LEVEL ANALYTES IN WATER
Citation:
Melkowits, R. B., T. L. Williams, AND T W. Collette. THE IMPORTANCE OF PROPER INTENSITY CALIBRATION FOR RAMAN ANALYSIS OF LOW-LEVEL ANALYTES IN WATER. Presented at Federation of Analytical Chemistry and Spectroscopy Societies Conference, Ft. Lauderdale, FL, October 19-23, 2003.
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
Modern dispersive Raman spectroscopy offers unique advantages for the analysis of low-concentration analytes in aqueous solution. However, we have found that proper intensity calibration is critical for obtaining these benefits. This is true not only for producing spectra with accurate intensity ratios, but also for consistently meeting the signal-to-noise requirement for this demanding application. While the former has received considerable attention in the literature, the latter has received far less. We will discuss the development and application of a technique for intensity calibration that has been particularly convenient in our laboratory.
One common method of intensity calibration relies on collecting the spectrum of a regulated light source with a traceable output of intensity versus wavelength. A spectrum collected from this source is stored and used to correct the instrument factors that distort the spectra of samples that are subsequently collected. These factors include the wavelength dependence of the quantum efficiency, and the pixel-to-pixel variation, of the CCD detector, as well as the varying transmission efficiency of the instrument. However, this approach has certain limitations. For example, the spectrum of the light source must be collected in close temporal proximity to the sample spectrum if any instrument factors are changing, such as those due to temperature fluctuations or realignment. Furthermore, the spatial orientation and geometry of the calibration source and the Raman sample are dissimilar. Our simple method shares the benefits of this convenient calibration method, but suffers considerably less from these limitations. The Raman spectrum of an analyte in water is generally produced by subtracting a water spectrum from a sample spectrum that was collected sequentially. If data collection and subtraction are conducted judiciously, the CCD pixel-to-pixel variation and other instrument factors are adequately corrected by this simple step, but the resultant analyte spectrum will exhibit biased peak intensities because the CCD quantum efficiency variation is uncorrected. However, this can be corrected by multiplying the analyte spectrum by a severely smoothed light source spectrum that has been ratioed with the source's traceable output function. Consequently, only one light source spectrum is necessary for long-term use on a single instrument.