Calibration Raman shift standards

American Society for Testing and Materials, Standard Guide for Raman Shift Standards for Spectrometer Calibration, Standard E-1840-96, 1998. 

Frequency Calibration with Raman Shift Standards... 

With a set of standard Raman shifts in hand, an instrument s Raman shift axis may be calibrated from a spectrum of one or more standards from Table 10.4. The same mathematical procedure used for atomic emission standards is employed, usually involving a quadratic or cubic fit of the observed to the actual frequencies from Table 10.4. As noted earlier, Raman shift standards provide a more direct calibration than an atomic emission lamp, with no need to accurately determine the laser frequency. The disadvantage of calibration... 

Calibrate Raman shift axis with neon source or Raman shift standard. Record spectrum of 4-acetamidophenol (or alternative ASTM standard from Table (10.4) to provide a record of Raman shift calibration. 

In the laboratory, a Raman-shift standard may also be used, after intensity calibration with the tungsten-halogen source, to perform a final calibration validation over the entire... 

Laser wavelength calibration (Raman standard). A Raman-shift standard such as cyclohexane (or another Raman standard) is used to calibrate the laser wavelength without bypassing laser notch filters in the probe head or base unit of the analyzer. In the case of cyclohexane, the laser wavelength can be calculated from the measured wavelength position of this Raman line and the knowledge that the shift of this line is 801.1 cm . ... 

In some on-line cases, the Raman-shift standard may be left in place to validate the total calibration of the system using the other Raman lines of cyclohexane. Alternatively, if the sample position is not physically accessible, then another standard (e.g., a process solvent if this material has known, defined bands which are stable) can be used instead. By comparing the Raman shifts and amplitudes of the measured Raman peaks with the stored shifts and amplitudes, it can be quickly established if the unit is within accepted calibration values. 

Other aspects of Raman instrumentation are reviewed elsewhere in this volume. For the purposes of this discussion, we assume a compact spectrograph and array detector system providing a spectral resolution of 20 cm" or better and simultaneous acquisition of a spectral window 1000 cm or more in width. We wish to direct our attention to elements employed in calibrating the system a white-light source for ordinate calibration and an atomic line source and Raman shift standard for abscissa calibration. 

Feedback correction must be applied to every sample measurement without operator intervention and without degrading the operating speed of the system. Of the calibration observations that are needed, all except the white-light spectrum need to be recorded simultaneously or nearly simultaneously with every sample measurement. To deal with laser position shift, a Raman-shift standard spectrum can be obtained through the sample channel—more on this later. 

Direct calibration of the Raman-shift scale has much to commend it, but this approach also has limitations that cause it to fail in achieving the accuracy required for the quantitative applications. A number of materials have been proposed for use as Raman-shift standards. A recent ASTM study considered eight naphthalene, l,4-bis(2-methyl-styryl)benzene, sulfur, 50/50 toluene-acetonitrile, 4-acetamidophenol, benzonitrile, cyclohexane, and polystyrene [2]. The limitations are several. The shift values are not known with sufficient accuracy. (Of the 25 lines reported for acetamidophenol, only 9 are reported with a standard deviation of less than 0.5 cm For low resolving power, wide spectral coverage systems there are too few resolved lines to provide accurate calibration. Finally, there is the question as to how one incorporates such material for routine calibration of a real-time measurement system. 

Guide for Raman shift standards for spectrometer calibration. In Annual Book of ASTM Standards. West Conshohocken, PA ASTM, 1996, El 840, vol. 03.06. 

Returning to the dispersive case, it is far more reliable to use many calibration lines than to use only one. Ideally, a large number of accurately known frequencies would be dispersed across the spectrum, then observed under the same conditions as the sample. Assuming the optical and data acquisition conditions are precisely reproduced for the standard and the sample, the sample frequencies may be accurately calculated from the standard spectrum. As noted earlier, Raman shifts may then be determined from the equally accurately known laser frequency. 

The example is illustrated by the results of Table 10.5. The Raman shift range from 400 to 2000 cm was calibrated with the 4-acetamidophenol shift standard, and the calibrated spectrum was recorded and stored on disk. Then calcium ascorbate was observed, with and without recalibration between spectra. Finally, spectra of calcium ascorbate were obtained approximately daily (each after recalibration) over a period of 2 months. The 769- and 1582 cm peaks were chosen for analysis, and their peak frequencies were determined by a center-of-gravity criterion included in the data analysis software (GRAMS 32). It is important that these qualification spectra duplicate the instrumental conditions to he used for real samples, at least as far as optical geometry, sampling mode, and calibration procedure. The objective is to provide an accurate indication of instrument performance in the intended application. 

The qualification of a Raman spectrometer is described in USP chapter < 1120>. In particular, the tests for the operational and performance qualification of a Raman spectrometer are described x-axis precision, photometric precision, laser power precision and accuracy. The x-axis of the Raman spectrometer is the Raman shift measured in wavenumbers. Before the Raman shift can be determined, both the laser wavelength and spectrophotometer calibration must be determined. The precision of the Raman shift can then be measured using an American Society for Testing and Materials (ASTM) Raman standard material [20]. A commonly used Raman standard material is acetaminophen. The peak position of the known reference peaks can be determined visually, but is better done with a peak location algorithm. The USP chapter on Raman specifies that the peak location should not vary more than... 

It should be noted at this juncture that it is possible to calibrate the Raman-shift axis directly by exciting a known Raman standard material, such as cyclohexane, and observing... 

Calibrations based on techniques other than fluorescence are also available. Lattice parameter calibrations from pressure dependent X-ray experiments have been reported for several systems. The most commonly used X-ray standards include MCI (M=Na, K, Cs) [89], Au [117,118], W [119], and Cu [120]. Spectral shifts of Raman lines in N2 [121] and diamond [122] have also been calibrated as a function of pressure. 

---