Accuracy wavelength/wavenumber

Since the thermal emission curve from a black-botdy radiator may be calculated from first principles, it can serve as a primary standard for an intensity vs. wavelength curve. Petty et al. (16) described a procedure for intensity calibration of an FT-Raman spectrometer using such a source. Provided the source acts as a black body to the required accuracy, its temperature is known, and the laser wavelength is known accurately, the black-body provides a known i(AT>) curve to use in Eq. (10.3). Since most modern spectrometers measure photons/second rather than watts, the proper units for b fAv) are photons per second per wavenumber. ... 

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... 

The distribution of the spectral lines of each individual element is not random. It was discovered first empirically and also later shown theoretically that the wavelengths of the lines of the simple atomic spectra can be fitted to simple series formulae with great accuracy. Furthermore, many of the lines in the simple spectra occur in small groups which are called multiplets, such as doublets of the alkali metals or triplets of the alkaline earths. There is also a constant difference between the wavenumbers of the two components of some doublets or two of the three components of some triplets. For example, the two lines of each doublet are separated by 17 cm" in the atomic spectrum of sodium (Table 3). This has been shown by Ritz to be a direct consequence of a general rule named the combination principle. According to this principle, for each atom or molecule there is a set of spectral terms... 

In the sequence of Na-like ions we note an accurate and comprehensive study of Sr " by Reader. Also, new measurements of the principal transitions 3s-3p, 3p-3d, and 3d-4/ have been made by Reader et for a number of ions from Cu " to Sn ". By fitting the differences between observed and calculated wavenumbers to simple expressions in Z they obtained least-squares fitted wavelengths for all Na-like ions from Ar to Xe with an accuracy that should make them useful as reference lines. 

Because NIR is typically a low-resolution technique, the importance of accurate wavelengths or wavenumbers was not emphasized. The proposed USP method for wavelength accuracy defines three peaks with tolerances of 1 nm at 2000, 1600, 1200 nm. This translates to allowable errors of 5.0, 7.8, and 13.9 cm A typical FT-NIR spectrophotometer has a precision of better than 0.01 cm . In Figure 5.13, a three-week monitoring of the water-vapor peak at 7299 shows a standard deviation of 0.0027 cm (0.0005 nm). This precision has required careful evaluations of the standards that are used for NIR. 

However, even in FT-NIR spectroscopy the actual wavelength accuracy depends to a small degree on the optical design of the instrument. The measured wavenumber i/ = v l — (1 /2)a where a is... 

Rotation of the prism or grating was controlled by a cam to allow the spectral trace to be recorded in either constant wavelength or wavenumber. This cam was coupled to the slits to vary their width in order to keep the radiation reaching the detector roughly constant. Resolution therefore varied over the spectrum. Positional accuracy and repeatability were poor owing to backlash and wear in the mechanical linkages most specifications quoted values around 0.5 cm k... 

Spectral measurements can be done either ex situ, changing the doping level or redox state appropriately for every measurement, or in situ, the preferred method for greater accuracy. All such spectroelectrochemical data can be presented with Absorbance (Optical Density) or %-Transmission on the ordinate, and wavelength (nm, fim) or energy (eV, X (in pm) = ca. 1.24/eV), or sometimes, wavenumbers (cm ) on the abscissa. Occasionally, difference spectra, usually referenced to the pristine state of the CP, are also presented. 

The mirror position in the stepping mode, and the sampling point in the constant velocity mode, must be accurate to a small fraction of a wavelength. To achieve this accuracy it is customary to employ a second interferometer with a monochromatic source of a wavenumber considerably higher than that of the radiation to be measured. The reference interferometer shares part of the main infrared interferometer... 

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