Calibration wavenumber

This function re-calibrates the x-axis of a spectrum including any peak pick lists. The new axis and new wavenumbers are calculated by using the two parameters Mand A. The formula is  

Open the wavenumber (frequency) calibration dialog box (see Fig. 10.31), select the spectrum as usual, enter the multiplication factor Mand the offset A, and then click on the Calibrate button to start the function. 

After the calibration the parameters M and A can be found in the spectrum data block. This gives you the possibility to revoke the calibration. 

Examples of the calibration are depicted in Fig. 10.32, where the original spectrum is the Raman spectrum in the spectral range 700-1800 cm . The original data set before the calibration is listed in Fig. 10.33. After the calibration the wavenumbers have changed, as evidenced by the wavenumbers of the first and last data points, see Fig. 10.34. Furthermore, two additional parameters Mult, for Freq. Calib. and Add for Freq. Calib. have been included in the list. 

The x-axis of a spectrum can be modified several times. The parameters M,otai and totab representing the entire changes in comparison to the original spectrum, then appear in the data parameter list. These changes can be reversed in just one step. 

The main thrust of the calibration of radiometric instruments concerns the establishment of an absolute intensity scale. However, generation of a trustworthy wavenumber or wavelength scale must not be overlooked. Clearly, in the identification of an unknown spectral feature an accurate knowledge of the wavenumber is crucial. Without a good wavenumber scale the identifications of the nitrile, hydrocarbon, and carbon dioxide features discovered in the Titan spectrum would have been difficult (see Section 6.4). 

Broad-band radiometers, where the spectral range is determined by intrinsic material properties, have generally not been calibrated in orbit with respect to wavenumber. In most cases the cut-off wavelengths of optical materials are predictable functions of temperature, but they are assumed to be relatively stable, otherwise. A major shift in the wavenumber range of such a radiometer is not likely, and the scientific objectives of broad-band measurements rarely depend on the cut-off wavenumbers in a critical manner. 

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Maki, A.G. and Wells, J.S., Wavenumber Calibration Tables from Heterodyne Frequency Measurements, NIST Special Publication 827, U.S. Department of Commerce, 1991. 

If the initial absolute numbering of the Fortrat parabola is incorrect (the relative numbering is assumed to be correct, but could easily be corrupted by inadequate precision in the wavenumber calibration of the measured spectrum), as it almost certainly will be, the set of a priori known A2F values will not match the set of trial combination differences computed from the Fortrat equation. It is necessary to examine a series of trial absolute numberings of the Fortrat equation, shifting m in integer steps, until the known combination differences match those derived from the Fortrat equation, either... 

Foremost is record a spectrum To accomplish this one needs a light source, a means of monochromatizing the light either before or after it interacts with the target molecules, a scheme for wavenumber calibration which is of absolute accuracy superior to the resolution of the experiment, a molecule source, and a signal detector. Often, a single device fulfills several of these requirements, such... 

Extending previous accurate laser studies of the IR spectra of metal hydrides in gas phase, including NaH, CsH, BaH, SrH, RbH, GaH, InH, TIH CdH, ZnH, KH, MgH, CaH and LiH and also of diatomic hydride radicals, for instance SnH, NiH, FeH, CoH and GeH, the infrared spectra of two isotopic forms of silver hydride [ Ag (51.35%) and Ag (48.65%)] in its ground electronic state ( S) have been recorded . Accurately known absorption hnes of nitrogen dioxide sulphur dioxide and formaldehyde served for wavenumber calibration 21 transitions in the bands from v=l -0uptov = 3 2 have been measured for AgH and AgH respectively, their assignments and equation wavenumbers were presented explicitly and fitted to the Dunham expression (equation 38) for the energy levels of diatomic molecules, and the parameters Yij for two isotopic forms of AgH were determined. ... 

Huorolube , Polytrifluorochloroethylene, [-C2CIF3-] Polystyrene Wavenumber Calibration Wavelength-Wavenumber Conversion Table Diagnostic Spectra... 

The most stringent need for wavenumber axis calibration is in determinations based on band position. For this reason, qualitative analyses are likely to be affected by drifts or inaccuracy in the wavenumber axis [14]. Likewise, quantitative determinations based on band position, such as strain in diamond films [6], will be affected similarly. Other quantitative analyses may also be affected by band-position error. It is common to use the raw spectral intensities (intensity at every wavenumber) in a multivariate analysis. Although this approach can be very powerful, any unexpected shift in wavenumber calibration can cause severe error in the model. In essence, the spectral pattern to which the model has been trained has been shifted. The mathematics of the model are expecting a particular relationship of intensity between adjacent variables (wavenumbers) and cannot usually account for shifts [31], To some extent, multivariate models can be desensitized to inaccuracy and imprecision by assuring that the calibration samples also exhibit some of the same shifting features, but model sensitivity may suffer as a result. Although not in common use, other deconvolution methods have been introduced which may be applicable to removing shift effects of inaccurate wavenumber calibrations [37]. 

Search systems are powerful tools, but as for most algorithms described in this chapter, care must be taken when they are used. The unknown and the hbrary spectra must both obey Beer s law. Care must be taken that the wavenumber calibration of an instrument is correct as wavenumber shifts may occur between the unknown and the hbrary spectra. The algorithms compensate for spectral shifts to some degree, but they cannot compensate for large shifts. In addition, the spec-troscopist must remember two important caveats (1) a hbrary search cannot identify an unknown unless the unknown is in the hbrary and (2) a high value of the hit quality index is not necessarily an indication of a correct match. The judicious spec-troscopist will visually compare the spectra of the unknown to the best matches to determine if they are indeed identical. In fact, the best test is to perform a spectral subtraction between the unknown and the hbrary match to see if there is a nonzero residual. If the two spectra are not identical, a nonzero subtraction will result. Finally, it should be recognized that spectral similarity does not indicate structural similarity. Certainly, some of the functional groups in structurally similar molecules will be the same, but the overall structure of an unknown and the best matches that result from a search may be quite different. 

In principle, the calibration of FT-IR spectrometers is performed with regard to intensity and wavenumber. However, the intensity calibration is not usually necessary because the absorbance scale is used, -ln(J/7o), where the ratio I/Iq automatically accomplishes the intensity calibration. The most important task in calibration is the wavenumber calibration. Usually the calibration takes place by simultaneous measurement of the known IR spectrum. For example, the user simply adds reference gas (known spectrum) into the sample gas under measurement. Using the known... 

Careful measurements have been made in the same conditions in the same laboratory. In particular, care has been taken to (i) check the wavenumber calibration, (ii) regulate the temperature at 298 K, (iii) purify and desiccate the solvent, (iv) use a long-pathlength cell (1 cm) in order to work with very dilute solutions of methanol and base and (v) deconvolute overlapping free and bonded OH bands, in the case of weak bases, and overlapping multiple bonded OH bands, in the case of polyfunctional bases. 

A third advantage of the Michelson interferometer, the ability to have a very precise wavenumber calibration by comparing wavenumbers of the planetary spectra... 

A physical measurement is of little value unless accompanied by a reasonable estimate of its uncertainty. For remote sensing investigations intensity calibration is of prime concern, but wavenumber calibration also is very important. In all measurements one must distinguish between precision and accuracy. Precision refers... 

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