Fine-structured background

A fundamental requirement for the correction of fine-structured background via reference spectra is that the spectrometer is equipped with an accurate mechanism for wavelength stabilization. This feature is included in all research spectrometers and described in detail in Section 3.2. At the beginning of each analytical cycle the actual wavelength position is checked and adjusted, if necessary, to ensure an exact pixel-to-wavelength correlation. 

In addition, the reference absorbance spectra ) of the number i of molecules causing fine-structured background at the analyte wavelength position must be known. These reference spectra are then used as independent linear functions in a least-squares fitting algorithm and fitted to each absorbance spectrum. The individual absorbance spectra corrected for fine-structured background are calculated as follows  

The molecule correction factors Uscan,. which are a measure of the strength of the different reference absorbance spectra for the corresponding scan, are obtained by omission of the pixel at the analyte wavelength position and its neighbors, since the actual analyte absorption may not influence the background correction procedure. 

Preferably, the molecule reference spectra, at least those of the most notorious troublemakers, such as PO and NO (refer to Chapter 7), should be pre-recorded with a good SNR and stored in a library. Fortunately, identification of the molecule responsible for the specific fine-structured background is not in all cases necessary, since a reference spectrum can be generated from the measurement itself, when the molecular structure can be separated from the analyte signal in time. By limiting the number of scans to the time interval of the presence of the unknown molecular structure, a reference spectmm can be obtained when calculating the time-integrated absorbance from the selected scans. 

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Figure 5.8 Molecule reference spectrum used for fine-structured background correction of the model measurement in Figure 5.6 analyte wavelength position dashed line...

Figure 5.9 Temporal behavior of the molecule correction factor as a measure of the fine-structured background absorption caused by the molecular species of Figure 5.8 present during atomization...

The occurrence of fine structures has already been noted in the sections on spectral information and ionization losses (Sects. 2.5.3 and 2.5.3.2). In the following text some principal considerations are made about the physical background and possible applications of both types of feature, i. e. near-edge and extended energy-loss fine structures (ELNES/EXELFS). A wealth of more detailed information on their usage is available, especially in textbooks [2.171, 2.173] and monographs [2.210-2.212]. 

Another noteworthy example is x-ray absorption fine structure (EXAFS). EXAFS data contain information on such parameters as coordination number, bond distances, and mean-square displacements for atoms that comprise the first few coordination spheres surrounding an absorbing element of interest. This information is extracted from the EXAFS oscillations, previously isolated from the background and atomic portion of the absorption, using nonlinear least-square fit procedures. It is important in such analyses to compare metrical parameters obtained from experiments on model or reference compounds to those for samples of unknown structure, in order to avoid ambiguity in the interpretation of results and to establish error limits. 

Figure 12. NeNePo spectra of the silver trimer taken with wavelengths of a) X = 390 nm, (b) X = 400 nm, (c) X = 415 nm, and (d) X = 420 nm. Each curve has its own axis of zero signal. The time-independent background increases steadily with decreasing wavelength. The fine structure around Ar = 0 is due to the interference of pump and probe pulses [9).

The presence of hydrous ferric oxide (FeOOH) has been confirmed by X-ray diffraction, Mossbauer spectroscopy, and magnetization measurements (7). Figure 11 shows the X-ray diffraction patterns for the region of 20 between 14 and 18°. The two peaks at 15.2 and 16.3° are diffractions from a-Fe203, but the large background is from the various forms of hydrous ferric oxide. The lack of fine structure excludes the possibility of identifying... 

In this protocol, commercially purchased carotenoid standards are dissolved in a suitable solvent and the absorbance measured at its maximum wavelength (A.max). Using published extinction coefficients and taking into consideration the dilution factor, the concentration of the standard carotenoid is calculated. The spectrum is also scanned in order to evaluate the fine structure (see Spectral Fine Structure in Background Information). The carotenoid solution should ideally be assayed by HPLC as described in unit F2.3 to establish chromatographic purity and thus correct the calculated concentration. 

Scan to allow measurement of the fine structure (see Table F2.2.3 also see Spectral Fine Structure in Background Information). 

Observe the UV spectrum (Table 11.6.5 also see Background Information, Spectral Fine Structure). 

Figure 1. Electron energy-loss spectrum of GdBa C O illustrating the various observable types of fine structures a) the low-loss segment including the outer shell ionization edges b) the innershell core-loss edges the background model AE r and the parameters used in microanalysis (IA, ea. A) are also shown.

Atoms are not rigidly bound to the lattice, but rather vibrate around their equilibrium positions. If we were able to examine the crystal over a very brief observation time, we would see a slightly disordered lattice. Incident electrons see these deviations, and this is for example the reason that in low-energy electron diffraction (LEED) the spot intensities of diffracted beams depend on temperature. At high temperatures the atoms deviate more from their equilibrium position than at low temperatures, and a considerable number of atoms is not at the equilibrium position necessary for diffraction. Thus, spot intensities are low and the diffuse background high. Similar considerations apply in other scattering techniques, as well as in extended X-ray absorption fine structure (EXAFS) and in Mossbauer spectroscopy. 

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