Electron spectra, quantitative analysis

Many complexes of metals with organic ligands absorb in the visible part of the spectrum and are important in quantitative analysis. The colours arise from (i) d- d transitions within the metal ion (these usually produce absorptions of low intensity) and (ii) n->n and n n transitions within the ligand. Another type of transition referred to as charge-transfer may also be operative in which an electron is transferred between an orbital in the ligand and an unfilled orbital of the metal or vice versa. These give rise to more intense absorption bands which are of analytical importance. 

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

Figure 11. Electron-energy-loss spectrum of crystalline boron nitride, showing the boron K-edge (at 190 eV) and the nitrogen K-edge (at 400 eV). The background intensity, delineated by the dashed curve arises from inelastic scattering by valence electrons. The hatched areas represent the measured values required for the quantitative analysis of boron ( see text) (50).

Figure 5. Spectrum extracted from a series of electron spectroscopic (ESI) images around a core loss edge. For a quantitative analysis a slit width is dE = 10...20 eV.

For qualitative analysis, two detectors that can identify compounds are the mass spectrometer (Section 22-4) and the Fourier transform infrared spectrometer (Section 20-5). A peak can be identified by comparing its spectrum with a library of spectra recorded in a computer. For mass spectral identification, sometimes two prominent peaks are selected in the electron ionization spectrum. The quantitation ion is used for quantitative analysis. The confinnation ion is used for qualitative identification. For example, the confirmation ion might be expected to be 65% as abundant as the quantitation ion. If the observed abundance is not close to 65%, then we suspect that the compound is misidentified. 

The change in the optical absorption of et7 with time (at 77 K) is shown in Fig. 5. It can be seen that electrons stabilized in shallower traps decay more rapidly due to which, in the course of the reaction, the absorption spectra shift steadily to the short-wavelength region, and the rate of the change of the optical density depends on the wavelength. This somewhat hinders the quantitative analysis of the kinetic data obtained for reaction (4) by the optical method. At the same time, the width and the shape of the EPR lines of et7 remain unchanged as kinetic measurements are made. This makes the analysis of the kinetic data much simpler since, in this case, the amplitude of the et7 EPR spectrum can be taken directly as a value characterizing the concentration of etr. For this reason most of the kinetic measurements for reaction (4) have been made by the EPR method. 

Another early application of the CASSCF/CASPT2 method was to the electronic spectrum of the benzene molecule [64], The spectrum of this molecule was very well described by the semi-empirical methods of the 50 s and 60 s. Actually a first semi-quantitative analysis was performed as early as 1938 by Goeppert-Mayer and Sklar [65], It turned out to be very difficult to reproduce these results with the ab initio methods that were developed in the 60s and 70s. The CASPT2 calculations... 

From equation 5, it is apparent that each shell of scatterers will contribute a different frequency of oscillation to the overall EXAFS spectrum. A common method used to visualize these contributions is to calculate the Fourier transform (FT) of the EXAFS spectrum. The FT is a pseudoradial-distribution function of electron density around the absorber. Because of the phase shift [< ( )], all of the peaks in the FT are shifted, typically by ca. —0.4 A, from their true distances. The back-scattering amplitude, Debye-Waller factor, and mean free-path terms make it impossible to correlate the FT amplitude directly with coordination number. Finally, the limited k range of the data gives rise to so-called truncation ripples, which are spurious peaks appearing on the wings of the true peaks. For these reasons, FTs are never used for quantitative analysis of EXAFS spectra. They are useful, however, for visualizing the major components of an EXAFS spectrum. 

The benchmark spectra used to analyze composite /-irradiated DNA spectra are shown in Figs. 1(B) to 1(E). By determining the low-dose yield [G value (pmol/J)] of each radical using dose-response curves, the low-dose composition of the free radical cohort is determined to be G (35 5%), T- (25 5%), C- [C(N3)H ] (25 5%), XN. (15 5%). Scheme 1 presents the structures of the first three radicals. XN. represents a composite spectrum of neutral radicals, which is assumed to originate mostly with the sugar-phosphate backbone (Cl, C3, C5, C3 gpj J. A semi-quantitative analysis indicates that, of the 15% of assumed backbone radicals, about 11% originate with electron loss and about 4% with low-energy electrons (Sec. 3.3). Thus, about 46% of the stabilized radicals at 77 K are electron-loss... 

The absorption of electromagnetic radiation of wavelengths between 200 and 800 nm by molecules which have n electrons or atoms possessing unshared electron pairs can be employed for both qualitative and quantitative analysis as such, it is known as spectrophotometry. As a wide variety of pharmaceutical substances absorb radiation in the near-ultraviolet (200-380 nm) and visible (380-800 nm) regions of the electromagnetic spectrum, the technique is widely employed in pharmaceutical analysis. 

There are many reasons that one might want to record, assign, and interpret the electronic spectrum of a diatomic molecule. These include qualitative (which molecular species are present) and quantitative (what is the number density of a known quantum state of a known molecule) analysis, detection of trace constituents (wanted, as in analysis of ore samples for a precious metal, or unwanted, as in process diagnostics where specific impurities are known to corrupt an industrial process), detection of atmospheric pollutants, monitoring of transient species to optimize a combustion process by enhancing efficiency or minimizing unwanted byproducts, laboratory determinations of transition frequencies and linestrengths of interstellar molecules, and last but certainly not least, fundamental studies of molecular structure and dynamics. 

Excitation of the outer ns electron of the M atom occurs easily and emission spectra are readily observed. We have aheady described the use of the sodium D-line in the emission spectrum of atomic Na for specific rotation measurements (see Section 3.8). When the salt of an alkali metal is treated with concentrated HCl (giving a volatile metal chloride) and is heated strongly in the non-luminous Bunsen flame, a characteristic flame colour is observed (Li, crimson Na, yellow K, lilac Rb, red-violet Cs, blue) and this flame test is used in qualitative analysis to identify the M ion. In quantitative analysis, use is made of the characteristic atomic spectrum in flame photometry or atomic absorption spectroscopy. 

The spectrum obtained either by CW scan or pulse FT at constant magnetic field is shown as a series of peaks whose areas are proportional to the number of protons they represent. Peak areas are measured by an electronic integrator that traces a series of steps with heights proportional to the peak areas (see Fig. 4.22). A proton count from the integration is useful to determine or confirm molecular formulas, detect hidden peaks, determine sample purity, and do quantitative analysis. Peak positions (chemical shifts, Section 4.7) are measured in frequency units from a reference peak. 

The instrumentation for EM uses the same type of X-ray spectrometers discussed in detail in Chapter 8, with an electron beam as the source and a UHV system that includes the sample compartment. An ED X-ray spectrometer allows the simultaneous collection and display of the X-ray spectrum of all elements from boron to uranium. The ED spectrometer is used for rapid qualitative survey scans of sample surfaces. The wavelength dispersive spectrometer has much better resolution and is used for quantitative analysis of elements. The WD spectrometer is usually equipped with several diffracting crystals to optimize resolution and to cover the entire spectral range. The electron beam, sample stage, spectrometer, data collection, and processing are all under computer control. 

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