Background, absorption chemical

Here, B is the background absorption in the absence of chemical relaxation, ca the angular frequency of the inflection point of the absorption ca = 2nf (rad s ) and X the relaxation time (s). The excess absorption due to chemical relaxation A is obtained from the low-frequency absorption where ca x and is given by ... 

Perturbation of a chemical equilibrium by ultrasound results in absorption of the sound. Ultrasonic methods determine the absorption coefficient, a (neper cm-1), as a function of frequency. In the absence of chemical relaxation the background absorption, B, increases with the square of the frequency f (hertz) that is, a/f2 is constant. For a single relaxation process the absorption increases with decreasing frequency, passing through an inflection point at the frequency at (radians sec-1 = 2nf) which is the inverse of the relaxation time, t (seconds), of the chemical equilibrium [Eq. (6) and Fig. 3]. 

The dominant bands in the resolved spectra obey Beer s law in that the magnitude of the absorbance (defined in terms of the peak heights relative to a linear baseline drawn between 880 and 1880 cm1) is linear in both concentration and thickness (Figures 5 A B). Futhermore, when these spectra are divided by the magnitude of the intrinsic absorption at 2800 cm the resultant absorbances are independent of sample thickness and concentration as shown in Figure 6. This supports the previous conclusion that both the broad background absorption and the superimposed spectra are directly related to the chemical structure of the black. The data also demonstrate that the spectral features of carbon black can be routinely measured to within 0.005 absorbance units. This capabilitity for the quantitative measurement of the effects of various modification procedures... 

Atomic spectrometric methods are relative methods. The signals produced by the sample solutions are compared to the signals caused by the reference solutions. If the samples and references are behaving differently during the measurements, interferences will be seen. Interferences in the flame AA techniques may be divided into chemical, ionization, physical, and background absorption interferences. 

Some spectrophotometric analyses focus on the properties of the target compound (its native spectrum), a chemical derivative, or the product of some separation or sample pretreatment. Direct analyses of target compounds are only possible if the wavelength used is not interfered with by other species or by background absorption analyses of mixtures of components are also possible with recent developments in chemometrics and derivative spectrophotometry. 

Photoexcitation spectroscopy is a powerful method for studying conducting pol5mers charges are injected into the 71-electron system without chemically changing the polymer and without introducing additional disorder. Since the background absorptions are completely suppressed in the photoinduced absorption spectrum. 

Although the high water background absorption degraded the S/N ratio of the measurement, the use of the PapMap method, including NA-PCA (see Sections S.2.3.2 and 5.2.4.1, respectively), allowed for the collection of excellent spectra of live cells and the effects of chemicals perturbing these cells. Furthermore, we could demonstrate that cells could be kept alive for at least 24 h, and that starvation of the cells by changing the medium to a buffer solution caused cell death in a relatively short time [68, 69]. 

Figure 15.15 Imaging NHj adsorbed on reduced graphene oxide, (a) shows the raw absorbance of the sampie at one wavelength (3300 cm ) as a means to visualize the distribution and thickness of the sample. The IR absorbance here is due to a broad electronic background absorption of the RGO. (b) shows an integrated chemical image over the N-H and O-H...

While the carbon dioxide/caiistic test method has become accepted, one should use the results with caution. The chemical reaction masks the effect of physical absorption, and the relative values in the table may not hold for other cases, especially distillation applications where much of the resistance to mass transfer is in the gas phase. Background on this combination of physical and chemical absorption may Be found earher in the present section, under Absorption with Chemical Reaction. ... 

A working curve was constructed for each element from counting data obtained on a number of chemically analyzed standards apparently no. background correction was necessary. By use of these simple curves, and without allowing for absorption or enhancement effects, satisfactory approximate results were obtained for both iron and manganese, as is shown by the data in Table 7-10, which are representative of those for a series of 40 samples. 

Practically all classical methods of atomic spectroscopy are strongly influenced by interferences and matrix effects. Actually, very few analytical techniques are completely free of interferences. However, with atomic spectroscopy techniques, most of the common interferences have been studied and documented. Interferences are classified conveniently into four categories chemical, physical, background (scattering, absorption) and spectral. There are virtually no spectral interferences in FAAS some form of background correction is required. Matrix effects are more serious. Also GFAAS shows virtually no spectral interferences, but... 

In this chapter, we briefly discuss the theoretical background of polarized x-ray absorption spectroscopy (PXAS). Many of the recent applications of synchrotron radiation to polarized absorption edge structure and to EXAFS are discussed, with particular emphasis being given to the study of discrete molecular systems. We present here some indication of the potential applications of PXAS to systems of chemical and biological interest. 

The large amount of sodium chloride in seawater samples causes nonspecific absorption [366-370], which can only be partially compensated by background correction. In addition the seawater matrix may give rise to chemical as well as physical interferences related to the complex physico-chemical phenomena [371-373] associated with vaporization of metals and of the matrix itself. 

Interferences in atomic absorption measurements can arise from spectral, chemical and physical sources. Spectral interference resulting from the overlap of absorption lines is rare because of the simplicity of the absorption spectrum and the sharpness of the lines. However, broad band absorption by molecular species can lead to significant background interference. Correction for this may be made by matrix matching of samples and standards, or by use of a standard addition method (p. 30 et seq.). 

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