Spectra correction

Fluorescence and phosphorescence spectra corrected for the instrumental sensitivity were measured with a spectrometer described previously (()). Corrected excitation spectra were obtained with constant excitation intensity controlled by a rhodamine B quantum counter. For phosphorescence polarization measurements the apparatus was set up in an "In Line" arrangement (j ) and equipped with a Glan-Thomson polarizer and a sheet polarizer (analyser) (10). 

Table 10.1 Integrated areas of 13C NMR spectra, corrected for variable TlpH sa... 

Free radicals are also produced by chain scission during deformation of polyethylene and FT-IR has been used to follow this process 237). The polyethylene samples were unaxially drawn and the resultant spectra corrected for orientation. An increase in the vinyl and methyl end groups created by decay of the free radicals occured in going from draw ratios of 5 to 20 44). A similar study involving deformation was made of polystyrene 246) and a comparison demonstrated between the results of thermal and mechanical degradation 24S. ... 

Another basic program for the Zeiss UMSP 80 is LAMBDA-SCAN, which is useful for spectral analysis. The light intensities at fixed wavelength intervals are measured automatically and printed out as a spectrum. The bandwidth can be precisely set to within 1 nm and the spectra corrected for the influence of ambient light. 

Samples spiked with known amounts of stable isotopes Fe> Cu, Zn, Zn were prepared and the measured ion intensities in the mass spectra were compared to theoretical values The response of the mass spectrometer was linear over a wide range of enrichment levels ( ) A computer program included in the mass spectrometer software package was used to calculate the theoretical values for peak intensities in the mass spectra Corrections for isotopic composition of the ligand ( C, H) were made as described previously (2) ... 

An example of the results for such an approach is shown in Fig. 2.5 (from Ref [1]), which we now discuss. Methylene Blue (MB) is used here as a probe molecule and is transferred onto gold NP arrays by dipping them into a 10 // M MB solution for 5 minutes. The arrays are plasma-cleaned before any dipping, to ensure that no contaminants are previously adsorbed on the NPs. The MB molecules are therefore adsorbed directly onto the gold surfaces, with possibly some molecules further away from the surface if several monolayers are present. Three NP arrays are used with distinct LSP resonances at 612 nm (array Al), 667 nm (A2) and 713 nm (A3), as evidenced in the extinction spectra of Fig. 2.5(b). The resonance of A2 is close to the peak absorption and fluorescence of MB and would be considered as the standard situation for most MEF experiments (except for the direct adsorption onto the metal). Al and A3 have resonances much further away on either side of the MB fluorescence spectrum. The MEF spectra (corrected for the ITO background), shown in Fig. 2.5(c), exhibit a broad spectrum underneath the SERS (Raman) peaks. The SERS peaks clearly confirm the presence of MB on the NPs surface. The accompanying broad signal is attributed to the modified MB fluorescence (MEF), initially for two reasons ... 

The mass spectrum of the GC/MS experiment with an empty reactor provided us with the fragmentation pattern of i3C-labelled 1-butene. Subsequently, isomerization experiments were carried out using fresh and spent HS-FER catalysts. The MS spectra corrected for the fragmentation pattern of the butene in question provided us with the label distribution in butenes. In Fig. 4 the results of the label distribution for isobutene, as obtained from isomerization at 350°C over the catalysts, are displayed. The scrambling of 13C over isobutene as observed with the fresh HS-FER after correction for natural abundance of 3C, is close to the values expected from complete (statistical) scrambling. Clearly, with the spent Ferrierite catalyst, which is still active for butene isomerization, scrambling has not taken place either in isobutene, or in the other butenes. Very similar results were obtained using i3C-labelled isobutene instead of 1-butene. 

The analysis and information content of an EPR spectrum from singlecrystal samples is much more obvious. For proteins, this possibility is limited by the difficulties involved in growing of large-enough crystals (> 10 mm for EPR applications). The analysis of these spectra may even be complicated by many non-equivalent sites in the unit cell of such a crystal. Additionally, one has to consider that crystallization effects may potentially change the structure of a protein or hinder internal conformational dynamics. Samples may also be partially aligned, for example, by external fields, on surfaces, membranes, or on thin films. In these cases, the ordering of the molecules is not perfect, and additional information on order parameters is necessary to simulate the observed spectra correctly. 

According to one of the present authors and Ugai [3], the lines of aluminum in AI2O3 shift toward shorter wavelengths by +0.48 eV and the maximum of the Kg band of aluminum in AI2O3 shifts toward longer wavelei hs by —4.85 eV relative to the maximum of the Kgjj band of metallic aluminum (this was deduced from the Kg spectra corrected by the column method for the distortions Introduced by the apparatus). 

The development of methods to obtmn excitation and emission spectra corrected for wavelength-dependent effects has been the subject of nummous investigations. Overall, none of these methods is completely satisfactory, especially if the corrected spectra are needed on a regular basis. Prior to correcting spectra, the researcher should determine if such corrections are necessary. Frequently, one only needs to compare emission spectra with other spectra collected on the san instrument. Such comparisons are usually made between the technical (or uncor-... 

Figure 1 shows absorption spectra (corrected for surface reflections) of a single unsupported film of Polaroid type HNP B material oriented parallel and perpendicular to the plane of polarization in a spectrophotometer Commercial HNP B polars usually consist of two such films sandwiched between isotropic cellulose acetate films. Absorbances in the perpendicular (Ag) orientation were not measurable between 550 and 670 nm because of instrumental difficulties (primarily depolarization at optical surfaces in the spectrophotometer). However, in measurements with the HeNe instrumentation described previously the dichroism was seen to exceed 5 at 6328A Obviously several layers of this material would be required in each polar if a dichroism of 5 were required throughout the visible region ... 

The spectrum of the unknown compound to be identified should not contain any spectral impurities as caused by coeluting components. If a spectrum is known to contain a spectral impurity, it should be corrected for this impurity before the comparison with the standard spectra. Corrections for coeluting impurities are very difficult. If the impurity does not coelute completely, identification may still be possible, depending on the concentration and elution time of the impurity, relative to the main peak. 

Generally, force fields available for the study of dense silica polymorphs and zeolites can also be used to obtain reliable structures and vibrational spectra for the frameworks of these materials. In the past it seemed necessary to use molecular mechanics force fields to be able to predict vibrational spectra correctly, but it was shown several years ago that shell model potentials also have this capability. 0 175 Shell model potentials, therefore, seem to be the best-suited force fields for silica polymorphs. These potentials model the more ionic character of the SiO bond, include polarization, allow coordination changes, and contain only a few adjustable parameters. Molecular mechanics potentials have value in molecular dynamics calculations and allow for easier extension with respect to studying the adsorption behavior of organic sorbates in zeolites, whereas the use of shell model potentials is cumbersome. 

The donor contribution in the acceptor channel (crosstalk) should be as low as possible the impact of this contribution on a bioassay is not obvious to anticipate starting from a lanthanide complex emission spectrum, since many instmmental factors, such as the filter settings (bandpass width), have to be considered. The intensity distribution between the emission lines is critical, particularly for europium complexes, with a strong impact of the ligand structure and symmetry (for terbium complexes, this impact is reduced). Care must be exercised in comparing published emission spectra, since many of the published spectra are not corrected for the photomultiplicator sensitivity (which falls off rapidly between 650 and 800 nm even using a red PMT ). The consequence is that the 690-nm ( Dq p4) band seems much smaller than its true value. Some articles do indeed show spectra corrected for the sensitivity of the detection system (which contains contributions from the PMT, but also from the monochromators and optics). Whenever such corrections have been applied, this is usually indicated in the experimental section of the article. 

We also investigated the paraxylene/HZSM-S syston with the same technique. Due to the lower intracrystalline diffusion rate the "non uniform" model was not able to simulate the NMR spectra correctly. A simpler "core shrinking" model was used [9]. The crystallites are divided into two zones a core free of diffusing molecules and a shell with a uniform hydrocarbon concentration. During the adsorption there is a diffiision front in the crystallites, the shell region increases at the expense of the core. Then, the NMR spectra is simply the sum of two lines whose intensity inversely varies widi time (Fig. 3b). 

Because of its highly discriminative capability, Fourier transform infrared (FTIR) spectrometry is valuable for identification of unknown compounds by comparison of sample spectra to reference spectra or by spectral interpretation (Somsen et al., 1995). IR spectra have been obtained on eluted samples or directly on TLC plates. About 5 pg is usually required for the elution method, which involves scraping of the zone and elution from the layer material onto an IR-transparent substance such as KBr (Issaq, 1983). Spectra can be measured directly on TLC plates by diffuse reflectance Fourier transform (DRIFT) IR spectrometry (Zuber et al, 1984). For in situ DRIFT-IR spectrometry, which requires 1-10 pg of compound, solvents must be removed from the layer and spectra corrected... 

Montalti M, Credl A, Prod L, Gandolfi MT (2006), Handbook of photochemistry, 3rd edn. CRC Press, Boca Raton. An essential reference book containing data tables for a wide range of compounds, and a variety of reference materials including quantum yields, lifetimes, quenching rate constants, electrochemical potentials and solvent properties as well as information on standard procedures used in chemical actinometry, determination of emission and excitation spectra correction factors, and quantum yield measurements and also information on equipment such as lamps and filters. 

Fig. 16. Low-energy INS spectra corrected with detailed balance from Pb2Sr2Pri jCa Cu30g (Staub et al. 1997d). Data are normalized per mole Pr ions (20 K).

Usually, fluorescence spectra are uncorrected , i.e. they contain wavelength dependent sensitivities of monochromator and detector (emission spectra) or excitation source and monochromator (excitation spectra). Correction may not be necessary, if only relative changes of individual emission bands are measured. However, for getting correct spectral intensities 1f(A), a reference source with a well known spectrum S(A)... 

Figure 5.6 Series of absorbance spectra corrected for continuous background and other continuous...

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 ... 

IL4 - [C8-CiIm][BF4] (A) - before correction for fluorescence ionic liquid (B)- pntre atl-frflMS P-carotene spectra corrected for fluorescence of ionic liquid. 

In some applications, one requires corrected spectra. Corrections in excitation spectra have to be made to account for the wavelength depemdemce of the lamp source and the efficiency of the excitation monochromator throughput, as well as for temporal fluctuations in lamp intensity (9). These corrections are commonly made within the fluores-cence spectrometer by use of a quantum counter. A quantum counter is a substance such as rhodfimine B which has a fluorescence quantum efficiency independent of the excitation wavelength. A small fraction of the incident light is directed onto the quantum counter, whose emission intensity serves as a reference for the emission intensity of the sample itself. The ratio of the two intensities is independent of distortions arising from the excitation source. 

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