Correction of spectra

Wreford NGM, Schofield GC. A microspectrofluorometer with on line real time correction of spectra. J Microsc 1975 103 127-130. 

It will be shown in Chapter 6 that fc depends the detector are wavelength-dependent, on the wavelength because the transmission Therefore, correction of spectra is necessary... 

As outlined above, the spectra are distorted by the wavelength dependence of several components of the instrument. Correction of spectra is of major importance for quantitative measurements (determination of quantum yields and calculation of overlap integrals), for comparison of excitation and absorption spectra, and for comparison of fluorescence data obtained under different experimental conditions. 

Figure 10.8. Schematic of procedure for correction of spectra for instrumental response variation. R represents the response function 0z. and

These numbers are related in the right sense to the a-values of the inert gases. This connection can be traced still further by considering the a-values of other (multiple-valued) ions of inert gas type, which may be determined partly from the Rydberg corrections of spectra of the ionised element (spark spectra), partly from the refractive indices of solid salts, (ionic lattice). In this way further support is obtained for the view that the Rydberg correction of the terms of the outer orbits in the spectra under consideration is due to the... 

This function performs normalizations and offset-corrections of spectra. Three different methods are available for spectrum normalization ... 

B. W. Wabuyele and P. de B. Harrington, Arutl. Chem., 66,2047 (1994). Optimal Associative Memory for Background Correction of Spectra. 

A-(9-Acridinyl)maleiinide (NAM) [49759-20-8] M 274.3, m 248 , 255-258 . Purified by chromatography on silica gel using CH2CI2 as eluant. Evaporation of pooled fractions that gave the correct NMR spectra gave a solid which was recrystd from Me2CO as pale yellow prisms. IR v (nujol) 1710 (imide) UV (MeOH) (nm), (e M- cm ) 251 (159 500), 343 shoulder (7 700), 360 (12 400) and 382shoulder (47... 

In contrast to these findings, two independent reports have appeared in which the ultraviolet spectrum of 2-amino-l,3,5-triazine has been interpreted to indicate that this compound exists in the imino form, but the correctness of these conclusions appears doubtful, especially since comparisons were not made with the spectra of the alkylated derivatives. Other investigators have interpreted the ultraviolet spectra of a series of amino- and diamino-1,3,5-triazines to indicate that the amino forms of these compounds predominate. ... 

In 1882 Baeyer and Oekonomides advanced formula 72 (R = H) for isatin on chemical grounds, but shortly thereafter the dioxo structure 73 (R H) was proposed since the ultraviolet spectrum of isatin resembled that of the N—Me derivative (73, R Me) and not that of the O—Me derivative (72, R = Me). " It was later shown, despite a conflicting report, that the ultraviolet spectrum of isatin is very similar to the spectra of both the O— and N—Me deriva-tives - the early investigators had failed to take into consideration the facile decomposition of the O—Me derivative. Although isolation of the separate tautomers of isatin has been reported, - these claims were disproved. A first attempt to determine the position of the mobile hydrogen atom using X-ray crystallographic techniques was inconclusive, but later X-ray work," dipole moment data, and especially the infrared spectrum demonstrated the correctness of the... 

Fig. 8.5 Corrected fluorescence spectra of partially purified dinoflagellate luciferin obtained from Dissodinium. From Njus, 1975.

Artifact removal and/or linearization. A common form of artifact removal is baseline correction of a spectrum or chromatogram. Common linearizations are the conversion of spectral transmittance into spectral absorbance and the multiplicative scatter correction for diffuse reflectance spectra. We must be very careful when attempting to remove artifacts. If we do not remove them correctly, we can actually introduce other artifacts that are worse than the ones we are trying to remove. But, for every artifact that we can correctly remove from the data, we make available additional degrees-of-freedom that the model can use to fit the relationship between the concentrations and the absorbances. This translates into greater precision and robustness of the calibration. Thus, if we can do it properly, it is always better to remove an artifact than to rely on the calibration to fit it. Similar reasoning applies to data linearization. 

The pressure being higher, all features of rotational structure disappear and the difference between spectra of various spin modifications becomes so smooth that any of them practically reproduces the whole contour shape. In Fig. 5.14 the theoretical contours are shown calculated with and without adiabatic correction of the impact operator for an ideal nitrogen solution in Ar. They are compared with the experimental one related to the same value of... 

Debye s theory, considered in Chapter 2, applies only to dense media, whereas spectroscopic investigations of orientational relaxation are possible for both gas and liquid. These data provide a clear presentation of the transformation of spectra during condensation of the medium (see Fig. 0.1 and Fig. 0.2). In order to describe this phenomenon, at least qualitatively, one should employ impact theory. The first reason for this is that it is able to describe correctly the shape of static spectra, corresponding to free rotation, and their impact broadening at low pressures. The second (and main) reason is that impact theory can reproduce spectral collapse and subsequent pressure narrowing while proceeding to the Debye limit. 

Procedures for determining the spectral responslvlty or correction factors In equation 2 are based on radiance or Irradlance standards, calibrated source-monochromator combinations, and an accepted standard. The easiest measurement procedure for determining corrected emission spectra Is to use a well-characterized standard and obtain an Instrumental response function, as described by equation 3 (17). In this case, quinine sulfate dlhydrate has been extensively studied and Issued as a National Bureau of Standards (NBS) Standard Reference Material (SRM). 

These methods can give us useful information on radicals in a manner similar to that for closed-shell systems, provided the exploitation is correct. Of course, in expressions for total energy, bond orders, etc., a singly occupied orbital must be taken into account. One should be aware of areas where the simple methods give qualitatively incorrect pictures. The HMO method, for example, cannot estimate negative spin densities or disproportionation equilibria. On the other hand, esr spectra of thousands of radicals and radical ions have been interpreted successfully with HMO. On the basis of HMO orbital energies and MO symmetry... 

The next step after apodization of the t time-domain data is Fourier transformation and phase correction. As a result of the Fourier transformations of the t2 time domain, a number of different spectra are generated. Each spectrum corresponds to the behavior of the nuclear spins during the corresponding evolution period, with one spectrum resulting from each t value. A set of spectra is thus obtained, with the rows of the matrix now containing Areal and A imaginary data points. These real and imagi-... 

An alternative combination pair of spectra is then formed, taking the geometric mean of two undulator spectra of positive helicity, and of the two recorded with negative helicity. These two spectra of given light helicity each contain corresponding corrected contributions from both undulator sources, and it can be shown [55] that the instrumental asymmetries are effectively canceled by this procedure. 

This yields an estimate for the bias (intercept) a and slope b needed to correct predictions yg from the new (child) instrument that are based on the old (parent) calibration model, b. The virtue of this approach is its simplicity one does not need to investigate in any detail how the two sets of spectra compare, only the two sets of predictions obtained from them are related. The assumption is that the same type of correction applies to all future prediction samples. Variations in conditions that may have a different effect on different samples cannot be corrected for in this manner. 

The H- and 13C-NMR spectra of gelsemine (1) have been reinvestigated with 2D homonuclear NOESY and heteronuclear COSY techniques (31). As a result some of the original assignments (4,32) have been revised. Thus the <5H of values of H-14a, H-14e, H-15, and H-16 are revised from 2.37, 2.0,2.83, and 2.30 to 2.83, 2.01, 2.30, and 2.43 ppm, respectively, while the most significant corrections of <5C values involve that of C-6 from 40.2 to 50.47 ppm and 1V-CH3 from 50.4 to 40.40 ppm. These adjustments will be helpful in future studies of alkaloids in this series. 

Table 2. EPR parameters of Mo(V) species from milk xanthine oxidase. Parameters have been measured from spectra of frozen solutions at about —150°, in some cases at both 9 and 35 GHz. Correctness of some of the interpretations was confirmed by computer simulations...

HYSCORE spectra of zeaxanthin radicals photo-generated on silica-alumina were taken at two different magnetic fields B0=3450G and B0=3422G, respectively. In order to combine the data from the two spectra, the field correction was applied (Dikanov and Bowman 1998). The correction consists of a set of equations that allow transformation of spectra to a common nuclear Zeeman frequency. The set of new frequencies was added to that of the former spectrum and plotted as the squares of the frequencies v2a and v2p. Examples of these plots can be found in Focsan et al. 2008. 

Fig. 14.1 (a) Red shift of Cr emission line peaks as a function of Ar bath gas pressure at 3,230 K. The three curves correspond to the three peaks of the triplet centred on 27,820 cm-1, (b) Corrected MBSL spectra (orange) and Cr emission from a hollow cathode lamp at low pressure (blue). Relative red shifts for each peak are indicated [11] (reprinted with permission from Annual Reviews)... 

Photophysical Processes in Pol,y(ethy1eneterephthalate-co-4,4 -biphenyldicarboxyl ate) (PET-co-4,4 -BPDC). The absorption and luminescence properties of PET are summarized above. At room temperature the absorption spectrum of PET-co-4,4 -BPDC copolymers, with concentrations of 4,4 -BPDC ranging from 0.5 -5.0 mole percent, showed UV absorption spectra similar to that of PET in HFIP. The corrected fluorescence spectra of the copolymers in HFIP exhibited excitation maxima at 255 and 290 nm. The emission spectrum displayed emission from the terephthalate portion of the polymer, when excited by 255 nm radiation, and emission from the 4,4 -biphenyldicarboxylate portion of the polymer when excited with 290 nm radiation. 

Equation (2.3) describes line positions correctly for spectra with small hyperfine coupling to two or more nuclei provided that the nuclei are not magnetically equivalent. When two or more nuclei are completely equivalent, i.e., both instantaneously equivalent and equivalent over a time average, then the nuclear spins should be described in terms of the total nuclear spin quantum numbers I and mT rather than the individual /, and mn. In this coupled representation , the degeneracies of some multiplet lines are lifted when second-order shifts are included. This can lead to extra lines and/or asymmetric line shapes. The effect was first observed in the spectrum of the methyl radical, CH3, produced by... 

Both extended spectra are now optimized by trial and error corrections of H(t) in such a way that, over the entire reduced frequency interval, G"(w) calculated by means of [6] approximates G (w) measured, and G (w) measured minus G (constant value. Of course, Equations [5] and [6] are now used with the extended integration limits. With some routine, this procedure yields two optimized spectra after about eight iterative adjustments of the entire extended spectra. 

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

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