Spectra code

Figure 17.17. Principal component analysis map of sample (left) and color-coded spectra (right) from a sample of marine suspended particulate matter. The lower three spectra are characteristic of low organic mineral phases, while the upper three organic phases have distinctively different C-NEXAFS spectra. Background regions are shown in black (J. Brandes, unpublished data 2007). See color insert.

The code SPECTRA assumes that the flux is a piecewise-linear function of energy i.e., for any energy group j, one can write... 

To code spectra and chemical structures in analytical databases and to learn about library search methods and the simulation of spectra. 

The Peek-a-boo system is easy to use in the laboratory, and has a finite number of cards, each corresponding to a specific band interval of the spectrum. A card can have as many as 10,000 holes, and the locations of the holes correspond to serial numbers of the coded spectra. For example, the card for 6.5 has holes punched to correspond to the serial numbers of all the compounds displaying an absorption band at 6.5 fi. Each of the spectral characteristics to be coded is accounted for by similar cards. 

All the central spectra were calculated with the SPENG code spectra as a function of position in the various cores were calculated by SPENG-MONDAY. 

Library search is usually performed in the "forward" mode, i.e., by comparing the coded spectrum of interest to every single spectrum within the library. The method of "reverse" search, i.e. comparing each library to the unknown, is slower but allows the detection of small components (poorly resolved) in mixtures (14). 

Several empirical approaches for NMR spectra prediction are based on the availability of large NMR spectral databases. By using special methods for encoding substructures that correspond to particular parts of the NMR spectrum, the correlation of substructures and partial spectra can be modeled. Substructures can be encoded by using the additive model greatly developed by Pretsch [11] and Clerc [12]. The authors represented skeleton structures and substituents by individual codes and calculation rules. A more general additive model was introduced... 

However, one of the most successfiil approaches to systematically encoding substructures for NMR spectrum prediction was introduced quite some time ago by Bremser [9]. He used the so-called HOSE (Hierarchical Organization of Spherical Environments) code to describe structures. As mentioned above, the chemical shift value of a carbon atom is basically influenced by the chemical environment of the atom. The HOSE code describes the environment of an atom in several virtual spheres - see Figure 10.2-1. It uses spherical layers (or levels) around the atom to define the chemical environment. The first layer is defined by all the atoms that are one bond away from the central atom, the second layer includes the atoms within the two-bond distance, and so on. This idea can be described as an atom center fragment (ACF) concept, which has been addressed by several other authors in different approaches [19-21]. 

The spectral signals are assigned to the HOSE codes that represent the corresponding carbon atom. This approach has been used to create algorithms that allow the automatic creation of "substructure-sub-spectrum databases that are now used in systems for predicting chemical structures directly from NMR. 

Figure 10.2-9. Application of a counterpropagation neural network as a look-up table for IR spectra sinnulation, The winning neuron which contains the RDF code in the upper layer of the network points to the simulated IR spectrum in the lower layer.

The right-hand side of the form gives the network map in the upper part and to its right the simulated IR spectrum is plotted, which can be downloaded as a JCAMP File (cf. Section 2,4.5, Section 4,2,4.2). By clicking on tbe neurons in the map one obtains the RDF code and the spectrum of the corresponding structure in the lower part of the form and compared with those of the winning neuron,... 

FIG. 4 PCA similarity map defined by the principal components 1 and 2 for the tryptophan emission spectra. Samples were coded NHO, NHP, HOM, and HOP for raw, heated, homogenized, and homogenized -I- heated milks, respectively. Each label corresponds to a spectrum. 

Fig. 14 The experimental geometries of benzene- -HC1 and benzene- -ClF (to scale) and the n-electron model of benzene. See text for discussion of the motion of the C1F subunit, as inferred from an analysis of the rotational spectrum of benzene- -ClF. See Fig. 1 for key to the colour coding of atoms... 

In applying RAIRS to CO adsorption, the contribution from CO molecules in the gas phase to the absorption spectrum at CO pressures above 10-3 mbar completely obscures the weak absorption signal of surface adsorbed CO. Beitel et al. found it possible to subtract out the gas phase absorption by coding the surface absorption signal by means of the polarization modulation (PM) technique applied to a conventional RAIRS spectrometer, p-polarised light produces a net surface electric field which can interact with adsorbed molecules, whereas both polarization states are equally sensitive to gas phase absorption because gas phase molecules are randomly oriented. By electronic filtering a differential spectrum is computed which does not show contributions from the gas phase and which has much higher surface sensitivity than a conventional RAIRS setup. 

These workers employed a new in-vacuum experimental set-up, and it can be seen that the analytical range of the UTW detector is between 0.2 and 6keV (which corresponds to elements from C to Mn), whereas that of the Be-windowed detector is above 4keV, corresponding to elements from Ti to U. A PIXE spectrum is usually quite complicated, and due to interferences between different elements and due to small peaks being hidden by larger ones. Deconvolution by computer using special codes is required in order to carry out the complete analysis of the spectrum. 

The discovery of the base-paired, double-helical structure of deoxyribonucleic acid (DNA) provides the theoretic framework for determining how the information coded into DNA sequences is replicated and how these sequences direct the synthesis of ribonucleic acid (RNA) and proteins. Already clinical medicine has taken advantage of many of these discoveries, and the future promises much more. For example, the biochemistry of the nucleic acids is central to an understanding of virus-induced diseases, the immune re-sponse, the mechanism of action of drugs and antibiotics, and the spectrum of inherited diseases. 

This generic procedure affords the powder EPR absorption spectrum, which should be differentiated to get the powder EPR spectrum. Note that the whole procedure consists of three nested loops with the computation of an exponential (Equation 4.8) within the inner loop. Coded in a higher language (C, FORTRAN95) and run on a standard PC, this program will generate the EPR spectrum of a simple S = 1/2 or an effective S = 1/2 system in a split second (of the order of 10 ms or less). It is, however, useful to think about ways to make it as fast as possible, because extending... 

Suppose we want to compare two spectra—let s call them spectrum-a and spectrum-P—taken over field sweeps that may be identical but with a slight difference in their micro-wave frequency. The spectra are digital arrays corresponding to amplitudes at equidistant field values. The procedure to convert spectrum P taken at frequency vp to frequency va of reference spectrum a is as follows For each field value B of spectrum-a we calculate the corresponding field for vp /ip = (vp/va)5a, and then we search in spectrum-p to the two digital field values that nearly match (that embrace ) the value /ip in order to interpolate the two corresponding amplitudes to an intermediate amplitude value for flp to be stored in a new array of P-amplitudes onto a B(J grid. In pseudo-code... 

Fig. 2. VLA detection of 3He+ in the PN J 320. We have modeled the radio continuum and line emission using the radiative transfer code NEBULA [1], assuming an expanding shell of ionized gas. The dashed line is the model including the H171 7 and 3He+ transitions. The solid line shows the observed spectrum and only includes the 3He+ transition. The model fits the data reasonably well even though the morphology is bipolar as indicated by the HST image [6]...

The equivalent widths were determined using gaussian fit and the atmospheric models were computed using OSMARCS code improved by [6,3]. When it was not possible to measure equivalent width, the abundance was directly determined by using spectrum synthesis. 

As we ve already seen, program code is precise but tends to force you into too much detail. In design, you want to describe only the main scheme before proceeding to the detail. There are experimental wide-spectrum languages that cover both specification and coding but unfor-... 

For maximum ENDOR enhancement, the Zeeman modulation amplitude has to be about one half of the width of the EPR line which is saturated at an extremum of its first derivative. However, in an EPR spectrum with line widths of typically 1 mT this Zeeman modulation contributes 20 kHz to the width of a proton ENDOR line. It turns out that in many cases a remarkably better resolution of the spectra may be obtained with a single coding in which only the rf field is modulated. 

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