Peak-searching

From that highest energy point, called the peak, search conjugate to the direction of the reaction pathway and find the lowest energy point. 

Reliable quantification is based on peak-search software that combines peak location, peak identification, and element deduction. Element deduction means that, for unambiguous detection, at least two of the principal peaks must be detected for each analyte of interest. In trace analysis, only the strongest peaks can be detected and special attention must be paid to interfering satellites and spurious peaks. 

Modem pulse height analysers essentially contain dedicated digital computers which store and process data, as well as control the display and operation of the instrument. The computer will usually provide spectrum smoothing, peak search, peak identification, and peak integration routines. Peak identification may be made by reference to a spectrum library and radionuclide listing. Figure 10.15 summarizes such a pulse height analysis system. 

For symmetric reflections the peak search may now begin. For asymmetric reflections, the specimen must be rotated about its normal until the desired diffraction vector lies in the incidence plane of the beam conditioner. This is normally the diffractometer surface. An accurate knowledge of the orientation of the specimen in two axes is required to set asymmetric reflections this is usually taken from the position of the orientation flat or groove. 

Switch on X-rays and search for a peak by scaiming the axis. The binary peak search described in section 2.6.1.1 is the most efficient if the position is unknown. An intense peak is easily seen, therefore the scan can be fast, with short detector counting times. 

If the laboratory worker does not know of a reference to the preparation of a commercially available substance, he may be able to make a reasonable guess at the synthetic method used from published laboratory syntheses. This information, in turn, can simplify the necessary purification steps by suggesting probable contaminants. However, for other than macromolecules it is important that at least the NMR and IR spectra of the substance be measured. These measurements require no more than two to three milligrams (which are recoverable) of material and provides a considerable amount of information about the substance. Three volumes on the NMR spectra [C.J.Pouchert and J.Behnke, The Aldrich Library of C and FT-NMR Spectra, Vols 1—3, Aldrich Chemical Co., Inc, Milwaukee, Wl, 1993], and one on the infrared spectra [C.J.Pouchert, The Aldrich Library of FT-IR Spectra, 3nd ed, Aldrich Chemical Co., Milwaukee, Wl, 7959], as well as computer software [FT-IR Peak-search Data Base and Software, for Apple HE, IIC and II Plus computers and for IBM PC computers, Nicholet Instruments, Madison, Wl, 1984] contain data for all the compounds in the Aldrich catalogue and are extremely useful for identifying compounds and impurities. If the material appears to have several impurities these spectra should be followed by examination of their chromatographic properties and spot tests. Purification methods can then be devised to remove these impurities, and a monitoring method will have already been established. 

The on-line acquired chromatogram is used to select a peak for the NMR measurement. The software must export the chromatograms for the necessary peak search. 

Such tables are very convenient for peak-by-peak search if the inverted files containing ID numbers of reference spectra are at hand. These files must be generated in advance (Fig. 4.7). 

The second problem inherently associated with peak search in the inverted file of peak vs. ID numbers is the tolerance limit within such a retrieval should be carried out. If the intervals in which the peaks are inverted are broad the search will probably yield the correct answer but the list of produced matches will be rather... 

In general, the detectors are combined with a preamplifier, an amplifier and a multichannel analyser, in which the pulses are sorted according to their pulse heights. Frequently, the multichannel analyser is operated by a computer and a program for peak search, peak net area calculation, energy calibration and radionuclide identification. 

The crystal used for data coUection was transferred to an Enraf-Nonius CAD-4 diffractometer. Automatic peak search and indexing procedures yielded the same monoclinic cell as derived from the X-ray powder diffraction data and precession photographs. Testing showed that the cell was indeed primitive and that there was no superlattice present. Table 5 gives the crystal data and X-ray experimental parameters, and Table 6, the interatomic distances and angles. Positional and thermal parameters are given in Table S3 (Supporting Information)... 

The coordinates of atomic positions can be located and a molecular model derived, either by contouring the map, or by use of a peak search routine.. An example is provided in Figure 9.6. In the most favorable cases this map will also give an indication of the locations of atoms that were not included in the phasing model. Such a map can also be used to improve the accuracy of a preliminary model by adjusting the model to best fit the electron density. 

Peak search, which is unbiased by any kind of structural information. 

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