Peak search manual

The results of an automatic peak search can be further improved by adding real and/or removing false peaks manually. An example of such a peak search is shown in Figure 4.8. 

Figure 4.8. Automatic peak search conducted using a second derivative method (top) and manually corrected reduced pattern (bottom). The upward arrow placed on the digitized pattern shows a false peak (which was eliminated manually) and the downward arrows show the missed peaks (which were added manually).

Every crystalline phase in a sample has a unique powder diffraction pattern determined from the unit cell dimensions and the atomic arrangement within the unit cell. It can be considered a fingerprint of the material. Thus, powder diffraction can be used for phase identification by comparing measured data with diffraction diagrams from known phases. The most efficient computer searchable crystallographic database is the PDF-4 from the International Centre for Diffraction Data (ICDD) [3]. It is used by very efficient computer-based search-processes. In 2007 the PDF-4-i- database contains information about Bragg-positions and X-ray intensities for more than 450000 compounds, out of which there are about 107 500 data sets with atomic coordinates. New entries are added every year. The positions of the peaks in the measured pattern have to be determined. This can be done manually, but effective, fast and reliable automatic peak search methods have been developed. The method can obviously be successful only if the phases in the sample are included in the database. However, the database can also help to determine unknown phases if X-ray data exist for another isostructural compound albeit with a different composition. 

No peak search uses the position saved in the interferogram if this value is known, it can also be entered manually. 

Figure 3 Peak search example using the NIST mass spectral database. The mass spectrum of a hypothetical unknown is from caffeine contaminated with a phthalate. Manual selection of relevant peaks easily allows the spectroscopist to consider probable contaminations (peaks at miz 149, 167). The correct solution is found after the input of four peaks by excluding the typical phthalate peaks. Note the wide intensity intervals applied. Should a peak at m/z 149 be required the correct compound is not found.

Two indices are used to present the data, each in the form of a unique digital code that permits either manual or computer search for identification purposes. A coded drug name index is supplemented by a base peak index designed to aid in identification of unkonwn spectra. 

All GCMS data files were examined with a general search procedure developed for scanning GCMS data for anthropogenic chemicals at trace levels. Hard copies of the mass spectral data were examined manually to verify computer matches and Identify compounds not selected by the computer program. Identified compounds were then quantitated by multiplying their peak area with appropriate response factors obtained from analyses of quantitative standards under Identical Instrumental tunes and... 

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