Finger print technique

FIGURE 10.9. A SiNW sample produced with the finger printing technique (see text). The scale bar on the left is 1mm, and 10 fim on the right. The darker regions are bare Si surfaces. [Chem Comm 2005]-Reproduced by permission of The Royal Society of Chemistry, (ref 54)... 

In this technique almost all groups absorbs characteristically within a definite range. Thus a strong IR band at 1800 to 1600 cm 1 in the IR spectrum of an unknown compound indicates that a carbonyl group is present. Identical compounds have identical IR spectra. Molecules with identical or similar shapes of their IR spectra in the finger print region have the same or a similar skeleton of atoms. 

Chemistry is often used to solve crimes. Soil samples from a suspect s shoes or from the tread of the car tyre used by the suspect is analysed for pH value. This is compared with the pH of the soil at the scene of crime. The DNA (from hair, fingernail, saliva etc.) of the suspect is determined by a special technique. This is called DNA finger printing . 

Figure 9.15 Typical mass spectrometry based experiment for protein identification/characterization. (1) Proteins are fractionated by chromatography, separated by sodium dodeyl sulphate (SDS) polyacrylamide gel electrophoresis (PAGE), then excised from gel (2) the protein of interest is digested into peptide fragments, that are then (3) identified by ESI MS/MS (4) the first dimension involves molecularion analysis for peptide mass finger-printing (ql, MS only). (5) Tandem MS/MS is used when protein identification is not unambiguous, in which case parent molecular ions are activated by CID (q2) and daughter (product) ions are characterized (ToF) according to the technique of product ion scanning (illustration from Aebersold and Mann, 2003, Fig. 1).

From the above remarks it may be seen that many factors affect the observed results. This means that although thermogravimetry can identify a substance from its decomposition temperature it should not be thought of as a finger print method like spectroscopy. In that technique a peak will always be at the same position in the spectrum, regardless of the make of the instrument or size of sample. 

In addition, a number of advanced analytical techniques, such as low temperature luminescence spectroscopy, tandem mass spectrometry (MS/MS), Fourier-Transform IR-spectroscopy and nuclear magnetic resonance spectrometry have been successfully applied to PAH analysis. For instance, low temperature luminescence spectrometry, sometimes in combination with laser excitation, was used for the analysis of PAH in various matrices without prior separation, which is attractive especially for screening or finger-printing purposes (10, 73). However, a wider application for routine analysis is at present inhibited by the limited availability of the required equipment. The same remark applies to tandem mass spectrometry, FT-IR spectroscopy and NMR. All three techniques, however, are increasingly used for the detection and identification of novel PAH species and derivatives and efforts are continuing towards coupling IR and NMR as detectors to GC and HPLC (74) respectively. 

To conclude our technique seems to be well suited for finger printing and classifying compounds according to their known or even unknown - modes of action. 

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