Identification of the emitted compounds

In many cases, solvent mixtures are used. Their composition must be identified. Rodofos is one example of such solvent used in Poland. Its composition was determined by gas chromatography. Analyses were performed using a Hewlett-Packard gas chromatograph model 5890 coupled with computerized mass spectrometer instrument, model 5970. 

Samples were collected by drawing a known volume of air through a bubbler containing 1 ml of carbon disulfide. Volume of the injected sample was 1-5 pi. 

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The second, more theoretical approach is based on the chemical analysis of the volatile compounds which are emitted. The scope is to find and to identify one or more volatile compounds which are specific for the current status of the cooking process. A possible disadvantage of this approach is that after the identification of volatile components it cannot be assured that suitable gas sensors are available or can be developed. 

Molecular fluorescence involves the emission of radiation as excited electrons return to the ground state. The wavelengths of the radiation emitted are different from those absorbed and are useful in the identification of a molecule. The intensity of the emitted radiation can be used in quantitative methods and the wavelength of maximum emission can be used qualitatively. A considerable number of compounds demonstrate fluorescence and it provides the basis of a very sensitive method of quantitation. Fluorescent compounds often contain multiple conjugated bond systems with the associated delocalized pi electrons, and the presence of electron-donating groups, such as amine and hydroxyl, increase the possibility of fluorescence. Most molecules that fluoresce have rigid, planar structures. 

Pheromones are powerful modulators of insect behavior. Since the isolation and identification of the first pheromone, (10E, 12Z)-hexadec-10,12-dien-l-ol, the sex attractant of the silk moth Bombyx mori, thousands of other insect pheromones have been identified. Our understanding of the sensory apparatus required for pheromone detection has also increased significantly. Coincidentally, B. mori was instrumental in many of these advances (see below). Volatile pheromones are detected by a specialized olfactory system localized on the antennae. The precise recognition of species-specific nuances in the structure and composition of pheromone components is essential for effective pheromone-based communication. The pheromone olfactory system of species studied so far exhibits remarkable selectivity towards the species-specific pheromone blend. Pheromones are emitted in low (fg-pg) quantities and are dispersed and greatly diluted in air plumes. Thus, pheromone olfaction systems are among the most sensitive chemosensory systems known. (Schneider et al., 1968). This chapter summarizes efforts (particularly over the past 10 years) to understand the molecular basis for the remarkable selectivity and sensitivity of the pheromone olfactory system in insects. The chapter will also outline efforts to design compounds that interfere with one or more of the early events in olfaction. 

McEwan, M., MacFarlane-Smith, W.H., Identification of volatile organic compounds emitted in the field... 

McEwan, M., MacFarlane-Smith, W.H., Identification of volatile organic compounds emitted in the field by oilseed rape (Brassica napus ssp. Oleifera) over the growing season. Clin. Exp. Allergy, 28,332-338, 1998. 

However, the satisfactory explanation of phosphorescence as a radiative transition from the lowest triplet state had to wait till the decisive paper of G.N. Lewis and M. Kasha in 1944. Moreover, considering that the characteristics of the phosphorescence phenomenon, in terms of wavelengths, triplet lifetime, quantum yield, etc., are specific of the emitting molecule, these authors also suggested the potential of phosphorescence for the individual identification of organic compounds in complex mixtures. 

Physical and chemical characterization methods are essential to assess aspects such as material and processing quality. Raman microprobe is an analytical tool coupled to an optical microscope. Elemental analysis using the x-rays emitted from the specimens in the electronic microscopy techniques can be used for local composition determination or to obtain a map of the distribution of a certain element in a wider area wavelength and energy-dispersive x-ray spectrometers are used for these purposes. Fourier transform infrared spectrometer is widely used for the qualitative and quantitative analysis of adhesives, the identification of unknown chemical compounds, and the characterization of chemical reactions. Thermal methods such as thermomechanical analysis and differential scanning calorimetry are discussed as valuable tools for obtaining information during postfracture analysis of adhesively bonded joints. 

Nevertheless, in certain cases anomalous liuninescence may be possible, identification of which may be based on the following aspects an abnormally large Stokes shift and width of the emission band a wavelength of emission that is not consistent with the wavelength anticipated from the properties of the compound an anomalous decay time and thermal behavior (Dorenbos 2003). Such luminescence may be red, for example at 600 nm in Bap2, with a decay time of about 600-800 ns. This is due to the fact that the emitting level contains spin octets and sextets, whereas the ground state level is an octet, so that the optical transition rate is slower because of spin selection rule (Dorenbos et al. 2003). 

Electron ionisation is still the most widely used technique for the analysis of volatile molecules. It is considered to be a hard ionisation process, which leads to reproducible spectra that can be compared to a library of mass spectra for compound identification. In this technique, ionisation occurs in the ion source by the collision of the sample molecules with electrons that are emitted from a filament by a thermoionic process (Fig. 16.15). 

Tandem mass spectrometry (MS/MS) is a new analytical technique applied to problems in food and flavor analyses. Rapidity of analysis, a high discrimination against chemical noise, and the ability to analyze mixtures for functional groups are attributes of MS/MS that make it attractive for such problems. Sanple collection and pretreatment differ frcm methods used in GC/MS. Correct choice of an ionization method is paramount. Daughter ion MS/MS spectra are used for conpound identification via comparison with those of authentic compounds, and parent and neutral loss spectra are useful in functional group analysis. Applications to direct analysis of volatiles emitted from fruits and to spice analyses are considered. 

Electron ionization is the most commonly method used for the analysis of volatile compounds. It is the case for organic molecules. This is a ionization in the gas phase that occurs in the ion source by the collision of the neutral molecules of the sample and electrons emitted from a filament by a thermoionic process (Figure 16.17). The ejection of the most weakly held electron leads to positive ions. This reproducible procedure facilitates the identification of a compound by comparing its spectrum with those collected in a spectral library, assuming the compound is registered within. 

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