Indirect methods volatile compounds

The perception of odour is due to the presence of volatile compounds in the inhaled air. Measurement of this perception can be carried out directly by psychophysical methods or indirectly by analysing the air for the odorous volatile compounds. Both methods however present a number of limitations and difficulties. 

The carbonyls are in general volatile compounds with an extensive chemistry which presents many problems as regards valence and stereochemistry. Some are reactive and form a variety of derivatives, as shown in Chart 22.1 for the iron compounds, while others are relatively inert, as for example, Cr(CO)6 etc. and Re2(CO)iQ. This rhenium compound, although converted to the carbonyl halides by gaseous halogens, is stable to alkalis and to concentrated mineral acids. A few carbonyls may be prepared by the direct action of CO on the metal, either at atmospheric pressure (Ni(C0)4) or under pressure at elevated temperatures (Fe(CO)s, Co4(CO)i2)- Others are prepared from halides or, in the case of Os and Re, from the highest oxide. The polynuclear carbonyls are prepared photo-synthetically, by heating the simple carbonyls, or by other indirect methods. 

AOCS has a recommended practice (Cg 3-91) for assessing oil quality and stability (AOCS, 2005) for measuring primary and secondary oxidation products either directly or indirectly. For example, peroxide value analysis (AOCS method Cd 8-53) (AOCS, 2005) determines the hydroperoxide content and is a good analysis of primary oxidation products. To determine secondary oxidation products, the procedure recommends p-anisidine value (AOCS Method Cd 18-90, 2005) volatile comlb by gas chromatography (AOCS Method Cg 4-94, 2005) and flavor evaluation. (AOCS Method Cg 2-83, 2005). The anisidine value method determines the amounts of aldehydes, principally 2-alkenals and 2, 4-dienals, in oils. The volatile compound analysis method measures secondary oxidation products formed during the decomposition of fatty acids. These comlb can be primarily responsible for the flavors in oils. The... 

Indirect methods for the determination of anions and organic compounds may be classified into the three groups (i) Methods based on the formation of compounds in solution (ii) Methods based on the formation of compounds in the atomization cell and (iii) Methods based on the formation of volatile compounds. 

Pyrolysis may be defined as the thermal transformation of a compound (single entity) into another compound or compounds, usually in the absence of oxygen. In modem pyrolysis the sample decomposition is rigidly controlled. One shonld keep in mind that pyrolysis gas chromatography (PGC) is an indirect method of analysis in which heat is used to change a compound into a series of volatile prodncts that should be characteristic of the original compound and the experimental conditions. 

Lewisite 1 per se is never found in the environment. Figure 18 shows that this compound hydrolyzes rapidly on contact with moisture to 2-chlorovinyl arsonous acid, which in turn slowly dehydrates to lewisite oxide (syn. 2-chlorovinyl arsenous oxide) (16). Both 2-chlorovinyl arsonous acid and lewisite oxide are nonvolatile. The most frequently used method for the identification of CWC-related chemicals is based on gas chromatography (GC) in combination with mass spectrometry (GC/MS). Indirect GC/MS analysis of lewisite 1 requires sample preparation, which involves conversion of lewisite oxide to 2-chlorovinyl arsonous acid in an acidic environment, followed by derivatization (12). The obtained species is both volatile and thermally stable, and thus amenable to GC analysis. Often, a mercaptan reagent is used as a derivatization agent. The reaction with 3,4-dimercaptotoluene is shown in Figure 19. 

The volatile nature of ethanol makes it eminently suitable for headspace analysis. The principle on which the method is based is that of partition theory, i.e., that the concentration of a volatile species in the headspace above a solution is proportional to the concentration of that species in solution. Procedures based on this principle must therefore be classed as indirect , but nevertheless they have some advantages over direct methods, the most important being that the procedure separates volatile species from the numerous nonvolatile compounds commonly found in alcoholic beverages. As most sensors used in instrumentation for the determination of ethanol are at best semispecific and are usually oxidative in... 

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