Volatility discrimination

For many routine applications, a solid or liquid sample may be introduced to the source of the spectrometer by direct mechanical insertion, i.e. it is held on the end of metal probe which is inserted into the source via a system of air-locks. All of the sample is presented to the source simultaneously, allowing no separation of the components, and the resulting spectrum is a function of the whole of the sample. For gas-phase ionisation techniques an added level of discrimination may be obtained by selectively vaporising the components of the sample. Volatility discrimination is achieved by acquiring data as a function of time while the... 

Another characteristic similar to A/ 100 is the Distribution Octane Number (DON) proposed by Mobil Corporation and described in ASTM 2886. The idea is to measure the heaviest fractions of the fuel at the inlet manifold to the CFR engine. For this method the CFR has a cooled separation chamber placed between the carburetor and the inlet manifold. Some of the less volatile components are separated and collected in the chamber. This procedure is probably the most realistic but less discriminating than that of the AJ 100 likewise, it is now only of historical interest. 

The lateral diverticulum cells in semi-terrestrial species such as toads can still detect a wide range of amino acids, comparable to the properties of fish neuroepithelium. Both water-soluble and volatile odourants are discriminated by the olfactory neurones of the Clawed toad (Xenopus) (Iida and Kashiwayanagi, 1999). When single olfactory neurones were tested with acidic, neutral and basic amino acids, over 50% of the receptors gave some excitatory response. 

HPLC solvents (PDMS-coated fibres are incompatible with hexane). PDMS fibres are more selective towards nonpolar compounds and polyacrylate fibres towards polar compounds such as acids, alcohols, phenols and aldehydes. Another feature of SPME fibre selectivity is discrimination towards high-MW volatiles. SPME has successfully been applied to the analysis of both polar and nonpolar analytes from solid, liquid or gas phases. Li and Weber [533] have addressed the issue of selectivity in SPME. 

Although the feasibility of direct probe MS for the analysis of additives in complex polymeric matrices has been demonstrated (Section 6.4), application is limited, difficult and requires above-average mass-spectroscopic expertise. Direct desorption in the MS probe is usually limited to screening of volatile components. Direct multicomponent spectroscopic analysis has other hurdles to overcome (UV/VIS lack of spectral discrimination IR/R functional-group recognition only, with no discriminative power for additives with similar functionalities NIRS unsuitable for R D problems NMR sensitivity). 

As a further example of a dog s ability to discriminate certain solvents, it is believed that dogs searching for the explosive C-4 as a target substance may be detecting the presence of cyclohexanone rather that the explosive itself. For example, cyclohexanone is a volatile solvent used in the purification process for... 

Data have been analyzed from a multivariate point of view. In this way the cooperative effects of the different materials is studied and the characteristics of each sensor are easily compared with those of the other sensors. PLS was used as a regression method for calculating the capability of the set of sensors to discriminate between the volatile compounds. Volatile compounds were checked at different concentrations in order to evaluate the response of sensors in a wide concentration range. Nevertheless, the concentration variation tends to shadow the reaction of sensors with analytes, since the sensor response contains both qualitative (sensor analyte interaction) and quantitative (analyte concentration) information. In order to remove the quantitative information, data have been normalized using the linear normalization discussed in section 3. 

The high water-solubility of surfactants and their, often more polar, metabolites prevents direct application of gas chromatographic separation (GC) with appropriate detection. The necessary volatilisation without thermal decomposition can be achieved by derivatisation of the analytes, but these manipulations are time- and manpower-consuming and can be susceptible to discrimination. Additionally, each derivatisation step in environmental analysis is normally target-directed to produce volatile derivatives of the compounds to be determined. Unknown surfactants that are simultaneously present, but differ in structure and therefore cannot react with the derivatisation reagent, are discriminated under these conditions. 

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