Ethanol detection, isotopic

The catalyst for the in situ FTIR-transmission measurements was pressed into a self-supporting wafer (diameter 3 cm, weight 10 mg). The wafer was placed at the center of the quartz-made IR cell which was equipped with two NaCl windows. The NaCI window s were cooled with water flow, thus the catalyst could be heated to 1000 K in the cell. A thermocouple was set close to the sample wafer to detect the temperature of the catalyst. The cell was connected to a closed-gas-circulation system which was linked to a vacuum line. The gases used for adsorption and reaction experiments were O, (99.95%), 0, (isotope purity, 97.5%), H2 (99.999%), CH4 (99.99%) and CD4 (isotope purity, 99.9%). For the reaction, the gases were circulated by a circulation pump and the products w ere removed by using an appropriate cold trap (e.g. dry-ice ethanol trap). The IR measurements were carried out with a JASCO FT/IR-7000 sprectrometer. Most of the spectra were recorded w ith 4 cm resolution and 50 scans. 

The various tautomers and rotamers of alloxan have been examined in detail by the MNDO method and it is predicted that the keto form is most important in the gas phase, although in solution the monohydroxy forms are also thought to contribute. A mass spectral study has been used to investigate the enol-keto tautomeric equilibria of a series of substituted salicylaldehyde and 2-hydroxynaphthaldehyde Schiff bases. In neutral, ethanolic solutions, the cis- and trans-tm forms of 4,5-dimethyl-2-(2 -hydroxyphenyl)imidazoles (393) and (394) have been found to exist in equilibrium in the ground state. However, in neutral aqueous solutions, the trans-eao and keto forms (394) and (395) were the only species detected. Deuterium isotope effects on... 

Due to the inability to reliably detect beet sugar additions, the site-specific natural isotope fractionation nuclear magnetic resonance (SNIF-NMR) method used widely in the wine industry was adapted for the maple industry (Martin et al., 1996). This method determines the site specific isotope concentrations of organic compounds by nuclear magnetic resonance of ethanol fermented from the suspect sample. 

As already indicated, a special problem with esters is their preparation from two natural precursor molecules by a chemical ester synthesis. Such products have to be labelled nature-identical. For an interesting positional H-NMR study on ethyl butyrate from enzymatic esterification of beet ethanol with butyric acid from milk see [317]. Another chance to detect a corresponding adulteration would be a positional carbon and oxygen isotope analysis of the ester components. Isotope effects on the esterification reaction in question seem to influence characteristically the 8-values of the atoms involved, and hence form a basis for the origin assignment of these compounds (for further details see 6.2.2.4.4). 

The three different second-order processes thus exhibit widely different kinetic behaviour towards the varying base concentration at constant buffer ratio. In theory this dependence should provide a means of assigning the mechanism. An advantage over the isotopic exchange approach is that it should be possible to detect carbanion intermediates that eliminate more rapidly than they protonate. Unfortunately, the kinetics are not always clear-cut. The E2 mechanism can, under certain conditions, follow specific base catalysis, especially if one base is of much greater catalytic efficiency than the other bases present (e.g. the E2 reaction of l,l,l-trichloro-2,2-di-p-chlorophenyl-ethane with sodium thiophenoxide in methanol) . Alternatively, the base may be sufficiently powerful to produce a kinetically significant concentration of lyate ions (e.g. the E2 reaction of alkyl bromides with phenoxides in ethanol) " . 

The extent to which ethanol is derived from added sugar in fermentation can be determined by the NMR spectroscopic measurement of the ratio of the hydrogen isotopes H to H. The method is based on the fact that the plant-specific H/ H ratio (R value, cf. 18.4.3) of the sugar also appears in ethanol about 2.24 (com sugar), about 2.70 (beet sugar), about 2.45 (wine). The detection... 

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