Furans, isotopically labelled

Among the facts supporting this mechanism (which is an A-Se2 mechanism because the substrate is protonated in the rate-determining step) are (1) lsO labeling shows that in ROCH=CH2 it is the vinyl-oxygen bond and not the RO bond that cleaves 497 (2) the reaction is subject to general acid catalysis 498 (3) there is a solvent isotope effect when D2O is used.498 Enamines are also hydrolyzed by acids (see 6-2) the mechanism is similar. Ketene dithioacetals R2C=C(SR )2 also hydrolyze by a similar mechanism, except that the initial protonation step is partially reversible.499 Furans represent a special case of enol ethers that are cleaved by acid to give 1,4 diones. Thus... 

A typical example for extended use of the isotope dilution method is the quantification of polychlorinated dioxins. Sixfold and twelvefold labeled dioxins and furans in the required chlorination degrees are used as recovery and surrogate standards during sample preparation and GC-MS analysis, as described in the widely applied EPA 1613 method. The nearly identical compound characteristics during clean-up, the chromatographic process and detection, guarantees most reliable quantitation results based on the sample individual recovery values in one analysis run (see details in the application Section 4.14 on dioxin analysis). 

The described isotope dilution method for the quantification of polychlorinated dioxins, furans and dioxin-hke polychlorinated biphenyls (dl-PCBs) follows the EPA 1613 Rev.B method (US EPA, 1994,1998a/b). The MID scheme on the accurate target masses typically uses isotope ratio qualifiers besides the specific retention time for all native dioxin/furan congeners, as well as for their specific labelled internal standards, one quantification mass and one ratio mass. The analytical setup for the high resolution GC-MS is given with the MID descriptor, as... 

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