Migration Control methodologies

In the following, several options for migration control testing are described briefly with a focus only on the essential parts of each strategy. The options are given in a sequence which tries to meets in the best way the considerations made above. 

Indirect migration assessment by compositional analysis of plastics Assessment by worst-case assumptions of total mass transfer 

In all cases under the premise of total mass transfer where an indirect migration assessment demonstrates the impossibility of exceeding a given legal SML restriction criterion, full compliance testing has been achieved and no further migration assessment or testing is necessary. 

Assessment by mass balance considerations under equilibrium conditions 

The use of these diffusion models to progress the evaluation process of a food packaging plastic will be discussed shortly. In those cases where assessment by mass balance considerations under equilibrium conditions, including partitioning effects, does not provide a clear picture of the plastics conformity status, then the different diffusivities of polymer types and the influence of the migrant molecule size or its molecular weight on its mobility within a plastic can be taken into account to achieve more distinguished views on QM/SML ratios. 

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An interesting extention of this methodology using chiral crotyl silane 474 produces tetrahydrofurans directly as a result of 1,2-silicon migration and heterocyclization [152]. Under boron trifluoride etherate catalysis (non-chelation controlled conditions), cis-2,5-disubstituted tetrahydrofuran (475) is produced in 50% yield, whereas tin tetrachloride (chelation control) gives the ra 5-2,5-disubstituted tetrahydrofuran (476) in 75% yield. Stereoselectivities in both cases exceed 40 1. 

Many workers prefer to use double diaphragm cells that is, vessels in which the anode compartment is separated from the cathode by two frits in order to further minimize mixing by diffusion or migration. If desired, the volume between the anode and cathode compartments may be periodically or continuously purged with fresh electrolyte to completely eliminate any possibility of contamination by undesirable electrolysis products. An excellent cell of this type has been described by Meites (43) who also gives a number of methodological pointers for precise analysis using controlled-potential coulometry. 

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