Migration limits verification

The verification of compliance with the specific migration limits (SML) provided in the Directives may be replaced by the verification of compliance with the maximum permitted quantity of substance in the finished product (QM), which corresponds to the SML. The correspondence between the specified SML and the QM shall be established by a calculation based on adequate experimentation of the finished product or by the application of generally recognized diffusion models. However, if the limit of QM is exceeded, for a judgement of non compliance of the material or article, it is necessary to obtain a confirmation that the SML has been exceeded . 

A second, and more chemical, verification is due to Finke et al.,21 who also invented the descriptive phrase persistent radical effect and gave a prototype example to the extreme. The thermal reversible 1,3-benzyl migration in a coenzyme B12 model complex leads to the equilibrium of Scheme 9. Earlier work had shown that the reaction involves freely diffusing benzyl and persistent cobalt macrocycle radicals, but the expected self-termination product bibenzyl of benzyl was missing. Extending the detection limits, the authors found traces of bibenzyl and deduced a selectivity for the formation of the cross-products to the self-termination products of 100 000 1 or 99.999%. Kinetic modeling further showed that over a time of 1000 years only 0.18% of bibenzyl would be formed, and this stresses the long-time duration of the phenomenon. 

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