Scattering by an isotopic mixture with two solutes

Equation (7.2.60) applies also to a mixture of two isotopic varieties D and H of the same monodisperse species. 

let us admit that the solvent (,e/ = 0) is unique. Thus, the cross-section per unit volume is 

we can replace the unique solvent by a binary isotopic mixture the composition of which is given by 

This is the result we were looking for and (7.2.62) is analogous to (7.2.60). (In the polydisperse case when the deuterated and non-deuterated chain molecules have the same distribution, the first term between braces is replaced by 

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