"enriched in" vs "enriched with"

Solution 1:

ODO includes the following different subsenses for enrich:

enrich [verb]

...

1.1 Add to the nutritive value of (food) by adding vitamins or nutrients.

porridge oats enriched with extra oat bran

[M-W has The drink is enriched with vitamin C.]

1.2 usually as adjective enriched Increase the proportion of a particular isotope in (an element), especially that of the fissile isotope U-235 in uranium, so as to make it more powerful or explosive.

enriched uranium

[M-W has the more general enrich ... to add or increase the proportion of a desirable ingredient] ...

Uranium for the first atomic bomb and for nuclear reactors was enriched in the 235 isotope, as compared to the more abundant 238 isotope, by gaseous diffusion.

So 'enriched with X' means X has been added, often where there was none originally;

'enriched in X' means the proportion of X [already] in the sample etc has been increased.