Accurate limits for the violation of the Principle of Equivalence have been found from the comparison of the redshifts of two identical nuclear species in different chemical environments.
Cite
@article{arxiv.gr-qc/9211005,
title = {M\"o\ss{}bauer null redshift experiment II},
author = {M. De Francia and G. Goya and R. C. Mercader and H. Vucetich},
journal= {arXiv preprint arXiv:gr-qc/9211005},
year = {2007}
}