  The relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ (PMN) has received attention due to
its potential applications as a piezoelectric when doped with PbTiO$_{3}$ (PT).
Previous results have found that there are two phases existing in the system,
one linked to the near-surface regions of the sample, the other in the bulk.
However, the exact origin of these two phases is unclear. In this paper, depth
dependant analysis results from negative muon implantation experiments are
presented. It is shown that the Pb content is constant throughout all depths
probed in the sample, but the Mg and Nb content changes in the near-surface
region below 100$\mu$m. At a implantation depth of 60$\mu$m, it is found that
there is a 25% increase in Mg content, with a simultaneous 5% decrease in Nb
content in order to maintain charge neutrality. These results show that the
previously observed skin effects in PMN are due to a change in concentration
and unit cell.
