  The plasma composition of the solar corona is different from that of the
solar photosphere. Elements that have a low first ionisation potential (FIP)
are preferentially transported to the corona and, therefore, show enhanced
abundances in the corona compared to the photosphere. The level of enhancement
is measured using the FIP bias parameter. In this work, we use data from the
EUV Imaging Spectrometer (EIS) on Hinode to study the plasma composition in an
active region following an episode of significant new flux emergence into the
pre-existing magnetic environment of the active region. We use two FIP bias
diagnostics: Si X 258.375 A/S X 264.233 A (temperature of approximately 1.5 MK)
and Ca XIV 193.874 A/Ar XIV 194.396 A (temperature of approximately 4 MK). We
observe slightly higher FIP bias values with the Ca/Ar diagnostic than Si/S in
the newly emerging loops, and this pattern is much stronger in the preexisting
loops (those that had been formed before the flux emergence). This result can
be interpreted in the context of the ponderomotive force model, which proposes
that the plasma fractionation is generally driven by Alfv\'en waves. Model
simulations predict this difference between diagnostics using simple
assumptions about the wave properties, particularly that the fractionation is
driven by resonant/non-resonant waves in the emerging/preexisting loops. We
propose that this results in the different fractionation patterns observed in
these two sets of loops.
