  For the International Large Detector (ILD) at the planned International
Linear Collider (ILC) a Time Projection Chamber (TPC) is foreseen as the main
tracking detector. To achieve the required point resolution, Micro Pattern
Gaseous Detectors (MPGD) will be used in the amplification stage. A readout
module using a stack of three Gas Electron Multipliers (GEM) for gas
amplification was developed at DESY. In a test campaign at the DESY II Test
Beam Facility the performance of three of these modules was investigated. In
this contribution results were presented on the particle identification
capabilities of the system using the specific energy loss (dE/dx). The results
from the prototype were used to extrapolate to the performance of the full ILD
TPC, where a dE/dx resolution of better than 5 % could be achieved. In
addition, simulation studies were performed to optimise the readout pad size
for improved dE/dx separation power. These studies also investigated the
possibility to measure the deposited energy by counting the number of
ionisation clusters (cluster counting). For small enough pads this approach was
found to give similar or better performance compared to the traditional method
of measuring the deposited charge.
