We propose to microscopically study the role of defects on spin transport in one of the best approximations to the ideal antiferromagnetic Heisenberg s = ½ chain. By measuring the longitudinal muon relaxation in magnetic fields we will study the processes governing the dynamical properties. A range of temperatures and fields will be used to identify whether there are any crossovers between different transport regimes and to see if they are related to the new pseudogap energy scale which appears upon introduction of impurities.