A compact, versatile, and simple rf plasma source with capacitive coupling compatible to ultrahigh vacuum (UHV) requirements was designed and built to allow sequences of sample surface modification in plasma and surface preparation and analysis in vacuum without breaking the vacuum. The plasma source was operated at working pressures of less than 1 to a few millibars. Sample transfer to UHV was performed at pressures around 10-9mbar. For easy integration into an existing UHV setup, the sample recipient and transfer system were made to accept standard commercial sample holders. Preliminary experiments were performed by exposing monolayers of colloidal CoPt3 nanoparticles to oxygen and hydrogen plasmas. The structural and chemical effects of the plasma treatments were analyzed with scanning electron microscopy and x-ray photoelectron spectroscopy.