AT@C-60, synthesized by heating primarily C-60 (with trace amounts of C-70) under a high pressure of argon gas, was purified by high-performance liquid chromatography (HPLC) and examined by Fourier transform mass spectrometry (FTMS). The sample, which consisted of 0.1% Ar@C-60, was dissolved in toluene and separated with HPLC using a COSMOSIL, 5PYE column as the stationary phase and toluene as the eluent (UV detection). AT@C-60 was enriched by a factor of about 400 relative to the starting material. AT@C-60 was also examined by X-ray photoelectron spectroscopy (XPS) but was found to be sensitive to radiation damage and subsequent depletion. Low-energy collisional dissociation of Ar@C-60(+) in the FTMS yields primarily C-58(+), generated by loss of the argon atom and C-2. The ionization potential of Ar@C-60 was bracketed by charge-exchange reactions and was found to be between 7.53 and 7.8 eV, which is indistinguishable from that of C-60 (7.65 eV).