The composition of gallium beams originating from an effusion source in an ultra-high-vacuum system is monitored with a mass spectrometer mounted in cross-beam geometry. Here, thermal cycles comparable with operating conditions as in an epitaxial growth chamber can be simulated and beam components with partial pressures of 10(-13) Torr can be reliably detected. Using different gallium charges, particular focus was placed on gallium-oxygen- and gallium-hydrogen-compounds, whose signatures fade after a few thermal cycles, but do not disappear entirely. No other impurities have been detected in the same sensitivity range. In parallel, employing Ga loads from the same batches, high-purity simple GaAs/AlGaAs heterostructures exceeding low-temperature electron mobilities of 10(7) cm(2)/V s were grown in a separate system. Mobility versus density dependence of these high-mobility samples points to the simple architecture of the heterostructures as being the current limit of the electron mobility. The Ga analysis suggests the Ga2O and GaH3O are the main reasons for background impurities in the very early stages of a growth campaign. (C) 2008 Elsevier B.V. All rights reserved.