Analysis of silicon oxide (SiO2) desorption was performed by a combination of thermal annealing in an ultra high vacuum (UHV), consecutive growth of a thin silicon (Si) layer by molecular beam epitaxy (MBE) and ex situ secondary ion mass spectrometry (SIMS). Together with the Si substrate the Si-MBE layer creates a uniform Si matrix which allows quantitative analysis by SIMS. The protective oxide on as-received wafers stored for a few months turned out to be 0.8-1.6 nm thick. Desorption in a vacuum takes place in a narrow temperature window below 875 degrees C. The main residual contaminations were carbon (C) with a surface concentration of about 10(12) cm(-2) and boron (B) with a concentration of 6 x 10(10)-6 x 10(11) cm(-2). Metallic impurities were below the SIMS detection limit.