The quantitative analysis of carbon levels in thin layers of semiconductor materials is usually limited by poor detection limits. While several qualitative techniques for detecting carbon exist, secondary ion mass spectroscopy (SIMS) has been the primary tool for obtaining depth profiles of carbon in thin films. The presence of carbon compounds in the residual gas background of the SIMS instrument typically limits the carbon detection limits to ∼10 17 cm-3 under standard operating conditions. If the carbon contamination in material processing could be studied with 13C labeled compounds, detection sensitivities should be far better. We have shown here that with this approach and the judicial choice of the analyte ion As13C- the detection sensitivity can be lowered by more than two orders of magnitude. We have applied this technique to the study of the incorporation of carbon from isotopically enriched CH4, C2H2, and C2H 4 during the epitaxial growth of GaAs by the metalorganic vapor phase epitaxy technique. We find no additional incorporation of carbon into these layers when substantial amounts of the hydrocarbons are introduced into the growth ambient. The use of isotopically enriched sources, together with these improved SIMS techniques, permits the routine detection of carbon at very low levels without the interference present in the conventional SIMS approach.