Measurements of the isotopic compositions of multiple elements (Mo, Zr and Ba) in individual SiC presolar grains were made by Resonant Ionization Mass Spectrometry. Grains were classified on the basis of their C, N and Si isotopic compositions. While the heavy element compositions of mainstream grains are consistent with model predictions from low mass (1.5--3 [special characters omitted]) stars of solar metallicity when viewed on single-element three-isotope plots, grains with anomalous compositions have been identified on multielement plots. These grains have compositions that cannot result from any neutron-capture process and can be explained by contamination with material of terrestrial composition. This contamination could occur within the host meteorite, or in the grain separation or sample preparation. Previous work, in which only one heavy element per grain was measured, was not able to identify contaminated grains, and all grains were interpreted as having compositions representative of their parent stars. Through the multielement measurements, contaminated grains were eliminated from consideration, resulting in a data set with a reduced spread on the three-isotope plots, corresponding to a smaller range of stellar ¹³C efficiencies in their parent stars. Due to the reduced spread of the data, the nature of the stellar starting material (previously interpreted as solar) is not certain. The constraint of the amount of ¹³C in the parent stars of the grains may aid in uncovering the mechanism responsible for the formation of ¹³C, the main neutron source for s-process nucleosynthesis in low mass stars.