Quantitative liquid chromatography electrospray mass spectrometry (LC-MS) analysis for pesticides in a complex environmental matrix using external and internal standard calibration was investigated. Various approaches to introducing different internal standard compounds to address quantitative errors associated with signal suppression were also examined. The study involved the analysis of pesticides in wheat hay matrix samples using three kinds of internal standard compounds: deuterium labeled (D3), carbon-13 (singly labeled), and structural analogs (derivatives) of the target analytes. Introduction of the internal standard by volumetric addition and direct post-column infusion were also studied and compared.