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Gas chromatography ("GC") and mass spectrometry ("MS") make an effective combination for chemical analysis. This article serves to demonstrate tools for an effective attack or defense of GC/MS evidence. To effectively use GC/MS evidence one must understand the process. First, the GC process will be considered, then the MS instrument will be presented. After a background in GC and MS is obtained, the reader will learn how to analyze the evidence produced by these instruments. The focus of this article lies in presenting the limitations to GC/MS analysis