The eicosapentaenoic acid (EPA) Quant-PAK has been designed for the convenient, precise quantification of EPA by GC- or LC-MS. It includes a 50 µg vial of EPA-d5 and a precisely weighed vial of unlabeled EPA, with the precise weight (2-4 mg) indicated on the vial. This unlabeled EPA can be used to quantify the EPA-d5 standard by constructing a standard curve of peak intensity ratios (deuterated versus unlabeled).