(±)5-iPF2α-VI-d11 contains 11 deuterium atoms at the 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, and 20 positions. It is intended for use as an internal standard for the quantification of (±)5-iPF2α-VI by GC- or LC-mass spectrometry. Isoprostanes are prostaglandin-like products of free-radical induced lipid peroxidation.¹ Although the isoprostanes derived from arachidonic acid are the best characterized, many other polyunsaturated fatty acids can form isoprostanes.² iPF2α-VI is one of dozens of possible stereo- and regioisomeric isoprostanes which can be formed from arachidonic acid. To date, the most extensively studied of these is 8-isoprostane (8-epi-PGF2α, iPF2α-III).^3,4 However, 8-isoprostane is a minor isoprostane constituent when compared to some of the other isomers which form in natural conditions of oxidative stress,5 including iPF2α-VI of the type-VI isoprostanes. This class has been shown to be one of the major isoprostane products, in contrast to 8-isoprostane. In addition to being produced in greater abundance than 8-isoprostane, Type VI isoprostanes form internal lactones which facilitate their extraction and purification from biological samples.^5,6,7,8