Docosahexaenoic acid-d5 (DHA-d5) is intended for use as an internal standard for the quantification of DHA by GC- or LC-MS. DHA is a long-chain ω-3 polyunsaturated fatty acid (PUFA) found in fish and algal oils.¹ It comprises approximately 40% of total brain PUFAs and is abundant in grey matter and retinal membranes.² DHA typically represents 0.52-7.5% of human total plasma fatty acids. It is produced from α-linolenic acid (ALA) via a series of desaturase- and elongase-catalyzed reactions, resulting in a docosapentaenoic acid (DPA) intermediate, which is elongated, desaturated, and β-oxidized to produce DHA.³ DHA can be liberated from cellular membranes by phospholipase A2 (PLA2) and converted to numerous oxylipins, including specialized pro-resolving mediators (SPMs), which are produced by lipoxygenases and include D-series protectins and resolvins, as well as maresins, that regulate host defense and the resolution of inflammation.⁴ DHA has roles in several physiological and pathological processes, including neural development, cardiovascular diseases, obesity, and inflammation.^3,5