Coenzyme Q-d (CoQ-d) is intended for use as an internal standard for the quantification of CoQ by GC- or LC-MS. CoQ is a naturally occurring quinone found throughout the body in cell membranes, primarily in mitochondrial membranes, with highest concentrations in the heart, lungs, liver, kidneys, spleen, pancreas, and adrenal glands. It is a component of the electron transport chain and participates in aerobic cellular respiration, generating energy in the form of ATP. In its reduced form, CoQ acts as an antioxidant, preventing the formation of reactive oxygen species. CoQ deficiencies have been associated with heart failure, hypertension, parkinsonism, mitochondrial encephalomyopathies, and other chronic diseases.