L-Glutamine-d5 is intended for use as an internal standard for the quantification of L-glutamine by GC- or LC-MS. L-Glutamine is a non-essential amino acid involved in many biochemical processes. It is synthesized in vivo by glutamate and ammonia. It serves as a substrate for glutamine synthetase in neurons for the biosynthesis of the major excitatory and inhibitory neurotransmitters glutamate and GABA. L-Glutamine decreases adhesion of sickle red blood cells (RBCs) to human umbilical vein endothelial cells (HUVECs) when incubated ex vivo with patient-derived autologous plasma either alone or with LPS. It has commonly been used in cell culture media. Formulations containing L-glutamine have been used in the treatment of sickle cell disease.