Normal L-glutamine has a relatively short half-life in solution, showing a temperature dependent loss of product with time. L-glutamine degrades through an intramolecular cyclization reaction forming as by-products, pyrrolidone carbonic acid and ammonia in equimolar concentrations. The rate of breakdown increases with increasing temperature and in the presence of phosphate or bicarbonate.
Therefore, never premix your glutamine in solution, and always store it in a dark container. Especially don't keep your glutamine in sunlight, as ultra violet light causes mutations and denaturization.