Semen contains citric acid, free amino acids, fructose, enzymes, phosphorylcholine, prostaglandin, potassium, and zinc. The average volume of semen produced in a single ejaculation varies from 2 to 5 ml. The semen from a single ejaculation may contain between 40 million and 600 million sperm, depending on the volume of the ejaculate and the length of time since the last ejaculation. Samples used for medical purposes are obtained by having the donor masturbate. If a sample cannot be produced without sexual intercourse, non-reactive condoms can be used to collect the semen.