Blood is collected by a phlebotomist in two grey-top tubes that should contain a preservative and anticoagulant (grey top tubes are normally used in medicine for glucose-testing). Typically sodium fluoride is used as preservative. One tube is for the crime lab to test and the other is preserved for the subject’s independent testing purposes. At the crime lab, one tube is opened and the contents are poured into a clean container. A measured amount of blood and a type of alcohol (usually n-propanol) as internal standard are mixed together into a headspace vial, which is then crimp-sealed.