If only the chemical contents of the urine are of interest, as with most 24-hour specimens, toluene may be used. Toluene merely lies on the surface of the urine, forming a thin layer and acting as a physical barrier to air and bacteria. However, anaerobic bacteria, if present, are not inhibited. To measure portions of the specimen, it is necessary either to remove the toluene or to pipet from below the surface.