Toxicology refers to the science of the chemical and physical properties of toxic substances. Samples from a body are analyzed for drugs or other toxic substances. The concentrations are measured and the substance's contribution to a death can be determined. This is done by comparing concentrations to lethal limits. The most common samples analyzed are blood, urine, kidney, liver, and brain. The samples are usually put through various tests, but the most common instrument used to quantify and determine a substance is gas chromatography-mass spectrometry (GC-MS). These instruments produce chromatograms of the sample, which are then compared to a database of known substances. In blood samples, the substance can usually be found, but in the liver, kidneys, and urine the metabolite may be the only substance that can be found. A metabolite is the broken down version of the original substance after it has gone through digestion and/or other biological processes. Substances can take anywhere from hours to weeks to metabolize and leave the body and have different retention times in different parts of the body. For example, cocaine can be detected in the blood for two to ten days, while it can be detected in urine for two to five days.
The results of post-mortem toxicology testing are  interpreted alongside the victim's history, a thorough investigation of the scene, and autopsy and ancillary study findings to determine the manner of death. 
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