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Forensic analytical chemistry plays a crucial role in the identification of physical evidence , such as body fluids, tissues, and inorganic specimens (e.g., artificial fibers , accelerants, gun powder) found at crime scenes. Substance-specific metabolites, derived from the physiologic transformation of medications, illicit drugs , poisons, or alcohol, can be identified in blood , saliva , and urine. Blood and semen are also the preferred sources for DNA extraction from both victims and suspects, although saliva may also contain epithelial cells from the oral tract, from which DNA can be extracted. Therefore, body fluids may be used to detect drugs or other harmful chemicals present in the body or for DNA analysis.

Toxicological tests provide both qualitative results (the identification of substances present in the body) and quantitative results (amounts of substances present the body). Legal medicine also uses these tests for several other purposes related to public health, such as the determination of acceptable levels of toxins in the food and water supply, and determining the toxic potentials of prescription drugs and their interactions with other drugs.

Blood alcohol levels can be detected even after three or more hours after drinking. Metabolites of cannabis, LSD, and other hallucinogenic drugs persist in the system much longer, up to 72 hours, and are detected in cerebrospinal fluid, urine, blood, and other tissues. Blood levels of morphine derived from heroin injection and methamphetamine metabolites may be identified in several fluids and tissues, such as blood, urine, liver, muscles, and cerebrospinal fluids. The same is true for a variety of other chemicals and toxic gases as well as animal toxins, such as poison metabolites from venomous animals or insect bites.

Forensic investigators processing a crime scene must collect, condition, store, and transport body fluids, following strict technical and legal protocols to avoid contamination in order to guarantee credibility in courts. Evidence should be handled by the minimum possible number of personnel, preferably only two: the crime scene technician who collects it and the crime laboratory expert in charge of forensic testing. This is important because by law, all those who handled the evidence between the time of its collection and its analysis must be prepared to testify in court. This procedure is termed the "chain of custody." A chain of custody usually consists of the officer who seized the evidence and the forensic toxicologist, forensic chemist, or other officer who entered in direct contact with the substance before or after it was packaged (but not those who touched the outer sealed container). The validity and acceptability of the evidence may be questioned in court if this procedure is not properly followed and reported.

Biological fluids from victims and suspects are analyzed and compared for matches in cases of murder , rape, or in paternity tests. The first blood test involves usually ABO and other blood typing. Because this procedure only narrows the population of probable suspects, DNA analysis from blood, or semen, or other tissue may be necessary to establish a more precise match. Sometimes blood at a crime scene, especially near a victim, contains DNA from both the victim and the aggressor. DNA analysis is known as DNA typing or DNA profiling , and consists of several molecular techniques that screen specific segments of human DNA where certain characteristics are almost 100% unique in each individual.

DNA profiling is useful either to exclude a suspect or to identify one, through the comparison of the suspect's DNA with samples taken from the crime scene. In the absence of a suspect, the sample may be compared against DNA data banks such as CODIS , where thousands of DNA profiles of known criminals and suspects are recorded. DNA analyses do not always offer a 100% certainty in all cases, because DNA content in samples could have suffered degradation, or because the quantity of a sample is not large enough. However, the combination of several DNA tests, each one specific to a different segment or locus of the DNA molecule, may provide a unique pattern of matches that allows a high degree of scientific certainty as to whom it belongs. The probability that a person other than the suspect would display that same genomic pattern is about one in every five trillion individuals chosen from the population, with the exception of identical twins. Because the world population is around six billion, DNA profiling is a powerful identification tool.

see also Assassination weapons, biochemical; Blood spatter; Bloodstain evidence; Breathalyzer®; Chemical and biological detection technologies; CODIS: Combined DNA Index System; Cross contamination; DNA evidence, cases of exoneration; DNA profiling; DNA sequences, unique; DNA typing systems; Luminol; Medical Examiner; Narcotic; Paternity evidence; Rape kit; Saliva; Serology; Toxicological analysis; Toxicology; Toxins.