microscopic analysis, of which a number of different types are available depending on the type of material to be analyzed, such as a stereomicroscope, scanning electron microscope (SEM) or comparison microscope. SEM is especially useful because X-ray analysis can be conducted on selected areas of the sample, so is a form of microanalysis. It is useful where chemical residues can show unusual elements present which may indicate a chemical attack of the product. A car accident caused by a diesel fuel leak, for example, showed traces of sulfur on the cracked tube indicative of an attack by sulfuric acid from the battery. Gunshot residue may be identified by elemental analysis using atomic absorption or with a scanning electron microscope equipped with an energy dispersive spectroscopy. Small amounts of explosives, volatile hydrocarbons, and other chemicals are identified with the use of analytical instruments, such as gas chromatography, mass spectrometry, and infrared spectroscopy, all of which separate out the components of the chemicals. Similar comments apply to damaged items from an accident scene, but care is needed in ensuring that the sample is not damaged by the testing or sampling for testing. Such nondestructive testing must always be used first before considering destructive methods which involve taking small samples from the item for more detailed tests, such as spectroscopic analysis. Use of all such methods must be done in consultation with other experts and the relevant authorities, such as lawyers on both sides of a case. Problems False positives and contamination by subsequent handling or nearby objects (e.g. mixing of blood from victim and attacker), for example, are problems owing to the presence of many common substances and the necessity of human involvement in the collection of trace evidence. Both can occur with DNA traces and fingerprints, and therefore evidence should be collected, analyzed, and presented in accordance with established guidelines. Partial fingerprints are even more vulnerable to false positives. Samples from accidents or crimes should therefore be protected as much as possible by enclosure in a sealable container as soon as possible, after an incident is under investigation. See also Evidence packaging Forensic chemistry Forensic engineering Forensic materials engineering Forensic polymer engineering Forensic photography Forensic profiling Forensic science Locard's exchange principle, which states that when two objects come into contact, there is an exchange of material Skid mark Digital traces Blowback, material drawn into the barrel of a firearm post discharge