a crime scene ( Bell , 2019 ). It is sprayed over a large area , in a darkened room and forms a blue colour to indicate that blood is present . It is also known as 3-Aminophthalhydrazide and 5-amino-1,4- dihydroxyphthalazine and 5-amino-2,3-dihydro-1,4- phthalazinedione in the literature ( Huntress , Stanley and Parker , 1934 ), and is considered to be one of the most sensitive blood detection techniques ( with sensitivities of one part per million ( Shimamoto , DeFrance and Adair , 2013 )) that is available for use in forensic investigations ( Cassidy et al ., 2017 ).
Luminol was first reported in 1902 ( Huntress , Stanley , and Parker , 1934 ) and has been used by forensic investigators since 1937 ( Adair and Shaw , 2005 ). Luminol exists as a crystalline solid that is dissolved in sodium hydroxide to make a stock solution and mixed with hydrogen peroxide and distilled water to make a working solution . The hydrogen peroxide acts as an oxidising agent and when in contact with blood , the ferric haem groups ( the iron part of haemoglobin ) decompose the hydrogen peroxide and act as catalyst for the oxidation of luminol ( Cassidy et al ., 2017 ). When this occurs , luminol releases energy in the form of bright blue light ( Bell , 2019 ) and this forms a chemical reaction known as chemiluminescence . This chemiluminescence is a result of an excited molecule that is produced due to the oxidation of luminol . The surplus energy in the molecule is released as a light photon in the visible region of the electromagnetic spectrum ( Bell , 2019 ). Once the area of interest is found , swabs of the luminescent area are taken and tested with confirmatory tests such as Kastle Meyer or leucomalachite green to establish the presence of blood ( Brenzini and Pathak , 2018 ).
However , luminol is prone to false positives whereby it can be catalysed by something other than blood and cannot differentiate blood between different species . For example , household and industrial chemicals such as bleach contain hypochlorite ions that catalyse the reaction thus causing a false positive ( Quickenden and Cooper , 2001 ). As such , further analysis with tests such as RSIDTM ( Rapid Stain Identification-Blood ), are used to establish that it is in fact blood that is present in the crime scene . This test , the RSIDTM , is dependent on the existence of a glycoprotein ( a form of protein ) called glycophorin A that transverses the membrane of erythrocytes ( red blood cells ) ( Howard , Chaseling and Wright , 2019 ).
Figure 2 is a schematic that shows the chemical reaction that takes place when luminol comes in contact with blood . The left-hand side of the reaction shows luminol ( C8H7N3O2 ) being activated by the oxidising agent hydrogen peroxide ( H2O2 ). The hydrogen from the two NH groups in the phthalate ring is removed , the two nitrogen
Blood was added to the middle of the swatch from the NH group leave the ring as nitrogen gas , and two oxygen molecules are added by the hydrogen peroxide to the areas previously occupied by the two NH groups ( Brenzini and Pathak , 2018 ). This forms 3-Aminophthalate , the middle structure , an excited and unstable molecule that has two negative electrons due to an electron moving from a low energy level to a higher one . The final part of the reaction shows 3-Aminophthalate emitting a bright blue light due to the molecule returning back to its ground state . This bright blue light is known as chemiluminescence . In order to view this chemiluminescence , a fully blacked out room is required . An example of this reaction between blood and luminol can be seen in Figure 1 , imaged as part of this project , where blood was added to a 100 % cotton denim swatch .
Experimental
a . Aims
1 . Firstly , this project investigated the sensitivity of luminol in detecting latent bloodstains on two different types of fabrics after being subjected to machine washing once , twice and three times .
2 . Secondly , this project investigated the role of non-biological and biological laundry capsules on removing bloodstains .
3 . Thirdly , the study aimed to obtain quantitative information on chemiluminescence using the number of pixels that gathered from pictures taken of the luminol reaction .
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