The Kastle-Meyer test is an alleged blood test, first described in 1903, in which the chemical indicator of phenolphthalein is used to detect the possibility of hemoglobin presence. It relies on peroxidase activity-such as hemoglobin in the blood to catalyze the oxidation of phenolphthalin (the colorless reduction form of phenolphthalein) into phenolphthalein, which is seen as a bright pink color. The Kastle-Meyer test is a form of catalytic blood test, one of the two main classes of forensic testing commonly used by crime laboratories in the identification of blood chemistry. Other test classes used for this purpose are microcrystalline tests, such as the Teichmann crystal test and the Takayama crystal test.
The test was named after the American agricultural chemist Joseph Hoeing Kastle (1864-1916), who in 1901, discovered and tested the raw blood test, and German physician and chemist Erich Meyer (1874-1927), who modified the test. in 1903.
Video Kastle-Meyer test
Metode
The assumed blood sample was first collected with cotton. A drop of phenolphthalein reagent is added to the sample, and after a few seconds, a drop of hydrogen peroxide is applied to the cotton. If the swab turns pink quickly, it is said to test the positive assumption for blood. Waiting for periods longer than 30 seconds will produce most pink colored swabs naturally as they oxidize themselves in the air.
Optionally, cotton may be first treated with a drop of ethanol to lyse the existing cells and obtain increased sensitivity and specificity. This test does not damage the sample, which can be stored and used in further testing in the laboratory; However, some laboratories will use the cotton used for the Kastle-Meyer test in further testing, choosing to use fresh swabs from the original stains.
Maps Kastle-Meyer test
Limitations
While the Kastle-Meyer test has been reported to detect blood dilution to 1:10 7 , there are a number of important limitations for the test. Chemical oxides such as copper and nickel salts will cause the Kastle-Meyer reagents to turn pink before adding hydrogen peroxide, so it is important to add reagents first, then wait a few seconds, then add hydrogen peroxide.
The Kastle-Meyer test has the same reaction with human blood as it does with other hemoglobin-based blood, so confirmatory tests such as the Ouchterlony Test should be done to infer definitively from where the species of blood originated.
Color catalytic test is very sensitive, but not specific. Positive color tests alone should not be interpreted as positive evidence of blood. The general negative result is evidence of no detected heme quantity, but a false negative can be generated in the presence of a reducing agent. The test can not provide specific evidence of what is in the blood.
Mechanism
The phenolphthalein used in this test has been modified from its conventional form, which has been reduced by two electrons and dissolved in an alkaline solution. This is usually achieved by boiling the alkaline phenolphthalein solution with a zinc powder, which reduces phenolphthalein to phenolphthaline. After reduction, a very intense pink color of the cationic form of phenolphthalein fades into a pale yellow color. This is a form of phenolphthalein present in the Kastle-Meyer test kit. To produce an intense pink color that indicates a positive test, reduced phenolphthalein must be oxidized back to its normal and colored form.
In the relevant reaction, hydrogen peroxide reacts with hemoglobin in the blood. Phenolphthalin does not directly participate in this process; instead, it acts as a source of external electrons. In reaction with hydrogen peroxide, the heme hemoglobin center behaves as peroxidase, reducing peroxide to water. This activity consumes electron hemoglobin which, in turn, is supplied back by phenolphthalin. Donation of electrons to hemoglobin converts phenolphthalin back into highly colored phenolphthalein. As long as the enzyme survives, the heme reaction with peroxide is catalytic, making this test very sensitive to the small amount of blood present in the swab test. The reduced peroxide of the hemoglobin-catalysts is shown in the reaction below. Both electrons are supplied by phenolphthalin:
- HOOH 2 e - 2 H -> 2 H 2 O
Consumption of protons during the reaction has the effect of increasing the pH of the solution, but the amount of base produced is negligible compared to the amount of base already present in the reagent mixture.
References
Other sources
- Culliford, Bryan J., Examination and Typing of Bloodstains at the Crime Laboratory, Washington, D.C.: US Government Printing Office, 1971.
- Gaensslen, Robert E., Sourcebook in Forensic Serology, Immunology, and Biochemistry, Washington, D.C.: US Government Printing Office, 1983.
- Kirk, Paul L., Crime Investigation, John Wiley and Son, 1974.
- Metropolitan Police Forensic Science Laboratory, Manual of Biological Methods, 1978.
- Ponce, Ana CastellÃÆ'³; Pascual, Fernando A. VerdÃÆ'º, "Critical Revision of the Blood Test for Estimates," Communication Science Forensics, vol. 1, No. 2, July 1999, pages 1-15.
- Saferstein, Richard, The Forensic Science Handbook, Prentice Hall, Inc., 1982.
Source of the article : Wikipedia
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