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Old 12th November 2020, 08:10 PM   #521
Chris_Halkides
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negative result in the test for blood

Originally Posted by JTF View Post
2) A portion of that garage lit up like a christmas tree when sprayed with Luminol.
Luminol is only a presumptive test, and one that gives a wider range of false positives than other presumptive tests, such as the Kastle-Meyer test. The Kastle-Meyer test gives false positives in the presence of substances that catalyze the same reaction as a peroxidase. Luminol reacts in the presence of catalysts but also in the presence of oxidants. A positive result from a presumptive test indicates the possibility that a substance is present. A positive result is a reason to move on to a confirmatory test. This is (almost literally) Forensic Chemistry 101. McHrozni mentioned some of these problems in the first thread, in comment #3553. Academic references available upon request.

Mr. Ertl agreed under cross examination that the reaction was faint. The claim that it lit up like a Christmas tree is, putting it charitably, an exaggeration, perhaps originating with Mr. Kratz's closing remarks. From what I can gather Mr. Ertl also acknowledged that he tested for blood and that the result was negative. A quick perusal of the trial testimony leaves me with the impression that Mr. Ertl used the phenolphthalein test (the Kastle-Meyer) test. This test is capable of detecting blood that has been diluted by a factor of 1000 or more, although it is still only a presumptive test. Only a positive result from a confirmatory test is conclusive evidence for the presence of blood.
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Last edited by Chris_Halkides; 12th November 2020 at 08:27 PM.
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