It was raining. Washed off.
The defendant’s supporters contend that no blood was detected on the cuffs and lower sleeves of Trayvon Martin’s two sweatshirts because the rain had washed away all of the defendant’s blood and the scrapings from only one fingernail were tested. They also claim that no blood was discovered on Trayvon’s hands because Trayvon covered them with plastic bags before he assaulted the defendant.
Let’s deal with the last claim first because it is the most ridiculous. Police routinely bag the hands of homicide victims to preserve trace evidence and that is what they did to Trayvon’s hands.
They also complain that Trayvon’s hands were not swabbed, but that is not surprising since the standard practice is to cut a homicide victim’s fingernails and examine them for trace evidence. Forensic scientists do this because trace evidence can transfer from a victim’s hands to another person, surface or fall off. Trace evidence, particularly blood and DNA gets trapped under fingernails and that is why fingernail cuttings are the preferred location to check for trace evidence.
They soak the fingernail cuttings in a solution of purified water overnight and spin it out in a centrifuge to obtain all of the cellular debris present. Then they examine the debris under a microscope. If they find intact cells, the extract the DNA and type it. They do not scrape one fingernail and throw the rest away. Whoever conjured up that explanation has absolutely no idea what they are talking about. It’s probably the same person who said Trayvon wrapped his hands in plastic bags before assaulting the defendant.
Pathetic and laughable nonsense.
What about the claim that rain washed away the blood and DNA?
That claim is almost, if not quite as ridiculous, because we are discussing whether the rain would wash away all blood present on fabric leaving no detectable trace amount. To understand why the rain would not wash away any detectable amount of blood, let us take a look at presumptive tests for blood and failed efforts to remove all detectable trace amounts of blood invisible to the human eye.
Experience has shown that it’s virtually impossible to clean-up blood spatter at a crime scene so that no DNA can be detected. And that is after using water, solvents, cleaning fluids and other assorted chemicals in multiple washings to remove all visible traces of blood. Even though the human eye cannot see trace amounts of blood residue, luminol will detect it. Luminol is so sensitive that it can detect the presence of blood in serial dilutions down to 1:100,000. If luminol can detect it, PCR testing certainly will type the DNA present.
Other chemicals used in presumptive testing for the presence of blood are leuchomalachite green, phenolphthalein, Hemastixs, Hemident, and Bluestarr. All are as equally sensitive to blood as Luminol except for leuchomalachite green (1:10,000). For more information, please read this Technical Note in the Journal of Forensic Science, published in 2006. JFS is a peer reviewed professional journal.
The defendant’s supporters do not realize that a complete DNA profile can be developed from the DNA contained in a single white blood cell by using the STR/PCR process (red blood cells do not have a nucleus). In practice, they require more than a single cell to avoid copying a contaminant DNA introduced extraneously into the testing process, but they do not need very much sample.