Luminol and the revealed footprints

This post talks about the possibility of the Luminol footprints being made in bleach instead of blood. For information about the footprints revealed in luminol, check out

The problem with establishing a prosecution case: Prints compatible with Knox are only revealed in luminol. Only three such footprints were revealed, in a path pointing away from each other; one going into the victim’s room, two in front of Knox’s door going  Knox’s room. There is no publicized prosecutoral evidence of footprints in the victim’s bedroom.  If other bloody prints were cleaned in the hallway, they were cleaned better then the ones revealed by luminol.

Are the footprints proven to be blood if revealed by luminol? No, not without additional testing of the substance, which can be difficult if the substance has been so diluted that it can only be revealed with Luminol.  According to one article: “Because it has several drawbacks as a presumptive test for blood, spraying luminol at a crime scene should be an investigator’s last resort for detecting blood.” It goes on to state, “The luminol reaction is at best a presumptive test for blood. If the stain is so diluted that it can only be visualized with luminol, then no further analysis can be performed to confirm the presence of blood.” On the other hand, “experienced forensic investigators know the difference based on how quickly the reaction occurs. After the Luminol test, forensic investigators still need to run other tests to verify that it is really human blood.”

Therefore, luminol does not prove if a print is blood. This is because Luminol also reacts to other substances, including bleach. As will be shown below, in the absence of  enough of the substance to test for blood, the person conducting the test could testify that based on the reaction, it is their professional opinion that the substance is blood.  However, even with the expert testimony, it is not proven to be blood.

I’ve been looking into the interactions of Luminol, bleach, and blood, since reading a theory put forth in the perguia-shock blog Luminol not only reacts to blood, but it also reacts to bleach. The theory was that the footprints that were revealed may have been the result of a clean foot pressing down on a cleaner of some type that included bleach,  instead of signs of a footprint in blood. More after the break:

The existence of bleach at a crime scene can interfere with the luminol test, because the bleach will react with the luminol. In an extreme case, where an entire scene is cleaned with bleach, luminol will react with the bleach present. From  wikipedia:  “Luminol chemiluminescence can also be triggered by (…) certain bleaches[citation needed]; and, as a result, if a crime scene is thoroughly cleaned with a bleach solution[specify], residual bleach will cause the entire crime scene to produce the typical blue glow, effectively camouflaging any organic evidence, such as blood.”  How then can the test be used to detect blood or other substances that have been cleaned with bleach?

A study on porous surfaces, like tile, implies a long drying period is necessary: “The results indicate that the drying method may very well overcome household bleach interference in luminol reaction tests, if the investigation allows for an appropriate waiting time.” According to one study, even blood that was cleaned with bleach can still eventually produce a glow similar to the one produced by the direct detection of blood: “However, when the tiles were cleaned with bleach there was an initial drop in chemiluminescence intensity, followed by a rise to a consistently high value, visibly indistinguishable from that of blood.” They also concluded, “Examination of bleach drying time suggested that any interfering effect becomes negligible after 8 h”

I don’t know if there is a standard operating procedure for when a luminol test is done at a scene if it is necessary. There are other factors that can affect this also; temperature can obviously be a factor in the drying time, and also effects the rate the bleach itself breaks down over time

Going back to the theory that the prints were bleach residue in a porous tile, the prints made with a cleaner containing bleach would have to be relatively recent to still be detectable by luminol. According to the study above, after about 8 hours the bleach wouldn’t be detectible. A lack of heat in the apartment in November would have significantly delayed the drying time of any potential bleach.  The timeframe the luminol test was done is important.

The scenario would be that after someone mopped the floor with a bleach mixture, someone ‘hopped’ (say, into the victim’s room) or stepped out unknowingly (from Knox’s room), but it would have had to have been in fairly recently. For this theory,  both sides could then argue that their client didn’t do the cleaning. For the defense, in the absense of any statements from the residents, the only feasible story would be that the victim cleaned the apartment herself that night, and did those actions herself.  For the prosecution, the theory would be the defendants/perpetrators of the crime did the cleaning, and left the footprints by stepping in the water/bleach after cleaning the floor, which, incidentally, puts them very close to the crime. 

Can an expert tell the difference between a bleach result and a blood result? In one study, a test of 250 substances revealed  “only a small number which produced” a glow similar to blood, including bleach. But they reported,  “The presence of hypochlorite-based bleaches on non-porous surfaces being sprayed is sometimes recognizable and can be identified by an experienced forensic practitioner as it leads to bright flashes of chemiluminescence as opposed to the more gradual development of chemiluminescence by blood.”

The problem is that still missing are footprints leading into the bloody print in the bathroom, and that the two footprints in the hallway are of a full foot. Could there have been a clean-up of some prints, but didn’t fully get the prints in the hallway? According to the study just mentioned,  “They observed that when a person attempts to remove bloodstains by washing the area with water or sodium hypochlorite solution, depending on the thoroughness of the clean, the effect on the luminescence spectrum could range from the complete absence of emission to various combinations of blood-initiated emission and hypochlorite-initiated emission (each peaking at its separate respective wavelength) which might be expected if the cleaning process is not complete.”  

One other study states that there is a compound that can be added to the luminol test to counter the reaction with bleach, but still react with blood; and also mentions that a sufficient drying time will counter the bleach as well:

“(…) that addition of 0.1 M 1,2-diaminoethane to Grodsky’s luminol formulation reduced the bleach interference on all substrates and only slightly affected the blood chemiluminescence. This solution is now included in the ESR protocol for luminol use if there is evidence that bleach may have been used at a crime scene. (…) Alternatively, if the crime scene is such that it can be left to dry for a few days (e.g., a car interior) then this delay will reduce or remove the
bleach interference.”

So, in the end, we’re left with prints compatible with Knox that reacted to luminol in the hallway.

The two theories remaining then are:

1. The footprints in the hallway are in a cleaner that contained bleach; however this could be 1-determinable by those experienced with using Luminol, and countered depending on how long after the crime the luminol test was done.

2. The prints are in the victim’s blood, but not provable via the Luminol test to be blood. Some prints were cleaned better then the ones in the hallway; perhaps by running out of bleach or forgetting to go over an area after an initial swipe.


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