We're not through. Laboratories performing LCN rely heavily on what are called “negative controls.” The following quote is from The Law Society of Scotland's publication at:
http://www.journalonline.co.uk/Magazine/52-2/1003857.aspx
“In forensic science the fact to be established is that the DNA profile originated from the material recovered from a crime scene or a suspect, not the investigator, the laboratory, packaging, or analytical instruments. A “negative control” is set up by simply processing a “blank” sample that has no DNA. All being well, this control will not show any DNA. The presence of DNA in the negative control illustrates that there has been a source of contamination in the analytical method. It does not, of itself, show where that occurred, merely that it has. The tradition over many years has been, for very sound reasons, that anything found in the “negative control” invalidates the analysis.
...even in a tightly controlled analytical procedure a significant number of supposedly negative controls give a positive result, i.e. they indicate the presence of DNA.”
Stefanoni apparently did not perform any negative controls with same system parameters as that used for the kitchen knife DNA. That's another deficiency.
And note here, the statement, “It does not, of itself, show where that (contamination) occurred, merely that it has.” When Sara Gino was pressed, under cross-examination, to say where and how Stefanoni contaminated the samples, she could not give a specific answer, but only cited, “the literature.” This was seen as a weak response. But how could she possibly know at what point in the handling or processing contamination by 50 picograms of material may have happened? This is something you test for the presence of, not something you can possibly see happen. Even if someone followed the sample with a microscope in real time, it would be virtually impossible to witness the inadvertent transfer of such a minute amount of material. Gino's response, that she could "cite the literature,” which lists typical sources of contamination to guard against, was correct.
But these negative controls do not check for all sources of contamination, they only check for contamination originating in the laboratory. If the kitchen knife were contaminated before being chosen at random from Raffaele's drawer, this type of control won't catch it. If it was contaminated while being picked up, while being transported, or anywhere else along it's path, this method will not show that. To do that, you would have to perform additional negative controls, say, on the other knives in the drawer, or from some other source, handled in the same manner as the kitchen knife. None of this was done. That's yet another, major deficiency.
[end quote]
I would also draw your attention to the comments about contamination and the need for negative controls. I sense a misunderstanding in some comments about how contamination may occur and may come back to this point in a few days if time permits.
Chris
