You're mistaken, there's a huge difference between identifiable substances which can be linked to a murder and trace elements that can get transferred much easier and are far more prone to contamination and require far more stringent laboratory environments and collection methods.
Here's something to read if you want to educate yourself on the topic. I look forward to the discussions that can occur when you actually know something about it.
What would make you say that? If you know that some safeguards,
any safeguards are required to prevent contamination, what would you think of a technician who left samples out for six weeks in a room filled with items and DNA where a murder had occurred and you knew the door to that room was broken as well as a window to the outside.
You are aware that not every speck of DNA in the house was identified, right? They went over the murder room pretty good--
twice. That's because a murder had been committed in there and then the door locked, and it discovered some ~14 hours later that's actually decent conditions for preserving the scene. Outside that room they can't and don't get everything, think of it: there would have been nothing discoverable when they went back.
Why was there any need to go back?
The prosecutor and the flying squad made international news with their arrest of Amanda and started a press frenzy, their arrest of Patrick only caused something on the order of twenty people to come forward and alibi him within
days. They still dragged their heels for a total of two weeks before releasing him, it wasn't until Rudy was successfully reeled in that they ever let Patrick go.
Letting Amanda and Raffaele go would have revealed that they were entirely incompetent and might have caused some to ask questions as to why they ever arrested them in the first place on such an incoherent 'witness' statement from Amanda Knox. Prosecuting Raffaele and Amanda and blaming
her for their arrest

rolleyes

fed into the narrative of Amanda being a demoness from hell who could manipulate males do her will, and they thought she would confess anyway or that they'd find the evidence they'd need to prosecute her. Once it became apparent she wouldn't confess and they couldn't find real evidence of murder, they took a pile of stinking slimy goose nuggets into court and pretended it was 'evidence' just 'cuz they said so.
After all, some have absolutely no clue that LT/LCN DNA isn't what convicts people in almost all countries of the world and isn't what people think of when they say 'DNA evidence.' They don't realize that 'mixed DNA' from people that
live together outside the murder room isn't evidence of much of anything, nor is mixed DNA that relies on the known murderer (and that's what Rudy is now) using fixtures and items used on an everyday basis by someone who
lives there is either.
I've got a question for you. Since 2008 the defense has demanded they receive the full data for the DNA analysis,and the rest of the forensics for that matter. Stefanoni dragged her heels and only released some of it on July 30th, 2009. That revealed a plethora of interesting things that had been denied the defense such as the (blood test) TMB negatives and the information to divine that there were about five total (including Meredith's) contributors to the bra clasp. However it was not all the information needed, thus the defense sought the entire time between the Trial of the First Instance and the Appeal to receive that information, and to have
independent experts look at the DNA evidence at the very least. That was granted by the appeals judge in about November of '10, and Profs Conti and Vecchiotti demanded the further information that had been denied the defense.
Stefanoni refused to comply, and the judge intervened and demanded she turn it over. She refused until the last instant, requiring a month long delay in the experts finding, and they never did receive the electronic data files, but as they put it (something to the effect of) 'we have all we need.' They sure did as it turned out, they practically whipped it out and watered down Stefanoni's DNA work, completely unreliable is what they found. Some of the comments were
scathing.
However, they never did receive those EDFs, the most important and revealing record of her DNA work. Why do you suppose Stefanoni refused to allow anyone to see those files, and now that the DNA work has been completely discredited why do you suppose she didn't produce them in order to try to defend herself in court?
What was Patrizia Stefanoni trying to hide?