I think anybody who's ever run a spectral analysis can say that unless you have 100 grams of a 100% pure substance before you do your sample prep and then your injection there is no way that you can get the spec to tell you that white poweder you just analyzed is 100% cocaine. The highest percent certainty I obtained from running a GC-MS library on an unknown was like 50 and it was accompanied by a host of other substances that the unkown could also be at lower percentages.
So many people probably think that Instrumental Analysis is easy peasy, they would never think about the amount of time you have to take doing sample prep, running your 50 samples and 10 standards, looking through all those data readouts, doing you final calculations and making some sort of conclusions...TV would have you think the computer makes the final decision and not the scientist!
Ahhh...GREAT discussion topic. Unfortunately, I am reminded of this each time I tell people that I have a Forensic Chemistry degree. They say "Oh like CSI!?" My typical response is "That is what everyone says, but no."
I find a handful of discrepancies between real life Forensics and the media's portrayal of it. Although the speed at which the actors perform the analyses is a large misconception, I am not sure if what pesters me more is 1.) the fact that the actors always seem to find that ONE key piece of evidence which makes or breaks the case or 2.) the fact that one scientist seems to do the job of six or 3.) how the crime scene is processed?
1.) The fact that the actors can find THE key piece of evidence is such a misconception. I find it quite amusing how they will be at the crime scene with just gloves on, zoom into a bullet casing, pick it up and then go analyze it - to of course provide them with the results they were looking for. I think to myself "IF only it were that easy".
2.) One person performs a variety of different analyses (e.g. firearms & toxicology & trace). In reality, those three aspects would be performed by different analysts. People educated about Forensic Chemistry/Science are well aware that there are specific disciplines within the field of Forensics. One analyst will not test a carpet stain to determine it's identity as well as test whether a spent shell casing came from a specific gun and analyze a tire impression to see if it matches a suspect's vehicle and compare a fiber collected to a victim's shirt.
3.) As for how many crime scenes are processed, at least on TV, I am just in disbelief. The actors walk in, sometimes guns drawn, and just put on gloves (no booties, hair nets, etc.) with no chain of custody or documentation being recorded. I think this is an extreme misconception, especially since the chain of custody is (one of) the most important things for a piece of evidence to be admissible in court.
Of course everything is highly dramatized to entertain a broader audience but I do think that some of these fallacies could be better represented.