Forum: Author Hangout

It was my understanding that they used luminol. I don't know how luminol is produced, so Phenolphthalein may be used in luminol.

That's what I recall, but while searching, about all I could come up with were rudimentary chemical tests performed in high school labs. Phenolphthalein is a straightforward chemical test which turns different colors when exposed to different biological markers, while (I believe) luminol causes dried blood (and other fluids, like semen) to show up under UV light.

What I'm going for is: someone is seen waving a bloody knife, the police pounce, but then cannot find any evidence (with only a cursory view) of any blood.

The natural follow-up would be to pull out a quick test, but if there's no evidence of a crime, they couldn't send it to a lab for a followup investigation.

What I'm going for is: someone is seen waving a bloody knife, the police pounce, but then cannot find any evidence (with only a cursory view) of any blood.

The concept of a bloody knife that has no blood sounds like a flaw. Unless of course, you intended to have some other red substance on the knife that was misidentified as blood.

CW's description of his scene included the patrolman pulling out his test kit and testing for blood and finding none. It also suggested that the knife wielder did not have time to properly clean the knife of all traces of blood.

Visible blood would probably be enough for probable cause to seize the knife and arrest the wielder even without a specific field test, unless the knife wielder has a reasonable explanation for having a knife covered in blood (it's animal blood and he's a butcher, a hunter who just field dressed a kill, hey we're shooting a movie here, it's fake blood.)

Don't forget, we're talking sci-fi here, so there are alternatives to their not finding evidence of the blood. But the whole point of the chapter rests of the cops having a ready access to the test while ON the street. If they have no direct evidence of blood (Hint: it wasn't wiped off), then they'd have no cause for seizing it for later testing.

Thus the whole exchange rests upon the cops having 1) access to the equipment and 2) being able to use it in bright sunlight on a busy street (I need a crowd for the scene).

As to why blood might disappear, I'll leave that to your imaginations, as often, these explanations often degrade into story spoilers.

Frankly, requiring UV light would be easy to incorporate, but requiring a dark room to observe the blue light wrecks the entire scene. :(

@Not_a_ID

The one problem with testing a knife for it having "encountered blood" is that unless that knife has been cleaned using atypical methods, it'll test positive months or even years later.

I know we're discussing 'traditional usage' here, but the point isn't what evidence could be used in a trial, but rather, given the complete lack of on-the-scene evidence, the cops would really have no justification to seize the weapon for later testing.

That's the fallacy of the entire scene, not the fact that someone quickly wiped the blade clean and there's some dead body lying there requiring a full forensic team and seasoned detectives.

P.S. I don't object to everyone discussing the use of luminol, but rather than continuing to discuss my story (and giving away vital story elements), I'd prefer moving the discussion to private emails. There are specific reasons for the 'lack of blood', but it's not open to discussion here.

Incidentally, whilst Phenolphthalein is normally used in titration to identify the degree of acidity it SEEMS to be connected chemically with luminol

I'm pretty sure it's one of the central components in luminol, as the side-effects (including the laxative effects) are identical in the two products (one natural, one produced in a factory).

@remarcsd

I never understood the confidence crime scene shows had in using it (phenolphthalene) to prove the presence of blood.

That's why they don't rely on it alone. It's mostly used for lab experiments in high-schools. It's got a high-rate of false positives, and all it detects is the presence of A blood substance, without the ability to distinguish between human, rat or pig's blood. Luminol is a better indicator, but it still needs to be followed up with a DNA analysis to determine who's blood it is. Even so, the police routine use it to argue guilt given a lack of other evidence when an area has been 'wiped clean' using bleach.

I'm pretty sure it's one of the central components in luminol,

In researching Luminol I wanted to know if phenolphthalene was a component of luminol. The article gave a chemical breakdown. Not my area of expertise. If it is I didn't recognize it in the description.

I know that was why did not to say it definitely did or did not contain the chemical.

Looking at the books, if the knife had been cleaned

Not a complaint, but one of the problems I've noticed in the Forum is our discussions are rarely limited to the described scenario. In this case, CW did not define the knife as being wiped clean. A second problem is scenario descriptions that do not provide full disclosure of the scenario's details until we hypothesize what-if situations that do not conform to the actual scenario.

Not a complaint, but one of the problems I've noticed in the Forum is our discussions are rarely limited to the described scenario. In this case, CW did not define the knife as being wiped clean. A second problem is scenario descriptions that do not provide full disclosure of the scenario's details until we hypothesize what-if situations that do not conform to the actual scenario.

That was a point I raised, as often, the 'requested context' requires a detailed explanation of the basic plot. Seeing as how the first draft isn't entirely written yet, I hate giving away plot details before they're even cemented yet.

For the described scene, there IS no bleach, no one wiped anything down, the blood simply Disappears with no chemical trace left whatsoever. However, that should sidetrack the larger discussion about how these products are used by police departments around the world. That's more apropos to most authors anyway.

As I read the details of the scene you've provided; crowd of people, on the street, etc. Just the situation of someone waving a knife around, with or without blood, would be sufficient cause for arrest and seizure of the knife. (Note: I did not say it would lead to successful prosecution of the individual.) That's unless you've built a rather unusual legal system into the story.

Ha-ha. Yeah, normally that's the case. However, if you want to see how the MC works around the issue, you'll eventually have to read the story! :)