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[Bob Mueller]

Samantha has been arrested for murder; the victim was cut numerous times. When she was arrested, a boxcutter was found in her purse, and assumed to be the murder weapon. Brown stains were found on the blade, and assumed to be blood, and tested at the lab.

What really happened was that Hollie planted the boxcutter in Sam's purse, after wiping it down. But Hollie only wiped down the outside, not the razor blade.

I know the stains can be tested and identified as human blood. Can they also be typed for human blood type? I want the blood type to be radically different from the victim, and I may have it match Hollie. Maybe not; maybe it's cow blood from the ground beef in the fridge. She was in a hurry when she planted the boxcutter, and wasn't thinking straight.

Would the testing of the blood (or the processing in general) affect the quality of the thumb and forefinger print I want to be found on the blade?

It's not so much that Hollie wants Sam convicted of murder; she just wanted her arrested and out of someone's life. Hollie wants the guy who's in love with Sam (but Sam loves someone else - my main character).

[JIM DOHERTY]

Bob,

Not only can blood be typed, but DNA can be derived from the blood.  The first time that DNA was used to break a criminal case, the DNA was derived from blood.  You can read about it in Joseph Wambaugh's The Blooding.

Before it was possible to make DNA matches, but after it was possible to get blood types, I seem to recall it was possible to determine if the donor was male or female.  It seems to me that there was a Quincy episode from the '70's about doing this.  Dr. Noguchi, the LA County Coroner, developed the male/female test, IIRC.

[Bob Mueller]

That's what I thought, but I wanted to make certain. So that whole sequence seems feasible? When Hollie planted the boxcutter, she didn't know victim had been stabbed, only that he had a slashed throat, so the depth of the injuries would rule out the boxcutter as the murder weapon anyway, but I want to make it point back to Hollie, to expose her unrequited feelings.
 

[Ingrid]

Not about the blood, but I'm wondering:  slashed throat? If the wound is serious enough to look fatal at first glance, it probably was fatal. There should be a lot of blood. If there isn't a lot of blood, the slashing was posthumous.  A police man would know that right away.

And if the box cutter was wiped down, how did Sam's fingerprints get on it?  Wouldn't the investigator wonder why it was wiped down before it was used by Sam? And then wiped down again, preserving only the fingerprints?

Making trouble -- but maybe your scenario has already taken care of these matters.

Ingrid

[Bob Mueller]

It gets pretty involved here.

Sam was the last one seen with the victim, via security video. When he's found dead, and the car is processed, her prints are found in several places inside and outside the car. Prints are on file due to a prior soliciting arrest. It just occurred to me that I'm going to have to lengthen the time between the murder and her prints showing a hit, I think. I know Columbus has an AFIS system, and her arrest was local, but I need to add in a day or two, at least. Right now, I think I've got her being identified within a couple of days. That may be another post.

At any rate, Sam is assaulted in the car by the victim, who's actually a mob enforcer. When he drags her out of the car, she gets a lucky shot in, and knocks him out, then escapes and goes to ground for several days.

Just after Sam leaves the scene, her bouncer (Donovan), who is in love with her (Sam doesn't know this, or refuses to admit it), finds the unconscious mob guy, wakes him up, gets some info from him, and then he's the one who stabs and cuts the guy.

Sam ends up hiding out at a friend's apartment. Hollie is also living there. Hollie, who is in love with Donovan, figures out that Sam is somehow involved in this death, so plants the boxcutter, and calls the tip line to turn Sam in. Hollie doesn't really think it through; she just wants Sam gone so she can convince Donovan that she's much better for him than Sam is.
 
In a freakish twist later, Donovan gives the mob guy's knife to Sam as a gift, thinking she won't recognize it. But Scott, my main character, gets suspicious, and takes the knife to a PI friend who does a field test on the knife which finds some blood down in the hinges, or the storage groove (it's a folding knife).
 

[Bob Mueller]

In a freakish twist later, Donovan gives the mob guy's knife to Sam as a gift, thinking she won't recognize it. But Scott, my main character, gets suspicious, and takes the knife to a PI friend who does a field test on the knife which finds some blood down in the hinges, or the storage groove (it's a folding knife).

Back to this part of the whole thing.

I'm aware of different tools to test for blood spatter or residue, etc. But I don't know much about them, or how they can be used. Is there an easily-used-in-the-field product that an ex-cop might have, now that he's working as a PI?

As I say in the quote above, I want my MC to have someone be able to test the folding knife quickly for the presence of blood. Donovan washed the knife, but only with soapy water, and there was a lot of blood.
 
This is a big folding knife, something like one of these.
 

[Bob Mueller]

I reasoned there was a decent amount of dried blood down in the storage channel for the blade. I was on a roll, and wrote the graph below. I can change it to have him scrape out some of the dried blood, and do it that way.

Is it reasonable to have him scrap the dried blood onto a glass slide or similar plate, then onto the test paper?

Why three reagents? Testing for different fluids or sources?

Is the black light still used?

When I got there, we headed into a room just off his main office. It looked like a kitchenette, but smelled like a lab. Bill rummaged around in one of the cabinets, pulled out a box, then dug a small light fixture out of a drawer. He set some paper towels on the counter, whistling the theme from “Law & Order” as he stuck his hand out for the knife. From the box came several bottles, and from the bottles, several liquids were measured and poured into a spray bottle. Bill sprayed a mist onto the knife in a few places, then reached for the light switch, making it really dark in the smallish room. I assumed Bill plugging the light fixture in caused the next sounds I heard, because there was suddenly a violet glow to the room from the black light. I saw several flecks of blue glowing on the knife blade, and a brighter blue glow inside the storage channel. The whistling stopped.

“Well, someone’s been playing with this.”