Crime Scene Day 2: Scientists at work!
Miss Day 1? Check it out here
Since Monday, our student detectives have collected blood, DNA, handwriting, and ink samples from all of our suspects.
Wednesday in class they analyzed their crime scene and suspect evidence to see if they would determine the culprit!
Blood typing was done to determine if all of the blood at the crime scene (two locations) was from the victim or if someone else’s blood was present. In this method, the blood samples are mixed with antibodies that bind to chemical signals on our red blood cells that determine our blood type (A, B, O, AB; Rh negative or positive). Changes in the color and clotting indicate the blood type.
The same group also used a method called Restriction Fragment Length Polymorphism (RFLP) to individualize the DNA left at the crime scene on the pop can. Each human shares about 99.9% of their DNA with other humans – this is the DNA that’s used to make proteins that we all have. But 0.1% of our DNA is unique. While this may
seem like a tiny difference, it comes out to about 3 million(!!) different bases – the building blocks of DNA. RFLP exploits these differences by using enzymes (restriction enzymes) that cut up DNA into smaller pieces. Everyone’s DNA will produce fragments of different lengths, which can be visualized on an agarose gel and compared to the evidence (polymorphism just means different forms – which these fragments are among a group of people). Tune in next Wednesday to see what the gel looked like when it was finished!
One group of detectives got a lesson in how weird it is to take someone’s fingerprints! And how hard it is to compare suspect fingerprints to partial prints found at a crime scene. They were diligent and are close to catching the perpetrator!
The document analysis group set out first to compare handwriting samples to the note they found at the crime scene. Distinctive handwriting can be a dead giveaway in some cases – what about in this case?
They also compared ink from the note to the pen samples they collected from suspects using a method called thin-layer chromatography (TLC). This method uses solvents like methanol (mixed here 2:1 with water) to separate components of a substance like ink which is composed of dyes or pigments. Even pens that all use black ink can look different when separated by TLC due to different formulations used by manufacturers.
Our last group was trying to figure out the identity of the white powder left at the scene. To do this, they use a combination of solution tests and chemical tests utilizing paper analytical devices (PADs), which is research that Dr. Barstis does in collaboration with the University of Notre Dame (PADs project). After putting drops of chemicals on the PADs in the white lanes, they scrape the powder across the whole PAD and then stand the PAD in a little bit of water.
The paper wicks up the water, which carries the powder to the chemical and the reaction and possible color change takes place. Color changes can indicate the makeup of the powder – for example, the dark purple spot in the middle lane of the PAD on the right tells the group one major component of the white powder.
So what were the results of all of these analyses? Tune in next Wednesday to see if we have identified a single perpetrator from our evidence. During class on Wednesday, each group will present their findings to a jury of the possible suspect’s peers (a team of science and humanities profs) who will vote on whether they can convict based on the evidence.