Crime Scene: The Verdict is In!

After collecting and analyzing the evidence from the crime scene, our detectives presented their evidence to a jury who in the end deliberated and settled on a guilty verdict. Who was convicted? Read through the detectives’ arguments and see if you agree with the jury!

Crime Scene Sketch

Before they began collecting evidence from the crime scene, each group made a rough sketch of the area. This is one of the first steps taken at a crime scene and must include every piece of evidence. As the case progresses, a final sketch is made and often presented in court. To be admissible, the finalized sketch may not contain any evidence that was not in the initial rough sketch done at the scene. Below you can see rough and finalized sketches from the group that analyzed blood and DNA:

Salgado-rough sketch

Rough Sketch

Sketch-DNA-bloodgroup

Finalized sketch

Chemical Analysis

powderThe Chemical Analysis group (Sally, Adrienne, Katy G, and Natalie) had the task of analyzing the white powder that was left on the benchtop at the crime scene. To figure out the components of this white powder, they used paper analytical devices, or PADs. While these PADs are designed to be used to identify counterfeit medicines, they can also be used to run pretty much any chemical test.

They were given a list of possible substances: ampicillin (an antibiotic), potassium clavulanate (an antibiotic mimic), salicylic acid (starting material in the production of aspirin), starch, chalk, sodium phosphate, sucrose, and glucose.

Using a variety of tests, they were able to eliminate starch, salicylic acid, and sodium phosphate right off the bat due to some negative results.

Negative tests

They started to narrow things down with the Folin-Ciocalteu test, which indicates the presence of phenols or reducing groups. The positive result they saw meant the powder contained ampicillin, acetaminophen (Tylenol), vitamin C, or chalk (calcium carbonate).

They also got a positive test with the biuret reagent – this chemical changes color in the presence of an amide bond. This is the chemical bond that makes up the backbone of all proteins and is also present in beta-lactam antibiotics such as penicillin and ampicillin.

From this information, they concluded that the mixture contained ampicillin and chalk and decided their lead suspect was me (!!) because I work with ampicillin as a biochemist and, of course, I have pretty good access to chalk.

Blood Typing

The group in charge of DNA and blood analysis collected two blood samples at the scene: one on a drawer handle and the second on the floor near the Pepsi can.

crime scene - blood evidence

Blood evidence at the crime scene

Their task was to collect both of these samples, determine if they were from the same source, and if possible, use this information to name a suspect.

More than 35 human blood groups have been identified, but the A-B-O and Rh systems are the most well-known. Blood is made up of red blood cells, white blood cells, and platelets which all float around in plasma – the fluid portion of blood. On the surface of the red blood cells are antigens, which look different for each blood type.

blood types-ABO

There are different antigens present on the red blood cells of each blood group. The four most well known groups are A, B, AB, and O. Image: InvictaHOG/Wikicommons

A fourth antigen, known as the D antigen or the Rh factor, may also be present on the red blood cells. This antigen is what gives your blood type the negative or positive connotation.

blood types - Rh

Rh positive blood contains a fourth antigen, shown here in yellow. Image: InvictaHOG/Wikicommons

Each of these antigens is recognized by specific antibodies and the body uses this specificity to fight against foreign antigens. This is the theory behind immunity and organ rejection. For example, if your blood type is A+, your body produces antibodies that will only bind to B antigens, not A antigens. This is because when an antibody binds to a red blood cell by attaching to the antigen, coagulation – or a clumping together of blood cells – occurs, which is not desirable in your arteries and vessels!

This group used a kit containing A, B, and Rh antibodies to determine the blood types of the crime scene samples. The mixed the blood samples from the crime scene and the samples they collected from possible suspects with a little bit of the antibody and looked for coagulation.

blood test - drawer-Haas

The sample of blood on the drawer matched Dr. Haas’s blood sample. The blood coagulated in the presence of A and Rh antibodies, indicating an A+ blood type. Other possible suspects with this blood type are Barstis, Bentley, Dunlap, and Fishovitz.

blood test - floor-Miller-Feigl

The blood sample on the floor tested as O-, indicating this blood did not come from the victim, Dr. Haas. Possible suspects that also had this blood type are Dr. Feigl and Mrs. Miller.

Based on the blood analysis, the group was able to determine that there was blood from two sources at the crime scene – one being Dr. Haas and the other, presumably the perpetrator. At this point, they narrowed it down to two possible suspects: Dr. Feigl and Mrs. Miller.

DNA Analysis

Along with blood analysis, this group was also responsible for collecting and analyzing any DNA evidence found at the scene. While DNA can be extracted from white blood cells, we didn’t have this option, so they had to look for other sources. Luckily, the perpetrator left a pop can which contained DNA from saliva on the top! The group collected this sample and samples from possible suspects. They then added an enzymes (restriction enzyme) which would cut the DNA up into smaller pieces. Restriction enzymes recognize specific short sequences of DNA, which are present in different amounts and at different places in everybody. Because these enzymes are so specific, they should be able to clearly match the DNA from the pop can to a suspect (or the victim). Your fragmentation pattern of your DNA is as unique as your fingerprints. In fact, this method is commonly called “DNA Fingerprinting”.

One restriction enzyme can cut DNA from two different people in different places. Here, the DNA from individual 1 gets cut at three places, resulting in four fragments of different sizes. DNA from individual 2 gets cut at two places, resulting in three fragments of different sizes. These fragments are in a mixture until they are run on a DNA gel, which separates each fragment by size which the largest fragments at the top of the gel and the smaller fragments at the bottom.

When analyzes the DNA evidence, this group was looking for a DNA sample from the possible suspects that matched the fragmentation pattern of the DNA from the crime scene.

DNA gel - labeled

The DNA gel of the crime scene and suspect samples. The ladder is a DNA sample that contains fragments of known size which can be used to approximate the sizes of fragments in other samples. This gel clearly shows that the DNA from the crime scene matches the DNA sample from Dr. Feigl.

DNA analysis is so specific that the odds of Dr. Feigl not being the source of the crime scene DNA are very slim (something like 1 in 13 billion!). Only if Dr. Feigl had an identical twin would someone else have the same pattern. From this analysis, the group was able to conclude that the DNA at the crime scene did not belong to Dr. Haas, the victim, but did belong to Dr. Feigl. While this doesn’t necessarily mean that she was the perpetrator, it does place her at the scene of the crime.

Between the results of the blood typing and DNA analysis, this group is pretty certain Dr. Feigl is the perpetrator of the crime!

Document Analysis: Ink and Handwriting

Because there were pieces of a handwritten note found at the crime scene, one group was focused on naming a perpetrator from this evidence.

document- evidence

A portion of a handwritten note was found in the overturned garbage can at the crime scene, written in cursive, in black ink.

The group collected pen and handwriting samples from all of the possible suspects:

Document - Becker sample

Pen and handwriting sample from Dr. Becker. All of the evidence that was collected in Ziploc bags with an evidence tag. Note the chain of custody: it’s important in evidence collection to keep track of everyone who has access to the evidence.

The analyzed the handwriting by eye:

document - handwriting

In the analysis of handwriting, the group decided to focus on the letter “a” found on the note at the crime scene (bottom right sample).

They saw that unlike the other suspects, only Dr. Feigl started her “a” with an extra stroke.

They next set out to determine if they could match the collected pen samples to the ink used on the note. To do this, they used a method called thin-layer chromatography (TLC). This method uses chemicals to separate the various components of ink, which includes dyes and additives. The theory behind this is similar to the theory of DNA fingerprinting, but not nearly as accurate.

document - ink tlc

The document analysis group used TLC to separate the components of different pen samples collected from the crime scene and suspects. They made a dot of ink at the bottom of the TLC plate (on the horizontal line), then set the bottom of the plate in a chemical solvent, in this case, a mixture of methanol and water. The TLC plate acts as a wick to draw the liquid to the top of the plate. As the liquid moves us, different components move with it in a distinctive pattern.

While many of the black pen samples that were collected from the suspects contained a yellow dye that easily separated, the crime scene sample and suspect sample #5 did not have this yellow component. Suspect #5 was Dr. Feigl.

Fingerprints

The final group was tasked with collecting and analyzing fingerprints from the crime scene. This is often very difficult and proved to be so for our crime scene investigators as well. They were able to pull prints from the pop can and from a door window, but many of these prints were only partial. They found the prints, which are left by the oils in our skin, by dusting with a carbon-based black powder. The powder sticks to the fingerprint and can be transferred to paper for record-keeping and analysis.

fingerprints-crime scene can thumb pinky

The best fingerprints collected were from the pop can, but are only partial and it’s hard to distinguish the features (especially on a computer screen!). Based on the placement on the can, the group suggested these were prints from the right thumb and little finger.

Modern fingerprint analysis is done with powerful computer programs, but our scientists had only their own eyes to use. Fingerprints are unique – not only to each person, but to each finger. It is impossible to remove your fingerprints, though some criminals have tried! Notorious gangster John Dillinger once tried to remove his fingerprints by putting acid on them which resulted in scars. However, scars also show up in fingerprints and are just another characteristic that can be used to identify the prints!

Fingerprints are unique because they are composed of different patterns like loops, arches, and whorls, but also each fingerprints contains up to 150 ridge characteristics like bifurcations, endings, enclosures, and dots.

fingerprints - patterns

Source: Saferstein, Richard. Forensic Science: From the crime scene to the crime lab, 3rd ed.; Pearson: Hoboken, New Jersey, 2014.

fingerprints - minutae

Source: Saferstein, Richard. Forensic Science: From the crime scene to the crime lab, 3rd ed.; Pearson: Hoboken, New Jersey, 2014.

By finding these patterns and characteristics in the fingerprints from the crime scene and the suspects, this group was able to find some matches to one person: Dr. Feigl.

fingerprints - feigl samples

The fingerprint analysis group was able to use the scars and ridge patterns and characteristics to match the crime scene fingerprints to Dr. Feigl.

Closing Statement

Based on the evidence collected at the crime scene, three out of the four groups put forward Dr. Feigl as the perpetrator. Her DNA fragmentation pattern matched the DNA found on the pop can at the crime scene, she has the same blood type as one of the crime scene blood samples, and her handwriting and preferred pen are similar to what was found on the torn note at the crime scene. While the chemical analysis of the white powder found at the scene doesn’t necessarily point to her, the class was confident in presenting Dr. Feigl as the perpetrator to a jury of her peers: Dr. Farmer (music), Dr. Kloepper (Biology), Dr. Ralston (Biology), and Dr. West (Modern Languages). The jury’s verdict?

Feigl - guilty

Disclaimer: No Chemistry professors were harmed in the staging of this crime scene. Dr. Feigl wouldn’t hurt a fly, let alone Dr. Haas! All blood and DNA samples were simulated and proper lab safety protocols were followed during analysis.

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