5.02 Other Clues on the Document

Task

Chromatography Virtual Lab

Scenario

The Crime

Mr. Ben Harper is a CEO of ABC Investments, Inc. He has a 16-year-old daughter, and they live in a house in the countryside. One day, he went back home around 6 P.M. after work. He noticed one of the windows of his house was broken. It seemed like someone broke in and a fight happened in the house. On the coffee table in the living room, he saw a ransom note that told him to pay $500,000, otherwise, he will never see his daughter again. Mr. Harper noticed the ink on the ransom note is black.

The Victim

Police talked to Mr. Harper.

The Evidence

The police immediately began to gather evidence. Based on the information from Mr. Harper, the police took the ransom note and 6 pens that were left on the coffee table. Of these six different pens, three are black felt-tip and three are black stick.

The Forensic Lab

As a lab technician at the forensic lab, you've received the ransom note and 6 different pens collected by the police. Your goals are:

• Identify the inks from different pens.
• Compare inks from different pens and the ink on the ransom note using chromatography.

You will use chromatography to carry out your investigation.

Background

The Science - Ink Analysis

• Composition of Inks
  • You can use many different writing instruments on paper, but in this lab, we will focus on the composition of inks.
  • No matter what type of ink is used, whether it is from a ball-point pen, a gel pen, a felt tip pen, or a printer, every ink is made up of a number of different dyes. Even a black pen will be composed of numerous dye colors, which can be dissolved in a specific solvent.
  
• Chromatography
  • In this lab, you will use chromatography, which is a lab technique to separate mixtures, to help you determine the composition of various inks. You will test the ink for each pen left at the crime scene to make a chromatogram.
  • A chromatogram is a picture that the results of your chromatography will display showing the separation of each component in the ink. A chromatogram can tell you what ingredients are used in the ink.
  

The Ink Library and the Secret Service

The U.S. Secret Service, Forensic Science Division maintains the International Ink Library, which contains data on over 12,000 inks, including printer ink and inks manufactured as early as 1920.

The samples in this collection can be used to determine the type and brand of the writing instrument (pens, markers, printer ink, etc.), the ink manufacturer, and even the year it was made.

Manufacturers often introduce unique dyes each year that include slightly different trace materials. Therefore, this collection can be used to date the "age" of the ink/pen being used; this process is called static ink dating.

Additionally, the International Ink Library contains six thousand different types of chromatographic patterns for comparison purposes, and it's constantly being updated as manufactures adjust their products.

Process Overview

• A Dot of Ink
  • In this lab, you will be creating a chromatogram for each of the six pens left at the crime scene. By placing a small dot of ink on the bottom of a porous strip of paper and placing the paper into a solvent such as water or alcohol, you will create a chromatogram which will show the separation of the components of the ink.
  • The dot of ink will go at the bottom of the paper strip. Label the dot with the brand/name of the pen.
  
• Chromatogram's Bands of Color
  • As the solvent travels up the paper, different components of the ink travel up the strip of paper at varying speeds, creating a colorful chromatogram. This happens because molecules of the same compound will generally move in groups. The compounds are separated into distinct bands within the chromatograph ("Liquid Chromatography").
  • The paper will be placed in a small amount of solvent, which will travel upwards.
  • Notice the dot at the bottom is black but the inks above it are different colors.
• Distinct Bands
  • A chromatogram will have patterns of various bands of color at various distances from the original spot of ink. Each chromatogram will be unique for each type of ink that is tested.
  • This means that your chromatogram will be unique for each pen from the crime scene, but one should match with the chromatogram you'll create of the ransom note ink, if the note was written with a pen from the crime scene.

  

Knowledge Check #1

Three different black pens will produce ____________.

1. three unique chromatographs
2. three identical chromatographs
3. Black ink does not produce a chromatograph.

Answer: three unique chromatographs

Link to Case

As the lab technician, you will be creating 18 chromatograms from the 6 pens acquired at the scene of the crime. Then, you'll create a chromatogram of the ink used in the ransom note.

Finally, you'll compare the 18 known ink chromatograms to the unknown ink chromatogram of the ransom note to determine which pen was used to write the ransom note.

Reflection

Different inks are composed of different materials.

How does chromatography help us identify which pen was used on the ransom note?

Pen Chromatography

The Equipment

• Pens
  • 3 pens, black felt tip
  • 3 pens, black stick
• Acetone
• Ethanol
• Butanol
• Distilled water
• Chromatography paper
• Chromatography tubes
  • 8 Dram glass vials
• Tape
• Gloves
• Pencil
• Chemical Splash Goggles
• Lab Coats

Safety Set-Up

The safety equipment includes your gloves, goggles, and lab apron or lab coat.

In this lab, you will be using butanol, ethanol, and acetone, which are poisonous. Do not allow the solvents to get on your hands and do not inhale the vapors. Allow your chromatograms to dry in a well-ventilated area or in a dry ventilation hood.

Step 1: Setting up the Chromatograms (Felt Tip)

• Step 1 Part 1
  • Select one of the felt tip black pens to complete your first chromatogram. At the top of the chromatography paper, write the name of the pen manufacturer in pencil. About 1 cm from the bottom of the paper, draw a line across the bottom.
  • Place a dot of ink in the center of the line across the bottom of the chromatography paper.

Open Adding Pen Chromatograms to Solvent in a new window

Note: The presentation may take a moment to load.

• Step 1 Part 2
  • Repeat this process on two more slips of chromatography paper, so that you will have 3 chromatography paper slips ready to test for each pen.
  • Repeat this step for the other felt-tip pens. You should have a total of 9 slips of chromatography paper with your ink dots on them.

Step 2: Felt Tip Pen and Distilled Water

• Step 2 Part 1
  • For your first solvent, you will use distilled water. Place enough water into your chromatography tube so that it is no deeper than 0.5cm. Place one of the chromatography papers you prepared into the tube, with the bottom of the paper slip touching the water.
  • Allow the solvent to move up the paper until you notice that distinct bands of color have become evident on the paper. Typically the water must travel up the strip of paper almost completely before the bands of color will be distinct.
• Step 2 Part 2
  • Once your chromatogram is complete, remove the paper from the tube, mark the solvent front, and allow it to dry. Tape the dry chromatogram onto a sheet of paper. Be sure to mark in pencil that water was used as the solvent for this chromatogram.

Open Adding Pen Chromatograms to Solvent in a new window

Note: The presentation may take a moment to load.

Felt Tip Pen and Distilled Water Chromatograms

Repeat the previous procedure using the other felt tip pen chromatography papers. Be sure to mark in pencil that water was used as the solvent for this chromatogram.

Step 3: Felt Tip Pen and Ethanol

Using ethanol as the solvent, repeat step 2 for all felt-tip pens' chromatography paper slips. Be sure to mark in pencil that you used ethanol as the solvent.

Step 4: Felt Tip Pen and Acetone

Using acetone as the solvent, repeat step 2 for all felt-tip pens' chromatography paper slips. Be sure to mark in pencil that you used acetone as the solvent.

Black Stick Pens

Repeat steps 1 – 4 for the black stick pens, but you will be using ethanol, butanol, and acetone for the 3 solvents. Be sure you label which solvent you use on each chromatogram. You should have 9 chromatograms completed from the stick pen ink.

Read below to see the chromatograms for the black stick pens using the different solvents.

• Ethanol



• Butanol



• Acetone

Viewing the Evidence

Now you can see the chromatograms for the pens from the crime scene.

Does each pen ink have its own distinctive bands of color?

Do you notice different resulting bands when using different solvents in the chromatography system?

Now it's time to see if the ransom note was written using one of these pens.

The Ransom Note

Analysis

We need to analyze the ink in the ransom note and compare.

You'll use the same procedure you just learned, but first you must decide which solvent is the best one to use. Select from these four:

1. Distilled Water
2. Acetone
3. Butanol
4. Ethanol

1. Distilled Water

Distilled water won't work for black stick pens. This won't be your best option, but let's look at how it will work.

Complete the ransom note chromatogram using ethanol as the solvent.

Let's compare the ransom note chromatogram with the three distilled water chromatograms you made with the pens from the crime scene.

How is the ransom note chromatogram similar to or different from the distilled water chromatograms from known inks?

2. Acetone

Complete the ransom note chromatogram using acetone as the solvent.

Let's compare the ransom note chromatogram with the six acetone chromatograms you made with the pens from the crime scene.

How is the ransom note chromatogram similar to or different from the acetone chromatograms from known inks?

3. Butanol

Butanol won’t work for felt-tip pens. This won’t be your best option. 

Complete the ransom note chromatogram using butanol as the solvent.

Let's compare the ransom note chromatogram with the three butanol chromatograms you made with the pens from the crime scene.

How is the ransom note chromatogram similar to or different from the butanol chromatograms from known inks?

4. Ethanol

Complete the ransom note chromatogram using ethanol as the solvent.

Let's compare the ransom note chromatogram with the six acetone chromatograms you made with the pens from the crime scene.

How is the ransom note chromatogram similar to or different from the acetone chromatograms from known inks?

Conclusion

Can you tell which pen wrote the ransom note?

You now have 18 chromatograms that you completed in your lab, as well as the chromatogram of the ransom note. In your lab report, you will write down how you analyzed the chromatograms you made during this lab. Be sure to note the differences between each chromatogram, and how you determined which pen was used to write the ransom note.

Your lab results will be shared with the police and used, in part, to determine a suspect in the missing child case of Mr. Harper's daughter.

Credits

Thanks to the following volunteers and their supervisor Dr. Paul Rupar for conducting this experiment: Brendan Rosamond, Michael Gilmore, Olivia Person, Kayla Veal, Emily Linn, and Hugh Hendricks.

"Liquid Chromatography" by LibreTexts is licensed under CC BY-NC-SA 3.0 US.