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Instructions for conducting an ink chromatography lab experiment, a forensic technique used to separate and analyze the pigments in ink samples for crime scene investigations. Students will learn about chromatography, the role of solvents, and how to calculate retention factors (Rf) to identify the specific marker or pen used. pre-lab questions, materials list, procedure, and instructions for calculating Rf values.
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Background: Chromatography is a method for analyzing mixtures by separating them into the chemicals from which they are made. It can be used to separate mixtures like ink, blood, gasoline, and lipstick. In ink chromatography, you are separating the colored pigments that make up the color of the pen. Even though a pen will only write in one color, the ink is actually made from a mixture of different colored pigments.
To perform ink chromatography, you put a small dot of ink to be separated at one end of a strip of paper. This end of the paper strip is then placed in a solvent. The solvent moves up the paper strip; and, as it travels upward it dissolves the mixture of chemicals and pulls them up the paper. The chemicals that dissolve best in the solvent will move up the paper strip further than chemicals that do not dissolve as well. What is produced from this method is a chromatogram.
Forensic scientists are able to use ink chromatography to solve crimes by matching documents or stains found at a crime scene to the marker or pen that belongs to a suspect. Forensic scientists analyze the unknown ink and compare it to writing utensils collected from possible suspects.
To prove that an ink sample is a certain brand scientists need to calculate the Rf (retention factor) values of the different colored chemicals present in the marker. Rf is a calculation that compares the distance the solvent traveled up the paper strip to the distance a pigment traveled up the same strip.
Pre-lab:
Materials:
Filter paper
3-4 different brands of black markers
Scissors
Small cups or beakers–one for each marker being tested
Toothpicks
Rulers
Calculators
Reference Table(s)
Water
Procedure:
Conclusion questions:
Known Marker Sample a-Tape here:
Known sample b-Tape here:
Known sample c-Tape here
Known sample d- Tape here
Total # of pigments: Total # of pigments: Total # of pigments: Total # of pigments: Total solvent distance traveled:
Total solvent distance traveled:
Total solvent distance traveled:
Total solvent distance traveled: Colored Pigment #4: Color: Distance: Rf:
Colored Pigment #4: Color: Distance: Rf:
Colored Pigment #4: Color: Distance: Rf:
Colored Pigment #4: Color: Distance: Rf Colored Pigment #3: Color: Distance: Rf:
Colored Pigment #3: Color: Distance: Rf:
Colored Pigment #3: Color: Distance: Rf:
Colored Pigment #3: Color: Distance: Rf: Colored Pigment #2: Color: Distance: Rf:
Colored Pigment #2: Color: Distance: Rf:
Colored Pigment #2: Color: Distance: Rf:
Colored Pigment #2: Color: Distance: Rf: Colored Pigment #1: Color: Distance: Rf:
Colored Pigment #1: Color: Distance: Rf:
Colored Pigment #1: Color: Distance: Rf:
Colored Pigment #1: Color: Distance: Rf: