Paper Chromatography
Purpose: to qualitatively separate mixtures using paper chromatography& then compare inks samples to establish which pen
was used on a document.
Background:
Recently, handwriting experts discovered a set of forgeries. Several museums in the
United States had been displaying documents supposedly signed by Abraham Lincoln.
The FBI suspects that this could be the work of Benny "Fingers" Smithson, who was
recently paroled from prison after serving time for his part in a phony Babe Ruth
autograph scam. Smithson denies his involvement. A search warrant was issued, and the
FBI found three pens in his apartment. The FBI is also investigating another suspect,
Thomas Banks, an employee of one of the museums. Three pens from his belongings
are also being held as evidence. Before the FBI can press charges, they need conclusive
evidence linking the pens and the phony signatures. Your goal is to determine which
pen was used in the forgery by preparing chromatograms for each of the pens using
two different solvents and then compare the chromatograms and decide which pen is the likeliest match.
Theory:
Paper chromatography is a method of separating mixtures by using a piece of absorbent paper. In this
process, the solution to be separated is placed on a piece of dry filter paper (the stationary phase). A
solvent (the moving phase) is allowed to travel across the paper by capillary action. As the solvent is
soaked up by the paper, some of the components of the mixture are carried with it. The components of
the mixture that are the most soluble in the solvent and least attracted to the paper travel the farthest.
The resulting pattern of molecules is called a chromatogram. In cases where the molecules are easily visible
such as in inks, this method distinguishes the components of a mixture.
Safety:
Always wear goggles for protection of your eyes. If you get a chemical in your eyes, immediately flush it
out at the eyewash station while calling to your teacher. Know the locations of the emergency lab
shower and eyewash and how to use them. Make sure you carefully read the labels and follow the
directions on all containers of chemicals that you use. Never return leftovers to their original
containers; take only small amounts to avoid wasting supplies. Because the isopropanol is flammable,
no Bunsen burners, hot plates, or other sources of heat should be in use in the room during this lab.
Always clean up the lab and all equipment after use, and dispose of substances according to proper
disposal methods. Wash your hands thoroughly before you leave the lab after all lab work is finished.
Procedure:
1.
Use a pencil to sketch a circle about the size of a quarter in the center of the piece of filter paper. Write the
numbers 1- 6 in pencil around the inside of this circle.
2.
On the circle beside the number 1, use pen number 1 to make a large dot. Use pen number 2 to make a dot beside
number 2, and repeat this procedure for each pen.
3.
Repeat steps 2 and 3 with a second piece of filter paper. (One will be used with water as a solvent, and the second will
be used with isopropanol as a solvent.)
4.
Using a pencil, poke a small hole in the center of two filter papers.
5.
Fill a one of your large beakers about ½ full with water and one of your small beakers about ½ full of isopropanol.
6.
Wet one pipe cleaner by dipping it in the beaker with the water. Push the wet pipe cleaner from the bottom
through the hole in the filter paper. Set the filter paper with the pipe cleaner onto the beaker so that only the wick
(pipe cleaner) is touching the water. Repeat this set up but dip the second pipe cleaner into the beaker with
isopropanol.
7.
Allow the chromatograms to develop for approximately 15 min (While you wait you can start step 9), or until the
solvent is about 1 cm away from the outside edge of the paper. Remove each piece of filter paper from the beakers,
placing them on paper towel or in an area that they will dry.
8.
Record the colors that have separated on the chromatogram from each of the six different black inks in your data table.
You may either describe the colors, use colored pencils or both to record this information.
9.
Obtain a forged signature and record the forgery letter in your data table.
10. Choose two segments that can be used to make chromatograms. (Hint: consider in which direction the ink will travel
after the wick brings solvent onto the paper.) Cut out the two segments, leaving as much blank paper attached to each
one as possible.
11. With one piece of the paper, make a chromatogram using a beaker, a paper clip, and water. Do not allow the ink from
the paper to come into direct contact with the solvent. Unbend the paper clip, and use it to support the strip with the
signature. Then make a chromatogram using the isopropanol as the solvent in another beaker.
12. After 15 min or when the solvent is about 1 cm away from the top of the paper, remove each chromatogram from
their beakers and allow them to dry.
13. Record the colors from the pieces of the signature and their arrangement in your data table.
14. The water may be poured down the sink. Chromatograms and other pieces of filter paper should be allowed to dry
and then stapled into your lab book. The isopropanol solution should be placed in the waste disposal container
designated by your teacher. Clean up your equipment and lab station. Thoroughly wash your hands after completing
the lab session and cleanup.
Data Table:
Prepare your data table with seven columns and eight rows. Label the boxes in the top row Pen number,
H20-center, H20-middle, H20-edge, Isopropanol -center, Isopropanol -middle, and Isopropanol -edge. In the
first column, add the labels Smithson 1, Smithson 2, Smithson 3, Banks 4, Banks 5, Banks 6, and Forgery (with the Letter).
Conclusions: (Answer the questions in complete sentences in such a way that the questions are understood.)
1. Which pen was used for the forged signature? Explain your reasoning, giving specific examples and providing
quantitative data as possible. (Hint: measure the distances that each component traveled.) Be sure to include which
forgery letter you’re sample was.
2.
When you labeled the pen numbers, a pencil was used because a pen would have smeared/ran into all the other
pen markings. Explain why this does not happen with a pencil.
3.
What would have happened to the chromatogram if the process had not been stoppped after 15 min, but instead
was allowed to proceed overnight?
4.
If the components of the mixture are most soluble in the solvent they travel farther faster. Compare the colors
of the ink from pen one in both chromatograms. What does this suggest about the solubility of mixture in water
compared to isopropanol?