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JJC CSI

PURPOSE

To learn about the technique of paper chromatography. To develop a paper chromatography technique to separate components in an ink sample. Identify an ink sample based on its chromatogram. Write a report that could be used as evidence in a criminal trial.

INTRODUCTION

Paper chromatography was developed in the early 1940s in England. The technique is based on the fact that paper contains a thin film of water around the cellulose fibers of the paper, called a stationary phase. A mixture of the components to be separated is placed in a

small spot at one end of the strip of paper and an organic solvent (the mobile phase) is passed over the spot and across the paper. Since each compound present has a differentsize, shape, and polarity, each compound will dissolve in the water and organic solvent to adifferent extent.

The net result is that if two compounds are started at the same place and solvent passedover them, one compound will move along the paper faster than the other. After a period oftime the flow of the mobile phase is stopped. The paper is dried and then sprayed with areagent that will produce colored spots, if the compounds are not colored. The materialused in our experiment is already colored, so the latter step is not required.

Chromatography works because of differences in polarity between the stationary phase, thesolvent, and the components of the mixture being studied. When using a solvent with low

polarity, the majority of the components of the mixture, if not all of them, will be held morestrongly to the stationary phase than the solvent molecules, so the solvent will not displacethem and carry them along with it. If a more polar solvent is chosen, it will displace themajority of the molecules in the sample, but depending on the polarity of each of thecomponents, some may travel farther from the solvent than others, resulting in separation ofeach of the components. The distance traveled with the solvent are characteristic ofdifferent molecules and make identification possible.

A solvent much more polar than the components of the mixture will displace all of themolecules and carry them all very easily on the solvent front, yielding no separation thatwould be useful in identification. Finding a solvent suitable for separation of a mixture or

identification of a sample usually involves trying several solvents/solutions of different polarity. In your experiment you will be using deionized water (most polar), a 20% ethanolsolution, a 50% ethanol solution, and a 90% ethanol solution (least polar).

Criminal investigators use chemistry routinely to solve crimes. Sometimes crimes involvethe use of ink pens. Forgery cases, kidnapping cases (involving a ransom note), and othercrimes involving writing are often investigated using chromatography.

Someone has kidnapped a beloved pet fluffy, leaving a ransom note at the scene of thecrime. The CSI team has collected black pens from the suspects. In this investigation,

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your team will be given a sample of the ink found at a scene of the crime. Your job will beto determine which pen was used to write the note.

MATERIALS

Black ink pens Parafilm squaresChromatography paper Ink sample from crime scene

Scissors Deionized water

4- 600 mL beakers 20%, 50%, and 90% ethanol solutions

Rulers

PROCEDURE

Black ink is usually a mixture of several different colors that can be separated that can beseparated by chromatographic conditions.

Designing chromatography experiments usually involves selection of the proper solvent forthe system of interest. In this investigation, your group of crime specialists has been

provided several solutions of ethanol with water. Your first task should be to determinewhich solvent mixture is appropriate for separation of ink components in the known pens. Inaddition to the ethanol solutions, you should try using deionized water as the solvent.

In addition to determining the proper solvent, you should develop a way of quantifying yourdata for your report; that is, you might consider ways of expressing your results in numericalterms that allow for easy comparison of results from one experiment to another.

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This experiment is best done in groups of four individuals. Expect further detailed directionsfrom your laboratory instructor about how you will go about the labs activities.

1. Obtain four pieces of 16 cm x 11 cm piece of chromatography paper. Use a pencil todraw a line along the 16 cm side of the paper that is 1 cm from the bottom of the paper.Fold the paper as you would a fan to generate 5 panels as shown below.

2. Use each of the confiscated pens to place a small dot (approximately 3 mm wide) of inkon each panel. Use a pencil to label the identity of each ink sample as shown below. It isOK to use abbreviations as long as you provide a key to what they are in your report.Allow the ink samples to dry for about one minute.

Flair Sharpie Vis-aid Vis-a-vis Calligraphic

3. Collapse your fan of paper slightly so it will stand up in a beaker on its own and so thesides of the paper will not touch the walls of the beaker.

4. Pour 20 mL of deionized water in a 600 mL beaker. Do not use a bigger or smaller size beaker.

5. Insert the folded chromatography paper into the liquid. The pencil line end of the papershould be closest to the liquid. Be sure the ink dots are above the liquid level.

6. Remove the protective paper from the square of parafilm. Keeping the beaker on the bench top, carefully stretch the parafilm over the top of the beaker to seal it. Do this verycarefully so that the piece of chromatography paper is not disturbed.

7. When the liquid has nearly, but not completely reached the top, remove the paper, use a pencil to mark how high the solvent traveled. Allow the chromatography paper to air dry before making any measurements.

8. Repeat the same procedure for the other solutions. Each member of the group should be

responsible for using one of the solvents/solutions.9. Your group will be given one or two samples of ink taken from a crime scene. Because

the sample is taken from a limited supply that will be used in court, do not use theunknown until your group is certain of the solvent system you will use. You are lookingfor the solvent that will allow you to easily identify each sample.

10. The set up for the unknown is a little different than what you used for the large pieces ofchromatography paper.

fold in paper

pencilline

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11. Place approximately 1 cm of the solvent of choice in a 250 mL erlenmeyer flask fromyou equipment drawer/tote tray.

12. Place the paper with the ink spot end closer to the liquid. Allow the paper to touch theliquid, but not the bottom of the flask. Carefully insert a cork or stopper in the flask tohold the paper in place. When the liquid has nearly, but not completely reached the top,

remove the paper and allow it to air dry.

Cleanup: All the solutions used may be disposed of down the drain. Thoroughly rinse and dry allthe 600 mL beakers before returning them.

Report The report must be typed and grammatically correct. It may be returned to you for

corrections if it is not acceptable. Your report should include a discussion of factors that mayaffect the rate at which the solvent and the ink components move up the chromatography

paper. Remember that your report may be used in a court of law. If directed by yourinstructor, include your chromatograms with your report.

QUESTIONS (Please put answers in your lab report)

Q1: What pen was used to write the ransom note

Q2: Why do you suppose some solvent systems work better than others do in the separation ofthe components of ink?

Q3: Why should the solvent depth of the chromatography solvent remain below the level of theink dots?

Q4: Some components of ink are minimally attracted to the stationary phase and very soluble insolvent. Where are these components located on the chromatography paper duringchromatography?