Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Prepare filter paper “wicks” for the students by cutting 12.5-cm pieces of filter paper into eight pie-shaped wedges. Prepare enough wedges so each student group has a fresh wick for their chromatogram.
2. Using masking or similar tape, create a label on each of the pens assigning them numbers 1, 2, 3, etc. Pens that are alike should have the same number for easy identification. Place them in a central area of the lab so students can share.

3. Select one of the pens to serve as the evidence chromatography sample. Record this number so that students’ work can be checked for accuracy after the lab is complete.
4. Follow the student procedure with the exception that only one pen should be used to make all six dots on the filter paper and the number should not be written in pencil on the filter paper.
5. Make enough so that when run, dried and cut, each group can have a wedge. Note: For example if you have 15 groups you will need at least three chromatograms.
6. Cut into wedges so that each student group gets a sample of the evidence.

Safety Precautions

While this activity is considered nonhazardous, follow all appropriate laboratory safety rules.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Water may be flushed down the drain according to Flinn Suggested Disposal Method #26b. The filter paper disks may be saved or disposed of in the trash according to Flinn Suggested Disposal Method #26a. Save all other materials for future use.

Lab Hints

• Enough materials are provided in this kit for 15 students, working in pairs.
• It is suggested that the markers be numbered (coded) and placed in a central area so that the students can share them.
• Pens, such as Vis-à-vis^®, Expresso, Le Pen, Papermate Flair™, Boldliner, Mr. Sketch^®, Prang and Vis-Aid, work well. Note that the specific brands of pens included in the kit may vary depending on availability.
• Experiment with a variety of different water-soluble markers or felt-tip pens to determine the composition of each. Many different brands are available at local stores. Have students bring in their own markers to try. Try using colors other than black.
• If a chromatogram is running too slowly or not at all, check to make sure that the wick has been inserted snugly into the hole and that there is good contact between the wick and the inside edge of the hole.
• Avoid excessive handling of the filter paper circles. Oils from the skin can interfere with the capillary action that draws the water through the paper.

Answers to Prelab Questions

1. What is the adsorbent and the eluent used in this laboratory activity?

   The adsorbent is the filter paper and the eluent is the water.

2. Describe how the process of chromatography works to separate organic compounds.

   Chromatography separates organic compounds, in this case ink pigments, on the principle that they will be slightly soluble in the eluent and spend some of the time in the eluent and some of the time adhered to the adsorbent.

Sample Data

Students should attach their chromatograms.

Answers to Questions

1. What colored pigments make up each black ink?

   The pigments are listed from least mobile (greatest affinity to the paper) to most mobile (least affinity to the paper) for each pen. Colors shown in parentheses may be indistinct or blend with other pigments. Remember, manufacturers may change formulations and pigments.
   Papermate, Liquid Flare™—Dark midnight blue, gray, darker gray.
   Blick Watercolor Marker—Dark gray, light gray, dark gray, yellow.
   Universal porous-tip pen—Dark gray, violet, lavender.
   Expo, Vis-à-vis—Yellow, pink/rose, purple, royal blue, turquoise
   Le Pen—Purple, green, yellow.
   Mr. Sketch—Yellow, orange, pink, blue.

2. Why do different black inks separate into different pigment patterns?

   Each ink is a mixture and has a different formulation—it is composed of different compounds. Each compound (pigment) has characteristic physical properties that will determine its relative attraction for the paper versus the solvent (water) in this experiment.

3. Knowing that water is a very polar solvent, what can you infer about the relative polarities of the various pigments in each ink?

   The pigments that traveled the farthest are the most soluble in water. For example, the Mr. Sketch pen separated into yellow first (least polar) and eventually separated out the blue pigments furthest away (most polar). Conversely, in LePen the yellow was the most soluble pigment as it traveled the furthest.

4. Based on evidence and your test results, which pen was most likely used to forge Alyson Marie Combs’s signature at the bank?

   The answer to this question depends on the pen selected in the Prelab Preparation section step 3.

5. What other types of forensics tests should be performed to further determine if this is indeed the pen used to forge Ms. Combs’s signature on the check?

   Fingerprint and handwriting analysis could be performed as ancillary tests to verify this was the pen used to forge Ms. Combs’ signature.

References

Cesa, I., Flinn ChemTopic™ Labs—Elements, Compounds and Mixtures; Flinn Scientific; Batavia, IL; 2005; Vol. 2, pp. 37–44.

Introduction

Imagine the panic of discovering that $20,000 is missing from your checking account! Use radial chromatography techniques to test possible different types of ink used to forge your signature.

Concepts

• Chromatography
• Separation of mixtures
• Physical properties

Background

Case Background

One afternoon Alyson Marie Combs decided to do some shopping to decorate a new home she had just purchased. At a local boutique she found a comfortable yet stylish leather chair she thought would be perfect in her study. Alyson was a successful author who had just purchased a summer cottage in Kennebunkport, ME. As she attempted to complete her transaction the sales clerk politely notified her that her debit card had been declined. “That’s impossible,” said Alyson. “Try it again.” The clerk apologetically said that she had tried it twice and asked if there was another card she could use. Alyson handed over her Visa card, and the purchase went through.

She briskly walked to her car thinking, “How could my debit card have been declined? I’ll just have to go back to the cottage, look at my online banking records and everything will work itself out.” Sitting down at her computer Alyson’s hands started to shake as her online banking screen appeared with the two words she did not want to see—“account closed.” Her bank was located in her hometown of Columbia, SC. Alyson immediately picked up the phone and called her personal banker, Mary Hayes. Mary pulled up Alyson’s account information and said that her records show that Alyson closed the account two days earlier at their Chapin, SC branch. “This is horrible,” Alyson exclaimed. “I just transferred $20,000 to that account because I knew I would be making several large purchases for my new home. How was the money withdrawn?” Mary replied, “By check.” Mary apologized profusely and vowed to immediately notify the state fraud department and the police. Alyson thanked Mary and said she would be on the next plane to Columbia so she could get home and straighten all this out.

Technical Background—Chromatography

Chromatography is probably the most useful method of separating organic compounds for identification or purification. There are many different types of chromatography, but most work on the principle of adsorbtion. The two important components of chromatography are the adsorbent and the eluent. A good adsorbent is usually a solid material that will attract and bind the components in a mixture. Paper, silica gel, or alumina are all very good adsorbents. The eluent is the solvent that carries the materials to be separated through the adsorbent via capillary action.

Chromatography works on the principle that the compounds to be separated are slightly soluble in the eluent and will spend some of the time in the eluent (or solvent) and some of the time on the adsorbent. When the components of a mixture have varying solubilities in the eluent, they can then be separated from one another. The polarity of the molecules to be separated and the polarity of the eluent are very important. This affinity for the eluent versus the adsorbent is what separates the molecules.

Paper chromatography is commonly used as a simple analytical separation technique. In paper chromatography, the adsorbent is the paper itself. The eluent can be any number of solvents; in this lab, the eluent is water. Water is a very polar molecule. The polarity of the eluent is very important in paper chromatography since a small change in polarity can dramatically increase or decrease the solubility of some organic molecules. The organic pigments in the inks, which will be “spotted” on the filter paper, separate out as they are carried with the water at different rates. Those molecules that have a polarity closest to the polarity of the water will be the most soluble and will move outward on the radial chromatogram the fastest.

Many materials, such as the ink in felt-tip pens, are actually mixtures made up of several different organic compounds, or pigments. Each of these pigments has a different molecular structure and, usually, a different polarity. Many of these pigments can be easily separated using paper chromatography, because even when mixed together, they tend to maintain their own characteristic physical properties.

Experiment Overview

Once Mary alerted the state fraud department of the Alyson Marie Combs situation they immediately searched for the check that was written to withdraw the funds. Once they located it they sent it over to the crime lab for ink analysis via chromatography. Several other customers that wrote checks left pens at the bank over the last week. They were collected and their ink will be analyzed in this experiment. The evidence sample will then be compared to test results to determine which pen was used to forge Ms. Combs’s signature. The sample will then be sent to the crime lab for further analysis.

Materials

Prelab Questions

Read through the Background and Procedure sections before answering.

1. What is the adsorbent and the eluent used in this laboratory activity?
2. Describe how the process of chromatography works to separate organic compounds.

Safety Precautions

While this activity is considered nonhazardous, follow all appropriate safety rules.

Procedure

Part A. Chromatography

1. Obtain a piece of filter paper. Use a sharp pencil tip or pushpin to puncture a small hole into the center of the filter paper.
2. Select one of the pens collected from the bank and make a dot on the filter paper about 1 cm from the center hole.
3. Using a pencil, write the corresponding number that is on the pen on the filter paper near the edge so it can be identified later (see Figure 1).
   {12485_Procedure_Figure_1}
4. Repeat steps 2 and 3 with the remaining five pens.
5. Fill a plastic cup roughly ¾ full with tap water. Use a paper towel to dry off any water drops that are on the rim.
6. Roll up a wedge-shaped filter paper “wick” into a tight cone and insert it into the center hole (see Figure 2).
   {12485_Procedure_Figure_2}
7. Set the prepared filter paper circle on top of the water-filled cup and observe as the water is soaked up through the wick and then outward radially across the paper. Observe how the advancing water front acts on the spots of black ink.
8. When the water has advanced to within 1–2 cm of the outer edge of the circle (should take 10–12 minutes), carefully lift the chromatogram up and set it on a paper towel to dry.
9. Once dry, compare the chromatography of the ink used to sign Ms. Combs’s check with your test results to determine which pen should be sent to the crime lab for fingerprint analysis.
10. Dispose of the water down the drain. The filter paper disks may be saved or discarded in the trash. Save all other materials for future use.

Student Worksheet PDF

12485_Student1.pdf