The Case of the Sticky-Fingered Kidnapper

Problem: Detectives Morgan and Grimes have been called in to investigate the apparent kidnapping of Lily Green, the eight-year-old daughter of a local real estate speculator. The only evidence of any consequence that the detectives have is a ransom note. The note was hand printed using some sort of felt-tip marker and has an orange stain on the edge, that when tested had a high sugar content consistent with the sugar coating found on a variety of candies. The detectives surmise that the kidnapper left this stain handling the paper while writing the note, but it was too smudged to detect a fingerprint.

Your assignment: Determine the probable type of marker used to write the note and what type of candy the kidnapper was eating prior to writing the note. Since most of you are novice forensic investigators, the CSI in charge points out that both of these should be able to be determined if you can separate the components of the ink and the orange dye and compare them to a variety of samples.

Using paper chromatography you should be able to accomplish the separation.

  1. Obtain samples of a variety of pens and orange candies.
  2. Use a simple absorbent paper like paper toweling or filter paper as the stationary phase.
  3. Draw a short line (about 1 cm) horizontally across the paper strip with each test pen or candy, about 2 cm from one end of the strip. To apply the dye sample from the candies place a drop or two of water on the piece of candy then rub the candy on the paper.
  4. Fold the end of the strip farthest from the sample over a stirring rod and tape or staple the strip to keep it in place.
  5. Place the rod across the top of a 400 ml beaker so that the strip is suspended and drops down into the beaker.
  6. Add water to the beaker until the bottom edge of the strip is just immersed in the solvent.
  7. Allow the water to move through the sample and toward the top of the strip until you can see definite separation of the component colors.
  8. If there is no separation of dye in your marker sample, it may be necessary to use a solvent other than water in order to achieve separation. Use ethanol as an alternative solvent
  9. Calculate the Rf for each marker and candy (see notes for calculation information).
  10. Obtain a sample of the separation for the pen and candy recovered from the crime scene and calculate the Rf of each.
  11. Compare the Rf of the sample to the test markers and try to determine the marker used to write the note.
  12. Repeat the comparison process with the orange dye

Helpful Information for Forensic Novices

Chromatography:

A mixture is a combination of two or more kinds of matter. The different kinds of matter in a mixture can be separated by physical means because each component in a mixture retains its own composition and properties. Properties of substances that can be used as a basis for separating mixtures include solubility, melting point, molecular size, and magnetic attraction, to mention a few.

There are a variety of methods for separating mixtures; some methods will work for some components, but not for others. Some methods allow you to identify the components but not easily quantify or reclaim them.

Separation of mixtures that contain different colored components can sometimes be accomplished using chromatography. Chromatography can indicate the number and colors of components in a mixture, but it is difficult to use as a method for recovering the components. Chromatography consists of three portions, the stationary phase, the solute (usually the unknown sample) and the mobile phase. The ability of a mixture to be separated by chromatography depends on two things: the solubility of the sample in the mobile phase and the difference in size of the particles in the components. If the mobile phase is water, the more hydrophilic molecules (those that are most soluble in the water) will move faster than those that are hydrophobic (those that tend to be insoluble in water). In addition the smallest of the hydrophilic molecules will move faster through the molecules of the stationary phase, just as small particles move more easily through a sieve.

In paper chromatography the stationary phase is a piece of absorbent paper. The substance to be separated (the solute) is applied to the solid phase. A solvent (the mobile phase) is then allowed to move through the sample. The motion can be radial, descending or ascending. In all cases the “wicking” action of the paper disperses the solvent in a particular direction. If the substance is reasonably soluble in the mobile phase, the substance will also move away from the point of application.

Quantifying the movement of the sample through the stationary phase can identify each sample. We quantify the sample by calculating the rate of flow (Rf), using the formula

Rf = the distance the compound travels

The total distance the mobile phase travels

The Rf can be used to help identify the parts of the mixture. The Rf depends upon a number of variables – type of stationary phase, temperature, starting point etc. It is often easier to calculate reference values by testing known substances before testing unknowns and comparing the Rf.

Anatomy of a dye:

Inks and dyes are sometimes pure colors but more often are made by combining different colored components. This is evident if you look at a colored poster that has been left out in the weather and you can see which colors the effects of rain and sun have worn away. When it is important to have resilient long-lasting colors the mixture may be different than in a dye that is not intended to be permanent. In food products additional considerations for the mixture will include the effect that the different dyes will have on the product to which they are applied, as well as the effect that product has in reverse upon the dye.

Knowing the type of dye used in a product can be import for a variety of reasons. Many people are allergic, for instance, to yellow dye #7. Knowing if a particular product contains this dye could help to avoid an allergic reaction. As illustrated in this lab, knowing the dyes that compose a particular type of ink can be important in a forensic investigation. The dyes used in cosmetics can also at times cause irritation or allergic reaction and some people may need to avoid using products that contain certain dyes. The chemical make-up of everyday substances including the dyes that give them their color can be important to many people. This is just one way that chemistry plays a role in your every day life.