8

th

Grade Forensic Science

T. Trimpe 2006 http://sciencespot.net/

What is chromatography?

From Wikipedia ...

Chromatography

(from Greek word for

chroma

, colour) is the collective term for a family of laboratory techniques for the separation of mixtures. It involves passing a mixture which contains the analyte through a

stationary phase

, which separates it from other molecules in the mixture and allows it to be isolated.

Which means ...

Chromatography is the physical separation of a mixture into its individual components. We can use chromatography to separate the components of inks and dyes, such as those found in pens, markers, clothing, and even candy shells. Chromatography can also be used to separate the colored pigments in plants.

http://members.shaw.ca/vict/chemistry_test3.htm

A few basics ...

Mixture

– Two or more substances that are mixed together, but not chemically combined.

Examples of mixtures ...

Air – mixture of gases Bowl of cereal – mixture of cereal and milk Soda pop – mixture of soda syrup, water, and CO

2

gas Fog –water suspended in air Kool-Aid – mixture of water, sugar, and flavor crystals

Compounds

– Two or more elements that are chemically combined.

Examples of compounds ...

Salt –Sodium and chlorine combined chemically Water –Hydrogen and oxygen combined chemically Carbon Dioxide – Carbon and oxygen combined chemically

Solutions

Solutions

are mixtures in which one substance is dissolved in another.

Solutions

have two parts: solute and solvent The solute is the substance that is dissolved.

The solvent is the substance that does the dissolving

Identify the solute and solvent in each solution ...

Solution

Lemonade Soda pop Ocean water

Solute Solvent Solubility -

A measure of how much of a given substance will dissolve in a liquid. A substance that does not dissolve in water is called

insoluble

. A substance that does dissolve in water is called

soluble

.

Types of Chromatography

Liquid Chromatography

is used in the world to test water samples to look for pollution in lakes and rivers. It is used to analyze metal ions and organic compounds in solutions. Liquid chromatography uses liquids which may incorporate insoluble molecules.

Gas Chromatography

is used in airports to detect bombs and is used is forensics in many different ways. It is used to analyze fibers on a persons body and also analyze blood found at a crime scene. Helium is used to move a gaseous mixture through a column of absorbent material.

Thin-layer Chromatography

uses an absorbent material on flat glass or plastic plates. It is used to detect pesticide or insecticide residues in food. Thin-layer chromatography is also used in forensics to analyze the dye composition of fibers.

Paper Chromatography

is one of the most common types of chromatography. It uses a strip of paper and capillary action is used to pull the solvents up through the paper to separate the solutes.

RF Factor

Because molecules in ink and other mixtures have different characteristics (such as size and solubility), they travel at different speeds when pulled along a piece of paper by a solvent (water). For example, black ink contains several colors. When the water flows through a word written in black, the molecules of each one of the colors behave differently, resulting in a sort of “rainbow” effect with the colors separating out at different rates.

We can determine the RF (or retardation factor) by measuring the distance the pigments traveled up the strip and the distance the water traveled. You will need to calculate the RF for each of the colors that are present in a sample.

RF =

Distance pigments traveled Distance water traveled

Directions

Tape – Label with marker Pencil • • Obtain the supplies you’ll need from your teacher.

– Beaker with a small amount of water – Paper strips – Pencil – Markers for testing – Masking tape – Timer Paper Ink Cut the strip so that it will be at the correct height – long enough to reach the top level of the water. Write the pen number on the strip using a pencil or permanent marker.

• Use one of the testing markers to place one line on the paper strip at about ¼ inch from the bottom. You don’t want the ink line to touch the water.

• Tape the top of the strip to a pencil and hang in the water for 5 minutes.

• Remove the strip from the water and use a pencil to mark the line between the wet and dry portions of the strip. • Use the ruler to make the measurements of each color for RF data and then allow the strip to dry.