In this lesson, we’ll discuss what chromatography is and why it’s useful to forensic scientists. We’ll focus on thin-layer chromatography and how it works, but we’ll also mention some other types of chromatography.
What Is Chromatography?
Did you come here to solve crimes or to do science? Or maybe you’re here to do both?Let’s pretend you’re on the case to solve a bank robbery. You know that, sometimes, when bank tellers are handing wads of bills to a bank robber, they include a special packet of red dye with the bills. This red dye packet is triggered to burst by a radio transmitter near the door of the bank, leaving red stains all over the bills and sometimes the bank robber. You have two suspects in front of you, Mr.
Handed and Mr. Herring. Both were caught with red dye on their hands, trying to spend $20 bills covered with red dye.
Your suspects don’t seem to be connected. Both claim that the red dye came from a broken red pen. You think one of them is lying.
Which one of them committed the crime?You might turn to chromatography, the science of separating mixtures into their component parts. Why would separating a mixture help you crack this case? Well, the dye used in dye packs is a special dye, called Disperse Red 9. The ink used in a red pen has a very different chemical composition. By separating out the components of the red markings on our suspects, we can see who’s touched the dye.
In this instance, you would use thin-layer chromatography, which is a fairly simple technique. In thin-layer chromatography, you put a spot of mixture (such as the dye found on your suspects) on a thin-layer chromatography plate. This chromatography plate is called the stationary phase because it doesn’t move.
You then put that plate in a beaker or watch-glass. In the bottom of your beaker is a solvent. This solvent is called the mobile phase because it’s a liquid (in other cases it’s a gas), and will move during the chromatography to provide your results. The solvent will slowly move up your plate, as though the plate were a sponge. When the solvent moves, the mixture will move with it. You know how if you have a stain on your shirt, and you get it wet, suddenly the stain spreads? Like that.
The different components of your dye mixture will move at different rates. Give the chromatography plate some time, and you’ll see that it looks like there are different colors on the same chromatography plate, from the same dye mixture.You can then compare the samples of dye mixture. Do either of them look like the mixture found in a red pen? Mr. Herring’s does, so it looks like he may be innocent. Do either of them look like the mixture found in Disperse 9? Mr. Handed’s does, so it looks like you might have found your bank robber.
You can also calculate how hard different components of the mixture are to move with a bit more precision. This is called the retention factor and is abbreviated Rf. You find your retention factor by dividing the migration distance of the component substance by the migration distance that the solvent has reached at its furthest point. So if your solvent reaches the 5 cm mark but a certain component has only moved 2.5 cm, your Rf would be 0.5.
If you perform your experiment under exactly the same conditions each time, your Rf will be the same if your components are the same. But keep in mind that subtle differences in temperature, solvent concentration, humidity, and so on will affect your Rf.Why do components of your mixture move at different rates? This has to do with polarity. Polar molecules, like water, have a partially positive side and a partially negative side, because electrons tend to spend more time in their negative end than the positive end. Non-polar molecules, like oils, have a neutral charge throughout the molecule. Polar molecules tend to stick to the chromatography plate, so the larger the Rf value, the less polar a molecule is.
Other Types of Chromatography
There are other types of chromatography besides thin-layer chromatography, and they all have different purposes. Paper chromatography is so simple you can try it at home, using a coffee filter as your stationary phase and rubbing alcohol as your mobile phase. Using paper chromatography on markers and pens can be fun!If you have a sample that is volatile – that is, portions of your mixture might turn into a gas, gas chromatography will better help you analyze your sample.
Size-exclusion chromatography can help separate mixtures by the size of their component molecules, rather than by their polarity. Each can help with a different type of suspect.
Chromatography is the science of separating mixtures into component parts. It can be used in forensics because it can help forensic scientists determine the type or origin of a sample. For instance, if a suspect is covered in red dye, we can use chromatography to find out whether it’s the same type of red dye used by banks to protect against bank robberies.Things you need to practice chromatography include the stationary phase, a solid onto which you put your sample, and a mobile phase, a liquid or gas that’ll move across the stationary phase. The mobile phase will move different molecules at different rates.
We can calculate how much a component molecule resists movement by calculating the retention factor, or Rf. This is calculated by dividing the distance moved by your component molecule by the distance moved by your mobile phase. The reason that different molecules move at different rates is because there is a difference in their polarity. This means that some molecules have a strong partially-negative end and a strong partially-positive end, while some molecules have weak charges, and some have none at all.
Thin-layer chromatography is popular because it is cheap, easy, and useful. However, there are many other types of chromatography to explore, including paper chromatography, gas chromatography, and size-exclusion chromatography.