As a kid, I always played with my food. After a trip to the grocery store, where my mom allowed me to select one candy to bring home, playing turned into experimentation. The candy I chose? Skittles! I had seen several YouTube videos of people taking these colourful candies and placing them in water, causing the vibrant colours to bleed out. Prompted by these videos, I grabbed a shallow dish, filled it with a bit of water and began placing my Skittles around the perimeter. Like magic, streams of color began to leech from the candy, converging towards the middle of the plate, creating an admittedly unattractive brown colour. Little did I know that this experiment was my introduction to .
Separating mixtures of compounds is a chemist鈥檚 bread and butter. Whether synthesizing drugs, analyzing water contaminants, or trying to replicate nature in the lab, there is always a separation step. Several methods of separating the desired product from impurities, side products or unreacted reagents are available, the most common being some sort of chromatographic process. But what does that even mean? If we start with some etymology, chromatography 鈥渃hroma鈥� (蠂蟻峥段嘉�) meaning colour and 鈥済raphein鈥� (纬蟻维蠁蔚喂谓) meaning to write. So, in its essence, this technique is 鈥渃olour writing,鈥� but there鈥檚 more to it than that.
In more scientific terms, chromatography is a method by which a mixture is separated by distributing its components between a . As you may have guessed, the mobile phase is the one that will move during the separation. This can be a liquid or gas and is essentially there to push the components of the mixture along the stationary phase that can be paper, silica gel or some other substance to which compounds may be attracted.
Chromatography essentially revolves around the idea that 鈥渓ike dissolves like.鈥� In other words, , and those with differing properties, won't. This is illustrated when trying to mix oil and water. Even after shaking a mixture of the two vigorously, they will still settle into their distinct layers. This is because oil and water have opposing , or separation of electric charge. Water is a polar substance, and oil is highly non-polar.
This summer, in the 9I制作厂免费 chemistry labs, I have had chance to perform a more sophisticated version of my Skittles experiment. I took a drop of the solution from a dissolved green Skittle and spotted it on the bottom of a strip of chromatography paper. After immersing the paper in a shallow dish of salt water, the solution began to migrate up the paper, taking the dye components along for the ride. Most remarkably, the originally green spot separated into its primary colours, yellow and blue, along the path of the liquid. The blue dye traveled further up the paper, indicating its greater affinity for the salt water (polar). The yellow dye was better retained on the paper, demonstrating its greater affinity for the paper (non-polar). That鈥檚 how we achieve separation with chromatography!
There are many, many variations of chromatography but they all fit into one of two categories: planar or column. The Skittles experiment is an example of planar chromatography. The seemingly magical migration of the liquid up the paper is due to .
The movement occurs because the mobile and stationary phase have very strong cohesive and adhesive forces that are strong enough to defy gravity. It is during the upward movement of the mobile phase that different compounds will travel different distances up the stationary phase depending on their relative solubilities in the two phases. This is the basis of both paper and 鈥�,鈥� a topic that continues to haunt me to this day after a poorly written lab report.
The field of chromatography started with planar chromatography in the 19th century. Dye chemists would analyze their creations by into their mixture and then watching the colour spread. Then came column chromatography, an invention often attributed to , a Russian botanist, after he discovered that he could using a column filled with chalk (calcium carbonate). When a plant extract was applied to the top of the column and a mobile phase, an , continuously added, the components of the mixture would travel down the column at different rates. If they were coloured, they would be clearly seen as and could be collected as they emerged from the column. This remains the fundamental principle of column chromatography: components of a mixture separate due to different solubilities in the mobile and stationary phases.
Now, I suggest you go to your nearest grocery store, pick up a pack of Skittles and experiment for yourself.
Angelina Lapalme is a BSc student majoring in Bio-Organic Chemistry at 9I制作厂免费.
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