 Continuing on from our previous video, we're going to start by answering the question, how can chromatography be used to determine the composition and purity of substances? In essence, chromatography is a separation technique, so it can be used to separate out different components of a mixture based on the strength of their intermolecular reactions. If our sample is not a mixture, but is instead a pure substance, we should have no separation into components. Often, however, our mixture will probably not be pure, and so we will want to quantify the purity of our substance. This means we need to determine how much of each component in the anolate mixture, which is extremely difficult to do with paper chromatography. And now we're up to our fourth and final question, what are some different kinds of chromatography? We're going to take a quick look at three different kinds of chromatography, and then summarize their characteristics. First up is thin layer chromatography, or TLC. TLC uses a thin layer of silica gel or alumina as the stationary phase, which is bonded to a glass slide. Just like in paper chromatography, the analyte is dotted onto the slide and the mobile phase can be almost any liquid solvent we choose. If, for example, silica gel is chosen, this means the stationary phase is highly polar, and in order to clearly see the effect of the differential interactions, a non-polar mobile phase such as heptane can be used. This technique is very similar to paper chromatography, and uses the polarity of different substances to separate them out. Next up, we're looking at liquid column chromatography. Though it looks a bit different, liquid column chromatography is very similar to thin layer chromatography. The stationary phase is also usually silica gel or alumina, but this time it's packed into a plastic or glass tube forming the column. The mobile phase is flowed through the column until it is uniformly wet, and then the analyte is added along with more of the mobile phase. Unlike in thin layer chromatography, the components of the mixture can be collected at the bottom of the column as they flow out. Once they're separated from the rest of the mobile phase, they can then be analyzed quantitatively using different techniques, so that the composition of the original mixture can be quantified. This can also provide a quantitative measure of the purity of our substance. Column chromatography, as I've described it here, uses gravity to push the mobile phase out through the column, and so the particles in the column need to be relatively large, and the column needs to be relatively short in order to process the sample in a reasonable amount of time. By adding pumps to the column however, we can use smaller particle size and longer columns, and that means that there is a greater separation between the components. That is, the resolution of the system is greater. The technique with all these pumps is then called high performance liquid chromatography. This photo shows an example of a high performance liquid chromatography setup. You can see the column in the center of the image, and two large pumps to the left. On the right is a system that analyzes the components of the mixture as they exit the column. The final chromatographic technique I'll talk about is gas chromatography. In this case, the analyte is dissolved into a solvent, which is then vaporized, so that it is in its gaseous form. The mobile phase is an unreactive carrier gas, like helium or nitrogen. The stationary phase can be either a liquid or a solid, which can be packed into a column, like in high performance liquid chromatography, or coated on the inside of a capillary wall. Another feature which makes gas chromatography a bit different is the oven that surrounds the columns, the temperature of which can be very precisely controlled. The analysis begins at a low temperature to separate out the low boiling point components, and the temperature then increases during the separation to resolve the less volatile and higher boiling point components of the sample. So the basis of separation in this case is the boiling points of the compounds. Now we've looked at four different kinds of chromatography. We can summarize their properties in a table by comparing their mobile and stationary phases, as well as the method of separation that they are based on. This brings us to the end of our videos on chromatography. Hopefully you can now identify the key components needed for a chromatographic analysis, and you understand the physical basis of separation in chromatography. You should also be able to describe some different chromatography techniques and understand how they can be used to determine the composition and purity of different substances.