Hey guys, Francis here. In this video, we are going to learn about what capillary electrophoresis is and how does it work. The instrumentation of capillary electrophoresis is pretty simple. First, we have a few silica capillary that stand between two buffer reservoirs. Inside these buffer reservoirs, there's a platinum electrode which is connected to a high voltage power supply. Our sample can be introduced at one end of the capillary while the detector is placed near at the other end. The sample valve is interchangeable with the buffer reservoir during the introduction of sample. In order to understand how capillary electrophoresis work, we need to take a look at two key mechanisms. The first mechanism is the electrophoresmotic flow, which governs the movement or the direction of the valve solution in the capillary. The second mechanism is the electrophoretic mobility, which is responsible for the separations of charge species in the solution. First of all, let's take a look at the electrophoresmotic flow. By definition, electrophoresmotic flow is the movement of the separation buffer through the silica capillary as a result of the assistance of a zeta potential at the solvent silica interface. One of the key words we see here is the solvent silica interface. So to understand what exactly is electrophoresmotic flow, we need to take a look at what is going on at the solvent silica interface. This diagram here shows the cross-section of a few silica capillary. Typically, the capillary we use in CE has an inner diameter of around 10 to 100 micrometers, with length of about 40 to 100 cm. It is usually coated with a thin layer of polyimide to protect the capillary. To give you some perspective of how small the capillary is, we can take a look at our own hair. A typical human hair has a diameter of about 100 micrometers. The inner diameter of CE capillary is even thinner than our hair. Remember the curl fuel experiment that we have done in CM-MOM-9-1? We use column chromatography to separate beta-carotene and curl fuels from a plant and stric. The silica gel that we use for column chromatography is Paula, because of the cylinder groups exposed on the silica surface. Similarly, on the surface of the few silica capillary we use in CE, the silica groups. And the chemistry of these silica groups are pH-dependent. When the pH is lower than 3, the silica groups on the silica surface are fully protonated. When the pH is greater than 9, the silica groups are almost completely deprotonated, making the silica surface negatively charged. In between pH-3 and pH-9, the silica groups are partially deprotonated. In this experiment, the buffer solution we will be using has a pH of 7.7, which is pretty close to pH-9. Therefore, it is safe to assume that the slioner groups on the silica surface are dominantly deprotonated. Because of the deprotonation of the slioner groups on the silica surface, the capillary surface is basically negatively charged. The negatively charged surface will attract the positively charged carions from the buffer solution, forming a layer of tightly absorbed carions, which is called a fixed layer on the silica surface. However, the tightly absorbed carions can only partially neutralize the negative charge on the capillary surface. Therefore, the negative charge surface can still attract more carions from the buffer solution. This forms a diffused layer, where the concentration of carions is higher than the concentration of anions. Together with the fixed layer, these two layers are called the electrical double layer. At the center of the capillary tube is the bulk solution, where the concentration of the carions and anions are about the same. To complete the cross-section of a capillary, we have the diffused layer and the fixed layer at the other end, too. Let's take a closer look at what will happen when we apply a voltage across the capillary. On one end, we have the end node, which is positively charged. On the other end, we have the cathode, which is negatively charged. When the voltage is applied across the capillary, the carions in the diffused layer will be attracted to the negatively charged cathode, dragging the bulk solutions along with them. This is what we call the electro-osmotic flow. Now, let's take a slightly more 3D perspective of the electro-osmotic flow. Imagine the capillary tube is fixed in position, and on top of the capillary wall, we have the fixed layer and the diffused layer, where the concentration of carions is higher than the concentration of anions. And then we have the bulk solutions in the middle. Imagine the diffused layer is like a movable tube inside the capillary. When we apply a voltage across the capillary, the carions in the diffused layer will be attracted towards the negatively charged cathode, dragging the bulk solutions along with them. That's why the electro-osmotic flow is generally towards the cathode. Electro-osmotic flow is a unique feature of capillary electrophoresis. Because the electro-osmotic flow essentially originates near the capillary walls, it has a flat flow profile, which is very different from the perabolic flow profile we usually see in pressure induced flow in the case of HPRC. Therefore, as compared to the pressure induced flow, electro-osmotic flow minimizes band broadening, which in turn improves the peak resolution and the separation efficiency. The unique flat profile of electro-osmotic flow is the main reason for the higher separation resolution and efficiency in CE as compared to that of HPRC. In summary, we have learned about the electro-osmotic flow in this video. Electro-osmotic flows tells us about the movement and the direction of the bulk solutions in capillary electrophoresis. And also because of the unique flat flow profile, it usually results in better separation efficiency and resolution. But it doesn't tell us about the separation of analytes. So, the next question we need to ask is, how do we separate N ions, cat ions and neutral species in capillary electrophoresis? We will be learning more about this in the next video when we talk about the electro-ferratic mobility. So see you guys in the next video. Bye!