Density gradients are used in many different operations:
- To separate particles of different densities (isopycnography, which is short for "equilibrium density gradient centrifuation)
- To separate particles of different sizes (sedimentation centrifugation)
- Column elutions that must smoothly go from one concentration to another
- Isolation of diamond dust (isopycnography)
- Isolation of bovine X-sperm from Y-sperm (dairy industry) (sedimentation without centrifugation)
There are several ways for making density gradients including those that use syringes, twin linked containers, and other devices. Here are two very simple additional ways to make linear density gradients:
- This might be simple but it is one that takes many hours is to fill a plastic centrifuge tube with - say - a 10% sucrose solution. Put it in the freezer. The first to freeze will be almost pure water, with only a little sucrose trapped in it, but as the overlying ice layer gets thicker, more and more sucrose is trapped within it. Thus when it is completely frozen, the top has little sucrose and the bottom has much. Upon subsequent thawing, the bottom melts first, and the melting proceeds upwards reinforcing the preparation of the gradient.
- Here is one that takes about 60 seconds of time from start to finish!
The beauty of this procedure, which was invented by this author with the help of a remarkable undergraduate, is that it is not limited to centrifuge tubes. Cylinders of any size will work. (Teachers: make a demo using a graduated cylinder which is half filled with water, and then underlayer it with an equal volume of very salty water into which has been put some dye. Cap it off so that no air is left in the cylinder and then roll it at the appropriate angle - perhaps about 20 rapid revolutions in 2 seconds. Upon standing the cylinder upright, the blue will become linearly more intense with depth.)
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