This is a link within the Waters HPLC Primer site. It contains very useful details regarding the properties of HPLC packing materials and how they impact performance. Since it is part of a larger site, it contains links to other information provided by waters on other aspects of HPLC as well as MS and other techniques. There is a higher level of detail in the instruction here than in most general sites discussing HPLC. Also, the site shows which Waters columns contain the different kinds of packing materials described, which would be helpful to a lot of people.