I have used guard columns that attach to the column with an adaptor or with a short piece of tubing and both worked well if properly connected (minimal void volume). There is usually little or no noticable loss in column efficiency with these types. The ones I have used are from vendors such as Waters, Supelco, Phenomenex, and Rainin and all were good. One way to choose the guard column is to be sure that you can get cartridges that match or are similar to the analytical column that you are using. Of course cost can also be a consideration when comparing different vendors. Knowing when to change the cartfidge is the most difficult question to answer since it depends on how clean (or dirty) your samples are. With some brands, you can visually inspect the cartridge for contamination. You can also watch your chromatograms and observe when the separation starts to deteriorate. You can then plan to change the next cartridge before that point if your samples are consistant and you have been monitoring your number of injections. Keep in mind that if you wait until your separation deteriorates, your analytical column is also dirty and needs to be cleaned (or replaced in the worst case). Also check the archives for past discussions on this topic. Good luck choosing!
