I regularly receive e-mails from readers with a wide variety of questions. Some of these blossom into full-blown columns, but many are answered with just a short response. As I cleaned out my inbox recently, I chose a few of these to share. There is no central theme this month, but hopefully you will pick up some useful information from these topics related to practical operation of the liquid chromatography (LC) system.

Should I use a Guard Column or an In-Line Filter?

If you are a regular reader of "LC Troubleshooting", you will know that I am a strong proponent of using an in-line filter to protect the LC column. The filter contains a frit, most commonly 0.5 µm in porosity, and is mounted directly downstream from the autosampler. Its purpose is to trap the stray particle that can be injected with the sample or that results from pump-seal or injector-rotor wear. Columns packed with 5 µm diameter particles use 2 µm porosity frits to hold the particles in the column, and 3 µm particle columns use 0.5 µm frits on the outlet and either 0.5 or 2 µm frits on the inlet. Thus, the 0.5 µm in-line filter will catch any particles that would otherwise block the frit at the head of the column packed with 3 or 5 µm particles. When the system pressure rises significantly, for example by 25%, change the frit for a new one and you should be back in business. For columns packed with sub-2 µm particles, 0.2 µm frits are used in the column, so the in-line filter porosity should be no larger than this or it will not be effective. Two common filter designs are available. One comprises a reusable holder that is disassembled to replace the frit. The other is a disposable unit that screws into the end of the column. I believe that the in-line filter is the least expensive way to protect the column from blockage.