A list of tubing and fittings and their use can be long and daunting. But in reality, making a choice can be rather simple (with a little experience). Match the female and male fittings of the same manufacturer(Rheodyne, SSI,Valco, etc); in some cases these can be mixed with no trouble (here is where some of that experience comes into play). You can use plastic (PEEK, etc) male fittings in steel female fittings, but the other way around is not recommended since stripping of threads becomes a problem. Stainless steel ferrules will be permanently swagged to tubing (the tubing will actually be dimpled) whereas after use PEEK ferrules can be moved on the tubing so can be replaced if necessary. For BASi® microbore columns use only PEEK nuts and ferrules.
PEEK and stainless steel tubing should be able to withstand the pressures most chromatographers encounter (<5000 psi or <350 bar). Stainless steel tubing of <0.020 inches ID is forgiving and can take 90° bends and stay in position, whereas PEEK requires large circumference turns (so as not to be weakened) and snaps back unless restrained by some sort of frame. PEEK can be cut (with a sharp single-edge razor blade or a cutter for PEEK tubing) into 90° butt ends; stainless steel is hard to cut and hard to make a 90° cut. This is why it is best to get precut stainless steel tubing for the lines that carry sample. The 90° end-cuts are important to produce zero dead volume connections. Sample moving in the tubing is dispersed/diluted by excess volume, so use 0.005 inch ID tubing for all sample delivery paths.
A few generalities: for high-pressure paths that have sharp bends use a stainless steel tube and stainless steel nut and ferrule, this same combinations is used for permanent connections; for not so permanent connections, like the inlet of an analytical column, you might prefer to use a two-piece PEEK nut and ferrule with either PEEK or steel tubing; for a PEEK column, PEEK injector or PEEK female fitting use a two-piece PEEK nut and ferrule; for PEEK tubing use PEEK nuts and ferrules; for low-pressure fittings, like the column exit and inlet/outlet of the cell, you can use a one-piece nut and ferrule. All manual injection valves on BASi® products are manufactured by Rheodyne. If your connection/junction has not been included here, call or email BASi and we can go over the requirements.
1p=one-piece nut/ferrule fitting
N/F=nut and ferrule