by
lmh » Wed Apr 25, 2018 11:01 am
In defence of PEEK, it's very easy to use, wherever the pressure isn't excessive (i.e. everything post-column in any system, and pre-column in HPLC applications where you expect pressures not to exceed 300bar). Metal tubing is a pain to cut, and a pain on any system where you make regular changes to the plumbing, because it doesn't like being bent, bent back, fiddled around with and bent again; it just gets more and more kinked until fittings don't fit. The thin metal tubing with normal-diameter ends is easier to use, but liable to breakage at the junction between thin and thick. I haven't tried PEEKsil, but it would worry me that where connections are continuously being changed, it would get compressed, and you can't just trim a few mm off the end, like you would with normal PEEK.
Yes, the previous poster is quite right about flow-rate. For what it's worth, we generally use red (0.005") on nearly all our systems, which are analytical and mostly equipped with UHPLC pumps, with methods running at up to about 0.6mL/min. We do change to 0.007 or 0.010, or even wider, downstream of detectors that don't like back-pressure (breaking fluorescence detector flow-cells is a hobby restricted only to the very wealthy). We have run semi-prep methods at up to 3.5 or 4mL/min on these systems, but the back-pressure isn't pretty, and I wouldn't try it with a viscous solvent mix. In these cases we keep the tubing as short as possible, and use the blue 0.01" where possible. Upstream of the column, everything is inevitably metal.
In the end, you choose the tubing that keeps your backpressure down to what you deem to be acceptable. It's worth having all the standard sizes available in the drawer.