by
lmh » Thu Dec 10, 2015 1:14 pm
I really wouldn't write off guard-columns as a universally bad idea. Of course it's possible to prepare samples so cleanly that they will never harm an analytical column, and that is certainly good sample prep, but it takes time, which is a limited commodity! Often it's possible to get fine results from really quite unpleasantly dirty samples, but with the disadvantage that back pressure creeps up and retention times start to move after a few hundred injections. If using, and changing a guard solves the problem, then it's worth considering.
To take a concrete example, Luna guard cartridges in the UK currently cost about £200 for a pack of 10, I think. It takes me about 5 minutes to find and change a Luna guard, so if I decide to change a Luna guard every 250 samples, my cost per sample is 0.08 pounds, and the time spent per sample is just over a second. If, instead, I decide to put the samples through microcentrifuge spin-filters, these cost about 1 pound per sample, and if I spin them in batches of 18 (the size of my centrifuge) and pipette them, I'm probably going to do 20 seconds work on each sample. Of course syringe-tip filters are cheaper if you have enough sample, but they're even more time-consuming to handle, not to mention the added plastic waste.
In many cases an inline filter would do just as well as a guard, of course. Removing highly-retained stuff by SPE is going to add a lot of time and expense to the sample prep, and it's something I would avoid unless the SPE gives me better results or allows me to concentrate up an unreasonably dilute sample.