A column frit is a small disk of sintered steel that is placed on each end of the HPLC column. Some frits are incorporated into the column end-fitting itself. And some frits are made of titanium or specialised alloys, instead of steel.
Frits have three things to do:
Protect the column packing from contamination.
Distribute the injected sample into a uniform band before the column head.
Keep in the column packing in.
Column frits are porous, and for obvious reasons, the pore size needs to be less than the particle size of the column packing. For an analytical 5μm column, the top frit may have a pore size anywhere from 0.2μm to 2μm. Bottom frits may have slightly larger pore sizes.
Which means of course, that frits make a significant contribution to the system backpressure. And it also means that frits can get choked and further increase the backpressure.
As we know, ignoring high backpressure is not a good idea.
Here’s how we deal with troublesome frits:
- Disconnect column from detector.
- Reverse the column, reconnect to injector.
- If it’s a C18 or C8 column, backflush column with AcN:water:70:30 @ 0.5ml/min for an hour. For any other bonded phase, check with the column data sheet for solvent compatibility. For a non-bonded normal phase Si column, I would back-flush with dry acetonitrile or iso-propyl alcohol.
- Keep an eye on the backpressure. Once it drops down to normal, continue backflushing for 15 minutes.
Backflushing works most of the time. If the backpressure hasn’t come down at all, then you have a really choked frit, and it’s time for drastic measures.
Remember, this is only a last resort.
- Clamp the LC column in a strong vise, if you have one. Or else, use two opposing spanners or wrenches.
- Carefully unscrew the end-fitting – of the top frit only.
- Leave the bottom frit strictly alone.
- If the top-frit is part of the end-fitting, that’s good. Or else, if the top-frit is a separate disk, then …
- Carefully slide the frit away from the column head. Don’t pull out the top-frit from the column head. Slide it out. Or, you will disturb the column packing.
- Place the frit in a 50 ml beaker, add enough acetonitrile to cover it, and sonicate the top-frit in an ultrasonic bath for 30 mins.
- Some chromatographers prefer a stronger sonication liquid like 50% nitric acid. Not advisable at all. Strong acids will physically corrode the frit.
- Sonication should clear the frit, and restore its normal perfomance.
Once again, you use this method as a last resort. If this doesn’t work as well, then you have a real problem. All you can do then, is to buy a new frit.
Or a buy a new LC column. C’est la vie.
Cheers … SKS