In a recent email, a reader enquired about the best way to clean the frits used in the HPLC system. He indicated that he had been sonicating them in nitric acid, but wasn’t sure if that was the best way to approach the problem.
There are four main types of frits that are used in the HPLC system: (1) the sinker frits in the mobile phase reservoirs, (2) frits internal to some pumps, (3) in-line filters, most commonly used between the autosampler and column or guard column, and (4) column frits. Years ago it was common to change the column frits, but with today’s highly compacted columns, removal of the column frit may allow the column packing to ooze out and ruin the column. For this reason, removal of column frits is not recommended. Let’s look at each of the other frit types and how they function to get an idea on whether or not they should be cleaned.
The sinker frits in the mobile phase reservoirs typically are sintered glass or stainless steel. Their function is twofold, as my name for them implies. They act as sinkers to ensure the transfer tubing from the reservoir to the pump is in the bottom of the reservoir. They act as very crude filters. Because the porosity of these is large – typically 10 µm – they trap dust or large particles, but are not effective at filtering the mobile phase. Mobile phase filtration usually is accomplished with an 0.2 or 0.5 µm porosity membrane filter. I think that cleaning these is not worth the trouble, and instead I recommend that you replace them every 6-12 months as part of a preventive maintenance program. I know people who, as the reader does, sonicate them in acid and claim success, but I wonder if the labour involved is more expensive than replacing the part with a new one. If blockage is common, it is best to find the problem and prevent it. For example, always use newly cleaned reservoirs (don’t refill a dirty reservoir) and discard the buffer after 1-2 weeks. Frits internal to the pump, such as the screens mounted on some check valves or on the outlet from the pump, serve to trap bits of pump seal or other debris from the pump. When I’ve looked at this kind of frit, they look like precision parts to me, with a tight fit and not much room for forgiveness if they get damaged. I wouldn’t try to refurbish these – just replace them with a new part.
The in-line filter traps debris from worn pump seals or injector rotor seals, as well as any particulate matter that might be remaining in your sample. As I’ve said before, I think these are the simplest and least expensive way to extend the lifetime of a column. I recommend that they be used just after the autosampler on every system.
I can’t imagine that it would be worthwhile to clean these. At a cost of <$5/frit and a lifetime of >100 samples/frit, we’re looking at <1% of the cost of sample analysis. With the total cost of a lab technician in the range of $50-100/hr in most labs, and only 10 min to clean a frit, you’d still be losing money – just replace the frit with a new one. And if you do try to clean a frit and don’t do a good job, and then put it in upside down in the holder, you may just wash any remaining debris onto the column, which you were trying to avoid in the first place.
So my bottom line is: no, I don’t know of any better way to clean frits than sonicating in dilute nitric acid. But I do know a more effective way to handle frit maintenance – replacement with new parts.
This blog article series is produced in collaboration with John Dolan, best known as one of the world’s foremost HPLC troubleshooting authorities. He is also known for his research with Lloyd Snyder, which resulted in more than 100 technical publications and three books. If you have any questions about this article send them to TechTips@sepscience.com