Guard cartridge packing

[Peter Skelton]

Conventional sayings have us using a guard cartridge packed with the same material as the column we use, ie. C18 column gets a C18 cartridge.

But is there any reason not to use a different guard cartridge? I've yet to see a method that suggests such a thing, but it seems to me that if you're aiming to get something to bind irreversibly (or practically irreversably) to something that isn't the head of your column, then a different functionality may be beneficial for this.

Anyone got any experience with this?

[tom jupille]

The usual purpose of a guard cartridge is to catch anything that *would* bind irreversibly to the head of the column -- which suggests that making it as much like the head of the column as possible is exactly what you want to do (think of the guard as a replaceable/disposable column inlet!).

The risk with using something different is three-fold:
1. Something might not stick to the guard but would stick to the analytical column
2. Something that you are interested in might stick to the guard and never get to the analytical column
3. The selectivity of the overall separation might be changed (this one is usually not that big an issue so long as the volume of the guard cartridge is small relative to the volume of the analytical column).

[Peter Skelton]

That seems like a rational enough reason to stick with what I'm doing. I know that my current setup is catching my oily injectables on the guard cartridge, but I also know that they're not retained indefinitely, as I can elute them from the guard with my impurities method.

[tom jupille]

My personal feeling about guards is that they have their place, but are not a panacea.

The purpose of the guard is to catch "column killers". If you have predictably clean samples, they are a waste of effort. If you have predicably dirty samples, they can be effective, but they are an expensive way to go; it's usually more cost-effective to do a good sample clean-up.

Where guard cartridges earn their keep is unpredictable samples; in that situation, they are like an insurance policy.

[Peter Skelton]

I'm not normally a fan of the things, as you suggest I'd rather do decent sample prep. However it's much cheaper to buy guard cartridges than it would be to do SPE on 50-100 samples per day, especially as I don't need to worry about validating the SPE process this way.

One cartridge is £20 and lasts for about 250 injections before I change it, and I could probably extend this too. The time alone for sample cleanup would be much more.