I always dried them with N2 gas flow, ... then I put them in an mini-excicator with floated with N2 in the fridge for using in a week, ....

worked good. Never did a dried storage. so i cannot compare.

Always use them while wet (wet extraction is just that). That is the reason for the conditioning steps. Never dry the support between, during or after steps, just storage.

Thank you for the responses everyone!

Just to let you know that I also sent this enquiry to Agilent (the provider) and this is what they said:

"

In case of reversed phase SPE (which PPL also is) the solvent that is used for elution is typically used to pre-treat (condition) the cartridge. This helps to eliminate any interferences that otherwise might be extracted from the cartridge during the elution step. For silica based RP SPE (especially longer chain such as C18 and C8) this is also critical for "activating" the sorbent. C18 chains collapse (bind to each other) when gone to dryness which will heavily affect the binding capacity of the sorbent. So before loading the sample you apply solvent to "break" the binding between the C18 and thus the sorbent has most capacity for sample binding.

However as this solvent is also used for elution, you must equilibrate the cartridge before sample loading. If you would skip this step, most analytes will go through unretained. In most cases this would be using 100% water but depending on the application could also be acidified or basified water, or a solvent/water or solvent mixture. In case of silica based SPE it is critical to not let the sorbent go to dryness before loading the sample, as the C18 will collapse again and thus no proper binding of your analytes. For polymeric based SPE like BE PPL this is less critical as there's nothing to collapse. It will only be a sort of rinsing/cleaning step. But the equilibration step is, you need to make sure all solvent (methanol in this case) is removed before loading the sample.

Once loaded and thus analytes are on the sorbent, it does not matter anymore if sorbent goes to dryness. This could even be beneficial to improve recovery and you find methods where a drying step is included in the method. This going to dryness is by the way not something that happens in seconds. You don't need to watch the meniscus and even have a small layer still on top. If the water has gone through then make sure to switch off the vacuum. But if it has still been on a few seconds that's fine. Just don't leave it for a minute or longer (of course also depends on how much vacuum you use).

Here's an app note that shows a method for using BE PPL: https://www.agilent.com/cs/library/applications/SI-02188.pdf.

"

Of course, any further comments, especially from those with experience using PPL cartridges on natural water samples would still be welcome.