Just take out the cartridge filter, run enough to purge the lines, then grab a sample, and reverse the process.

Or not.

If you have old iron lines, the situation is different. Iron can be released into the tap water continuously -- I assume your cartridge shows up orange? But, it can also be introduced in large quantities, on a no-notice basis. Among other occasions, this will tend to happen anytime the fire hydrants are used or a water main breaks . . . or even is just closed. So, depending on when you collect the sample, it may not tell you anything. The condition of your particle filter will tell you more. Orange = iron.

Also, with water lines, you may be introducing insoluble iron particles directly.

The good news is, this sort of iron can be handled even more effectively than well water simply by filling SL-O-WLY via the skimmer with the pump running. But, using HEDP and polyquat will help.