This answer includes this schematic showing the overview of the water processing system on the ISS, found on page 341 (page 375 of the pdf) of The International Space Station - Operating an Outpost in the New Frontier.
The top part of the diagram contains an "Archival Sample".
- How often are the samples collected?
- If they are continuously collected, what fraction of the water is collected?
- Are they returned to earth? If so, how often?
- How are they used? Have they served important roles yet? (If so how?)
- These samples seem to be potable water collected from the Water Delivery ORU. Wouldn't it make more sense to collect waste water for analysis (as well)?