sample storage
Sample storage is keeping a sample unchanged from the moment it is collected until the moment it is measured — fighting the slow spoilage that would otherwise rewrite the answer. It is the same battle as keeping milk fresh: leave it out and it turns, but chill it, seal it, and use the right container, and what you pour tomorrow is what you bought today.
In the lab, this means choosing the container, temperature, and any preservatives so the analyte's concentration holds steady over the wait. Samples can change in many ways: volatile compounds evaporate, microbes consume the analyte, light or air drives chemical reactions, metals plate out onto glass walls, or contaminants leach in from the container itself.
It matters because a perfectly taken, perfectly analyzed sample still gives a wrong answer if it changed in between. The fixes are specific to the threat: refrigerate or freeze to slow biology, acidify water samples to keep metals dissolved, use amber bottles against light, fill containers to the brim to exclude air, and pick container materials that neither leach into nor adsorb the analyte.
Water samples for metals are stored in acid-washed plastic bottles, acidified to pH below 2, and kept cold — so the dissolved metals stay in solution instead of sticking to the bottle walls before analysis.
Container, cold, and preservative keep the sample from changing.
Storage rules are analyte-specific and often have a holding time — the maximum interval after which the result is no longer trusted. The same water sample may keep for months for metals (acidified, cold) but only hours for dissolved oxygen, which shifts as soon as the bottle is opened.