Cross-contamination between samples
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Reused containers
Washing a container to standard is harder than it looks, particularly for trace work, and a bottle that held a high concentration can carry enough residue to matter in the next sample. Single use is the norm in regulated sampling for exactly this reason.
Swapped caps
Caps set down on a bench and picked up at random move material between samples through the liner. It is the most mundane cross-contamination route there is and it happens whenever several containers are open at once.
Preservative decanting
Pouring acid from a shared stock into sample bottles in the field moves contamination both ways and risks the person doing it. Pre-preserved containers remove the step entirely, which is much of why they exist.
Storage next to the wrong thing
Permeation runs both directions, so samples stored near solvents or strongly volatile standards can gain analyte through a plastic wall. Separating sample storage from reagent storage is a layout decision with a measurable consequence.
Gloves are for the sample as much as for you
Hands transfer material onto rims and into caps. In this property the gloves protect the result as well as the person, which is the opposite way round from the resin property and the same discipline.
Trip blanks and field blanks find it
A blank that traveled with the samples and never had anything added shows whether contamination happened in transit or in handling. It costs one container per run and it is the only way to distinguish those from a laboratory problem.
Open one container at a time
Most cross-contamination happens in the seconds when several are open together on the same bench or tailgate. Working through them one at a time costs very little and removes the swapped-cap route entirely.