Preservation, and what it does to the container

Cold is the most common and the easiest to get wrong

Table II rows specify cooling, frequently at or below six degrees C and for the bacterial tests below ten. That is a transport problem rather than a container one, and it is where field samples most often fail: ice packs that did not last, a vehicle in summer, a sample left in a boot.

Acid, to keep metals in solution

Acid is added for many inorganic determinations to stop analytes adsorbing onto the container wall. It is the direct countermeasure to one of the three mechanisms, and it means the container and its liner must tolerate a low pH for the whole holding time.

Base, and thiosulfate

Some parameters are preserved with base, and chlorinated samples for bacteriological work take sodium thiosulfate to quench residual chlorine so it does not keep killing organisms in transit. Table II names the concentration.

Pre-preserved containers are a different product

Containers supplied with the preservative already in them remove a field step and an error, and they are specialist stock with documentation. They also make the empty container a hazardous item in transit, which is a shipping consideration people meet unexpectedly.

Adding it in the field has its own risks

Decanting acid at a sampling point is a real exposure and a real chance of cross-contamination. If you do it, the hazard communication rules apply to the acid you are carrying just as they would on a bench.

The liner is the part in contact

A cap with a pulp liner over a preserved sample is a failure waiting to happen. Preservation makes the liner a specification rather than a detail, which is the closures section.

References