Ultrapure Water Applications – Water Sample Preservation Measures

1. Fill the container to overflow and seal.

To avoid sample vibration during transportation and interference from oxygen and carbon dioxide in the air on the sample composition and analytes, and to prevent impact on pH, BOD, DO, etc., the water sample should be filled to overflow and sealed for storage. However, for samples intended for freezing, the container should not be filled completely; otherwise, the expansion of volume after freezing will cause the container to break.

2. Refrigeration

The temperature for refrigerating water samples should be lower than the temperature at the time of sampling. Immediately after collection, place the water sample in a refrigerator or ice-water bath in a dark place, generally at 2–5°C. Refrigeration is not suitable for long-term storage, and the storage time for wastewater is even shorter.

3. Freezing (-20°C)

This generally extends the storage period, but requires mastering thawing and freezing techniques to ensure the sample quickly and uniformly recovers to its original state upon thawing. Water samples expand in volume when frozen; therefore, plastic containers are generally used.

4. Addition of Protective Agents (Firming Agents or Preservatives)

Adding certain chemical reagents can fix some analytes in the water sample. Protective agents should be added to an empty bottle beforehand; some can also be added to the water sample immediately after sampling.

Commonly used protective agents include various acids, alkalis, and biodegraders. The amount added varies depending on the needs.

The added protective agent should not interfere with the determination of the analyte. If there is any doubt, necessary experiments should be performed first.

The volume of the added protective agent affects the initial concentration of the analyte and should be considered in the calculation results. However, if a sufficiently concentrated protective agent is added, its dilution effect can be ignored due to the small volume.

The added protective agent may alter the chemical or physical properties of the components in the water. Therefore, the impact on the analyte must be considered when selecting a protective agent. For example, acidification can cause the dissolution of colloidal components and solids suspended on particulate matter. If the analyte is a dissolved substance, it must be acidified and preserved after filtration.

For certain assays, blank tests are necessary when using fixatives. For example, when measuring trace elements, it's essential to determine the amount of the analyte that the fixative can introduce. (For instance, acids can introduce significant amounts of arsenic, lead, and mercury.)

It is crucial to note that some protective agents are toxic or harmful, such as mercuric chloride (HgCl2), chloroform, and acids. Therefore, strict safety precautions must be taken during their use and storage.