There is no fixed standard for the storage time of water samples for water quality testing. The key lies in the stability of the testing items and whether the storage conditions are standardized. Moreover, it is necessary to combine the operation details of the water sampler to ensure the initial state of the water samples. Specifically, it can be summarized into the following three points:
The retention period is determined by the testing items: indicators that are easily affected by the environment (such as pH, dissolved oxygen, residual chlorine) need to be tested on-site after water sample collection and are almost impossible to preserve (water samples collected by water samplers need to be analyzed immediately with portable instruments). Relatively stable indicators (such as heavy metals, total nitrogen and total phosphorus) can be stored for several hours to several days under standardized storage conditions (for example, under refrigeration conditions, total phosphorus can be stored for 24 hours and heavy metals for 7 days). Organic indicators (such as VOCs) need to be fixed with fixatives and should not be stored for more than 48 hours.
Storage conditions directly affect the validity period: Three key factors need to be controlled - temperature (most water samples should be refrigerated at 0-4℃ to prevent microbial reproduction), container (when collecting with a water sampler, a suitable clean container should be selected, such as polyethylene bottles for heavy metal measurement and brown glass bottles for organic matter measurement), and fixative (for example, sulfuric acid should be added to adjust the pH when measuring ammonia nitrogen, and the fixative should be added immediately after collection by the water sampler in accordance with the specifications Otherwise, the indicators are prone to change. If the conditions are not met, the preservation time will be significantly shortened (for example, at room temperature, the total nitrogen water sample may become ineffective within 6 hours).
Core principle: "Test as soon as possible" : Regardless of the project, it is recommended that water samples collected by the water sampler be sent to the laboratory within 24 hours (except for special indicators) to avoid indicator deviations during storage (such as increased COD due to microbial decomposition and decreased dissolved oxygen due to temperature changes), which may lead to distorted test results.