What is a common term for a sample that is taken without prior planning and represents a specific time?

A grab sample is defined as a sample taken at a specific moment in time without prior planning or systematic sampling methodology. This type of sample provides a snapshot of the conditions at that exact time, which is particularly useful in situations where immediate results are needed, such as testing for contaminants or immediate water quality issues.

In contrast, a composite sample is collected from multiple locations or over a period to provide an overall picture of water quality, while continuous samples are taken at regular intervals over time to track changes. Random samples, although they involve some degree of unpredictability, are typically collected in a way that tries to ensure a representative cross-section of a larger set, rather than focusing on a specific point in time as a grab sample does. Thus, the defining characteristic of a grab sample is its spontaneity and its ability to reflect conditions at a particular moment, making it particularly significant in water quality assessments.