What Type of Sample to Collect

What Type of Sample to Collect

After determining where to collect samples, the next step in designing a sampling plan is to decide what type of sample to collect. Three methods are commonly used to obtain samples: grab sampling, composite sampling, and in situ sampling. The most common type of sample is a grab sample, in which a portion of the target population is removed at a given time and location in space. A grab sample, there- fore, provides a “snapshot” of the target population. Grab sampling is easily adapted to any of the sampling schemes discussed in the previous section. If the tar- get population is fairly uniform in time and space, a set of grab samples collected at random can be used to establish its properties. A systematic sampling using grab samples can be used to characterize a target population whose composition varies over time or space.

A composite sample consists of a set of grab samples that are combined to form a single sample. After thoroughly mixing, the composite sample is analyzed. Because information is lost when individual samples are combined, it is normally desirable to analyze each grab sample separately. In some situations, however, there are advantages to working with composite samples. One such situation is in deter- mining a target population’s average composition over time or space. For example, wastewater treatment plants are required to monitor and report the average compo- sition of treated water released to the environment. One approach is to analyze a se- ries of individual grab samples, collected using a systematic sampling plan, and av- erage the results. Alternatively, the individual grab samples can be combined to form a single composite sample. Analyzing a single composite sample instead of many individual grab samples, provides an appreciable savings in time and cost. Composite sampling is also useful when a single sample cannot supply sufficient material for an analysis. For example, methods for determining PCBs in fish often require as much as 50 g of tissue, an amount that may be difficult to obtain from a single fish. Tissue samples from several fish can be combined and homogenized, and a 50-g portion of the composite sample taken for analysis.

A significant disadvantage of grab samples and composite samples is the need to remove a portion of the target population for analysis. As a result, neither type of sam- ple can be used to continuously monitor a time-dependent change in the target popu- lation. In situ sampling, in which an analytical sensor is placed directly in the target population, allows continuous monitoring without removing individual grab samples. For example, the pH of a solution moving through an industrial production line can be continually monitored by immersing a pH electrode within the solution’s flow.