A biological sample is material taken from an organism or biological setting for observation or analysis, such as blood, a tissue biopsy or a cell-culture aliquot. The source, collection time and handling define what the material can represent. A sample of extracellular fluid and the cells suspended in it need not report the same molecular pool. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 8, Manipulating Proteins, DNA, and RNA: preparation and measurement of biological material
Sampling and measurement are separate processes. If a preparation preferentially loses fragile cells, repeated measurement of the surviving cells can be precise yet biased relative to the starting cell population. Replicate aliquots assess one kind of variability; independently collected specimens assess another. Explicitly identifying the sampling unit prevents technical repetition from being mistaken for independent biological evidence.
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