sampling
Sampling is the act of choosing and physically collecting the bit of material you will actually analyze, out of a much larger world you cannot test all of. It is the moment a pollster picks which thousand people to phone, or a cook decides which spoonful to taste — the deliberate step of saying, this small part will speak for the whole.
In the lab, sampling is the first link in the analytical chain: deciding where, when, and how much to take from a lot — a river, a warehouse of ore, a patient's blood — and how to physically grab it without changing it. The collected piece is called the sample, and from it everything else follows: preparation, measurement, the final number.
Sampling is easy to underrate because it happens before any chemistry, often outdoors and by hand, far from the gleaming instrument. Yet errors made here can dwarf every later one and cannot be undone in the lab. A good rule of thumb: the sampling step is frequently the largest single source of uncertainty in a whole analysis.
An environmental team samples a river by dipping bottles at several points across its width and depth, since pollutants are rarely spread evenly across a flowing stream.
Where and how you collect decides what the lab can ever learn.
Sampling and sample preparation are distinct steps, though people blur them. Sampling is getting the material out of its source; preparation (digestion, dilution, extraction, and so on) is turning that material into a form the instrument can read.